(V/STOL Handling-Qualities Criteria and Specifications Apply

Home , Takeoff, VTOL

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AGA RD-R-5779'

AGARD REPORT No. 577 on (V/STOLHandling I -Criteria and Discussion

DlSTRlBUT10 N AND AVAl LAB1LlTY ON BACK COVER

NORTH ATLANTIC TREATY ORGANIZATION

ADVISORY GROUP FOR AEROSPACE RESEARCH AND DEVELOPMENT

(ORGANISATION DU TRAITE DE L’ATLANTIQUE NORD)

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V/STOL HANDLING-QUALITIES CRITERIA

This report was prepared at the request of the Flight Mechanics Panel of AGARD-NATO. Published December 1970

629.7.014.16:629.7.0a3

Printed by Technical Editing and Reproduction Ltd Harford House, 7-9 Charlotte St, London. WiP iHD This report prepared by a V/STOL Handling Qualities Committee sponeored by the ACARD Fqght Mechanics PanelGsents criteria on handling-7 qualities for VTOL and'STOL aircraft. Included wlth each criterion is a discussion pointing out the pilot's reasons for including a particular handling quality feature. The criteria are based on results of tests using piloted ground-based simulators, variable stability aircraft, particular models of VTOL and STOL aircraft, ani variable stability helicopters. -I

ii i ACKNOWLEDGMENTS

The following personnel were members of the V/STOL Handling Qualities Committee who helped prepare this report:

Seth B. Anaerson, U.S.A., NASA, Ames Research Center , H. William Chinn, U.K., RAE, Bedford Antonio Filisetti, Italy, Fiat Dr. Xavier Hafer, W. Germany, Lehrstuhl und Institut fiir Flugtechnik Douglas M. McGregor, Canada, NAE Laurel G. Schroers, U.S.A., Army Aeronautical Laboratory George Ville, France, Service Technique de 1’ Aeronautique

iv TABLE OF CONTENTS

Introduction 1 Background 1 Revision Procedures 1 Report Format 1 Criteria Ap p 1i c a t i on 2 Interpretation 2 Level of Criteria 2 Wind Conditions 2 Classification of Aircraft 2 Terminology 2 Basic Terms 2 Flight Regimes . 3 Control Systems 3 Engineering Terms 3 Symbols 4 Pilot Rating Scale 5 References 5 1.0 Characteristics of the Control Systems 6 1.1 General 6 1.2 Control Breakout Forces 6 1.3 Control Force Gradients 6 1.4 Control System Free Play 8 1.5 Powered Control Systems 8 1.6 Trim Systems 8 1.7 Height Control Systems 8 1.8 Thrust Vector Controls 8 1.9 Control Travel Limits 9 1.10 Augmentation Systems 9 2.0 Longitudinal Stability and Control 11 2.1 General 11 2.2 Pitch Control Power 11 2.3 Control Sensitivity 12 2.4 Pitch Damping 13 2.5 Control System Time Lags 13 . 2.6 Static Longitudinal Stability Characteristics 14 2.6.1 Trim Speed Stability 14 2.6.2 Stability with Respect to Speed 14 2.6.3 Thrust Vector Stability 14 2.7 Longitudinal Control Characteristics in Maneuvering Flight 14 2.8 Dynamic Stability 15 . 2.9 Longitudinal Control Characteristics in Takeoff 15 2.10 Longitudinal Control Characteristics in Sideslip 15 2.11 Longitudinal Control Characteristics in Landing 16 3.0 Lateral Directional Stability and Control 17 3.1 General 17 3.2 Roll Control Power 17 3.3 Translational Control 18 3.4 Roll Control Sensitivity 18 3.5 Cross-Coupling 19 3.6 Roll Angular Damping 19 3.7 Roll Control System Time Lags 20 3.8 Peak Roll Control Forces 20 3.9 Spiral Stability 20 3.10 Dihedral Effect 21 3.11 Yaw Control Characteristics - General 21. 3.12 Yaw Control Power 21 3.13 Yaw Control Sensitivity 22 3.14 Control System Time Lag 23 3.15 Peak Yaw Control Forces 23 3 :16 Cross-Coupling 23 3.17 Directional Characteristics in Steady Sideslip 24 3.18 Side Force Characteristics in Steady Sideslips 24 3.19 Lateral-Directional Dynamic Stability 24 4.0 Hovering and Vertical Flight Path Characteristics 26 4.1 General 26 4.2 Ground Effect 26 4.3 Vertical Flight Path Control 26 4.4 Hovering Precision 27 4.5 Vertical Thrust Margins 27 4.6 Vertical Velocity and Thrust Response 28

V 5.0 Transition Characteristics 29 5.1 General 29 5.2 Acceleration-Deceleration 29 5.3 Flexibility of Operation 29 5.4 Tolerance in Conversion 29 5.5 Control Margins 30 5.6 Trim Changes 30 5.7 Rate of Control Movement 30 6.0 Miscellaneous Characteristics 31 6.1 General 31 6.2 Ground Handling - General 31 6.2.1 Landing Gear 31 6.2.2 Control Effectiveness During Takeoff, Landing Rollout, and Taxi 31 6.2.3 Power Checks Prior to Takeoff 31 6.3 Cross-Coupling Effects - General 31 6.3.1 Gyroscopic Effects 31 6.3.2 Inertial Cross-Coupling Effects 32 6.3.3 Mechanical Cross-Coupling 32 6.4 Minimum Flight Speeds - General 32 6.4.1 Loss of Lift 32 6.4.2 Warning of Approach of Vmin 32 6.5 Warning of Approach to Hazardous Flight Condition 33 6.6 Aircraft Behavior Following Systems Failure 33 Appendix 34 Maneuvers for V/STOL Aircraft Handling Qualities Evaluation 34 Summary of Criteria Tables 39

TABLE INDEX*

1.1 6 V/STOL Control Breakout Force Criteria 1.2 7 Control Force Gradients (lb/in.) for Hover 1.3 7 Control Force Gradients for STOL 1.4 7 Control Force Harmony Ratio Criteria 1.5 9 Cockpit Control Travel Limits (in.) 2.1 11 Pitch Control Power Characteristics 2.2 12 Pitch Control Sensitivity Characteristics 2.3 13 Pitch Angular Velocity Damping Criteria 2.4 13 Pitch Control System Time Lag Criteria 3.1 17 Roll Control Power Characteristics 3.2 18 Roll Control Sensitivity Characteristics 3.3 19 Cross-Coupling Characteristics in STOL Operation (Yaw Control Free) 3.4 19 Roll Angular Velocity Damping 3.5 20 Roll Control Lags 3.6 22 Yaw Control Power characteristics 3.7 23 Yaw Control Sensitivity Characteristics 3.8 24 Yaw Cross-Coupling Characteristics 4.1 26 Minimum Vertical Flight Path Control Characteristics in (STOL) Operation 4.2 28 Vertical Velocity and Thrust Response Characteristics

FIGURE INDEX*

2.1 15 Longitudinal Dynamic Stability Criteria 3.1 25 Lateral-Directional Dynamic Stability Criteria 4.1 27 Vertical Thrust Margins Criteria

*All tables and figures are summarized in the Appendix, Page 39.

vi 1

V/STOL HANDLING-QUALITIES CRITERIA

I - CRITERIA AND DISCUSSION

INTRODUCTION

Background. The first AGARD work relating to V/STOL handling-qualities criteria began in 1960; this work is documented in reference 1, published in October 1962. Following a general review in 1963, the report was revised to include comments on recommendations from industry, military services, and government agencies. This report (ref. 2), published in October 1964, has been used extensively as a guide in design-. ing new V/STOL aircraft. The report has received criticism, not unexpected, in two areas: scope and specific recommendations. It was directed primarily toward VTOL aircraft and did not clarify a large number of requirements for STOL aircraft. In addition, recommendations did not include both VFR and IFR operation for all types of VTOL and STOL aircraft, because the background data used to establish many of the recommendations had been obtained chiefly from helicopters. Perhaps the most serious deficiency was that the report attempted to provide quantitative requirements based on limited flight experience obtained for the most part from test-bed type aircraft. As a result, some inconsistencies would be expected when applications were made to operational V/STOL designs. In particular, the effect of vehicle gross weight or size on aircraft response was continually a source of disagreement. Further, the consequence of providing only minimum acceptable values of each handling-quality item was not fully appreciated (and perhaps is not yet understood); a VTOL aircraft that individually met all the recommendations could still be too demanding of pilot skill. In many cases, achieving a given set of numbers did not ensure a completely satisfactory aircraft,

At the request of the Flight Mechanics Panel, a second AGARD V/STOL handling-qualities committee was organized in 1966 for the following purposes: 1. To revise and update AGARD Report No. 408A, changing its emphasis to reflect criteria rather than specifications. 2. To explore the latest VTOL flight-test techniques and recommend how to prepare a report on this activity . The committee consisted of one representative each from Canada, France, United Kingdom, West Germany, and Italy; the chairman and technical secretary were from the United States. Item (1) above was accomplished by (a) reviewing comments from users of TR No. 408A, (b) examining data from second-generation V/STOL aircraft, and (c) incorporating results from recent piloted simulator tests and analytical studies, Work directed at item (2) indicated an urgent need to standardize flight-test techniques to help reduce inconsistencies in measurements of flight-test data. Although it was beyond the scope of this comittee to set up flight-test techniques, a set of standardized flight-test maneuvers was devised for pilots to use in evaluating handling qualities during flight tests of V/STOL aircraft.

Revision Procedures. In revising TR 408A it was agreed that a more meaningful and useful document would include: 1. Evaluation of the various handling-qualities items in terms of criteria, rather than requirements or specifications 2. A discussion section to explain the purpose of each criterion 3. Data and reference material to support the criteria.

Criteria can be defined as evaluation standards based on numbers that are meant only to be typical and can vary widely. Meaningful criteria can serve as a guide in establishing specifications to be used by a contractor for the design and testing of a particular aircraft.

In the past, handling-qualities requirements have been presented without an explanation of why the pilot desires a particular characteristic; in many cases neither the purpose nor the various factors related to the requirements were understood. Without an understanding of all the possible tradeoffs, there may be a tendency to apply the specifications too rigidly to a particular aircraft design, thereby compromising its utility.

Finally, it is helpful to provide background data and reference material for each of the criteria, If the user understands the limitations of the data on which the criteria are based, he can evaluate the criteria with respect to their optimum application to his design, and the contracting agency can then write more effective specifications.

Each committee member was responsible for supplying quantitative back-up data derived from tests carried out in his respective country. In particular, considerable experience had been obtained from flight tests of advanced jet-lift VTOL aircraft in Europe, while in the United States a variety of experience had been gained from both VTOL and STOL aircraft. An early review of test data disclosed that existing quanti-. tative data unfortunately pertained solely to performance or safety of flight. There were no data from flight test maneuvers that could be used with pilot opinion to establish criteria for stability and control characteristics. The committee focused attention on the lack of quantitative data and stimulated some interest in obtaining handling-qualities data from existing V/STOL aircraft and specially designed, variable- stability vehicles. However, information pertaining to many advanced aircraft was always "just around the corner," and it was decided to publish only the criteria part of the report initially, with documentation to follow as a separate report when sufficient data became available. In many cases, therefore, the criteria are based on incomplete quantitative data, and controversial sections will have to be revised further as data from.operationa1 experience with advanced aircraft become available.

Report Format. It was agreed that organization of the original report (AGARD Rep. 408) was acceptable and should be followed generally, thus making it easier for readers of the basic report to follow this revised version. In addition this report is organized around the following elements: 1. Criteria: A description of the goals for each handling-qualities item, stated in qualitative and/or quantitative terms. 2. Discussion: For each criterion, pilot reasons for including a particular handling-quality feature.

Recommended standardized flight-test maneuvers to be used in evaluating V/STOL aircraft so that more L

consistent pilot ratings of handling qualities may be obtained are presented in the Appendix.

Criteria Application. The criteria in the revised report are intended to apply to all types of VTOL aircraft regardless of the lift method used except for certain phases of helicopter operation, since the helicopter is covered by Military Specification H-8501A and its revisions, STOL aircraft treated only lightly in reference 1 are covered more comprehensively here (primarily obtained from reference 3).

As in the earlier report, the criteria presented apply only to the low-speed regime, i.e., up to a speed where the aircraft is configured for conventional flight. It would be desirable to have an AGARD report covering the complete flight envelope to ensure compatibility of handling qualities of both speed ext.remes .

Interpretation.

Level of the criteria. The criteria proposed here are intended to define handling qualities that will result in satisfactory behavior of the pilot-aircraft system during normal operating conditions. At certain times, operation outside of the normal flight envelope will occur, and some systems will fail. Under these conditions, some degradation in handling qualities should be allowed, depending on the probability or remoteness of the failure. Because of the lack of experience in this area, it was not possible to establish criteria that relate the probability or remoteness of failure to a change in handling qualities; however, designers should make a strong effort to apply this philosophy to new aircraft.

Ultimately, all types of V/STOL aircraft must be capable of operating routinely in all weather situations. There is insufficient operational experience with V/STOL aircraft under IFR conditions to completely and accurately assess the effect of restricted visibility on aircraft handling-qualities requirements. Thus, while no distinction has been made here to reflect different handling qualities for VFR and IFR conditions, the criteria can serve as guidelines for IFR operation, provided that proper discretion is applied.

Wind conditions. In assessing the effect of wind conditions on handling qualities criteria, both steady wind and gusts or turbulence must be considered. In many of the criteria, operation in certain steady wind conditions is stated; however, no attempt has been made to specify the magnitude of wind velocity or even the direction, since these items were considered to be more appropriately handled by the procuring agency. A few of the criteria are written to reflect the effects of operation in a particular atmospheric turbulence. Although desirable, it is extremely difficult to specify a turbulence model and intensity for all appropriate criteria because of lack of data and the requirements of the procuring activity.

Classification of aircraft. It has been customary in some aircraft handling-qualities documents to relate handling qualities to classes of aircraft. For V/STOL aircraft there is less need since a given class of aircraft may carry out several tasks during a particular mission. Specifications for different classes of V/STOL aircraft may be required by a procuring activity for some of the handling-qualities items, particularly as they relate to mission effectiveness. Rather than catalog handling qualities as a function of aircraft class, the report discusses some of the handling-qualities items, such as control power required for maneuvering, in terms of requirements for a particular mission. Perhaps the requirement to relate handling qualities to classes of V/STOL aircraft and to the mission can be clarified when more operational experience with V/STOL aircraft has been obtained.

TERMINOLOGY (Reference 4)

Basic Terms.

VTOL: Vertical takeoff and landing. Pronounced by letter or as a word and used as an attributive adjective designating heavier-than-air aircraft.

-STOL: Short takeoff and landing. Pronounced by letter or as a word and used as an attributive adjective designating heavier-than-air aircraft. The ground roll distance, or the total air distance to clear a given obstacle must be defined by the procuring agency.

V/STOL: Vertical and/or short takeoff and landing used as an attributive adjective to designate aircraft that have vertical and/or short takeoff capability.

-VTO: Vertical takeoff. Pronounced by letter and used to describe a maneuver wherein a VTOL aircraft clears a given obstacle vertically.

-STO: Short takeoff. Pronounced by letter and used to describe a maneuver wherein V/STOL aircraft clear a given obstacle in a minimum distance. Takeoff may be initiated from a hover, but forward speed is required to gain additional lift necessary to clear the obstacle or to operate outside the height-velocity envelope.

-RTO: Rolling takeoff. Pronounced by letter and used to describe a maneuver wherein a ground roll distance is required to accelerate V/STOL aircraft to takeoff speed.

&: Approach speed. Used to define the stabilized landing approach speed that produces maximum performance and/or the landing approach speed for which the landing qualities criteria apply.

Conversion air speed. The air speed at which an accelerating transition is completed and the aircraft -Vcon: enters the aerodynamic flight regime.

Vrec: Reconversion air speed. The air speed at which a decelerating transition is initiated and the air- - craft enters the power lift flight regime. -Vmin: Minimum flight speed. The lowest steady flight speed at which all relevant handling criteria can be 3 met with any given configuration and power setting. This includes configuration appropriate to takeoff, landing approach (including both shallow and steep approach), landing and waveoff.

-SAS: Stability augmentation system. Any system (e.g., mechanical, electrical, fluidic,) that supplements or modifies the stability and/or the damping of the basic aircraft.

-IGE: In-ground effect.

-OGE: Out-of-ground effect.

Flight Regimes.

Aerodynamic flight: Flight supported primarily by the forward flight dynamic pressure.acting on nonrevolving aerodynamic surfaces at air speeds above the power-off stall speed.

Power-lift flight: That flight regime of any aircraft where controlled, sustained flight is possible below the power-off stall speed and in which part or all of the lift and/or control moments are a function of power. (V/STOL handling-qualities criteria and specifications apply. )

Hover: To remain stationary relative to the air mass. Flight primarily supported by power plant(s) derived lift.

Spot hover: To remain stationary relative to a point on the ground. Flight primarily supported by power plant(s) derived lift.

Translation: Horizontal or vertical movement relative to a fixed point.

Accelerating transition: The act of flying an aircraft from the hover flight condition to the conversion air speed.

Decelerating transition: The act of flying an aircraft from the reconversion air speed to the hover flight condition.

Conversion: The act of making the configuration changes necessary to go from the appropriate takeoff configurations to the aerodynamic flight regime.

Reconversion: The act of making the configuration changes necessary to go from the aerodynamic flight regime to the appropriate landing configuration.

Transition envelope: That portion of the aircraft's flight envelope where steady, controllable flight is possible in the powered-lift flight regime. The envelope is defined by such factors as air speed, height, power, thrust vector angle, control margins, angle of attack, rate of climb or descent, etc.

Downwash: The downward component of the efflux from the lift system.

Ground effect: Any effect on the aircraft performance, stability, or control caused by the aircraft itself due to its proximity to the ground.

Recirculation: The phenomenon wherein a part of the lift system efflux is returned to flow through the lifting system again.

Reingestion: Recirculation of exhaust gases into the engine inlet(s), which usually occurs near the ground and can result in loss of power.

Height-velocity envelope: A family of curves that define the probability of aircraft damage on ground con- . tact that would result if a failure of the power system occurred within a specific height velocity portion of the power-lift flight regime.

Reference speeds: Any speed (e.g., surface wind, landing approach speed, takeoff speed) that is specified by the procuring agency for which the handling-qualities criteria apply.

Control Systems.

Acceleration: A type of control system in which the steady-state acceleration is proportional to the control displacement. The control system variables pertinent to this system are control power and control sensitivity.

Rate damped: A type of control system that incorporates an angular rate feedback circuit to assist the pilot in controlling the aircraft. The control system variables pertinent to this system are control power, control sensitivity, and angular rate damping.

Rate command: A type of control system that causes the steady-state angular rate to be proportional to the appropriate control displacement by direct comparison of these two parameters. The pertinent system parameters are the same as for the rate-damped system.

Attitude command: A type of control system that causes the attitude changes to be proportional to the appropriate control displacement by direct comparison of these two parameters. The system variables pertinent to this system are control power, control sensitivity, angular rate damping, and attitude feedback gain.

Engineering Terms.

Breakout force: Total force required to initiate motion of the cockpit control, including friction, preload, 4 and inertia of the control system.

Friction: A force that acts to resist control motion.

Preload: A control force, built into the control force feel system by pretensioning of a spring, that holds the trim control position selected by the pilot and/or assists the pilot in returning the cockpit control to trim position after a control input.

Free play: The lost motion between the cockpit control element and the movement of the moment-generating device.

Force gradient: That portion of the control force that opposes control displacement and returns the control to the trim position as the pilot input force is relaxed. It is the total force for a given control displacement, minus the breakout force including friction, divided by the control displacement, pounds per inch.

Control displacement: Control travel of the pilot's cockpit control element.

Control effectiveness: The ability of the control surfaces or moment generation devices to produce the moments commanded by the pilot that enables him to maintain and/or change the aircraft attitude, velocity and acceleration throughout the flight envelope.

Control sensitivity: The initial acceleration per unit step control displacement from a trim condition.

Control power: The maximum acceleration produced by a full step control displacement from a trim condition.

Control margin: The amount of control parer remaining before, after, or during a maneuver that is available to the pilot to initiate a maneuver, recover from a maneuver, and/or correct for gust disturbances.

Rate damping: A moment-opposing motion, proportional to rate. For a first-order system, it is measured as the negative reciprocal of the time required to obtain 63 percent of the final steady-state angular velocity resulting from a step control input.

Damping ratio: Ratio of actual damping to critical damping. (Exponential attenuation envelope for oscillatory modes. )

Symbols.

g Gravitational constant, 32.2 fps/sec.

Time to damp to one-half amplitude, sec. '/2 Tp Time to double amplitude, sec.

Zw Vertical velocity damping, l/sec.

Aileron deflection, deg, or cockpit roll control input, in, 6a Elevator deflection, deg, or cockpit pitch control input, in, 6e Rudder deflection, deg, or cockpit yaw control input, in. 6r 5 Damping ratio

T Time to 63 percent of steady-state value, sec.

U Product of damping ratio and natural frequency(5w ), rad/sec.

wd Damped frequency, radlsec.

w Natural frequency, rad/sec. n 5

HANDLING QUALITIES RATING SCALE

DEFINITIONS FROM TN D-5153

COMPENSATION PERFORMANCE The measure of additional pilot eHon The precision of control with r(111pec110 and anentian required to maintain a aircraft movemarit that a pilot is able 10 given level of performancein tho face of achieve in performing a task. (Pilot- deficient vehicle characteristics. vehicle performance is a measure of handling pdormance. Pilot pertom- HANDLINQ OUALlTlES ince io B measure of the mnnnei or Those qualities or chareCteristics of an efficiency with which a pilot moves the aircraft that govern the ease and preci. principal conlr01s in performing a task.) don withwhichapilotillIlbl~IOperfarm the tasks required in suppon of an ail- ROLE oah role. The function or purpose that definer the pimary use of an aircrah. MISSION The composite of pilot-vehicle functions TASK that must be performed 10 fulfill opera- The acluaI work assigned a lobe tional raquirsmantr. May be specified for a role. complete flight. flight phase. or prfo'med in Of Or rOpre- sentalive of designated flight segment. flight subphase. LI

WORKLOAD :: The int~ratedphysical and mental eHon required to perform I specified piloting task.

REFERENCES

1. Anon: Recommendation for V/STOL Handling Qualities. North Atlantic Treaty Organization, Advisory Group for Aeronautical Research and Develop- ment, Report 408, October 1962.

2. Anon: Recommendations for V/STOL Handling Qualities With an Addendum Containing Comment on the Recommendations. North Atlantic Treaty Organization, Advisory Group for Aeronautical Research and Develop- ment, Report 408A, October 1964.

3. Innis, R. C.; Holzhauser, C. A.; and Quigley, H. C.: Airworthiness Considerations for STOL Aircraft. National Aeronautics and Space Administration, Ames Research Center, Technical Note D-5594, January 1970.

4. Ransone, R. K.: Report of the Ad Hoc Committee on V/STOL Terminology. U.S. Air Force Flight Test Center, Special Report 67-1001, 1967.

5. Cooper, G. E.; Harper, R. P., Jr. (Cornel1 Aeronautical Labs): The Use of Pilot Rating in the Evaluation of Aircraft Handling Qualities. National Aeronautics and Space Administration, Ames Research Center, Technical Note D-5153, April 1969. 6

Section 1

CHARACTERISTICS OF THE CONTROL SYSTEMS

1.1 GENERAL The mechanical control systems for VTOL and STOL aircraft must have characteristics that enable the pilot to rapidly and precisely position the controls in hover and low-speed flight where the inherent stability and damping are low. Important mechanical characteristics include friction, preload, free play, force gradients, mass unbalance, inertia, nonlinear gearing, and rate limiting (including servo valves and actuators). These characteristics directly relate to the pilot's feel of the aircraft (and, therefore, the handling qualities), and although discussed individually here, they ultimately must be considered together in an overall assessment of the acceptability of the flight control system.

Characteristics of other systems also influence the pilot's impression of the handling qualities. These include powered control systems, stability augmentation systems, trim systems, and control travel.

1.2 CONTROL BREAKOUT FORCES Criteria. Breakout forces including friction, preload, etc., should be maintained within the limits shown in table 1.1, regardless of the position of the primary controls and for any setting of the thrust vectoring system. These values apply for stick and wheel type controls. The breakout forces should be symmetrical for each control with a maximum differential of 10 percent of the low value.

Table 1.1

I V/STOL Control Breakout Force Criteria I After failure Acceleration Rate Control Attitude of power control system, system, system, axis system, lb lb lb lb L I I 1 I I Pitch I 0.5-1.5 I 0.5-3 I 0.5-3 I <5 I Roll 0.5-1.5 0.5-3 0.5-3 <4 Yaw 1-10 1-10 - ~15 Height collective 1- 3 1-3 - <5 throttle 1- 3 1-3 - <3

Forces should be measured at the cockpit control under conditions resembling those of flight as closely as possible, e.g., with engine(s) operating. When adjustable friction devices are provided, it should be possible to obtain desired values regardless of control position.

The combined effect of breakout force, including friction, preload, and force gradient, should result in positive centering in flight for pitch, roll, and yaw controls at any normal trim setting and must allow smooth operation of the throttle. Although absolute centering is not required, the degree of centering should preclude objectionable flight characteristics.

Discussion. The upper range values of breakout force should be low for two reasons, First, in hover and low-speed flight, the pilot may be required (depending in part on the type of control system and SAS) to make rapid and frequent movements of the controls, which tend to induce pilot fatigue and affect precise- control positioning. Second, in forward flight, they are needed to avoid masking the inherently low values of aerodynamic stability peculiar to low-speed operation. Positive centering helps to reduce pilot workload and thus is desirable, particularly for IFR operation. In general, the pilot desires positive centering during phases of flight requiring stabilization of the aircraft with respect to the air mass. When the task requires stabilizing the aircraft with respect to a point or a path on the ground, on the other hand, positive centering is less desirable because of the need to continually hold some value of control force, particularly under gusty, crosswind conditions.

The pilot desires some breakout force for all types of controlling methods to reduce inadvertent control inputs, such as may occur when he replaces his hand on the control.

After failure of a power control system, the resulting manual control breakout forces are allowed to increase somewhat since only a short duration of operation would be anticipated. An upper limit is needed, however, to avoid serious loss of precision control.

1.3 CONTROL FORCE GRADIENTS Criteria. Control force gradients for hover should be kept within the limits shown in table 1.2 for each type of control system.

The slope of the control force versus displacement curve, beyond the breakout region, must be positive and should be linear with no discontinuities. If nonlinear, the slope for the first inch of displacement from trim should be greater than the slope for the remaining stick travel, In addition, the total force for the first inch of travel from trim should not be less than the breakout force.

With increasing forward speed, a smooth increase in gradients is desirable. A multiplecstep force gradient device may be used provided that large out-ofctrim conditions do not occur during the stepping phase. 7

Control System Control axis Acceleration Rate Attitude

Pitch 1-2.5 1-3.5 1-3 Roll 0.5-1.25 0.5-1.75 0.5-1.5 Yaw 2.5-10 2.5-10

Control force gradients for STOL operation should be in the ranges specified in table 1.3.

~ ~ ~~ Table 1.3

Control Force Gradients for STOL

Control axis lb/in.

Pitch 2-5 Roll 1- 3 Yaw 10-35

To ensure satisfactory control force harmony, control force gradient ratios should be within the limits shown in table 1.4.

Table 1.4

Control Force Harmony Ratio Criteria

Control force Aircraft Minimum Optimum Maximum ratio I type 1 Ratio I Ratio 1 Ratio I Pitch V/STOL

2Roll Yaw - IVTOLISTOL 16 Roll f 1; 111

Discussion. For a number of reasons, the pilot desires low control force gradients for operation of V/STOL aircraft. In controlling these aircraft, for example, the pilot operates with one hand a primary control to adjust aircraft attitude, and with the other hand he adjusts power, vector angle, or both to control flight path angle or height. Because of the need for one-hand operation, the force values are similar for wheel or stick. Another reason is that more frequent control movements are required, particularly for the acceleration type control system. For the same reason, the effects of power boost system failures are more serious. As noted previously, no force gradient may be desired for tasks where prolonged hovering is required with respect to a point on the ground. As a general rule, higher force gradients are accegtable when control travel is small.

Pilots prefer linearity of force displacement because it improves accuracy of controlling flight close to the ground. Nonlinear systems with low force gradients near the neutral control position reduce control precision.

Because most aircraft become more responsive to aerodynamic forces with increase in airspeed, increases in force gradients are helpful in preventing excessive excursions in angle of attack or sideslip. At STOL operating speeds, forces must nevertheless be moderate to reduce pilot fatigue, since frequent and rapid control inputs are usually required, particularly in gusty air.

Selection of control force harmony is based primarily on a pilot's relative strength capability about each axis. For hover and'very low-speed operation, control harmony should be such that smaller force gradients are used for lateral than longitudinal control; even though these controls are used at comparable frequencies, the pilot can exert a push-pull force repeatedly with less fatigue. With increasing forward speed, longitudinal forces should increase with respect to lateral forces to help protect against inadvertent excessive load factors. At STOL speeds, proper control force harmony will aid the pilot in obtaining co- ordinated turn maneuvers. 8

1.4 CONTROL SYSTEM FREE PLAY Criteria. The free play in each cockpit control system-that is, any motion of a cockpit control that does not produce corresponding movement of a moment-producing device (e.g., reaction nozzle, propeller pitch, engine power, etc.)- in combination with other mechanical characteristics of the control system should not result in objectionable ground handling or flying qualities.

Discussion. The amount of free play in control systems for V/STOL aircraft is important; if it is excessive, precision of control is directly affected because of overcontrol tendencies and pilot-induced oscillations. To the pilot, free play appears similar to a control system phase lag; that is, aircraft motion lags the control input. The tolerable amount of free play depends on a number of interrelated factors: (1) the type of control system, such as acceleration, rate damped, or attitude command; (2) other characteristics of the control systems, such as control system time constant, breakout forces, and control sensitivity; and (3) aircraft stability and overall aircraft handling qualities.

1.5 POWERED CONTROL SYSTEMS Criteria. The powered control system in combination with other mechanical control characteristics (free play, friction, inertia, etc.) should not restrict the operational maneuvers required for the mission; and limited rate of movement of the moment-producing system should not restrict the aircraft from operating in the designated atmospheric disturbances. In addition, control-system oscillations should not adversely affect precision of control, make trimming difficult, or cause pilot-induced oscillations. For aircraft having two or more independent powered control systems, the foregoing criteria should be met on failure of one system, including the period of transfer from the defective system.

Discussion. Adequate response characteristics of powered control systems, commonly used on V/STOL aircraft to meet desired force gradient values, are needed to obtain precise control of the aircraft. In maneuvering V/STOL aircraft in atmospheric turbulence, the pilot may frequently require rapid, large-amplitude control inputs. It is desirable that the powered control system isolate the pilot from snatching of the aerodynamic surfaces during tail-to-wind hovering. A powered control system in which the actuator rate lags the pilot's control input will result in nonlinear force and control deflection rate characteristics; the pilot is given the impression of reduced rate (angular) damping and tends to overcontrol.

1.6 TRIM SYSTEMS

Criteria. The trim systems should be capable of reducing the pitch, roll, and yaw control forces to zero at any condition of flight where prolonged steady operation is required. The trim devices should be continually adjustable and capable of easy and comfortable operation by the pilot for all positions of the primary and secondary cockpit controls. In addition, actuation of the trim system should not produce "stick-jump," nor should trimming of one axis result in an out-of-trim condition in another axis. In otner parts of the operational envelope or following a failure of any trim system, the permanent out-of-trim forces should not exceed 10 lb for pitch, 7 lb for roll, and 20 lb for yaw.

All trim devices should maintain the setting selected by the pilot or automatic interconnect devices, or by the operation of an augmentation scheme. When powered, automatic, or augmentation devices are used, provision should be made to prevent "runaway" trim. Failure of a powered control system or SAS should not affect the ability to trim.

Trim operation should be sufficiently rapid to provide low control forces during configuration and speed changes, and during maneuvers without causing overcontrol tendencies.

Discussion. The intent of these trim criteria is to assure precision of control by minimizing out-of- trim forces. Although various trim-control methods can be used, special considerations must be given to provide compatibility for both low- and high-speed flight. It is important that the pilot know how much of his control is being used for trim and what his true control margin is, especially if trim does not change the position of the cockpit control.

1.7 HEIGHT CONTROL SYSTEMS Criteria. The height control should remain fixed at all times unless moved by the pilot or some automatic system. A zero force gradient plus an adjustable friction damper that provides a constant force, at least equal to the breakout force (noted in table l.l), should be present during movement of the height control. A friction device capable of being adjusted without releasing the height control is desirable. A forearm rest should be provided for power-lever controls.

If an aircraft is equipped with both power-lever and lift-stick (collective) controls, the conversion from one system to another should be accomplished easily with a minimum of procedural complexity. If a lift stick is used, the full down position should not require body motion, and the full up position should not require excessive bending at the elbow.

Discussion. The height control system must have satisfactory control characteristics to obtain the necessary precision required for both hover and approach tasks. Some friction is necessary to prevent inadvertent movements, and it should be adjustable since less friction is needed for hover than forward flight. In general, no force gradient is desired because for most height control tasks there is little need to return the control to a given (trim) position. When both power-lever and collective controls are used, it is important to avoid procedural complexity that adds to the pilot's workload when making a transition. A forearm rest for power-lever controls assists the pilot in making precise control inputs. In addition, for either type of control, the range of travel should be comfortable to allow precise movements and reduce fatigue.

1.8 THRUST VECTOR CONTROLS Criteria. It should be possible for the pilot to adjust the thrust vector control as desired without compromising his ability to manage other flight controls. 9

A variable thrust vectoring rate, controlled by the pilot, is desirable; however, thrust vectoring rates may be automatically programmed as a function of thrust vector angle, provided the rates selected are compatible with a safe aircraft forward speed and the mission.

The acceleration and deceleration required in transition should not be limited by the rate at which the thrust vector can be rotated. Performance and repeatability of the takeoff and landing maneuvers should not be limited by the minimum and/or maximum thrust vectoring rate, or by accuracy with which a chosen angle setting can be selected.

A selected setting of the thrust vector control should be maintained indefinitely without pilot attention. If a separate thrust vector control (lever type) is located near the height control, means should be considered of preventing accidental movements of either control that would result in an uncontrolled or cata- strophic condition. If a mechanical friction device is employed to hold selected thrust vector control positions, it should be a separate device from that used to secure power lever positions. Similar motions of the controls should be avoided when accidental movement would be dangerous or objectionable to the pilot.

No single failure of any control system should cause the thrust vector to rotate to a position, or at a rate, such that the aircraft cannot maintain height or make a safe landing. Discussion. Thrust vector controls must have satisfactory mechanical characteristics with fine adjust- . ment to allow the pilot to accurately control flight path angle and airspeed as desired during transition as well as for fore and aft and lateral positioning in hover. The pilot prefers to use varying rates of thrust vector rotation. The maximum effective thrust vectoring rate depends on the magnitude of the vector and whether the thrust vector can be turned independently of the aerodynamic lift vector. As a general rule, in transition, large thrust vectors (high engine powers) must be turned slowly to minimize altitude changes, while small vectors (low engine powers) may be turned rapidly provided turning is independent of the aero- dynamics vector (for example, on the P.1127). It follows that the pilot will use slow rates in the acceleration portion of the transition where high engine powers prevail to permit a gradual (safe) shift over from powered lift to aerodynamic lift. Finally, on vectored jet aircraft it may be desirable to preselect specific nozzle angles to facilitate STOL operation.

Inadvertent operation of thrust vector angle by either the pilot or systems failure must be avoided regardless of the rate of movement because of the serious consequences of sudden height loss.

1.9 CONTROL TRAVEL LIMITS Criteria. Cockpit control travel should be kept within the limits shown in table 1.5. If necessary to achieve the required sensitivity, full hovering control may be obtained by less than full cockpit control travel.

Table 1.5

Cockpit Control Travel* Limits (in.) I I Longitudinal I I f4.0 - 26.5 f3.0 - 26.5 I 22.5 - f4.5

Although a stick-type control is preferable for hover, a wheel may be used, provided that the wheel-arm position is compatible with precision control for hover, and the wheel throw necessary to meet the lateraJ control recommendations is readily obtainable with one hand. In STOL operation, maximum wheel displacement should not exceed f60'.

Discussion. The amount of cockpit control trave provided for VTOL and STOL aircraft can have an important effect on the "apparent response" of the aircraft and on precision of control. In general, when cockpit control travel is kept ,small (for proper sensitivity), the aircraft appears to the pilot to be more responsive. However, small cockpit control travel has an adverse effect on mechanical advantage of the control system, resulting in increased forces felt by the pilot from friction and aerodynamic inputs. These problems can be overcome by power boost.

If it is not possible to let the pilot know how much control moment remains, either by proximity to stick or wheel stops or by some other means, it is extremely important in the design of the aircraft that sufficient control power is available to meet all the demands of the pilot.

Travel of wheel-type controls must be compatible for one-hand operation to achieve precision of control required for operation close to the ground, particularly in gusty air. In addition, improper or awkward position of the wheel with respect to the pilot's body can adversely affect controllability. It is customary for the pilot to support his arm to improve precision of control.

1.10 AUGMENTATION SYSTEMS Criteria. When stability or control augmentation systems are used to improve one part of the aircraft behavior, other handling qualities must not be adversely affected. In addition, consideration must be given in the implementation of SAS to ensure satisfactory operation in gusty air and during ground roll. Limits on the authority, saturation, or response of SAS should not result in objectionable handling qualities in any part of the flight envelope.

A check of normal operation as well as failure of any part of the SAS should be made evident to the pilot, and failures should not result in objectionable out-of-trim moments or other unsafe flight conditions. 10

The addition of SAS should not result in marked resonances in the frequency response characteristics, should not introduce objectionable phase angle lags, and should be free from noticeable limit cycles that would impair the pilot's ability to carry out precision tasks.

Discussion. Stability augmentation systems are expected to be common on all types of V/STOL aircraft to permit satisfactory, routine operation in all flight conditions. Consideration therefore must be given to mechanical (and other) characteristics of these systems that influence the pilot's opinion of controllability and precision of control. For example, the use of a device to improve turn coordination for STOL operation can adversely affect sideslip control in aligning the aircraft for touchdown in a cross wind, or the addition of a system to reduce dihedral effect can seriously degrade spiral stability.

Consideration must be given to the method of implementation of SAS input signals, for gusty air and recirculation can seriously limit the usefulness of a sensor system employing vanes for measuring angle of attack or sideslip. Problems may arise on authority or saturation limits because of the requirement for large control inputs inherent in low-speed flight. Therefore, reliability must be emphasized, and redun- dancy undoubtedly will be commonplace. Further, when the addition of SAS markedly changes the dynamic behavior of the aircraft, its complete failure should have a low probability, and the pilot will wish to test the SAS as a routine check before takeoff. Further, the pilot must know how much of his control authority is being used for stability and damping purposes in normal operation, and he must have enough control moment remaining to overcome any SAS failure. 11

Section 2

LONGITUDINAL STABILITY AND CONTROL

2.1 GENERAL The performance potential of VTOL and STOL aircraft in routine day-to-day operations can be fully real- ized only if the aircraft can be positioned accurately along or about some selected flight path during approach and climb out, and can touch down and take off consistently in some prescribed area. VTOL and STOL aircraft must be more precisely controlled in and out of restricted ground space in unusual turbulence and operate more routinely in cross winds than conventional aircraft, yet with minimum penalty on performance. VTOL aircraft are particularly sensitive to detailed control requirements, because there is a direct tradeoff between performance in term of payload and range and the amount of control needed for hover. Pitch control is used for adjustment of attitude for hovering over a spot, performing quick stops, or changing forward speed. For STOL aircraft, engine power is used in combination with pitch control for changing flight path angle becpse only small values of normal acceleration are available from aircraft rotation at low speeds. To accomplish these functions accurately without exceeding acceptable pilot workload, attention must be given to several items, all of which are interrelated and establish the pilot’s impression of the overall aircraft behavior. These include control power, control sensitivity, linearity of response, pitch angular damping, control system time constant, control forces, and cross-coupling.

2.2 PITCH CONTROL POWER Criteria. From trimmed conditions in hover, at the selected reference approach speed in STOL flight, and for the environmental conditions and the mission specified for each type of aircraft, the pitch control should be sufficient to achieve at least the values shown in table 2.1. Aircraft whose missions require extensive hover and low-speed maneuvering should as a minimum meet the upper levels of values shown, while those for which maneuvering is only incidental to the mission and for which thrust vectoring can also be used, should as a minimum meet only the lower values.

The maximum cockpit control force from trim to achieve che total values shown in table 2.1 should not exceed 20 lb for hover and 40 lb for STOL.

Table 2.1

Pitch Control Power Characteristics

Minimum levels for satisfactory operation Parameter to be Control power required Type of measured for: :ontrol system Hover STOL

Attitude com. Pitch angular acceleration, Maneuvering Rate 0.05 - 0.2 , rad/sec2 Acceleration 0.2 - 0.4

Attitude com. Rate y-j-77 Acceleration - 2-4

~ ~~ Pitch control Sufficient control in EXCESS of MANEUVER- deflection at zero ING REQUIREMENE to trim over designated A1 pitching velocity, 1 speed and c.g. range and for most critical engine failure

Sufficient control in EXCESS of MANEUVER- ING and TRIM REQUIREMkNTS to balance mo- Time to recover to Upset (due to gusts, ments due to a specific gust; for example, attitude’ recirculation, ground 30 f/sec gust: sec or control All effect, etc.) deflection, in. Building up in Building over a 1 sec I 100 ft distance Typical range of Pitch control values used by V/STOL power angular aircraft for maneu- A11 acceleration, 0.4 - 0.8 0.4 - 0.6 vering, trim, and rad/sec2 IL

Discussion. The total amount of pitch control desired by the pilot for VTOL and STOL operation is determined chiefly by three individual requirements: (1) how rapidly the aircraft must be maneuvered for a particular task; (2) the magnitude of the moments that must be trimmed out to maintain a given attitude or speed when power, flaps, or thrust vector are changed; and (3) the amount of pitch attitude change resulting from gusts, recirculation, or other disturbances. Pitch angular rate damping will indirectly affect the pitch control moments desired by the pilot because of its influence on dynamic response, When large values of inherent rate damping are present, larger values of control power (and higher sensitivity) are needed to avoid the feeling of sluggish response.

The amount of pitch control moment for each of the above requirements will vary for the following rea-- sons. First, maneuvering control requirements depend directly on how rapidly the pilot must rotate the aircraft for the mission. For example, the pilot will tend to rotate VTOL transport aircraft less rapidly in part because there is less demand for the performance of rapid maneuvers and also because large values of normal acceleration would be imposed on the pilot or passengers when displaced large distances from the center of rotation. Conversely, for special missions, abrupt maneuvers such as quick stops and rapid changes in flight direction place a higher demand on control moments, In a sense, the time available to execute a particular maneuver will define control power values. For example, higher control moments are needed if the landing must be made quickly to conserve on fuel or to avoid military opposition. The pilot may use thrust vectoring to maneuver, thereby reducing pitch angular acceleration requirements. Control of pitch attitude then becomes a secondary task, at least in the low-speed range where normal force is controlled by thrust. Thrust vectoring also helps avoid the disconcerting loss of view when the nose of the aircraft is raised to achieve a quick stop. In addition, STOL aircraft may require less maneuver control power because less abrupt attitude changes are used in the flare compared to hover quick stops.

Second, pitch trim moments due to engine failure, ground proximity, changing forward speed, power, or thrust vector angle will vary markedly depending on the VTOL or STOL concept and should be estimated from analytical or wind-tunnel studies. Of primary interest to the pilot is the capability of the control system to maintain zero angular pitch rate such that unwanted speed or attitude changes do not occur.

Third, control moments are needed to offset pitch attitude changes induced by turbulence or self- generated disturbances in ground proximity. The pilot requires sufficient control to maintain the desired pitch attitude for approach and touchdown. Larger pitch attitude changes will obviously occur for a given nose-up gust value on small aircraft with low pitch inertia and those with low inherent pitch rate damping. As noted previously, the pilot desires to be able to maintain zero pitch rate in the presence of disturbances to hold a given position over the ground or maintain a given approach speed or angle of attack. The intensity and size of atmospheric turbulence must be specified to provide meaningful values for control requirements for disturbances.

2.3 CONTROL SENSITIVITY Criteria. It should be possible to achieve the values of aircraft response shown in table 2.2 following an abrupt step input of the pitch cockpit control. These criteria should be met starting from nonaccelerated flight for the environmental conditions specified for each type aircraft in hover and at the selected reference approach speed in STOL flight.

In addition, the linearity of aircraft response with cockpit control deflection should be as noted in table 2.2.

Table 2.2

Pitch Control Sensitivity

Minimum levels for satisfactory operation Parameter to Type of Item 1 be measured control system Hover STOL

, ~ I I Attitude change per unit control Attitude deflection command 3-5 degf in.

Pitch angular acceleration per unit control Rate 0.06 - 0.1 0,08 - 0.12 Sensitivity deflection rad/ sec2f in.

acceleration per unit control Acceleration 0.08 - 0.16 deflection radfsec2fin.

pitch angular Constant or should not abruptly increase All I Linearity I acceleration with I nor change sign, I 1 control deflection 13

Discussion. The amount of control movement needed for a given pitch acceleration or attitude change affects the pilot's impression of controllability. Large control movement (low sensitivity) is particularly undesirable when rapid pitch changes are needed in accelerate/quick-stop maneuvers and in flare and corrections for upsets due to turbulence. Since large, repetitive control movements increase the physical workload, the pilot prefers not to move the control rapidly and can suffer degradation of control precision during critical phases of flight.

The dependence of aircraft pitch response on cockpit c.ontro1 deflection is an important aspect of control gearing. Constant control effectiveness is desirable with conventional control systems because the pilot uses control position in part to indicate margins available to correct for possible trim changes and for turbulence. Proper design is required to ensure that nonlinear control gearing improves precision of control without compromising maneuverability aspects. Overcontrol can result if large abrupt increases in pitch response occur during some portion of the cockpit control travel because of the pilot's inability to predict the final aircraft response.

2.4 PITCH DAMPING Criteria. For the flight conditions specified, the aircraft should possess the pitch angular velocity damping characteristics of at least the values given in table 2.3.

I ~~~ Table 2.3 I Pitch Angular Velocity Damping Minimum levels for satisfactory operation Parame t e r to 5Pe of be measured control system Hover S TOL Angular velocity -2.0 -1.0 Attitude command I Damping ratio I I ~15%overshoot Angular velocity 1.0 damping, l/sec I I-.0.5 to -2.0 I- Numbers of control Rate reversals required -- Not more than 1 to stabilize

Levels for satisfactory operation Parameter to Type of control system be measured Hover STOL 1 Time from Attitude command CO. 2 3 input to 63% of CO. peak angular Rate c0.2 3 acceleration, sec

Time to 90% of demanded attitude Attitude command >1 and <2 change, sec I I 14 I' Discussion. The control system time constant is an important consideration for precise pitch control since control system lags, if excessive, force the pilot to anticipate (lead) the final response, resulting I in over-control tendencies. The amount of control lag allowable depends in part on the type of control system used and the flight mode. The pilot will tolerate more lag when large amounts of rate angular damp- I ing are available, because the damping acts to reduce the tendency to over control, Precise hover tasks require small values of control lag, while STOL operation permits a somewhat larger value since extreme "tightness" of control is usually not demanded.

2.6 STATIC LONGITUDINAL STABILITY CHARACTERISTICS - GENERAL When operating V/STOL aircraft in the powered lift flight regime, the pilot is interested in several forms of stability. The primary purpose of stability is to reduce divergences in airspeed, attitude, or angle of attack, which if undetected by the pilot could result in an unsafe condition in the form of either large attitudes or insufficient control for recovery.

With the most critical loading, for all steady forward flight conditions at which the aircraft might be operated continuously, the aircraft should possess the stability characteristics outlined below.

2.6.1 TRIM SPEED STABILITY Criteria. With the pitch cockpit control forces continuously trimmed to zero at airspeeds covering the range from V to hover, the curve of pitch control position versus airspeed should not be negative con - that is, forward control deflection associated with decreasing trim airspeed and vice versa. These requirements should be satisfied in level, descending, and climbing flight,

Discussion. When going from V to hover or vice versa, the pilot usually prefers to continuously con trim the pitch control forces to zero as thrust vector angle is varied. Minimum pilot effort would be expended if no change in pitch control position were required. In any case, it is undesirable to encounter an unstable variation of control position with speed because of the increased pilot attention required.

2.6.2 STABILITY WITH RESPECT TO SPEED Criteria. With the pitch trim, thrust vector, throttle, or collective controls at the trim setting, the variation of pitch control position and force with air speed should be in a stable direction over a range of approximately t10 knots about the trim speed. If speed stability is obtained by means of a SAS, SAS failure should result in only mild instability (more than 5 sec for divergence to double in amplitude). In addition, the pilot should be made aware of any unstable variation in pitch control as a warning of the possibility of insufficient control for recovery.

Discussion. The pilot desires stability with respect to speed so that air speed will not diverge excessively during unattended periods of flight and for ease in trimming. This is similar to static stability, normally specified for conventional aircraft, that covers the complete speed range. V/STOL aircraft do not cover a large speed range without a configuration change; thus, the stability is considered only over a small speed range about the trim speed.

2.6.3 THRUST VECTOR STABILITY Criteria. With the fuselage attitude and engine power held constant in level, descending, and climbing flight, the variation of air speed with thrust vector control position should be in the stable sense; that is, decreases in aft thrust vector angle from the vertical should result in increased air speeds and vice versa.

Discussion. The intent of this criterion is to provide the pilot with a positive means of controlling air speed during transition. For example, thrust vector systems that do not direct the thrust from vertical to horizontal in the plane of symmetry of the aircraft would be more difficult to control precisely during decelerating or accelerating flight since the variation of vector angle with speed would be nonlinear making it more difficult for the pilot to associate a given vector angle with a particular air speed.

2.7 LONGITUDINAL CONTROL CHARACTERISTICS IN MANEUVERING FLIGHT I Criteria. At the most critical loading in turning flight at constant air speed or in turns with vary- I ing air speed, increasing pull forces and aft motion of the cockpit pitch control should be required to maintain increases in normal acceleration, angle of attack, or nose-up pitch rate, and vice versa,

The variation in pitch cockpit control force with normal acceleration should be approximately linear. The maximum local value of the gradient should be in the range of 20 to 40 lb per g but never less than 3 lb , per g at or above the reference STOL approach speed.

When trimmed in unaccelerated flight at the designated speeds, it should be possible to develop at the trim speed the designated limiting attitude or angle of attack. i

Discussion. Although maneuvering flight at low air speeds does not produce normal acceleration of any consequence from a structural standpoint for V/STOL aircraft, it is important to the pilot that there be a restoring tendency that returns the aircraft to unaccelerated flight or to a reference value of angle of attack following a disturbance. Of primary concern is the avoidance of pitch-up tendencies that can limit the operational envelope by reducing the desired approach angle and descent rate. This restoring tendency is obviously more important in the critical phases of IFR operation where pilot attention and instrument scan time is demanding.

The minimum value of force gradient per g is needed to reduce the tendency for pilot-induced oscillation, An upper limit on force gradient is needed to avoid sluggish response as well as for physical workload considerations. 15

2.8 DYNAMI'C STABILITY Criteria. The responses of the aircraft should not be divergent (i.e., all roots of the longitudinal characteristic equations should be stable). In addition the damping ratio of the second-order pair of roots that primarily determine the short-term response of angle of attack and pitch attitude following an abrupt pitch control input should be at least 0.3 for the most critical undamped natural frequency.

The frequency and damping characteristics of any oscillation superimposed on the normal control modes for VTOL aircraft in hover and V/STOL aircraft at the approach reference speed should meet at least the values shown in figure 2.1. Any sustained residual oscillations should not degrade the pilot's ability to perform the required tasks.

These criteria apply with the pitch cockpit control €Tee and fixed.

5 4.19 - 4.49 p 483

5 785

12.56 1 15.m 2393 8 31.40 8 6280 I-L m-1.2 -1.0 - .B U = -cwn Figure 2.1 Longitudinal Dynamic Stability Criteria.

Discussion. The dynamic characteristics of V/STOL aircraft differ in general from conventional aircraft in that for V/STOL aircraft the short-period response has a longer period and the phugoid period has a shorter period. When the modes are well separated, flight path deviations associated with the phugoid occur at a slow rate, and the pilot can make corrections at his leisure. The short-period response, on the other hand, can be of more immediate concern if not adequately damped, particularly in the lower frequency region where the longer response period makes it more difficult for the pilot to anticipate the growth of the aircraft response. When the response time of flight path angle to pitch attitude changes becomes too long, the pilot finds it necessary to change the method of controlling flight path from the rotation of the aircraft by use of the elevator, to use of more direct control of lift by means of spoilers, thrust vectoring', or circulation control through slipstream effects. Further, when any oscillation generated by aerodynamic forces and moments or by improper operation of SAS is superimposed on the normal control mode, satisfactory pitch frequency response and damping characteristics of these oscillations are needed to avoid deterioration of the pilot's control of flight path angle.

2.9 LONGITUDINAL CONTROL CHARACTERISTICS IN TAKEOFF Criteria. Longitudinal control effectiveness should not restrict the takeoff performance for STOL operation. If rotation is necessary to achieve liftoff, the pitch control effectiveness should be sufficient to initiate rotation at the designated speed or at least 2.0 sec before liftoff, and adequate control margins and sufficient pitch rate damping should exist to prevent overrotation to undesirable attitudes. In any case, pitch control effectiveness should be sufficient to achieve the required rotation after liftoff, but while still in ground effect.

With trim setting optional but constant, the pitch cockpit control force required during short takeoffs should not exceed 25 lb pull or 15 lb push, including the effects of thrust vector rotation or any configuration change associated with the takeoff procedure.

For vertical liftoffs at the designated wind conditions, it should be possible, in conjunction with other controls' as necessary, to prevent fore or aft translation during engine runup for takeoff. For all types of takeoff operation, it is desirable to be able to check for proper control functioning during runup at less than takeoff thrust.

Discussion. Adequate pitch control effectiveness is necessary to adjust the nose-up attitude at the desired rotation speed during takeoff so that short field performance is optimum, Because horizontal acceleration capability is large for V/STOL aircraft, the time between brake release, rotation speed, and liftoff speed is short, and it is difficult to optimize takeoff performance by monitoring air speed. As a result, the pilot desires a time period before liftoff during which he can start rotation at a rate appropriate for the particular takeoff environment.

2.10 LONGITUDINAL CONTROL CHARACTERISTICS IN SIDESLIP

Criteria. For STOL operation with the aircraft trimmed for straight flight at the reference approach speed, the values of pitch control deflection required for increasing sideslip should be small with the control forces not to exceed 10 lb pull or 5 lb push at the maximum sideslip. For the maximum sideslip condition specified, there should be a sufficient margin of control to counteract disturbances and to flare the aircraft when sideslip angle changes occur as a result of aligning the aircraft with the runway in cross-wind landings.

For VTOL operation at the designated speeds in sideward flight, the requirements for pitch control inputs should be sufficiently small to prevent unwanted fore or aft translations or attitude changes. 16

Discussion. Pitch cross coupling encountered in sideslips in STOL operation should be minimized to improve precision of flight path control in landing approach and to avoid compromising precision of touchdown in cross-wind landings. In VTOL operation, changes in pitch attitude as a result of sideward flight are undesirable and will adversely affect hovering precision. Not only is fore and aft positioning made more difficult, particularly in gusty air, but also any required pitch attitude changes will affect vertical positioning through changes in the thrust vector.

2.11 LONGITUDINAL CONTROL CHARACTERISTICS IN LANDING

Criteria. At the most critical loading, with the aircraft trimmed at the designated approach speed, the pitch control, in conjunction with other controls, should be capable of flaring the aircraft and achieving the desired landing attitude from both steep and shallow approach angles.

The pitch cockpit control force required to meet the landing requirements should not exceed 25 lb pull or 15 lb push.

Discussion. The requirement for pitch control effectiveness can become critical in STOL landings because of possible adverse trim changes as the aircraft approaches the ground. Proper attitude at touchdown is essential not only to avoid nose wheel contact before the main gear, but also to achieve consistent landing performance into restricted field lengths. For operation in restricted areas, it is desirable to have precise and sufficient height control to place the aircraft on the ground at a particular spot and at the optimum attitude for maximum braking effectiveness rather than hold the aircraft off the ground after flare until some minimum air speed is reached. 17

Section 3

LATERAL.-DIRECTIONAL STABILITY AND CONTROL

3.1 GENERAL The lateral-directional characteristics for V/STOL aircraft in hover and low-speed flight are different than for conventional aircraft because of the need for more quick, precise lateral positioning and the larger influence of cross winds during landing. The important interrelationships among the factors that influence the pilot's impression of the lateral handling qualities of V/STOL aircraft require that the various criteria be considered individually and collectively. The primary factors in the overall lateral control characteristics include roll control power, control sensitivity, angular rate damping, control system time constant, control forces, and cross coupling.

3.2 ROLL CONTROL POWER Criteria. From trimmed conditions in hover, at the designated reference approach speeds in STOL operation, and for the environmental conditions specified for each type aircraft, the roll control should be sufficient to achieve the response values shown in table 3.1. Aircraft whose missions require extensive maneuvering should be capable of at least the larger values indicated, while those for which maneuvering is only incidental to the mission and those for which direct side force control can also be used should be capable of at least the lower value noted.

The maximum cockpit control force to achieve the total values shown in table 3.1 should not exceed 10 lb for hover and 20 lb for STOL.

Table 3.1

Roll Control Power Characteristics

Minimum levels for satisfactory operation Parameter to Control power required Type of be measured for: control system Hover I STOL

Atti tude 0.2 0.4 command - Roll angular acceleration, Maneuvering Rate 0.2 - 0.4 0.1 - 0.6 rad/sec2 Acceleration 0.3 - 0.6

Attit ude command Bank angle after Maneuvering Rate 2-4 1 sec, deg 2-4 Acceleration 2-4 I 2-4 Roll control Sufficient control in EXCESS of MANEWER- deflection at ING REQUIREMENTS to trim over designated Trim Ai1 zero rolling speed and c.g. range and for most critica velocity, in. engine failure

Sufficient control in EXCESS of MANEWER- ING and TRIM REQUIREMENTS to balance mo- rime to recover to Upset (due to gusts, ment due to a specific gust; for example, initial attitude recirculation, ground All 30 ft/sec gust or control effect, etc.) deflection, sec Building up in , Building over a 1 sec I 100 ft distance Attitude Typical range of 0.4 - 1.5 0.2 - 2.0 Roll angular values used by V/STOL acceleration, aircraft for maneu- Rate 0.8 - 2.0 0.3 2.5 radlsec' vering, trim, and - upset Acceleration 0.8 - 2.0 - Discussion. The pilot desires certain values of roll control for maneuvering, for trimming in sideward flight, and for controlling upsets due to turbulence or self-generated disturbances. Control power requirements depend on many factors: (1) the mission to be performed, (2) the susceptibility of a particular configuration to unsymmetric moments resulting from aerodynamic or thrust-induced cross flow as well as turbulence and ground-induced disturbances, (3) aircraft size, since in general, the pilot tends to maneuver large aircraft less briskly and they are disturbed less by turbulence, (4) the type of control system used (more stabilized systems require less control power), and (5) the amount of angular rate damping available.

The amount of control required for maneuvering, listed as a range of values, reflects mission requirements. Lower values of control power are needed by the pilot if the mission requires only routine vertical takeoff or landing, while rapid lateral quick stops, needed for special missions, require larger control power values.

For trim in hover, various amounts of roll control moment are needed to maintain desired velocities in sideward flight. These differ for each VTOL concept because of the different magnitude of rolling moment 18

introduced from both aerodynamic and engine-induced flow sources. For aircraft with inherently large rolling moments induced by side velocity, ample control moment is needed to avoid development of excessively large bank angles, which may occur very abruptly with a sudden loss in altitude when the aircraft is suddenly turned sideward from a headwind approach. Some types of V/STOL aircraft require that any asymmetric rolling moments associated with power plant failure be trimmed out. Further, amounts of trim required depend on the cross-wind magnitudes specified for a particular mission and VTOL concept.

The amount of control power available to counteract upset due to gusty air or self-induced flow effects in ground proximity (which are also configuration dependent) directly affects the precision of the approach and touchdown. In vertical takeoffs and landings, the pilot needs to adjust attitude rapidly to avoid excessive side drift. Bank angle excursions are undesirable-in STOL approaches because of the tendency to induce large heading errors. In these cases, the pilot is interested primarily in returning to the initial bank angle in a given time. In addition, the type of control system used has a pronounced effect on control power requirements for upset. More sophisticated control systems, such as attitude command, automatically reduce or eliminate the need for the pilot to correct for the upset, Because corrections can be sensed and made more quickly by the SAS, large amplitude excursions in bank do not develop with a resultant savings in control power requirements.

3.3 TRANSLATIONAL CONTROL Criteria. For aircraft that use direct side force as a primary lateral positioning device, starting from trimmed conditions in hover for the designated wind conditions, the direct side force control should be sufficiently poiJerful to obtain lateral acceleration values between 0.08 to 0.12 g in wings-level sideward flight.

Discussion. Translation control may be desired to maneuver sideways to correct for lateral offsets as well as to provide a trim function, particularly for large aircraft for which large amounts of roll inertia severely limit the angular response. When direct side force control is to be used, consideration must be given to the aircraft mission in selecting values of side acceleration needed, Low values of lateral acceleration may be acceptable in maneuvering only for vertical landing and takeoff, while larger values may be needed for rescue operations. Regardless of the mission, however, the pilot requires some means for maintaining wings level to avoid conflict of control authority, since it is difficult for him to determine to what degree the sideward motion is due to bank angle or to translational control input. The residual angular acceleration control power needed in conjunction with this type of control system will vary depending on the aircraft's susceptibility to disturbances in gusty air and trim effects. To provide proper turn "feel' for the pilot, translational control authority must be reduced progressively with increases in forward speed as well as to avoid directional problems from possible fin stalling.

3.4 ROLL CONTROL SENSITIVITY Criteria. It should be possible to achieve at least the values shown in table 3.2 following an abrupt 1-in. step of the cockpit roll control. This criterion applies starting from nonaccelerated flight for the designated wind conditions in hover and at selected reference approach speeds in STOL operation. In addition, the linearity of roll response with cockpit roll control deflection should be in the direction noted in table 3.2.

Table 3.2

Roll Control Sensitivity Characteristics Minimum levels for satisfactory operation Parameter to Tvue of I tem _. be measured control system Hover STOL

Attitude change per unit control Attitude command 3-5 deflection, degfin.

Roll angular acceleration per unit control Sensitivity Rate 0.15 - 0.30 0.05 - 0.25 deflection, radfsec2fin.

Roll angular acceleration per unit control Acceleration 0.2 - 0.8 deflection, radfsec2f in.

Variation of roll angular Constant or should not abruptly increase Linearity A11 acceleration with nor change sign. control deflection

Discussion. Control sensitivity has very.. important effects on the pilot's impression of the amount of control moment required. For a given value of rolling moment available, high sensitivity reflects brisk response and low sensitivity suggests sluggish response. The pilot prefers high sensitivity in correcting for upsets in gusty air, because bank angle excursions can be reduced quickly before large side velocities or heading changes develop. Low sensitivity is particularly bothersome to the pilot for rapid roll reversal maneuvers such as in an offset approach in gusty air. Increased physical effort results when the roll control is moved rapidly through large angles to achieve a desired bank angle response. The pilot usually prefers not to increase physical exertion, for the result may be reduced control precision during a critical 19 phase of flight; thus when he does not moveethe control rapidly, the additional time required is reflected in more sluggish aircraft response.

An upper limit on sensitivity depends on the amount of angular rate damping available and the type of control system used. With high 3ensitivities and only small values of damping available, an upper limit on control sensitivity will usually be dictated by mechanical advantage aspects.

A smooth variation of aircraft roll response to control deflection is important for hover and STOL operation. For conventional control systems, the pilot tends to use cockpit control displacement as an indication of margins available to control trim changes and disturbances. In addition, control precision can be degraded. Of the various types of nonlinearity encountered, the one with an abrupt increase in roll response per unit control deflection is most objectionable. When an abrupt increase in roll response occurs midway during control deflection, precise adjustments for maneuvering and upset are more difficult, resulting in a tendency to overcontrol.

3.5 CROSS COUPLING Criteria. From trimmed conditions at the selected reference approach speeds in STOL operation, the magnitude of cross coupling about all axes following abrupt-roll control inputs (yaw control free) up to that required to meet the roll control criteria should have the characteristics noted in table 3.3.

I Table 3.3 I

Parameter to be measured Level for satisfactory operation

Maximum sideslip to bank Not greater than approximately angle ratio (AB/AI$)~,, 0.3 to 0.5 and not to exceed or maximum sideslip angle 20’ sideslip angle

Pitch angle change A0 Not obj ectionab le

Normal acceleration change Ag

Discussion. When using roll control to maneuver at STOL operating speeds, large disturbances may occur about other axes with an undesirable effect on precise control of flight path. Roll control inputs can cause changes in heading, pitch attitude, and vertical lift. Because of reduced directional stability and damping inherent in low-speed operation, moments generated by roll control inputs tend to result in larger sideslip angles than in conventional flight. These large sideslip angles are generated by (1) the yawing moment due to lateral control deflection, and (2) the yawing moment due to roll rate. Because these effects are large at high lift coefficients, their influence is greater in STOL aircraft and they increase the requirement for turn coordination to reduce sideslip. As a result, precision of heading control deteriorates, cross-wind landings become more difficult, and the possibility exists of stalling the vertical fin. Of primary interest to the pilot is the maximum allowable sideslip angles developed for various bank angles. Another factor affecting pilot opinion is the rate of sideslip divergence or the elapsed time before the aircraft starts to turn, since larger aircraft, with their more sluggish response, are more difficult to control precisely.

Coupling of roll control inputs with pitch (pitching) and lift loss, although usually less serious than yaw disturbances, degrade pilot opinion because of their effect on precision of flight path control. Pitch- ing moments generated when using outboard spoilers for roll control on sweptqing aircraft, for example, make it more difficult for the pilot to precisely control flight path. This is particularly bothersome during the final stages of an IFR epproach. 3.6 ROLL ANGULAR DAMPING Criteria. For the flight condition specified in hover and STOL operation, the aircraft should possess the roll angular velocity damping characteristics of at least the lower values shown in table 3.4.

I Table 3.4 I Roll Angular Velocity Damping

I I Minimum levela for satisfactory operation Parameter to 5Pe of be measured control system Hover I STOL

Angular velocity -1.5 to -4 -0.5 to -3 Attitude command Damping ratio ~15%overshoot

Angular velocity -2 to -4 -0.5 to -3 damping, l/sec I’ I I I Rate .Number of control reversals reauired t Not more than 1 Not more than 2 1to stabilize I I I I 20

Discussion. The amount of roll damping available can have important effects on the pilot's acceptability of the lateral control Characteristics for VTOL aircraft in hover and STOL aircraft in landing approach. In hover, adequate damping is needed for lateral quick stops to avoid overshoot and to reduce pilot control activity, particularly in gusty air or in ground-induced recirculation when hovering over a spot. Although the amount of damping required will vary somewhat depending on the type of control system used, loss of artificial rate damping is most serious in an attitude command system because of inherent instabilities and resulting pilot-induced oscillation tendencies. For VTOL aircraft, some damping usually must be supplied by augmentation schemes because inherent damping is not sufficient for all types of flight operation. In contrast, 'most STOL aircraft have ample inherent roll damping resulting from aerodynamic lift associated with forward flight. However, in the presence of vertical gusts, the same aerodynamic forces that produce damping also induce roll upsets. To the pilot, a STOL aircraft with too much damping not only will tend to feel sluggish to maneuver but may also behave poorly in rough air.

Levels for satisfactory operation Parameter to Type of be measured control system Hover STOL

Time from control Attitude command input to 63% of peak angular acceleration, sec c0.2 CO. 3

Time to 90% of . demanded attitude Attitude command >l'and <2 change, sec

Discussion. Control system lags due to either slow engine thrust response or poor control system design can significantly affect the pilot's opinion of lateral controllability. In hover, excessive control lag is evident from the inability to precisely reposition the aircraft in a lateral quick stop or to hover precisely over a spot in gusty air. In STOL operation, control lag is evidenced in turn entry by the requirement for large values of odposite control and a number of reversals to stabilize to a desired bank angle in the turn. Excessive lags downgrade pilot opinion because too much lead is required to anticipate the final response, overcontrol tendencies result, and the pilot is given the impression of low roll damping. In fact, it is difficult for the pilot to distinguish between excessive control lag and low roll damping. The pilot will tolerate more lag if damping is high since overshoots are reduced. Further, lag has an effect on the apparent roll response. Since lateral control power may be assessed in terms of an attitude change after a given time, an increase in control lag requires an increase in moment to produce a given attitude change.

3.8 PEAK ROLL CONTROL FORCES Criteria. The cockpit roll control forces required to obtain the designated roll response should not exceed 15 lb for hover and 20 lb for STOL operation.

Discussion. The maximum roll control forces specified are intended for one-hand operation and therefore are generally independent of whether a stick or wheel control is used. Low peak forces are needed to improve precision of control by reducing the physical workload associated with rapid and frequent control inputs usually required in low-speed flight. The peak forces cannot be too low, however, or roll-pitch control harmony will be destroyed and overcontrol problems will arise.

3.9 SPIRAL STABILITY Criteria. With the aircraft trimmed for wings-level, zero-yaw-rate flight for all normal flight conditions up to V , the spiral characteristics should be such that with the lateral control free and following an intentionafogmall bank angle input ($ = lo'), no increase in bank angle buildup is desired; however, in no case should the bank angle double in less than 20 sec. In addition, there should be no objectionable coupling between the conventional roll and spiral modes.

Discussion. Low divergence rates are desired for spiral stability to help the pilot control flight path during a landing approach when his attention is diverted momentarily. Because larger turn rates are associated with small bank angles at low speeds, large heading errors can develop quickly when only moderate spiral instability is present. Although the spiral mode can be easily controlled by the pilot when full attention is devoted to this phase of flight, there are many circumstances when constant attention to the spiral mode is not possible. As a consequence, poor spiral characteristics can quickly result in an intol- erable situation.

Optimum spiral characteristics depend on a number of factors. The combined effect of flight control system characteristics, the lateral-directional trim change with speed, and the aerodynamic spiral mode all contribute to the apparent spiral behavior noted by the pilot. Foremost is the lateral mechanical control system characteristics (control centering).

The lack of centering makes it difficult for the pilot to judge the true (aerodynamic) spiral characteristics and is perhaps the primary reason for most of the spiral mode problems of V/STOL aircraft. Any lateral-directional asyrmnetry that develops with change in air speed can also contribute to poor spiral characteristics. Finally, the amount of aerodynamic spiral stability is important. Too much positive spiral stability is undesirable because of conflicting requirements with dihedral effect, the Dutch rola mode, and the need to hold lateral control into the turn. For these reasons neutral spiral stability is desired.

When the conventional roll and spiral modes couple and form a second oscillatory mode, in addition to the conventional Dutch roll oscillatory mode, the pilot's control of bank angle is seriously affected since the response from a lateral control input appears to have low roll damping causing the pilot to "check" or reverse all lateral control inputs to manually damp the roll motion.

3.10 DIHEDRAL EFFECT Criteria. For operation at and above the reference approach speed and for the sideslip conditions specified, the rolling-moment variation with sideslip should be such that for conventional control systems left roll control deflection and force are required ii left sideslips and vice versa.

The variation of roll cockpit control deflection and force with sideslip angle should be essentially linear.

The positive effective dihedral should be limited so that sufficient roll control remains to correct for gusts or other self-induced upsets and to maneuver as required with a force not to exceed 10 lb for maximum sideslip angles that may be experienced in operational flying.

Discussion. Positive dihedral effect (rolling moment due to sideslip) is desired by the pikt because (1) it provides an alternate method of raising a wing, and (2) it serves as a warning of sideslip development. Linearity is desired for a consistent calibration of the magnitude of sideslip. The degree of positive dihedral effect desired by the pilot is strongly influenced by other characteristics such as roll control power, turn entry coordination, lateral damping, and spiral stability. When the turn coordination in turn entries is good, the pilot is more tolerant of large values of dihedral effect because little sideslip will develop in maneuvering. In addition, positive dihedral effect has a favorable influence on spiral characteristics. On the other hand, the ,aircraft may be more difficult to control laterally in gusty air due to inadvertent development of sideslip.

3.11 YAW CONTROL CHARACTERISTICS - GENERAL The requirements for use of yaw control for V/STOL aircraft change between hover and forward flight conditions. For the VTOL aircraft in hover, yaw control is used primarily to change heading without consideration for coordination with lateral control; while for forward flight and STOL operation, the rudder is used primarily to trim the aircraft to maintain trim sideslip angle in turn entries and to obtain a specific sideslip angle in a cross-wind landing. The different requirements for yaw control and the interrelationships among the various parameters are important considerations for handling qualities because of the need to minimize heading deviation particularly in IFR operation.

3.12 YAW CONTROL POWER Criteria. From trimmed conditions in hover, at the selected reference approach speeds in STOL operation, and for the wind conditions specified, the yaw control should be sufficient to achieve at least the values of aircraft response shown in table 3.6. 22

Table 3.6

Yaw Control Power Characteristics

Parameter to Control power required Minimum levels for satisfactory operation I be measured for: Hover

Yaw angular acceleration, Maneuvering 0.1 - 0.5 0.15 - 0.25 rad/sec2 I Tim for 15' ,i heading change, Maneuvering 1.0 - 2.5 (2.0 sec I I Steady-s tate sideslip angle, Trim deg

Yaw control Sufficient control in EXCESS of MANEUVER- deflection at Trim ING REQUIREMENT to trim over designated zero yawing speed and c.g. range and for most critical velocity, in. engine failure 1 Time to recover o initial .heading, Upset (due to gusts, sec or control recirculation, ground deflection, in. effect, etc.) Building up in Building over a 1 sec 100 ft distance

Yaw angular Typical range of values used by V/STOL acceleration, 0.35 - 0.8 rad/ sec2 aircraft for maneuvering, trim, and upset I I

Discussion. Like the control requirements for the pitch and roll axes, yaw control power requirements must be examined in terms of inputs required for maneuver, trim, and upset. For VTOL aircraft in hover, the pilot will accept less yaw control power compared to roll and pitch, due primarily to the fact that rapid maneuvering is not nsrmally required and heading errors can accumulate with less regard for safety of flight. In addition, guts or recirculation effects generate relatively small yawing moments for most configurations. Trim requirements in sideward flight, which are configuration dependent, can be very important for configurations chat develop large rolling moments with side velocity (or sideslip). In these cases, adequate yaw control power is needed to reduce sideslip angle before large bank angles are attained. The pilot requires two manifestations of yaw control power: (1) the time for a given heading change since large changes in direction are commonly made in hover, and (2) angular acceleration as a measure of how quickly he can correct for unwanted sideslip.

In STOL operation, yaw control power requirements are dictated by the frequent need to sideslip for cross-wind operation, and for some configurations, by asymmetric yawing moments produced by power plant failure as well as for turn coordination in turn entries. For cross-wind landings, combinations of wing-down and crab techniques are normally used. When using low approach speeds without crabbing in cross-wind operation, the sideslip angles required can become very large. The pilot needs adequate angular response to I decrab the aircraft quickly for alignment purposes prior to touchdown before sideward drift becomes excessive, ,

3.13 YAW CONTROL SENSITIVITY Criteria. It should be possible to achieve at least the values of response shown in table 3.7 following an abrupt 1-in. input of the yaw control. These criteria should be met starting from trimmed, zero-yaw-rate flight in hover and at the selected reference approach speeds in STOL operation.

The linearity of yawing acceleration with yaw control deflection should be in the ,direction noted in table 3.7. 23

Table 3.7

Yaw Control Sensitivity Characteristics

Minimum levels for satisfactory operation Parameter to Item Type of control system be measured 1 Hover I STOL Yaw angular acceleration Rate per unit control deflection, rad/sec2/in. Sensitivity Yaw angular acceleration Acceleration per unit control 0.05 - 0.2 deflection, rad/sec2/in. Variation of yaw Linearity 1 All angular acceleration Constant or should not abruptly increase .with control nor change sign.

Discussion. Satisfactory levels of yaw control sensitivity are needed on V/STOL aircraft for the pilot to be able to make precise heading changes, correct for turbulence and recirculation effects, and compensate for cross-coupling. Large yaw control moments generally are not available with most VTOL concepts, and sluggish directional response is likely. For this reason, the pilot prefers to obtain maximum response with only small total rudder movements. It is equally important chat sensitivity be adjusted to avoid over-control tendencies for high-speed flight.

Linearity of yaw response to control inputs is also important to the pilot. Smooth, proportional response is obviously desirable for precise heading control, particularly in gusty air and for IFR operation. For VTOL aircraft in hover, nonlinear response to yaw control inputs can result from engine inlet and other aerodynamic moments when turning out of wind. In addition, saturation of an augmentation device can cause nonlinear behavior. Because VTOL aircraft must make large heading changes in winds, linearity of response is desirable out to sideslip angles of 90". In forward flight, linearity of directional control is limited by vertical fin stall, and +15' sideslip traditionally has been accepted by the pilot.

3.14 CONTROL SYSTEM TIME LAG Criteria. Following an abrupt displacement of the cockpit yaw control, the time required to reach 63 percent of the maximum angular acceleration should not exceed 0.3 sec.

Discussion. A lag in yaw response, although not as critical as for the roll and pitch axes, is undesirable and an upper limit is required to reduce the tendency for pilot-induced oscillation, which is more likely to occur in tracking a heading in IFR approaches. Yaw control lag is more bothersome to the pilot in large aircraft where the directional period is .long, because precise phasing of rudder inputs is more difficult.

3.15 PEAK YAW CONTROL FORCES Criteria. The cockpit yaw control forces required to obtain the designated yaw response should be within the range of 15 to 50 lb for hover and between 50 and 100 lb for STOL operation.

Discussion. The values of peak yaw control forces are intended to prevent the overcontrol problems that can occur if forces are too low and the possibility of destroying precision of control and causing pilot fatigue if they are too large.

3.16 CROSS COUPLING Criteria. From trimmed conditions in hover and at the selected reference approach speeds in STOL operation, the magnitude of cross coupling about all axes following yaw control inputs should meet the criteria shown in table 3.8. 24

Table 3.8 1 I Yaw Cross-Coupling Characteristics I Levels for satisfactory operation Parameter to be measured Hover STOL

Positive, but not Apparent dihedral, rolling requiring more than moment variation with 50% of roll control yaw rate to trim for 6 max

Not objectionable; Response about pitch axis AI3 Pitch angle change pitch down with less than 2" increase in sideslip

Response in vertical direction Less than 0.05 g Not noticeable

Discussion. Cross coupling from yaw control inputs occurs in varying degrees primarily as roll, pitch, and height changes in hover and as pitch and roll disturbances in forward flight. In hover, height changes are usually more bothersome to the pilot during precision tasks because yaw control inputs are held for relatively long time periods to make large heading changes. In forward flight, it is preferred that pitching moment changes be small so that precision of flight path control is not adversely affected and be in nose-down direction as sideslip angle is increased to avoid higher angle of attack regions where pitchup may occur.

The rolling moment due to yaw control input can result in serious control problems for the pilot because rolloff can occur very abruptly and change in magnitude with angle of attack and air speed during transition. The pilot needs a relatively large margin in lateral control moment to keep bank angle excursions to a satisfactory level while rapid yaw control inputs are made, since the correct use of yaw control to reduce sideslip angle may not be apparent to the pilot during certain dynamic maneuvers.

3.17 DIRECTIONAL CHARACTERISTICS IN STEADY SIDESLIP Criteria. For the sideslip angles obtainable in the speed range from 30 knots to V right yaw cock- con' pit control deflection should be required for left sideslips and vice versa.

For angles of sideslip around f15O, the variation of yaw cockpit control deflection with sideslip angle should be essentially linear. For larger sideslip angles, an increase in deflection should always be required to increase sideslip.

The variation of yaw cockpit pedal force with sideslip angle should be essentially linear for sideslip angles of i15". At greater angles of sideslip, a gradual lessening of force is acceptable; however, the pedal force should never reduce to one-half the maximum value.

Discussion. The static directional characteristics of V/STOL aircraft in sideslip are nominally eval- uated by the pilot in terms of the variation of yaw cockpit control deflection and force with sideslip angle. As for other axes, linearity of pedal deflection and force is desired to aid the pilot in setting up and maintaining some specific value of sideslip that may be desired, for example, in a cross-wind landing.

3.18 SIDE FORCE CHARACTERISTICS IN STEADY SIDESLIPS Criteria. For the sideslip conditions and speeds specified in STOL operation an increase in right bank angle should accompany an increase in right sideslip and vice versa.

Discussion. The requirements for a proper sense of bank angle variation with sideslip angle is desired by the pilot primarily to indicate the direction and magnitude of sideslip angle developed. The increase of side force with sideslip should be small, however, so that sensitivity to side gusts is not excessive and to avoid the need for large angles of bank in cross-wind landings. The pilot is concerned about ground clearance of wing tips or engine pods, which may be critical in determining cross-wind operating limits.

3.19 LATERAL-DIRECTIONAL DYNAMIC STABILITI Criteria. Any roll-yaw oscillations superimposed on the normal control mode due to a disturbance input should exhibit at least the frequency-damping characteristics shown in figure3.l(c)over the speed range Speci- fied. Also, there should be no tendency for perceptible small-amplitude oscillations to persist or for pilot-induced oscillations to result from the pilot's attempts to perform the required flight tasks.

After a failure of a SAS, minimum damping characteristics should be at least those of the single failure curve in figure 3.1. Small amplitude residual oscillations are permitted, provided they are not objectionable to the pilot. 25

- 449- $4.@3- b 5.23- ;; 5.71 - 6.28 - d 698- 7.05 - 3 897- 8 1047 -

L -1.0 -a -.6 -.4 -.2 0 .2 .4 .6 U * -&In

Figure 3.1 Lateral-Directional Dynamic Stability Criteria.

Discussion. The roll-yaw oscillatory characteristics superimposed on the normal control mode tend to be more of a problem for V/STOL aircraft because low inherent damping and low frequencies associated with low-speed flight make flight path excursions more difficult to control. Undesirable values of yaw-roll coupling can seriously degrade pilot rating at low speed because of the difficulties of turn coordination, the amount of attention required to control roll excursions, and the immediate loss of altitude when the resulting oscillations produce large bank angles. 26

Section 4

HOVERING AND VERTICAL FLIGHT PATH CHARACTERISTICS

4.1 GENERAL Altitude control is needed in varying degrees of precision for all phases of operation of V/STOL aircraft. Some important factors in the pilot's ability to control altitude for a VTOL aircraft in hover and V/STOL aircraft in forward flight include: ground effect, vertical thrust margins (control power), height control sensitivity, time constant (thrust response), and vertical damping. The interrelationship of these factors strongly influence the pilot's impression of the final aircraft response.

4.2 GROUND EFFECT Criteria. Downwash/ground interference should not result in unsatisfactory characteristics during hover at any altitude in any designated wind condition or at any airspeed required for STOL operation. In addition, there should be no objectionable effects on the cockpit controls or aircraft response from unsteady aerodynami forces and moments or other factors.

Following failure of a power control system or SAS, it should be possible to make a safe landing through the ground interference region.

Discussion. During operation close to the ground, both VTOL and STOL aircraft may experience unsteady dynamic behavior and changes in vertical thrust resulting from recirculation of downwash or exhaust gases. This behavior varies in intensity with height above the ground, with forward speed, and with each V/STOL concept, depending on how the flow is altered through the powered lift system. The change in lift (either positive or negative) and erratic rolling, pitching, or yawing behavior can be particularly bothersome to the pilot because it may occur suddenly and with unpredictable aircraft response.

The acceptable magnitude of disturbance and change in vertical lift will vary with the mission and task. Precise and prolonged hovering requires that ground effect disturbances be virtually nonexistent. When prolonged hovering is not required in the mission, as for a fighter, the pilot would accept larger disturbances, especially for VFR operation.

Hovering precision is adversely affected when ground effect disturbances are felt by the pilot in the cockpit controls as random, inadvertent inputs. In addition, when wind vanes used for SAS inputs are affected by disturbed flow around the aircraft, the pilot cannot be sure whether his input alone is causing a particular aircraft response.

4.3 VERTICAL FLIGHT PATH CONTROL Criteria. For satisfactory flight path control during all phases of STOL flight operation below V (including approach, landing flare, touchdown, and waveoff), the vertical aircraft response characteris$% obtained at a constant attitude resulting from any combination of inputs from throttle, collective, and thrust vector controls should meet the values listed in table 4.1.

Table 4.1

Minimum Vertical Flight Path Control Characteristics in STOL Operation

Level for Minimum level for Item Mode* Parameter to be measured I I .I satisfactory operation acceptable operatior

Incremental normal +o .lg Insufficient data acceleration I Incremental normal B lg Insufficient data acceleration fO. Control power . I C I Steady-state climb angle I 6' or 600 ft/min I 200 ft/min

2' greater than selected All Incremental descent angle approach angle Insufficient data

Achieve mode IA in less A Aircraft response Insufficient data than 0.5 sec

Achieve mode IB in less Response time B Aircraft response Insufficient data than 1.5 sec

Achieve mode IC in less Achieve mode IC in Aircraft response lcI I than 2.0 sec I less than 4.0 sec Cross' coupling All Pitching moment Not objectionable Not objectionable

*Mode A: For flare and touchdown control when less than 0.15 g can be developed by aircraft rotation using pitch control alone. Mode B: For flight path tracking when more than 0.15 g but less than 0.30 g can be developed by pitch control alone. Mode C: For gross flight path changes regardless of the normal acceleration developed by pitch control. 27

Discussion. During low-speed flight, insufficient normal acceleration is available by pitch control alone for vertical flight path control for STOL operation of V/STOL aircraft, and the pilot must use additional methods of developing normal acceleration. Powered lift is used for flight path control in three general modes: (1) controlling rate of sink at flare and touchdown, (2) acquiring and tracking a particular flight path angle during approach, and (3) making gross changes in flight path for waveoff and turning flight. Satisfactory performance of the foregoing tasks depends on the amount of normal acceleration available from powered lift, the aircraft response time, and the degree of cross coupling. The values needed by the pilot depend on how critically the particular flight mode must be controlled. For example, altitude control during flare and touchdown must be precise and requires a short response time for this critical maneuver.

It is important that cross coupling between powered lift and aircraft rotation be minimized so that the pilot can adjust rate of sink and aircraft attitude independently for optimum landing performance.

4.4 HOVERING PRECISION Criteria. At the design VTOL gross weight and depending on the mission requirements, the aircraft should demonstrate the following capabilities in the designated wind conditions: 1. It should be possible to take off, hover continuously in ground effect (IGE), and land all within an area on the ground that is 1.1 span wide and 1.1 length long. 2. It should be possible to hover continuously out of ground effect (OGE) within an area on the ground that is 1.2 span wide and 1.2 length long. 3. For rescue or sling load operations, it also should be possible to control vertical height to f1 ft both IGE and OGE. This should be accomplished with a minimum amount’of height control motion and should not require more than f0.5 in. displacement of the height control.

Discussion. The overall objective of the hover-precision criteria is to ensure that a VTOL aircraft can operate in a confined space. The degree of horizontal hover precision desired by the pilot depends on the task. For example, less precision is needed for a fighter that may merely take off or land either individually or in formation, while precise accuracy may be required to carry out a rescue operation with a winch or use of a sling load. A requirement for vertical height positioning is also needed since all types of VTOL aircraft must be able to adjust altitude with varying degrees of precision for landing, flying in formation, or operating with a sling load. Because of his work load, the pilot desires that altitude positioning be carried out with minimum height control activity.

The precision requirements for OGE may be less stringent for two reasons: (1) less concern for hitting objects; and (2) for aircraft operating with sling loads, the longer cable length will result in less off- center displacement on the ground for a given displacement of the aircraft.

It is desirable to establish an upper limit on the magnitude of height control movement to aid the pilot in setting a reference height control position. For example, if control movements are too large, it is more difficult to return to the reference position.

Since the pilot desires a certain precision in positioning the aircraft horizontally as well as vertically, such factors as vertical thrust margin, control sensitivity, vertical velocity damping, ground effect, control system time constant, mechanical control characteristics, and cross coupling assume particular impor- tance.

4.5 VERTICAL THRUST MARGINS Criteria. In hover and low-speed flight sufficient control of vertical ascent and descent rate should exist at the design gross weight, with due consideration for the effects of control moment inputs required for maneuvering and trim for the environmental conditions specified. The values shown for takeoff and landing in figure 4.1 should be met for hovering in still air for inputs up to the permissible maximum. In addition, the combination of vertical velocity damping and thrust-to-weight ratio available to the pilot should result in a climb rate of at least 600 fpm.

TAKE OFF

UNSATISFACTORY (MAXIMUM RX

SATISFACTORY

> LANDING

E Y

1.0 1.05 1.10 1.15 1.x) MAX THRUST-TO-WEIGHT RATIO AVAILABLE

Figure 4.1 Vertical Height Control Characteristics. 28

Discussion. The vertical thrust margins (height control power) needed by the pilot to carry out the total mission depend on (1) the flight condition, i.e., hover, takeoff, or landing: (2) the vertical height damping; (3) the lift changes experienced IGE due to recirculation and ground effect; and (4) the mission constraints such as field size, height of surrounding obstacles, and whether the specific operation is for rescue, transport, or fighter aircraft. Consideration must be given also to the loss in vertical thrust margin resulting from control moment inputs required for trim and to offset disturbances when operating in the environmental conditions specified.

The pilot needs a relatively large thrust margin as measured OGE for hover-type takeoffs to provide sufficient forward acceleration capability during climbout. So far as control of height rate is concerned, vertical height damping influences thrust margins in an opposite sense for takeoff compared to landing. If the inherent vertical damping is high, the pilot desires higher values of thrust margin to avoid sluggish height response in hover as well as to provide the desired rate of climb during an accelerating transition. For some VTOL concepts, the pilot will accept less thrust margin by using rolling takeoffs, which reduce the influence of adverse ground effect disturbances, and will take advantage of the appreciable amounts of aerodynamic lift provided by rapid forward acceleration. Smaller thrust margins are satisfactory for landing if good vertical height damping exists and the damping is not obtained artificially at the expense of vertical thrust. Further, marginal operation is expected at low thrust margins because of the limitations the pilot may put on sink rate. The pilot should be able to completely check the sink rate before touchdown.

4.6 VERTICAL VELOCITY AND THRUST RESPONSE Criteria. The required change in normal acceleration and vertical velocity in one second following an abrupt input of the cockpit height control, as well as the required thrust response (total time to achieve 63 percent of the commanded thrust increment) should be as noted in table 4.2

~~ Table 4.2

Vertical Velocity and.Thrust Response Characteristics

Level for satisfactory operation Item Parameter to be measured Minimum Maximum

Height control sensitivity Normal acceleration, g/in. 0.1 to 0.4 I

Vertical velocity Rate of climb (after response 1 sec), ft/min I 150 to 775 I First-order time Thrust response Not greater constant, sec than 0.5

Discussion. When an abrupt 1-in. input of the cockpit height control is made, the pilot desires a change in vertical velocity that must not be so small as to appear sluggish, or so large as to cause overcontrol. The magnitude of vertical velocity obtained depends chiefly on three factors: (1) g per inch of control displacement (control sensitivity); (2) vertical velocity damping; and (3) thrust response time. A major consideration is the provision of adequate thrust response. When large values of lag are present, the pilot has difficulty in anticipating the final aircraft response, and overcontrol tendencies may result. 29

Section 5

TRANSITION CHARACTERISTICS

5.1 GENERAL Good transition characteristics are essential for successful use of V/STOL aircraft for a number of reasons. First, it may be desirable to perform transitions quickly to minimize time spent in the terminal area. Second, transitions are usually performed in the critical landing approach phase of flight, where the pilot must be able to maintain precise control of flight path particularly for IFR operation. Finally, transitions occur at a time of high pilot work load, including configuration changes such as selection of landing gear and flaps, and starting lift engines, etc., as well as communications and navigation duties. In the following paragraphs attention is given to those handling-qualities items that govern aircraft behavior in going from powered lift flight to aerodynamic lift regime and vice versa for both VTOL and STOL aircraft.

5.2 ACCELERATION/DECELERATATION Criteria. For VTOL aircraft, it should be possible to accelerate rapidly and safely from hover to Vcon in climbing flight or at constant altitude; and from V it should be possible to declerate rapidly and safely at constant altitude or in a descent up to the k%mum approach angle required by the mission, to acquire and maintain both shallow and steep flight path angles, and to stop quickly and precisely over a preselected hover spot. Depending on the mission, acceleration and deceleration values up to 0.5 g in level flight are desired. In addition, it is desirable to be able to accelerate continuously from a rolling takeoff (RTO) to Vcon and decelerate smoothly to a rolling landing.

For STOL aircraft, it should be possible to accelerate from V to Vcon in level flight or climbing flight, and to decelerate quickly from V to V and to precisef!fPacquire and maintain both shallow and con app steep flight path angles.

It should be possible to carry out the above maneuvers with the precision and performance specified for the mission without restriction due to control power, trim, stalling or buffeting, engine thrust, or response characteristics.

The pilot should be required to operate only primary flight controls, power setting, and thrust vector tilt. If other devices required for transitions are operated automatically, it should be possible for the pilot to easily monitor their performance, and inadvertent operation of any transition control should be prevented.

Discussion. The purpose of these criteria is to ensure that in going from powered lift flight to aerodynamic lift flight and vice versa, the pilot can perform the necessary maneuvers as expeditiously as needed without undue attention to aircraft attitude, angle of attack, airspeed, and trim-factors that would com- promise his ability to fly the aircraft accurately along a chosen flight path in all environmental conditions. Further, good control characteristics are needed for STOL operation when going in an out of ground effect because ground-induced recirculation may cause unsteady flow over the aircraft. In addition, the pilot should have the capability to decelerate as needed at any portion of the speed range to quickly attain a particular approach speed or to avoid overshooting a desired touchdown area.

The time required for making a transition can vary according to the mission; however, it is necessary from safety considerations that the rate desired by the pilot should not be governed by limitations in controllability about 9 axis. If the pilot must handle a large number of separate operations to accomplish the transition, his performance in terms of airspeed, angle of attack, and flight path angle control will suffer during this critical flight phase. Due consideration should be given to multicrew functions in transport configurations where, for example, lift engine startup and shutdown could be handled by a copilot.

5.3 FLEXIBILITY OF OPERATION Criteria. It should be possible to stop, operate steadily as required for a particular mission, and reverse the direction of transition quickly and safely at any speed without undue complicated operation of the powered lift controls.

In the event of a single engine failure on a multiengine aircraft or a failure of SAS, automatic trim, or power control system, it should be possible to make a landing or waveoff safely within a selected range of airspeeds.

Discussion. For operation in confined areas, the pilot must be able to abort the transition quickly in either direction. When cockpit procedures are complicated, longer time periods are needed and too much pilot attention must be directed at such items as proper switch sequencing, stowing of collective controls, etc. Further, flexibility of operation is needed for safety when, for example, excessive trim changes are encountered due to a malfunction of an automatically programmed stabilizer during a transition. Unless the pilot can immediately stop the transition and revert back to the conventional flight mode, excessive pitch attitudes may develop from which recovery may not be possible.

5.4 TOLERANCE IN CONVERSION Criterion. It should be possible to change from hovering flight to conventional flight and vice versa with minimum requirements for large attitude changes, precise programming of engine power, wing tilt, thrust vector angle, etc., and with sufficient tolerance in speed or time so that the conversion can be made safely and routinely.

Discussion. The intent of this criterion is to provide sufficient leeway in executing the transition to ease the amount of pilot attention required during a critical period of flight when other dbties also demand his time. If transition programming must be very precise, as with certain VTOL concepts, the utility of the aircraft can be severely compromised, especially for routine day-to-day operation in all types of weather conditions. 5.5 CONTROL MARGINS Criteria. The remaining margin of control power about any axis needed to maneuver and correct for upsets at any stage of transition should not be less than the maneuvering values presented in the previous sections, regardless of how rapidly the transition is made or how much simultaneous control input is used. In addition, the true amount of control margin available should be made apparent to the pilot, preferably by usable control position. A warning device or a means to restrict the amount of angle of attack or sideslip attainable must be provided on VTOL concepts for which unsafe attitudes may occur during transition.

Discussion. The purpose of these criteria is to provide sufficient margin in control power during transition to enable the pilot to maintain a wings-level, straight, fixed attitude in turbulence; to change the flight path both in direction and elevation; and to aid the rapidity of the transition by changing aircraft attitude. Unless these margins are ample, the pilot will be forced to make the transition more slowly than desired or give undue attention to aircraft attitude. Serious control problems may occur on aircraft that exhibit sensitivity to angle of attack (resulting in pitchup) and sideslip (resulting in rolloff). The latter instability occurs when the aircraft is sideslipping, and the rolling moment due to sideslip rolls the aircraft away from the sideslip. Since rolling in this direction results in an increase in incidence, which can make the rolling moment still larger, an extremely rapid divergence in bank may occur. The pilot may not have been aware of the sideslip and therefore may not instinctively take the necessary recovery action, which is to reduce sideslip.

The amount of control margin required will depend on the maneuvering desired for the mission; the degree to which a particular VTOL concept is upset by turbulence; and the amount of trim change associated with speed, angle of attack, and sideslip change.

The need to provide some warning as to the amountof control remaining may be particularly acute for some types of control systems in which cockpit control position does not provide an accurate indication Of control moment available for recovery.

5.6 TRIM CHANGE Criteria. Trim changes about axis (e.g., for changes in configuration speed, etc.) during the transition should be small and gradual; without retrimming, the control force change should not exceed approximately 15 lb pull or 7 lb push for pitch control, f10 lb for lateral control, and f50 lb for yaw control. Trim changes associated with switchover from one control mode to another (e.g., from attitude command to rate damped) should be small, gradual, and compatible with the trimming rate available.

Discussion. The intent of these criteria is to keep the force values low enough so that the pilot can maintain the desired flight path comfortably with only one hand on the primary controls.

5.7 RATE OF CONTROL MOVEMENT Criterion. During transition, with the maxinum acceleration or deceleration values used, it should be possible to maintain wings-level, straight flight with a low rate of control movement. The maximum pitch control movement should be in the range of 1/2 to 1 in. per sec.

Discussion. The intent of this criterion is to restrict the rate at which trim changes occur in transition; rapid changes require constant pilot attention to monitor aircraft attitude to prevent the development of unsafe attitudes. Changes in forward speed and induced flow effects when vector angle and power are changed may result in changes in trim, which if rapid, will degrade control of attitude and the pilot's awareness of control margin. This is particularly important in aircraft prone to instability such as pitchup or rolloff. The pilot must be able to detect an impending attitude change, preferably through control feel. As noted previously, warning may not be apparent in certain types of control systems. Roll upsets as a result of excessive sideslip are.particularly disconcerting to the pilot because the proper recovery techniques may not be instinctive. 31

Section 6

MISCELLANEOUS CHARACTERISTICS

6.2.2 CONTROL EFFECTIVENESS DURING TAKEOFF, LANDING ROLLOUT, AND TAXI Criteria. It should be possible to maintain the desired path and aircraft attitude in takeoff and landing by normal use of cockpit controls and steering controls in calm air and for designated values of cross winds. This criterion also should apply during landing rollout when reversed thrust is used, for any condition when one or more thrust systems are inoperative, and for any desired STOL configuration including that for optimum takeoff and landing performance.

It should be possible to make a 360" turn in either direction while taxiing within a circle whose radius equals the major dimension of the aircraft, in the designated wind conditions. In addition, for any desired selection of lift configuration (such as wing-tilt angle), it should be possible to taxi in a straight line at any desired aircraft/wind angle with adequate control margins to adjust aircraft attitude in gusty air,

Discussion. The pilot needs good control and stable behavior of V/STOL aircraft during ground roll in cross winds during takeoff or landing for a number of reasons. First, operation is likely to take place on narrow uneven runways where low values of ground reaction moments associated with narrow track landing gear designs, and soft, long-stroke oleos struts create a tendency for rollover, particularly for V/STOL configurations with high dihedral effect. Second, corrective control moments, provided in some form by engine power, may be reduced drastically when reverse thrust is used, a propeller is feathered, or a particular configuration (for example, wing tilt) is selected. Finally, recirculation and ground effects may result in instabilities and loss of control effectiveness due to random flow disturbances. 6.2.3 POWER CHECKS PRIOR TO TAKEOFF Criteria. For VTOL operation, it should be possible to maintain a fixed position on any designated surface for the wind conditions specified using only brakes and cockpit controls during engine and control system checks prior to takeoff, and there should be no objectionable attitude change during starting and runup to takeoff power. For VTO, STO, and RTO operation, it is desirable that proper control functioning be checked prior to reaching takeoff power.

Discussion. The purpose of these criteria is to ensure that sufficient control power exists during engine runup so that the pilot can adjust the attitude of the aircraft or the direction of the thrust to avoid undesired movement of the aircraft over the ground. Since the aircraft may operate in a remote area, the foregoing should be possible without the use of wheel chocks. Because of obstacle hazards it may be desirable to run up and lift off in any direction regardless of wind direction. Although it is desired to prevent 9 movement when running up to takeoff power, the pilot is generally more concerned about the lateral direction because of the likelihood of overturning when striking a wing tip or propulsion unit as a result of the large overturning moments generated by the high thrust.

6.3 CROSS-COUPLING EFFECTS - GENERAL Because large gyroscopic moments may result from the engines or lifting system of V/STOL aircraft, the effects of aircraft'motion or control input about a given axis may result in objectionable aircraft motion or control force about other axes.

I 6.3.1 GYROSCOPIC EFFECTS Criteria. The effects of engine, fan, or rotor gyroscopic moments on the dynamic behavior of the air- 32

craft in flight or in ground operation should not result in objectionable aircraft motions.

6.3.2 INERTIAL CROSS-COUPLING EFFECTS

Criteria. The application of any control input necessary to meet roll, pitch, OK yaw response criteria (the other controls being held fixed) should not result in aircraft angular rates OK attitude changes that cause objectionable or dangerous flight conditions. This criterion should also apply following the failure in a powered control system or SAS.

6.3.3 MECHANICAL CROSS-COUPLING Criteria. A displacement of one cockpit control should not produce objectionable forces at any of the other controls for normal operation and following failure in a powered control system OK SAS.

Discussion. The purpose of the various cross-coupling criteria is to aid the pilot in obtaining satisfactory control of aircraft behavior by reducing extraneous aircraft motions in hovering and forward flight. The degrading effects of gyroscopic cross-coupling are particularly severe in gusty air where the pilot is unsure of the cause of aircraft motion. Because of the profound effect on precision of control, the pilot prefers that no control deflection be required to counteract gyroscopic coupling, and any mechanical CKOSS- coupling effects should be overcome with very low forces (compatible with the need for one-hand operation).

6.4 MINIMUM FLIGHT SPEEDS - GENERAL Both minimum flight speed and maximum incidence must be considered when defining handling qualities in low-speed flight. Dynamic behavior, loss of lift, and buffet are chiefly related to maximum incidence, while the speeds at which they OCCUK vary widely with engine power and thrust vector angle.

6.4.1 LOSS OF LIFT Criteria. The dynamic behavior of the aircraft when loss of lift is experienced at V (OKmaximum incidence) should be characterized by mild nose-down pitching, moderate settling, and mildminto moderate buffet (aircraft StKUCtUKal integrity is not impaired). Unintended lateral attitude or directional heading changes are undesirable. If oscillatory motions of large magnitude OCCUK about any axis, the oscillatory period should be long enough (at least 5 sec) such that control action can be taken to avoid possible secondary stall problems.

It should be possible to avoid the attainment of the minimum flight speed (OKstall) by normal use of the controls at the onset of the warning. In the event of attaining Vmin, recovery should be possible by normal use of the controls, including thrust and vector angle as necessary, and without excessive loss of altitude OK excessive increase in speed.

On multi-engine aircraft with the critical engine out (or with the critical propeller inoperative), it should be possible to recover safely from a stalled condition for all engine powers, vector angles, or thrust conditions specified.

Discussion. The purpose of these criteria is to take advantage of the aircraft maximum performance potential by ensuring that it can be flown safely at speeds close to the stall. Regardless of the type of lift concept, the pilot may desire to use as much aerodynamic lift as possible in slow-speed operation of V/STOL aircraft from considerations of safety and fuel economy. Since aerodynamic lift can be generated by the direct action of free-stream dynamic'pressure on surfaces such as wings or ducts, and also induced flow from engine thrust or propeller slipstream, the loss of lift from any source is of concern because of its effect on flight path control. Because the pilot may choose to minimize loss in altitude and partial loss of controllability at the stall by uslng engine thrust, quick thrust response can facilitate stall recovery directly by improving flow conditions and indirectly by accelerating the aircraft to a higher speed, thus reducing the angle of attack.

6.4.2 WARNING OF APPROACH OF Vmin Criteria. The approach to the minimum flying speed should be accompanied by an easily perceptible warning. Acceptable warning consists preferably of natural aerodynamic shaking OK buffeting of the aircraft, shaking of the cockpit controls, OK visual and aural cues. Intensity of the warning should increase progressively as the stall is more deeply penetrated. Other types of warning devices, such as a stick pusher, may be acceptable provided no dangerous flight conditions result from actuation of the device. In addition, artificial devices should not unduly limit aircraft performance.

The warning should OCCUK at a margin that allows the pilot to avoid attainment of the limiting condition by normal use of the controls. The margin from V should be no less than 5 knots or 5" angle of attack. min Discussion. The purpose of these criteria is to provide the pilot with a usable warning of the impending loss of aerodynamic lift so that corrective action can be taken in time to prevent undesired deviation from the approach or takeoff flight path. Aerodynamic buffeting of the aircraft is preferable because of its inherent reliability. However, it is important that buffet intensity increase with penetration into the stall to indicate that a more dangerous flight condition is developing. When stick pushers are used on aircraft confighations that can approach a dangerous, nonrecoverable stalled condition with no apparent attitude change, it is important that this and other types of artificial stall warning devices do not restrict the pilot from obtaining the desired low-speed performance by being preset too far ahead of Vmin, It is also important that premature actuation of a stick pusher device does not occur at high speed where StKUCtUKal damage could occur, OK near the ground where a dangerous nosedown condition could result.

The margins needed for satisfactory warning depend on the behavior at the stall and the relative amount of aerodynamic and powered lift being used. For configurations for which powered lift predominates and no undesirable dynamic behavior exists at Vm n, little OK no warning is necessary, On the other hand, if little powered lift is used and excessive altituae is lost at the stall, a larger margin would be required to allow sufficient time for corrective action. 33

6.5 WARNING OF APPROACH TO HA2ARW)US FLIGHT CONDITION Criteria. Clear and unambiguous warning should enable the pilot to avoid hazardous flight conditions. Warning devices should not unduly limit the performance envelope of the aircraft.

Discussion. VTOL and STOL aircraft conceivably could be flown inadvertently into hazardous parts of the flight envelope. Such behavior as a large nose-up trim change (pitchup) in transition, an excessive rolling moment (dihedral effect) in sideward flight, and loss of control from excessive use of engine bleed air from a demand-type system can lead to unrecoverable situations. Penetration into these hazardous areas can occur quickly and imperceptibly to the pilot. Further, excursions into hazardous areas are likely to occur at low altitude where the pilot has insufficient time to explore the use of several recovery techniques. When an attitude command control system is used, the true trim condition (margin of control) may not be apparent through position of the cockpit controls.

6.6 AIRCRAFT BEHAVIOR FOLLOWING SYSTEMS FAILURE Criteria. The dynamic behavior of the aircraft following a sudden failure of an engine, thrust device, or control system should not result in excessive attitude changes, angular accelerations, or sink rates that preclude corrective action by the pilot to avoid a hazardous condition, permit an emergency landing, or allow sufficient time for the crew to escape.

Failure of high-lift systems such as boundary layer control should not result in asymmetric moments, uncontrollable pitch changes, or excessive loss of altitude.

When SAS failures result in marked changes in dynamic response of the aircraft, the pilot must be warned of the failure, even though there may not have been any marked change in the behavior of the aircraft such that an alternate form of control can be selected. The pilot also must be provided with a means of discon- necting the SAS quickly in case of failure.

Discussion. The purpose of these criteria is to provide protection for the pilot and crew so that an emergency landing or successful escape can be made, The consequences of system failures on VTOL and STOL aircraft are more serious than for conventional aircraft, and may result in large deviations from normal flight due to large disturbing moments and the low aerodynamic stability moments associated with low-speed flight. Some form of protection must be provided to ensure the pilot that safe operation is possible in low-speed, low-altitude flight; otherwise, the true performance potential of the aircraft may not be realiz- able in routine operation because the pilot will not have the confidence to practice low-speed flight,

An inadvertent or sudden failure of the SAS can be more serious on some aircraft where dynamic response changes are large, and it may take 2 to 3 sec for recognition of the problem and the pilot's alteration of his mode of control (compensation). It may be preferable to allow the pilot to manually disengage the SAS to avoid transient disturbances and prepare his altered mode of control to the new aircraft dynamics. 34

Appendix

MANEWERS FOR V/STOL AIRCRAFT HANDLING-QUALITIES EVALUATIONS

This appendix outlines maneuvers suggested for the evaluations of the handling qualities of V/STOL aircraft. Sufficient tests should be conducted to evaluate the characteristics over the complete weight and center-of-gravity ranges and aircraft configurations utilizing all reasonable combinations of stability augmentation features in wind conditions from calm to the specified wind. (A wind of 35 knots is considered a reasonable maximum.)

As the various maneuvers are attempted, starting from trimmed flight conditions, control system characteristics such as those outlined below should be noted:

a) Control deflections required, breakout forces and force gradients b) Control backlash and free play c) Rate of change and the ease of operation of any trimming devices d) General characteristics such as the view from the cockpit, particularly during decelerating transition and landing

The Pilot Opinion Rating Scale should be used along with detailed comments to indicate the pilot's assessment of the handling qualities. (See page 5.)

Clearly, all types of V/STOL aircraft will not be flown in the same manner, and what is a necessary maneuver for one type may not be for another. For example, jet V/STOL aircraft must be able to make rapid decelerations to conserve fuel, whereas a tilt-wing aircraft can afford to be more leisurely. The suggested maneuvers should illuminate the main handling-qualities characteristics for each aircraft when it is operated in the manner expected of its type. Since severalsfactors can produce similar effects on the handling qualities, the pilot's comments should be directed to the effects, while recorded flight data should be used to establish the causes.

The reference numbers for each section indicate the sections of the main report relevant to the maneuvers described.

A1.O HOVERING AND LOW SPEED MANEUVERS Al.l Longitudinal Translation and Pitching Characteristics. Several methods may be available to the pilot for control of longitudinal position and/or pitch attitude such as adjustment of the thrust vector direction (tilting wing or engines) and/or fuselage attitude. The following maneuvers should be investigated using each appropriate method of control.

Al.l.l Vertical takeoff and steady hovering. Lift the aircraft into the surface wind, +45', 'and k90' from the wind direction. Note the following:

a) Difficulties in maintaining constant height, position, heading, and attitude b) Power required for comparison with Al.2.1.

If it is necessary to perform a rolling takeoff (to avoid hot-gas ingestion or other ground effect problems, also note the ground roll required and the difficulties of the following:

a) Thrust and lift management b) Adhering to the takeoff schedule c) Obtaining the lift-off attitude d) Attaining a steady hover over the desired spot (Reference 4.4 and 4.5).

Al.1.2 Longitudinal translations. Face the aircraft into the wind and while maintaining constant altitude translate forward and aft at a rate comfortable to the pilot through a distance approximately five times the length of the aircraft, stopping as precisely as possible over a predetermined spot. Repeat the foregoing maneuvers as quickly as possible.

During this series of maneuvers the following should be noted:

a) The speed stability (by the pitch control movement with speed change) b) Control sensitivity and angular rate damping c) Acceptability of thrust vector tilt rate d) Any tendency to rise or sink e) The ease with which the thrust vector direction and fuselage attitude may be selected and detected (Reference 1.8. 2.3, 2.4 and 2.6).

A1.1.3 Rapid controlled pitching. Rapidly apply at least 1 inch of pitch control in the nose-down direction; after a moderate attitude change has occurred, arrest the longitudinal translation and level the fuselage as quickly as possible. Repeat with increasingly larger inputs until either full control is reached or until motions approaching the controllability limits are obtained. Repeat using nose-up inputs. These maneuvers should indicate:

a) Control sensitivity and control power b) Angular rate damping- c) Control system lags d) Gyroscopic effects and mechanic cross-coupling e) Tendency for pilot-induced oscillations in pitch (Reference 2.3, 2.4, 2.6, and 2.7).

Al.1.4 Pulse inputs. Apply a rapid pitch input of at least one inch; then return the control to its original position. The input should be sufficiently long to excite the longitudinal dynamic characteristics 35

and simulate the response to a gust upset, Let the resulting motions continue for as long as possible and note:

a) Any tendency for this motion to converge or diverge b) And cross-coupling into other axes (Reference 2.3, 2.4, and 2.8).

Al.2 Height Control

Al.2.1 Flight in-ground effect. After establishing a steady hover out-of-ground effect with the aircraft facing into the wind, +45', and +90° to the wind, let down until the wheels are approximately one foot above the ground to investigate:

a) Self-generated disturbances b) Positive or negative ground effects c) Control manipulations required to hold a constant position and attitude d) Feedback of unsteady aerodynamic forces to the cockpit controls e) Power setting for comparison with that used out-of-ground effect (Al.l.1).

Repeat at several heights up to the disappearance of the ground effect (Reference 4.2 and 4.4).

A1.2.2 Vertical ascent and descent. Select a comfortable rate of climb and descent changing altitude approximately 50 feet. Repeat this maneuver, accomplishing it as quickly as possible. This series of maneuvers should indicate:

a) Height control sensitivity, control power, and vertical velocity damping b) Maximum usable descent rate c) Cross-coupling effects d) Any tendencies for pilot induced oscillations (Reference 4.2, 4.5 and 4.6).

A1.2.3 Vertical landing. While facing into the wind, +45', and +goo to the wind, land as accurately as possible on a predetermined spot noting:

a) Ease with which this may be accomplished b) Any tendency to move after the touchdown.

If it is necessary to perform a "roll-on" landing (to avoid hot-gas ingesting or other ground effect problems) the pilot should also note:

c) Ground roll required d) Ease of arresting the aircraft after the landing e) Any other difficulties encountered (Reference 4.2, 4.4, 4.5 and 4.6).

A1.3 Lateral-Directional Characteristics

A1.3.1 Slow "pedal" turns. While hovering out-of-ground effect, turn over a spot at a comfortable rate stopping at +-goo and 2180' to the surface wind. If control appears critical at any angles from the wind, investigate these more fully, noting the following:

a) Amount of yaw control remaining while holding a steady hover in the critical directions b) Required control forces and trim changes in roll (dihedral effect and side7force variations) c) Required control forces and trim changes in pitch (speed stability and longitudinal force variations) d) Required control forces and trim changes in yaw (weathercock stability) e) Angular rate damping levels as indicated by the response following gust disturbances (Reference 3.4, 3.10, 3.13, 3.14, 3.15, 3.16, 3.17, 3.18 and 3.19).

Al.3.2 Rapid "pedal" turns. Execute rapid turns over a spot by quickly applying at least one inch of yaw control and attempting to stop at f90' from the original heading. Repeat with increasingly large inputs until either full control is reached or until motions approaching the controllability limits are encountered. This series of maneuvers should indicate:

. a) Yaw control sensitivity, control power, and angular rate damping b) Cross-coupling effects c) Control system lags d) Any tendency for pilot-induced oscillations (Reference 3.2, 3.3, 3.4, 3.5, 3.6, 3.8, 3.10, and 3.18).

A1.3.3 Directional pulse inputs. Apply a rapid yaw input of at least one inch; then return the control to its original position. The input should be sufficiently long to excite the lateral-directional dynamic characteristics and simulate the effects of a gust disturbance. Let the resulting motion continue as long as possible and note:

I a) Tendencies for this motion to converge or diverge b) Any cross-coupling into other axes.

A1.3.4 Lateral translations. Face into the surface wind and translate at constant altitude both right and left at a rate comfortable to the pilot through a distance approximately five times the span of the aircraft, stopping as precisely as possible over a predetermined spot. Repeat, executing the maneuver as quickly as possible. This series of maneuvers should indicate:

a) Controls-free (control force) and control-fixed (control position) lateral and directional stabil- ities b) Side force variation with lateral velocity 36

c) Roll control sensitivity, control power, and angular rate damping (especially during the final leveling of the wings) (Reference 3.3, 3.4, 3.6, 3.7, 3.8, 3.10, 3.17, 3.18, and 3.19).

A1.3.5 Rapid roll reversals. Rapidly apply at least 1 inch of roll control, and after a moderate attitude change has occurred, arrest the lateral translation by reversing the roll and leveling the wings as quickly as possible. Repeat, using progressively larger control inputs until either full control is reached or motions approaching the controllability limits are encountered. The series of maneuvers will indicate :

a) Roll control sensitivity, control power, and angular rate damping b) Cross-coupling effects c) Control system lags d) Any tendency for pilot-induced oscillations

A1.3.6 Roll pulse inputs. Repeat the exercise outlined in A1.3.3 using roll control inputs. A2 .O ACCELERATING CONVERSION Three methods of accelerating to conversion speed from a steady hover should be assessed. These include (1) accelerating at a rate comfortable to the pilot, (2) accelerating in increments to various intermediate speeds at which stabilized flight is maintained for short periods before progressing to the next speed, and (3) accelerating as quickly as possible. Each method should be attempted in level and climbing flight with and without cross-winds. During these maneuvers the pilot should note:

Flexibility of performing the conversion Need to control incidence and pitch attitude separately Pitch control position and pitch attitude as a function of speed and configuration changes Ease of effecting any required change in aircraft configuration, (e.g., shutting down lift engine, tilting wings, etc.) Control requirements in other axes Changes in control force Effects of losing visual cues Influence of turbulence (Reference 5.2, 5.3, 5.4, 5.5, 5.6, and 5.7).

A3.0 TESTS AT INTERMEDIATE TRANSITION SPEEDS The aircraft should be stabilized at intermediate speed-power combinations and the maneuvers outlined in A3.1.1 through 3.1.4 should be attempted. While attempting to establish the desired conditions the pilot should note:

a) Trim changes b) Uncontrolled motions c) The ease of lighting lift engines, unlocking wings, and actuating thrust vector control.

A3.1 Pitch, Roll and Yaw Inputs

A3.1.1 Step inputs. From the trim conditions apply step inputs through each control separately, progressively increasing the input size until either the maximum input is reached or the controllability boundary is approached. During these maneuvers the pilot should note:

a) Angular rates developed and their variations b) Changes in attitude and sideslip c) Oscillatory and overshoot behavior d) Control forces, control positions and their variations e) Control or inertial cross-coupling effects f) Adverse yaw, vibrations, and the like (Reference 2.3 through 2.8, 3.4 through 3.8, 3.10, and 3.13 through 3.19) .

A3.1.2 Pulse inputs. Separately about each axis, apply pulse inputs of approximately 1 inch with sufficient duration to excite the dynamic characteristics and simulate the effects of a gust disturbance. From the resulting motion the pilot should note the effects outlined in A3.1.1 (Reference 2.3 through 2.8, 3.4 through 3.7, and 3.13, 3.14, 3.16.)

A3.1.3 Coordination of banked turns. Set up a steady 15' banked turn, maintaining the "ball" in the center and note the following:

a) Yaw and roll control displacements and control forces necessary to coordinate the turn b) Pitch control displacement and control force.

Without altering the rudder position, rapidly roll to bank an equal amount in the opposite direction and note:

a) The rolling and yawing characteristics.

Repeat the entire maneuver, using larger bank angles up to 30" if attainable (Reference 2.7, 3.9, 3.10).

A3.1.4 Steady sideslips. Establish steady sideslips from f5" to the maximum, noting:

a) Changes of control position and control forces b) Difficulties of maintaining steady conditions c) Critical sideslip angles d) Maximum sideslip angle attainable and whether sufficient for the intended use of the aircraft. 37

The maximum angles achievable will indicate the following:

a) Weathercock stability b) Dihedral effecL c) Roll and yaw coptrol effectiveness

From steady sideslips release the yaw control and note:

a) If and how the aircraft returns to lower sideslip angles and if it oscillates while doing so b) Damping and period of the "Dutch roll" mode (Reference 2.7, 3.10, 3.17, and 3.18).

A3.2 Lift Power Inputs

A3.2.1 Slow and rapid inputs. From trimmed conditions, slowly increase power until maximum power is reached while maintaining a constant airspeed and note the effects on trim. Decrease power and until either the engine flight idle position or the aircraft aerodynamic boundary due to wing stall, pitchup, etc., is approached and note:

a) Control movements necessary to effect the desired changes b) Warnings available to the pilot that the aerodynamic boundary is being approached c) Motions while approaching these boundaries d) Control actions necessary to recover from these motions.

Repeat, using rapid power changes (Reference 4.5, 4.6, 5.5, 5.6, and 5.7).

A3.3 Descent Control

A variety of methods may be available for controlling speed and flight path on an approach. Examples include the following:

a) Speed may be controlled by aircraft attitude, wing-tilt, thrust vector angle, or propulsion thrust b) Flight path may be controlled by incidence or lift power.

Each reasonable method should be investigated to determine:

a) Descent rate available b) Limitations on the descent rate c) The optimum approach procedure

Assessment of the optimum method of approach conirol should include the landing maneuver in case a change of technique from that used on the approach is needed for the touchdown (Reference 4.3).

A4.0 DECELERATION CONVERSION

A4.1 Decelerating in Level Flight

Using all reasonable techniques, reduce air-speed from V to several lower speeds, while holding the altitude constant and flying both into and at right angles tocFEe wind directions. Both zero sideslips and sideslipping techniques should be investigated with the pilot noting:

a) Trim changes b) Uncontrolled motions c) Ease of effecting configuration changes d) Loss of visual cues due to changes in attitude e) Any other difficulties encountered (Reference 4.3, 4.6, and 5.2 through 5.6).

A4.2 Decelerating During Descending Flight

While on a straight, constant angle descent of not less than 3", reduce air-speed from V to several lower speeds, including hover. These tests should be conducted both into and at right anglescFg the wind direction using both zero sideslip and sideslipping techniques with the pilot noting the same characteristics outlined in A4.1 (Reference 4.3, 4.6, and 5.2 through 5.6).

A4.3 Decelerating During a Descending Sidestep Maneuver

Establish the aircraft on a straight, constant angle descent path of at least 3". At 200 feet above ground level initiate a sidestep maneuver to displace the aircraft laterally from its original approach track by 300 feet. During this maneuver, decrease the air-speed and attempt to establish hovering flight over a spot 300 feet laterally displaced from the point at which the original glide path intersected the ground. The sidestep should be attempted to both the right and the left, noting the characteristics outlined in A4.1 (Reference 2.10, 3.17, 3.18, 4.3, 4.6, and 5.2 through 5.6).

A5.0 MINIMUM FLIGHT SPEED

The flight characteristics at the minimum flying speed should be checked from several intermediate transition speeds in the following ways:

a) Reducing power at constant speed while maintaining the configuration constant, as outlined in A3.2.1; b) Reducing speed at constant power while maintaining the configuration constant c) Changing configuration at constant power and altitude.

The characteristics should also be checked at constant configuration and speed, while increasing the 38

normal acceleration in a level banked turn up to the limits of the design flight envelope or until handling qualities difficulties prevent further increase in normal accelerations (Reference 2.7, 6.7, and 6.8).

A6.0 STOL OPERATIONS

A6.1 Short Takeoffs. Using all reasonable aircraft configurations and methods of control, attempt a series of short takeoffs at various liftoff speeds both into the wind and at right angles to the wind. The pilot should note:

a) Difficulties encountered in maintaining the aircraft straight on the ground roll. b) Pitch roll, and yaw excursions as the aircraft passes through the ground effect region c) Lateral and directional control effectiveness when in cross-wind conditions (Reference 2.9, 4.2, 6.4, 6.5 and 6.7).

A6.2 Short Landings. Using all reasonable aircraft configurations and methods of control, attempt a series of constant glide slope, constant speed approaches, both into and across the surface wind, concluded by full stop rollon landings. Both the zero sideslip and the sideslipping methods should be attempted. Note the effects outlined in A6.1. (Reference 2.11, 4.2, 4.6, 6.3, 6.4, 6.7, and 6.8). 39

SUMMARY OF THE TABLES

Table 1.1

V/STOL Control Breakout Force Criteria

After failure Acceleration Rate Control Attitude of power control system, system, system, axis system, lb lb lb lb

Pitch I 0.5-1.5 I 0.5-3 I 0.5-3 I <5 I Roll I 0.5-1.5 I 0.5-3 1 0.5-3 1 <4 1 Yaw 1-10 1-10 - <15 Height collective 1- 3 1- 3 - <5 throttle 1- 3 1-3 - <3

Table 1.2

Control Force Gradients (lb/in.) for Hover Control System I I Control t I I Acceleration Atti tude I Pitch 1-2.5 1-3.5 1-3 Roll 0.5-1.25 0.5-1.75 0.5-1.5 Yaw 2.5-10 2.5-10

*After a failure in a power control system the gradients should not be more than twice the values shown.

Table 1.3

Control Force Gradients for STOL

Control lb/in.

Pitch Roll 1- 3 Yaw 10-35

Control force Aircraft Minimum Optimum Maximum ratio type Ratio Rat io Ratio

Pitch V/STOL Roll -Yaw , Roll Yaw - STOL 4 16 Roll a

Table 1.5

Cockpit Control Travel* Limits (in.) 1 Longitudinal 1 Lateral 1 Directional 1 k4.0 - k6.5 k3.0 - 26.5 f2.5 - k4.5

*Considered as hand movement for stick or wheel. 40

Pitch Con 01 Power Chara :eris tics

Minimum levels for satisfactory operation Parameter to be Control power required Type of me as ured for: control system Hover STOL

Attitude com. 0.1 - 0.3 Pitch angular acceleration, Maneuvering Rate 0.05 - 0.2 rad/sec2 Acceleration 0.2 - 0.4 Attitude com. 1 Pitch angle after Maneuvering Rate 2-4 2-4 1 sec, deg I Acceleration 2-4 2-4

Pitch control deflection at zerc Trim All pitching velocity in.

Sufficient control in EXCESS of MANEUVER- ING and TRIM REQUIREMENTS to balance mo- Time to recover t( Upset (due to gusts, ments due to a specific gust; for example, initial attitude recirculation, ground All 30 f/sec gust: sec or control effect, etc.) deflection, in. Building up in Building over a 1 sec I 100 ft distance Typical range of Pitch control values used by V/STOL power angular aircraft for maneu- A11 0.4 0.8 acceleration, - 0.4 - 0.6 vering, trim, and radlsec upset

Table 2.2

Pitch Control Sensitivity

Parameter to Item be measured control system Hover STOL

Attitude change Attitude per unit control 3-5 deflection command deg / in.

Pitch angular acceleration per unit control Sensitivity deflection rad/sec2/in.

Pitch angular acceleration per unit control Acceleration 0.08 - 0.16 deflection rad /s ec / in.

Variation of pitch angular Constant or should not abruptly increase Linearity A11 acceleration with nor change sign. control deflection 41

Table 2.3

Pitch Angular Velocity Damping

Minimum levels for satisfactory operation Parameter to 5Pe of be measured control system Hover STOL

Angular velocity damping, llsec -2.0 -1.0 Attitude command Damping ratio I ~15%overshoot Angular velocity damping, llsec I -0.5 to -2.0 I -1.0 Rate Numbers of control reversals required -- Not more than 1 to stabilize

1 Table 2.4 I I Pitch Control Lags I Levels for satisfactory operation Parameter to Type of be measured control system Hover I STOL

Time from control Attitude command input to 63% of 1and <2 change, sec

Table 3.1 ,

Roll Control Power Characteristics

Parameter to Control power required 5Pe of be measured for: control system Hover STOL

Attitude command 0.2 - 0.4 . Roll angular I acceleration, Maneuvering I Rate 0.2 - 0.4 I 0.1 - 0.6 rad/sec2 Acceleration 0.3 - 0.6 I Atti tude command Bank angle after Maneuvering Rate 1 sec, deg 2-4 1 2-4 Acceleration 2-4 I 2-4 Roll control Sufficient control in EXCESS of MANEUVER- deflection at ING REQUIREMENTS to trim over designated Trim All zero rolling speed and c.g. range and for most critica velocity, in. engine failure

Sufficient control in EXCESS of MANEWER- ING and TRIM REQUIREMENTS to balance mo- 'ime to recover to Upset (due to gusts, ment due to a specific gust; for example, initial attitude recirculation, ground All 30 ft/sec gust or control effect, etc.) deflection, sec Building up in Building over a 1 sec 100 ft distance

Attitude Typical range of 0.4 - 1.5 0.2 - 2.0 Roll a9gular values used by V/STOL I I acceleration, aircraft for maneu- .. Rate 0.8 2.0 0.3 2.5 rad/sec2 vering, trim, and - I - upset Acceleration 0.8 - 2.0 - 42

Table 3.2 I Roll Control Sensitivity Characteristics I Minimum levels for satisfactory operation Parameter to Item 5Pe of be measured control system Hover

Attitude change per unit control Attitude command 3-5 deflection, deglin. Roll angular acceleration per unit control 0.15 - 0.30 0.05 0.25 ensit ivi ty Rate - deflection, rad/sec2/in.

Roll angular acceleration per unit control Acceleration 0.2 - 0.8 deflection, rad/sec2/in.

Variation of Constant or should not abruptly increase roll angular All Linearity acceleration with nor change sign. control deflection

Parameter to be measured Level for satisfactory operation

Maximum sideslip to bank Not greater than approximately angle ratio (AB/A$)max, 0.3 to 0.5 and not to exceed or maximum sideslip angle 20' sideslip angle

Pitch angle change A8 Not objectionable

Normal acceleration change Ag

Table 3.4

Roll Angular Velocity Damping

Minimum levels for satisfactory operation Parameter to 5Pe of be measured control system Hover I STOL

Angular velocity -1.5 to -4 -0.5 to -3 damping, l/sec Attitude command Damping ratio

Angular velocity -2 to -4 -0.5 to -3 damping, l/sec Rate Number of control reversals required Not more than 1 Not more than 2 to stabilize 8 43

I Table 3.5 1

~~~~ Roll Control Lags 1 Levels for satisfactory operation Parameter to Type of be measured ccntrol system Hover STOL I Time from control -1 input to 63% of Attitude command I co.2 I c0.3 peak angular Rate acceleration, sec <0.2 I c0.3

~ ~~~ I I Time to 90% of demanded attitude Attitude command >1 and <2 change, sec I

Table 3.6

Yaw Control Power Characteristics

Minimum levels for satisfactory operation Parameter to Control power required be measured for: Hover I STOL Yaw angular acceleration, Maneuvering 0.1 - 0.5 0.15 - 0.25 rad/sec2

Time for 15" heading change, Maneuvering 1.0 - 2.5 <2.0 sec

Steady-s tate sideslip angle, Trim I sin-l ("cross wind) deg 'approach

~~ ~~ Yaw control Sufficient control in EXCESS of MANEUVER- deflection at ING REQUIREMENT to trim over designated Trim zero yawing speed and c.g. range and for most critical velocity, in. engine failure

Sufficient control in EXCESS of MANEUVER- Time to recover ING and TRIM REQUIREMENTS to balance moment Upset (due to gusts, to initial heading, due to a specific gust; for example, recirculation, ground sec or control 30 ftlsec gust effect, etc.) deflection, in. Building up in Building over a

Typical range of Yaw angular values used by V/STOL acceleration, 0.35 0.8 aircraft for maneuver- - rad/sec2 ing, trim, and upset

Table 3.7 1 Yaw Control Sensitivity Characteristics I Minimum levels for satisfactory operation 5Pe of Parameter to Item control system be measured Hover I STOL Yaw angular acceleration Rate per unit control 0.08 - 0.2 deflection, rad/sec2/in. Sensitivity I 0.05 - 0.10 Yaw angular acceleration Acceleration per unit control deflection, rad/sec2/in. Variation of yaw Linearity All angular acceleration Constant or should not abruptly increase with control nor change sign. deflection 44

Table 3.8

Yaw Cross-Coupling Characteristics

Levels for satisfactory operation Parameter to be measured Hover STOL

Positive, but not Apparent dihedral, rolling requiring more than moment variation with 50% of roll control yaw rate to trim for 6r max I--I I Not objectionable; 1 Pitch angle change pitch down with Response about pitch axis AB less than 2' increase in sideslip

Response in vertical direction Less than 0.05 g Not noticeable

Table 4.1

Minimum Vertical Flight Path Control Characteristics in STOL Operation

Level for Minimum level for Item Mode* Parameter to be satisfactory operation acceptable operation

Incremental normal +o. lg Insufficient data lAl acceleration I I I Incremental normal tO.lg Insufficient data B acceleration Control power C Steady-state climb angle 6' or 600 ftfmin 200 ftfmin

2' greater than selected All Incremental descent angle Insufficient data I I I approach angle I Achieve mode IA in less Aircraft response Insufficient data IAI than 0.5 sec I I Achieve mode IB in less B Aircraft response Insufficient data Response time than 1.5 sec

Achieve mode IC in less Achieve mode IC in C Aircraft response than 2.0 sec less than 4.0 sec

Cross coupling All Pitching moment Not objectionable Not objectionable

*Mode A: For flare and touchdown control when less than 0.15 g can be developed by aircraft rotation using pitch control alone. Mode B: For flight path tracking when more than 0.15 g but less than 0.30 g can be developel by pitch control alone. Mode C: For gross flight path changes regardless of the normal acceleration developed by pitch control.

Level for satisfactory operation Item Parameter to be measured Minimum Maximum

Height control Normal acceleration, g/in. 0.1 to 0.4 sensitivity I Vertical velocity Rate of climb (after 150 to 775 response 1 sec), ftfmin I First-order time Not greater Thrust response I constant. sec I than 0.5 45

$ 4.19

FLIGHT LIMIT 5.71 SATISFACTWY0

12.56 s 23.93 b 31.40 8 6280 L P Y.2 -1.0 -8 --.6 -A -.2 U -€Un

Figure 2.1 Longitudinal Dynamic Stability Criteria.

$ 4.19 - 4.49 463

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B m-1.2 -10 -.8 -.6 -A -.2 0 .2 .4 .6 U = -cwn

Figure 3.1 Lateral-Directionel Dynamic Stability Criteria.

TAKE OFF

UNSATISFACTDRI

> LANDING

1.0 1.05 1.10 1.15 1.20 MAX THRUST-TO-WEIGHT RATIO AVAILABLE

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