General Disclaimer One or more of the Following Statements may affect this Document This document has been reproduced from the best copy furnished by the organizational source. It is being released in the interest of making available as much information as possible. This document may contain data, which exceeds the sheet parameters. It was furnished in this condition by the organizational source and is the best copy available. This document may contain tone-on-tone or color graphs, charts and/or pictures, which have been reproduced in black and white. This document is paginated as submitted by the original source. Portions of this document are not fully legible due to the historical nature of some of the material. However, it is the best reproduction available from the original submission. Produced by the NASA Center for Aerospace Information (CASI) • 6 I 1 13 A Review of Facilities and Test Techniques Used in Low-Speed Flight By Seth B. Anderson and Laurel G. Schroers Ames Research Center, NASA Z Moffett Field, California 94035 c.' 3.a V^ ^a INTRODUCTION 4 New classes of aircraft being developed require new and improvea Ww Loo w1 01 AU113VA flight test facilities and techniques. The high cost of designing novel transport aircraft, in particular a supersonic transport, makes it essential to be able to predict flight characteristics accurately. And safety ib a major consideration, especially in flight testing new VTOL dc-signs. For these reasons a number of new facilities have been devel- oped to support flight test programs, and new flight test techniques have been devised to help explore problem areas. This paper will dis- cuss how these facilities and techniques have been applied. The rela- tive merits and limitations of each facility and technique will be pointed out, and future needs will be discussed. Many phases of flight testing are normally involved in developing a new aircraft; however, this paper will deal with facilities and test techniques for Ptudying only low-speed problems of handling qualities, stability and control, and performance. The material for this paper has been gathered from flight tests made by the NASA, other research organizations, and various aircraft companies. The discussion is divided into two parts, the first dealing with flight test facilities, and the second, flight test techniques. A common problem has been chosen to illustrate the interrelationbhip between facilitiec ) and the relative merits and limitations of each. SOL - 0 a 0 2 . FLIGHT TEST FACILITIES In recent years a number of new facilities have been employed to aid the development of new types of aircraft. Among these are; Piloted ground-based simulators Ground-test rigs Flying-test rigs Variable-stability aircraft In the following discussion, examples are given to show how these facilities are used in studying control power requirements in low-speed flight . Piloted Ground-Based Simulators A number of simulators used in V/STCL research are reviewed in refer- ence 1. The piloted simulator n<'xs become a valuable aid in Flight test programs. For example, in an attempt, to reduce the roll control power in hovering VTO1j aircraft , it wras planned to install a movable vane iii the engine exhaust to vector the thrust, thereby producing sideward translation without banking the aircraft. We needed to know (1) how to control the vane from the cockpit (i.e., by a thumb controller or by the stick deflection), and (2) how much to deflect the vane for satis- f; °y maneuvering. To answer these questions a piloted simulator sti .y was conducted on the NASA-Ames six-degree-of-freedom motion simu- lator (fig. 1). The cab of this simulator is free to travel within a cube approximately 18 feet on a side and with angular motion capability of ±450 about all axes. According to pilots' comments, the overall motion capabilities of the simulator closely corresponded to actual a a a 3 The results of the simulator study in which three methods of control were used are presented in figure 2. Two points can be observed: (1) The vane improved (lower number) pilot rating; (2) programing the vane as a function of bank angle was not as desirable as actuating the vane by a thumb controller on top of the stick. Both bang-bang and proportional controller methods were evaluated for possible flight application. The simulator tests indicated that a lateral acceleration of approximately 0.10 g would be the maximum desired for rapidly positioning a VTOL air- craft. In addition, attitude stabilization in roll was desired to reduce the effects of inadvertent upsets caused either by the pilot or by gusts . Flight tests with the vane on the X--14A VTOL aircraft (fig. 3) have borne out trends shown by the simulator. The flight tests also emphasized the need for attitude stabilization when the vane is used for control. The thumb controller method of regulating the vane, estab- lished by the 60 simulator studies, was adequate and remained essentially unchanged for the flight program. In ,summary, the foregoing is only one of many examples of the vital support of flight ,search provided by simulators. It is expected that the piloted simulator will become more valuable in flight research in the future ^ :htdications are that cab motion contributes significantly to the realism of the simulation, thereby producing more accurate results, which in turn, can simplify research, increase safety, and reduce -;he amount of actual flight time required to prove a new design. Even when sophisticated, multimotion piloted simulators are used, experience has shown the need for flight validation. 17 a 0 Ground-Based Test Rigs Ground-based test rigs have recently become popular with the advent of VTOL aircraft. They are used To provide a functional check of the control systems used in hovering flight including SAS failure and engine out. To study ingestion and recirculation problems due to engine exhaust or lift system. To measure hover performance in and out of ground effect. To check engine operation, calibration of various equipment, and initial pilot familiarization. An example of the use of a ground-based test rig for checking control systems is given for the XV-5A fan-in-wing VTOL aircraft shown mounted on the NASA Ames Research Center adjustable height test rig in figure 4. Before flight the aircraft was tested on the stand at various heights above the ground. These tests revealed that close to the ground, an erratic rolling oscillation was produced by recirculation of the nose fan exhaust. The XV-5A aircraft is controlled laterally by means of louvers at the wing fan exit which close off the fan exhaust differen- tially to produce a rolling moment. Measurements of the available control moment indicated that lateral control would be only marginal. Subsequent checks indicated that the control system mechanism deflected under dynamic pressure from the fan exhaust. Therefore, a more rigid system was installed before flight tests were begun. The flight tests confirmed the unsteady flow behavior in ground effect; but the diffi- culty of operating in this region had been alleviated by making the roll control power adequate and by use of attitude stabilization. 0 8 5 The foregoing was an example of a ground test rig adjustable only in height. More sophisticated ground-based rigs with more motion freedom have been used, particularly In West Germany. The rig shown in figure 5 for testing the VJ--101-X2 is a telescope type with a mounting capable of angular freedom in roll, pitch, and yaw. It is desirable, of course, to mount the pivot as close to the center of gravity of the aircraft as possible to avoid large static moments when the aircraft is tipped. The maximum free movement, which is restricted by cable, is X25 0 in pitch and roll and t8° in yaw. It is possible to "fly" the aircraft on the telescope and this is done subsequent to any changes or maintenance performed on the control system including the SAS (stability augmenta- tion system). By this means it is possible to check such items as the time constants of the control system (thrust modulation is used for pitch and roll), hardover failures in the SAS, the effect of one engine thrust loss, and the difficulty of flying as the system fails progres- sively from an attitude command, to a rate damped, and finally an acceleration system. It is ironic that the VJ-101-XI was not checked on the telescope prior to the loss of that aircraft. The crash resulted from a yaw gyro which was installed with the wrong polarity. The flight was made as a conventional flight with no hovering intended; howErer, the yaw gyro which in conventional flight serves as a yaw damper, caused a divergent Dutch roll oscillation. In summary, ground test rigs have been extremely useful not only for examining potential control problems for hovering flight, but also for the-king the functioning of the entire aircraft in a partial-flight environment. Because of their inherent limited motion capabilities, However, this type of equipment cannot be used for determining the .., CIA . ,.. .. .. t, .. .. _.... _. .-. ^. ^...., _-.,. _ .. .. ... ... ....-..-... _.... .. _ ... ., _. .. .... ._.. .. _. ._._... __._._... to g • 6 I desired values of control power needed for hovering, The ground test rig needs to continue to grow in sophistication as VTOL aircraft become more complex. Flying Test Rigs Airborne test rigs were first used in the 1950 1 x; the Rolls-Royce Flying Bedstead in the U.K. and the Coleopter in France proved the practicability of attitude control for VTOL flight. Recently, the air- borne test rig has been used (1) as a flying simulator in the develop- ment of hover and low-speed handling qualities requirements, and (2) as a test bed for the propulsion and control systems hardware which will ultimately be installed in a specific aircraft.