David W. Levy The University of Michigan Department of Aerospace Engineering Ann Arbor, MI to copy or republish, c and Astrona~lcs esi sin leron Controls David W. Levy * The University of Michigan Department of Aerospace Engineering Abstract Gyro axis angular rate Laplace transform variable The use of a control tab in a simple autopilot is dis- Aileron surface area cussed. The system is different from conventional in- Rate gyro tilt angle stallations in that the autopilot does not move the Aileron deflection main control surface directly with a servo actuator. Tab deflection A servo tab is used to provide the necessary hinge Feedback error signal moment. A much smaller control actuator may then Laplace transform operator be used. A further benefit of this approach is that Control system natural frequency the system may be operated full-time with only mi- nor control force feedback to the pilot. For the case of Acronyms: the wing leveler system, the result is a full time sta- bility augmentation system in the lateral axis. With IFR Instrument Flight Rules improved stability, a large number of accidents due IMC Instrument Meteorological Conditions to loss of control could be prevented. Pilot workload SSSA Separate Surface Stability Augmentation is also reduced. The failure modes of such a system VFR Visual Flight Rules are benign, eliminating the need for redundancy and VMC Visual Meteorological Conditions the associated costs. The system is shown to be sta- ble and effective using either angular rate or attitude feedback. For the case of the light, four seat airplane Introduction studied, the basic wing leveler would weigh less than nine pounds and would cost no more than a compara- The vast majority of airplanes in service today have ble conventional autopilot. Potential applications to near neutral stability or are mildly unstable in the other autopilot modes and to decoupled flight control spiral mode of dynamic lateral/directional motion. systems are also discussed. A divergent spiral is characterized by a gradually in- creasing bank angle resulting in a turn of increasing load factor. At the same time, the airplane begins a Nomenclature descent and the airspeed will increase. This combi- nation is often termed a ‘graveyard spiral’ for, if left C Wing chord length uncorrected, will result in loads beyond the airplane Ca Average aileron chord length design limits and/or collision with the ground. This ch Hinge moment coefficient instability is easily controlled by the pilot. In oper- HM Hinge moment ations conducted under Visual Flight Rules (VFR), IC Moment of inertia the pilot maintains a wings-level attitude by reference P Roll rate to the horizon. Under Instrument Flight Rules (IFR), Q Dynamic pressure the pilot uses an artificial horizon, among other in- r Yaw rate struments, to maintain level flight. A significant number of accidents every year are *Lecturer; Senior Member, AIAA. Copyright 01992 by the American Institute of Aeronautics attributed to in flight loss of control by pilots. Ac- and Astronautics, Inc. All rights reserved. cident data from the National Transportation Safety 1 All Accidents Fatal Accidents All Accidents Fatal Accidents Year No. % No. % Year No. % No. % 1978-81" 5 17 13.3 233"" 34.7 1980 140 3.9 102 16.4 1982 311 9.5 123 20.3 1981 167 4.8 114 17.4 1983 40 1 12.9 147** 26.2 1982 127 3.9 98 16.6 1984 383 12.6 132 23.7 1983 116 3.8 91 16.4 1985 367 13.2 115 22.6 1984 96 3.2 74 13.6 1986 350 13.4 120** 24.6 1985 93 3.4 69 13.9 1987 326** 13.1 113" 25.7 1986 68 2.6 52 11.2 1988 346** 14.5 135" 29.7 1987 74 3.0 NIA N/A Average: 13.2 29.5 1988 71 3.0 N/A N'/A Average: 3.6 15.3 * Data for 1978-81 are yearly average values. ** Inflight loss of control was the most common Table 2: Accidents Where 'VFR Flight into IMC' is cause. Listed as a Cause or Factor, 1980-1988, A41 Aircraft, All Operations. Table 1: Accidents Where 'In Flight Loss of Control' is Listed as the First Occurrence, 1978-1988, All Air- craft, All Operations. all too often such operations exceed the pilot's ca- pabilities. As with the statistics for loss of control accidents, not all of the VFR into IMC cases would Board for the calendar years 1978-88 [l] are shown have been due to the pilot simply losing control of for all aircraft and all types of operations in Table 1. the aircraft. Some of these accidents may have been For this period; single reciprocating fixed wing air- caused by weather conditions which posed a direct craft accidents accounted for 80% of the total of all hazard to the aircraft, such as severe turbulence or ic- accidents. These data are taken from tables of the ing. The VFR pilot is less likely than the IFR pilot to 'first occurrence' which led to the accident. It is not be knowledgable about weather conditions conducive clear exactly how many total accidents were due to to such hazards. the characteristics of spiral divergence. Certainly not Loss of control by IFR pilots also occurs occasion- all accidents in the categories shown are of this type. ally, due to a number of different causes. In light air- For example, mechanical control system malfunctions craft operations, there is usually only one pilot and would be included here. An unstable spiral, however, the workload during IMC can be very high. could easily have contributed to accidents listed in Automatic pilots are often used by both IFR and other categories. The amount of concentration re- VFR pilots to reduce workload. In extreme cases they quired to simply 'fly the airplane' will certainly de- have been used by VFR pilots in IMC to maintain tract from a pilot's ability to deal with any type of control of the airplane. The simplest type of autopilot emergency. Notice that, in several years, in flight loss is often called a wing leveler, which commands zero of control was the most prevalent first occurrence. In bank angle by sensing either the bank angle or the every year, this cause was at least the second most yaw rate. Many general aviation airplanes, however, common cause of fatal accidents. On the average, in are not equipped with even simple autopilots. flight loss of control contributed to 13.2% of all ac- Conventional autopilots use servo-actuators to cidents and 29.5% of fatal accidents. This type of drive the entire control system in the axis desired. accident is much more likely to be fatal than most For example, a wing leveler uses the aileron controls other types, such as engine failure. to correct excursions in bank angle. The servo must There is a surprising number of pilots without an be sized not only to overcome the forces due to aero- instrument rating who either deliberately or inad- dynamic hinge moments, but forces due to friction, vertently enter Instrument Meteorological Conditions stiction, and inertia. (IMC). Accidents where 'Continued VFR into IMC' Another characteristic of conventional autopilots is listed as a cause or factor are listed in Table 2 for is that pilot-in-the-loop operations are not possible. the years 1980-88. The average for this period was In other words, either the pilot is in control, or the 3.6% for all accidents and 15.3% for fatal accidents autopilot is in control, but never both at the same (1980-86). Although all VFR pilots receive specific time. This is a consequence of the design of the pri- training in instrument flying, these data show that mary flight control system. General aviation aircraft 2 almost always use reversible controls, which use C& A different approach to the general problem of blea or pushrods to drive the aerodynamic surfaces. automatic flight control is Separate Surface Stabil- Conventional autopilot servo8 are directly connected ity Augmentation (SSSA) due to Roskam. This ap- to the control cables/pushrods. If the autopilot is proach has been demonstrated for both a simple wing engaged, then the pilot must physically override the leveler system [3] and as a full stability augmentation servo to make an input. Note that the autopilot must system [4]. In principle, the wing leveler system can be designed so that the pilot has this capability, in be used for the purpose as intended here: a full time case of a malfunction. system that stabilizes the spiral mode, without any An alternative to the conventional installation is to feedback to the pilot. The implementation as a sep- use a control tab to drive the primary flight control. arate surface requires one aileron to be cut into two For example, a wing leveler system would use a tab pieces, and the extra section of aileron is used only to drive the aileron controls. The principle is exactly for autopilot functions. the same as a servo tab, which has often been used on Although not an automatic stabilization device, an large aircraft with cable driven controls (most mod- electrically driven trim tab has been developed for ern large aircraft use powered controls now). For an light airplanes [5]. It is manually controlled from the autopilot installation, a small actuator is connected cockpit and is used for trim in the roll axis. The directly to the tab, and does not interfere with the system has been certified by the Federal Aviation primary aileron control system. Such a system is Administration for installation on a broad range of lightweight, inexpensive, and can be easily retrofitted aircraft from light singles up through twin-engine air- to existing light airplanes.