Stability and Control.-Figure 39, (RA71-3153) Stability and Control, illustrates the three major aspects of the stability and control efforts directed toward general aviation aircraft. Analytical techniques, wind-tunnel tests and flight validation are being used to provide designers with improved capability to specify, predict and achieve better stability and control characteristics. Several analytical procedures for predicting stability and control characteristics currently exist and are used in design. However, significant variations in results occur depending on which technique is applied, and the experience of the designer. The use of full-scale wind-tunnel data will be the basis for modifications to existing techniques and the development of new procedures where appropriate. Flight validation of predictions will be accomplished as required. Use of a variable control system airplane as a test bed for the evaluation of control concepts is providing data that ultimately will permit the specification of improved control system characteristics.

Figure 40, (RA71–3154) General Aviation Yaw Damper Flight Evaluation, illustrates an additional aspect of the work in the stability and control area. A comprehensive flight evaluation has been completed on the effect of incorporating a yaw damper in general aviation aircraft. While general improvements were noted in all areas, significant improvements were seen in the four areas listed on this figure. Representative of the improvements, as illustrated, was the effect on aircraft controllability during a simulated engine failure in a light twin-engine airplane. In this example, recovery was delayed as long as possible after the simulated failure. As shown, the time available to the pilot to effect a safe recovery when a yaw damper is engaged is nearly twice that of the basic aircraft.

Simplified Control.-A major step toward increasing both the safety and utility of general aviation aircraft would be a reduction in the reliance on highly developed piloting skills and judgments for operation, and the attendant requirement for constant practice and proficiency. Simplification of the control task and the display of proper information is a first step toward this objective.

The PA-30 variable control system airplane is being used to examine various levels of vehicle response to pilot control input. Acceleration and rate command systems have been examined, and the aircraft is now being modified to permit evaluation of an attitude command control system. Further extensions will permit direct pilot command of the airplane flight path.

Two specific flight investigations are directed at the evaluation of devices to permit the generation of lift and drag forces without the conventional rotation of the vehicle. The first is the use of spoilers for lift and drag control and the second is the planned modification of a U-3A airplane to incorporate slotted flap-spoilers for direct lift control.

Figure 41 (RA71-15497), Simplified Flight Path Control, illustrates a flight program in which a typical four-place single-engine airplane has been modified to include aerodynamic spoilers similar to those used on sailplanes. The spoilers are installed as shown, opening on both top and bottom of the wing. For the experimental installation, the spoiler plates were divided into segments, inboard and outboard, top and bottom, to facilitate changes in geometry for the test program, As shown in the upper right of the chart, there is a marked improvement in rate of descent with spoilers throughout the airspeed range compared with the use of conventional flaps. Not shown on this plot but of importance is the lack of significant trim change with spoiler deflection and the extremely rapid response in regaining lift upon spoiler retraction.

Connecting the spoilers to the throttle would further simplify control of the aircraft and this is under study and initial results are shown at the lower right of the chart. Throttle travel was extended beyond the normal point of engine idle. The interconnect point was established through flight test, and as shown starts spoiler deflection just below the power level required for level flight. Results obtained using this single lever control of both engine and spoiler have shown an increase in touchdown precision, better ground handling during roll out, and easier control with greater confidence in the final approach.

Cockpit Protection. Much effort is being expended by the industry to fabricate cockpit structures and restraint systems that will increase the chances of survival in the event of a crash. As these results are incorporated into new aircraft, the range of survivable crashes with severe enough structural damage to post a post-crash fire threat will increase. A measure of cockpit fire protection can be achieved through the use of fire resistant foam as insulation. Properly tailored, this material also has the potential for reducing the cockpit noise levels as well as providing structural stiffening and impact attenuation.,

Figure 42 (RA71-15494), Cockpit Protection, illustrates current efforts being initiated to evaluate one specific material, polyisocyanurate foam in a real application to small aircraft. The basic fire protection ability of this material is illustrated in Figure 43 (RA71-3155), Foam Fire Protection Test. These data were obtained from a special test conducted as part of an overall foam application effort. An instrumented section of an aircraft fuselage was subjected to a severe fire. The inside temperatures for both the protected and unprotected halves are shown versus time from ignition of the fire. Projected survival time before temperatures reach 250° F is increased from approximately 11⁄2 minutes to nearly nine minutes.

A less catastrophic detriment to the cockpit environment is the high noise level as shown in Figure 44 (RA71-3156), Measured Cockpit Noise Level. As indicated, continued exposure without protection creates a high risk of permanent hearing damage. Laboratory tests are being conducted to determine the proper application of the foam material to obtain maximum reduction in sound levels. Results from these experiments and the fire protection tests will be used to define the most efficient use of this material. The results of these studies will be verified subsequently in flight.