FLYING QUALITIES OF PILOTED AIRCRAFT - CAFE Foundation

MIL–STD–1797AAPPENDIX Athose of FAR Part 23 for the Aerobatic Category. Amendment 2 deleted all altitude bounds,on the premise that wing loading and drag are set by other considerations, leaving only turnsfor recovery to determine altitude loss, and that these bounds on turns for recovery could notreasonably be reduced further. Amendment 2 also deleted a number of Amendment 1’sspecifics on departure techniques, as well as an Amendment 1 requirement that the start ofrecovery be apparent within 3 seconds or one spin turn. Those specification featuresindicated desirable tests and characteristics, but added considerable detail in areas wheredesign capability is lacking. That material is felt to be more pertinent to a flight demonstrationspecification such as MIL–S–83691.Changes from MIL–F–8785C reflect pilots’ views on spin recovery. The specification of recovery in terms ofaltitude loss, as in Amendment 1 of MIL–F–8785B, based upon what the pilot really is concerned about, wasconsidered. For example, the piloted simulation of AFWAL–TR–80–3141 included an airplane model thatwould not spin, but showed a...low–frequency wallowing that masks departure. At the same time, the wallowing does notgenerate sufficiently rapid motion to excite inertia cross–coupling and PSG. All pilots tendedto continue fighting to maintain control well past full stall, incurring excessive altitude loss.However, if controls were released at any time the aircraft would immediately go into anose–low spiral and recover by itself.The high–α characteristics were otherwise considered quite good, but the excessive loss of altitude wasunacceptable: “pilot commentary indicated the overall departure ratings were heavily influenced by altitudeloss and mission phase.” Specification of altitude loss was, however, deemed impractical.AFWAL–TR–81–3108 also shows preference for an altitude–based metric:Altitude loss per turn can vary drastically with different spin modes (e.g., steep versus flat),and a given vehicle may exhibit more than one spin mode. The allowable altitude loss, whichis highly mission–related (e.g., air–to–ground versus air–to–air), appears to be a moreappropriate recovery metric than turns for recovery.But with rate of descent in a spin roughly proportional to wing loading, W/S, it would seem extremely difficultfor a high–W/S fighter to recover in much less altitude than presently required. Ideally the altitude–lossrequirement would also be a function of altitude above the ground, since a PSG at (say) 80,000 ft would not beas critical as one at 2000 ft above the ground. Although air density variations exert some influence on themotions, such a requirement is not felt to be practical.This requirement will be verified in flight test only for aircraft that must be designed to withstand the forces ofpost–stall gyrations and spins. For other aircraft the requirement is only to determine that post–stall/spincharacteristics are satisfactory, by appropriate wind–tunnel, spin–tunnel or free–flight model testing andanalysis. This should provide some confidence in the pilots’ handbook material and thus help to save theaircraft when they inadvertently get beyond the prescribed flight limits. The requirement then has implicationsfor design of the structure and other subsystems. The procuring activity should weigh the benefits of assuredrecovery against any design penalties, so as not to unduly compromise the aircraft.A recovery technique independent of the direction of motion—releasing or centering the controls, forexample—is very desirable because pilots easily become disoriented in violent post–stall motions. Suchrecovery characteristics, however, may not be achievable without some automation. A “panic button” hasbeen suggested.644