FLYING QUALITIES OF PILOTED AIRCRAFT - CAFE Foundation

MIL–STD–1797AAPPENDIX Amight influence pilots’ perceptions. Even for Class III aircraft, which will have no spin flight tests,stall/post–stall wind–tunnel tests and analysis are in order.VERIFICATION GUIDANCEWhere allowed by the structural specification (for example MIL–A–8861), insert “flight test” in the blank.Stall angle of attack (or C Lmax ) is dictated by performance requirements. However, experience with theF–5 series aircraft and the F–15 leads to the conclusion that a sharp increase in longitudinal stability, startingslightly below stall angle of attack, allows the pilot full use of the transient pitch performance for air combatmaneuvers. This aerodynamic characteristic limits angle–of–attack overshoots during abrupt pullup androlling maneuvers. It also provides rapid recovery at low dynamic pressure with neutral pitch control. Thoughyaw departures occur in a limited portion of the flight regime, the F–15 does not continue into a spin but pitchesdown due to its inherent longitudinal stability at high angle of attack.A configuration that is longitudinally unstable at or above stall is undesirable for Class IV aircraft.Angle–of–attack limiters are usually implemented in this case; limiters can be defeated, however, duringlow–speed maneuvers such as zoom climbs and high–angle–of–attack rolls. To preclude angle–of–attackovershoots as the limit is approached, a rate anticipation system is usually incorporated into the flight controlsystem. This feature reduces the transient maneuvering performance of the vehicle. Pitching moment curveswith a strong unstable break are poor for Class IV applications. When departure occurs, it is violent and canpreclude safe ejection of the crew. Even if there is a large amount of nose down control power, the low dynamicpressure encountered as the aircraft pitches up results in slow nose down recovery.Wind tunnel data that present rolling moment (C ) and yawing moment (Cn) as a function of angle of attackfor zero sideslip should be evaluated to ensure that no excessively large moment values occur (e.g., fromasymmetric vortex shedding from the nose) that could cause departures. The aerodynamic effect of the flighttest boom, if located on the nose, should be determined.Over the last decade an open–loop “Directional Divergence Parameter”, Cn β dyn, has been extensivelyutilized. It has been partially successful in predicting departure susceptibility for several Class IV tactical jetaircraft, including the F–5E, A–10, F–15, YF–17 and F–18. Cn β dyn is defined as follows, in terms ofprincipal–axis stability derivatives:C n β , dynC nβcos α z x Cβsin αFigure 258, from Titiriga et al. in AGARD–CP–199, gives a Northrop criterion based on this parameter. Figure259, from ASD–TR–72–48, presents Weissman’s departure boundaries in terms of this and the LCDPalready presented.637