FLYING QUALITIES OF PILOTED AIRCRAFT - CAFE Foundation

MIL–STD–1797AAPPENDIX AFIGURE 173. C–5A flight test data (from AFFDL–TR–75–3).The roll axisOf major importance in designing and producing an acceptable aircraft is the definition of the roll axis used incompliance. The roll axis is not specified exactly in these requirements. Its desired orientation varies with thepilot’s intent: turns (or straightening out) to modify the flight path, barrel rolls to slow down, aileron rolls to startsplit Ss, .... The most frequent, usually most important, use is the first–named, for turn entry or exit. Withrespect to the direction of flight, a roll axis tilted up corresponds to adverse yaw (nose lagging the turn entry) instability axes; while a nose–down tilt indicates proverse yaw. At high angle of attack, rotation about any axisother than the flight path will generate sideslip, thus exciting dutch roll motion or even departure fromcontrolled flight. Other studies have shown that a major contributor to departure is the p term in theside–force equation:Σ Y = mV ( β+ r – p )αand pαo is, of course, zero in stability axes.However, compared to conventional body axes the cockpit is higher above a flight–path–aligned roll axis athigh angles of attack. The result is spurious responses to roll control inputs: lateral acceleration as in theC–5A, F–15, etc.; visual slewing, e.g. of a runway threshold, found troublesome for the YF–16. These effectsinvolve the kinematic relationships:442