ON THE ROLL-COUPLING INSTABILITIES OF HIGH-PERFORMANCE AIRCRAFT

High-performance aircraft configurations, characterized by a small spa n and swept wings, have rolling moments of inertia that are significan tly smaller than the pitching or yawing moments of inertia. As a resul t., nonlinear coupling during high-roll-rate manoeuvres produces signi ficant yawing and pitching moments. For certain critical flight condit ions, inertial coupling causes jump phenomena called roll-coupling ins tabilities. These jump phenomena typically occur as a result of turnin g-point bifurcations of the aircraft steady states. Analysis of the mo ment balances along the steady solution branches provides physical ins ight into the causes of these instabilities and potential means of eli minating them. Analysis performed by using the full eight-degree-of-fr eedom equations of motion shows that the critical control-surface defl ections are essentially the same as for the fifth- and sixth-order equ ations of motion. Solving the full eight-degree-of-freedom equations a llows one to determine the orientation of the aircraft before and afte r the instability. For the aircraft model studied here, roll-coupling instabilities result in a change in sign of the angle of attack of the aircraft. The equilibrium state of the aircraft changes from a spiral dive, with the bottom of the aircraft closest to the axis of the spir al, to a spiral dive where the top of the aircraft is nearest the axis of the spiral, or vice versa depending on the trim angle of attack fr om which the manoeuvre was initiated. Pitching moment balance is shown to be central to the instability.