Longitudinal Control for Recovery
Tactical airplanes are able to reach supermaneuvering angles of attack by low or even negative static longitudinal stability. Full nose-up control starts the pitchup; unstable or nose-up pitching moment keeps it going. Recovery requires a nose-down pitching moment that will overcome the unstable pitching moment and leave a margin for nose-downward angular acceleration.
A rule of thumb for recovery nose-down pitching moment has been proposed, based on simulation studies and practical fighter design (Mangold, 1991). A pitching acceleration of 0.3 radians per second squared is said to be adequate. This leaves a margin for inertial coupling due to rolling during the pitching maneuver. A related problem is the amount of longitudinal control power required for very unstable airplanes, not necessarily during supermaneuvers. For that problem, Mangold correlates required pitching acceleration control with time to double amplitude.
The recovery control problem also has been attacked using the classical Gilruth approach (Nguyen and Foster, 1990). Satisfactory and unsatisfactory recovery flight characteristics are used to draw a criterion line in a plot of minimum available pitching moment coefficients with full-down control versus a moment of inertia and airplane size parameter. With only five flight data points, Nguyen and Foster call their criterion preliminary.
10.5 Concluding Remarks
Current tactical airplane maneuverability research spans all aspects of the stability and control field, from linearized transfer functions to unsteady aerodynamics and the complex, vortex-imbedded flows found at very high angles of attack. Further advances and new theories appear likely with the advent of thrust-vectoring and direct side and normal force control.