Nonlinear Stabilizing Control of High Angle of Attack Flight Dynamics.
Abed, Eyad H.
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This paper discusses a new approach to the feedback control of aircraft at high angles of attack. This approach is based on recent results on control of nonlinear systems at bifurcation points [3, 4, 5]. The lateral dynamics of a slender aircraft studied by Ross  provides-a convenient model for illustrating this application. It is shown how local bifurcation control can be used in the stabilization of the so-called "wing rock" phenomenon. The appropriateness of this approach derives from the fact that unstable motions at high incidence, such as stall, spin entry, wing rock and roll coupling, have been attributed to bifurcations in the vehicle dynamic equations. The instability can indeed be predicted via bifurcation analysis, and depends critically on the nonlinear aerodynamic force modeling. Bifurcation control results can facilitate the synthesis of nonlinear stabilizing feedback controls for aircraft at the onset of vortex separation; an angle of attack limiter is not employed in this approach. Possible means of achieving the required control laws include the control surface deflections and thrust vector control. The bifurcation control technique considered here, developed in [3, 4], provides an analyticallybased, algorithmic approach to the stabilization of vehicles at the onset of flow separation. In classical terminology, the goal of the nonlinear control design is to render the stability boundary at high incidence "safe" (as opposed to "dangerous"). Practically the result would be to enlarge the flight envelope, by ensuring that any near-stall sustained deviation from trim is stable and of small amplitude, and therefore tolerable. Divergence cannot result from such a state without the introduction of a large disturbance. Thus, pre- and post-stall dynamics are stabilized. The remainder of this paper is organized as follows. Section 2 presents a brief discussion of relevant background material on high angle of attack dynamic modeling and associated nonlinear effects. In Section 3, the bifurcation control framework of [3, 4] is reviewed. In Section 4, application of the bifurcation control technique to a model for the high incidence dynamics of the Handley Page 1-15 research aircraft model of Ross  is presented.