Abstract
Based on bifurcation theory, nonlinear frequency response analysis is a recent development in the field of flight dynamics studies. Here, we consider how this method can be used to inform us on how to devise the control input such that the system transitions from an undesirable equilibrium solution—an aircraft deep stall solution in our case—to a desirable solution. We show that it is still possible to induce a large-amplitude oscillation via harmonic forcing of the pitch control device and escape the otherwise unrecoverable deep stall, despite very little control power available in such a high angle-of-attack flight condition. The forcing frequencies that excite these resonances are reflected as asymptotically unstable solutions using bifurcation analysis and Floquet theory. Due to the softening behaviour observed in the frequency response, these unstable (divergent) solutions have slightly lower frequencies than the value predicted using linear analysis. Subharmonic resonances are also detected, which are reflected in the time-domain unforced responses. These nonlinear phenomena show strong dependency on the forcing/perturbation amplitude and result in complex dynamics that can impede recovery if the existing procedures are followed. The proposed method is shown to be a useful tool for nonlinear flight dynamics analysis as well as to complement the rather thin literature on deep stall analysis—a topic of relevance for recent research on unconventional landing techniques in unmanned aerial vehicles. A full description of the aircraft model used, the unstable F-16 fighter jet, is provided in the appendix.
Highlights
Aircraft can potentially encounter flight conditions in which rich and nonlinear dynamics cause sudden and dangerous responses, especially in high angles-ofattack or sideslip conditions
The method opens up another dimension of nonlinear analysis that can be of great interests for researchers in flight dynamics and control analysis
We have shown show how bifurcation theory and nonlinear frequency analysis can facilitate a systematic study on identifying possible deep stall escape manoeuvres
Summary
Aircraft can potentially encounter flight conditions in which rich and nonlinear dynamics cause sudden and dangerous responses, especially in high angles-ofattack or sideslip conditions. Nonlinear frequency response is employed to devise a recovery manoeuvre from deep stall—a dangerous condition in which the aircraft is locked into a high angle-of-attack attitude that results in a steep descending trajectory. Recent research into advanced landing techniques for small unmanned aerial vehicles involves deliberately bringing the aircraft into a deep stall to minimise the landing distance [17,18,19] These developments emphasise the need to improve our understanding of the flight characteristics in the deep stall regime. All bifurcation analysis was carried out using the Dynamical Systems Toolbox [25], which is a MATLAB/Simulink implementation of the numerical continuation software AUTO [26]
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