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Interest in oscillation susceptibility of an aircraft was generated by crashes of high performance fighter airplanes such as the YF-22A and B-2, due to oscillations that were not predicted during the aircraft development. Flying qualities and oscillation prediction, based on linear analysis, cannot predict the presence or the absence of oscillations, because of the large variety of nonlinear interactions that have been identified as factors contributing to oscillations. Pilot induced oscillations have been analyzed extensively in many papers by numerical means. Interest in oscillation susceptibility analysis of an unmanned aircraft, whose flight control system fails, was generated by the need to elaborate an alternative automatic flight control system for the Automatic Landing Flight Experiment (ALFLEX) reentry vehicle for the case when the existing automatic flight control system of the vehicle fails. The purpose of this chapter is the analysis of the oscillation susceptibility of an unmaned aircraft whose automatic flight control system fails. The analysis is focused on the research of oscillatory movement around the center of mass in a longitudinal flight with constant forward velocity (mainly in the final approach and landing phase). The analysis is made in a mathematical model defined by a system of three nonlinear ordinary differential equations, which govern the aircraft movement around its center of mass, in such a flight. This model is deduced in the second paragraph, starting with the set of 9 nonlinear ordinary differential equations governing the movement of the aircraft around its center of mass.In the third paragraph it is shown that in a longitudinal flight with constant forward velocity, if the elevator deflection outruns some limits, oscillatory movement appears. This is proved by means of coincidence degree theory and Mawhin's continuation theorem. As far as we know, this result was proved and published very recently by the authors of this chapter (research supported by CNCSIS-–UEFISCSU, project number PNII – IDEI 354 No. 7/2007) and never been included in a book concerning the topic of flight control.The fourth paragraph of this chapter presents mainly numerical results. These results concern an Aero Data Model in Research Environment (ADMIRE) and consists in: the identification of the range of the elevator deflection for which steady state exists; the computation of the manifold of steady states; the identification of stable and unstable steady states; the simulation of successful and unsuccessful maneuvers; simulation of oscillatory movements.

Oscillation Susceptibility of an Unmanned Aircraft whose Automatic Flight Control System Fails