Aircraft departure resistance prediction using structured singular values

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Virginia Tech

Research has been conducted in recent years to determine the dynamic behavior of aircraft in unusual flight attitudes, particularly at very high angle-of-attack or post-stall conditions. The possibility that future advanced fighter aircraft will have the ability to perform controlled maneuvers at such attitudes is indicated by the current military aircraft flying qualities specification, MIL-STD-1797. As it becomes more important to understand the dynamics of aircraft at such flight conditions, the need for a meaningful and useful assessment of aircraft departure resistance in varying attitudes will increase proportionally. This thesis surveys some of the measures of departure susceptibility currently in use and examines a candidate for a new departure resistance criterion which offers distinct advantages over the traditional metrics.

The new departure resistance criterion, called DPSSV is essentially a measure of how much uncertainty the nominally stable plant can tolerate before being driven unstable. DPSSV is calculated using structured singular values. In this thesis, DPSSV is calculated over various flight conditions for a typical high-performance fighter aircraft which is represented by a full six degree of freedom, nonlinear simulation. The results are compared with those obtained by using a traditional departure susceptibility metric and by examining the eigenvalues of linearized forms of the aircraft model.

The new criterion DPSSV is shown to provide more information about the departure susceptibility of an aircraft than CηβDYN traditional metric, and to produce results in good agreement with the eigenvalue analysis of the stability of the aircraft for the conditions studied. The interpretation of DPSSV is discussed, and suggestions for future investigation are also presented.