Aircraft agility

Abstract

A definition of an aircraft agility vector is given as the time rate-of-change of the applied forces acting on an aircraft and agility is characterized as being representable by instantaneous and integral time-scales. A unified framework for evaluating instantaneous and integral agility is developed based on the notion of a new dynamic model for aircraft motions. This model may be viewed as intermediate between a point-mass model, in which the body attitude angles are control-like, and a rigid-body model, in which the body attitude angles evolve according to Newton's Laws. Specifically, we consider the case of symmetric flight and construct a model in which the body roll-rate and pitch-rate are the controls. Accordingly, we refer to this new dynamics model as the body-rate model, (BRM).

Instantaneous agility is presented as the locus of achievable agility vectors and the construction of such agility sets is demonstrated from aerodynamic and propulsive data for a modern jet fighter. Figures depicting this locus are displayed with indications of the limiting control. An integral performance flight problem is presented and subsequently solved via the optimal control theory. Agility metrics are suggested for this problem based on the transients which exist between the dynamics of the BRM and those of the point-mass model. Suggestions are also provided on the use of instantaneous agility sets and integral agility metrics in the design of aircraft and in performance comparisons of competing aircraft.