Effects of Flexibility on the Stability of Flying Aircraft

Traditionally, flying aircraft have been treated within the confines of flight dynamics, which is concerned for the most part with rigid aircraft. On the other hand, flexible aircraft fall in the domain of aeroelasticity. Although some attempts have been made to include aircraft rigid body degrees of freedom, aeroelasticity is concerned mostly with the vibration and flutter instability of wings fixed at the root. In reality all aircraft possess some measure of flexibility and carry out rigid body maneuvers, so that the question arises as to whether flexibility can have adverse effects on the stability of flying aircraft. This paper addresses the question of flexibility effects on the stability of flying aircraft by solving the eigenvalue problem associated with the following three cases: 1) the flight dynamics of a flexible aircraft regarded as rigid and whose perturbations about the flight path are controlled by feedback control, 2) the aeroelasticity of a corresponding flexible aircraft prevented from undergoing rigid body translations and rotations and 3) the control of the actual flexible aircraft using the same control gains as in the first case. This investigation demonstrates that it is not always safe to treat separately rigid body effects and flexibility effects in a flying flexible aircraft. Indeed, it is shown that on the one hand the flight of a flexible aircraft treated as rigid can be stabilized by means of feedback controls and on the other hand that the motion of the same flexible aircraft prevented from undergoing rigid body motions tends to consist of damped oscillation. But, when the flying flexible aircraft is treated as it actually is, namely, a single system, the elastic vibration and the perturbations from the flight path can be rendered unstable by feedback controls designed by ignoring the elastic variables.