Active Flutter Suppression by Feedback Compensation of Transport Logs

Abstract

A IRCRAFT wings are deigned to be flexible, and as a consequence of this feature they are known to be continuously oscillating under the action of wind loads leading to the well-known phenomenon of “flutter.” Flutter in wings was conventionally inhibited by passive design techniques based on altering the mass and stiffness distributions of the wing. However, the recent trend in building highly flexible aircraft has demonstrated the inadequacy of passive techniques for inhibiting the occurrence of flutter. To overcome the inadequacy of passive techniques in inhibiting the occurrence of flutter within the design envelope of the aircraft, a new technique, based on feedback control, was developed in the early 1970s, called “active flutter suppression.” Two distinct approaches emerged for the design of the control law for activeflutter suppression, the first based on optimal control, and the second approach [1], emerging from the concept of passivity, considers the energy flowing in and out of the “system.” There are several applications of this method to real aircraft, and [2] is archetypal of these examples. References [3,4] are typical examples in which the techniques based on the modeling methods were applied to the synthesis of optimal and near-optimal control laws for active flutter suppression. In this paper we propose a third approach towards controlling flutter. By careful consideration of the unsteady aerodynamic loading, those components that contribute to the delay in the growth of the loading in response to changes in the wingmotion are isolated. These delays or transport lags are then eliminated by an appropriate control law that ensures the constancy of circulatory component of the unsteady lift. With secondary compensation one could increase the flutter speed, which is the primary objective of active flutter suppression. Furthermore, the constancy of the circulatory component implies that the flow is less likely to separate and the circulation is more likely to remain steady. To evaluate the method we consider the archetypal problem of the typical section and implement such a control law. Themotivation for this work arose from the need to develop smart, flexibly actuated morphing control surfaces for future aircraft wings in low-speed flight. The method described in this paper can be routinely implemented with control surfaces. Morphing control surfaces can be deflected in a multitude of modes and the method itself is based on observations of bird flight.