TABLE OF CONTENTS As already noted, the potential for structural coupling exists in any flexible vehicle fitted with an automatic FCS. It is probable that the problem first became well known and analysed in guided-missile design [1,2], although it was certainly anticipated, and has also been experienced, in aircraft control [1-4]. The total loss of guided missiles owing to poorly understood or inadequately analysed structural-coupling interactions is by no means unknown, although none of the obviously structural-coupling-related encounters experienced in manned aircraft has proved to be catastrophic [1,4-6]. The loss of X-15 number 4 and its test pilot Mike Adams in 1967 has been attributed to structural coupling by some sources, but the details generally available [7] are not conclusive.
Notwithstanding the X-15 incident, structural failure in the manner of a flutter encounter is unlikely to occur as a result of structural coupling, since the FCS cannot, in general, input sufficient energy, and since FCS non-linearities would limit the amplitude of excitation through the control surfaces anyway. However, structural coupling has been predicted to have a profound effect on fatigue life [8] even subject to these constraints, and this has been borne out by experience of airframe and actuator fatigue-life usage during typical ground tests. Further concerns are that coupling with flutter modes may occur [1,4] which could lead to structural damage, and that the propagation of high frequency signals through the FCS could seriously degrade actuator performance for rigid-aircraft control of the rigid-aircraft dynamics [9].
Development...
