The Effect of Tab Mass-balance on Flutter Part I Ternary Tailplane-Elevator-Tab Flutter Part II Experiments on the Influence of Tab Mass-balance on Flutter

Abstract

Part I. Following an accident to a Mosquito fitted experimentally by the Royal Aircraft Establishment with a g-restriction device involving heavy mass-overbalance of an elevator tab, an investigation has been made into the flutter characteristics of tailplanes carrying elevators and tabs. The degrees of freedom considered were vertical bending of the fuselage, elevator rotation and tab rotation. The tab was assumed to be spring-connected to the elevator, while the elevator was taken to be free. The effect of variation of the stiffness ratio, of the states of mass balance of the tab and elevator, and of horn balance of the elevator, was investigated. It was found that, with a statically balanced elevator and a statically overbalanced tab, ternary flutter could occur at low speeds while all binary motion involving two only of the degrees of freedom was stable at all speeds, Such flutter could be eliminated by a mass overbalance of the elevator. It is thought that similar results would apply to spring-tab systems, but this is to be investigated. It is considered that flutter of this nature was a likely cause of the Mosquito accident, and it is recommended that in no circumstances should tabs be overbalanced unless a detailed investigation involving at least three degrees of freedom has shown the system to be flutter free. Part II. Tests made to investigate the effect of tab mass-balance on wing-flexure-aileron-tab flutter show that the ternary flutter may arise from over mass-balance of the tab although the binary types of flutter are stable. This conclusion is in agreement with that reached theoretically in Part I for flutter involving tabs and elevator with fuselage vertical bending.