Elevator Flutter Involving Two Tabs

An account is given of a theoretical flutter investigation in connection with an accident to a Sea Venom aircraft. The investigation covers both symmetric and antisymmetric flutter of the tailplane-elevator-tab system. The main (symmetric) calculations include six degrees of freedom, comprising three structural modes and movements of elevator, spring-tab and trim-tab respectively. The parameters that are varied include elevator mass-balance, position and magnitude of mass-balance on each tab independently, structural damping and stiffness of the tab circuits, chord and mass of the trim-tab, and flexibility of the elevator mass-balance attachments. The investigation shows the symmetric flutter to be a probable cause of the accident and confirms the efficacy of the remedial measures adopted, which consisted of modifications to the tab mass-balances combined with a reduction in chord and inertia of the trim-tab. Several points of general interest are discussed. 1. Introduction.-In the summer of 1952 an accident to a Sea Venom occurred at the start of a high-speed run. It was suggested that flutter might have been the cause, and a theoretical investigation, in which the R.A.E. Flutter Simulator 1 played an important part, was made of this possibility. The present paper describes the investigation. It is concluded that flutter is a likely cause of the accident, and Suitable modifications to the aircraft are discussed. The aircraft differs from the Venom Mark 1, on which considerable flight experience has been gained, in having folding wings and an extended nose (being a two seater). The extended nose produced a relatively forward centre of gravity and therefore a more stable aircraft than the Venom 1, and this in turn required the use of more powerful tabs on the elevator. The present investigation shows that these larger tabs are probably the main cause of the accident, a conclusion which is strongly supported by the fact that the trim-tab and part of the spring-tab broke away from the aircraft in flight. The calculations show that a violent form of symmetric flutter involving both tabs could occur at medium to high speeds. The calculated frequency is about 24 c.p.s., so that very high local stresses would be induced. It is logical to suppose that this type Of flutter caused one or both elevator mass-balance weights to break away (neither was recovered), as well as the failure of the trim-tab itself. With the elevator unbalanced flutter would certainly persist, and in fact part of the spring-tab …