Criteria for the prevention of flutter of tab systems

Abstract

An investigation has been made into the flutter characteristics of an idealised tab system in which the three degrees of freedom normal translation of the lifting surface, rotation of the control surface, and rotation of the tab are represented. Specific cases of this idealised system represent similar idealised forms of the standard trimming, spring, servo, and geared tab systems. From a consideration of tile relationships existing between the systems, criteria for flutter prevention have been developed from the criteria evolved earlier for trimming tabs. As initially derived, the criteria are applicable to the stick-fixed condition (in the case of spring and servo-tabs), to the case with no aerodynamic balance on either control surface or tab, and to tile case where the control surface is statically balanced about its hinge. Comparison is made between the criteria for spring-tabs and the existing Collar-Sharpe criteria. Design implications are deduced from the criteria for spring-tabs, and the general application of the criteria to actual systems is considered in some detail. A comprehensive survey of the results is given in section 11. Points of major importance are as follows:-- (a) The criteria are liberally provided with generalised constants whose values can if necessary be adjusted in the light of practical experience. (b) The backward limit set to the tab centre of gravity will normally be less severe than in the case of the Collar- Sharpe criterion. (c) Satisfying the criteria of this report is likely to be most difficult with elevators carrying a tab on one side only, and from a flutter point of view such systems should be avoided if possible.