The flutter properties of a simple aero-isoclinic wing system

Abstract

The work dealt with in this report is of an exploratory nature primarily intended to provide some understanding of the flutter characteristics of a Hill aero-isoclinic wing. Essentially, this is a swept-back wing elastically designed so that the lifting loads do not affect the aerodynamic incidence. The aero-isoclinic property is dependent on the change of incidence due to torsion of the wing being neutralised by that produced by wing bending. The simple aero-isoclinic system used in the tests consisted of a rigid aerodynamic lifting surface having the two essential freedoms, rotation about a swept-back axis, and rotation about a perpendicular axis at the root. Flutter critical speeds and frequencies were measured over a range of the ratio of the bending and torsion frequencies; the results show that the aero-isoclinic condition is not sufficient to prevent flutter, and further that the critical speed for flutter may be low. As usual, forward mass-loading of the wing raises the critical speed. A subsidiary part of the report deals with a method of flutter-speed calculation in which the aerodynamic terms are restricted to static derivatives only. No damping terms whatever are present in the equations of motion. The solution yields a boundary between constant amplitude and growing oscillations, which is regarded as the flutter critical condition. The method gives critical flutter speeds that agree surprisingly well with the experiments.