Some Experimental Results on Wing Flutter

Abstract

The present paper represents a progress report of some experiments being carried out in the aerodynamics laboratory of Harvard University, The first part of the experimental investigation consisted in the measurement of the vibration response curves of a wing in the wind tunnel. A wing, free to vibrate in bending and in torsion, was subjected to a forced vibration at various air speeds up to the flutter speed. These measurements are of interest in that they show how the two natural frequencies of the wing come together as the flutter velocity is approached. The peaks of these response curves are also of interest in that they determine the maximum stresses which are experienced, for instance, by a propeller which is subjected to periodic impulses from the engine. The second part of the investigation deals with the problem of flutter above the stall. Measurements show that at angles of attack above the positive stalling angle and below the negative stalling angle the flutter speed is decreased to less than one-third of the flutter speed of the unstalled wing. This flutter seems to be associated with an instability which arises when the slope of the lift curve becomes negative. Experiments on a metal wing supported as a cantilever indicate that the flutter at the stall may also be associated with an excitation from a system of Karman vortices.