The Gyroscopic Effect of a Rigid Rotating Propeller on Engine and Wing Vibration Modes

Abstract

In many wing vibration analyses it is found necessary to take into account the effect of flexibly mounted engines. Hence, it is reasonable to ask what vibratory gyroscopic effect this flexibility may give rise to when propellers are whirling. An engine mount may be thought of as a horizontal beam cantilevered from the wing, having both horizontal and vertical flexibility. If this beam were infinitely rigid horizontally, then, when it vibrated, the gyroscopic moments induced in the propeller due to the resultant pitching motion of its axis would not produce propeller axis yaw. However, engine-mount lateral stiffness (which allows propeller axis yaw) may actually be less than vertical stiffness, so that gyroscopic effects will play a role as the propeller axis undergoes pitching vibrations at the tip of the cantilever engine mount. The purpose of this paper is to investigate this role under the assumption that the propeller itself is a rigid disc. The paper is divided into four parts. Part (I) deals briefly with classical gyroscope theory. Part (II) presents engine vibration mode studies—experimental photographic techniques on a model gyroscope mounted at the ends of two different cantilever beams. Part (III) presents the theory of the coupled motion of an elastic wing upon which a gyroscope is mounted to simulate an engine-propeller system on an airplane. Part (IV) consists of an example of the theory of Part (III) , in which, by taking what are thought to be reasonable parameters, results are obtained showing how the whirling of a rigid propeller may materially affect wing normal mode shapes and frequencies.