Coupled Engine Torsional and Propeller Flexural Vibrations

Abstract

In the initial treatment of resonant torsional vibration in engine crankshaft propeller systems, the basic assumption was made that for practical purposes a propeller could be regarded as a rigid pulley in the plane of rotation. At an early stage, however, it became evident that theoretical results ensuing from the rigid pulley hypothesis could not explain certain engine-propeller failures, and thus it came to be realized that the flexibility of the propeller blades must have an important influence in coupled crankshaft-propeller vibrations. In this paper an analytical method is given for the determination of coupled engine torsional and propeller flexural vibrations, in which account is taken of blade section, blade twist, hub moment of inertia, pitch setting angle, and speed of rotation. The propeller blade is divided chordwise into six portions which are regarded as having their masses concentrated at their centres of gravity, while account is taken, so far as is possible, of the actual elastic properties of the blade. In the case of an aircraft propeller, the problem is simplified in that the mass axis is practically straight and coincident with the flexural axis, so that flexural and torsional vibrations of the blade itself are not coupled. The theory has been applied to a three-blade propeller used on two different engines, in both of which agreement between the calculated critical speeds and those observed experimentally on the cable hangar was found to be satisfactory. Moreover, the calculated fixed root frequencies for the fundamental and six overtones accorded very closely with the corresponding values evidenced on a -model of the blade by bowing and calibrated oscillator tests.