Coupled Oscillations of Aircraft Engine- Propeller Systems

The coupled oscillations of a flexible propeller and an aircraft engine with arbitrary number of cranks is investigated. In the first part of this paper a practical method is developed to deter- mine the complete dynamical characteristics of a rotating vibrat- ing propeller of arbitrary shape, and the legitimacy of the ap- proximations is checked with a numerical example. In the second part the first nine torsional frequencies of a V-12 engine, with a flexible propeller are calculated and compared to the frequencies of the free-wheeling propeller and those obtained by considering the propeller as rigid. The simultaneous influence of the engine speed on all nine frequencies and the general procedure to com- pute the complete critical speed spectrum are also indicated. T HE call for light weight and highly efficient metal propellers, together with the high speeds and power of aircraft engines, has considerably increased the danger of propeller fatigue failure. In fact the vi- bration characteristics of the coupled engine-propeller system has become one of the primary factors which govern the choice of a propeller for a given engine. The calculation of these oscillations is a rather compli- cated problem and it is made especially difficult in case the propeller is coupled with an in-line engine as will probably occur more frequently in the future. In a previous paper' the torsional oscillations of a crankshaft-propeller system with an arbitrary number of cranks has been calculated with the assumption that the propeller is rigid. In the present paper the flexibil- ity of the propeller is taken into account, the crank- shaft and the propeller being considered as two coupled vibrating systems, and a method is developed by which it becomes practically feasible to predict with fair accuracy the vibrational behavior of the system for all speeds of the engine. It is clear that the propeller is a dynamical system with an infinite number of degrees of freedom, and that the problem of coupled oscillations implies the knowl- edge of the dynamical characteristics of the propeller. They are completely defined for the present purpose by a function of the frequency referred to hereafter as the "dynamic modulus" of the propeller. This concept is very similar to that of impedance in electrical engi-