Minimization of Unsteady Propeller Forces that Excite Vibration of the Propulsion System

A simple method is presented for predicting the periodic forces Which act on a propeller operating in a spatially nonuniform wake flow. The method, based on a 10w aspect ratio type of approximation, yields results with an accuracy comparable to that of elaborate computer calculations but with considerable physical insight. More important, the analysis has been inverted to create a design method explicitly yielding the propeller blade shape which will in principle produce zero net force for a chosen periodic component in a given wake field. A rational method for the design, of the propeller blade outline in terms of the distribution and total amount of skew, tailored to the wake, is thereby provided. Model-scale measurements have demonstrated periodic thrust· fluctuations for propellers with blade forms designed by this method which are less considered theoretically minimum for the total blade skew incorporated. The analysis indicates, and the measurements tend to verify, that the significant blade skew is that of the leading edge rather than the midchord line. The amount of skew required for force minimization may be half that previously believed necessary, but there are any number of skew distributions of greater amount which will in principle null a selected force component, each tailored to the same wake distribution. Construction and operation of propeller blade forms designed by this method appear quite feasible.