The number of comparisons that have been made of calculated and measured blade-frequency thrust, torque and other force and moment components are very few because of the paucity of data. In this chapter comparisons of theoretical predictions with experimental data will be given. Results obtained by various theories will also be compared. The chapter concludes with presentation of a simple procedure, based upon the KT-J curve of the steady case, for a quick estimate of the varying thrust at blade frequency.

The measurement of blade-frequency forces on model propellers requires great care in the design of the dynamometer which must have both high sensitivity and high natural frequencies well above the model blade frequency. After a number of failures a successful blade-frequency propeller dynamometer capable of measuring six components (three forces, three moments) was evolved at David Taylor Research Center (DTRC) about 1960.

Measurements were made with a triplet of three-bladed propellers of different blade-area ratio designed to produce the same mean thrust. This set was tested in the DTRC 24-inch water tunnel alternately abaft threeand four-cycle wake screens which produced large harmonic amplitudes of the order of 0.25-U in order to obtain strong output-to-“noise” levels! The three-cycle screens give rise to blade-frequency thrust and torque whereas the four-cycle wake produces transverse and vertical forces and moments about the y- and z-axes which in general come from the fourth and second harmonic orders of blade loading on a three-bladed propeller, as was demonstrated on p. 367.