Second report on the twisting of propeller blades. Supplementary to R. & M. 454

Abstract

The method of investigating the twist of propeller blades, which was developed in R. & M. 454, is interpreted mathematically by making a certain assumption as to the shape of the cross-sections. A general equation expressing the twist as a function of the radius is obtained, and an experimental method of solving it is evolved. It is shown that blades of certain shapes may be peculiarly liable to torsional vibration, and that a plan form.common in current practice possesses this property to an appreciable degree. It is further shown that the maximum stress due to torsion may determine fracture in this case. A method of calculating the shape of plan form in any given case, in order that the blade shall not twist, is deduced, and it is shown that this leads to a nearly symmetrical form in one instance. The effect of the large torsional hysteresis of timber in damping out vibrations is discussed, and it is suggested that herein may lie the reason for the comparative failure o5 metal propellers up to the present. Finally, suggestions are made for the modification of current practice in accordance with the indications of the present theory.