The Torsional Oscillations of Airscrew Blades at Low Mean Incidences

An attempt is made to explain the observed torsional oscillations of an airscrew by means of the 'wake' theory. It is shown that the aerodynamic torsional damping becomes negative at certain blade angles and frequencies; the frequencies, or speeds, at which a single blade becomes unstable are shown to differ slightly from the critical frequencies of a multi-blade airscrew. At low incidences it does not appear to be possible to completely avoid this type of oscillation, but it is suggested that the oscillatory stresses might be reduced by moving the flexural axis towards the leading edge. 1. Introduction. Usually the problem of airscrew flut{er is only serious at blade angles for which some or all of the blade is stalled, but recent tests I have shown that a second form of flutter can be important at small (positive and negative) blade angles. The essential features of the experimental results are as follows: