The Torsional Oscillations of Airscrew Blades at Low Mean Incidences

Abstract

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.