Helicopter blade slap

Abstract Blade slap is the sharp increase in helicopter rotor noise, at the blade passing frequency, that is characteristic of certain model helicopters during some regimes of flight. An investigation has been conducted to determine the nature of the phenomenon. This has included flight tests with the Wessex and Belvedere helicopters and a comprehensive operational survey, military and commercial, in the United States and United Kingdom. Blade slap appears to be caused by a blade passing through the tip vortex shed by another blade in its proximity. This has been simulated on a rotor whirl stand under controlled conditions and the effect of various parameters investigated. A theory has been developed to predict the noise generated during a blade slap condition. There is good correlation between the flight tests, model tests and theory. The paper discusses all aspects of the investigation.