Experimental Investigation of Rotor Slipstream Oscillations in the Ground Effect

Unsteadiness of pressure in a helicopter rotor slipstream may bring a significant problem during operation from the elevated heliports, as the oscillating slipstream acts on the heliport plate and causes vibration of building's structure (Ref. 1). However, still it is an unappreciated issue, discussed mainly in the literature focused on the brownout (e.g. Nathan and Green (Ref. 2), Ramasamy et al (Ref. 3)). In this case researchers usually neglect the loads acting on the ground. On the other hand, investigation of interaction between rotor slipstream and helicopter's surroundings (e.g. Gibertini et al. (Ref. 4), Dziubinski (Ref. 5)) is often limited to a time-averaged case. The investigation presented below has been carried with the 2-bladed rotor, having diameter of 8 meters, in conditions of IGE hover. Pressure measurements showed that the amplitude of its oscillations is nearly constant along the radius of the rotor and achieves the value up to 30 percent of the disc loading.