Induced Aerodynamics of Helicopters

Abstract

AT low forward speeds the slipstream from a helicopter rotor is substantially downwards in direction and will cause a drag force to be generated on any body immersed in it, the drag acting in the direction of the slipstream. In most performance methods the effect of this vertical drag is ignored, bat it cart in fact substantially modify calculated performance, being equivalent to a weight increase of over 10 per cent even on some single rotor designs. The basic parameter is the equivalent flat plate area (area of body drag coefficient) which is immersed in the slipstream, and this is expressed as a ratio of the rotor disk area, i.e. ACD/πR2.