Equation 2.30 shows the power required to climb and generate downwash. To this is added the profile power. If we examine the equivalent power consumption in descent we find the following equation:
P = T (-VD + Vi)+ Pp = – T • Vd + T • Vi + Pp
The profile power expression is independent of the climb/descent speed so the hover result (Equation 2.16) is applicable here. Equation 2.58 now has the possibility of attaining the value zero. This would mean that no power input to the engine is required. Assuming this to be the situation, we have the following expression:
While this is the simplest form, with no factors applied, it illustrates the importance of disc loading – hence downwash – in the descent rate that is required to establish this condition of no
power input. This is the autorotation condition used by helicopters to land safely when power to the main rotor is lost. It is not surprising that this autorotation speed is kept as low as possible, so the downwash should be limited in value. This places a minimum limit on the main rotor radius.