In hovering flight, the vertical upward lift from the rotor equals the weight of the entire aircraft plus an additional quantity to account for the drag of the rotor wash, or slipstream, over the body (Fig. 15.2).
The Froude efficiency criterion used for propellers is of little meaning for helicopters since, when hovering, it is zero. The efficiency of a rotor in hover is sometimes expressed as a figure of merit, which relates the engine power delivered at the drive shaft to the minimum power required to support the aircraft The more efficient the rotor and the higher the figure of merit, the less engine power is wasted.
15.3 GROUND EFFECT
When near the ground, the rotor wash spreads out and the high pressure region under the helicopter forms a cushion similar to that which supports a hovercraft, though without the restraining side curtains. Because of this ground effect, power needed near the ground is less than higher up. This may be of assistance when taking off but can cause difficulties if power is insufficient to climb away afterwards. On landing, the ground effect tends to check the rate of descent and may cause an aerial ‘bounce’ (Figure 15.3, p.220).