The Art of the Helicopter John Watkinson - Karatunov.net

(a)
(c)
(b)
Introduction to helicopter dynamics 85
Fig. 3.19 (a) When hovering close to the ground, a pressurized air cushion builds up under the rotor, reducing
the induced power needed to hover (b). Hovering close to a building (c) may result in the downwash being
returned to the rotor. Recirculation causes loss of lift.
control. If the machine descends, it will obtain more lift from the air cushion; if it climbs,
the lift will fall. In many respects the helicopter in ground effect is like a hovercraft.
A helicopter in ground effect over water will displace some of its weight in a saucershaped
depression in the water surface. This is not often seen because hovering low
over water is a recipe for spray in the engines.
The presence of the rotor head and blade root cut-out creates a hole in the middle
of the disc through which air can escape upwards in ground effect. This is actually
another form of tip loss, which is called the fountain effect. In practice the fountain
effect can be beneficial because it puts some of the hull into upflow and reduces the
hull download. There will be vortices trailing from the inboard end of the blade. As
the lift gradient is small here these are relatively weak and their main contribution is
that in ground effect they make the hover conditions chaotic especially if they interact
with the tail boom or tail rotor.
It is often stated by pilots that hovering over rough ground ‘dissipates’ the ground
effect, when theory would suggest that restricting the ability of the downwash to escape
would enhance ground effect. This aerodynamic effect ought to be very small, and the
real effect may be psychological. When hovering over a crop, the pilot will estimate his
height from the top of the crop, not from ground level. Thus the ground effect may not
appear as powerful.