User Guide

Ground Effect

If you were very slow and cautious in raising the collective, you may find that the helicopter

slowly rises a short distance and comes to the hover a few feet off the ground without any

change in the collective setting. If this happens, you can congratulate yourself on having

demonstrated ground effect. The helicopter is, in effect, riding on an air cushion produced

by the rotor downwash. Ground effect magnifies the lifting power available for a given

collective setting, but the effect falls off quite rapidly with height, and disappears altogether

at a height equal to the diameter of the rotor disc. Rough or sloping ground, violent

manoeuvering or strong, gusty winds will all tend to spill the air cushion, push it off to one

side, or prevent it forming in the first place, so a wise pilot is cautious about depending on

ground effect for the lift needed to stay airborne.

In any case, for our first transition to forward flight we want to climb rather higher

than this – say to a minimum of 100 feet/30 meters. As you approach the desired height,

ease the collective down slightly and wait to see the effect. Remember that the helicopter

has momentum – the faster you were climbing (or descending), the longer it will take for

your vertical speed to change till it actually reflects the new collective setting. As you lower

the collective, you’ll also need to coordinate with pedal input. With practice, you can

anticipate the effects of your control inputs, but be careful in the early stages – it’s hard to

avoid over-correcting.

2: Transition From the Hover to Forward Flight

Once you’re above the minimum height and your climb rate is reduced to a low figure (a

perfect hover is too much to expect, but DON’T start this exercise while descending!),

check that there is a long clear run ahead, with no high obstacles. If necessary, use the

pedals to turn onto a clear heading. Now, without changing the collective setting, ease the

cyclic a little forward and hold it there, watching the HUD altitude readout. You’ll see three

effects from this control input:

1) The helicopter tilts forward.

2) The helicopter starts to accelerate forward.

3) The helicopter starts to lose height.

Effects 1 and 2 are easy enough to understand – we’re tilting the rotor disc, which tilts the

helicopter and directs some of the main rotor thrust forwards, accelerating us forward. The

third effect is more indirect, but obviously important. We started in the hover (or very nearly

so), with the main rotor producing just enough thrust (directed straight down) to support the

helicopter’s weight. Now we have tilted the rotor disc (and its thrust vector) in order to

accelerate forwards. This leaves less thrust acting straight down to support the helicopter’s

weight, so it starts to descend. In order to maintain height, we must raise the collective

slightly (not forgetting the pedal input), increasing the total main rotor thrust so that its

downwards component is large enough to support the full weight of the helicopter. At the

risk of stating the obvious, it should be pointed out that this effect applies whenever the

cyclic is tilted away from the central position in ANY direction – the further away from the

central position, the smaller the lift component.

GROUND SCHOOL