User Guide

Imagine yourself sitting in a swivel chair, with your feet tucked up so the chair can spin
freely. Your arms are above your head, supporting the middle of a long heavy plank. The
plank is the helicopter’s main rotor and you are the engine. The swivel chair is the rest of
the helicopter, off the ground and free to pivot. Now start spinning the plank round and
round like a rotor. As you do this, you’ll find yourself spinning around in the opposite
direction to the rotor. The harder you spin the rotor, the faster you spin yourself - and in this
example, there’s nothing you can do to stop yourself spinning except put your feet on the
ground, which equates to landing the helicopter. This tendency for the engine to spin the
whole helicopter in the opposite direction to the main rotor can be called Main Rotor Torque
Effect.
The tail rotor solves this problem by creating a thrust in the opposite direction to the
main rotor torque effect. Its small size is compensated by the fact that it’s mounted at the
end of a long lever (the tail-boom) which magnifies its effect. Also, by changing the
amount of thrust the tail rotor produces you can pivot the whole helicopter on the spot, in
either direction.
How Rotors Work
A rotor is simply a set of long thin wings attached to a central hub. The wings are
more commonly called Rotor Blades, and when the rotor is spinning, the whole assembly
is often referred to as the Rotor Disc. Just as in an ordinary aircraft, the wings generate a
lift force when they are moved through the air. How much lift a wing generates is governed
by three factors:
1: The Density of the Air
The atmosphere is densest (and provides most lift) at sea level. As you climb above sea
level the density decreases and the wing produces less lift. Air temperature also affects
density – hot air is less dense than cold air, and gives less lift. ‘Hot and high’ is the worst
combination of conditions, and in practical terms this means you can lift less weight and
have less ‘performance’ available.
2: The Wing’s Speed Through the Air
The faster a wing moves through the air, the more lift it generates. In sophisticated modern
helicopters the rotors spin up to a set flying speed before take-off and hardly change speed
in flight, unless you demand more power than the engines can provide or something goes
wrong with the engines or the transmission system. You don’t control lift by changing the
rotor speed, so at first sight this factor seems irrelevant – and it is indeed irrelevant in
hovering or vertical flight. However, when the helicopter is moving forward at high speed
this factor becomes critically important, and determines the maximum safe flying speed –
and what happens when you exceed it [see page 6.19 – Retreating Blade Stall].
GROUND SCHOOL
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