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The helicopter is an aeroplane with a difference, because it can hover and remain completely still in the air. It is able to do this because its wings continue to move through the air even when it is stationery.

The wings of a helicopter are the blades which turn all the time it is in the air. The blades of a helicopter have an aerofoil shape like the wings of an aeroplane. Their top surface is curved, with the undersurface almost flat. As the blades turn, there is greater air pressure under the blades than above them, and this produces an upwards lift force.

The amount of lift produced by the turning blades depends upon the pitch (angle) of the blades as they turn. If the front edge of a blade is higher than its back edge, it produces more lift than it would if it were flat. To take the helicopter up, the pilot increases the pitch of the blades. 

To take the helicopter down, the pilot decreases the pitch of the blades. To fly forward, the pilot changes the pitch of the blades as they turn. As each blade moves past the tail of the helicopter, the pitch is increased.

Then as the blade sweeps past the front of the helicopter, its pitch is decreased. The result is that the blades at the back of the helicopter produce more lift than those at the front. This moves the helicopter forward. To move backwards or to one side, the pilot adjusts the pitch of the blades to produce more lift at the front  or to one side.

Most helicopters have another set of blades at the back. These prevent the helicopter spinning around in the air, in the opposite direction to the turning blades.

(Source: The big book of How things work by "Peter Lafferrty")

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