As a helicopter doesn t speed down a runway, how does it get off the ground

A.� Helicopters and airplanes obviously get off the ground in different ways.

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An airplane uses speed to obtain lift. As it zooms down the runway, the shape and angle of its wings forces air to flow faster over the top surface of each wing than under the bottom - so�air pressure above the wing drops below that underneath. As a result the wings - and, of course, the plane to which they are attached -�are lifted off the ground. Naturally, this depends on the wings being big enough to provide sufficient lift.

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A helicopter, on the other hand, gets its lift by spinning�its 'wings'�- or�rather,�blades - even when the�aircraft itself is stationary. But the effect on air pressure is similar: air flows faster over each blade than beneath it, creating an upward force on each blade. The helicopter uses this upward force to get off the ground, to fly and to hover. It can hover in the same spot in the air because the moving blades are able to control upward lift.

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Q.� Why doesn't a helicopter just keep going up How can it move horizontally as well

A.� Once up in the air the blades of a helicopter can be tilted, so that the air pressure can be used to propel the helicopter in a horizontal�direction.

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Q.� Helicopter blades look very thin compared to airplane wings -�how do they propel the helicopter without breaking off

A.� Helicopter blades are under much more tension than plane wings. Normally an object,�traveling�without the constraint of other forces,�moves�in a straight line (we call that inertia). To make a helicopter work, the blades must be anchored to�a central pivot�(pictured left)�and pulled in a circle -�experiencing huge tension at the same time.
This tension stiffens, and so strengthens, the blades, in the same way that a string instruments strings are tensed and strengthen by stretching them. Obviously, though, the material from which they're made has to be capable of withstanding these enormous tensions - and this is rigorously tested when they are�being manufactured.

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by Lisa Cardy