Forces

How does an airliner fly?

An airliner takes off from San Francisco Airport.An airliner takes off from San Francisco Airport.Click to play videoHow does an airliner, weighing 350,000 kilos (nearly 400 tons), lift off from the ground and fly through the air? It relies on its engines to drive it forward at high speed and its wings to provide lift force. As a plane moves forwards through the air, air hitting the leading (front) edge of the wing separates above and below the wing. Because of its aerofoil shape—curved more on its upper side than on its lower side—air flows over the top of the wing faster than underneath it. This creates higher air pressure under the wing than above it. The higher pressure on the lower side produces an upward force called lift. The plane will climb into the air if this lift is greater than its weight. The faster the airflow, the greater the lift, which is why the plane must travel very fast along the runway before it can take off.

How lift is producedHow lift is produced

How the plane’s wing surface is extendedHow the plane’s wing surface is extendedMaximium lift is needed at take-off and landing, when speeds are slower than when the plane is in the air. Flaps fitted to the wings are used to extend the wing surface and exaggerate its curved profile in order to provide extra lift at these times. When an airliner takes off or lands, the flaps extend outwards at the trailing (rear) edge of the wings. The airliner’s wings may have a layered “skirt” of three flaps with slots in between through which the air is channelled. Hinged flaps at the leading edges of the wings also project forwards to extend the wing surface even more. 

You can see the lift force in action by trying this simple experiment. Hold a piece of paper very close to its edge and blow across the top of it. You’ll see that the paper rises up. This is because the air flowing over the top of the paper is faster than the air underneath it, which remains still.

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