When you watch one of the big jets heaving itself almost effortlessly up into the sky it seems amazing that something weighing up to 500 tonnes can even fly. Yet the facts about flight are as basic as they would be with a model airplane. There are four forces involved in flying: thrust (which comes from the power of the propellor or jet); drag (the friction of the air on the body and wings of the airplane); weight (the weight of the airplane); and lift (which is produced by the air pressure on the upper surfaces of the wings being lower than that on the lower surfaces). Weight, thrust and drag are fairly easily understandable but many people initially have a problem with the theory of "lift".

The shape of an aircraft wing has been continuously developed over the last century and we have now reached a highly efficient shape. The wing is thicker at its front (leading edge) than at its rear (trailing edge), and this helps create two flows of air as the plane travels forward. The air pushed above the leading edge of the wing has a lower pressure than that flowing beneath the wing. Imagine aiming the water from a hosepipe at a solid object. Before it reaches the object it is flowing in a steady stream (high pressure). But once it hits the object it breaks up and does not have the same power – that is, its pressure is lower. This is what happens when air hits the front of the wing. The air flowing below the wing is still at high pressure.

As this lower pressure air flows above the wing (at very high speed) it creates a partial vacuum and, as objects, gases or liquids will try to fill a vacuum, the airplane wing "rises" into this partial vacuum. At the same time the high-pressure air beneath the wings is being pushed downwards, creating a pocket of higher pressure. Just as in a balloon when you inflate it, the air inside (which is at a higher pressure) pushes against the sides, so the higher pressure under the wings push against the wings, pushing them upwards. This, very basically, is what holds the plane up. But the plane must maintain a minimum speed through the air for this to take effect – any lower speed results in a stall, where the plane loses lift and thus height.

During take off and landing the angle of the leading and trailing edges of the wings is altered slightly (by flaps and trim – see below) to give extra lift and better low-speed control.

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