An aerofoil (or airfoil) is shaped to produce `lift' as it travels through the air. The most usual example of an aerofoil, is an aircraft wing. The same principle applies to many other objects. For example, an aerofoil is utilised in the driving force of fan blades, propellers and also helicopter rotors. Aerofoils are sometimes installed upside down on racing cars. The reason for this is to force the car downwards, thereby holding it firmly to the ground at high speeds.
The upper side of an aerofoil is curved, while the lower side is more or less flat. As it travels through the air, the leading edge divides the air into two streams, one passing over the aerofoil, the other underneath. The streams then rejoin one another behind the trailing edge.
In comparison, the curved upper surface of the aerofoil is longer from front to back than the straighter lower surface. The air stream that passes over the longer upper route therefore has to move faster in relation to the stream passing under, in order to reach the rejoining point (trailing edge) at the same time. The faster air stream travelling over the top of the aerofoil, has a lower pressure than the slower one beneath creating a lift on the underside.
If the aerofoil is tilted so that its leading edge is higher than the trailing edge, it increases the distance travelled by the upper air stream and will therefore increase the `lift.' The angle of tilt on an aircraft wing is called the `angle of attack.' The angle of the tilt required is determined by the speed of the aircraft. The slower an aircraft flies, the greater the angle needed to allow enough lift to keep it in the air. The increased `nose-up' of an airliner is noticeable when the aircraft is coming in to land.
