Introduction to Aerofoils

Updated on August 4, 2013

Aerofoil

An aerofoil is a surface designed in order to achieve a desirable reaction from the air in which it moves. The profile (shape) of a conventional wing is a good example of an aerofoil. The top surface of the wing will usually have a much greater curvature than the lower surface. This difference in curvature between the upper and lower surfaces of the wing builds up what is known as the Lift force. This occurs because the air flowing over the top surface of the wing, must reach the trailing edge of the wing in the same amount of time as the air flowing under the wing. In order for this to happen it requires the air flowing over the top surface to be at a higher velocity than the air flowing on the bottom. According to Bernoulli's principle, this will in turn create a decrease in pressure on the surface. This will create a pressure differential between the upper and lower surfaces of the wing, thus forcing the wing in an upwards direction.

Terminology and Definitions

The Chord Line: the straight line joining the centres of curvatures of the leading and trailing edges.

The Chord: defined as the distance from the leading edge to the trailing edge of the section measured along the Chord Line.

The Mean Camber Line: the line drawn halfway between the upper camber and lower camber.

The Thickness/Chord Ratio: the ratio of maximum thickness to the length of the chord is comparable to the fineness ratio of a streamlined body.

The Angle of Attack: the angle between the Chord Line of the section and the direction of the relative airflow.

The Angle of Incidence: the angle between the Chord Line and the longitudinal axis of the aircraft.

(Definitions / Terminology from British Airways handbook).

Aerofoil Shape

Firstly you should know that the word camber is curvature. For an aerofoil increasing the camber on the upper surface causes an an acceleration in the airflow, thus producing higher levels of lift at the same angle of attack. Thick aerofoils will produce high lift but can sometimes also increase the amount of drag. Thin aerofoils will give a much higher speed. Changing the camber over the top surface of the wing has a much bigger effect than changing the camber on the lower surface. The shape of an aerofoil can also be changed during flight, this is done by the use of flight control surfaces. These flight control surfaces are the Leading Edge Flaps, Trailing Edge Flaps and Slats.

The efficiency of a wing is measured by it's ratio of lift over drag. The ratio will be different depending on the angle of attack but will always reach a maximum level. At this level the wing is at it's maximum efficiency. The shape of an aerofoil is what determines were this level is.

Stalling

When the angle of attack is increased gradually up to a positive angle of attack (front of aerofoil lifting upwards) the lift component will increase rapidly. This will happen up to a certain point that we call the angle of maximum lift or the Stall Point. This angle is called the critical angle of attack which once reached will cause the air to stop flowing smoothly over the top surface of the aerofoil. This causes air to break away from the upper camber line of the wing producing turbulent air over the aerofoil. Once this has occurred the amount of lift being produced will drop tremendously and drag will become much more excessive. This point can be seen by the dropping of the nose of the aircraft.

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