An aerofoil is the term used to describe the cross-sectional shape of an object that, when moved through a fluid such as air, creates an aerodynamic force. Aerofoils are employed on aircraft as wings to produce lift or as propeller blades to produce thrust. Both these forces are produce perpendicular to the air flow. Drag is a consequence of the production of lift/thrust and acts parallel to the airflow.
Other aerofoil surfaces includes tailplanes, fins, winglets, and helicopter rotor blades. Control surfaces (e.g. ailerons, elevators and rudders) are shaped to contribute to the overall aerofoil section of the wing or empennage (Skybrary, 2011).
Several terms are used to describe aerofoils (Dynamic Flight, 2002).
- Leading Edge = Forward edge of the aerofoil
- Trailing Edge = Aft edge of the aerofoil
- Chord = Line connecting the leading and trailing edge. Denotes the length of the aerofoil
- Mean Camber Line = Line drawn half way between the upper and lower surface of the aerofoil. Denotes the amount of curvature of the wing
- Point of Maximum Thickness = Thickest part of the wing expressed as a percentage of the chord
By altering each of the above features of an aerofoil, the designer is able to adjust the performance of the wing so that it is suitable for it's particular task. For example, a crop duster may have a thick, high camber wing that produces a large amount of lift at low speed. Alternatively, a jet would have a thin wing with minimal camber to allow it to cruise at high speeds.
How it works
The basic principle behind an aerofoil is described by bernoullis theorem. Basically this states that total pressure is equal to static pressure (due to the weight of air above) plus dynamic pressure (due to the motion of air).
Air that travels over the top surface of the aerofoil has to travel faster and thus gains dynamic pressure. The subsequent loss of static pressure creates a pressure difference between the upper and lower surfaces that is called lift and opposes the weight of an aircraft (or thrust that opposes drag).
As the angle of attack (the angle between the chord line and relative air flow) is increased, more lift is created. Once the critical angle of attack is reached (generally around 14 degrees) the aerofoil will stall.