Airfoil
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An airfoil (in American English, or aerofoil in British English) is the shape of a wing or blade (of a propeller or ship's screw) as seen in cross-section. It is used to provide lift or downforce, depending on its application. Subsonic-flight airfoils have a characteristic shape, which is that of a curved streamline with a rounded leading edge and a sharp trailing edge.
For an understanding of the various ways of explaining lift, see lift. This force can be harnessed to support an aircraft in flight. As well as the wing, an aircraft's horizontal and vertical stabilizers are airfoils as well. Airfoils are also found in propellors, fans, compressors and turbines. Sails are also airfoils, and the underwater fins of sailboats, such as centerboards, are also lifting foils and operate on the same principles as airfoils (technically they should called hydrofoils, but this term has already been taken; generally they are just referred to as "foils"). Birds rely on airfoils for wing-lift.
An airfoil in an inverted position will create a downward pressure on an automobile or other motor vehicle, improving its traction and reducing its likelihood of becoming airborne. The term "lift" can mean a force generated in any direction in any medium.
It is important to note that any thin object at an angle of attack with respect to the airflow, such as a flat plate or even the deck of a bridge, will generate lift; there is nothing "magic" about the shape of an airfoil. However, the airfoil shape ensures that lift is generated with the minimum of drag, so it is important for efficiency.
The optimal design of airfoils has been much studied and is a key element in aerodynamics. Different applications will call for a different airfoil--there is no one "true" airfoil design. A supercritical airfoil, with its low camber, reduces transonic drag divergence, while a symmetric airfoil may be more suitable for frequent inverted flight. Some profiles look like symmetric profiles with flaps at the back producing the lift. Supersonic airfoils are much more angular in shape and can have a very sharp leading edge. New airfoil design techniques continue to develop.
Aifoils have a thickness for two reasons:
- stiffer and lighter => Medium sharp leading edge.
- more space for fuel => Round leading edge.
Various systems have been devised to describe and characterise airfoils—the most common and prevalent is the NACA system. Before this, various ad-hoc systems were used. An example of a general purpose airfoil that finds wide application, and predates the NACA system is the Clark-Y.