Camber

Camber, in aerospace engineering, is the asymmetry between the top and the bottom curves of an airfoil in cross-section. Camber in its relation to planing surfaces was first discovered and utilised by Sir George Cayley in the early 19th century in England.^[1]

Overview

Camber is often added to an airfoil to increase lift and/or increase the critical angle of attack (the angle at which the airfoil begins to stall). The camber of a wing may vary from wing root to wing tip. Camber is not necessary for the generation of lift, and some airfoils have no camber. Airfoils with no camber (symmetric airfoils) do not generate lift at 0 angle of attack, however. Traditionally the upper camber of an airfoil has been greater than the lower, but some recent designs use negative camber. One such design is called the supercritical airfoil. It is used for near supersonic flight, and produces a more efficient lift to drag ratio at near supersonic flight than traditional airfoils. Supercritical airfoils employ a flattened upper surface, highly cambered (curved) aft section, and greater leading edge radius as compared to traditional airfoil shapes. These changes delay the onset of wave drag and move that drag further aft on the airfoil.

Adding camber doesn't necessarily increase lift; it depends on the airfoil shape. If too much camber is added, the flow over the airfoil may not stay attached to the wing even at an angle of attack of zero. When this occurs, we say the flow has separation over the airfoil; if the entire top of the wing has separation, the wing is stalled. Wings with camber don't, as a result, have the ability to produce more lift in all cases; however, adding moderate camber does generally result in more lift, especially when compared to non-cambered wings at zero angle of attack.

A designer may also reduce the camber on the outboard section of the wings to increase the critical angle of attack (stall angle) at the wing tips. When the wing approaches the stall angle this will ensure that the wing root stalls before the tip—giving the aircraft resistance to falling into a spin.^{[citation needed]}

Definition

The camber of an airfoil can be defined by a camber line, which is the curve that is halfway between the upper and lower surfaces of the airfoil. Call this function Z(x). To fully define an airfoil we also need a thickness function T(x), which describes the thickness of the airfoil at any given point. Then, the upper and lower surfaces can be defined as follows:

Example - An airfoil with reflexed camber line

An airfoil with reflex camber.

An airfoil where the camber line curves back up near the trailing edge is called a reflexed camber airfoil. Such an airfoil is useful in certain situations, such as with tailless aircraft, because the moment about the aerodynamic center of the airfoil can be 0. A camber line for such an airfoil can be defined as follows (note that the lines over the variables indicates that they have been nondimensionalized by dividing through by the chord):

An airfoil with a reflexed camber line is shown at right. The thickness distribution for a NACA 4-series airfoil was used, with a 12% thickness ratio. The equation for this thickness distribution is:

Where t is the thickness ratio.

References

References

• Desktop Aerodynamics Digital Textbook. Retrieved 9/7/08. [1]
• Theory of Wing Sections-Ira H.Abbott and Albert E.Von Doenhoff(Dover Publications-1959),ZeRO LiFT

See also

• Chord
• NACA airfoil