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An airfoil is said to have a positive camber if its upper surface (or in the case of a driving turbine or propeller blade its forward surface) is the more convex. Camber is a complex property that can be more fully characterized by an airfoil's camber line , the curve Z(x) that is halfway between the upper and lower surfaces, and thickness ...
The aerodynamic center of an airfoil is usually close to 25% of the chord behind the leading edge of the airfoil. When making tests on a model airfoil, such as in a wind-tunnel, if the force sensor is not aligned with the quarter-chord of the airfoil, but offset by a distance x, the pitching moment about the quarter-chord point, / is given by
m is the maximum camber (100 m is the first of the four digits), p is the location of maximum camber (10 p is the second digit in the NACA xxxx description). For example, a NACA 2412 airfoil uses a 2% camber (first digit) 40% (second digit) along the chord of a 0012 symmetrical airfoil having a thickness 12% (digits 3 and 4) of the chord.
For a thin airfoil of any shape the lift slope is π 2 /90 ≃ 0.11 per degree. At higher angles a maximum point is reached, after which the lift coefficient reduces. The angle at which maximum lift coefficient occurs is the stall angle of the airfoil, which is approximately 10 to 15 degrees on a typical airfoil.
The aerodynamic center is the point at which the pitching moment coefficient for the airfoil does not vary with lift coefficient (i.e. angle of attack), making analysis simpler. [ 1 ] d C m d C L = 0 {\displaystyle {dC_{m} \over dC_{L}}=0} where C L {\displaystyle C_{L}} is the aircraft lift coefficient .
Some important parameters to describe an airfoil's shape are its camber and its thickness. For example, an airfoil of the NACA 4-digit series such as the NACA 2415 (to be read as 2 – 4 – 15) describes an airfoil with a camber of 0.02 chord located at 0.40 chord, with 0.15 chord of maximum thickness.
A typical lift coefficient curve. A cambered aerofoil generates no lift when it is moving parallel to an axis called the zero-lift axis (or the zero-lift line.)When the angle of attack on an aerofoil is measured relative to the zero-lift axis it is true to say the lift coefficient is zero when the angle of attack is zero. [1]
A lift-producing airfoil either has camber or operates at a positive angle of attack, the angle between the chord line and the fluid flow far upstream of the airfoil. Moreover, the airfoil must have a sharp trailing edge. [6] Any real fluid is viscous, which implies that the fluid velocity vanishes on the airfoil.