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Computer drawing of an airfoil showing
 the aerodynamic center  - AC. AC = 1/4 chord
 for low speed airfoils.

The Wright brothers approached aerodynamics in a thorough, practical, experimental way. From their writings, it is evident that they were very concerned about accurately determining the aerodynamic forces on their aircraft. But they were more practical engineers than theoreticians and many of the factors which affect lift and drag are better understood today than they were by the Wright brothers. For your more complete understanding, we present here a page which describes the modern method for accounting for the movement of the center of pressure (cp) with angle of attack. The brothers understood the theory of their day, which incorrectly described a steady movement of the cp from the center of a flat plate forward to the leading edge of the plate as the angle of attack decreases from 90 degrees to zero. With the wing of the 1901 glider, the brothers cleverly mapped out the motion of the cp which showed a motion initially forward with decreasing angle, then a reversal to the trailing edge. The brothers determined that the theory, as presented to them in textbooks, was incorrect. The modern use of the aerodynamic center to perform design and analysis, instead of the center of pressure, began several years after the brothers had successfully flown.

As an object moves through a fluid, the velocity of the fluid varies around the surface of the object. The variation of velocity produces a variation of pressure on the surface of the object. Integrating the pressure times the surface area around the body determines the aerodynamic force on the object. We can consider this force to act through the average location of the pressure on the surface of the object which we call the center of pressure in the same way that we call the average location of the weight of an object the center of gravity. In general, the pressure distribution around the object also imparts a torque, or moment, on the object. If a flying airfoil is not constrained in some way it will flip as it moves through the air.

If we consider an airfoil at angle of attack, we can (theoretically) determine the pressure variation around the airfoil, and calculate the aerodynamic force and the center of pressure. But if we change the angle of attack, the pressure distribution changes and therefore the aerodynamic force and the location of the center of pressure also change. Since the pressure distribution changes with angle of attack, the torque created by this force also changes. So determining the aerodynamic behavior of an airfoil is very complicated if we use the center of pressure to analyze the forces.

For a single angle of attack, we can compute the moment about any point on the airfoil. The aerodynamic force will be the same, but the value of the moment depends on the point where that force is applied. It has been found both experimentally and theoretically that, if the aerodynamic force is applied at a location 1/4 chord back from the leading edge on most low speed airfoils, the magnitude of the moment is always the same, regardless of the angle of attack. Engineers call the location where the moment remains constant the aerodynamic center (ac) of the airfoil. Using the aerodynamic center as the location where the aerodynamic force is applied eliminates the problem of the movement of the center of pressure with angle of attack in aerodynamic analysis. (For supersonic airfoils, the aerodynamic center is nearer the 1/2 chord location.)

For symmetric airfoils, the aerodynamic moment about the ac is zero for all angles of attack. With camber, the moment is non-zero and constant for thin airfoils. For a positive cambered airfoil, the moment is negative and results in a counter-clockwise rotation of the airfoil. With camber, an angle of attack can be determined for which the airfoil produces no lift, but the moment is still present. This set of conditions is used experimentally to determine the aerodynamic moment which is then applied for all other flight conditions. For rectangular wings, the wing ac is the same as the airfoil ac. But for wings with some other planform (triangular, trapezoidal, compound, etc.) we have to find a mean aerodynamic center (mac) which is the average for the whole wing. The computation of the mac depends on the shape of the planform.



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Editor: Kelly Sands
NASA Official: Nancy Hall
Last Updated: May 10 2021

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