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I'm confused by wikipedia's page on aerofoils, the aerodynamic center, and center of pressure; it seems to contradict itself.

The airofoils page says the center of pressure is at the same position as the aerodynamic center. It says they are both the position on the wing where the moment is zero, and that they are one quarter of the chord length from the leading edge.

Yet the Aerodynamic center page says:

For non-symmetric (cambered) airfoils the quarter-chord is only an approximation for the aerodynamic center.

And the center of pressure page says:

The location of the center of pressure varies with changes of lift coefficient and angle of attack.

I would appreciate an explanation of the difference between center of pressure and aerodynamic center - e.g. are they both the point at which there is no moment? What's the difference between the two? Please keep terminology as close to layman's terms as possible, particularly as little maths as possible.

Thanks!

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2 Answers

I think I've found the answer. On the pitching moment page it says:

The aerodynamic center is defined to be the point on the chord line of the airfoil at which the pitching moment coefficient does not vary with angle of attack,[2] or at least does not vary significantly over the operating range of angle of attack of the airfoil.

However the question remains, what is the position of the aerodynamic centre and centre of pressure?

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Center of Pressure

By definition, an airfoil produces lift when placed in an airstream. Different points along the airfoil produce lift in different amounts and in different directions. However, if you take the sum of all of these different force vectors, you end up with a single upward and a single downward force vector.

The points on the airfoil at which these force vectors are located is called the center of pressure.

The location of the center of pressure along the airfoil changes continuously with changes in angle of attack.


Pitching Moment

In a perfect world, these upward and downward force vectors would always be located the same distance away from the leading edge (that is, at the same chord length along the wing). If that was the case, there would be one and only one center of pressure.

Unfortunately, the world is not perfect and most of the time, the upward and downward force vectors are separated by some distance. When this happens, the airfoil tends to rotate. The tendency of the airfoil to rotate due to upward and downward forces located at different points along the airfoil is known as a pitching moment.


Aerodynamic Center

As the angle of attack changes, the centers of pressure will move and therefore the pitching moment experienced at any given point on the wing will change.

To put it another way, when the angle of attack changes, the total amount of lift and the rotational force felt by the wing at a given point both change.

However, there is one point on the wing, usually located at the one-quarter chord position, around which the pitching moment (rotational force) will never change - only the amount of lift changes.

The point at which the pitching moment remains constant with changes in angle of attack is called the aerodynamic center of the airfoil.

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