# What happens to center of pressure when the angle of attack increases in symmetrical airfoil.?

Any guess ?It won’t vary with angle of attack.

## Center of pressure

First let me explain from basics. Center of pressure is a point at which the resultant aerodynamic or lift force acts in the chord line.

For unsymmetrical airfoil, when the angle of attack increases, first the center of pressure will move towards leading edge up to or near (may be located in front or back) to the aerodynamic center. And when it is increased further, it will return back towards trailing edge.

So, for an unsymmetrical airfoil, the center of pressure will vary when angle of attack varies.

### How is that so ?

When angle of attack increases, the pressure distribution varies. If we take the resultant pressure and shear stress distribution, center of pressure will take a new location to increased angle of attack.

For symmetrical airfoil, when angle of attack varies, the pressure distribution will also vary. But for any angle of attack if we consider the resultant lift force position or center of pressure it will lie on the aerodynamic center. It won’t vary with angle of attack. Because aerodynamic center is at constant location  & usually lies at 25% of the chord. Aerodynamic center and Center of Pressure are same in symmetrical airfoil.

But a question may pop up into your head that, the moment about aerodynamic center is constant, but why the moment about center of pressure is zero ? It is because the moment arm distance is zero

But what if both , aerodynamic center & center of pressure in symmetrical airfoil lies at same point, then how will it accord ?

It will tally because zero is also a constant.

Center of pressure in symmetrical airfoil won’t vary because it lies on aerodynamic center of the airfoil.