# Airfoil pressure distribution

#### ruzfactor

I need help to determine the angle theta at each point of the surface so that i can apply numerical integration to get the lift and drag using the pressure distribution. How can I define theta?

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#### RandomGuy88

Do you have the airfoil coordinates? If so you could just use simple geometry to get the angle relative to the horizontal.

Divide the airfoil up into panels where each panel is in between two points. Then the angle that panel makes with the horizontal would be arctan((y2-y1)/(x2-x1)) where 1 and 2 denote point 1 and point 2. Then to get the angle from the horizontal to the normal of the panel you would just add 90 degrees. You may have to be careful with the sign of the angle though you calculate from the equation above. So watch out for that.

#### ruzfactor

@RandomGuy88: thnx..

I have calculated the lift but I'm facing some problem regarding the moment coefficient around quarter chord point. I need moments to calculate Cm. When calculating moment at panel 3 (e.g lift at the surface between panel 2 and 3 x distance of panel 3 from 1/4 chord), I'm multiplying the lift with the distance between 1/4 chord point and that panel. Is this the correct way to calculate moments?? The confusion is, I used trapezoidal rule to calculate lift between say panel 2 and 3. So when calculating moment I'm multiplying only the distance of panel 3 from the 1/4 chord. Please explain.... :(