Useful Approximations
A useful approximation is that
where
- is the 3D lift coefficient for elliptical circulation distribution,
- is the 2D lift coefficient slope (see Thin Airfoil Theory),
- is the aspect ratio, and
- is the angle of attack in radians.
The theoretical value for is 2. Note that this equation becomes the thin airfoil equation if AR goes to infinity.
Lifting-line theory also states an equation for induced drag:.
where
- is the drag coefficient for induced drag,
- is the lift coefficient, and
- is the aspect ratio.
- is the planform efficiency factor (equals 1 for elliptical circulation distribution).
Read more about this topic: Lifting-line Theory