# Does induced drag of wing change with speed for fixed AoA?

In wind tunnel we put 3D wing at fixed AoA, then tested at two different subsonic airspeed.

At higher speed, wing will produce more lift and more overall drag. Will induced drag also rise at higher airspeed?

If yes that mean induced drag change with AoA but with speed as well. (Pressure drag also change with AoA and speed.)

The question suggests some possible confusion around the difference between the actual value of a variable, and the value of the coefficent of that variable.

If we ignore effects related to the increase in Reynold's number as the airspeed increases, then--

Coefficients of lift, drag, and induced drag will stay the same. Because they depend on angle-of-attack, and are independent of airspeed.

Actual values of lift, drag, and induced drag will increase. Because for any wing at given-angle-of-attack, since the related coefficients are fixed, the actual values of these variables will increase according to the square of the airspeed.

Induced drag is, by definition:

$$D_i =qS \cdot\frac{C²_L}{\pi AR e}$$

where $$AR$$ is the aspect ratio, $$e$$ the Oswald coefficient and $$q=½ \rho V²$$.

From this equation you can infer all the information that you need. In your experiment the only thing that changes is speed, then the induced drag is going to increase proportionally to it squared. At least until effects due to Reynolds number don't change $$C_l$$ or the spanwise lift distribution changes modifying $$e$$; but this can be neglected as a first approximation.

Anyway this is a pure academical exercise: in reality lift doesn't assume random values but equals the weight of the aircraft (except during manoeuvres). Substituting therefore the weight $$W$$ and doing some simplifications, one gets:

$$D_i = \frac{W²}{q \pi e b²}$$

where $$b$$ is the wingspan. This time $$q$$ is by the denominator and therefore the induced drag reduces with the speed squared i.e. the opposite of before.

Btw, induced drag belongs to pressure drag.