# Can I measure induced drag with a simple wind tunnel test?

Device can measures total drag force and lift of wing for given AoA and airflow speed. Wing both wingtips are far away from side walls. Airflow speed is constant all the time:50m/s

TOTAL DRAG=MEASURED DRAG

TOTAL DRAG =ZERO LIFT DRAG + INDUCED DRAG

TOTAL DRAG=PROFILE DRAG +INDUCED DRAG

profile drag=skin friction drag + pressure drag

Step 1.

•First I put wing at zero-lift angle: it show 500N drag force

Step 2.

•Then I put wing at 15° AoA:it show 700N drag force

Results:

So for 15° AoA and 50m/s:

"Profile drag"(Skin friction + pressure drag) is 500N and induced drag is 200N(700-500).

Is this correct calculation?

• Does the wing span the full width of the wind tunnel's test cell? Or is it attached by its root to only one wall, with its wingtip distant from all walls? – Camille Goudeseune Sep 3 '20 at 20:49
• @CamilleGoudeseune "wing" has two wingtips,and both are far away from side walls,I will add this in text.. – user52248 Sep 3 '20 at 21:02

Yes, by the textbook definition $$C_D=C_{D_0}+KC_L^2$$.
Your measurement at AoA=0° gives $$C_{D_0}$$, 500 N.
Your measurement at another AoA=15°, by subtraction, gives $$KC_L^2$$, the induced drag at that AoA, 200 N.
If measurements at many AoA's yield a consistent $$K$$ and $$C_L$$, then you gain confidence in those values.
If the airfoil is strongly cambered, a better fit to the equation is made if you add another term proportional to $$C_L$$ (not squared).