ORIGINAL: Rotaryphile
A way to measure drag would be to use onboard airspeed telemetry, kill the engine, dive your airplane straight down from high altitude until it reaches its maximum airspeed. This may produce a dangerously high airspeed and cause the airplane to break up from flutter, turning it into an unguided missile. The drag of the stationary propeller can be calculated reasonably accurately; a windmilling propeller produces tremendous drag that is also difficult to estimate. At maximum speed, vertically down, drag is equal to weight.
Interesting. A terminal velocity dive is not without its shortcomings but involves fewer unkowns:
TERMINAL DIVE: mg = F(drag)
vs.
LEVEL FLIGHT Vmax: P(engine) x Eff(prop) = F(drag)
Assuming I could keep the dive perfectly straight and reach terminal velocity (TV) before hitting the ground this is far simpler than a level flight approach. It's conceivable I could have the plane towed to altitude allowing me to remove the prop beforehand. The plane should come out to about 10 pounds dry. If I put it on a starvation diet for the test that should allow a shorter dive duration, lower TV, and reduce airframe stress during pullout. I'll crunch some numbers based on a ballpark Cd range and see what kind of start height and TV I could expect.