John,

No, the efficiency of a prop has nothing to do with slippage. It is just a measurement of energy conversion efficiency. I don't think that our racing props slip all that much, but I don't have any trustworthy data. Props are the extremely complex to understand, really more difficult than the rest of the airplane.

About the only way to accurately determine the drag of an airplane (short of a wind tunnel) would be to haul it up to altitude on a "mother ship" and then drop it without a prop. With a data logger, especially one that could measure both the rate of altitude loss, and airspeed, a "J Curve" graph could be generated. Since the rate of descent is the power inputed vs the speed of the plane, you can calculate the equavant flat plate drag of the airframe with a series of drop tests.

Once you have this data, then you could compare engine power inputed vs actual speed and determine prop efficiency. Of course this too would require a bunch of testing where you measure engine rpm and torque at multiple loads and rpm's to determine the horsepower curve vs rpm. And there are also the other variables of atmospheric pressure, temperture, and humidity. Then the internal variables in the engine setup of crank, and sleeve timing, as well as head clearance and design (combustion chamber). Throw in a few different fuels, props, and plugs and the data universe explodes.

In full scale, this curve tells some interesting things about the airframe. The very bottom of the curve is the minimum power point at which the airplane can maintain level flight. This speed is usually the one where you can glide the furthest (this also depends on the wind, as with a headwind, you must glide faster than the so called best glide speed to get the maximum distance).

Then there is Carson's number which is a line from the origin to a point on the curve that is tangent to the curve. This is the speed that give maximum fuel efficiency for greatest range. We race a lot further up the curve

The left side of the curve which is slower than the best glide speed is the so called "back side of the curve". That is where giving more up causes the airplane to drop out of the sky faster while going slower. A good place to be IF you are too high and want to lose altitude to make a landing. But to land, you have to drop the nose and speed up to get back to normal pitch response or your landing is likely to be a very bad arrival.

recap:
So for airframe development, glide and measure.
Test engine setup to adjust shims for conditions and prop so the engine delievers peak hp. Simple eh?