Dave,

It is a lot like the gas mileage stated on the window sticker of a new car. Not very meaningful in the real world, but pretty accurate for comparison. What your club needs to do is establish a specific measurement standard of its own. How high off the ground is the plane, how high is the mic, where is the mic relative to the plane and exhaust, and what surface it is measured over.

After all, these measurements are not very accurate compared to what is being produced in the air (no reflictivity, higher prop rpm, etc.).

Good luck,

Bedford