That equation has never worked for me either. If it makes anybody feel any better, Joe Horton reminded me today that he can reproduce 180 mph TAS on a regular basis, and his ASI system is well calibrated. He also has a nosewheel out front. Temperature and altitude are factors, and of course RPM and weight of the plane.
I flew the KR2 (old wing) with a 54x50 prop today at WOT at 168 mph TAS at about 2500 ft, so I'm getting over 100% efficiency. Bottom line is this equation doesn't fit KRs. Properly built, they are simply more efficient than most airplanes out there, especially at the time that equation was probably hatched...I'm guessing the early 1940's. Mark Langford ML at N56ML.com http://www.n56ml.com
