Tim,


Your engine is under-propped!

In the 05/03 MAN magazine 'shootout', Dave Gierke found the Tower Hobbies .61 peaked with 1.59 actual HP @ 14,150 RPM.

Your .75 is breathing through the same passages as the .61 with similar timing numbers, and as such, peaks lower in the RPM band. It peaks around 13,500 (don't read what Tower claims...), with slightly greater HP numbers than the .61; perhaps 1.80.

Your numbers (on an MA prop) suggest this is right, but the RPM is too high.

You are running your engine over peak power RPM and as your plane speeds up and the prop unloads, the higher RPM would bring it further off peak; off the meatiest part of the HP curve.

The engine will, of-course, sustain no damage, but your model will fly much better, even faster, if you load it down into the fat part of the band (~12,500 RPM), with a larger, higher-pitch prop.