Some time back when I was having trouble with the Swanson Associates NOS Fireball Red Hot standard long plugs, for a song, I bought a grody looking Enya .09-III TV with unmuffled original exhaust baffle. It cleaned up nicely, found to be a very low time engine hardly broken in. The Enya 30 plug that came with it looked aged, but it fired up right away no problems, run fine since.

Reason why I mentioned Sceptre Flight engine tests is that the testers such as Peter Chinn have been intimate with the engine industry. Articles refer to various props tried, RPM's achieved, recommendations what sizes to stay away from as they take the engine outside its power band.

I did a test on my old worn out, beat up 40" wingspan foam Hobby Shack Cessna originally for a Cox .049 reedie. Peter made a recommendation not to use props smaller than 7x4, because it would run it beyond peak power, outside the power band. He was right. I used an APC 7x3 prop, it flew very .049 like. The engine develops its optimal power of 0.16 HP when unloaded to 14,500 RPM.

I put a wood 7x6 prop, now it moved out like it had a .09 on it, really became fun flying it.

Peter recommends selecting a prop that when run static, is around 1,000 RPM less than peak. Once in the air, the prop unloads with the forward motion of the plane, then approaches peak.

Also, the whole plane and engine is a system. Sometimes a lower pitched larger diameter prop is more optimal for thrust because of the airplane's wider cross sectional area than say a narrow profiled speed plane. Those old ignition .60's of the 1930's with the 1/4 horsepower output of a modern .15 glow was able to power those 7 foot (1 meter) wingspan cabin or pylon free flight airplanes spinning say a 14" - 16" prop. A modern .15 glow on such a plane would not work today, because there is not enough prop arc area past the fuselage spinning a 7" or 8" prop.