Does anyone know; Is there any reason why, You cannot run a smaller prop than recommended on a gas engine?. Example; a 16x8 or 16x10 on a 40 cc engine. Naturally,I'm talking about not overreving it, using either mechanical or transmiter programing to limit max. rpm.

Without going into over revving or increases heating effect, there's no reason not to use a smaller prop. However... there will be clear decreases noted in thrust levels, with a somewhat lesser increase in speed performance. It comes down to what you want to do, work hard through high thrust levels at reduced speeds, or go faster through higher rpm levels. With any engine you can only develop a finite amount of power, so eventually a point is reached where further gains in speed are no longer available due to reaching the point of maximum available power at any rpm.

I think quite a few might be interested in hearing more about the reasons for the smaller prop, if you didn't object.

Reasons for running a smaller than optimal thrust sized prop:
1. Better engine response with faster spool up,
2. Engine reaches max rpm much faster out of a hover, since it really doesn't have to unload (plane move forward) to reach top rpm.

I am personally running a Biela30x10 om my DA150 on cans in a 39 lb 37.5% Yak54. It is pure fun and will crack out of a hover. I know that Mark Trent likes to run a Mejzlik30x10 on his DA150 on pipes in a Composite-ARF 3.3m Yak, for all the same reasons.

Running a smaller prop and letting the engine rev and breath is much better for it than having it lug a large prop. More rpm at lower load means more engine/oil through the engine, means better dispertion of heat and better cooling and less pre-ignition.

Also, I seriously doubt one can over rev these gassers, short of doing really crazy things to/with them.

DKjens

We run GT80s on glow fuel at 11,500 in the air on our Formula 1 racers..19-18 carbon props...No problems....