General question on prop sizing
 

I have an OS 75 Ax and get 10,200 on an APC 13 X 8. I am running 10% Omega and a # 8 OS glow plug. engine seems to run great.

My question is: should I try a lower pitch to increase RPM and get the engine more into its power cruve? Possibly a 13 x 7

What is the ideal RPM for this engine?

Thanks

RE: General question on prop sizing
 

13x8 sounds a little big for that engine.

I run the Tower .75 on any of the following props:
12x8, 13x5, 13x6, 13x7

RE: General question on prop sizing
 

Try a few props and find out. 10,200 rpm with the APC 13 x 8 is not a bad number - about par or slightly better than an average .91 four-stroke with the same prop.

OS has tended to promote the use of larger props with its two-stroke engines of late - presumably as a "responsible corporate citizen" with respect to noise issues. One can therefore assume that OS has designed these engines to happily handle the prescribed heavier loads without deleterious effect.

There is always "the right prop" for every engine/airframe/pilot combo. Regardless of what good theorycraft may suggest and what the guys at the field might recommend, oftentimes it takes a good day at the field, a few props, a bit of patience and some experimentation to find out just what that "right prop" is.

Good luck and do report back in the interests of science :)

RE: General question on prop sizing
 

Harry, that's good stuff you said, when I got my Webra P-5 .91 people had trouble wrapping their minds around the smallest recommended prop being a 14x12, (which it turns at 8,500 on 5% nitro) and the largest a 16x8. I tried that 14x12 on a SuperTigre .90 and it only could manage 7,000 and quickly got hot. The P-5 has an absolutely huge muffler.