I am running an OS 91 Surpass on a Hanger 9 P51D using a 15x8 prop. The engine has some serious pull but the engine, on 10% nitro fuel (albiet it is still relatively new) is only giving 7,500 rpm at full throttle.

More power to take off, more power to climb, but with the wooden prop (14x6), I might have had more speed in flight. Not that speed really matters on a scale war bird, but my question is - am I harming the engine? Is 15x8 too large? I like the thrust I get - what should I use for a prop on a 91 Surpass?

Thanks,

Nomi

I'd try a 14x8 or a 15x6. I believe the 15x8 is lugging that engine.
(That's more a 120 sized prop than a 91 sized)
The 15x6 will probably give you more thrust, in the first place.

I agree with C_Watkins, you're over-propping the engine. I'd use a 14x8 on the P-51.

Doh... missed the P-51 part.
You're right, the 14x8 is probably a better match for that airframe.