I am building a Stick for sport flying with an FA-82 motor.
I live at 5400' and I was told by some of you to go with a 12x6 or 12x8 prop and I am not sure why.
The benchmark prop is the 13x8 for this motor although I see that Saito says a 12x8 to a 14x8 will work.
So with the recommended 12x6 or 12x8 what am I doing, compensating for the thinner air at 5400'?
Just trying to understand and I guess I missed something.

I would think you could take more prop with thinner air as it wouldnt bog the engine down as much. less prop and thinner air would over rev the engine.

......but the engine will have less air for the combustion, delivering less power.

http://www.rcuniverse.com/forum/m_36...tm.htm#3660632

you may want to consider up to a 15x8 at that altitude, but a 15x6 should work fine</p>

but its methanol plus nitro methane, both have oxygen in their molecular make up...
so air density playsa bigger role in prop rpm than available oxygen

I am fully aware I will be looking at a loss of power for altitude, that's why I have selected the biggest motor for my plane to compensate.
My problemnow is understanding the prop selection.
I now have differing opinions at to what size prop to use.

try a 14x6 or 14x7 apc....if you rev over 10k rpm then try a bigger prop

Start with the prop the engine manual ask for, and grab a couple larger ones and a couple smaller ones. Prepare them all balance, ream etc. Try them all and find the one you like best.

let logic guide yuor thnking.

you can compensate for power drop by running slightly more rich on 4s fuel.

you still need to create the prop load so you need a bigger prop.

start with largest prop recommended by the engine manufacturer and work from there.

OK, I will give that a try. Thanks.

The prop you need is the one which works best. That is a matter of fly and try, so get some different size props of different makes and find the best one. Save the rest as one will probably be the best one for some other airplane/engine combo. You may be surprised at which prop will turn out to be the best.

shoot for a rpm between 9500 and 10500