orstraycat

I am wondering how to determine what pitch 3 blade prop would equal an 11X 8 2 blade prop for thrust. I want to use a 3 blade pusher prop for the Nitro Planes Bobcat 50. Does a 9 x 7p 3 blade prop come close? Using a OS .61 engine.

nauticom

orstraycat:
10-9 three blade wiil get you very close.
BobM

da Rock

You know, the real problem isn't going to be figuring out the perfect numbers for the airplane/engine. And it sounds like you've already seen a bit of the problem.

It's hard enough finding ANY 3-blade prop that comes close to our engine sizes. Master Airscrew only sells what? 12-15 tractor 3-blades, and they go from 5" to what? 16" diameter. And they only have about 4 pushers. And nobody sells Graupner 3-blade pushers at all in the US, do they?

So your choices for the 61 would be either the 9x7(3) or the 14x7(3), right?

I've recently been testing 3-bladers on my OS61FX and my OS75AX. I've only had the 12x6(3) and the 12x8(3) to test. The two engines both seem happy with the 12x8(3). The 61FX does ok with the 12x6(3) but it's not the best prop for it if I include it's 2-blade favorites. So I trimmed a 12x6(3) to be a 11.5x6(3) and it's very good with that one. But the engine is turning top rpm with that modified prop. Diameter on 3-bladers appears to affect rpm more. It ought to. And I'd not wish to turn the 61 any faster so don't plan to even try an 11x7(3).

So......... I'd say you've only got one 3-blade pusher that sounds like it's specs are in the ballpark, and that'd be the 9x7(3). And I'd bet money it won't be enough load on your engine to work. And I'd bet the 14x7(3) would be way too much load.

So....... If I was in your situation, I'd try the 14x7(3) fully expecting to need to trim the diameter until the engine responded with enough revs to work. And then see if what was left of the blades pulled.... oops..... pushed the airplane ok.

orstraycat

What about the graupner 12.5 X 7 (3) available from www.hobby-lobby.com?

da Rock

Horizon and Hobby-Lobby both appear to have no 3-blade pushers advertised anywhere on their websites. They both have a 12.5x7(3) advertised, but it's listed with all the other tractors.

Now, I find both those websites really tedious to try and search with their search functions, so that sucker could be there somewhere. Got a direct link?

I'm about to get around to testing more 3-bladers and would like to know if I've missed any other Graupners than I found on those sites.

da Rock

Anybody know of ANY 3-blade pushers other than the couple of Graupners and handful of Master Airscrews?