Hey all, I'm going to buy an OS 55AX and I want to know what the best speed and 3D props are for this engine? I'm at 670 ft. above sea level(I've been told that affects performance). I'm giong to put it in a H9 Twist 40 and I'd like a good prop for hovering and one for scooting around fast doing rolls and loops. I currently own an 11x7 prop for my old engine, and was wondering if that would also work for this new one?

I use a 13x4w and a 12.25x3.75 on my katana profile. I like the 12.25x3.75 better.

You won't notice any problems at all at 670 ft. Not a bit.
Also I would think a 13x4 would be worth a try on that engine.

For 3D I've tried the APC 12x4, 13x4, and 13x4w on my 55AX in a profile Katana. I like the 13x4 best.

I've tried both of these sizes in my Reactor, although the 13x4 I used was a Pro Zinger. The 12.25x3.75W APC definitely has a lot more thrust, so that is what I am currently using. Unlike the 120AX, the 55AX has its peak power at high rpm, so to get maximum thrust you want a relatively small prop.

My Reactor really likes to tip over on takeoff/landing so using the 13x4 wood prop would also get expensive.

Malcolm