Sorry to keep bothering everyone with my questions, but I need to learn about correct prop selection, without buying a whole heap and experimenting to much and not knowing what I am doing. It seems for 3D you want a bigger diameter prop with less pitch? Why is this?

And for speed you want less diameter and more pitch, which I can kind of understand for resistance and RPM reasons, or am I completely off track?

Any advise would be greatly appreciated, specifically pertaining to 55AX and ST G-51

Thankyou

you are right with a larger prop and a lower pitch will slow you down and give you good vertical......some props for those engines could be a 12x4 12x5 and a 13x4

Propellors are a bit like the gears in your car.

A larger diameter with smaller pitch is kind of like first-gear: lots of pull but not much speed.

As you increase the pitch and reduce the diameter (so as to keep the RPMs the same), you go faster but have less acceleration or pull.

If you've every tried to pull away from a stop in top gear or try to break the speed limit in first gear you'll know how important using the right gear (prop) really is.

For 3D (where you spend a lot of time flying slowly or not moving at all (hovering) you want to be in first-gear all the time so you use a large diameter prop with a small pitch. As well as providing maximum static thrust (ie: best hovering and pull-out) a prop like this also helps avoid the situation where you fly so fast that the wings (or control surfaces) flutter off your plane.

On a 45-55 sized engine you should try props ranging from 11.5x4W to 12.25x.3.75W. My personal favorite is the APC 12x4 but that's on a TT46Pro which likes to rev a little more than the ST 51. The only way to find out what is best for your engine and airframe is to buy one of each (11.5x4W, 12x4, 12.5x3.75W) and try them.

APC seem to be the most efficient props in these sizes and efficiency = better flying.

However, if you're still at the stage where not all of your landings are "good" ones, you might want to consider using a less fragile prop such as a Master Airscrew. They have a 12x4 in their K series which provides a pretty good compromise between performance and durability on a 3D bird.

Prop selection is fairly simple. You want props that let the engine wind up to the rpm that produces best power and, for 3d, a pitch that produces maximum static thrust at that rpm. For example, a 55ax will turn a 12x6 about 12,000 rpm which is good for that engine. So will a 13x4 and a 11x7. For 3d I prefer 4" pitch so the plane won't want to go too fast.Many prefer 6" pitch and I use those on my planes some also. No big differences. If you put a 13x6 on that engine it may reduce the rpm 1000 rpm. The larger prop would produce more thrust if the engine could rev it to 12,000 but it can't and the lower rpm it can generate gives somewhat less thrust. The larger prop will spin up slower when you pop open the throttle. Slow spin up is annoying.
The preceding is an example. I don't know exactly what rpm a 55ax will spin a 12x6 , maybe 12,500. It doesn't matter.
I fly a Mojo with a OS 50 with a 12.25x3.75w, get about 13,000 rpm and 8 lbs thrust. Good enough for me.