Can someone explain how the pitch and length of the prop effect the speed of the plane/engine?

I flew my WM Super Sportster 40S today about a half dozen times, being this is my first plane after the trainer, it is FAST. I am running a 46FX on it, and this baby hums.

I want to slow it down a little, and not just with the throttle wise guys, but with the prop. I put an 11x4 on it, and it helped a little, but should I go with a larger prop with the same pitch? Any help is appreciated.

for a 46 fx, it should run slow enough with a 11x4, but u dont want a prop with little pitch because it will overheat ur engine. But use an 10x6, Thats the one I use on mine, works great. But follow my advice and dont use the 11x4

An 11 X 4 might be a little underpropped on the FX, but I've never had one so not sure.

A 10 X 6 however, will definitely give you more speed, which is what you say you don't want. If you have the ground clearance, I'd suggest trying a 12 X 4.

Not sure why pulling back the throttle isn't sufficient, unless you're looking for a "braking" affect from the prop.
Dennis-

On the .46Fx an APC 12.25X3.75 will safely give you the kind of performance change you are looking for.

John

I second the 12.25x3.75

10x6 all the way

If I have this right, at 100% efficiency, on a 10x6 prop, the plane would move forward 6" for every revolution. Right? If this is so, then I want to go with the lowest pitch possible (Such as the 12.25 x 3.75).

I don't understand how the 11 x 4 could overheat the engine, is this possible?

Yes your description of pitch is correct but of course unattainable. When power off drag is desireable to enable you to control approach better by increaseing the props diameter and reducing the pitch you are in effect reducing the propellor disc loading which will impart greater low speed acceleration (and vertical) but a lower top speed and greater drag power off. Propellor disc loading is the aircrafts weight divided by the total area swept by the propellor.


John

What prop are you using now, you don't say?

You won't hurt the engine with an 11x4, though it might not be enough prop for the engine to turn in the best portion of the torque curve. Basically, the engine would turn up to a higher than necessary RPM. You'll get a lot of noise, and the plane won't fly as well as it might with some other props.

If you put way too small of a prop on an engine, it will, in fact overheat. Basically, the engine tries to over-rev, which leads to too much heat. But you won't get those effects with an OS FX until you really get up there in RPM. www.osengines.com lists the max RPM at 17k, but I suspect you won't hurt anything until you excede 20k (haven't actually tried it with this engine though) (then again, OS engines "used to" have a liner peeling problem, so keeping the RPMS down might be more important on an OS than some other makes).

The 12x4 and the 12.25x3.75 look like the best bets, as others have said.

Btw, sometimes you actually go a bit faster by dropping a bit of pitch. The reason is that engine RPM doesn't remain constant, a lower pitch at the same diameter will allow the engine to turn up to a higher RPM, which may lead to a higher top speed. So you drop the pitch and increase diameter in part to keep the RPM about right. And the extra diameter helps all by itself, as noted above.