Ok guys, I have a st40 ringed engine. The prop sizes that ST. gives is a 9x7, 9x8, 10x5, 10x6, 10x7, 11x4, and 11x5. My question is witch size will give me the best results. I need the most or best power I can get out of this engine, the engine is a little under sized for the project.. thanks again.

It really depends on what you are trying to power with it. A 10x5 is the middle of the road, and will probably make your machine fly, but it probably is just a good starting point, and what you end up with at the end could change quite a bit. The good thing is that all the props in this size range are rather inexpensive, so experimentation will be fairly inexpensive at about 2 - 3 $ per prop. Good luck!

If the engine is a little undersized than you probably want to prop the engine for the most thrust, i run a 11x5 on my TT PRO.35 and OS .40LA for best results, so would say either the 11x4 or 11x5.

First of all, I always get the best ''power" with A.P.C. props. They're simply more efficient. We
really need to know what airplane you are using. For example, if you are flying a high-winged
trainer with a flat-bottomed airfoil, you'll want a lower pitched prop for slower flight and landing
speeds. Most novice pilots try to land too fast! I know it can seem a bit complicated and it is
when you are trying to maximize performance. Think of it this way; lower pitch is like a lower
gear in your car, better acceleration but lower top speed and visa- versa. O..K? Don’t forget to
adjust the prop diameter to maintain a similar load on your engine. Good luck and fly safely!

You are correct.
Higher diameter and lower pitch=more torque and slower speed.
Smaller Diameter and higher pitch=less torque and more speed.
I don't know exactly what you mean by most power but I would guess you want more torque to get a bigger plane off the ground so I would suggest the larger diameter prop with lower pitch.

I've found my Super Tiger GS-40 (the ringed version) runs good on a MAS 10 X 6, even better on a Xoar 10 X 6. It'll turn just under 13,600 all day long on that prop. Guys I know who run 9 X 6 are getting closer to 14, 500 out of theirs. If you are flying a trainer, then go with an 11 X 5 to keep the speed down, but for sport a 10 X 6 will work good. I also run 10% - 15% fuel with no issues.

Hogflyer.

I have a OS.91 FX with a pitts style muffler in a Minnow. The .91 lists 2,000-16,000 rpm and several prop/rpm stats at the towerhobbies web site. A couple examples are 13x10 @ 9500 rpm or 14x10 @ 8400 rpm or 14x8 @ 9500 rpm. I am wanting as much speed as I can get so which prop best suits my needs? or should I drop down to a something like a 12x12 or 12x10 to get towards the high end of the .91 rpm range? & would a tuned pipe help as well?

I don't see how anyone can recommend a prop without knowing anything about the airplane.

Exactly, weight of the plane and how you want to fly the aircraft plays a very big part of the equation.

8.5 lbs, 775 sq/in wing area, 25 oz/sq.ft. wing loading are the specs listed witht the plane. It isn't really a matter of my plane, just in basic performance, what gives faster speeds? ex. Say a 14x8 spinning 10,000 rpm makes a plane go 50 mph. Then say a 12x10 spins about 10,000 rpm also Which is faster? Kind of weird question I know. It may not have a simple anwser. Is the 12x10 faster because of the greater pitch even though it is the same rpm?

It is more than just that.
How fast the engine will come up to speed also comes into play. Yes you may be able to clock a prop on the ground with a tach but in the air is a different matter all together. Some combos will not be able to hit top speed before the airplane is already too far away and you need to turn.
All that said there is a simple rule that has proven true time after time.
Higher diameter and lower pitch results in slower flying speed but better climbing ability, good for aerobatics.
Smaller diameter and higher pitch is for speed, good for stuff like pylon racing.
Any specific size for what you want to do depends on the specific engine and airplane combo but engine maufacturers recommendations are a good place to start.

Sounds reasonable enough Missileman. I guess its kind of the try and see method really. I think I'll start with a 13x8 and see what happens, and then try props around that size and see what works best like always. Thanks for your inputs. Sorry I had such an odd question, trying to take some of the trial/error out of getting the best prop.

You won't need a tach, maybe an hourglass.

When choosing a model airplane propeller you must keep in mind that you are choosing the propeller based on how you want the airplane to fly. This really has nothing to do with the engine other than the fact that you must stay within the recommended window of props to prevent damaging it. The same engine used on two different airplanes may be using completely different propellers. If you have an airplane with low drag designed for speed then you will want more pitch. If you have a slow airplane with a lot of drag, such as a bi-plane, you will want more diameter (thrust) and less pitch (speed).

[link=http://www.hooked-on-rc-airplanes.com/model-airplane-propellers.html]choosing the right size propeller for your airplane[/link]

As others mentioned, there is no simple answer. You may have to try several props to find the one that YOU like best on your combination.

You can download ThrustHP from http://www.hoppenbrouwer-home.nl/ika...rusthpv20d.htm

It is a little software that estimates speed, static thrust, horsepower, etc.

It is not the holy grail of prop analysis, but it gives some numbers that could possibly be useful for comparisons.

In theory, using your example, a ** x 8 @ 10000 = 75.76 mph while a ** x 10 @ 10000 = 94.70 mph. This assumes perfect efficiency and no losses due to drag.