First of all, think about the numbers when you are looking at props. For example: 12-5. This is a 12 inch diameter prop with a pitch of 5 inches. The diameter is self explanatory. The pitch means that, in a perfect world, one complete rotation of the prop will move 5 inches of air through it, or it will move 5 inches forward (if turning in the right direction).

Now consider the transmission in your car. The lower the pitch, the less air it will grab or move, but it will have more power to pull something through the air because it will rotate faster, similar to lower gears on your car. If you want to climb a steep hill, you put your car into a lower gear. The motor turns at a higher RPM for a given speed, thus more torque, more energy passed on to the wheels through the lower gear, but the tires will turn slower than if you were in a higher gear.

Same with the prop. SO, if you want 3D, you will want a lot of power to hang the plane on the prop, so a lower pitch would be necessary. If you want a higher top end speed, then a higher pitch is what's needed.

This is a very basic description of what happens. There are limitations, of course, like maximum RPM capabilities of the particular engine, and of course the capability of that engine to provide the necessary power to do what you want it to do. Of course, increase displacement (increase engine size) and you get more power, but that is at a sacrafice to a lower RPM range. However, you can get more power at lower RPM's so you can prop it accordingly.

Someone already did most of the work in prop selection. There are charts that show ranges for props vs. engines, that will range from 3D performance to speed performance.

But, always remember one big point. Noise does not equate to power. So, if it makes a lot of noise, rotates at 16,000 RPM, the only thing you are doing is annoying people around you and destroying that engine. In other words, prop according to specs for the engine and don't exceed practical... PRACTICAL rpm ranges for your engine. Those numbers, such as 3.1hp at 17,000 RPM are nothing but numbers that mean absolutely nothing because you will never run the engine at those numbers.


Check this link by Top Flite. It provides a typical prop chart for various size engines.

http://www.top-flite.com/accys/topq5000a.html

CGr.