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You'd be forgiven for just seeing your airplane's prop as the thing at the front of the plane that spins very fast, but understanding a bit about how it works is no bad thing.
Propellers for rc airplanes are nothing more than vertically mounted rotating wings. Their job is to convert the engine power in to thrust, to pull/push the plane through the air. Thrust is generated in exactly the same way as lift is by the wing, and that's why props have a profiled airfoil section.
Propellers are designated two measurements, both given in inches...
The first number is the diameter of the imaginary disc ('arc') created by the spinning prop ie the length of prop from tip to tip. The second number is the pitch, and this is the harder of the two to understand - but we'll give it a go...
The pitch measurement of a prop indicates how far that propeller will move through the air per every single revolution of the engine (ie every complete turn of the prop). However, the pitch measurement of your prop must only be taken as a guideline because real-life factors come in to play to influence this distance eg the material of the prop, its condition, air density etc.etc...
So, pitch measurement is really only a theoretical value but is good enough to let you choose the right size propeller for your airplane.
Essentially, the higher the pitch, the faster your plane will go. One way to see it is to imagine the gauge of two different screw threads, coarse and fine, and picture both screws turning at the same speed. The coarse thread will cut in to the material a lot faster than the fine one will. It's the same for propellers.
In the illustration below, the two arrow lines represent the path of each propeller tip. You can see that the higher pitch prop (eg 10x8) takes only one and a half turns to cover the same distance that the lower pitch prop (eg 10x4) takes 3 turns to. So, conversely, with both engines and props spinning at identical RPM, the higher pitch prop will travel further in the same amount of time.
Again, remember that this is only true for a theoretically perfect world!
Selecting the correct propeller size for your gas powered rc airplane is very important, if you want to get the optimum performance from your glow engine. The subject of prop selection can be a bit of a minefield, but there are some common recommendations of prop size for each and every engine.
The first and foremost plan of attack is to follow the airplane manufacturer's recommendations for the plane (and engine) that you have. Failing that, the chart further down the page should help you out
Understanding propeller size
So, you can see that selecting a different pitch prop is going to significantly change your airplane's performance, speed being the biggest factor.
The diameter of the propeller (10" in the example above) will also effect how the airplane flies, but also how the engine runs and, again, following your airplane manufacturer's recommendations is the place to start. Roughly speaking, diameter influences the amount of torque generated, but an ever-increasingly and non-performance related issue these days, linked to prop diameter, is that of noise.
A faster turning prop (and most props spin in excess of 10,000 RPM) generates a lot of noise as the tips cut through the air. In fact, when you hear a gas rc airplane flying, it's more than likely the propeller that you're hearing more than the engine!
So it's important to note that a larger diameter prop reduces the engine's RPM at any given power setting, because there is more for the engine to turn over and hence more work to do. And slower turning props generate less noise - therefore, larger diameter props run quieter than smaller diameter props, all else being equal.
As already mentioned, following the prop size recommendations made by your airplane's/engine's manufacturer should always be your first point of reference. But there are generally recognized prop size ranges for each engine size, and these are the sizes to choose if you're unsure about propeller selection.
The following propeller size chart (© Top Flight, reproduced with permission) is easy to use; select your engine displacement along the bottom scale, then follow the vertical line up to the shaded area, which gives the prop range for that engine
Prop Size Recommendations
