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From the Suffolk Aeromodelers, Long Island, New York

Sizing the Model Airplane Propeller

originally from Hooked-on-rc-airplanes.com

The manual for every engine will give you a range of propellers that is safe to use with that engine. The manual does not specify the exact size propeller because the propellers must be sized for the airplane they are used with. It is very important to stay within this recommended range.

You can also refer to the Top Flite propeller selection chart below to determine the range of propeller sizes that are acceptable for your engine size. Keep in mind that the Top Flite chart is sized for 2-stroke engines. Consult the manual for 4-stroke propeller sizes because these engines produce more torque at the slower speeds and will use a larger propeller.

The propeller puts a “load” on the engine. If the load is too small or too large it will damage the engine. You must choose a propeller within the recommended range that best suits your airplane and your flying style.

The characteristics of a propeller are defined by the diameter and the pitch. The diameter is the distance from one tip to the other. The pitch is defined as the distance the propeller would move the airplane forward in one rotation in a “perfect” world. Perfect world meaning that the propeller is 100% efficient and the air does not compress; neither of which is practical in the real world.

The “twist” of the propeller is what determines the pitch. Basically the length of the propeller and its twist defines its characteristics.

A model airplane propeller size is always referred to as its diameter x pitch. An 11-inch-diameter propeller with a 6-inch pitch is called an 11 x 6 propeller.

Generally speaking, the larger the diameter of the propeller the more thrust will be produced by the engine. The larger the pitch the more speed you will get out of your engine. A small diameter, larger pitch propeller will move a small volume of air really fast. A large diameter small pitch propeller will move a large volume of air at a slower speed.

Increasing either the pitch or the diameter puts a larger load on the engine. To keep the proper load on the engine, you generally change the pitch and diameter together. For example, 9 x 7, 10 x 6, and 11 x 5 propellers would all put a very similar load on the engine.

If you want to change the maximum RPM, then you change the load on the engine. Replacing a 11 x 6 propeller with a 10 x 6 propeller, or replacing an 11 x 6 propeller with an 11 x 5 will decrease the load on the engine and raise the maximum RPM. Changing from a 10 x 6 to a 10 x 7 propeller, or changing from a 10 x 6 to an 11 x 6 will increase the load and lower the maximum RPM.

If the propeller load is too large, the engine will not turn fast enough to fly the airplane and could cause the engine to overheat. If the load is too small the engine will turn too fast damaging the engine. So it is important to stay within the window recommended by the engine manufacturer. It is also important to know that the engine must be tuned each time the propeller size is changed because of the change in load the engine sees.

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 propellers to prevent damaging it. The same engine used on two different airplanes may be using two completely different propellers. If you have 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 biplane, you will want more diameter (thrust) and less pitch (speed).

Choosing a propeller that best fits your airplane and your flying style is a trial-and-error process. Pick up several propellers within the recommendation range. If your airplane seems too sluggish when taking off and accelerating, then change to a lower pitch, larger diameter propeller. If your airplane has plenty of pep and you want to make it go faster, then change to a larger pitch, smaller diameter propeller. It’s really fun to experiment with different propellers and observe how the airplane reacts. Q
 

How to use the chart to find the right propeller for your engine:

1. Find your engine size along the bottom axis.
2. Follow the line to where it intersects with the shaded area.
3. Follow each point within the shaded area to its corresponding propeller size on the left axis. This will be your approximate propeller range.

Note: Four-cycle engines are typically higher torque engines and should use the larger propellers indicated in the range. Recommended propeller ranges will vary depending on your particular engine and airplane. This chart represents average propeller usage and should be used only as a general guideline. Always refer to the manufacturer’s instructions included with your engine.

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