Click image to enlarge.

A typical propeller adapter shown installed on a larger SPEED-600 ferrite electric motor. This particular adapter uses the collet-type method of attachment.

The screwdriver points to a propeller adapter installed on a tiny MTM Colobri S-050 electric motor. This adapter is held in place with a set screw.

A propeller adapter sold by E-Cubed RC that is intended for a 2 mm motor shaft. You can clearly see the collet at the extreme right in this photo. Notice the price is only $3.

This is the same propeller adapter as in the previous photo but it's fully assembled and is ready to slip onto the motor shaft.

Q-29: I keep seeing references to "prop adapters" with regard to the electric motors used to fly model aircraft. Can you tell me why they are needed and where I can purchase them?

A-29: Electric motors in general have rather slender (small diameter) output shafts. Mounting a propeller to an electric motor shaft is not a "direct job" as it is with a glow-fueled engine. The only way to mount a propeller with a relatively large hole to an electric motor shaft is with what we call a "prop adapter."

Many electric motors sold today, especially the more premium brushless types, come with the appropriate propeller adapter. The inexpensive ferrite, brushed motors hardly even come with the necessary adapter and that becomes your problem. The idea behind the adapter is to have it slip over or attach to the motor shaft. One type of adapter uses a simple set screw to fasten it to the motor shaft. Other popular adapters use a collet-type approach, which means it grips the shaft when you tighten down on the propeller nut.

When purchasing a propeller adapter, the first consideration is the diameter of the electric motor shaft. You must choose the correct size or you will end up with a "wobbling" propeller. The other consideration for a propeller adapter is having an output shaft that the propeller can slip over easily. This isn’t quite as important since the propeller hole can be enlarged with a drill or reamer. If the propeller hole is too big, an insert can be used to reduce the hole size. Many propeller manufacturers—especially APC—provide such inserts for the modeler's convenience.

The use of a propeller adapter should not be treated too casually. If the adapter uses a set screw, it is a good idea to file a small flat on the motor shaft and allow the set screw to rest on the flat. That will hold the adapter tightly to the shaft without any chance of it slipping.

Adapters that use the collet-type approach have to have tight propeller nuts. I’ve seen several instances where the propeller adapter became slightly loose, but it was hardly noticeable. One reader mentioned that he wasn’t getting the power level I described in one of my published design articles. I suggested that his propeller adapter might be loose and as a result the propeller was unable to turn at the full rpm. He tightened the adapter and the airplane flew great from then on. Make sure your propeller is tightly attached.

By the way, there are several sources for propeller adapters including Azarr at E-Cubed RC (www.ecubedrc.com/propsandadapters.htm), Hobby Lobby International (www.hobby-lobby.com), Kirk Massey at New Creations RC ([936]-856-4630), and Maxx Products International (www.maxxprod.com). There are others, of course, but this will give you a start!

—Bob Aberle