making direct connection with electric moror shaft and prop

[DenisCullinan]

Has anyone ever tried to connect the shaft of an electric motor directly to a prop shaft by using a length of heat-shrink insulation plastic? The idea appeals to me as being really easy, but I'm counting on the experts on the forum to let me know if I'm not totally nuts to have come up with this idea. (I'm a 75% novice, I estimate.)

Thanks everyone,

from Denis Cullinan

[Steve Blanchard]

It won't have enough grip to hold the two together.

[David Lewis]

Silicone fuel tubing might protect the prop better -- more elasticity and lower spring constant. I believe somebody years ago used a rubber band and it worked well.

[DenisCullinan]

Hello Steven and David,

Thanks for your valued input.

I read in some forum where a modeller suggested using a (narrow-diameter) extension spring (see attached pic) as a connector "to reduce vibration." What do you think of this idea?

When all is said and done, what is the usual modeller's solution to connecting the motor shaft to the prop shaft? (As you can see, I'm not much of an engineer.)

Thanks.....................Denis

[goose]

Most of the time, adding anything between the motor and prop is an invitation for making any little vibration into a big oscillation. If the prop is not perfectly balanced, it will oscillate wildly when the motor starts to spin, that is of course, unless the prop is held in place with some sort of bearings to keep it from getting off-axis. Then all you are doing is adding friction, increasing heat, and reducing thrust.

I must ask, Why do you not want to attach the prop directly to the motor? Electric motors for todays RC models either drive the prop directly, or through a reduction gearbox, either way the propshaft is held in place by bearings to keep it running true. If you are concerned about saving the prop in the case of it striking some immobile object, there are "prop savers" that hold the prop on the shaft, but allow it to pop free in case of a hit.

Just curious.

[DenisCullinan]

I have to admit that I haven't done any work with electrics at all. My motors are old very inexpensive things (I bought them quite a while ago) with shafts that are too short make it practical to directly put the prop onto. I think I really have to go and do a little homework on this.

Thanks......................Denis

[David Lewis]

You can use brass or steel tubing to make a motor shaft extension. The small diameter coil tension spring is often used to create an angled shaft arrangement (functions similar to a U-joint).

[David Lewis]

DenisCullinan wrote: "I haven't done any work with electrics at all. My motors are old very inexpensive things (I bought them quite a while ago) with shafts that are too short make it practical to directly put the prop onto."

The efficiency and thrust-to-weight ratio of direct drive can motors is low, so you probably will want to use a geardrive. To calculate the gear ratio, first determine how many cells to use in the battery. Battery voltage should be equal to the highest voltage at which the motor can produce maximum power without overheating.

If the battery has too few cells, you won't get very much thrust, and if the battery has too many cells, it will be unnecessarily heavy and you'll have to be careful on the throttle.