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Multi-Blade Propellers

Please note that all props made in this way are at your own risk and that great care should be taken at all times when running. They are best used for static display only and replaced with standard production propellers when being run.

First I only use plastic props NOT glass filled or any other material props as these must not be scratched or cut in any way.

First machine up two dural cups as in Figure 1, the inside hole 3mm smaller thean prop boss for props no larger diameter than 11"

Then cut three slots as Figure 2 the size to suit blades used and radius edges (x) to avoid cutting into blades.

Make a jig to fix prop blades and cut at exactly 120 degrees.

Place the three blades together and measure diameter of three blade boss.

Machine out hole in dural cup 1mm smaller than boss the force the blades into cup.

Then make a flat washer to clamp on prop shaft Figure 3.

The thickness of the cups must be at least 2mm thick and thicker for larger props.

I test the safety of props by running full speed inside a metal or plastic drum as the centrifugal force can be quite high then check they are still tight in the cup.

This CANNOT be done with 4 blades as it really is not safe and I can't guarantee that your work is safe.

As far as making 4 blade props I suggest you read up how they were made in the old days (eg WWI types). I think they were lapped joined EG 2 two bladers.

Please note that all props made in this way are at your own risk and that great care should be taken at all times when running. They are best used for static display only and replaced with standard production propellers when being run.

Bob Lane



With regards to the multi-blade prop holder that Mr. Bob Lane has offered in the hints and tips section, I would add that:

Instead of only using a flat washer to clamp up the three prop blades, one should turn up a "lipped cup washer" (for want of a better description) that fits over the prop holder edges by say 3mm, like a deodorant cap fits over the can edge.

This way there is no way that the flexing of the blades can cause the "wings" of the prop holder to break as a result of fatigue.

I've done exactly what Mr. Lane has done, but with the "lipped cup washer". It has worked for years like that.

Just a saftey thought.

regards,

Kuhn Oberholtzer, Limerick Ireland.

See also: Engines, Propellers, Propeller Sizes