Contra rotating props - interesting?

[norman]

YouTube link.

[martinbg]

It's something that we have been looking into but is on the back burner at the moment.

The props stopped very quick when the throttle was closed. A bit too much drag from the gearbox maybe? I would have expected them to do a couple of turns before stopping due to the inertia of the blades. Although the inertia will be a lot less as I estimate that they were much smaller in diameter than the original (Reference the cage diameter).

[norman]

Yes Martin, I wonder at the interaction between flows off both props too... Mind you, he seemed to get airborne fairly quickly.

I wonder how much power the gearbox itself absorbs; his motor didn't look massive... interesting tho' isn't it?

[martinbg]

Hi Norman

If you read the comments below the video the pilot admits to putting in too little oil, so the losses through the gearbox would get considerably higher as it got hotter.

As for the interaction between props. - I senced a prop "beat" like you get on a twin engine when the rpm isn't matched. on the motor in the video the rpm should be matched but each time the props pass each other it will have an effect. It's not recommended that you use a two blade metal prop on a rutan canard design because as the blades pass the trailing edge at the same time (opposite sides obviously) it sets up a prop vibration. I believe they either fit wooden, composite or 3 blade ones to overcome this. I suspect on the paramotor the props might benefit from a wider spacing axially, there are a few aircraft with counter rotating props to take an educated guess at a good spacing.

I didn't think that he looked that big either. The motor was quite a small machine. But it certainly worked on small prop diameters. Trouble with really small props is they get shielded by fat wotsits like me.....

As you say, very interesting.

Martin

[t_andrews]

I traded some mail with this fellow, who didn't seem to have much interest in making these for any commercial endeavor and suggested I ask a capable machinist to do it for me

He did note that he was experimenting with differently pitched props to modify the thrust and audio signature.

I expect it's come a long way since I saw his mail, but certainly a worthy result. Torque steer removal and added thrust as a result I expect despite gearbox losses.

[pete_b]

Miniplane with Top 80 engine

Pete b

[Guest]

i too mailed this guy some time ago and he didnt seem bothered bout doing any more