- Dec 26, 2021
- Reaction score
- Kawerau, New Zealand
If I built a tailsitter, then 2x 250 watt electric motors running 7 inch props would be sufficient."These engines are no good to me unless I can build a contra-rotating prop drive, for a vertical takeoff aircraft."
You do not need a contra rotating prop for a VTOL prop driven model aircraft, just go on the web and see the 1000's of youngsters prop hanging electric RC models .
I would suggest that you stand less chance of VTOL with a contra rotating prop just due to the weight complexities and complications in the system.
Using these largish petrol engines, the prop torque is quite high.
Possibly a "Monocopter" layout would work, with large deflector vanes.
I have seen it done in electric.
Altitude control would be problematic, as the petrol engine does not provide the rpm control
that electrics are quite good at.
Additional fast-acting thrust deflectors would be needed.
An engine under 2 kgs has over 10 kgs static thrust, so there is a lot of reserve.
Possibly a reversing system with some acetyl gears would be light enough- maybe another 2 kgs.
I have one designed, but I need a better lathe and possibly a mill to make it.
For satisfactory lift performance, at least 1.5 to one thrust to weight is needed to gain altitude.
1.8:1 would be better.
Contra-rotation is more efficient at thrust to power ratio so that makes up for some weight gain.
The front support bearing runs at 12-13,000 rpm, and possibly needs a circulating oil system to control heat build up.
I would also want to fit an electric start, at about 500g .
The whole assembly extends about 160mm out from the normal prop position, so the gear case needs a lot of bracing back to the engine.
I have an electric start system on hand, but it is not a simple fit to the engine, and it gets in the way of the main case for the contra-rotating gearbox, and makes a larger diameter casing necessary.
There is a large steel gear that sits behind the prop drive flange, on a one-way freewheel.
The gear has large holes in it and is quite narrow, so it is not that heavy, though.
You don't want to handle an engine setup with a vertical axis with 20 inch props while it is running!