View Full Version : Lift engines - torque convertors?

profqwerty - n/a
2-Aug-07, 07:48 PM

Is it possible to use a small motor bike engine as the lift engine? I assume it's possible to turn a 2 stroke one on its side and rotate the carb upright?

How would these perform? I've never used bike engines before - are they simple to hoverify (remove gearbox and use PTO and the like?) Also it would have to be air cooled! What sort of bike size is useful though - are they lightweight?

Thanks for the help!


Hornet - n/a
3-Aug-07, 10:16 AM
It can and has been done but its a fair bit of work to convert a bike engine. You will need to remove the gearbox internals and also get drive from the crankshaft. Something like a 250cc single cyclinder 2 stroke from trail bike would work well. These engines usually put out between 23hp to 30hp. Only problem might be that a lot of bikes in this size have been heading down the 4 stroke route for a while. You will also need to adapt some type of starting mechanism for the engine.

kevinf - n/a
3-Aug-07, 10:17 AM
Hi Toby

The simple answer is yes, they can be and are used. I used a Yamaha DT250 for lift in an F1 craft many years ago, I'm pretty sure Fred Wilson uses a 100cc Yamaha bike engine for lift, and Bryan White has done the same. Many of the Swedish drivers use high powered mini-moto engines.

Hoverising the engines is easy, as long as you stick to 2 strokes. Tap a connection into the crankcase or the carb inlet for the vacuum fuel pump. They don't have to be air cooled either, there is no reason why you cannot put a radiator under the fan. The biggest problem is usually starting them, you will need to fit a pullstart cup to the flywheel for a rope start. Pullstarts are possible but generally need to be anticlockwise like seagull outboards.


u415276 - n/a
3-Aug-07, 11:28 AM
Bike engines have been used many times with varying success.

We used a DT175 (aircooled trail based bike engine) with direct drive (still have it in the loft) about 10 years ago. Others have used more modern air and water cooled engines (moped, minimoto and bike engines) including the TZR125. Main prblem is matching the fan and engine speeds.

Direct drive is the simplest and cheapest approach but the engine ends up running slower than design and / or the fan a bit too fast.

The most succesfull approach (in terms of lift for racing) has been to run a belt drive to allow the engine to make power at high revs but turn a slower fan. This is more complicated and expensive, but seams to work well.


profqwerty - n/a
3-Aug-07, 01:06 PM
The Kawasaki KDX200 seems to be ok - it's 37hp @8000rpm, so if it's run slower it would be ok, juts use lots of fan blades (550mm duct)

as you said, matching speeds - could the gearbox be left on, and the chain drive output be connected to a fan hub?

What about fiddling the gears inside to just have the one set - how complicated are bike gear boxes?

How much does the avaerage small bike engine weigh - the specs only give the bike's gross weight.


Paul Fitz - n/a
4-Aug-07, 02:25 PM
A very crude calculation shows that although the KDX200 engine produces around 38 Hp at 8000rpm (I assume this is PS), at 4000 rpm (a rough estimate of required fan speed) this could be as little as just 8 PS due to the very peaky nature of the output. If more power than this is required to maintain the cushion a belt drive would be needed as Dan T suggests.

The problem with retaining the gearbox. - most gearboxes run with oil in the 'sump' formed at the bottom of the gearbox chamber. Turn this on its side and the oil forms a pressurised pool around the oil seal on the output shaft. These seals are not designed to be permanently immersed in hot oil and will invariably fail, with a very messy outcome. http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

profqwerty - n/a
5-Aug-07, 01:09 PM
How well would an Aprilia rs 125 engine work with this? - there's quite a few cheap ones around:)

It's 39 bhp and electric start:)

I see the problem with the gearboxes - what's a usual lifespan of this condition though?

How does the oil get round these gearboxes - is it a gear in the sump splashing oil, or a pump?

Would it be overkill to use one of these horizontally and use a 90 degree shaft link?

It's just not really worth having a whole belt drive mount on the front - it would be easier just to use a 4 stroke tractor engine!

profqwerty - n/a
6-Aug-07, 09:02 PM
What if I put a small torque convertor onto the engine?

The normal output shaft would be conneted to the crankshaft, and the casing connected to the fan, so the fan could turn at whatever speed the blade engle permits, absorbing the power of the crankshaft at it's speed?

Could be worth a try, but how much do these things weigh?