View Full Version : Gearboxes + new hover questions

profqwerty - n/a
28-Jun-07, 06:06 PM
I have a 447 with an unknown gearbox; is there an easy way to find out what it is? It came off of a microlight so i think it may be a high ratio one. I used this with a 750mm duct on the Eagle 1 (RIP), and it was revving about 7000rpm with the blades turned almost 50degrees!

How would a Mirage hull with a belt driven fan from the 447 look? I will upgrade the thrust into something more powerful at a later stage - what weight thrust engine will the Mirage take?

The Mirage is only avaliable with 1100mm duct, so what ratio gears would be recommended? These would be changed at a later date when I put a more powerful engine in...

hovmart - n/a
28-Jun-07, 10:45 PM
its probaly 2.58:1

jon_curtis - n/a
29-Jun-07, 02:08 PM
them gearboxes are not that hard to tell apart! it should have the ratio stamped on it. However if its a oldish points engine it might be A box, they didn't always!


take the plugs out and the starter off, get someone to turn the fan ten rotations, and count how many times the end of the crank turns, / by 10 and you have your gearbox ratio!

I would suspect a lower ratio than 2.58:1 would be required for the 750mm duct setup. probably 2:1 ergo your high reving problem.

with the larger duct, the 2.58 ratio would be more like it. depending on the fan setup

profqwerty - n/a
30-Jun-07, 09:02 AM
Ahh yes, I didn't find that part of their site!

It's an A type box, as it has the adapter plate between gearbox / engine.

If I put the 447 suspended on a frame, with the existing gearbox directly driving a 1100mm fan, will this be OK? (kind of a vague question I know!)

I woulg think the fan blades would be pretty flat, as there is only 40 HP to absorb. The main advantage is that it can be upgraded later.

Would it be better to use a belt drive from the engine without gearbox to a fan, low blade pitch.

when I put a bigger engine in, I can make the blades steeper.

hovmart - n/a
30-Jun-07, 09:17 AM
stay with the gearbox its more relyable and you allready have it

kevinf - n/a
30-Jun-07, 12:30 PM
With a 2.58 reduction you will be getting a fan tip speed of 152m/s at 6800rpm. This is considerably above the recommended maximum of 137m/s in the HCGB construction regs and will inevitably lead to a very noisy fan. If you over speed the engine to 7200rpm or so, you are close to the maximum permissible tip speed for your fan blades (assuming Hasconwing or Multiwing).

I'm assuming you will use a separate lift engine on a Mirage hull as an integrated craft with that large a duct and that power engine would not generate any lift because you would only be able to turn 2 or 3 blades at any sort of angle. Expect thust to be fairly low too.

With any other lower reduction ratios; 2.1 or 2.27, expect disaster. Tip speeds will be way too high.

You would be better off selecting a more appropriate reduction to suit the engine power and a more sensible tip speed and using toothed belt transmission.


profqwerty - n/a
30-Jun-07, 04:13 PM
Yeah; I would like to have the option of easily upgrading the 447 to something much more powerful later, so having the larger duct is useful.

There will be a separate lift engine / system.

I can't really change the gearbox, due to costs.

I worked out ideally the diameter of the fan should be 0.99m, (giving 137m/s tip speed with 2.58:1 gearbox with engine @6800rpm).

Are 1m ducts possible to find?

May be it's better to use a 900mm duct, and slow the fan down by turning the blades more.