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Old Jan 18, 2012, 05:57 PM
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Joined Jan 2011
11 Posts
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Torque Roll

Anyone have problems with torque roll on their hovercraft?

I have a 2' x 2' skirted hovercraft I made about a year ago that had a serious problem with turning right.

On the street I had less of a problem with this but on the water I had to be full left rudder in order to go straight. When I went full right rudder it would spin out in an incredibly fast turn. (More drag on water?)

The thing is, the way the prop spins you would expect the torque to be pushing the left side down.

I read about a problem someone had with a gyrocopter where the prop wash hitting the prop wash stabilizers would cause torque. That would cause a right lean.

Any ideas?

I ask because I have the bug again and am planning to build another one but if I can't figure this problem out I'll go simpler this time. Sort of a Axon Razor back EDF style and give up on water.

I do have some counter rotating props for a dual pusher style though, assuming that counter rotating props would cancel the torque lean. I want to be sure though.

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Old Jan 19, 2012, 09:41 AM
MHA webmaster
United Kingdom, England, Reading
Joined Mar 2011
206 Posts
Have you considered flow straighteners and adding weights on one side to compensate? Also I notice that the air enters the plenum at the back immediately under the fan - you may find it is a bit nose heavy as a result. If you can change the flow ducting so the air distribution is more even it may help with stability and steering. Finally pay attention to the skirt as any ruckles or unevenness can upset steering and drivability.
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Old Jan 19, 2012, 06:34 PM
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Joined Jan 2011
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Here's some video so you can see how it drives on water.


Hovercraft Right Turn on Water.avi (1 min 59 sec)


It doesn't seem to have much of a balance problem forward and back. I figured that had to do with the low loading and the weight of the fan and housing. The Fan housing is aluminum wrapped in fiberglass. Running a 4000mah 3S lipo in that front bay.

The reason the duct is in the back is that there is a counterpart duct in the plenum. I could maybe duct it back to the center or spread it out with ducts but I don't have very much clearance. I seem to remember blowing up the skirt design 4X normal to get that couple of inches clearance.


I could try adding weight to the left side to counter the torque but I would rather figure out how to get rid of it.

Angling the flow straighteners to counteract the torque would probably be the best solution. Still, I'd like to know the definite cause of the torque so that I can minimize it if possible.
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Old Jan 20, 2012, 11:51 AM
MHA webmaster
United Kingdom, England, Reading
Joined Mar 2011
206 Posts
It does look to be heavy on the right side, which is probably a result of the prop torque. The only other way to reduce the torque is to go for a much lower pitch prop and up the revs to compensate, but it's usually a case of trial and error to get the best prop/motor combination. I know of someone in the MHA who has a large IC powered model hovercraft and he has to weight it on one side to compensate for the torque problem, so it may be something you just have to live with. I didn't see flow straighteners on yours unles you mean the struts in the back of the duct, but if so they are too far away from the prop to be effective - they need to be right next to the prop to work.
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Old Jan 20, 2012, 01:56 PM
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Joined Jan 2011
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Thanks a lot for the advice.

I'll try putting some flow straighteners right behind the prop and see what happens. I like this idea best because as the throttle and torque increase so does the compensation. If that's not enough I'll just weight it.

When I made this I was just getting into RC so I didn't have that many parts and had to fabricate everything. Today I have motor mounts and hinges etc.


From what you guys have told me I feel confident in making another hovercraft.

I did buy some squirrel cage fans for a second hovercraft but my welding friend said he couldn't figure out a way to couple a motor to it. He didn't think he could weld it true enough. I might try taking the center fan out and see if the shaft that goes into the computer fan motor is long enough to get some sort of couple on.

I also got big counter rotating props. The small prop, high kv idea sounds good but for what I want to do, water and rough ground, higher torque on your prop is better right? Usually a small prop is better for high top speed, not raw push.
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