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teslaguy

Sat Jul 11 2009, 07:55AM

Registered Member #617 Joined: Fri Mar 30 2007, 07:29PM
Location: Buffalo NY
Posts: 49

I have been planning on building a small remote control hovercraft for a while and I finally picked some nice DC brushless motors that can pull 41 Amps at 14 Volts. I am planning on incorporating three of them in my design, my concerns how to power them, I dont think a standard 14.8v 4000mah lipo battery can take 1.7kW without melting, and if it could it would only power the craft for about 2 minutes at full power. I suppose that my main questions are: Does anyone know of a good book on hovercraft physics or anything about dealing with load balancing in hovercraft, and what should I do for power? I think that I will be able to lift a small motorcycle battery as a worst case scenario but I would prefer to have something much lighter with a fast recharge time and more cycles. Thank you.

Bjørn

Sat Jul 11 2009, 12:43PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

The most sensible option is lithium unless you want to save money and settle for low performance.

30 C, 4 Ah lithium battery packs can take 120 A discharge currents, which turns out at about 1700 W. A better option would be to go for higher voltages or several battery packs.

Most things you need to know can be found here:

Sulaiman

Sat Jul 11 2009, 12:50PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

A colleague of mine is into R/C and this is info I remember from him;
LiPo batteries should not be pushed beyond 20.C and 10.C is 'safe'
so a 1 A.h cell can supply 10A to 20A.
I suggest that you buy battery packs designed/sold for R/C
OR
LiPo battery packs that were for older laptops that many places are selling off NOS cheaply.
e.g. I recently bought a 14v4 4.4 A.h pack for GBP6 ! from CPC.Farnell
Take out the fuse and electronics for a basic battery pack.

Steve Conner

Sat Jul 11 2009, 05:54PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

When I was at high school, I built a RC hovercraft out of a 18" square lump of styrofoam for a hull, a trash bag for a skirt, and two 540 size RC car motors fitted with model aircraft propellers. One blew down through a hole in the foam to provide lift, the other was mounted on a swivelling pylon at the back to provide thrust and steering. The whole lot was powered by two ordinary 6 cell RC car packs, one for each motor.

Using a propeller as a lift fan was probably quite inefficient, a centrifugal blower would have worked better. The lift motor battery lasted less than 5 minutes. But my point is that I don't think it takes kilowatts to do this, those 540 motors are less than 100 watts, and my craft worked fine.

In case anyone wonders, it died when I crashed it into a wall at high speed, causing the thrust motor to break off its pylon, fall into the electronics and shred them. It went fast, but it was really hard to steer.

teslaguy

Sat Jul 11 2009, 09:00PM

Registered Member #617 Joined: Fri Mar 30 2007, 07:29PM
Location: Buffalo NY
Posts: 49

BjÃ¸rn, thank you very much for a link to that forum, I will look through it.

Sulaiman thank you as well, I may take your suggestion.

Steve, You have been an idol of mine ever since I started building tesla coils. Thank you for sharing your experience. I have built a couple RC hovercraft using a 24" diameter peice of foam with a 140mm computer fan for lift and 2 140mm fans for thrust which have worked well, with a 14.4v 2000mah battery I am able to use the craft for over 30 minutes easily, but I found these motors which I couldn't pass up. I may try to find a cheap centrifugal fan for lift, but like almost all other members on this forum I want to take an idea and design something that's completely ridiculous with it. I am aware of how hard it is to pilot a hovercraft and that I may not actually be able to use those fans at full power without destroying my craft, but I want to try.

I may mount just one of the fans for thrust on a servo capable of 360 degree rotation and see how that works instead of using two for thrust but I am going to find out the hard way. I will update you guys with my decisions.

Everyone, Thank you once again.

Steve Conner

Sun Jul 12 2009, 06:05PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hey Teslaguy,

Those ducted fan units look sweet. If I were you I'd just mount two of them on the back and use tank-style steering, and have a smaller fan for lift. Or even a third one of those mega fans running at reduced power.

Reduced power is the key. Your batteries might not be able to crank 120 amps to power all three motors fully, but the motors are hopefully so powerful that, as you said, you can't use full power because it'll flip your craft into a tree or tear it in half. Although I believe the Li-po batteries meant specially for model aircraft (Kokam etc.) are pretty powerful, and of course there are the new nanophosphate cells that DeWalt use in their power tools now, also found in the Killa-Cycle electric drag bike, and maybe available to hardcore RC hobbyists if you're really lucky

Wolfram

Sun Jul 12 2009, 08:05PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Those nanophosphate cells that Steve is talking about are indeed available to RC hobbyists. See here

for example. Check out the other stuff on that page as well, I believe they are quite cheap compared to many other RC model vendors.

Anders M.

teslaguy

Mon Jul 13 2009, 03:35AM

Registered Member #617 Joined: Fri Mar 30 2007, 07:29PM
Location: Buffalo NY
Posts: 49

Steve: That is exactly what I was planning on doing. I will look into nanophosphate cells, thank you once again. I am aware that I will not be running the craft at full power for the majority of the time and that I probably dont need 120 amps but I would like something that wont melt or burst into flames incase I somehow pull that. I have used model aircraft lipo batteries on almost everything else I have built and they seem to work wonderfully but I still dont think that a 14.4v 4000mah will be adequate.

One of the major problems that I am going to have with this project is balancing the craft, which is always hard to overcome, especially when you only have one central lift motor. The easy way of dealing with balancing I think has always been to turn your lift fan down enough that your craft wont spin out of control on its own but that often means that part of your skirt will actually touch the ground. I am contemplating building a stabilization system similar to the one used in Avrocars, but I may settle. I would love to build a circular craft on account of perimeter to area ratios but rotational momentum can be a horrible problem to overcome. I dont want this hovercraft to turn into a freak autogyro. I need to finish building the electronics but when I do I will get back to you.

Anders: Thank you for the link, they seem to have great prices.

Thank you once again everybody.

Steve Conner

Mon Jul 13 2009, 10:09AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

For balancing, all I needed was a weight on the front to counterbalance the weight of the motor and RC control gear at the back. If I'd thought, I would have moved the battery packs to the front, but I put them in the middle, so I ended up having to add about 300 grams of ballast. I just placed it by trial and error.

My craft never seemed to spin from lift fan torque. If it did, I'd look at putting vanes into the plenum chamber to take the swirl out of the airflow from the lift fan. The skirt always does practically touch the ground, which is why you make it thin and flexible. It radically reduces the amount of lift fan power you need to get hovering, and lets you get over bad terrain by lifting the hull an inch or two off the ground, deforming around rocks and so on.

aonomus

Mon Jul 13 2009, 03:53PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

If you have the money, look for the Dewalt tool brand's battery packs, they have 10 large LiFePO4 cells inside (they are A123s cells) and can sometimes be had cheaper than the cell kits from A123 systems, and come with the balancer/charge controller all internally contained (so all you need to do is find a charging base). Alternatively you could take the cells apart and repackage them into smaller packs, but then you need the charger.

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