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Old Dec 09, 2001, 09:30 PM
maciek is offline
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Hovercraft design - need help


Here are my questions:
1 Does the weight govern the pressure that needs to be produced by the " lifting" fan/prop?
2 If the answer to the above is "yes", what is the relationship between the "pressure produced" and the "pitch speed" (psi/MPH)?
3 Does the amount of air flow depend on the outer perimeter of the hovercraft (the "skirt" length around the hovercraft)?
4 If "yes", what is that relationship?

Required pressure and flow are probably partially dependent on each other. Anyone know the a relationships? I'd like to use my "airplane" stuff to make a model hovercraft. I'd use geared speed400 or speed280. With an 8 cell 1100mah battery the whole thing will come out to be no more than 16oz. Size should be oval, about 12" x16"?
A recent school project got me interested in this. From the problem statement, 0.7" water column was gonna lift an 8lb model, and 300 cu.ft./min was supposed to be just right for a 15" x 20" envelope.
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Old Dec 09, 2001, 10:38 PM
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Hmnnn I think I can answer some of your questions. Most of these answers are based on simple physics/aerodynamics but I can't say I'm an expert in this area.

1. Yes, in theory the pressure required is equal to the weight of the vehicle divided by the skirt area. In reality probably some fudge factor more pressure might be needed. But I don't really know.

2. Again in theory pitch speed or flow velocity makes the dynamic pressure in regards to 1/2*rho*v^2. However, be careful since a too high pitch can result in the prop stalled at the zero inflow case (hovercraft) and drastically less pressure/flow.

3. Yes, but I don't know the exact relationship. Since the skirt circumference length should be proportional to the amount of air lost through the air gap, a larger skirt would require more air volume moved. Note this isn't pressure but sheer volume. Ie a high pressure low volume pump/fan would be useless except in small bursts since the craft would bob up and down...you need enough flow volume to maintain the required pressure below the skirt.

4. So okay if dynamic pressure is equal to 1/2*rho*v^2 and flow volume is equal to the prop disc area multiplied by the flow velocity, what else do you need to know?

All I can say from my experience in a semi-failed attempt at making a hovercraft for a lego robotics lab..using a geared speed 400 and a 9-5 prop way depitched is that some serious engineering or trial and error is in order.

You need to find a prop/compressor that works well at zero inflow ie like a computer cooling fan type prop. Also it should probably be multiple bladed or massively overpowered, this assures that little leakback occurs out the original intake entrance. It should be ducted for that same reason. I tried a prop suspended below a CF plate with inlet holes that comprised the top of the skirt boundary and had massive leakage since I didn't used a duct around the fan. It would only work at light weight and high power settings...ie full throttle 8 cells.

I would find a old style comp fan with METAL blades and power it using a speed 480 or more power. Avoid the plastic ones since they can fragment at high rpms (VERY BAD plastic shrapnel) That way you get a decent prop design and a duct built in with very little work. For the skirt I was using a 20" bicycle tube.

The old tyco hovercrafts used an centrifugal compressor fan ie like a blower (squirrel cage fan??) with better vanes. This prevented leakback and allowed a more efficient design. They also ducted air through the skirt to reduce friction loss from skirt drag on the surface. I was using a filled skirt that remained filled regardless of power.

That's about all I can remember. Goodluck with your project and let me know how it turns out.

-Bernard
Old Dec 09, 2001, 10:54 PM
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C'mon..this certainly isn't Rocket Science.. althouh one certainly could complicate it enough to be something similar..
Just cut'n try it's most of the fun.. I did this when I was in Grade 5!!.. and it worked jest fine thanx.. cut some cardboard and try discovering for your self.. it's most of the fun.
Old Dec 09, 2001, 11:34 PM
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Bare-
I know what you mean, but I wanna actually design this thing. I wanna get rid of the habit of guessing. In a year i should be an engineer. I should at least try to use the education that I' m paying for. The problem is, that I haven't taken Fluids yet. Your experience gives me hope though. Thanks.

Bernard-

This project that I mentioned was for a senior design class. I was given fan-scaling relationships, and the task was to pick the right size fan and the motor for max duration. Now it would be awesome if I could get my hands on some performance curves for these metal comp. cooling fans and for speed 400 or 480 motors, since I already know how to match these components. But it looks like I might not be able to locate those. I just wanna make informed decisions instead of guessing, so that the hovercraft is efficient.
<4. So okay if dynamic pressure is equal to 1/2*rho*v^2 and flow volume is equal to the prop disc area multiplied by the flow velocity, what else do you need to know?> I need to ask my prof. if the figures he gave us could actually make a hovercraft work. Then, the required flow is 300cu.ft./min for 70" circumference. If he just made that up, then it would be nice to know the actual figure.
<They also ducted air through the skirt to reduce friction loss from skirt drag on the surface.> - I don't understand that part.

Thanks for responding. This helps a great deal.

Maciek
Old Dec 10, 2001, 03:21 AM
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There's a load of good information and links available at the RCHovercraft mailing list HQ http://groups.yahoo.com/group/rchovercraft

Don't foget that, for a normal hovercraft, your lift fan is both keeping the skirt inflated and creating the lift pressure underneath the craft inside the skirt.

HTH - Steve
Old Dec 10, 2001, 04:55 PM
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Maciek,

So what are you looking at, Mechanical Engineering? The engineering fields in general are a terrific life choice! I'm a Facilities (Plant) engineer. More of a generalist who knows something of electrical, HVACR, Mechanical, Chemical and Civil engineering without specializing in any of them.

I can understand your desire to apply theory and wanting to see it prove out. Great satisfaction in that, and Francois seems to have helped steer you in the direction you're looking to go.

Can also see what Bare's saying though. Ever watched a television program called "Junkyard Wars"? They pit two teams of 4 against each other. They're assigned a project and given ten hours to construct it out of bits of junk. Good Stuff!

The reason I mention the show and Bare's philosophy together is; one of the shows required the teams to construct hovercraft. The finished projects were run over mud flats (on the coast of Britian I think), and really performed quite well. They didn't use engineering theory.

Anyway, good luck with the project. And if you get it operational and want to show it off, send me an email. I'm not that far away.

Dennis-
Old Dec 10, 2001, 06:37 PM
maciek is offline
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DBCHerry-

Yes, I'm doing mechanical. Do you go to Hog Heaven LHS in Sturbridge?
Old Dec 11, 2001, 08:06 AM
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Maciek,

Yeah, Hog Heaven has become my hobby shop of choice. They have a good selection, and actually pay some attention to electric and park flyers in particular.

There's a guy in Worcester who used to be pretty good; but he's now only open Friday nights 6 - 9! He's strictly IC stuff too.

Used to go to RC Buyers in Nashua, NH.; but that's a haul!

Dennis-
Old Dec 11, 2001, 11:17 AM
maciek is offline
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slewin -

Thanks for that link. Lots of information on that discussion board.
Old Dec 08, 2006, 10:36 PM
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I built this today in about 2 hours.....still need to rig some steering.
Old Dec 08, 2006, 11:27 PM
dnorm is offline
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1.21 Gigawatts!
videos? that thing looks fantastic!!

hats off!

dn
Old Dec 08, 2006, 11:35 PM
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Quote:
Originally Posted by dnorm
videos? that thing looks fantastic!!

hats off!

dn
dude try it it is soooooooooooooo easy!
Old Dec 08, 2006, 11:55 PM
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1.21 Gigawatts!
where'd you get the foam...do you hvae a template and electronics suggestion?

dn
Old Dec 08, 2006, 11:55 PM
dnorm is offline
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1.21 Gigawatts!
do you have vanes on the tail? how do you move left and right?

dn
Old Dec 09, 2006, 01:16 AM
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finished


steering works good!


As far as a template or suggestions on the electronics, I guessed on everytihng I am new at this and just used what was laying around! I used two 300 gws motors


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