What is the best design for a miniature hovercraft for a race?

[ank_gl]

naaaaaaaaaaa, you still didnt get it.
cant a steady flow exist between large inlet cross sections and few small outlet cross sections??

 

[Gambit]

naaaaaaaaaaa, you still didnt get it.
cant a steady flow exist between large inlet cross sections and few small outlet cross sections??

Yes. But I am not able to understand where we will put the small outlet cross-sections when the whole underside of the craft is open to the ground/water. The skirt is only surrounding the craft right? then where shall we put the holes? on the sides? The skirt has nothing to do with enveloping the air beneath the craft. It only acts as a bellowing medium so that sir won't escape from the sides.

 

[Danger]

The skirt has nothing to do with enveloping the air beneath the craft. It only acts as a bellowing medium so that sir won't escape from the sides.

Those statements are mutually contradictory. The second one is correct; the first is absolutely wrong.
Gambit, you don't make holes to let the air escape. You don't want it to escape. That's a natural consequence of the machine regularly and unpredictably losing contact with the ground. The purpose of the skirt is to minimize that by conforming to the shape of the ground far better than a solid wall can. If the air didn't leak out, you could shut off your lift fan once you achieve a hover.
Remember that the lift is accomplished because the air in the plenum is above atmospheric pressure. Since the bottom is open to the ground, it leaks out to seek equilibrium. The lift fan compensates for that.

 

[ank_gl]

think it like this
you have a cuboid, top face is deck where you put your prop and fan, side faces and the bottom faces are the skirt. you put the holes in the bottom face. air leaks out(thrust is proportional to density*velocity^2, you keep the small holes in the bottom), and ,woao, you are airborne.

 

[Danger]

So then the lower part much be exactly even. Like there shouldn't be any curvy parts, then all the all will escape from that portion, won't it? How do we make sure that the air in the skirt escapes evenly around the craft? The skirt must be perfectly made I guess...

I forgot to comment on this in my last post. There should be 'curvy parts'. My drawing is wrong in that regard, due to time constraints in making it and the fact that it's irrelevant for this particular application. For a load-bearing (such as passenger) craft, at the very least you want the front corners rounded. In most, the entire front end is round. That prevents the skirt from snagging on things.
It doesn't matter where the air leaks out, unless it's so unbalanced that it causes a propulsive force. (And, indeed, simply venting the rear of the plenum can serve as your thrust, as mentioned in an earlier post.)

 

[Gambit]

Thank you Danger. Sorry about the late reply. I was a little busy with my work schedule. Anyways, I will be using motors and I understood the mechanism. Now, I only have 1 doubt. About the skirt, Shall I make a skirt with lower portion open to the ground, or the lower portion will only have holes for the air to escape and give the lift? Please clarify on this. Thank you. !

I included two pictures when the hovercraft is over-turned. The Orange colour is the skirt and the blue colour is the hole. In the first, the orange colour skirt's hole is smaller but it is around the hull, hence the up-skirt problem is taken care of. But the air coming out is significantly higher I suppose? Now, in the 2nd pic, where the craft will have holes on the skirt on the underside. From these holes, the craft will atain lift and glide on the ground. Please clarify this to me. Thank you.

 

[gaming_addict]

Hi Gambit, You still might have 'upskirt' problems with the first. Instead of 'upskirt out', it will 'upskirt in' or flutter in. Your second illustration might work better :)

for the skirt if you really want to minimize flutter or flying around, the upper portion should be rigid, polystyrene could be used, the lower mid portion, should be flexible but rigid enough, then finally, a small portion of the lowest part should be very flexible to seal the ground.

If you want higher levitation, you could have double skirt, to create a column of wall of air that acts like an invisible skirt so you can have a more rigid skirt design, but I don't know if it will work the same for small scale designs with low power fans.

 

[Danger]

Gambit, I can see now where some of the misunderstanding is coming from, given the way that you explained your diagram.
Let's get right down to the basics to try eliminating that. To start with, there are no holes in the curtain (or skirt, or whatever you want to call it). The holes that you indicate in your drawings are in the deck, which enter into the plenum. The curtain is the containment material to hold the air within that plenum.
The hole (that's one hole per lift fan, so one in your case) should be just the same diameter as the fan. Where is is placed doesn't matter at all as far as pressurising the plenum is concerned. If you wanted to, you could run a pipe under the curtain from your fan and blow it up from outside. For the sake of balancing and controlling the vehicle, the heavy bits should be near the centre (unless, as previously mentioned, you have to offset a cargo of some sort with it).
It might help if you build yourself a very simple toy to help you see what's going on. Get an aluminum pie pan (the disposable sort) and some light hose such as surgical tubing or aquarium hose. Poke a hole of the appropriate size in the middle of the pan and glue the hose in so that it aims into the 'chamber' of the pan when it's upside-down. If you blow into the other end of the hose, the pan will lift off of the ground. As long as you keep blowing into it, you can poke the pan from any side and it will effortlessly glide in that direction. That is the basis of the hovercraft.

 

[ank_gl]

i did a small scale of this miniature hovercraft(hehe), n i did all the three cases on the bottom part of the skirt viz. the bottom skirt without any holes. with few holes, and without a bottom part. they all worked pretty much same.
holes are only provided to provide an easy escape for the air for the SAFETY REASONS of the skirt(correct me if i am wrong DANGER sir, i guess i am).
i ll see if i can put up a video or a pic of that smaller scale of small scale hovery thing(hehe).

 

[Danger]

Don't call me 'sir'. I'm only 'sir' to people that I don't like.
You are still confusing the hell out of me with all of this talk about holes in the curtain. There shouldn't be any.
Also, there is no safety factor that any number of holes could address. The thing won't pop like a balloon if you pump too much air in; it'll just rise a bit higher until the leak rate again comes into equilibrium with the lift fan output. The only hazard to the curtain is the possibility of running over something sharp that can tear it. (Which is one reason that individual 'mini-bags' are preferred over a single inflated curtain.)
We really have to figure out how to get all 3 of us on the same page; this just isn't working as it should. I'm not giving up on it, but it is getting a bit frustrating.

 

[ank_gl]

its all so much confusing. i am not talking about the real big thing.
i am trying to sit with the Gambit's problem. at that scale, skirt can't be too heavy(that means strength), it ll only add up to the weight. n if i want to minimize on weight n optimize flexibility, i ll go for this material whose name i don't know(sorrrrrryyyyyyyy, i ll put up the name as soon as i get). but yes in my small scale(still smaller) i hooked up a hair drier on a 20cm X 20 cm board n used a polythene as the skirt n sealed it with the board n ran the hair drier. the polythene did blow up. so next time, i put up some holes n then it survived.
so u see its my lil silly experiment, bcoz of which i said that.
honestly speaking, i am also(now especially) a bit confused that the material i have, ll blow up or not?
aaaaaaaaaaaaahhhhhhhhh, why do always i get bad results to show my dumbness

 

[Danger]

You're really going to have to post a picture, as you suggested. This is making absolutely no sense to me whatsoever, so I have to see the setup to even begin understanding what you're talking about. A sketch or two should be sufficient.

 

[ank_gl]

ok.. will do in night.

 

[Gambit]

@danger, sorry about the confusion. Now, what you are telling is correct. I should have mentioned this ealier. Sorry about this. now the track where the hovercraft should run is straight and is made of cement. so, i guess, there will not be anything major which might poke the hovercraft from the bottom and tear it or something.

Now, like Ank, even I will do some experiments and find out. But the biggest problem is that my job schedule doesn't give me much time to do all this. I might use the material used for umbrellas as the skirt. But, as of now, i ll try with polythene bags and my hair drier.

Now, like gaming addict has mentioned, there might be no issue with upskirt-out, but the upskirt can take place towards inside. Thats the reason I don't to go with upskirt kinda skirt. I hope you can understand. I feel its safer and better to fo with holesrather than an open skirt thing. Sorry if I didnt enter the right terminology.

I will have to use a motor with small fan. Using holes kinda skirt thingy will be better i guess. Correct me if am wrong. I have to useonly 1 motor. Thats y I have to getit up and running with little effort. If I have the balloon kinda skirt, the craft can lift faster and easily. I was telling about the second pic in the two pics I put up.

Although I don't want to get on your nerve, which I surely will with a couple of more questions, please tell me if its feasible or not. My above mention craft !

 

[gaming_addict]

Here's from wiki http://en.wikipedia.org/wiki/Image:Hovercraft_-_scheme.svg

Notice that it uses some kind of 'double skirt'. I haven't experimented that approach yet but from numerous sources I've read. Such design can permit you to use stiffer skirts, so upskirt problems are reduced or eliminated.

It's easy to imagine that it's blowing stream of air round the sides of the craft directed under, towards the middle of the craft, this creates some kind of 'air wall' that gets in the middle and gets 'trapped' there.

Some pressure is created and the 'air wall' actually is now working like an invisible skirt. This design I'm positive will get your craft floating higher above ground than single skirt design, and will also be more efficient as well.

Anyway, I hope you also read my advice to use or design a skirt with varying thickness or flexibility. Just for the interest of eliminating upskirt, it should be most stiff at the top and very flexible at the bottom. It should be efficient as well

 

[Danger]

I still can't figure out what you guys are talking about with 'holes'. I've also never heard of, let alone seen, an 'upskirt' problem. You would have to have an incredibly weak or badly designed curtain for that to happen.
Gaming Addict, that illustration that you put up from Wiki does not have a double skirt; it has a double-hull with the bottom part serving as the top of the lift plenum. (It's going to complicate the terminology a bit because the gap between the hulls is a distribution plenum that feeds the lift plenum.) That design certainly has its advantages for heavy-lift machines such as ferries or gunships, but is absolutely unnecessary on something of this scale. The bottom part that's shown in red should be eliminated, and the curtain can easily be made to come straight down rather than curve out and back as shown.

 

[gaming_addict]

Yup I agree Danger. It has to be more accurate in construction because you will need clean flow of air around, as the air column will virtually serve as a 'skirt'.

I've really thought of 'two hull design' because some years ago, I haven't had much success with the 'simple design' I also had upskirt problems. I didn't come to stiffening the skirt but it might be a solution.

 

[mach 8]

mini hover craft project

hey gambit,i am onto this hover craft thing.its in iit chennai not in guwahati.
hi danger,gaming_addict,ank_gl and gambit,i was looking for some sort of guidance on this project when i stumbled upon your forum(good god i did it).all my doubts on the skirt thing where blown to pieces,i could do a little bit better with the drawings that danger has posted,so i request for the same.i have done a small craft before hitting upon this forum.it didn't kickoff.i shall post the pictures to you at the earliest possible time(a week at the most).meanwhile i want you to suggest the material for the hull(as usual strength and weight factors come into play in addition to economy).

 

[Danger]

i could do a little bit better with the drawings that danger has posted,so i request for the same.

Welcome to PF, Mach 8. I don't quite understand that statement. Are you asking for more detailed drawings?

meanwhile i want you to suggest the material for the hull(as usual strength and weight factors come into play in addition to economy).

As mentioned earlier, lots of materials would be suitable. It should be strong and fairly stiff, but a bit of flexibility would be okay. You could use balsa, dense-core Stryofoam with wooden edging and engine anchor plates, or even the lid from a Tupperware bin. Doped fabric over a wooden framework (such as with WWI aircraft) would also work. Those are just a few examples; there are dozens of others.

 

[captaincabo]

Try a rubber tire intertube, insert a 12v DC house fan using bungie cords to tie it off to the inflated tube. Some sort of battery pack and even a solar panel to keep it fully charged. Center the fan and back and use a remote switch for the variation in the fan to control it. As far as steering it, you would just descend the control until you get the direction then increase the control.

 

[mach 8]

Welcome to PF, Mach 8. I don't quite understand that statement. Are you asking for more detailed drawings?
@danger-i was talking about the drawings you mentioned to gambit.i don't know how to access them.i wish you would mail them to me.without them i cannot accurately understand what you people where talking.
also i hit upon a knew idea about the skirt.tell if it works.i would take a balloon (the long thin tube like version),two 70cms ones and two 40cms ones,wrap each of them in an umbrella cloth(probably glue their surfaces) so each of them forms a tyre (umbrella cloth) and tube(balloon) combination.then i would attach them (after inflating)on to underside of the hull surrounding it completely on the perimeter,after that i would seal their intersection points with cellotape and glue,so that in the end we have a hull resting on four tube like inflated skirts

 

[Danger]

I'm not sure what the difficulty is. You should have the drawing appear by clicking on the underlined part of post #32 (the ImageShack link).
Your curtain idea will work just fine, although the 'umbrella cloth' is unnecessary. The only problem will be surviveability. In particular, make sure that the adhesive you use doesn't eat latex. (And don't use this machine near a cat. )

 

[mach 8]

@danger -sorry for being sooooo dumb,anyway now i got it.i wanted to use the umbrella cloth to prevent punctures to the ballon(or a condom,though i suppose they are not that long).do you really think it is not neccessary?
@captaincabo -i could not understand what you intended to convey.please explain.

 

[Danger]

(or a condom,though i suppose they are not that long

Mine are, but that's an exception.

Ahem... That's actually not a bad idea in one way; condoms are manufactured to a lot more stringent strength requirements than balloons. Unfortunately, you would need the cloth covers in that case in order to confine the shape. Otherwise, they would get too fat and not long enough.
Come to think of it, though, I suppose that such covers would actually provide a reasonable amount of puncture resistance. I was thinking at first that something pointy would just poke through, but something only semi-sharp would probably be stopped. Good idea.

 

[mach 8]

@danger-that i think solves the problem of curtains (theoritically)until further execution.now coming to the motors i hit upon an idea that a 1/12HP motor(we use them in our place to motorise a manual sewing machine so that it uses electricity)may do the job.i went straight to the place where it is sold and upon enqiry found it quite light (about 400gms to 600gms not sure just going by the feel of it),but the problem is i don't know if it meets the 12v dc power supply condition.i don't know the conversion standards(i belong to information technology stream in engineering course),i will try to get it clarified with my mechanical counterparts but i want your advice just in case.also this 1/12th motor does not have any rotors.going by the hoverhawk calculator i want you to tell what diameter fans would be enough to provide the required lift(only if that motor could provide enough power upon meeting the required 12dc condition)and the probable source of those fans.
p.s- the weight of the motor could be further shaved by removing the plastic casing which is enclosing the motor itself.

 

[enginelover]

props and engines

am not sure if this info is already up there
but i get my aero modelling supplies from this person called mr eshwar
he is in bangalore pm me for his number if u want it :)
he has props and os engines
but if i were you i would go for a fx .18 or similar which would be more than sufficient
and a standard 6X18 prop would do me think how much does ur model weigh ?
plus don't think too much about motors :)
cause the ones that work are just too expensive and for it u need a battery which will weigh a ton
or u can get a LI-po battery which will burn a hole in your pocket...
either way

 

[Danger]

Mach, as Enginelover pointed out, you have to know what weight of machine you are dealing with before you can even begin to choose your fan.
As for the motor, if it plugs into mains electricity, it isn't 12 volt and probably isn't even DC.

 

[ank_gl]

ok, i am almost done with my hover machine. i used a 50X30 cm sq. wooden sheet(its really light), n had a hole of 5 inch dia punched through it in the centre, n mounted a 12 V dc motor over it with a fan 5" dia n abt 3" pitch. rpm of the motor is lil over 5000, n used PVC sheets for the skirt. hollaaa, system is more than working. now only the propulsion system is left.
the whole thing is about 2 kgs. i ll see if i can put up a pic or some vid of it soon(i left it at my cousin's place, so i ll hav to get it back first)

 

[Danger]

Right on, Ank; that's good news. I look forward to seeing the thing in action.

 

[Gambit]

Hey Ankit..! Great going buddy...!

too bad that i am not able to do anything as of now...I guess my work schedule hates me doing all such things...I literally am not getting enough sleep now-a-days. Extra training hours and all...anyways, Congrats...I woul love to see a picture or two of your hovercraft. Did you use motors or engines? let me know...

 

[Danger]

Guys, I'm kicking myself for not thinking of this sooner.
I've got the perfect way to make a multi-segment double-layer curtain with approximately zero effort. Just attach a bunch of 1 or 2 litre milk bags to the edge of the deck. They're approximately the same material as the poly vapour barrier stuff that I was talking about, and are already the right shape. You can seal them with air inside, or just leave them open.

 

[Gambit]

@Danger

Dont kick yourself too hard...lolzz...

Well, coming to your idea, I can get a 1 litre milk packet but not a 2 litre packet. So, I guess, i might have to make the skirt myself. I just Ankit would reply soon with the pics of his hovercraft...with regards to the lower side of the craft, if it has holes or not.

I get the idea you want to convey, that there will not any holes under the craft, But still, I can't seem to understand if we use a milk packet, how will we provide the outlet for the air to escape and provide lift? I hope you are able to understand my doubt...!

 

[Danger]

if we use a milk packet, how will we provide the outlet for the air to escape and provide lift? I hope you are able to understand my doubt...!

Unfortunately, I still can't understand it. The whole point of the curtain is to prevent the air from escaping; you don't want an outlet. It's the trapped air that provides the lift.

 

[ank_gl]

oh it ll be atlest sunday before i could post the pics. its at my cousin's place.

 

[dopa]

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