# Miniature hovercraft

I have never done something like this before, so I need a little help. Basically what I want to do is build a miniature hover craft, about 8''x3'' if possible. I do not want to propel it forward, I merely want to construct a craft that will hover about half an inch to an inch about a table, being propelled by air. I am planning on using 2 9-volt batteries as a power source, but does anyone have an idea what I could use to generate the air needed to lift the board off the table? Also, will I need anything besides a board, batteries, and energy source?

## Answers and Replies

Gonzolo
Are you sure 2 9-volt batteries can provide enough power to get anything of the ground? Let alone themselves? I'd figure this out first, and find out what's the maximim weight*time of flight you can have with these.

Propellers used for model airplanes seem to be the natural choice.

I haven't really taken into consideration how much energy I will need to power an engine because I do not know what type of engine I will be using yet, I guess I shouldn't have already stated I will be using 2 9-volt batteries. Do you have any ideas on what I could use as an engine or an air source? Once I figure this out, I will determine what batteries will be needed to lift the mass.

Also, I will check out a hobby store as soon as I get a chance to see what types of fans or engines they have, thanks for the suggestion.

There shouldn't be any problem with using 9V batteries. You don't need much power. The way a hovercraft works is like this: air is forced into the space inside it, which causes an increase in pressure. That makes it lift up. The air bleeds out from underneath, so it reaches a height that's just a little off the ground. If you were trying to make a helicopter, which requires a thrust from its rotor equal to the craft's weight, you'd need a lot more power. But a hovercraft is a different sort of beast.

when building make sure you use balsa wood as the board as it is light, easy to shape but fairly strong use about 12mm of this. secondly you need to build a skirt of rubber around the base of the hovercraft make sure it is air tight all around the sides but do not close up the bottom as then you will have no lift. thirdly 2 9-volt batteries is not enough power source i suggest that if it is not going anywhere outside you could use a modellers transformer these are in expensive and a found in your local modellers shop for model trains etc. next you will need to find a inverse propeller as normal propeller's found on model planes will blow the outbackwards but you want a propeller to blow the air out forwards into the skirt once you have found this measure accurately the diameter of the prop and cut a circular hole on the top of the board according to this size but leave about 5mm gap around the edge as you can never get a motor to sit still properly. now the motor must a big model motor capable of taking high revs around about 200-300 rpm make sure the motor is secured properly and then you shpould be away i done the exactly the same thing a few years baack and once i had all components it took just several hours to complete if you need any help with supporting the motor just buzz me a message
good luck and build well

day without sunshine.............is like............well................an eclipse

I'd use a '400' size electric motor - a 6V one. Power it from 6 or 7 'AA' rechargeable cells (I know that's more than 6V, but it will be OK) use a Gunther 5.5x3.5 propeller. These can be reversed depending on which way you wire the motor to run, and whether you want the propeller to suck or blow. You can buy the motor (about $5) and propeller ($2) from a model shop. The rechargeable cells and a charger you can also get at a model shop, or Wall Mart etc. You will also need a battery holder, some wire, a 5A switch, and the tools and skill to do some simple soldering. Probably best to get two, 4-cell holders and then you can position them either side of the motor to get the hovercraft's centre of gravity roughly in the middle. Although you'll have capacity for 8 cells, start with just 6 and work up.

For the initial prototype, you can just use a cardboard shoe box, turned upside down. Cut a circular hole about the diameter of your propeller from the top, and position the motor and prop over it using balsa wood, or similar, struts. Vary the amount of batteries to get the hover height you want. For use on a flat surface, it will work without a skirt, though not as efficiently.

If you have an excess of lift (I think you will with 7 cells), you can make some holes in the back of the box to let some of the air escape and provide some forward thrust.

Watch your fingers on that little Gunther prop. It looks like a toy, but will produce some nasty cuts if it bites your fingers.

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Like I said, the Gunther prop is the one to go for, on a direct drive (no gearbox) 400 motor. It's the white one - the first one in the non-folding propellers section in the site you linked to. Its number there is 60112. It's also known as a 'Zagi prop' (a Zagi is a popular flying wing delta R/C model).

I guessed the price right ($2) and you don't need any yoke or hub for that propeller, it's just a push fit straight onto the motor shaft. The 400 motor I mentioned is on the ferrite motors page of the same site: http://www.rc-dymond.com/order_motor_ferrite_lst.htm Number 22943 I was a little bit out with my price guess for the motor - it's actually$7-50

you should use a lawn-mower engine to power your craft, design the rest of the craft around that...such as size of propeller, and weight. I suggest a lawn-mower engine based purely on the fact that it'd probably provide enough power for you to sit on your hovercraft...and that'd be rad.

Thanks a lot for the help. Once I get this stuff in I am sure I will have more questions, so I will keep you posted as I go. Also, would it benefit anyone if I took pictures and I made it and wrote a little "how-to make a hovercraft"? If you want me to do that, just ask. Also, mikej_45, I am not going to use a lawn mower engine because I do not want a hovercraft which I can fit on, I merely want to construct a "miniature" one, maybe once that is built and working properly I will up scale it, but for now I want to keep it small.

Also, could you just verify some things:

Is this cell holder alright?
http://www.megabatteries.com/item_details.asp?id=10124&cat_id=47

Is this charger/these batteries ok?
http://www.megabatteries.com/item_details.asp?id=10276&cat_id=37

To purchase some additional batteries because the charger comes with 4, which ones would you recomend?
http://www.megabatteries.com/items.asp?cat_id=60

Is this 5a switch good?
http://www.electronelec.co.uk/acatalog/Electron_Electronics_Slide_switches_81.html

or to get everything from one place, which 5a switch would work here?
http://www.rc-dymond.com/order_speedcontrol.htm

What wiring/other stuff will I need from here?
http://www.rc-dymond.com/order_battconxwire_lst.htm

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transit442003 said:
...se balsa wood...12mm of this....[T]he motor must a big model motor capable of taking high revs around about 200-300 rpm...

day without sunshine.............is like............well................an eclipse

I think that 12 mm (1/2in) thickness is not necessary; 1/4 or 1/8 in should be OK. Also, 300 rpm is way too slow. That's only 5 revs per second. Probably, several thousand rpm would do. You're looking at a pretty small prop, if the width of the machine is 3 in. Two more comments: The torque of the spinning propellor will cause the craft to want to spin in the opposite direction. Two identical motors with identical but contra-rotating propellors would be an easy way to counteract that. Also, if the machine isn't perfectly balanced, it will travel in the direction it slopes toward. That might be good or not, depending on what you want. If I were making the machine, I'd use ribs cut from 1/16 in flat balsa with 1/16 X 1/16 in stringers going all around, and covered with tissue paper, for the body under the platform.

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Hmmmm....one more comment....I don't think it will hover at a 1/2 in. Maybe 1/16 or 1/8 in, just as a wild guess.

pack_rat2 said:
I think that 12 mm (1/2in) thickness is not necessary; 1/4 or 1/8 in should be OK. Also, 300 rpm is way too slow. That's only 5 revs per second. Probably, several thousand rpm would do. You're looking at a pretty small prop, if the width of the machine is 3 in. Two more comments: The torque of the spinning propellor will cause the craft to want to spin in the opposite direction. Two identical motors with identical but contra-rotating propellors would be an easy way to counteract that. Also, if the machine isn't perfectly balanced, it will travel in the direction it slopes toward. That might be good or not, depending on what you want. If I were making the machine, I'd use a ribs cut from 1/16 in flat balsa with 1/16 X 1/16 in stringers, and covered on the inside with tissue paper, for the body under the platform.

Do you think you could draw up a crude sketch just so I can picture what you are saying better, concerning the ribs. Also, I think 3'' might be a little too narrow but great idea with the 2 counteracting propellors, I might give that a try. Do you suggest I make the base round or rectangular to achieve the best results? If round, I would probably want a radius of about 4''..

pack_rat2 said:
Hmmmm....one more comment....I don't think it will hover at a 1/2 in. Maybe 1/16 or 1/8 in, just as a wild guess.

I was just throwing a height out there, thanks though, I am guessing you are will be right.

alex caps said:
Also, could you just verify some things:

Is this cell holder alright?
http://www.megabatteries.com/item_details.asp?id=10124&cat_id=47
No. I don't think so. That looks as though it is just a container to keep the batteries in when you're not using them. You want one with the little tabs and springs that make contact with the battery terminals, so that you can get power from them. Something like this:
http://www.thomas-distributing.com/battery-holders.htm
alex caps said:
Is this charger/these batteries ok?
http://www.megabatteries.com/item_details.asp?id=10276&cat_id=37

To purchase some additional batteries because the charger comes with 4, which ones would you recomend?
http://www.megabatteries.com/items.asp?cat_id=60
Yes. That looks fine, though a little pricey. Any AA Ni-Cad or Ni-MH cells will work - just find the cheapest ones you can. It's best not to mix different types together though - try to keep all the cells on the hovercraft at any one time the same type, and don't fit partly charged and fully charged batteries together, or it will shorten their life.
alex caps said:
No. If you look carefully, that is a 0.5A (half amp) switch. It won't last for many cycles when switching the sort of current your motor will take (about 5A depending on how many cells you use.)

At that site they have a toggle switch http://www.electronelec.co.uk/acatalog/Electron_Electronics_Minature_toggle_82.html rated at 3A, so you'd even be overloading that a bit, but it would be OK for probably 1000 or so On/Off cycles.

alex caps said:
or to get everything from one place, which 5a switch would work here?
http://www.rc-dymond.com/order_speedcontrol.htm
The electronic speed controls are great, but they're designed to work with radio control gear. Unless you want to buy a R/C transmitter and receiver, they're not any use.

But if you do decide to go R/C in the future, the speed controls don't just switch the motor on and off - you can vary the propeller speed anywhere from nothing up to full speed, by moving a control stick on the transmitter.

alex caps said:
What wiring/other stuff will I need from here?
http://www.rc-dymond.com/order_battconxwire_lst.htm
I don't think you need anything other than the battery holder, some hook up wire, the switch and the tools to solder. Have you done soldering before? If not try to find someone who has a soldering iron to help you, and show you how it's done.

I think you can probably get everything you want, except the propeller and maybe the motor, from somewhere like Wall Mart or Radio Shack. It will save on shipping costs, and will probably be cheaper - well at Wall Mart it will be anyhow.

I plan on taking a 4''x9'' piece of balsa wood, about 1/16'' thick and then cutting out a square on either end. Mounting a motor to fit over the cut out, having the part that spins under the board, and connecting the propeller there. Doing this one both sides. Then in the middle, mounting 2 6-AA Cell holders on the inside of where the motor sits, and then wiring that to a 5a switch. Doing this to each motor, so 6 batteries effect each motor. I do not know how, but I will try and make one motor spin clockwise as the other spins counterclockwise. I will also use a bicycle toob to make a skirt (not exactly sure how this works if anyone cares to explain.) Do you think this will be enough power to lift the mass? Also, I have not had any experience with soldering before, I will see if I can find anyone that can help me, but if not, is there anyway to do this at home?

Do you think there willl be any problems if I do this:

Buy 2 of these motors:
The 400 motor I mentioned is on the ferrite motors page of the same site:

http://www.rc-dymond.com/order_motor_ferrite_lst.htm

Number 22943

Buy 2 of these:
60112 Gunther white 5x3 with press on hub for 2.3mm shafts (Zaggi Prop)
found at http://www.rc-dymond.com/order_propeller.htm

Buy 2 of these:
6 Cell "AA" Battery Holder "
found at http://www.thomas-distributing.com/battery-holders.htm

http://www.megabatteries.com/item_details.asp?id=10276&cat_id=37
along with 8 other batteries of the same kind, totaling 12 batteries, 6 for each motor. (unless you know where to get a cheaper charger?)

Buy 2 of these:
http://www.kelvin.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=K&Product_Code=270085 [Broken]

And where could I get some hook up wire?

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Here's the basic idea in a .psd attachment. It shows the "skeleton," which would be covered with tissue paper.

Here it is (I think)...

No, it isn't. I'm having trouble uploading the file with this computer. I'll get it up sooner or later

Do you really want to use two motors? I think there are easier ways of overcoming any 'torque problem', and there isn't so much of a torque problem as you think anyway:

It's not like a helicopter where the torque of the rotor drive shaft makes the helicopter spin the other way. A helicopter generates that torque, by producing a spiral flow in the downwash it creates.

But with a hovercraft, you capture the downwash (spiralling or not) inside the cushion, and this gives it chance to react against the hovercraft body, cancelling out the torque from the motor.

And anyway, you can't just use two of those propellers - to cancel out the torque you'd have to make one of them spin the opposite way - but you can't do that without making it blow upwards, and then you'd need a complicated ducting arrangement to direct the airflow back down into the cushion.

You'd be OK if you could buy a reverse rotation version of the propeller, but they don't make them, as far as I know. (This is like the difference between right and left handed screw threads, for those of you not used to propellers).

Oh and those propellers are about 5 inch diameter, so your hovercraft will have to be at least that wide.

Ok, if I used one propeller, would you recomend I make the whole hovercraft round? Also, would those items I listed earlier be ok?

ceptimus said:
...to cancel out the torque you'd have to make one of them spin the opposite way...
Obvoiusly. I said "contra-rotating." You can get matched right-hand and left-hand propellors in a hobby shop.

pack_rat2 said:
Obvoiusly. I said "contra-rotating." You can get matched right-hand and left-hand propellors in a hobby shop.
Yes you sometimes can. But they tend not to be made in the small sizes normally used on the small electric motors we are considering here. Certainly the Gunther prop is not available in reverse rotation form, as far as I know.

The opposite hand versions tend to be used as 'pusher propellers' with Glowplug motors, These internal combustion engines often can't run in reverse, so if you want a pusher prop, rather than a tractor, you have to buy the opposite hand propellor.

But electric motors are normally so easy to reverse, that there is not a perceived need to make 'pusher' versions of small electric motor type props. When you need a pusher, you just swap the wires over on the motor, and turn the prop around.

If you know of a source of reverse rotation electric props, I would be most interested. They would be nice to have on some of the multi-engine planes I build, so that the props on opposite wings could turn in opposite directions. I've never been able to find any, and it's not through lack of trying.

Here's a simple wiring diagram for Alex. Sorry about the crude drawing.

This shows three batteries per battery box, but you'd use the same 'nose to tail' connections for the batteries, no matter how many you use.

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