Mini Hovercraft DIY: Build an 8''x3'' Craft with 9-Volt Batteries

[alex caps]

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?

 

[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.

 

[alex caps]

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.

 

[pack_rat2]

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.

 

[transit442003]

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

 

[ceptimus]

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.

 

[alex caps]

I just realized that the closest model shop is about an hour away and they aren't too good, but I found this site, I was wondering, which propeller would you recomend?

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

 

[ceptimus]

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

 

[mikej_45]

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.

 

[alex caps]

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.

 

[alex caps]

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

 

[pack_rat2]

...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.

 

[pack_rat2]

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.

 

[alex caps]

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''..

 

[alex caps]

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.

 

[ceptimus]

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

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 anyone time the same type, and don't fit partly charged and fully charged batteries together, or it will shorten their life.

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

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.

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.

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.

 

[alex caps]

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

Buy this:

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

And where could I get some hook up wire?

 

[pack_rat2]

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

 

[pack_rat2]

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

 

[ceptimus]

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).

 

[ceptimus]

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

 

[alex caps]

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

 

[pack_rat2]

...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.

 

[ceptimus]

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.

 

[ceptimus]

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.

 

[pack_rat2]

...But they tend not to be made in the small sizes normally used on the small electric motors we are considering here...

A long time ago, I saw some little 3-blade propellors that were made for 0.10-0.20 ci model airplane engines. They were about 2.5-3 in in diameter. Something like that would probably be ideal.

 

[swither]

About how much thrust will be needed from the engines to levitate the hovercraft?

Also, wouldn't it be possible to put on another engine for moving it forward and then use a rudder controlled by some kind of RC servo to steer it? Or would the hovercraft get to heavy then?

 

[ceptimus]

About how much thrust will be needed from the engines to levitate the hovercraft?

It's not so much about thrust, as what pressure difference the fan can create. Say your model hovercraft is 8 inches wide and 11 inches long, then you have 88 square inches of lifting area. If your model weighs a pound and three-quarters, then you only need a pressure of one fiftieth of a pound per square inch to make it hover.

If you have the same pressure, but double the size of the hovercraft, then you can lift 4 times the weight.

Also, wouldn't it be possible to put on another engine for moving it forward and then use a rudder controlled by some kind of RC servo to steer it? Or would the hovercraft get to heavy then?

It's quite easy to do what you say, and no, it won't be too heavy. It's also possible to use just the one engine and propeller to do both jobs - you have the fan pointing backwards, and have a duct that catches a lot of the air and directs it down into the cushion, while letting the remainder of the air blow out past the rudders.

 

[ceptimus]

To go into the thrust calculation a bit deeper.

In my last post I explained how a pressure difference of 0.02 psi is enough to raise the hovercraft. Now with a fan (propeller) that is 5 inches diameter, the area of the propeller disk is $$\pi 5^2/4$$ so that pressure difference acting on the disk produces a thrust on the motor shaft of 0.4 pounds. A '400' motor driving a Gunther prop easily makes this much thrust, and the fact that it is working inside a primitive duct makes it produce more thrust anyway. Also, you can build such a hovercraft a lot lighter than 1.75 lb. With care, it need only weigh 1 lb - including the motor and batteries.

In practical terms, if your hovercraft is too heavy to hover, and you can't get more thrust from the fan, you just have to extend the area of the cushion, without adding too much weight.

 

[alex caps]

To go into the thrust calculation a bit deeper.

In my last post I explained how a pressure difference of 0.02 psi is enough to raise the hovercraft. Now with a fan (propeller) that is 5 inches diameter, the area of the propeller disk is $$\pi 5^2/4$$ so that pressure difference acting on the disk produces a thrust on the motor shaft of 0.4 pounds. A '400' motor driving a Gunther prop easily makes this much thrust, and the fact that it is working inside a primitive duct makes it produce more thrust anyway. Also, you can build such a hovercraft a lot lighter than 1.75 lb. With care, it need only weigh 1 lb - including the motor and batteries.

In practical terms, if your hovercraft is too heavy to hover, and you can't get more thrust from the fan, you just have to extend the area of the cushion, without adding too much weight.

Isn't the trust equation $$\pi r^2/t$$ ?

if that is right, where does the 4 come from?

 

[ceptimus]

Isn't the thrust equation $$\pi r^2/t$$ ?

if that is right, where does the 4 come from?

The area of a circle is $$\pi r^2$$ but $$\pi d^2/4$$ works out exactly the same (where d is the diameter, twice the radius).

The thrust formula for a propeller, if you assume that a pressure difference, p acts evenly over the area of the disc the propeller sweeps out is:

$$t = p * \pi d^2/4$$

We have been working in non-SI units of pounds (lb), inches and pounds per square inch (psi), so if we have a pressure difference of 1/50 psi and a propeller diameter of 5 inches, we get a thrust on the propeller disk of:

$$t = 0.02 * \pi * 5^2/4 \approx 0.393 lb$$

Remember the lifting force on the hovercraft is much more than this, as the pressure difference acts over the whole area of the hovercraft, not just the fan disk.

 

[swither]

I've made some simple 3d models over how i thought i might build a hovercraft. I'm not that skilled in modeling and 3d studio is not supposed to be used for this, but anyway...

The blue/purple stuff is s upposed to be balsa tree and some kind of rubber. the yellow circles are the propellers, the motors are green, battery packs red and the orange thing is a servo. I'll probably add an antenna and a servo to control the speed of the engine used to move it forward.

I was thinking to use 2 '400' engines with günther props as you said would be a good idea. The power should be supplied from the two 7,2 v battery packs. I think i will power the servos from one of the two packs. The picture should be in scale if I'm not mistaken.

Are there any direct problems with this design?

Should i use the servo to move the whole engine instead to steer or should i do it this way (use the servo to move a rudder).

 

[ceptimus]

I think that's great swither. Nice work!

I think for an R/C version, with two motors, it might be better to run both the motors off the same battery pack (wired in parallel), and then it would probably be better to use the next size up of battery (the common ones are called Sub-C cells, and you can buy a ready made pack of 6 or 7 of these cells shrink wrapped together - they are often used in R/C model cars.)

Also, to carry the extra weight of the second motor, the bigger battery pack and the radio control gear, you might have to increase the area of the hovercraft a bit - maybe go to about 15 inches long and 10 inches wide.

You would probably want to use an electronic speed controller for the thrust motor at least (so you could slow down) - maybe another one for the lift motor as well. These electronic speed controllers also have a 'battery eliminator' circuit built in. This regulates the 7.2V or 8.4V from the battery pack down to the 5V that the receiver and servo use, so there is no need to carry a separate small battery to power the receiver and servo.

I think your idea of the rudder is easier to engineer than trying to pivot the whole thrust motor around. Maybe you could have two or even three rudders in parallel behind the drive motor (all worked by the same servo) and this would give you more powerful steering.

 

[swither]

thanks :)

hm, i think i have one of those packs lying around so that sounds like a good idea :)

this one is actually 35x25 cm, so i hope it will be sufficient. i first thought of making it smaller, but when i realized that there will be extra weight from the servos and so on i made it bigger.

i was thinking of using a speed controller for the thrust motor but just set the lift motor to what's needed to make it levitate and leave it there while it's on, i mean, there won't be any real reason to change it while it's in use right?

thanks for your adviced by the way:)

 

[ceptimus]

If the radio gear you intend to use has a third control channel, then it would be nice to control the lift motor too. Then if you are heading towards a wall at high speed, you can switch off the lift, so that the hovercraft sits down and stops.

But I agree, it's not essential - the main two controls you want are to vary the forward thrust and the steering (rudder) control.

I'll be interested to see how you get on. If you go ahead and make it, perhaps you can post a photograph of it? That also goes for Alex, and anyone else who makes one as well, of course.