# Miniature hovercraft

ceptimus said:
...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.

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?

swither said:
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.

swither said:
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.

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.

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ceptimus said:
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?

alex caps said:
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.

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... :tongue2:

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).

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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.

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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 realised 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:)

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.

I picked up 2 4-AA cell holders today, some batteries, a soddering iron (with sodder), some hook up wire, a 5a switch, and I am waiting for the motor and propeller to come in the mail. I have a piece of wood about 8x4'' or so but might go out and get one a little larger. I am planning on just having a switch to turn it on, having it merely hover above the ground, and if all goes well.. I will start adding some stuff to propel it forward. If this all goes well I am probably going to eventually do another project much like the one swither is doing, so that I can control direction and speed and such. Good luck with yours and I will keep you posted on how mine is going..

i'll probably build mine next month, really don't have the time at the moment. But i'll post some pictures, maybe even a video when i'm done!

swither said:
Are there any direct problems with this design?

I think the skirt should drop down much lower, so the air being injected can more easily travel around to maintain the internal pressure.

pack_rat2 said:
I think the skirt should drop down much lower, so the air being injected can more easily travel around to maintain the internal pressure.

how much do you recomend it should drop down?

Don't extend it too much , or the hovercraft won't be stable. It will wobble about on a deep flexible skirt, and maybe even fall over. For operation over a smooth surface, the cushion only has to be about two inches deep, and you don't need a flexible skirt at all.

Try to keep the heavy items as low as you reasonably can. The thrust motor is the main problem, as it has to be mounted high enough to allow propeller clearance.

If you're going to operate over rougher terrain, it's a compromise between stability and ground clearance. The flexible skirt needs to be tall enough to ride over the largest bumps you're likely to hit, and the fan/propeller should be high enough to prevent any chance of it grounding.

ceptimus said:
Don't extend it too much , or the hovercraft won't be stable. It will wobble about on a deep flexible skirt...
Not so. Remember, it's the air pressure inside that keeps it up. You can use flabby rubber from a balloon, without any support, and that would work--it would collapse when the motor shut down. I don't recommend that approach, though. I think a skirt that's 2-3 inches would be good. By the way, many people have built hovercraft, both small models and ones that can carry a person. I read an article in an old "Popular Mechanics" that had a project for making a one-person hovercraft. Why don't you search the Web and see what others have done, and have found successful?

a question on wiring

The 5a switch has 3 spots to attach the wire. The cell holder has 2 extruding wires and the motor has spots for 2 wires. I used a soldering iron and soldered the red wire from the cell holder directly to the motor. I then soldered the black wire from the cell holder to the 5a switch. Then I took a separate piece of hook up wire and touched it to one of the empty spots on the 5a switch, and the other end to the other spot in the motor.. nothing happend. When I took one end of the hook up wire to the motor and touch the other directly to the black cable from the cell holder, the motor spins. I have tried connecting it to the 5a switch while it was switched in either direction and neither seemed to work. Any ideas on how to get this to work?

never mind i got it

Does anyone know how I could figure out rpm?

Ivan Seeking
Staff Emeritus
Science Advisor
Gold Member
In case you all missed this one.

http://www.hovercraft.com/UH-18SPW_Hoverwing.wmv [Broken]

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some help needed for lift

Ok, I got a box about 6''x5'' and the motor is 400v, the propellor has a diameted of 5'' and it is wired to a 5a switch and a 4 AA-cell holder with 4 AA batteries in there. All the wiring is correct and works. The box has a base, then walls going up it, but no bottom. So I cut a hole in the base in the middle and put the motor in there, it is farely secure. I put the battery pack on top of the base and the 5a switch up there and rest the whole thing on the table, so the propellor is face down and the walls of the box act as a skirt, or atleast hold the whole thing up so the propellor doesn't hit the table. When I hit the 5a switch, the propellor spins but it acts alost as a vacume, pulling the whole thing down to the table and basically suctioning itself there. I remember some one saying a skirt was not needed, and it seemed heavy, so I cut off the walls of the box and was left with just the base of the box with everything still attached. Of course now I run into the problem of the propellor hitting the table, so I put 4 little pieces of cut off cardboard on the corners to hold it up. Now when I turn it on, it spins but will not lift. Does anyone have any ideas? I can't figure out how to lose weight due to the 4-AA cell holder being so heavy. I tried one where I held the 5a switch and batteries and then hit the switch but all it did was spin, it did not lift. Please help me and let me know if you want any pics.

If the system is working as a vacuum, swap the wires over on the motor (or where they go to the batteries if that's easier) You need to get the current flowing in the other direction through the motor, so it spins the opposite way.

You may have to reverse the propeller on the motor shaft too. Reversing the propeller alone won't make it blow the other way, but if the propeller is on back-to-front, it won't be as efficient. The concave (hollow) side of the blades should face into the hovercraft air cushion, so the convex side will face upwards. With the Gunther propeller, you'll find it's easy to reverse the white plastic part on the same (black) hub.

It ought to have enough lift to raise the whole thing, including the batteries, providing the propeller is spinning the right way round and the propeller is facing the right way.

Quantum_Prodegy
Just a thought, if you made the hovercraft circular, it would be more stable, especially if you are only using one source of power and propeller. I built a hovercraft in school with some friends, a bit bigger, it was circular, 4 ft in diameter, and could carry a person. It was powered by 2 leaf blowers (lol) and used the school's power through a recepticle and an extension cord.....it was sweet man.

Low voltage HC

I built a hovercraft with only 3v of power and it works just fine! Whys everyone think you need a power house to run one? I constructed mine out of paper thin fiber glass. It's mainly in the design and getting the most of your power source and motor.

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I'm building a miniature hovercraft for my high school Science Fair Project. I'm just getting started and I'm not quite sure where to begin. I want to test how high it can go depending on the temperature it is in. Do you think you could give me a step-by-step for how to build one and what materials I will need. I'm thinking of using a micrometer to measure it's height. Will that work?