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Author Topic: RC hovercraft  (Read 10420 times)

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Offline gamefreakTopic starter

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RC hovercraft
« on: October 14, 2007, 04:41:39 PM »
Well i've started on this and all i know is that i need a fan, plastic bag, base, and a way to propel it.

So far I have decided to use three computer fans, one connected to the base aimed into a plastic bag with some holes in it, then take the other two fans and use them to propel and steer the bot and build a base out of balsa.

Now I know it needs to be light weight and have a center of gravity near the middle, but im not sure about the fan used for the lift, should that be centered or would it be fine more to the back?

Now the computer fans(1) require 12 volts and since this is a small light bot i don't have room or the power to lift and hold that much weight, so i have decided to use 3 4.8 batteries(2) wired in series. Then use a fourth to power the radio receiver(3). I then plan to use micro switches that are positioned around servos(4) so that the servos steer the bot by turning the fans used for thrust on and off, i also found some fans that use PWM, so could i just plug those into a channel on the receiver and one would go faster while one goes slower, but if this is the case how can i make that work on just one channel?
So is there a way to keep this limited to 3 batteries and have the receiver and fans share a battery?

without the weight of the fans or balsa i get a total weight of: 8.84 ounces with 3 batteries and 10.44 ounces with 4 batteries, so can the computer fan lift this weight plus the weight i am missing and is there a way to keep this limited to 3 batteries and have the receiver and fans share a battery?

then for the amount of time it can last i get: 26 minutes, 270 MaH divided by 613 MaH multiplied by 60 minutes

now the fan specs give me the info of 28.4 CFM(i think cubic feet per minute) which has to deal with air flow, but does anyone know how to calculate that into a hovercraft for speed and lift?

Does this sound like a solid plan or one that is just ready to fall apart?

(1) http://www.pctoys.com/cg84leda.html  |  http://www.pctoys.com/840556081067.html
(2) http://www.servocity.com/~servo/html/4_8v_270mah_ni-cad_battery.html
(3) http://www.servocity.com/~servo/html/hp-2rnb_am_rx.html
(4) http://www.servocity.com/~servo/html/hs-311_standard.html
« Last Edit: October 14, 2007, 08:38:14 PM by gamefreak »
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Offline hgordon

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Re: RC hovercraft
« Reply #1 on: October 14, 2007, 10:44:53 PM »
I doubt that a computer fan could lift its own weight, let alone the weight of a frame and batteries, but this is easy enough for you to test.

To get sufficient lift, you're going to have to look at ducted fans like these -
    http://www.hobby-lobby.com/ductfan.htm
« Last Edit: October 14, 2007, 10:47:50 PM by hgordon »
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Offline gamefreakTopic starter

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Re: RC hovercraft
« Reply #2 on: October 15, 2007, 05:30:08 AM »
yes a fan wont have enough to lift its own, but I do not think that it has to, All it has to do is create an air cushion for everything else to ride on, i'm not sure if it has to be enough lift for the whole thing or only a fraction of the weight, and i cant seem to find the information i need anywhere.

Also, those fan are expensive, 20 dollars per fan will bring my price into the two hundreds, so unless there is no alternative then I dont plan on using those.
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Offline paulstreats

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Re: RC hovercraft
« Reply #3 on: October 15, 2007, 06:26:16 AM »
the bag has to be tightly sealed around the fan or the base to make sure that air doesnt escape upward instead of downwards.
the entire bottom of the bag wants to be open around the perimeter of the base like a skirt.
if the bag is tightly sealed around the fan or base there is only one way the pushed air can escape and that is out of the bottom, so it must push the base up in order to escape from under the bag


Offline gamefreakTopic starter

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Re: RC hovercraft
« Reply #4 on: October 15, 2007, 08:50:00 AM »
yes i knew about the skirt thing, but i also thought tht the skirt should be closed with small holes cut in it, not compleatly open as your 3D model suggests. But how much lift does the fan need to produce so that the bag can create a sufficient air cushion? And does the placement of the fan create huge differences in how the skirt should be made or how fast the craft can go?

Right now the only thing stopping me from spending the 100 dollars on this is the uncertainty of if the single computer fan listed in the first post be enough to create the air cushion needed.
« Last Edit: October 15, 2007, 08:55:39 AM by gamefreak »
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Offline paulstreats

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Re: RC hovercraft
« Reply #5 on: October 15, 2007, 05:08:28 PM »
if the bag has holes in it, the air can probably escape without having to lift the base first.
the bottom edge of the skirt needs to be in complete contact with the ground so there are no gaps when it starts. it is forced to lift up the base to escape then.

sorry, i dont know if a computer fan will work. At a guess i would say no. its not nescesarily the motor itself, its also the fan blades you need to look at, the tilt angle of the blades determines how much suck or pull the fan will have.

your best idea is to look at the motors and blades used for rc air planes or helicopter.

ive got a small indoor rc helicopter with ultra miniature motors and long blades, i have no doubt these would work but there are no part number printed on them to give you.

also the longer the blade you had the more air force you can generate. 

Offline gamefreakTopic starter

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Re: RC hovercraft
« Reply #6 on: October 15, 2007, 06:45:52 PM »
Well for the bag i mean have it secured tightly to the base and then cut some holes so that before the fan starts the holes are all shut because of the surface they are on, not just make holes randomly in the bag, only in the bottom, or make one large hole in the exact center of the bag, wouldn't that create a better air cushion then just a bag draped over the edges?

http://4wings.com.phtemp.com/tip/skirt.html
I originally planed on something similiar to the bag skirt

What i mean is that if you have a skirt designed like that then the air would escape faster and the air cushion might not be as reliable, but if you take a skirt the is solid and cut multiple small holes on the bottom portion of the skirt(that part that would normally be on the ground) then you have multiple parts holding the cushion together and more skirt touching the cushion and lss skirt on the ground, if you use a lager hole in the center bottom of the skirt then you have less skirt being touched by the air cushion but more still isnt touching the ground. If you simply have a skirt without any bottom then no part of the skirt is touching the cushion and more air can escape out the sides of the skirt.

I also want to use a RC receiver and not add the extra weight of servos and micro switches to control motors, so i need PWM and i have found some computer fans that use it.

Also due to the fact that i dont want to spend a hundred dollars on parts just to have something work, i've talked to a physics teacher at my school and they gave me a few calculations:

F=ma   Force(N) equals mass(kg) times acceleration(10N/kg)
so   f=0.5 * 10    f=5 newtons

O = V / A ( i think that P = F / A is equivalent)
Pressure equals force(N) divided by contact area(m^2)

so i took an average fan and put 30 CFM into my calculations, turned that into m^3/s and got 0.014158
Then i took the area of the fan of 80 mm, so PiR^2 gets me 0.005027 for the area of the fan.
My physics teacher then took those and got an air column with a length of 2.816569
so using sig fig it is 2.817 or for simplicity 3

so it is expelling 3 meters of air per second

but how do i use that to get the pressure it produces?
« Last Edit: October 16, 2007, 07:13:30 PM by gamefreak »
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Offline crossroads1946

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Re: RC hovercraft
« Reply #7 on: October 16, 2007, 10:26:37 PM »
Hovercraft and all components being lifted = X kg.
We'll assume the surface area of the base of the craft is equal to the area at the base of the skirt and call it Y square meters.
Force of gravity = -9.81m/s^2 = g

In order to lift the craft on a cushion of air, you need to maintain a pressure of: (X*g)/Y kilograms per square meter.

To convert this to pounds per square foot (PSF), multiply by 0.2048.
To convert to pounds per square inch (PSI), multiply by 1.422*10^-3.

This is the pressure you have to maintain inside the skirt. The next thing you have to calculate is the perimeter of the skirt and the height at which the base of the skirt will ride off the ground, because it won't actually be touching the ground if designed right. This will get you the size of the gap through which the air escapes under the skirt. Then use this to calculate the flow rate at which the air escapes at the given pressure. Your lift fan will have to have enough juice to maintain that flow rate PLUS enough to maintain the desired pressure. All fluid dynamics... I'd have to look up the calculations again for all this, but you shouldn't have too much trouble googling for it. Just remember that the flow rate of the fan is not the only number that's important here. You could have a fan that moves 500 cubic meters of air per second, but if it's a foot wide and only rotating at 20 RPM, it's never going to work.

Essentially, a computer fan isn't going to cut it for any craft that is more than a couple ounces in mass. Take a grocery bag, tie off its open end to make it air-tight and roughly the size of your craft. Make sure to have air inside the bag, but no pressure. Then place a weight equivalent to your craft's weight on top of the bag so that it's held up by the air trapped in the bag. You'll get an idea of the kind of pressure you'll need to maintain inside the skirt. Experimentation would be the best idea here. Get some fans and see what works and what doesn't. If you can't afford to buy a ton of stuff, ask some people that know. Here are a couple links to some RC Hovercraft pages, some with plans and parts.

http://www.model-hovercraft.com/
http://www.rc-hovercrafts.com/
http://www.aeroglisseur.com/Liens_Internet_e.htm

Hope it helps.

-Baker
 
« Last Edit: October 16, 2007, 11:00:39 PM by crossroads1946 »

Offline gamefreakTopic starter

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Re: RC hovercraft
« Reply #8 on: October 17, 2007, 04:58:08 PM »
alright, i talked to my physics teacher again and we found the fan can just barely lift 250 grams, this is 30 grams to little with the fans didnt add to the total weight, with the fans there is even more weight. So are there any cheap alternatives to computer fans?

I need a minimum of 20 ounces of thrust to lift the bot and i dont want to spend 20 dollars on the fans...
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Offline gamefreakTopic starter

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Re: RC hovercraft
« Reply #9 on: October 18, 2007, 08:27:37 PM »
alright crossroads i started on the math you gave me.

x= 0.5 kg(hopefully less)
g=9.81
y=0.16*0.2 = 0.32 = 0.3

(0.5 * 9.81) / 0.3
    4.905
       5         / 0.3
             16.67
               20 kg per square inch
         20 * 0.2048
           4.096=4
       4 * (1.422*10^-3)
        4 * 0.001422
          0.005688 PSI

which seems like an awful little amount of PSI

you might be wondering why i rounded everything, this is because my phys and chem teachers keep yelling at me that i need to do sig fig or else my calcs are inaccurate.

Also, one of the sites you gave me stated i need a high rpm motor, would 26500 at 7.2 volts with a 3 inch blade cut it? I intend to buy a block of styrofoam and sand out the center to create as little of a gap as possible.
« Last Edit: October 19, 2007, 06:02:17 AM by gamefreak »
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Offline paulstreats

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Re: RC hovercraft
« Reply #10 on: October 19, 2007, 09:16:59 AM »
i should think that 26500 rpm would be adequate.
you also need to make sure that the fans are well balanced, i would try and use a 2 or 4 blade fan then each side would be balanced against each other and prevent vibrations/gyrations? if thats the right terminology.


Offline crossroads1946

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Re: RC hovercraft
« Reply #11 on: October 19, 2007, 02:30:36 PM »
gamefreak, I think your math is wrong, but it's hard to tell without units. But don't worry, I don't think what I said earlier is 100% accurate. Going from kg (a mass) to pounds (a force), I think I missed something to do with converting from newtons to pounds. Don't have time to figure it out right now though.

As for skirt pressure, (assuming your craft is 16x20 cm, which is a very small craft), I figure about 0.03 PSI would be ball-park. The models on model-hovercraft.com range between 0.5 and 2 pounds per square foot.

motor... It's hard to say what would and wouldn't work without knowing more details. Blade pitch, flow rate, thrust... can the battery you're using actually get it to it's full capacity? A lot goes into it. For a craft your size, I'm guessing that 26500 RPM motor would most likely be over-kill unless the prop has got a really small pitch.
http://www.aeroglisseur.com/Motorisation_e.htm   has a good description of the power sources he used.
http://www.model-hovercraft.com/ also has a wealth of good information.

-Baker

Offline paulstreats

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Re: RC hovercraft
« Reply #12 on: October 19, 2007, 07:34:02 PM »
ive just checked the motor link from your other thread, and it suggests that at full power, that tiny little motor can provide 22 ounces :o of thrust (i presume with a standard airplane propellor). I cant see there being much difference between the motor pulling an aircraft and lifting a hover craft except providing too much lift.

Offline bulkhead

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Re: RC hovercraft
« Reply #13 on: October 20, 2007, 12:14:07 AM »
Calculations will not predict how it will actually work in this case as there are too many variables.

CPU fans are designed to move air with very little load.  They do not have sufficient torque to force air into a skirt.  Motor's torque ratings are generally their peak (stall) torque.  The blades' pitch, diameter, shape must be chosen so that the power is maximized (like the transmission in a car).  They also run on 12V.

This one uses a CPU fan blade fixed onto a small 3V DC motor for lift and another motor with airplane prop for steering (ruder style).  The skirt has the bottom completely open.  It works.

« Last Edit: March 22, 2008, 12:43:46 AM by bulkhead »

Offline paulstreats

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Re: RC hovercraft
« Reply #14 on: October 20, 2007, 08:12:44 AM »
aww i want to build one now.... where did i put them props

any chance of a small video showing it in action bulkhead??
have you tested how much it will lift?

Offline bulkhead

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Re: RC hovercraft
« Reply #15 on: October 20, 2007, 06:18:12 PM »
They can lift quite a lot.
« Last Edit: March 22, 2008, 12:42:56 AM by bulkhead »

 


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