For our Science fair this September, our Physics teacher asked us to build a mini balloon-run car. Of course materials should be as recycled as possible but I can still buy some stuff from the store I guess. Basically the car runs, as I have said, on air blown into a small balloon. When the air is released pointing backwards obviously the car would go forward. Simple, right? Well we have to submit THREE entries per group, and there we need some help.. The three cars are:

1.) A car that would run the most distance in one go. I was thinking it should be light and the car must have the least amount of friction so it would stop at a later time. What materials would you recommend for this? And what should the wheels be made of? I'm thinking of water bottle caps, but are there any more...? Secondly, would aerodynamics affect the car? Anyways the car is really small, and it wouldn't really run that fast, but still. (I was also thinking the design would be an XRR. )

2.) A car that would run the fastest. I don't know if our teacher would allow two balloons on this one... Hehehe. Again, should probably be light, what materials would you recommend again? This one I'm planning to base on an F1 car.

3.) A show car. We just have to show this on the Science fair's actual event.

Help would pretty much be appreciated. Thanks in advance...

EDIT: YES the balloon has to be inflated.

Regards,
Ryder

Balsa wood is always a cheap and light material to use. Aerodynamics will come into the equation because even though it's not going to be travelling very quickly it is only a small scale model so lower wind speeds still have an effect. The wheels don't have to transmit any drive so they just need to hold the chassis up and, as you say, provide the lowest rolling resistance they can. You could use thin balsa wood discs with a sharpened rim. There are a few important questions though.

• What size are the balloons?
  
• Are you only allowed to use 1 balloon (it looks like you need to clear that up with your teacher anyway).
  
• Are you allowed to control the rate of airflow from the balloon? E.g. can you put a restricter on it so the airflow is slower but lasts longer?
  
• Are you allowed to direct the airflow from the balloon (e.g. with a nozzle)?
  
• Does the balloon need to be filled with air?
  

What I bet most people will fail to consider is the drag from the balloon once it's deflated. This is going to be the biggest restrictor of all. So make sure your design has the balloon contained and not just held up on a narrow band that it will fall off about halfway.

At the same time I'd also consider holding the balloon in place so the transfer of force is consant, rather than bouncing around all over the place.

Would you be allowed to direct the airflow onto vanes on an axel? (similar to a paddle steamer)
I don't know if this would provide more thrust compared to just exhausting air.

if you can use more than one balloon i recommend you to try putting one balloon inside another one and inflate so there will be much more pressure inside and faster air flow / lower air resistance because of smaller surface. And using a nozzle should help constant speed...
Making the tyres as thin as possible is a must Balsa is a good choice but what about white strafor (i don't know it's name in english but it's something used mostly for packaging the things) ? i guess it should be lighter.

Quote from P1lot :

Would you be allowed to direct the airflow onto vanes on an axel? (similar to a paddle steamer)
I don't know if this would provide more thrust compared to just exhausting air.

I think that the force of the air hitting the paddles would cancel out any extra forward thrust produced from harnessing that force to drive wheels, in a similar way that you can't sail a boat by blowing a fan mounted on the hull into the sails.

EDIT: Actually, thinking about it it's a bit more technical than that. It would actually be a really good thing to test to find the best car and I bet your teacher would be impressed. You could try no balloon-powered-paddles, small paddles and long paddles (as longer ones wouldn't turn as fast but would give more torque)


You could try and get hold of some ball-races for use in R/C cars to minimise friction at the axles.

Quote from ZORER :

what about white strafor (i don't know it's name in english but it's something used mostly for packaging the things) ? i guess it should be lighter.

Polystyrene? It would be lighter than balsa but less workable and less sturdy.

Yep Mini-z ball bearings are the smallest i know.But they're made of metal and might take away the less-friction advantage because of extra weight. you should do tests to decide if they do good or not.

Quote from amp88 :

Polystyrene? It would be lighter than balsa but less workable and less sturdy.

yep that's the one thanks.


right it's harder to work on it.

Quote from ZORER :

you should do tests to decide if they do good or not.

This is the best idea

You can run three cars, but there's no limit to the number of cars you can run for yourself beforehand. Just have fun trying out ideas and use the three fastest ones.

Personally I would hunt for an off-cut of carbon fibre from nearby race/car manufacturers for the chasis, but balsa is sufficient.

One option for bodywork is to use a radio control car bodyshell, these are lightweight ( polycarbonate / lexan ) , aerodynamic, cheap, and give you sufficient space to work internally for your running gear.

Although I would personally do away with bodywork, it's weight that is not necessary. You're looking at it as if it is a car, it's really a device with the intention of completing a task, so I wouldnt worry too much about how pretty it is.

Aerodynamics will have an effect if the device is light or fast enough, it's some equation relating to weight and mommentum, but dont blow weight on adding aerodynamic features as this will reduce your overall range, simply reduce your speed - obviously not on the car being built for speed, if it becomes apparent the vehicle is being restrained by the airflow then go for the bodywork option.

can you use any shape balloon? as a long thin balloon would store a lot more air for a given surface area

I for one would ditch anything that would involve a body. You pretty much only need a base to hold your axles and the balloon together, without letting the ballon get in the way with the axles and wheels, or the ground.

As already said, use wheels as thin and as smooth as you can get them, as traction only hinders the vehicle, becaue a) the wheels are not driven, b) you don't need to turn and c) you don't want them to brake.

Pay attention to the axles and bearings, as you want them to have as little friction as possible.

Also, I'd try to experiment with a three wheeler, as one wheel less means 25% less friction on the axles and the ground.

I guess the shape of the balloon could also influence the airflow, so if you can chose the shape yourself, try out different ones for range and speed.

And lastly. I think you need to strike a balance on the weigth, as momentum will be what keeps your car going once the air is out of the balloon.
So a very light vehicle will stop more quickly than a heavier one, but it will reach higher speeds.

Put the balloon in an aluminium camping bottle, pump it up to 30, maybe 40 ish psi and BAM, pocket rocket. Might even work on 2 wheels
€: Hope it goes without saying that this might be a tad dangerous and is almost certainly considered cheating :P

make sure the balloon is facing up about 45 degrees to stop it from lifting off.

Bad idea. You'd have half the energy pushing the car on the ground, adding huge drag.

Luke, I think it's safe to assumed that the balloon should be attached to the vehicle in some way.

Also, you might be able to store more energy by twisting the rubber of the balloon rather than relying on air power. Could use that to drive some wheels or a fan. Or maybe some hybrid system.

Also, it's always worth leaning from those who tried before you. No point repeating their mistakes.

Quote from Bob Smith :

Also, you might be able to store more energy by twisting the rubber of the balloon rather than relying on air power. Could use that to drive some wheels or a fan. Or maybe some hybrid system..

I like your thinking. Who said the balloon has to be inflated? I ust imagine a tiny wee car being propelled forward via a slingshot using the balloon as the rubber.

You could use the balloon shooting air onto paddle technique if you ran an elastic band from a paddle attached above the wheels down to a drive shaft with the wheels on :P.

But of course there's alot of friction in that.. although if made well it might be worth a shot!

Hmm, get a 1lt coca-cola, 2 mentos and woooosh ... you win.

Quote from amp88 :

Balsa wood is always a cheap and light material to use. Aerodynamics will come into the equation because even though it's not going to be travelling very quickly it is only a small scale model so lower wind speeds still have an effect. The wheels don't have to transmit any drive so they just need to hold the chassis up and, as you say, provide the lowest rolling resistance they can. You could use thin balsa wood discs with a sharpened rim. There are a few important questions though.

1. What size are the balloons?
   
2. Are you only allowed to use 1 balloon (it looks like you need to clear that up with your teacher anyway).
   
3. Are you allowed to control the rate of airflow from the balloon? E.g. can you put a restricter on it so the airflow is slower but lasts longer?
   
4. Are you allowed to direct the airflow from the balloon (e.g. with a nozzle)?
   
5. Does the balloon need to be filled with air?
   

Thanks for the answers everyone. To answer amp's questions...

1. The balloons are party balloons. Not the long ones, the small kind of balloon... about 4 handfuls?

3. Wouldn't that make the car go slower? Unless it would traver farther, that would have no use.

4. Yes.

5. But of course.

Again, thanks for the answers everyone. Will have a look at the site you posted Bob.

Quote from only.one.RydeR :

3. Wouldn't that make the car go slower? Unless it would traver farther, that would have no use.

Yes, it would make the car travel slower but it MAY make it travel further. It depends on factors like the rolling resistance of the car, the mass of the car, the difference in airflow between the restricted one and the normal one etc. The only way to really find out is testing, but it's a good idea to know and understand all of the rules before you start designing.

I still love the idea of putting the balloon inside a pressure cannister so you can fillit with 10 times as much air as normal.

You could always dress as princess Leia and break into the LHC to steal parts....

30 Years ago... The famous "Yps" magazine, a must have for every 9-12 years old boy!

Quote from only.one.RydeR :

Will have a look at the site you posted Bob.

The second site is the better one. If you are not allowed to use the balloons elastic properites directly, but can only stretch the rubber using compressed air, then this vehicle seemed to the best, but generally reading around all of them gives lots of engineering tips and tricks. I'm finding that site quite an interesting read, it makes me want to have a go myself!