Hovercraft B/C

[RJohnson]

SPP SciO,
Thanks for sharing your experience. The students had terrible experience with 9V batteries in another project and completely avoided them. The lead team member wants to use LiPo2s 7.4 v 1000mAh (or 3000mAh if necessary). For performance, I agree with his thoughts, but not without concerns.
The rules were modified once to allow using brushless motors in DC proportion devices most of which are 12 V DC fans. Although the kids understand probably all SO contestants face the same 9 V battery /12 V DC fan issue, they wish the rules would be modified again to allow 12 V power supply, unless they can miraculously find 9 V DC fans for optimization. - A Parent Mentor

I would recommend the 3000mah batteries. I use 2700 mAh and they work fine. As mentioned above, some 12v batteries can do significant harm, and while batteries under 9v can two, the risk is compounded by no voltage regulator circuits and moving parts. Ultimately it comes down to the fact that the objective is to optimize the device under the rules, not the componentry in the device(s). While it would make it easier to be able to use 12v power supplies, the point of SO is not to be easy. My 12v, brushless fan lifts up to 3.3 kg operating at 7.4 volts. My brushed, DC motor rated for 18v will move 2kg with a 7.4 volt battery. I don't think a change is necessary.

The last thing I'd throw out is that optimization is finding the arguments (inputs) that yield the minimum or maximum result (output) given the parameters of the situation. This event is a beautiful example of having to do that. It's hard, but with research, trial, error, and a little ingenuity, it's very achievable. Sorry if I went ranty... I would love to help; I just see little utility in appealing for a parameter change rather than seeking a solution.

Good luck, and more than willing to answer questions

[chalker]

So after I built my circuit for the thrust fan (back facing fan), my 0.15W potentiometer burned out. After doing some calculations: P=IV --> P=(1A)(7.4V), I get that I need a 7.4W potentiometer. Is this right? The potentiometers I've seen rated at ~8W are upwards of $100. Is a potentiometer not a viable way to vary speed then?

Avoid using pots. When you increase he resistance on the pot, the wire gets hot like a lightbulb filament which is why it pops. That heat is energy loss, so what you want is a way to control voltage. This can be accomplished with pulse width modulation (PWM). Essentially it cycles the circuit on and off at different intervals so that it behaves like a lower or higher voltage is applied. These are also discrete circuits, meaning they are legal

Just because a circuit can be made of 'discrete' components doesn't mean it's allowed. Note that the rules prohibit integrated circuits (ICs). In most cases, if you have a small black box with more than 2 wires coming out of it in your circuit, you have an IC. While it's technically possible, realistically I don't think most people would be able to construct a PWM circuit that doesn't use any ICs such as flip-flops, 555 timers or op-amps.

Properly sized potentiometers will work just fine in these devices.

[chalker]

Is dry ice permitted to be used as the repulsive force to make it hover?
It doesn't say anywhere that you cant.

I'm not sure that dry ice itself provides 'repulsive force'. It might lower the friction on a surface and hence make it easy to slide around, but it doesn't necessarily meet the definition in the rules of 'hovering'.

There also is the general 'unsafe operation' concern. Some people might view dry ice as a dangerous substance to be messing around with.

[sciolyqa]

Thanks for sharing the info. on batteries, Johnson. While having the wishful thinking (typical middle schoolers!) , the kids are diligently seeking solutions and experimenting to achieve per the rules.

[sciolyFTW_aku]

Hi,

I am also having a problem with batteries. I keep on having to use new 9V batteries as the quickly run out of "juice". I was just wondering if there are any other batteries that are rechargeable and have a high current? Thanks!

Thanks,
sciolyFTW_aku

[slowpoke]

Just because a circuit can be made of 'discrete' components doesn't mean it's allowed. Note that the rules prohibit integrated circuits (ICs). In most cases, if you have a small black box with more than 2 wires coming out of it in your circuit, you have an IC. While it's technically possible, realistically I don't think most people would be able to construct a PWM circuit that doesn't use any ICs such as flip-flops, 555 timers or op-amps.

Properly sized potentiometers will work just fine in these devices.

If a fan has more than two wires coming out of it, it would still be allowed as it is "imbedded into a commerically available electronic or computer cooling fan," correct?

[Zioly]

What's being used for the body of the hovercraft? I've been trying to cut plates of styrofoam, but the cut surfaces are always uneven, which proves a problem. (It could be due to the type of styrofoam I bought.) Is anyone making the structure out of wood? I'm curious as to how far this is taken, as the majority of focus seems to be on skirts, vents, and fans.

[RJohnson]

What's being used for the body of the hovercraft? I've been trying to cut plates of styrofoam, but the cut surfaces are always uneven, which proves a problem. (It could be due to the type of styrofoam I bought.) Is anyone making the structure out of wood? I'm curious as to how far this is taken, as the majority of focus seems to be on skirts, vents, and fans.

I'm successfully using 1/8th in plywood cut on a table saw and hole for fan cut with jig saw.

[55555]

If a fan has more than two wires coming out of it, it would still be allowed as it is "imbedded into a commerically available electronic or computer cooling fan," correct?

Yes, I believe so.

[Zioly]

I've been looking for a thrust fan. Would this one fill the niche?

https://www.youtube.com/watch?v=rLmxUP4Dku4
http://www.digikey.com/product-detail/e ... ND/5209819

Same fan. One is a video I found of it.