Hovercraft B/C

[windu34]

My bet's on a microwave.

Never even thought to try one of those lol

Vacuum cleaner? (one day I will figure it out XD)

Nope XD

[Zioly]

Never even thought to try one of those lol

Vacuum cleaner? (one day I will figure it out XD)

Nope XD

Had a sneakin' suspicion... leaf blower...? xd

Prob not...

[0ddrenaline]

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?

I'd like to bring back this topic in lieu of the thread's... important discussion

Would anybody be willing share with us the process of choosing an appropriate potentiometer?

[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?

I'd like to bring back this topic in lieu of the thread's... important discussion

Would anybody be willing share with us the process of choosing an appropriate potentiometer?

There are a lot of possibilities here, but I think part of the issue is you might be miscalculating things.

Let's start with V=I*R, and based upon the numbers you gave above, I'll assume the fan you are using draws a max of 1A and the max allowed 9V (if not, you can redo these calculations below with the actual numbers). That means the internal resistance of the fan is V = I *R -> R = V / I = 9V / 1A = 9 ohms.

To slow the fan down, you need to reduce the voltage, which means you need to setup a voltage divider circuit with the potentiometer (pretty much the most basic circuit you can construct) - i.e. put the potentiometer in series with the fan. To make it work well, you probably want the voltage to range from about the max of 9V to maybe as low as 5V. Any lower and the fan likely will essentially stop working.

Hence, you want the resistor to consume a range of 0 - 4V (assuming you are using a 9V battery as the power source). That means the resistor needs to dissipate a max of P=I * V = 1A * 4V = 4 Watts.

However, the critical calculation is to also determine what the value of the max resistance should be. R = V / I = 4V / 1A = 4 Ohms.

4 Ohm, 4 Watt potentiometers aren't that expensive. Here's a 5 ohm, 5 Watt one I quickly found on Digikey that only costs ~$5:
http://www.digikey.com/product-detail/e ... -ND/203771

[windu34]

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?

I'd like to bring back this topic in lieu of the thread's... important discussion

Would anybody be willing share with us the process of choosing an appropriate potentiometer?

Chalker gave a pretty mathematical explanation which makes sense to me. Watch the hovercraft video posted on youtube by science Olympiad or the maglev video for more ideas

[sciolyqa]

We started conception design of the hovercraft by using the hovercraft calculator developed by Alex Olshove based on James Perozzo's book. http://www.hoverhawk.com/lcalc.html

Based on the calculation, the demand of a fan capable of lifting the max. load -2 kg. is quite high.
For Lift spacing: 12.5 mm (0.5 in)
Required air flow feed: P st = 44.5 mm water, flow rate = 402 CFM, Fan power + 137 w Fan diameter 6.35 in

Questions:
1. Is this a conservative estimate ?
2. Anyone has succeeded in running with max. load 2 kg?
3. Where to procure the brushed motor and propeller or axial fans for the design? We had difficulty in finding a) a brushed motor of 12 v and propeller to build our own fan, b) DC axial fans without PWM.

Any advice would be appreciated.

[RJohnson]

Questions:
1. Is this a conservative estimate ?
2. Anyone has succeeded in running with max. load 2 kg?
3. Where to procure the brushed motor and propeller or axial fans for the design? We had difficulty in finding a) a brushed motor of 12 v and propeller to build our own fan, b) DC axial fans without PWM.

Any advice would be appreciated.

1. Not sure, I have had success lifting well over 2 kg with the motor in this thread from Digi-Key.

2. Yes, see above. Also, skirt design makes a huge difference. Look at your current skirt, observe where you unnecessarily lose air, and fix it.

3. I bought a brushed, DC motor and a pack of plastic, 140mm propellers at my local hobby shop. I used a torch to melt the propeller and fit it to the motor, running at very low speed to center the prop, then stopped the motor and it adhered without skew (visible to the eye, that is). A) Most brushed motors can have their voltage overdriven. Also, the internal resistance of the battery pack(s) will cause a voltage drop over the pack, reducing the actual driving voltage. B) Check fans driven with PWM how they operate. Some are based off of proportion of a duty cycle, some require discrete signals (ie. constant high signal doesn't work well). If it has to do with the duty cycle, hooking it straight up to the battery should cause it to run at full speed.

Good Luck!

[sciolyqa]

Thanks for the prompt reply.

What is the part number of the motor from digi-key that suffice? We have been there and must have missed that.
We experimented changing the skirt opening area, it did not seem to affect the lift. We are in the process of changing the way of adhesion.

[chalker]

We started conception design of the hovercraft by using the hovercraft calculator developed by Alex Olshove based on James Perozzo's book. http://www.hoverhawk.com/lcalc.html

Based on the calculation, the demand of a fan capable of lifting the max. load -2 kg. is quite high.
For Lift spacing: 12.5 mm (0.5 in)
Required air flow feed: P st = 44.5 mm water, flow rate = 402 CFM, Fan power + 137 w Fan diameter 6.35 in

Questions:
1. Is this a conservative estimate ?
2. Anyone has succeeded in running with max. load 2 kg?
3. Where to procure the brushed motor and propeller or axial fans for the design? We had difficulty in finding a) a brushed motor of 12 v and propeller to build our own fan, b) DC axial fans without PWM.

Any advice would be appreciated.

RJohnson gave a pretty good response already, but to chime in on a couple other things:

-12.5 mm is REALLY high. I suspect most devices are more like 1-2 mm.
-Plugging 2 mm into that calculator results in very reasonable specs, which align pretty well with the previous discussion on this thread (326 Pa, 60 CFM, 15W).
-Note that that calculator doesn't seem to scale well to small devices - it's designed for human scale devices. I've personally witnessed several 2 kg devices that didn't use anything exotic in the design.
-Search this thread for Digikey to find some recommended part numbers. Otherwise, go to Digikey and you can filter the cooling fans based upon static pressure, which is the key parameter you should be looking at (other than max voltage).

[RJohnson]

What is the part number of the motor from digi-key that suffice? We have been there and must have missed that.

Here is the link: http://www.digikey.com/product-detail/e ... ND/2560506