# Quirky question about fans & air movement ...

#### TheInvincibleMustard

##### Senior member
Just reading through this thread over on the General Hardware forum ... approximately 84cfm from a small fan.

Now, my question is with regards to how much that could "lift" ... theoretically, could you rig up some sort of suspension mechanism holding 12 or whatever of these fans that could lift a small laptop? I know a little bit about hovercraft technology, but could you do something similar without a cloth around the fans? How many 84cfm fans would you need to support 10lbs?

Quirky questions, I know, but something that struck me as being very interesting ...

#### CTho9305

##### Elite Member
For hovercraft methods, you need to find the static pressure for the fan. For helicopters, you need something I can't remember (mass-flow? change in velocity times mass). Given that information, you can figure out the amount you can lift in an ideal situation. Usually you can find this information on fan manufacturers' websites... in the form of graphs.

edit: for a overcraft, you can lift the static pressure times the amount of area you have, until you are leaking too much air out the sides and the pressure starts to drop.

#### TheInvincibleMustard

##### Senior member
OK, so I'm looking at Sunon's website and found some monsters (pdf), specifically the SP100A model which is a 120x120x38mm fan with 0.39 Inch-H2O Static Pressure.

So, now if I take 12cm x 0.3937 = 4.7244 in
Then squared = 22.31995536 in2
Then times 0.39 = 8.7047825904 .... what? This is the area times static pressure, so does this work out to pounds or something? I've dealt with unit conversions before, but this is my first foray into pressures of any sort ...

#### rgwalt

##### Diamond Member
Originally posted by: TheInvincibleMustard
OK, so I'm looking at Sunon's website and found some monsters (pdf), specifically the SP100A model which is a 120x120x38mm fan with 0.39 Inch-H2O Static Pressure.

So, now if I take 12cm x 0.3937 = 4.7244 in
Then squared = 22.31995536 in2
Then times 0.39 = 8.7047825904 .... what? This is the area times static pressure, so does this work out to pounds or something? I've dealt with unit conversions before, but this is my first foray into pressures of any sort ...

Pressure is Force over area. 0.39 in of static pressure means that the fan produces a pressure differential sufficient to lift 0.39 in of water in a pressure measuring device known as a manometer.

A 1 in H2O pressure differential is equivalent to a 0.036 psi pressure differential. So, these Sunon fans can each lift approximately 0.3 pounds if the force is applied over their output area (120x120mm). This does not include the weight of the fan itself, which is probably more than 0.3lbs.

Sorry, no hovering laptops for you.

R

#### TheInvincibleMustard

##### Senior member
Originally posted by: rgwalt
Pressure is Force over area. 0.39 in of static pressure means that the fan produces a pressure differential sufficient to lift 0.39 in of water in a pressure measuring device known as a manometer.

A 1 in H2O pressure differential is equivalent to a 0.036 psi pressure differential. So, these Sunon fans can each lift approximately 0.3 pounds if the force is applied over their output area (120x120mm). This does not include the weight of the fan itself, which is probably more than 0.3lbs.

Sorry, no hovering laptops for you.

R
Aww ... but thanks for clearing that up anyway! So I guess those fans that power the hovercrafts are really powerful, huh? I've never done much research into this, but it's proven pretty interesting so far ...

Thanks!

#### CTho9305

##### Elite Member
It is more that hovercraft fans are different. Computer fans are designed to move a LOT of air, but in relatively open space, with a heatsink interfering with the airflow on one side. A hovercraft fan is probably designed to create a large pressure difference, and only needs to move enough air to get around leakage. (Or something like that. I'll shut up now before I say something that is wrong )

#### TDSLB

##### Member
I was watching a episode of 'junkyard wars' were they developed hover crafts, I forget what the engines were that powered the fans but they were big.... If you wanted to use energy from your battery to power a fan strong enough to create a hovercraft underneath your case you would probably want one that is fairly slow but large fins and has a very ready access to air flow.... If I'm not mistaken you were wanting to use the fans from the computer to power the 'craft'? the only problem I would see with that is getting a ready enough supply of air through your computer. Anyways, Interesting thought, I think it might take too much energy to make it advantagious for a notebook...

#### zixxer

##### Diamond Member
they used motorcycle engine for one.. then they had a duct type thing would divide half the air to go down, and the other half to go back (to propel it)

the other design used an electric fan to go down, and a large lawnmower engine for propulsion

#### Smilin

##### Diamond Member

Hovercraft use really small engines relatively speaking - they aren't lifting like helicopters with raw thrust/lift from blades. Hovercraft get their lift from the shape of their bottom side. The engines only need to be powerful enough to keep filling in the air that's leaking out from the bottom of the craft. The craft just floats on a bubble of airpressure underneath it.

Think about an air hockey table - if you take 20 of those little jets you wont have enough thrust to lift the puck. The puck has that rim around the bottom edge that allows the jets to fill the space under the puck with enough pressure for it to float. Lift the puck up by 1/8 of an inch and all lift is gone.

It might be possible to float a laptop with a few powerful fans if you employed some kind of skirt design. A 1/4 inch strip of foam around the bottom might do it. Batteries in a laptop might just be too much load tho.

#### FrankSchwab

##### Senior member
Actually, even the 0.036 psi pressure differential from one Sunon fan is enough to lift a laptop.

Assume a heavy laptop, 10 pounds. To lift it using a 0.036 psi pressure source, you simply need 10/0.036 = 275 in^2 of area, or a square about 17" on a side. A bit over twice the area of a normal laptop.

So, build a platform 17" on a side, with a skirt hanging down to contain the air, set the laptop on top, and it should float.

The practical implementation is left to the engineers.

/frank

#### CTho9305

##### Elite Member
Originally posted by: FrankSchwab
Actually, even the 0.036 psi pressure differential from one Sunon fan is enough to lift a laptop.

Assume a heavy laptop, 10 pounds. To lift it using a 0.036 psi pressure source, you simply need 10/0.036 = 275 in^2 of area, or a square about 17" on a side. A bit over twice the area of a normal laptop.

So, build a platform 17" on a side, with a skirt hanging down to contain the air, set the laptop on top, and it should float.

The practical implementation is left to the engineers.

/frank

Correct, assuming that the fan puts out enough CFM to cover the leakage from the 68-inch circumference.