# Help with air pressure

1. Feb 19, 2005

### Aki

I'm building a hovercraft, and I am using an old computer fan to blow air into a plastic garbage bag and fill it up. I need to poke a few holes on the bottom of the garbage bag to let the air out, and I want to have a less friction as possible. What size should the holes be? And how many?

2. Feb 19, 2005

### FredGarvin

3. Feb 19, 2005

### Andrew Mason

You need to start by figuring out the physics of a hovercraft's lift.

The hovercraft lifts off the ground when the downward force of the air is equal to the weight of the hovercraft.

Since
$$F = dp/dt = vdm/dt = v\rho dV/dt = vA\rho ds/dt = \rho v^2A$$

where $\rho$ is the density of the air, v is the air speed and A is the total area of the holes, if you know the speed of the air leaving the bag and the total area of the holes, you can work out the lift. What you want to do is keep the speed of the air high since it contributes lift as v^2.

Since v^2 is proportional to the pressure, you want to keep the pressure in the bag high and let out a lot of fast jets of air through a lot of very tiny holes. If you don't make enough holes, the bag will burst. If you make too many, you won't get enough lift (pressure will be too low). I think the easiest thing to do is experiment with it.

AM

Last edited: Feb 19, 2005