How to model pressure outflow from hovercraft Air constantly escapes from beneath the skirt of a hovercraft in operation. Is there an equation that will roughly model the rate at which pressure is lost?
 A: My own notes indicate that I gleaned the following from "Theory of Ground Vehicles 4e", Wong J.Y.  I don't seem to actually own this book so it probably came to me indirectly through another source -- forum post, website, Google Book, etc.
Caveat:  I'm not an engineer and I've have not used them, yet.
$$P = h \cdot l \cdot C_{d} \cdot \left( \frac{W}{A} \right)^{\frac{3}{2}} \cdot ( \frac{2}{d} )^{\frac{1}{2}}$$
where:  P is the power required to sustain the cushion beneath the vehicle, "h" is the clearance height within the cushion beneath the vehicle, "l" is the perimeter of the cushion area, "C_d" is the "coefficient of discharge", "W" is the total weight being lifted by the cushion of air, "A" is the cushion area, and "d" is the ambient density of the air.
The cushion pressure required to lift a specific weight is:
$$F_{pres} = \frac{W}{A}$$
which I'm sure you can see is already accounted for in the first equation.
My notes are obviously incomplete as I don't have a definition for "discharge coefficient".  I just did a Google search and found a paper entitled, "Construction and Analysis of a Remote Control Hovercraft".  It defines the discharge coefficient as:

"the ratio of the mass flow rate at the end of the nozzle to what the
  mass flow rate would be if the nozzle were ideal"

I've just realized that this answer does not technically answer your question, but I think it probably gives you the information for which you are looking.  I wish I could offer more, but that's all I seem to have on this subject.  Good luck with it.
