MadSci Network: Physics
Query:

Re: how does weight affect how high off the ground a hovercraft flies?

Date: Tue Jan 21 08:44:36 2003
Posted By: Martin Smith, Engineering, B.E., M.EngSc., Uni of Qld / airline pilot
Area of science: Physics
ID: 1042651353.Ph
Message:

First of all I am not a hovercraft specialist, however...

This is actually a fairly complex question.  Basically hovercraft tend to 
(or try to) hover at a fairly constant height.  This height is determined 
by the lift from the air cushion and the weight.  Lift from the air cushion 
is primarily a function of pressure in the air cushion.

A hover craft sits on a nearly static cushion of air (as opposed to wing in 
ground effect vehicles).  This air cushion is almost always contained using 
a "flexible bag" of some sort (the skirt).  Within this "bag" there will be 
a certain pressure of air, and it is this pressure which determines how 
much weight the hovercraft can lift.

At very light weights the bag will be less deformed and there may even be a 
area between the bag and the ground.  There is often an "air curtain" just 
below the skirt that also helps contain the air cushion, so the skirt may 
be hovering some height above the ground but the air cushion is mainly 
contained within the bag itself.  (Very early hovercraft models had no 
skirt and could only hover very close to the ground, also some basically 
skirtless hovercrafts or skates are used for shifting loads in factories 
where clearance is not an issue).

So as the hovercraft is loaded any hover height below the skirt will be 
lost (as it is hard to maintain air pressure here), and the bag may deform 
slightly as the load increases.  However this change is generally small 
compared to the total height of the skirt.  As the hovercraft is loaded the 
air pressure in the cushion is usually increased.

So basically if you have a light load and max power going on your engines, 
producing max pressure you will hover at your skirt height plus a bit more 
underneath.  The reason you are not blasting skywards even though the 
pressure in the skirt is much greater than that required for the load is 
because that pressure is lost rapidly below the skirt.  Load up your hover 
craft and the hover height below the skirt goes first (hardest place to 
maintain pressure) until your engines are producing just enough pressure to 
lift your weight plus the loss of air around the bottom of the skirt.

The max weight that a hovercraft can take is a function of this max 
pressure of the air that can be maintained in the skirt (the air cushion). 
 Load beyond this limit and height starts diminishing rapidly.
 http://
www.hovercraft.org.uk/download/download.htm 
http://
collections.ic.gc.ca/science/english/eng/projects/air.html

for wig
 http://home.mira.net/
~radacorp/ground_effect.html 
http://www.ae.metu.edu.tr/~gulkiz/
wig.html



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