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Air Pressure of a Hover Craft
Name: Stephen L.
Status: Student
Age: 13
Location: N/A
Country: N/A
Date: November 2002
Question:
How can I measure the air pressure under the hover craft
as it is hovering? I am trying to test the pressure on concrete and grass.
Replies:
Interesting question. The detailed answer is undoubtedly very complicated
because it depends upon the aerodynamic shape of the hover craft. However,
you could make some simplifying assumptions to get an estimate. Assume that
the hover craft has a circular cross section (or an "effective" circular
cross section) even though the craft doesn't have a circular base. For the
craft to hover and not move up or down the weight / "effective circular
cross section" is going to be a downward pressure (force / area) due to the
weight of the craft. This downward pressure then will have to be equal to
that pressure, if the craft is to remain suspended. Run those numbers
through and see if they make any sense.
Vince Calder
An interesting question. After all, the overpressure generated by the
hovercraft times the area under the hovercraft struck by the air blown
down by the hovercraft must equal the weight of the hovercraft, which
is far from negligible. This is true since the force upward of the air
on the hovercraft propeller blades must equal the reaction force down
on the air by the propeller blades. The resulting downward velocity
of the air must be stopped by the upward force of the ground on the
air and the reaction to that, of course, is the downward force of the
air on the ground.
I would think a pressure gauge could be made easily and cheaply in the
following manner. Find a metal can, stretch a rubber sheet over the
top and run a pipe or tube connected to the inside of the can to a
region outside the area affected by the hovercraft. The pressure
inside the can will then be the prevailing atmospheric pressure and
the pressure on the top of the rubber sheet will be increased by the
effect of the hovercraft. You can perhaps glue a vertical ruler to
the center of the sheet and send it through a slit in a narrow wood
plank fastened to the rim of the can so the ruler measures the
deflection of the center of the rubber sheet. That deflection will be
a function of the overpressure caused by the hovercraft.
You could easily callibrate this pressure gauge by mounting an
ordinary pressure gauge to show the pressure inside the can and then
reducing the pressure inside the can, perhaps by sucking on the tube.
A reduction of the pressure inside the can will have the same effect
on the stretching of the rubber sheet as an increase in pressure above
the sheet.
I would start with a rather large can (1 or 2 ft in diameter) and
fairly light rubber. I do not know the numbers, but a rough guess would
put a 50,000 pound hovercraft over an area 10 ft x 20 ft which gives
250 lb/ft^2 or around 1.5 lb/in^2. Incidentally, if the rubber makes
an airtight seal to the can, the tube is unnecessary (except for
calibration purposes).
If you do this, I would be most interested in hearing of your results!
Best, Dick Plano...
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Update: June 2012
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