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Name: Stephen L.
Status: Student
Age: 13
Location: N/A
Country: N/A
Date: November 2002

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.

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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