### Hovercraft design issues

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I have an application in mind that would lend it's self an industrial strength hovercraft. However, there are a great many questions that remain in my understanding concerning the crafts tolerance to irregular surfaces, skirt design (with corner design) and materials issues, air volume required as a function of gross vehicle weight, and control. One of my kids thought that I might be well served to carry an anchor on board to throw overboard when quick braking is indicated. Anyway, I have an ongoing interest in a project of this type and would be interested in networking with others with a common desire.

Regards, John G.

-- W. John Gammage (jgannage@koyote.com), March 20, 2001

I understand "skirt erosion" is a major issue in hovercraft maintenance- and a good reason why you don't see them used on land all that much. As for an anchor to stop- no need: Just drop the air volume/motor speed of the skirt pump, and it comes down on it's hull- the whole thing is one big brake! If you must run it on land, a single front pilot wheel under the skirt helps reduce (but not eliminate) lateral sliding in a turn. Oh, and make sure the beast will float on water with all the engines off. Very important that.

-- Chip Haynes (ehaynes@co.pinellas.fl.us), March 20, 2001.

1) Hovering doesn't require very high pressure but does require sufficient volume (cubic feet per minute in the US). You can calculate the required pressure in the plenum chamber by dividing the area under the craft by the total weight you want to carry (including vehicle of course). The speed that the air escapes is determined by the air pressure and the height of the air gap between the skirt and ground. Use the Bernoulli equation that can be found in first year college physics textbooks, etc. The fan requirements then = escaping air speed times perimeter (length) of skirt, times gap height.

2) Irregular surfaces are very bad for hovercraft designs without lots of reserve CFM(cubic feet per minute). The craft settles so the average gap is maintained, scraping the skirt on the high spots. Fixes: More Power, multiple small plenums so the ones over the high spot don't lose their air to the low spots, "bead curtain" fringe on skirt to fill gaps.

3)Skirts can be inflated like innertubes (better) or just plain skirts. Using a car tire sized inner tube for the corner piceces might work. Skirts are subject to wear, even when not dragging, as the escaping air carries debris.

4)A single "pusher" fan with a rudder in the airstream works, but better control is obtained with multiple fans. Emergency stops can be done by killing lift, but spinning the craft aroud and using the propulsion system to stop is easier on the craft.

5) Anchors would be good things to carry, but not for stopping. Since the craft is light in relation to it's area, it is vulnerable to wind. Also, if it floats, you may need to park on water (e.g. anchor).

-- John Calderwood (jkc@cts.com), March 23, 2001.