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peterd51 - n/a
13-Jul-07, 07:18 AM
Hi,



please can someone explain the basics of hovercraft design without getting too technical? OK, I understand how the engine turns the fan and pushes air into the skirt or out the back...



I'm looking for more on the pressures, etc, that are involved.



I've read in various messages about the low pressure under a hovercraft, 12 - 15 lbs per sq inch. I read a message somehwere that states it's simply the total weight of the hovercraft plus people, etc, divided by the base area.



Does it make any difference if the hover height is higher/lower?



I assume that the hover height is set by the height of the skirt but would be limited by how much the skirt 'blows out', ie, leaks from underneath?



Would a much higher hover height just take longer to inflate the skirt to get up to maximum? I assume the engine/fan can only shift a set amount of air and once it's up to height the skirt can't hold any more and the 'new' air just drifts out from under the skirt?



Also, I've read messages about static and dynamic pressure...can someone cover the differences for me please?



Regards

Peter

jar2 - n/a
13-Jul-07, 04:11 PM
Hi,

please can someone explain the basics of hovercraft design without getting too technical? OK, I understand how the engine turns the fan and pushes air into the skirt or out the back...



I'm looking for more on the pressures, etc, that are involved.



I've read in various messages about the low pressure under a hovercraft, 12 - 15 lbs per sq inch. I read a message somehwere that states it's simply the total weight of the hovercraft plus people, etc, divided by the base area.




Correct. If the pressure was higher than the weight/area then the craft would continue to rise - if lower it wouldn't get off the ground.

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Does it make any difference if the hover height is higher/lower?


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No - see above. The pressure is always (virtually) the same otherwise the hover height would vary. One exception is when the craft crests a rise or climbs a slope - gravity will temporarily cause a change in cushion pressure (because the craft weight is changed by gravity).

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I assume that the hover height is set by the height of the skirt but would be limited by how much the skirt 'blows out', ie, leaks from underneath?



Would a much higher hover height just take longer to inflate the skirt to get up to maximum? I assume the engine/fan can only shift a set amount of air and once it's up to height the skirt can't hold any more and the 'new' air just drifts out from under the skirt?


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The thing that holds the craft up is the air cushion underneath - the skirt is there to stop most of the cushion air escaping. The skirt to surface gap acts like a pressure regulator valve and stabilises the hover height. The lift system (fan, engine, ducting and skirt) is designed to operate with a small skirt to surface gap all around (usually about 10-20mm) which leaks air continuously (thus making a hovercraft frictionless). If too much lift air is pushed into the cushion this gap opens up a bit more and releases the excess air/pressure. Too little air causes the skirt gap to close up and increase pressure in the cushion keeping the craft hovering.



The generally accepted maximum hoverheight (skirt height) is 1/8th of the craft width. This height will give a stable craft.



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Also, I've read messages about static and dynamic pressure...can someone cover the differences for me please?



Regards

Peter


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My (simplistic!) understanding is that static pressure is pressurised air that isn't moving (air in a compressor tank for example). Dynamic (or velocity) pressure is the energy stored in moving air (the wind is an example) - it doesn't do anything until it hits something solid.

Keith Oakley - n/a
13-Jul-07, 09:34 PM
I agree totally with Johns feedback. Just as an aside it is possible to have a higher hoverheight simply by making a bigger skirt. I remember an early craft (of todays size) with a hoverheight 2-3 feet high but it took two people walking alongside to keep it upright. As John says about 1/8 the width is the generally accepted rule of thumb for reasonable stability.

Keith

Paul Fitz - n/a
13-Jul-07, 10:09 PM
Peter, I have just sent you a copy of the article "Hovercraft Principles..." which used to be on the downloads page. This explains the basic principles with simple worked examples and also explains what is meant by 'Pressure' and the different types of pressure.



HTH



Webby....Hint http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif)

peterd51 - n/a
14-Jul-07, 11:35 AM
Hi,



thanks for the input.



I'd guessed it would be possible to make a hovercraft unstable if the hover heigth was too great but it's nice to known there's a known ratio to work to.



Paul: I've not seen anything yet.. I assume you sent it to peter&#64;padco.net?



Unfortunately I'm studying for an exam next tuesday so I didn't get my email for a couple of days and my mailbox got full.



I've cleared it now, could you re-send it please?



Regards

Peter

kach22i - n/a
19-Jul-07, 07:52 PM
FYI: At this year's National Hovercraft Rally (USA) someone had a standard finger skirt custom made so that it is two inches longer. The craft operated very well, even over the short grass, and as you can see from the photos he could lean it easily. One thing the pictures do not show is that very often the last two inches of fabric would "knuckle under". The craft had enough thrust to overpower this condition, and the craft would unknuckle the finger as he leveled out.



I think the ability (rate/speed) of the fingers or bag skirt to "recover" is diminished when increasing it's height over stock, but I can't explain why.

andycollins - n/a
20-Jul-07, 08:05 PM
... the article "Hovercraft Principles..." which used to be on the downloads page. This explains the basic principles with simple worked examples and also explains what is meant by 'Pressure' and the different types of pressure.



HTH



Webby....Hint http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif)








Hint taken http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif It's there now.





Cheers.



Andy.

tonybroad - n/a
20-Jul-07, 08:55 PM
In answer to your question on skirt pressure, 12-15psi is way off, pressure is very low - more like 0.09psi (600 pascals) is a fairly regular pressure but you are right that it depends on the weight of craft+driver/area - change the weight or skirt contact area and the pressure is affected



hope that helps



Tony

Paul Fitz - n/a
2-Aug-07, 08:17 PM
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Hint taken It's there now.
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You must have a newer computer than me Andy,

it's not on mine. http://hovercraft.org.uk/images/icons/smiley_icons/icon_razz.gif

andycollins - n/a
2-Aug-07, 08:42 PM
Hmmm, Your right.



....



Now it really is there. Properly there. Actually there, on the site. I think.



Andy http://hovercraft.org.uk/images/icons/smiley_icons/icon_wink.gif