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charlietg - n/a
1-Mar-06, 07:56 PM
Hi all,



Im currently undertaking a University project to design and manufacture a hovercraft. We have been looking into the skirt design, and the airflow around the chambers. Our sports hovercraft is an integrated design, with a wooden chassis (cheapest not quality we are going for!) and a segmented skirt.



I have a couple of questions and wondered whether anyone could help? I had originally planned to put air holes of diameter 3 inches 70mm from the top of the planning surface, from which the air flowing around the air chamber will flow into and feed each segment of the skirt, however I have read in a previous thread that it is advisable to have smaller holes at the rear of the craft and larger holes at the front of the craft to allow for a greater pressure at the back, could someone explain why you would want this, or is it as simple as there is more weight at the rear of the craft thus you need more pressure supporting it? And what are the size of the holes feeding the air to the skirt dependent on?



My other question is i have large triangles around the outside of the central box of the craft, of which the planning surface and the deck affix to, leaving a small gap for the air to travel around the chamber to the air feed holes, would anyone think this is a problem? or would it be an idea to hollow out the triangles further to give a larger area for the air to flow through? bearing in mind i am planning on filling half of the chamber with polyurathene foam?



Hope that someone can help, 4 weeks left to complete, very stressed!



Charlie

Sean Atterbury - n/a
2-Mar-06, 05:42 AM
Good question about the feed holes, I was wondering the same as when the air does go nto the skirt and under the craft, I would think that no matter what the size hole, the pressure at the front and back will be the same, lest see what the others say.



As for the triangle bits, chuck some holes in them at the top near the deck, that should allow enough air through, seems you are at university, you do the math on how many are needed, maybe give it to your students, me, I'll just guess...(maybe thats Y it took 4 & a half years to finish mine. http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif )



Good luck for the rest of your build.

tonybroad - n/a
2-Mar-06, 08:08 AM
Hello



the reason most go for smaller holes at the rear is that the rear segments contribute very little to lift and simply need to remain inflated, it's the sides and front segments that need to shift air through them to generate the lift and lubricate the skirt as air exits



as for your triangles inside the plenum - any obstruction to air flow will reduce efficiency so reduce the obstruction as much as you can - drilling holes in doesn't necessarily mean weakening them



Hope that helps



Tony

Sean Atterbury - n/a
2-Mar-06, 08:55 AM
What size hole you got at the back Tony?

lazza - n/a
2-Mar-06, 05:34 PM
In my craft i have made it so that the rear holes are pretty much blocked off completly by having a air deflector panel mounted in front of rear holes. That way the air being forced into the plenum chamber has no choice but to go immediately up the sides and out the front of the craft without losing a portion of the air to the rear air holes, and my craft lifts perfectly fine even with a heavy Subaru engine in it http://hovercraft.org.uk/images/icons/smiley_icons/icon_razz.gif .

So i would agree that the rear segments have little contribution to the lift of the craft and can be made considerably smaller if wanted.

http://www.hovercraft.geek.nz/construction7.htm this page shows my rear holes blocked off



lazza

Hovertrekker - n/a
2-Mar-06, 11:32 PM
Charlie,

Larger holes in the front give you more lift in the front and hence a slight nose up attitude which helps prevent plow-in. Larger holes allow increased air volume. This is high volume, low pressure air flow.

tonybroad - n/a
3-Mar-06, 08:17 AM
I realise this may open up the skirt/pressure/flow/plough-in debate, but to have a skirt nose up and large holes at the front is asking for trouble



i won't make a statement like that without my reasoning, but i reckon there are three reasons plough in occurs



firstly lack of pressure, the skirt collapses either by contact with the water (the sudden friction drags the skirt out of shape) or the wind resistance at a certain speed collapsing the skirt (the most common in racing i believe)



secondly lack of lubrication - air escaping from the skirt acts as a lubricant, lack of lubricant and the skirt drags in the water collapsing the skirt



thirdly - many attempts to reduce plough-in see drivers shifting all their weight to the back, which causes the rear to drag more which forces the front end down which causes a plough-in rather than preventing it



larger holes at the front mean the skirt is less likely to resist collapse (less pressure) and with a nose up attitude (rear down) may well cause plough in



we all look for methods of preventing plough-in when the answer is simple - a well designed skirt doesn't plough-in - now what that design is, is another question



just my opinion



Tony

Hovertrekker - n/a
3-Mar-06, 01:02 PM
Certainly the rules will differ between racing craft and cruising craft. I have one of each, and as much as I try, the rules don't neccessarily apply equally. Wether larger holes in the front brings a benefit really depends on the craft, and they're all so different in handling characteristics. I know what works well for my craft based on my experience, but that may not work for someone else. I also run pressure skirts at the front on both craft, so that changes things too. Generally I use 4 inch holes in the front, 3 inch holes at the sides, and either no holes at the back (in the case of my Hovetrek), or 3 inch holes at the back in the case of my Revtech. I'm still experimenting with that though (are'nt we all) http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

tonybroad - n/a
3-Mar-06, 02:32 PM
I have 2" holes at the back and 3" everywhere else

charlietg - n/a
3-Mar-06, 11:01 PM
Thank you all for your replies! Its great to finally have some proper help with this project, its not been easy. Basically i just need reassurance that the lift system will work as thats the main area of my design, other group members are designing the rest of the craft. The hole thing seems like a good idea, logically speaking! just a problem with the triangle mounts interfeering with where id want to cut the air feed holes, and keeping the skirt segments equal widths, if that makes sense? I have approximately 70 segments going around the perimeter of the hovercraft, does this sound reasonable?



Thanks again for all the help,



Charlie

Sean Atterbury - n/a
4-Mar-06, 09:11 AM
I have 68 so it would be fine, they are going to move around anyway when the craft is going to the legth shouldent make that much a difference, over time those that are longer will just ware down to the propper length, see http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif



All good.