Hovercraft - Experimental Skirt Project

Someone recently posted a picture of their mock UH paper skirt fitting. Thank you to who ever that was, it inspired me to work in paper at model and full scale and saved me from repeating mistakes.





Squaring out the stern was done for several reasons.

1. Make the skirt fit

2. So it floats more level

3. Is similar to a large "dream hovercraft" I have on paper and in 3-foot foam model form

Getting the square end up the boat ramp and on to the trailer was not fun. I also suspect it shortened the stern skirts life with this geometry.



The "DARTS" went full for the final mock-ups, early model shown.



Like a fish with the meat cleaned off leaving only the head, sad to see the Flying Tiger Teeth go bye bye.

 

Getting the hull to fit the skirt was worth it, should of done this the first time.


I call this the "paper canoe", I love it when a plan comes together.




The tail was a little rough going at first, it's symmetrical with the front.





The paper bow fit perfectly, I forgot to click a picture of it's final fitting. I did the darts on the outside with a stapler to save time.



You can see in this picture why the tail had to be suspended from a hard hull. The Russians with the Mars hovercraft have a stacked skirt, the top one is Zodiac-like and foam filled. I don't know if it's rigid stiff foam or soft like a matress. I'm guessing it's stiff as I know of no soft foam which will not act like an open celled sponge and absorb water.


Original configuration:

 

UPDATE: 07/02/07

I've been meaning to do some work on the old Porsche and the old house, but the hovercraft is still fresh in my mind so I started to take it apart for repairs and updates.

To do list:

1. Replace stern skirt and modify the rear bottom hull lip so it does not chop it in half again. Thinking round tube covered in rubber all across the edge.

2. Repair bow skirt and add outer fabric wear material. I have several complex and cool ideas for this, but I have to make it simple so my wife doing the sewing will still say yes to helping out.

3. The side skirts have no wear at all. All the damage to the skirt was done by contact with the hull. There are a couple of holes made by sticks and sharp river rocks; changes in materials should take care of this.

4. I'm sealing up air leaks to get more lift (closing off weeps and duct joint leaks) and balancing out the air pressure between the captured air bubble under the craft and the bag skirts. This should give me more or better lift under conditions such as banking/turning and stopping.

5. Finish up duct lining and feather the edges for more thrust and revise the splitter so there is enough thrust to get over hump speed on water.

Link to pictures:
http://s184.photobucket.com/albums/x295/kach22i/

My brief moment of glory, mixed in with a future idea at work............how I see it playing out.

 

UPDATE: 09/04/07

I wrote down what I'm doing to the craft, ten things at once - yikes. At two weeks an item that comes to 20 weeks of work. I'm happy when I'm busy, so I guess I'm happy.


SKIDS:
I have to add a 45 degree wood pieces to each of the the sides of the skid pads, and add fiberglass all the way around.

Leading and trailing edges are beveled/canted 45 degrees.

BOW PLANE:
Everyone says that the Scat and Scat II have plow-in problems and most of that is because it has no plow plane, where a UH or Sevtec (approx. 10 degrees) have plow planes.

I marked off the bow with some duct tape for a cutting line (approx. 15 degrees). Pulled out my reciprocating and cut away the offending edge. I then took a bread knife and cut a 1/2" recess in the foam for a MDO plywood insert. I took out my jigsaw and cut a half circle piece which was then glued flush. The entire area will be fiberglassed next week (I hope).





If you were to do this for a stock fingered skirt, I imagine some additional modification to the bow skirts or at least their loop ties and lengthen the plastic zip ties.

On my craft there will be no connection to the hull as the front drape is attached to each side skirt. I found the connection points on my previous set-up was a souce of wear and stress as ithe connect got trapped under the bow hull.

 

More plow plane and skid pads.

Gallery:
http://s184.photobucket.com/albums/x295/kach22i/







I did not like they way the 45 degree wood outside edge skid pieces were fitting, they would have been only 18" long. I decided not to fit them just yet.

Stern-Plane-1:
The round tube and rubber hot tub hose at the stern edge are to keep the sten skirt from being severed in half. I just don't trust the roughly 3 inches of total rise. The tube cuts down on rise height by a little, but it's worth it to me.

 

Very nice post Kach22i. Thanks for the all the great pics.
I'm working toward a Hov Pod at the moment.
Can't wait!

 

1. If racing - get the largest 2-stroke you can, the Rotax 503 will not make you competitive.

2. If cruising - get a four stroke, it vibrates less, is quiter, better on gas, cleaner to run and often more reliable. However, it also will run you more money up front and will weigh more. The up side is that you can carry less gas, which is a weight savings on longer trips.

 

Not a perfect cone but close enough to be fixed on top of the splitter.

http://s184.photobucket.com/albums/x295/kach22i/




No injuries in the making of it, I'm happy so far.

NOTE: I looked into using plastic flower pots before. The problem was I am very cheap and went to the 1-Dollar Store when the stuff from China cracked in my hands by droping it once and picking it up. Perhaps a nice metal mixing bowl from a garage sale would be better.

 

UPDATE: June 02, 2008

The key to working upside down I found is to get your resin into a paste-like state. You do this by adding fillers. Use the graphite additives for a slippery hull bottom and the recommended filler for body. It also helps if you duct tape one end of the cloth so you can have a good starting point which does not move around on you. Also the duct tape allows you to pull on the fiberglass cloth so there is no stretching or slack.

Much of the cloth texture is left exposed using this method, even if laying it on thick. I used one thin layer of paste-like resin followed later by a thinner finish coat to really gloss it up and seal it good.

http://s184.photobucket.com/albums/x295/kach22i/
Duct tape both ends and each side so that the fabric is stretched flush.


After curing, cut away with a mat knife unresined or unused cloth and any lose parts which you can pull off with a little tug.


Cut away openings so that a second lap layer can be done.


I used 2 inch wide cloth at the edges and pressed in with more resin mud. Reinforce high wear or high stress areas too. Then the whole thing gets painted with resin to soak in and protect intersections of differing materials.


Just another picture angle, this area received more colored resin in the air lift supply holes to seal the open cell foam. Open cell foam is bad, some of the expanding foam in a can is almost as bad when it comes to absorbing water.

 

UPDATE: June, 02. 2008

Cone in place.

Finish quality is pretty shoddy, but even at idle it's effects are very noticeable. Full throttle absolutely rocks.

http://s184.photobucket.com/albums/x295/kach22i/


Thin layer of light weight fiberglass cloth. Finish coat of West Systems resin with low friction graphite (black) 423 & 406 (filler).

I never finished the cone and duct with (410) feather filler before applying the finish black coat, which is why it looks so funky. I'm on a deadline now, and have to start a new skirt ASAP.

 

Once I figured out a simple way to make operable controls I had to give it a try. Single cable operating the top elevator, five little pulleys and a bungee cord. I had it all working fine with the operator end tied down until I could get to making a hand lever. Then while playing around with it and giving it some shock and load tests a sudden SNAP!

The over the pivot end of the acrylic leading edge broke off. I cut off the other side to match and fastened everything in a fixed position.

I glued in the air brakes at the front, the operable cable, travel agents and springs will have to wait until the skirt is installed.

I'm done with operable controls (for now), I need to study up on this topic big time. I really seem to suck at it, but have to admit it was a material failure - my choice of material though - hey it was free.

http://s184.photobucket.com/albums/x295/kach22i/

 

FYI:
The other half of the wavy splitter (another part of the experiment - to cut down on sound).

http://s184.photobucket.com/albums/x295/kach22i/

 

George,

With all these modifications you have done , how much does your hovercraft weigh?

 

I have no idea.

I've lost 9 pounds working on it and my old cars this spring.

I have taken out the electric starter, the battery and cut out the seat.

I've put a bunch of holes in it, what do holes weigh anyway?

The new skirt will weigh less than the original finger skirt, a lot less material.

My biggest worry regarding weight gain is from moisture absorption. The spray can foam I used early on (before I knew any better) is in the lift duct corners, the thrust duct cavity and behind the bow plane.

 

Update: 07/17/2008

First Gen, Transparent skirt:
http://www.hoverclubofamerica.org/forum/in...p?showtopic=412

Second Gen, Heavy Yellow Skirt:
http://www.hoverclubofamerica.org/forum/in...p?showtopic=962

Third Gen, Gray Balloon Fabric Skirt:
http://www.hoverclubofamerica.org/forum/in...p;hl=gray+skirt

Fourth Gen, Black Thermal Polyurethane Skirt:

Video Link-1:
http://www.youtube.com/watch?v=zs_H4uOQP0o

Video Link-2
http://www.youtube.com/watch?v=Fh99M1YpsA8

I would like to do an article for the HoverNews about how I ironed my skirt together. So I'll make this thread more about the design as it is, what it took to get this far and what the next skirt modification might be.

Pass:
1. The heat welded (ironed) skirt held, no seams opened up or popped.

2. Fan duct lining held up and helped lower the sound levels.

3. Tail cone held up and helped with thrust and sound levels.

4. The wavy edge diverter/splitter worked fine, I think it also helped lower sound levels by breaking up nodes.

5. The skirt material shows no signs of wear or abrasion.

6. The landing skids did not fall off, neither did the vinyl crush tubes at the bow.

7. The pointed nose bow skirt worked out.

8. No plow in, plow plan did not come off.

9. Was able to get back on cushion after trying to turn to tightly, good thrust.


Fail:
1. The sewn zipper holding the bow drape, about 12" of stitching near the contact line came out when I accidentally went backwards (half off cushion) in a tight turn.

2. The double bag stern skirt when inflated pushed the tail up and the nose down. When weeps were cut to equalize the pressure the bottom loop of the stern skirt failed to inflate and was dragged along for the ride.

3. About 1/4 of the side skirt length located just aft of the center of lift and center of gravity were dragging in the water. I need to adjust the skirt for a small irregularity in the hull which pushes the skirt out or down.

4. Contact cement will not hold the vinyl teeth to the bow for more than one use, must mechanical secure, no regular one step glues will bond to Polyurethane.

5. Repairs to Polyurethane skirt are not resolved to a point I like. Patches are sewn on, heat welded patches are difficult because the material is not coated on both sides.



In General:
1. I want to try my hand at a full flow but not a conventional combination skirt. I'm inspired by the old air cushion landing systems, and leave it at that.

2. I hope to resolve the weird pressure issue in the side skirts and get more lift air flow. This change follows my intent to explore a skirt system for a craft four times the Scat's size. See item #1 above.

3. I want to regain loss lift area by extending the bow into a point, another two feet. This change in center of lift will allow me to remove tail weights (steel bars).

4. I want to get a real tune-up and "going over" of the engine to find power lost by age and hours of use.