Hovercraft - Experimental Skirt Project

Discussion in 'Boat Design' started by kach22i, May 30, 2006.

  1. kach22i
    Joined: Feb 2005
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    kach22i Architect

    HOVERCRAFT - MY FIRST FLIGHT:

    After three years of work, I have video of my first flight with a new skirt which I tested just last week.

    Let me know if you can't view these, it's supposed to be in the "Science & Technology" catagory.

    One:
    http://video.google.com/videoplay?docid=1663313081488662208

    Two:
    http://video.google.com/videoplay?docid=4563104873090699357

    ...........and a bunch of other video's on the Google Video site by others:
    http://video.google.com/videosearch?q=hove...age=2&lv=0&so=0

    We had a mishap getting it back on the trailer, the front noodle at the bottom of the leading edge drape was tucked under the bow. The trailer was tilted at it's typical steep angle with it's perimeter "C" channel facing the hovercraft like a hungry mouth. The tube like bottom edge fitted nicely into the "C" channel and the front drape pulled down and off. We caught this on video but I'm going to fix the design flaw and trailer flaw so I can do a proper "before and after" story. I need a winch, getting too old for all this pulling and heaving up a trailer anymore.

    The rear part of the side sponson bag can open up on turns and on impact with the street curb allowing large amounts of air to escape. I need to "in-fill" that corner with more fabric or anti drag flap.

    I flew across a large grass field but had problems coming back across it. The top edges of the skirt are just clipped every three or four inches and the fabric stretches a fair amount. This resulted in too much air escaping at the top edge, so I hopped off and walked it back with help from my assistant Cris.

    I've found my old top of skirt plastic edge strip cleannig my basement this weekend, I'm hoping it still fits. Otherwise 5/8" washer hose with a slit in it may be my sealing top edge.

    The stern/rear bag had wear at the corner rivets, but no where else. From this I know I need to adjust it so it seals better (more slack) in the middle. Larger drain holes will be added after a second dry land test, something I can get my fingers into to remove rocks. I read something about 2 inch diameter holes, maybe that's what I'll end up with. Easier to make them larger than smaller as I go.

    Nothing all that new about what I'm doing, just slightly different application/combination.........none of which has proven to be better than the stock finger skirt (yet). Even a simple bag skirt would of been easier to do, but hey - it's an experiment, the long way, the joy of getting there and all that.

    My thread at the Hoverclub of America (go to end - long):
    http://www.hoverclubofamerica.org/forum/index.php?showtopic=412

    SmileWavy
     
  2. Figgy
    Joined: Feb 2006
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    Figgy Senior Member

    Cool videos Kach22i! I'm curious though, in the fly-by video you keep looking behind you, what at? Sure looks fun!!
     
  3. kach22i
    Joined: Feb 2005
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    kach22i Architect

    I kept looking back to see that if while under full air-pressure/cushion the skirt kink had straightened out. You see while at rest/idle the side sponson skirts are not fully inflated, and there is a fold or break in the skirt tube as it cantilevers and forms the tail. I can see in the video and from what my camera person told me that the pucker does go away as intended.

    I have to get some mirrors, would really help sometimes.

    The arrows show where the "at rest" kinks show up.
     

    Attached Files:

  4. kach22i
    Joined: Feb 2005
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    kach22i Architect

    UPDATE:
    Ford Lake, Sunday October 08, 2006.

    Took it out three or four times into the lake, same result everytime. Gets out there, starts to slow down, nose starts pushing water.

    From what I can figure the lower front bow skirt drape edge where the pool noodle is, is being sucked down.

    The bow drape going straight across, and the pool noodle addition were not my first configuration choices for this craft. I wanted to slope it inward like a boat but all the extra cutting and fitting would of taken more time than I wanted to spend this spring.

    Worked great in the parking lot, and getting it on the trailer was pretty easy to.

    I think it work just fine on Gun Lake this winter. Come spring time, I'm doing some major modifications for water operation. It's going to look like a boat by the time I'm through with it.

    The sharp lower tail/sponson corners with their weep holes are catching water too. You can see it in the second video pretty clearly. If I can shape it like a canoe, that's what I'm going to do.

    6 Videos:

    http://www.youtube.com/watch?v=QQkzh0Yd0qE

    http://www.youtube.com/watch?v=Jq6yx7061lI

    http://www.youtube.com/watch?v=0S8rW3cmhT0

    http://www.youtube.com/watch?v=_eghLJ0dU0w

    http://www.youtube.com/watch?v=p7YHk0hq7Ek

    http://www.youtube.com/watch?v=NNUioMLU1_A
     
  5. kach22i
    Joined: Feb 2005
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    Location: Michigan

    kach22i Architect

    UPDATE 12/25/06 Chirstmas
    Test flight No. 7 went alright yesterday, there is good and there is bad to report.

    Good:
    Elevators gave me much needed control
    Craft was balanced
    Parking lot trials were good, I did not hit any wood post this time.
    Straight line water travel and large arc turns were good to fair

    Bad:
    Elevator clips for adjustable angles slipped up the shaft(s).
    Balance still allowed some mid-ship/mid-bag spray blasts of water when coming off cushion.
    I found myself sliding sideways when letting off the gas going down a steep drive.
    Tight water turns were slow to respond and resulted in coming off cushion.

    Conclusion:
    About the last "bad" item. This may indicate there is still large amounts of skirt drag under certain conditions. These conditions get worse very quickly because of the heavy skirt material. This heavy skirt drag once it happens does not allow the hovercraft to "recover" and get back on cushion.

    Weather Conditions:
    Christmas Eve, sunny 47 degrees F, and very little wind. What state am I in? Michigan, I know it's crazy - unheard of.

    Merry Christmas everyone. :)

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    Video going out on to the lake:
    http://www.youtube.com/watch?v=UEWQS8TCnDo

    Video coming back in and dragging it:
    http://www.youtube.com/watch?v=55L1HwYOvKQ

    I went in and out of the lake several times, and "at will". The results were always the same. Sharp turn would slow the craft down and force me off cushion, dragging the skirt. Large arc type turns did okay (270 degrees) until facing a slight headwind then there was no way to keep speed up, not enough thrust.

    There is not enough lift and thrust to overcome the problems the heavy skirt material has caused. What I have now will make a fine "ice skirt" but I don't see it ever see it becoming a good water skirt without more development. As it is now I can just experiment with it, and not enjoy it in any other manner.
     
  6. Figgy
    Joined: Feb 2006
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    Figgy Senior Member

    Cool vids! That looks like fun, you'll get it all sorted out!
     
  7. kach22i
    Joined: Feb 2005
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    kach22i Architect

    I've been thinking about the rear wake which follows the hovercraft up to shore.

    What would cause that?

    There is no bow wake, so the skirt must be catching/contacting at the tail end.

    I thinking of removing what I've called "The Cape" (large anti-drag flap) under the stern of the craft. I will then put a new stern bag skirt on the craft, one designed to seal the rear corners by it's self - trapazoid in shape.
     
  8. Albert.L
    Joined: Dec 2006
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    Albert.L Junior Member

    I should think the only reason you would get a rear wake is that the air is not leaving the skirt equally all the way around. It seems to me that it is all coming out the front.

    Regards,
    -Albert
     
  9. kach22i
    Joined: Feb 2005
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    kach22i Architect

    Yes, this is similar to what someone said on the hovercraft forum (HCA). Looks like I over-corrected to get a bow up attitude and still have to balance the craft out level.

    Of course throwing more power at the problem rarely hurts.:)

    As with any craft the center of gravity shifts with speed.

    When you accelerate it's thrown back, when you brake it shifts forward.

    Even though I used calculations and a pipe to balance the craft, this balance is at static hover, not at speed.

    I added too much weight in the back, but how do I determine the center of weight/gravity at speed?

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    Let's say you have a body in motion, a car, an airplane or hovercraft - does not matter, looking for basic laws of physics here.

    1. You know the weight.

    2. You know the center of gravity at rest.

    3. You know the center of lift (if applicable).

    Let's say our object weighs in at 600 lbs.

    Lets say our object is going 30 mph.

    Q1:
    How much does the center of gravity or weight shift back?

    Q2:
    Lets say I need to start or stop very quickly, how do I calculate the location shift of CG backwards and forwards?

    I think the key is taking the the actual weight and turning it into mass, then take into account speed the horizontal force, which in turn may reduce the gravity force or weight over a given time peroid.

    An actual example would help, then I can apply it to my situation.

    EDIT..........................................................
    Perhaps I've tried to find a universal solution that just does not apply.

    Example of what I meant.

    Weight Transfer-1
    http://www.msgroup.org/TIP074.html
    Stern/rear hovercraft skirt drag might be a version of torque or braking power applied.

    1. Car/Motorcyle; weight and center of gravity stay the same unless braking torque is applied or aerodynamic forces have an influenence.

    2. Boat; weight and center of gravity are the same until displacement mode is replaced by planing mode, then there is a shift to the "center of lift". This is countered by tabs at the rear which induce drag counterforces and angle changes.

    3. Airplane; weight and center of gravity are the same until speed increases lift on the wings causing a shift which is countered by the elevator (typically at the back).

    4. Hovercraft; once it's over the "hump speed" which is also a boat term with a slightly diffenerent context, it is in level flight. Weight is centered until torque (paracitic skirt drag) is encountered in the form of wave impact and aerodynamic resistance.
     
  10. Albert.L
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    Albert.L Junior Member

    I have to ask, what good will knowing the center of gravity do you? I should think that adding weight to balance the craft would kind of defeat the purpose.

    It would seem to my untrained eye that you have too much skirt or a skirt that is too heavy (something you mentioned before). So why not cut back a few inches from the diameter, aside from practical considerations (how much time would that take? Or would you just tape the extra up inside?), and see what happens? I might fix your problem.

    [edit]As to your CoG question: How about a tube of water that sits paralell to the ground? When you acellerate, tilt it forward, and vice versa when you decellerate. Its an idea that popped into my head just now. :)[/edit]

    Regards (Oh, and nice work!),
    -Albert
     
  11. kach22i
    Joined: Feb 2005
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    kach22i Architect

    Just as in a boat or airplane the center of mass/weight/gravity should centered on the center of lift - or be near enough to it to be balanced. Cars/trucks/motorcycles just have more weight on a front or back axle.

    Part of the goal was to increase the obsticle clearence height, which means more skirt material. What I failed to take into account is two things.

    1. More height means more energy going into the air cushion system to do "work".

    2. The closest design to the one I now have uses more lift energy than simple bag skirts and proven finger skirt designs.............but has the added bonus of more obsticle clearence.

    I put more lift energy into the system by stealing thrust energy (intergrated system). I need the thrust in turns because of the added skirt drag from contact surface area in banking or "lean-in".

    I plan to taper the skirt an inch up in the back as I go along the sides. I will then cut a new stern skirt, perhaps sticking with the double bag but eliminating the anti-drag flap which I'd like to call the big-drag flap at this time.
     
  12. kach22i
    Joined: Feb 2005
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    kach22i Architect

    UPDATE: USA, Michigan, Gun Lake, March 10 2007

    Video- on the ice:
    http://www.youtube.com/watch?v=N5tKir2Or9s

    I need to take about a foot of fabric out of the skirt, refit to the front and the tail to get rid of wrinkles and pinches (see arrows). Secure the sponson tails to the "curtain rod" to prevent side-way drift openings between the stern skirt and inflatable sponson skirt (see lines drawn in).

    My made in China $15 boots are a similar material to the skirt, and the same color- I'm styling man.


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    Note: If the ten photos above do not appear, click the link below to see them.

    http://forums.pelicanparts.com/showthread.php?threadid=297431&perpage=20&pagenumber=3

    More pictures here:
    http://www.hoverclubofamerica.org/forum/index.php?act=module&module=gallery
     
    Last edited: Mar 15, 2007
  13. kach22i
    Joined: Feb 2005
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    kach22i Architect

    UPDATE 06/28/2007

    The original finger skirt was taken off four years ago to repair the hull and duct, which were damaged. The damage was caused by losing a bottom aluminum rail to a rock in a river, this caused a gap between the two hull halves which came undone in a dramatic way about a year later and almost left me stranded.

    I never liked the constant spray in my face of the original skirt; the sorry obstacle clearance and the inherent instability which made it a poor cruiser yet gave it fair maneuverability and banking ability.

    I sought to change and improve what I thought I could, even tried to make it quieter in the process.

    The point of the continuing experiment is to learn as much as possible using the school of hard knocks method or perhaps an artistic rather than conventional engineering method. The excitement and possibility of doing something which has not been done before defies practicality and logic yet is the sprit of many inventions and innovations. Graduating from the theories found reading and the play of models, I sought to “scale up” my experience and knowledge base within a reasonable budget and investment of time.

    In retrospect I could of built a hovercraft from plans with the same amount of effort and time, but I don’t think I would of gotten out of it what this furrowed path has brought.

    Concept: Hovercraft operates in water the majority of the time, make the skirt more like a boat hull and see what happens.

    One of the rescue workers said of my Sunday morning river run; we had to do a double take, it looked like a Zodiac at first.

    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

    This thread is the Third Generation, and it may not be the last - we will just have to wait and see.

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    It's a work in progress, I'll try to back-track as much as possible.

    The stern skirt which kind of looks like a diaper with temporary straps got torn up on some rocks before it could be adjusted for air pressure and final connection to the two side skirts. There is some serious re-engineering going on in my head about this right now.

    The gray skirt is ballon material, a large bag filled (almost 90 feet x 4 feet) weighed in at about 10 pounds, I used 7 pounds. The old yellow skirt weighed in at 60.5 lbs....................a weight savings of over 50 lbs!!!!!!!

    I also redid the stern counter weights by removing the dumbells, and placing steel bars on the twin tail farther from the center of lift and center of mass/gravity. This was another weight savings, I'd like to take all the weights off by just having a larger engine one day.

    The hot air balloon material is not very abrasion resistant, I may be going to a conventional material or combination material skirt in the fall.

    Big thanks to my wife Kim who did all the sewing.
     
  14. kach22i
    Joined: Feb 2005
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    kach22i Architect

    This sketch may help you understand the inner workings.

    http://www.hoverclubofamerica.org/forum/in...si&img=1145
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    Video of first flight on grass, later flight on wet grass in the morning were much faster and encouraged me to go out on to the water several times.

    Flight 10a (grass):
    http://www.youtube.com/watch?v=Mt-EhS40788

    Flight 10b (going out in water):
    http://www.youtube.com/watch?v=xs_58qICi0c

    Flight 10c (coming back in to shore):
    http://www.youtube.com/watch?v=ywfD2-c_4lw

    The pressure in the skirts was much higher than the capture air bubble under the craft. When I tried to turn the stiff skirt burried its self into the water slowing the craft down enough to take it off cushion. With no reserve thrust I could not power out of this situation and had to make it to shore flying across the sandbar and getting back on cushion. Water operation on straight high speed runs was encourging, but not being able to slow down (no brakes) really was scary as the water level was low and many sharp rocks were everywhere.

    I tried to balance the air pressure, there are two (2 inch PVC pipes) feeding the stern skirt, and only one 3 inch hole feeding air from the duct to each port and starboard spnoson/pontoon skirt.

    I can either bleed air out of the bags like an Aerocruiser or attempt a combination bad/finger skirt of some sort. I could also just have very tiny holes feeding the bag skirts which will give me more lift. I closed off six other air holes each side, when they were all open the bags were ready to burst and over the grass performance was not as good.
     

  15. kach22i
    Joined: Feb 2005
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    Location: Michigan

    kach22i Architect

    The "diaper strap" cause some drag, but less than I had feared.
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    Here I am stranded out on racer's island, turned out I made it back to the boat launch on my own, but still needed the assistance of the rescue workers to get it on the trailer- great guys.
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    The reversing of the air flow at the bow at an acute angle does not happen as I once planned, which is why I by-passed it and installed the round HVAC ducts - they work but look very retro fitted and "after thought" like.
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    Cutting up a Scat is more fun than it looks, I'm still looking for things to cut up. :lol:
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    I'm getting air leaks from the air traveling back through the weeps, it's making my dash look like Jiffy-Pop popcorn. :huh:
     
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