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Way, Way OT: DIY hovercraft?

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  • Way, Way OT: DIY hovercraft?

    I am bored, and toying with the idea of starting another useless project. I have always wanted to build a HC since I was a little kid. Actually got quite a ways on one when I was about 14 or so, then girls and cars happened, and well, you know...

    So, has anyone on here built one? I have a good idea on how to accomplish the basic design, and what to power a small one with, but I am not turning up much in the way of how to design a prop or fan.

    I realize that there is a lot of engineering that goes into fan and prop designs, but for a scrap built HC, can you trial actual fans (like cooling fans) of the approximate diameter, or do you have to adhere to a strict pitch/surface area combo? What are some good sources of serviceable fans? I think for a 6Hp or so lift motor, I want a fan of approximately 28" diameter. I looked at ebay, looking for props and fans, and there really isn't much. New stuff will make this project just a dream, and never a reality, so I really can only do surplus if I try.

    When I was a kid, my best friend and I actually hand made the prop for the lift fan. Hand carved from IIRC a birch 2x4 which was recommended to us by some old aviation enthusiast we talked to. It came out well, and had good balance. The project kind of stalled out though when we bolted it on the old lawn motor engine, fired it up, and found that we cut the airfoil for the wrong direction

    Someday, down the road, I would live to build a ground effect craft. I see them advertised on line, but you rarely hear anything about them. If they really work, they would be a blast...

    Later,
    Jason

  • #2
    Well if you need some good skirt material I have some.Company I do work for makes the skirts for the LCAC.The LCAC itself is built not far from here by Textron Marine.

    http://www.fas.org/man/dod-101/sys/ship/lcac.htm
    I just need one more tool,just one!

    Comment


    • #3
      Originally posted by wierdscience
      Well if you need some good skirt material I have some.Company I do work for makes the skirts for the LCAC.The LCAC itself is built not far from here by Textron Marine.

      http://www.fas.org/man/dod-101/sys/ship/lcac.htm
      I will be certain to look you up if I get the project off the ground (ha ha, that was puny)

      What do you have? Is it just heavy gauge nylon? Looking at the link, the stuff that goes on those big bastards might be way to heavy, and significant overkill for this...

      Comment


      • #4
        I worked for a short time for a company in Edmonton that was building several different models of hovercraft. One I worked on was a personal single seat prototype. I also was the "test pilot". The most important thing we learned is that you need power, lots and lots of horsepower. Estimate what the most hp is that you can hang on the beast and then find a way to double it. Your best bet is to use separate engines for lift and thrust, just like the big ones do. The lift system takes way more power than it seems it should.

        You also need to build light, as light as possible. Every ounce matters. Although it didn't exist back then foam composite construction would probably work well. The reason for the lightest possible weight isn't so much for lift, it's to minimize inertia and momentum since every move you make has to overcome that. Also, if you intend to fly it over water make sure you install positive flotation. Don't ask me how I know that. Foam composite construction will provide that automatically.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          Separate engines are already the plan. My thoughts on the hull are to use laminated foam, thin birch plywood and fiberglass shell. I am pretty good with hand laid fiberglass. This side of stuff should be the more easily accomplished side of things. It will just take time to do it right. For a first machine, I am thinking about 3'x10', with a traditional boat type V shape. Ducted lift fan in the bow and thrust fan in the stern.

          I have found some similar sized crafts on line and the guys are using a leaf blower for the lift fan, and weed wacker for the thrust fan. Supposedly hitting about 25Mph. So I'm thinking the 6Hp B&S I have for a lift motor will be more than adequate, and as for thrust power, well, I don't specifically have any small engines that would be good, but I have several small auto engines which are likely far to heavy, or a 10Hp or so horizontal shaft engine can be had for reasonable money as surplus.

          Keep it coming! Do any of you know about fans?

          Later,
          Jason

          Comment


          • #6
            I built a small electric powered one for a friend. He had the bright idea to slide into our physics class on a hovercraft and ask his girl to prom

            Many rolls of duct tape later, it worked but his entrance was not as "cool" as he would have liked. We found that standing while riding a hovercraft is not the optimal posistion

            We used a leaf blower and a piece of plywood. The electric leaf blower provided both the lift and the thrust, but this was on linoleum tile floors carrying one lightweight guy.

            she said yes, btw. I guess she had a thing for geeks...

            Comment


            • #7
              You need a multi blade ducted fan for the lift fan with as many blades as possible. It needs to resemble a turbine because it must be able to compress air. A regular prop cannot compress air and even something like an electric rad fan doesn't have enough blades.

              BTW, if you live in a normally dry area forget the idea. A hover craft on really dry ground resembles a sand storm.
              Free software for calculating bolt circles and similar: Click Here

              Comment


              • #8
                Here you go this may be away to get your fix and not break the bank http://www.hovercraftmodels.com/
                I have both the books and found them interesting, The RC book also has the info about the problems one encounters.
                Anyways the kits look neat or you can always go to tower RC as they always seem to have the low buck RTF ones on sale.
                http://www.towerhobbies.com/index.html
                Glen
                Been there, probably broke it, doing that!
                I am not a lawyer, and never played one on TV!
                All the usual and standard disclaimers apply. Do not try this at home, use only as directed, No warranties express or implied, for the intended use or the suggested uses, Wear safety glasses, closed course, professionals only

                Comment


                • #9
                  Originally posted by Fasttrack
                  I built a small electric powered one for a friend. He had the bright idea to slide into our physics class on a hovercraft and ask his girl to prom

                  Many rolls of duct tape later, it worked but his entrance was not as "cool" as he would have liked. We found that standing while riding a hovercraft is not the optimal posistion

                  We used a leaf blower and a piece of plywood. The electric leaf blower provided both the lift and the thrust, but this was on linoleum tile floors carrying one lightweight guy.

                  she said yes, btw. I guess she had a thing for geeks...


                  One of the kids in my daughters class built one for their science fair. A Toro leaf blower, a piece of plywood, some blue tarp material and lots of ducttape. I weigh over two hundred pounds and it lifted me.

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                  • #10
                    It is far from dry here most of the time. It should be a fun toy, even in the winter.

                    Back to fans again... Evan, what you are telling me is that I need a fan with several blades, and a very high pitch? Is a fan something I could potentially tackle making in my HSM?

                    According to some math I did last night, it looks like for an approximately 3x10 sized machine, I need about 5500 CFM and .375 inches of water. That is based on a total weight of somewhere around 800lbs and .5" lift height (bellow the skirt). There are lots of fans out their that come in around that airflow. The thing that I am not turning up for your average fan, is the kind of pressure it can make, but .375 inches of water is not that much pressure.

                    Later,
                    Jason

                    Comment


                    • #11
                      The thing that I am not turning up for your average fan, is the kind of pressure it can make, but .375 inches of water is not that much pressure.
                      That is because a fan is not a compressor. When designing for subsonic airflow at less than about half the speed of sound air is treated as an incompressible fluid for most purposes including the design of propellers and fans.

                      On the prototype I worked on we had an interesting fan system that included a hub with a multitude of slots. It also included a box full of blades that could be installed in the slots to make whatever number of blades you wanted as long as it was balanced. We ended up installing all the blades, maybe 20 or so.

                      As I said, you want something that resembles a turbine wheel on a jet engine. That is the only type of fan that can compress air and that is why they are called compressor fans. As for pitch, low pitch produces the most compression and high pitch the most volume. You trade one for the other. Of course, more volume requires more horsepower. You might want to try calculating how much mass per minute you are moving. Air has non-negligible mass.
                      Free software for calculating bolt circles and similar: Click Here

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                      • #12
                        A motorcycle engine seems the way to go if you want it light. 2strokes even provide the rpm you'll need.
                        Should you make the same mistake again... most two-strokes are quite happy running in either direction after a few minor tweaks.

                        Runt

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                        • #13
                          Well, I don't have any motorcycle engines, but I know guys into dirt biking, maybe they can point me toward surplus. I think I would prefer not to go with a 2 stroke.

                          Evan, I saw the fans you are talking about last night in my research. I didn't turn up any hubs that could take that many blades, but the adjustable fixed pitch, replaceable blade fans are out there and widely used for bigger HC's from what I found. It looks like a fixed pitch non adjustable fan designed specifically for this application is about $249. I will have to keep looking for surplus...

                          Later,
                          Jason

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                          • #14
                            While you are "designing" make sure to include the weight of material to shroud the fan. It needs to be a scatter shield, not just a duct, in case the fan inhales something it can't digest.
                            Free software for calculating bolt circles and similar: Click Here

                            Comment


                            • #15
                              In high school we built a hovercraft as a special class. We bought the prop from a company that specialize in these props. They wanted to build one like in the plans but there was no way I was going to trust a prop turning at 3600 RPM buil by high school students. It was about 24" in diameter, 4 blades. I think around $200. Made a fiberglass duct for the fan. Powered by a 8HP (May have been bigger) Briggs vertical shaft engine.

                              Worked pretty well. The skirt was one problem though. Most of the plastic I found wasnt heavy enough and would wear through. I did find some cloth reenforced rubber but we never got around to installing it.

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