Lift Duct







    All right, the fun has begun.  Over Spring Break, I finished sealing the bottom of the craft with fiberglass tape and epoxy.  I also started work on the lift duct.  To cut the hole in the bottom, I measured to find center where the hole was to be cut, and drilled a 3/8" hole.  To draw the circle, I put together a make shift compass from some scrap wood.  Raising the compass arm above the height of the main stringer allowed the arm to move freely.  I drilled a hole through all of the pieces and sandwiched everything together with a 10" 3/8" bolt.  I measured the radius of the duct hole on the compass arm and drilled another hole for a long pencil.  This allowed the pencil to the contour the hovercraft, giving me a near perfect circle.

    The point of no return.  I started cutting out the hole with a jig saw.  Note, when cutting the hole, do not cut through the main stringers, cut around them.  They need to be left in place.  Cutting through them would be BAD!

     The hole is cut.  There's still some ruff work to do. I thought it best to take the rest of the skin off with a belt sander.

    Here's the lift fan.  I bought it from Universal Hovercraft fully finished and balanced.  Under the fan is one of the wood discs used to form the lift duct.   The disks were cut as round as possible with the help of my old high school shop instructor, Greg Adams, and a table saw.

    This is the lift engine.  It's a 20 hp Kohler Command, vertical shaft engine.  I never imagined I'd buy an engine off of the web, but I did, from Tulsa Engine Warehouse.    The engine is so new, I had problems finding a manual for it.  The metal hub on top will connect the fan to the engine shaft.  The hub came from UH.

    I went home for Easter and of course, worked on my hover.  The duct was formed using the plywood disks and the same 1/8" ply' used for the skin.  The disks were spaced with 2X4s all cut at exactly the same length using a stop on the miter box.  The ply' was wrapped around the form, grain vertical, and attached using a brad nailer and 3/4" nails (sparingly).  I put 2 or 3 nails at each seam and 1 every 6" or so which worked great.

     I purposely made the duct a little bit taller then needed.  I dry fitted the skin,  put the duct at a 90 degree angle to the top skin, then scribed a line on the duct and cut it to size.

    And this is what it looks like right now.  I ran out of time to pure the foam, so it'll have to wait until mid May when I come home for the summer.  The foam used for the duct is a 2 part expanding urethane foam, which when cured, has a compression strength of 40 lb psi!  That's after it expands up to 30:1!  1/2" pink foam board will be used to contain the foam around the duct.

    The foam's been poured and is now set.  I used a 2X4 and 2 3/8X10" bolts and nuts to support the duct while the foam was being poured.

    Cardboard was used to hold the foam within the hovercraft, but as you can see, the foam expands so much that it pushed the cardboard outwards a bit.  No problem, just cut it away.  Also as you can see, the foam sticks to everything making removal of the cardboard rather difficult.

    I cut away the excess foam with a 3" long heavy duty utility knife.  This would have made sanding much easier.  However, it made my job a lot harder and will prolong work on the duct.  See, while I was cutting through the foam, I came to a denser foam patch which required more force with the knife.  Well, after I cut through the foam, the knife kept going, with a lot of force, too....right into my left hand.  4 hours in the ER and 5 stitches later, I'm without the use of my left hand for 2 weeks.

    Another shot

    The next step is to sand the lip radius.

   I decided to fill the entire bow with foam.  This added both duct strength for the engine mounts, and flotation.

    In preparation of installing the skin, I cut away the excess foam with a small toothed wood saw and a long utility knife.  I also laid wire down for the bow light and sanded the lip radius with a belt sander, random orbit disk sander, and a block sander.  Then, I applied a layer of wood filler to the lip radius to fill all of the air holes and imperfections.  When sanded and fiberglassed, the lip should be very smooth and flow nicely.

    As the foam expanded, it pushed the skin up a bit.  You can see it's a little uneven.  This shouldn't be a performance problem, more of an aesthetic annoyance.

    I drilled the bow light hole and ran the wire through it.

    More of the wire.

    Before I attached the skin, I coated the underside of the skin with epoxy.  I would be unable to coat it after it's been installed.  I also brushed on epoxy to the stringers and ribs.

    All of the skin, aside from the flat bottom skin, and pieces that had to be form fit were cut slightly larger then needed, glued and nailed on, then trimmed and sanded off after the epoxy set.  This gave the hover near perfect exterior edges, ready to be taped.  A note on taping corners.  It's best to round off the corner slightly because fiberglass doesn't like to make sharp bends and stay while the epoxy sets.

    Next step, sand and glass the upper duct.  After that, I'll flip the hover upside down for the last time, finish up the bottom, then flip it back upright where it will remain.  Notice I haven't attached the side skin on the nose yet.  It's best to wait until you've located and installed the lift engine mounts before you skin the side.  This way you don't have to cut through the skin to install the mounts.

    The hover has been flipped over and work on the bottom of the duct is underway.  As you can see, the floor aft of the duct has been angled up (when right side up) so to reflect water down preventing it from splashing against the duct wall and up into the duct.  Also, I cut the bottom sides of the duct opening back a few more inches to promote more air flow back.  That, and it just looks damn nice.

   I used 2" foam triangles to keep the floor angled up.  Their epoxied to the floor and main stringers.  I ended up cutting the floor totally off because with it attached, the center bowed down.  This way I was able to achieve a uniform angle across the floor.

    The bottom of the duct has been fiberglassed now.  The joints where the floor meets the foam and where it was cut away were given extra fiberglass and epoxy.  Next, I'll do some sanding, then fiberglass in the air splitter for the skirt feed.  After that, I'll paint the bottom, then flip it back over and start work on the cockpit, steering controls, and thrust duct.

    Notice the duct lip radius, how it flows into the duct.  This helps the lift fan suck in air with less effort then a 90 degree inlet.

    Another view of the duct.

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