Thrust Duct Construction
This page illustrates some of the steps taken to fabricate the 60" thrust duct for my UH-19P.
The method used here is not the same method Univeral Hovercraft describes in their construction booklet. However,  several builders choose to use this method  to obtain a more
uniform duct.
To construct the 60 1/4" disks 2 each, I used 4  1/2" X 4'X8' sheets of plywood. Butt spliced the plywood together, then used a router with an extension arm to cut out the disk. The router will allow you to cut out a nearly perfect circle. The disks then were seperated by 2"X4"s  used as spacers.
I used a holesaw to cut the 1 1/2" hole in the center of the duct to install the drive shaft . I would strongly recommend using a reamer to obtain a close tolerance hole.
After the disks were formed the process of installing the 1/8" ply is nexted. Unlike the disks, the ply will remain as a intricate part of the duct. Just before adding the 1/8" ply, trimmer line was wrapped around the disks and taped in place. After the duct is completed and mounted, the trimmer line will pulled out and the disk removed. The 1/8" ply is held in place  on the disk using  a few 1" small wire nails.
A stack-up of 4 each 1" blue foam board is used for the core material in the duct. I used epoxy resin to glue the foam to the 1/8" ply.  wanting to get a "feel" of how to install the foam,  I installed just half of the first layer, it went well! 
Just a note: If you plan on using  the "foam in the can" to glue the foam boards together, you will need a couple of extra hands to help align the foam board before the "foam in the can" begains to set up.
As you can see, the first layer of foam board is installed and strapped down.
After the 4 1" stack-up of foam boards were installed, and after the epoxy resin had time to cure, the duct was ready to form. A simple hot wire was fabricated to cut the foam as seen in the image above. I used .020" aircraft safety wire and a battery charger to do the job. The hot wire bow is spring loaded to keep tension on the wire as it heats up.
I clamped the bow to a sawhorse, positioned it at the right angle, and begain to cut the foam.
I rotated the duct into the hot wire and shaved off the foam. Did the the home made hot wire bow work well you asked? Like a warm knife through butter!
After trimming the duct, I discoverd a few gaps that were easily filled with "Great Stuff" foam in the can.
Here's where the real fun begains, shaping the duct! As illistrated in the images above, I used a 1/2 H.P  electric motor, 16" home made wooden pulley to spin up the thrust duct.
I bolted the pillow blocks to the saw horses, installed locking collars on the pillow blocks, thrust duct, and the wooden pulley. Plugged in the electric motor,  and stood back! The duct began to spin up, I would guess at max. speed around 200 to 400 R.P.M. Standing in front of this thing was a bit intimidating. The thrust duct isn't very heavy, 25 to 40 l.b.s. max,  however, at the speed it was turning if it were to come loose and hit me, I'm sure one would have had to pry me out of the alminum garage door!  Anyway,  I began to sand down the outside of the duct. It worked great!  The sanding process didn't take long at all, but, it did make a mess!
After the outside of the duct was pretty much were I wanted it, I begain to form the inlet lip. This was a bit tricky.  I made a templet to maintain the proper radius, however, making the inlet perfectly round  even more difficult. There are several high and low spots that I still need to contend with. Over all the duct is turning out pretty good!

Oh yeah, Hey Rod, thanks for letting me use that great electric motor!!
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