Thrust duct foam layout

Duct construction method

(Note this is a 2-man job)

I have started to build the thrust duct and have written out a guide on how I have done it.

Many other hovercrafts have done this job before me, and this is how i have done mine. please bear in mind that this is only a guide and should you choose to do yours in the same way that is your decision and yours alone. i claim no responsibility for your efforts. 

thanks for looking and good luck with your build.

1.    Make two 1530mm disks from 12mm plywood using a router. The measurement from the centre of the disk to the outside edge (that is, the radius) should be 765mm.

first forming disc

2.    After cutting the disks, drill a 35mm hole in the centre of each disk. Then mount them both onto a 35mm shaft.

3.    Support the disks and shaft on a frame. The frame should hold the disks so that the centre of the disks is at least 900mm from the ground. This will allow the disks to rotate on the frame while you wrap the plywood around the forming disks (see no.4).

mounted to frame

4.    Cut ten 2”x4”x6” blocks. These blocks will be used to hold the disks apart.  Screw each block in place, using 2 screws in each end of each block.

block of wood to hold discs apart

5.    At this point you could tape some strong fine nylon string around the edges of the forming disks. This could be helpful when it comes to removing the formers from the completed duct.

6.    Cross-cut 4 pieces of 3mm X 300mm plywood. Join together to get the 4,852mm length needed. Use 6 oz plain weave glassfiber cloth on topside and underside of each of the three joins.

cross cut plywood

7.    Sand and feather the forward edge of the 3mm plywood to start the inlet lip radius. Mark out a centreline along the full length of the ply and a line approximately 63mm in from each edge, these lines will help to keep the whole thing even when wrapping around the former.

1.    Pin down one end of the plywood. The two outer guidelines should line up with the outside edge of the forming disks. Then tighten in place around the forming disks using a ratchet strap. When you are happy that everything is centred and tight all the way around the forming discs it is time to nail the plywood down along the outer edges and end, glue the open ends together. Use as few pins as possible to make removing the disks easier.

Top tip: It can be a good idea to give the plywood a coat of resin to seal it first as the resin will be absorbed by the plywood.

Next to cut out the foam for the outside of the duct

pined down

9.    My sheets are 600mm X 25100mm. First place a sheet on the table and centre it. I screwed blocks to the table to hold the sheet in place like a jig, now your jig is set to take the next sheet and so on. (Every sheet should now be exactly the same cut.)

50mm foam

blocks to hold in place

10.    I marked out my sheets by making a compass 4m 40mm long

11.    Drill a hole at the top end of your compass and a second hole 457mm from the centre of the first hole. Locate your felt pen at the centre of the top edge of the sheet and begin to move the compass from left to right; do the same for the second hole. You should now have an arc shape, the two parallel lines will be 457mm apart. 

12.    Now measure in from the end of the sheet 152mm to the top line and 280mm from the end of the sheet  to the bottom line.

13.    Measure between the two parallel lines at these two points to confirm that you have 457 between them and draw a line from top to bottom to join these lines together.

perfect arc shape

14.    Now you should have the perfect shape for your first segment. Repeat this process for the next two sheets. (Note although you have cut 3 sheets you will use only about half of the third sheet.) The overall length of the outside radius should be 543.4cm.

15.    Lay the segments down on a flat surface and butt join together using 75mm screws, screwed in from the edge to the next sheet. I used one screw per sheet per side.

butt join

75mm screws

16.    You need to leave a 25mm margin along the forward edge as this needs to be kept free of cloth and resin. Mark a line 25mm in from the leading edge then apply masking tape along this line.

Top tip: before working with the glassfiber cloth, clean the bench and the foam sheet of all dust and grime as this could become trapped between the cloth and foam sheet

applying masking tape

17.    When you’re happy that this is correct, fiberglass the top side using 6oz plain weave glassfibre cloth. Only fiberglass from the trailing edge towards the forward leading edge.

glass finer cloth

glass finer cloth

glass finer cloth

epoxy resine

epoxy resine

top side coated with resign and cloth

18.    Immediately after epoxying the cloth to the sheet it is advisable to remove the masking tape. Remove tape by pulling it away in a forwards direction away from the cloth. It may be necessary to check that the leading edge of the cloth has not lifted after removing the masking tape.

19.    Flip the foam over and measure 230mm from the trailing edge towards the leading edge. Draw a line the full length of the foam. And another line 100mm from the leading edge.

line 230mm and 100mm

20.    Then draw another line from the point where you marked the 230mm down the edge of the foam to 5mm.

21.    When this has been done, it is now time to cut the foam along the line between the line and the trailing edge. Use masking tape along the top line as a guide. Masking tape can also be used along the full length of the 5mm line.

flat sanding the wedge shape after hot wire cutting

22.    Now you should have a length of foam that resembles a wedge shape with a flat sloped surface of 230mm. The wedged surface should be sanded as flat as possible.

23.    Using a hand saw cut slots across the width of the foam, being careful not to go all the way through. Use 12mm plywood or make a jig to stop the saw from cutting all the way through. The saw cuts should be 25mm apart. Do this along the full length of the foam as this should make the foam easier to wrap around the duct.

Top tip: Practice wrapping the foam around the duct before glueing.

24.    Now, if you are confident, it is time to glue the foam to the former using the epoxy resin. Make sure you have an assistant; this is not a one-man job.

25.    Use ratchet straps to hold the foam in place, the trailing edge will need to be clamped, (clothes pegs can be used to do this).

26.    It may be necessary to add filler pieces of foam to the leading edge.

27.    Sand the inlet radius as shown in the drawing and fiberglass with the 6 oz cloth overlapping the plywood by about 25mm.

28.    Sand and fill any low spots using epoxy filler. Do not use anything other than epoxy based products as anything else will react with the foam.

29.    Paint epoxied surfaces a light colour to protect from sunlight.