Airframe Construction: Part 1, the fuselage|
The constructional techniques used for the DIY cruise missile are deceptively simple yet extremely effective.
The goal was to produce an airframe with a relatively low radar signature and which involved the use of materials that could be sourced readily without raising any suspicions. For this reason, foam and fiberglass composite techniques were used.
The result is an exceptionally light yet very strong airframe that is aerodynamically sleek but which could be built in any garage with only a minimum of tools.
GRP is used to make many different items including boats, car bodies, snowmobile body-panels, etc. However, in most cases these are items that are produced in quantity so special molds are made so that the composite material will take on the required shape. After the resin has cured, the boat, body panel or whatever is popped out of the mold and another can be made.
It makes no sense to go to all the trouble of making a mold for just one (or two) missiles so I've used a different technique for ensuring that the GRP takes on the correct shape.
To get the right shap, a polystyrene foam plug is created using a hot-wire cutter. Once this plug is cut and sanded to shape, the fiberglass cloth and resins are applied directly to it so as to form a layer that hardens into a very rigid composite sandwich.
Creating the fuselage plug
The first step is to create a foam plug for the tubular section and that's done
by cutting four pieces of polystyrene foam then gluing them so that they form
a box-section as in these photos.
I found that the best glue to use when joining the four strips of polystyrene is one of the polyurethene adhesives such as the one sold as "Gorilla Glue".
It is important that glue only be applied to the *inside* edge of the join line because if you glue towards the outside edge it may interfere with the hot wire cutting process.
The next step is to cut two plywood squares that will slip snugly into the end of the square foam plug. A hole is drilled smack dab in the center of these squares so that a rod can be slipped right through. This can be seen in the right-hand picture above.
Now we have a plug on a spit -- and we can rotate it on the rod like a pig on a BBQ.
Square pegs and round holes
To do this, a simple jig must be constructed and wood is a good material for this. As you can see in the picture below, the jig is simply two squares of plywood or particle board and three lengths of 2"x1".
The square plug is supported at each end by passing the rod through the end of the jig -- which also enables the plug to be rotated as required.
It's important that the side-runners of this jig aren't glued in place, just screwed. That's because they will be gradually lowered as the square plug is formed into a cylindrical one.
Initially the runners should be positioned so that their top edge is about an inch (25mm) lower than the highest point on the plug when it's rotated so that it's positioned such that it looks like a diamond with the edges at the 12, 3, 6 and 9 o'clock positions. If the hotwire bow is then passed along the length of the plug, supported by the runners, you should end up with something that looks like this.
If you've built things accurately, the amount removed from the plug during this first pass of the hotwire bow should be even along the entire length and remove the same amount of material from each corner as it is rotated between passes.
Once you're happy that your plug is nicely centered and that the hotwire is cutting properly you can progressively lower the side-runners on the jig and continue to whittle the square plug into a circular cross-section ny taking multiple length-wise cuts and then rotating the plug as shown here.
Eventually you'll end up with a very multi-sided plug that is almost circular in cross-section. At this stage you can get out the sanding block and sandpaper to complete the operation. Once finished you should end up with a very nice and smooth tube of polystyrene foam as shown above.
Making the nose-cone plug
Well surprisingly enough, this is no more difficult and uses almost exactly the same technique.
The only difference is that this time, instead of using straight runners to guide our hotwire bow, we use a pair of curved runners cut from plywood or particle board.
Once again we glue pieces of polystyrene foam together to make a (this time solid) block. We then pass a rod through the center of that block so it can be rotated just as the fuselage plug was.
There's no need to make a whole new jig because the one we've just used for the fuselage can be modified to do the job as shown in these pictures.
Again, once the nose cone plug is whittled down to near-circular cross-section it can be sanded until perfect.
The end result of all this work will be a tubular fuselage and nicely aerodynamic nose cone ready for fiberglassing.
Next: Fiberglassing the plugs (coming)
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