Now that the forms are made I'm ready to form the fuselage shells. This method is based on techniques worked out by Harpye several years ago. The key innovation was the use of packing tape as the female mold. The main benefits are that the formed foam is inherently stiff for it's weight, it's easy to build from a simple 3-view or even just photographs, it's cheap, and once you have the forms made, additional copies can be made quickly.
I'm using 1/4" fanfold foam but I've used Dollar Tree foam and depron too. The taping of the foam (and the oven I built) has been covered in detail is these threads:
so I won't bore you by repeating it here.
Once the foam is taped to the form it's baked at 100C for 15 minutes. Thinner foam takes less time and you have to be careful not to cook it too long or the form can be damaged. After it comes out of the oven I trim the shells flush with the top of the backer board and mark the former locations using the saw cuts in the backer boards. From this point everything is done in situ
so I draw an accurate reference line as shown below to make the wing and stabilizer cutouts.
This is the 8th model I've done with this technique and I've successfully(?) destroyed 3 of them so I've seen how they fail. The formed shells make a very stiff fuselage so no structure is needed for that. And whether covered with fiberglass, paper, or Styrospray 1000 (aka liquid sheeting) they don't fail under tension. The weakness of the foam is compressive strength: on impact the foam collapses under the skin. This is harder to repair than a clean break. So I've been experimenting with adding carbon or bamboo rods or sheet balsa to add some compressive strength to the system.
On the He-219 the sides are almost flat so I glued in a piece of 1/16" balsa from nose to tail. This will also form part of the wing saddle. I placed the shell with the glued balsa stiffener back on the form (covered in plastic wrap) and wrapped it more plastic wrap to clamp the balsa stiffener tightly to the fuselage while the glue set.
I made new templates for the formers by slipping cardboard into the form slots and tracing the form. After cutting out the formers I glued them into one side. Then I added the flexible pushrod sheath for the elevator. After checking that both sides would come together tightly and after tweaking a couple oversized formers, I glued the halves together.
The weight of the fuselage at this point is about 120g (minus the tape) after covering with Styrospray 1000 and adding the wing saddle and battery hatch, it should come in well under 200g. I'll leave the tape on the outside to protect it until I'm ready to coat it with Styrospray.