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I origionally intended to build Parseval PL.19 but I cannot see myself finishing this in 2014 and I want to build something that reflects the 100 year aniversary of the outbreak of the Great War. I therefore chose instead an observation balloon as this should take alot less time to complete. The Sigfeld-Parseval Drachenballon was in my opinion one of the most iconic objects in the Great war so it seems fitting to add one to my collection of WWI flying machines. I will be building this out of dense foam, the type used in cavity wall insulation. I have been experimenting for a while with this stuff to see how it can be plausible to make large models of balloons and dirigibles.
I started like I usually do by getting GA’s. I got these from the internet and blew them up on the photocopier. The next step was to cut from the sheet of foam the body of the envelope. I decided to do this in sections and to get an exact 90 degree hole thru the middle I put three right angled triangles on the top as drill guides. Once I had cut out enough of these I put a rod thru each one and sandwiched them between two cardboard discs that I had made to match the cross section size (+ 2mm for sanding) of the envelope. Although I have a hacksaw in the picture I found a really sharp bread knife did a better job. The outer layers of the foam were really rough and unsuitable so I drilled a hole thru a cutting block and fitted a rod at exactly 90 degrees to the base. The segments were then slid onto the rod and I cut across the block to make a flat surface. I then took off the slither I had removed, put it on the bottom of the cutting block, placed on top of the slither the segment again and cut across the other side. I now had a segment with both sides smooth (I have though since discovered foam board that has almost perfectly smooth sides and will use this in future).
To make the round ends I made a grinding tool out of plastic card. This I made to the same size and shape as my GA’s and glued sandpaper to the inside curve. A rod was placed thru the middle at exactly the same axis as the envelope. I then simply pushed one of the cylinder shaped segments onto the grinding tool while it was spinning in the drill. This quite literally took seconds to do. Once I had two I had all the segments I needed to make the envelope.
I found gluing a problem when experimenting because the foam becomes very hard when it has absorbed glue. This makes it impossible to sand down because the hardened parts sand down less than their surroundings, in other words the glued joins stick out like flanges and are extremely difficult to remove. The glue I ended up using was UHU clear glue which I smeared on the segments but made sure no glue went within about 10mm of the edges. It went on all surfaces and was left for about 15 minutes. I then slid each segment on the rod, pushing them together one by one. The whole lot was then sanded down and this took only a few minutes because this stuff sands really easy.
When experimenting I found that dense foam can be bent so the plan was to bend the metal rod and slid it thru the envelope to give a slight upward bend just like in the real thing. Well it worked in the practice run but not on the real thing. When I tried bending it, it broke in two places. I was able to glue it back together again and used this to my advantage, gluing it back so it acquired the upward bend I want. If I make another I will sand the segments at a slight angle so they sit naturally at a bend on the rod.
This is as far as I have got so far. I must say that building stuff this way is alot easier than the paper mashe and fibreglass method I used on the SS-Blimp and if this all goes tickedy-boo I can see myself knocking out alot more LTA stuff in the future. Next I need to cut/sand detail into the envelope but more importantly soak some varnish or paint into it to harden it as it is extremely soft at the moment. The giant air bag / scoop on the back will be made from plastic card heat moulded around a pattern. I somehow cannot get pictures on this post but I will have another go later.