The weather looks like a copy of last Sunday, but, for various reasons, I can’t get out to fly this week. I’m just going to have to make do flying the Q4 quadcopter inside. I have been making some progress on making a canopy for the ATOM autogyro though. I’ve bought myself a heat gun from Maplin and I’ve been drinking 17 pence worth of sugar free lemonade all week. OK, I could have just tipped it straight down the sink, but it’s actually quite nice and it took all week to make the canopy pattern.
I’ve been reading the September issue of RCM&E (the one with the Supermarine S6B seaplane on the front) and, by some amazing coincidence, there is an article on making canopies out of plastic drinks bottles with the ATOM canopy as an example. Having read it though, it would be good to have included a few more details as the author has obviously got a lot more experience at this than I have. The last canopy I made was for my Vampire, which I built from the Flying Scale Models plan featured in the March/April 1998 issue. That just shows how old it is. Actually, now I think about it, I also made a canopy for my Extra 300 some time after this, but it was never that good.
Both the Vampire and Extra canopies were made using a domestic iron and covering the plastic with cloth to avoid burning and it sticking to the iron. I should point out that I had a special iron for model making where the steam mechanism was broken so it had been long since retired for use on any clothes. Having said this, while the technique obviously does work, I wouldn’t recommend it unless it’s your only option. This time I wanted to do it properly and so went out and spend £20 on a Maplin heat gun.
Now, one thing missing from the RCM&E article is how much heat you actually need. The type of plastic will determine this, but I had already realised that, if they were talking about an iron used for Solarfilm in the magazine, then a 2KW paint stripper is probably a bit over the top. It is a really good heat gun though, and it made a really good job of melting my 17p lemonade bottle. The trigger has two settings, so I selected setting one (1300W) and had the heat deflector attachment fitted (see picture), which is what the instructions recommend for shaping plastic. This worked really well, with the large heat gun blowing lots of hot air onto the plastic and the deflector preventing it from getting too hot. It’s also a lot of fun shaping plastic in this way.
I made my pattern out of balsa and plywood by sticking together all the scraps from by bits box. While this was quite time consuming, it is hugely satisfying to have used up so much of my scrap wood. The way I made the pattern was to take a piece of lite-ply and cut it so that it fitted flush on top of the ATOM’s fuselage sides up against the front and back of the hole for the canopy. I then built up layers like bricks using 10mm wide and 5mm high strips cut from old balsa sheet. When I ran out of 5mm, I used 3mm and so on with larger blocks and bits of triangle being used on the top. The first few layers up from the plywood base are hollow, with 10mm balsa “bricks” along the outer edge, so the pattern is partly hollow. Once I got to a certain height, I just stuck a piece of sheet flat across the top and started glueing bigger bits together in a 3D jigsaw. One problem I had was that the rails for the elevator and aileron servos were installed in the fuselage, so I had to leave a gap at the back of the pattern in order to allow me to fit it into place on the fuselage for shaping. Once the shape was right, I then had to fill in this hole with balsa block. Then I increased the size of the pattern so that it’s bigger than the hole in the fuselage. This was done by adding sheet to the bottom, front and back to increase the size by about 15mm all around. The extra space will be needed for the plastic to overlap the fuselage sides to make a good fit.
As this was my first attempt, things didn’t go completely to plan, and I did manage to damage the plastic in two places. I washed out the bottle and dried it thoroughly before cutting the bottom off and wedging the the pattern inside using the pine block and some balsa wedges that you can see in the picture. This is very important, as the pattern must not move while the plastic is being formed. The first thing to do is to heat the bottom of the pattern (pine block) and the back where the bottom was cut off the bottle and where the pattern can escape if you were to shrink the neck of the bottle. After wedging the pattern securely in place, just heat gently to shrink the bottle around the pattern. This is where I went a bit wrong as I assumed that the white colour I could see at the back of the canopy was condensation from where I had previously washed the lemonade out.
Some white discolouration can be seen at the top right of the picture, where I’ve heated the plastic too much. Of course, you can’t tell this very easily when everything is still hot, but closer inspection of the pieces once they’ve cooled down shows the mistake.
The right hand side of the canopy is a bit more of an issue as a white circle is visible where the plastic has become very thin and weak. I’m hoping that this is low enough not to be an issue with the finished product.
OK, now to remove the plastic from the pattern. This is actually a lot harder than I was expecting as the plastic is thin and easily torn, while there isn’t much to get the knife into without it slipping or damaging the pattern. I think the best policy is to cut the bottom of in stages.
And here’s the finished result:
It doesn’t look that good in the picture because it needs trimming, but that’s not bad for a first attempt in quite a few years. The beauty of it is that I can have another go once I’ve drunk all the lemonade again.
So now all that’s left is to build it into a removable hatch for the ATOM. I’m thinking of making a base with pilot, dashboard and attach it all with magnets. I’ve also got the ATOM head mechanism attached with the servo pushrods in place, so the only major bits left to do are the tail section. Maybe it will actually fly this Summer after all?