Bending the Snoopy's windscreen into shape

Suomeksi

In the restoration work of the OH-XEA “Snoopy” (“Ressu”) light aircraft, built by the Hietanen brothers from Turku, it was time to bend the plexiglass sheet to serve as the Snoopy’s windscreen. The original windscreen was bent from 2 mm thick acrylic sheet to cover the front section of the cockpit. The sheet was fastened at the edges with screws to brackets welded to the frame tubes. The original cockpit windscreen has not survived, so a new one had to be made.

Photo: Esko Keskinen

Photos: Timo Kopranen

Departing from the original, we made the windscreen from a 3 mm thick transparent polycarbonate sheet, which is very easy to shape and work with. First, we made a cardboard template of the windscreen, based on which we acquired from ETRA a polycarbonate sheet cut to the shape of the template. The straight sheet’s sides are bent against the sides of the aircraft’s nose to form the sides of Ressu’s windscreen. Before bending the sheet, we tested with the cardboard template how the sheet should bend to match the cockpit’s front frame structure. We had never previously attempted to bend a polycarbonate sheet like this, so there was some excitement in the air, but - as the saying goes - fortune favours the brave.

Photo: Erkki Rossi

The method involved heating the polycarbonate sheet with a hot air blower at the point of the front side tube of the cockpit, and as the sheet warmed up, each side was bent in turn against the sides of the front section of the cockpit. We started by bending the left side of the sheet. To enable heating, the protective film covering the plexi sheet was removed from the area to be heated and bent. Likewise, a sturdy wooden strip was attached at the bend, at the location of the frame tube, to keep the sheet firmly in place during bending.

Photo: Ari Aho

We heated the bending area with a hot air blower. Care must be taken during heating not to overheat the surface of the sheet, as this can cause it to “burn” and turn grey. As the sheet warmed up at the bend, we began to carefully press it downwards towards the side of the cockpit. The bending seam of the sheet was heated for as long as it took for the left-hand side of the windscreen sheet to be pressed tightly against the left side of the aircraft’s nose. The sheet bent surprisingly neatly, and at least bending the left side of the windscreen was accomplished quite easily. It was then time to bend the right side of the windscreen.

Before bending the right side, it was ensured that the sheet anchored to the frame had remained at a right angle to the frame during the bending of the left side. It was noticed that it had shifted slightly during the bending, so the upper corner of the left bend of the sheet was trimmed. This allowed the upper right corner of the sheet to be moved about a centimeter higher, bringing the sheet back to a right angle with respect to the frame.

Now the protective film was removed from the area on the right side of the sheet which would be heated and bent. Before heating, the front edge of the sheet was fixed with two screws to the mounting brackets on the frame tube. This ensured that the sheet would remain properly aligned during bending. Next, a sturdy wooden strip was attached on top of the right-side frame tube in the cockpit to keep the sheet pressed firmly against the frame tube.

Heating of the bending area in the plexi sheet began with two hot air blowers. While the sheet was being heated, its right side was pressed downwards slowly with a sturdy wooden strip until it was firmly pressed against the right side of the cockpit. It was found that the sheet did not bend as neatly as the left side, but a slight bulge appeared in the middle. The reason was probably that when we heated the bending area on the sheet with two blowers from each end, the centre of the area unintentionally remained cooler. As a result, it did not bend as flexibly around the frame tube as both ends of the bending area did. However, the bulge was corrected by heating the centre of the crease and pressing the bulged area against the frame tube with a wooden support. As a whole, the right side of the sheet was bent perfectly. This was evidenced by the fact that the rear edge of the right side of the plexi sheet was pressed tightly against the side of the upright frame tube in the cockpit.

After the windscreen sheet had been bent, we noted that there was no need to trim any excess material from the edges, as the lower edges of the windscreen sides lined up perfectly with the rows of mounting brackets on the cockpit frame tubes. Therefore, 6 mm holes were drilled in the plexi sheet's edges at each bracket location to secure the windscreen to the brackets with screws.

Threads matching the screws had been made beforehand in the brackets, so the sheet edges were screwed onto the threaded brackets on the cockpit frame tube. As the bracket with the threads is quite thin, the fastening may be reinforced by putting nuts on the ends of the screws to ensure the sheet is pressed tightly against the brackets. The left side of the windscreen was locked to the upright cockpit tube with a small clamp, as was done with the original windscreen.

Once the contact surface between the rear edge of the windscreen and the front edge of the roof window, which is already in place, is covered with a protective strip, both the roof window and the windscreen are removed, wrapped in protective bubble wrap, and stored. This way, we ensure the windscreen and roof window do not get scratched while the Snoopy’s frame is still being restored.

Satisfied with the result, we concluded that we had succeeded in bending the polycarbonate sheet to serve as the Snoopy’s windscreen, even though we had never bent a plexi sheet in this way before.

Photos: Lassi karivalo, unless otherwise mentioned

Translation to English: Erja Reinikainen