Source: 2025Q2 Beartracks, Paul Gyger
When considering a skylight for my Bearhawk 4-Place, I had the usual difficulties and decisions, all mentioned in various threads of the Bearhawk forum. I remember reading somewhere that shaping the skylight proved difficult, mostly because the Bearhawk has 2 contours, one for the shape of the wing, and another perpendicular to the wing for the “Bearhawk bump”. This bump is required due to the elevator trim wheel, and if it is to be removed requires significant modifications to both the trim wheel and the stringer supports. Somewhere in the vast history of the forums I read about the process of thermoforming plastic, and so my long journey began to form a plastic skylight that followed the natural shape of an original Bearhawk 4-Place. The driving force of the experiment was that if the plastic was contoured it might solve two problems. First it would keep the original shape and avoid rethinking the trim wheel, and second I imagined that shaping the plastic would lend strength to it while reducing stresses from forcing it around curves. Time will tell.
Like so many of these side projects or modifications, if one sat down and responsibly made an estimate of the time and resources required for these fanciful substitutions, no one in their right mind would even consider them. Somewhat providentially some of us are not burdened with accurate assessments of how long things will really take to perfect, or how much material we will burn through in the process. It was in that spirit that I set out to first make a form, then make an oven, and then thermoform my skylight.
Thermoforming plastic can happen in a number of ways; vacuum molds and pressure molds both with weights and with air pressure. In the case of the Bearhawk a relatively simple drape type mold would suffice, basically just a copy of the natural shape of the surface in question. I accomplished this with plywood ribs that were held in place with a light frame that could then be removed and transferred to a base sheet of MDF. The voids between the ribs were then filled with whatever wood scraps were lying around the shop. The one exception to this is where the curves were only in one direction, say the center portion of the skylight. I was able to use a flexible plywood product meant for going around curves. Two examples of this product are Flexyply and Wigglewood.
The form was then covered with an old, mostly cotton bed sheet to prevent scratches in the final product:

When using a drape type mold, some consideration must be given to how the material will be held down and made to conform to the mold. This doesn’t seem like a big deal until you actually try to form a part and realize that some parts of the skylight are stretching and some are contracting, and even though things are quite flexible and pliable, it still requires a significant amount of persuasion. When planning a drape type mold having proper generous edges that extend past the actual part dimensions is highly recommended, as well as having some type of nesting area or starting point where the hot plastic can be locked into place as weights and clamps are applied. A little extra time on the mold will go a long way in making a repeatable part.

While plastic can be thermoformed with local heat application such as a heat gun, infrared, or torch, using an oven to thoroughly heat the entire part is essential. Plastic that is heated locally tends to get a wavy look that is very difficult to assuage. So the building of an oven is really inevitable, and anyone that is embarking upon this process must include this in their “time and resources” balance sheet. The design of such an oven is relatively simple, a large structure that is able to withstand heat and is insulated in some fashion. I used scrap metal to make a frame, I covered that with scrap sheet metal, and then insulated that. My heat source was two stove top burners sourced from Amazon for $30, a thermocouple and temp control unit. Now there is one design feature of the oven that is critical, it must allow the piece of plastic to stand vertically. Adapting a horizontal oven, ie the local pizzeria’s oven won’t work. There is a bit of an art to fine tuning things as far as placement of the burners and the thermocouple etc. I found that burners at the bottom worked best, with a piece of sheet metal separating the burners and the plastic. I did have a significant temperature difference between my oven floor and the top, around 10° C. I think this could be mitigated with a fan to circulate the air but I had success without. I used two metal tubes to clamp my plastic at the top, which allowed it to hang unimpeded. I won’t step into the debate of Lexan vs Plexiglass, but I will say that Lexan (polycarbonate) was significantly harder to work with. This is probably due to poor oven design, poor temp variation control, and inexperience on my part. Both of my attempts at making this part from polycarbonate failed. Lexan must be heated to a higher temperature, which exacerbates problems with oven design as a temp differential will be greater with an oven that is working harder. Another problem with Lexan is even though it’s hotter when forming, it cools faster, necessitating a fast transfer from the oven to the mold. After I built my oven I had some scrap acrylic that I used as a practice piece to test the oven and get a feel for things. This acrylic proved much easier to work with. It requires less heating (145°C vs 175°C for Lexan) , it cools slower, and it just seems to be much easier to work with for an amateur. Lexan also requires a drying phase to get the moisture out which is really quite long- for my piece it was 6 hours. Acrylic can be heated to temp without this drying time.

Eventually the time comes, the mold has been made, the oven is at temperature, and the plastic is ready. Nothing left to do but get everyone in the house fitted with welding gloves or oven mitts and bake some parts. I found that after about 25 min in the oven with a temp set to 145°C my plastic was just the right blend of pliable and able to take shape, without it being too molten. When a piece of acrylic is heated it will first bend or rise, this is a natural occurrence and it means that your plastic is not yet at temperature. A little more heat, or just a little more time baking and that curve or rise will settle down and the whole piece will be perfectly flexible. Going much past this temperature isn’t a great idea as it opens you up to deforming wherever weights or clamps are applied. It is a sweet spot, I found mine at 145°C.
Small deviations in adherence to the mold shape can be touched up later with judicious use of a heat gun. Great care must be taken as this local heating can give a distorted look. I had a couple small areas that raised up a bit as they cooled, but I was able to use the heat gun and get them proper.
In the end my acrylic skylight was a success. I have a part that follows both contours of my frame almost exactly. My skylight is under no stress from bending, actually quite the opposite it has added strength from the bending reinforcement. The view through the acrylic is almost perfect, I did end up with a few slight waves but they aren’t really noticeable unless you suffer from the OCD only a fellow builder will be racked with. Ultimately I decided to make two skylights and keep one on the shelf for the inevitable replacement required down the road.