Finishing the Bubble Doors

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 I used a black spray to finish the inside face of the Plexiglas called “Lacryl” which is a specially formulated Lacquer used in the sign business.  It has a very nice eggshell appearance on the sprayed side and a very glossy black look on the outside.  I used a metal reinforced black plastic edge to finish the plexiglas flange that extended beyond the door frame and I attached the plexiglas to the steel door frame using special black sheet metal screws with a waxed black plastic washer.  A Plexiglas drill bit was used to drill slightly oversized holes in the plexiglas to eliminate cracking or splitting when the sheet metal screws were attached and tightened.

The Forming Secret

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 The Fabricator basically uses a big pizza oven to heat up an oversized sheet of Plexiglas. When it arrives at the correct forming temprature it is removed from the oven and draped over the lower part of the mold.  The upper plywood frame is then clamped over the Plexiglas sheet and air is immediately pumped in from the center of the lower mold.  There is a “Secret” as to how the air is introduced to the lower mold. If you just attach an air hose nozzle the result of the form will look very localized around the air nozzle location. The preferred shape is a balanced uniform inflation of the bubble around the entire frame shape.  The way this is done is amazingly simple and crude.  A piece of cardboard approximately 12 inches square is stapled on the corners over the air inlet and that’s it. That is enough air diffusion to uniformly lift the heated sheet of acrylic to its lofty formed shape.

The Photos above show the finished parts trimmed and reclamped in the mold. For Insurance, I had the Fabricator make two sets just in case I broke one later and that’s exactly what happened a few years later when I ground looped the airplane and shattered the left door with a safety cable flinging off the landing gear!

Completing the Tooling

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 The last component of the tooling was a continuous plywood frame approximetely 3 inches wide that covered the shape of the steel door frame and adjacent flange.  The object of this plywood frame was for clamping over the heated Plexiglas sheet while forming the bubble.

The Fabricator also instructed me to add a silicone bead around all joints and edges to make the Tooling as air tight as possible.  With both Left and Right Tooling Fixtures completed it was time to bring them over to the Fabricator for the forming operation. The goal post shown in the photo above was for measuring the maximum height of the bubble during forming.

Fabricating the Bubble Doors

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 The Bubble Doors utilize the same fabrication method as Plexiglas Skylights – the same skylights as typically seen in school gymnasiums and factory spaces.  Luckily I had a local company who made these Skylights.  They had the equipment and know how to make my Bubble Doors and instructed me on how to make the tooling.

The first thing was to weld together a steel door frame that followed the contour of the airframes door opening.  I used a 1″ square 4130 steel tube and carefully tack welded the door frame inside the airframes frame.  With one of the wings attached I could determine if the future Gull Wing type Bubble Doors would interfere with the bottom side of the wing when it was raised.  I also  designed the latching method and gas lift spring locations needed to open and close the doors.   After I finish welded both left and right steel door frames I then began to build the wooden molds around the front and back sides of each door frame.