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More specifically, how is the composite frame of the canopy manufactured? Attached picture as an example.

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  • $\begingroup$ It's done with a mold, I'm not sure what you're asking though, do you want to know the molding process? $\endgroup$ – Ron Beyer Nov 27 '20 at 18:55
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    $\begingroup$ Yes! How to make the joggle on the fuselage that receives the canopy and the canopy frame that sits in it. $\endgroup$ – Mridul Nov 28 '20 at 9:34
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That is a complicated process. You need:

  1. A positive mold (mandrel) for the canopy.
  2. A negative mold for the canopy frame. For a single prototype you can also use the unfinished fuselage itself.
  3. A fuselage mold with a molded window sill.

First the fuselage positive mold core is built. From that the negative mold is taken, but also a negative mold only for the canopy area. From that negative mold the mandrel for canopy manufacture is built. If you have enough money, you can of course CNC mill the mandrel directly from a large block of aluminium.

The plexiglass canopy is deep-drawn over this mandrel by the canopy manufacturer, e.g. Mecaplex. There aren't many who get this process right!

Meanwhile, the window sill has been added to the fuselage mold and can be used for manufacturing the canopy frame mold. Once that is finished, the canopy frame is built by first lining the mold with diagonal weave fabric (glass or carbon, as you like) and filling the core of the frame with unidirectional rovings. Then another couple of diagonal weave layers is added on top. After removal from the mold the frame is trimmed and now the transparent part is added. The frame has a recess where the plexiglass fits in and the plexiglass is fixed to the frame with a special polyurethane adhesive. A couple of screws make sure it stays in the correct location.

To finish the canopy, the fittings are added and the frame is painted.

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  • $\begingroup$ The canopy frame on my PL-2 is f-glass built over steel tubing, with the f-glass joggled around the perimeter to accept the moulded plexi flush, but the builder attached it with screws through the plexi into drilled and tapped holes right in the epoxy matrix.They were leading to hole cracks and bits of the edge of the plexi breaking off and after I repaired the plexi I thot of replacing the screws with that polyur glue like the RV guys do,but instead just enlarged the plexi holes and installed the screws with large c-sunk washers so there are no lateral forces on the holes. No problems so far. $\endgroup$ – John K Nov 27 '20 at 22:52
  • $\begingroup$ Peter Kämpf, the last paragraph is where I'm having difficulties. How exactly did we move from a solid mold to a mold with window sills? I know that a tool is attached to the fuselage molds to get that joggle, but I can't think of a way to manufacture of the pattern for the tool other than CNC. $\endgroup$ – Mridul Nov 28 '20 at 9:29
  • $\begingroup$ @Mridul It's a long time since I did it, so I'm no longer exactly sure. One way would be to cut the profile of the frame in high density styrofoam with a hot wire and bend that strip into shape and fix it to the fuselage mold, then cover it with packaging tape. But that was for a prototype, so serial production would use a layer of glassfiber and filler for a smooth surface instead. $\endgroup$ – Peter Kämpf Nov 28 '20 at 19:40
  • $\begingroup$ @JohnK The cracks emanating from the screws is a known problem. The trick is to not tighten the screws all the way so the plexiglass is not compressed by the screw head. And countersink liberally, so the screw head does not stick out a bit. $\endgroup$ – Peter Kämpf Nov 28 '20 at 19:43

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