A small step forward every day

Steadiness and stamina is the key to success, but it’s the small and unimpressive steps taken every day that let great things happen. It’s some time ago since we started to make the shell of the cabin. We started by making a plug, moved on to producing a three piece mold, made a die for the canopy and ended up finally in production of the shell. Now, the cabin’s shell is ready. Well, almost …

It still has to be joined to the center section and the canopy’s opening has to be cut into the right shape. However, this is going to happen after the seat’s spar was installed. the reason is simple: We want to control the sight forward and to the sides before we start to trim anything.

Last week was intensive. We worked a couple of days till midnight or even longer, though we had to go work the next day. It’s only about two weeks ago since we prepared the core of the last piece. After that, we prepared all materials and consumables needed for the outer skin: one layer of 105 g/m² glass (Interglas 91111) two crossed layers of 80 g/m² spread two carbon (GRM Systems), vacuum foil and sealing tape. It’s better to prepare as much as possible before the lamination session. This prevents unexpected problems and reduces the needed time to a minimum. Either way, lamination sessions need always at least two to three hours without pause. Hence, we prefere to split the job into two different days.

Production of the shell is anyway in two sessions. In the first, the outer skin and core are laminated and cured in vacuum. Then, the inner face sheet is laminated on the core in a second session. Here are some pictures of the first session:

When the pieces are flat, production can be done in one shot. Are the pieces strongly curved—as in our case—it’s better to split production into two separate sessions. Else, the inner face sheet could wrinkle because the core shifts while it’s being pressed. The core would ideally be thermoformed in one piece before use. This would avoid having to use scarf joints and the shell could be made in one shot. however, thermoforming needs a large oven and a temperature resistant die. Nothing that could be done at home for large pieces and is rather something for serial production.

After the resin cured enough—say overnight—the foil can be pulled off carefully. Firstly, premature demolding is avoided by being careful. Secondly, the vacuum film can be reused for the next step. The piece produced need to get a scarf joint prepared before the inner face sheet is laminated. This time, we used much heavier carbon fabrics (400 g/m², GRM Systems) and a layer of flax fabrics (100 g/m², R&G Faserverbundwerkstoffe) for the inner face sheet. The piece will be subjected to much higher loads than the others, because it will be used as a handle to jump in and out of the cabin. As for the center section, flax is used as a protection against splintering.

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Since some time we use Compoflex® SB 150 peel ply and breather (Fibertex, sold by Mühlmeier Composites and GRM Systems). The force needed to peel it off is much lower than for usual peel ply. Danger of premature demolding is, thus, much lower. Also, by being a combination of peel ply and breather, it’s faster and easier to finalize the vacuum setup. There is also a variant with flow media instead of breather for vacuum resin infusion.

After the final piece was made, it was time to join the three parts. The left and right aft parts were easy to trim, as the edge was accesible. The front part was a little bit more difficult, as it had to be partially demolded, so that the edge could be cut without damaging the mold.

Having all three edges match, we joined the part in two steps. In the first step, the joints were made airtight by using thickened resin and a layer of glass fabrics (105 g/m²; IG 91111). The used fabrics have 1/3 twill weaving and are very tight producing a sealed perfectly surface. The aft edge was filleted with thickneed resin and joined with ±45° non-crimp carbon. All joints were covered with usual peel ply and cured overnight without application of vacuum. The next day, strips of ±45° carbon, scarfed PVC-foam, and some flax fabrics were laminated to strengthen the joint and cured under vacuum (pressure delta of ca. 700 mbar). Using vacuum prevents that the reinforcement does not shift and that it is perfectly pressed on the joint. Besides, the surplus resin ends up in the Compoflex®.

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At least one day curing should be considered before demolding. The time needed by epoxy composites until demolding is safe is longer than expected: Slow resins need up to 5 days at 20 °C (68 °F), though they have only a couple of hours of pot life. The parts joined were at least a week old and had sufficiently cured, while the less critical joint had more than 24 hours to harden before we demolded: