Getting the sandwich ready

This weekend we started to organize everything needed to get the missing face sheet finished. Many things have to be done before we start with it, for example filling the core. The honeycomb core has to be filled with a glass bubbles-resin mixture where it is subjected to high localized loads. This prevents the core from failing and is, among other places, needed on the left side of the cabin where the pilot will walk on top of the shell to get into the airplane. Though the glass bubbles reduce much of the weight of the mixture, it is still much heavier than an air filled honeycomb core. Thus, the places to fill should be considered very thoroughly.

Last time we filled the core and glued the face sheet in one shot, which was extremely stressfull. It takes ages to mix enough filling material, and having a wating face sheet that is slowly but surely curing does not make it easier. This time we will do it in two steps and started today to mark the places to fill:

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Christmas already?

Yesterday, we got a very nice and awaited for package from R&G GmbH:

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Most probably you are wondering what these strange black rings are. Well, these are pultruded carbon profiles. To be precise twelve rolls à 50 metres with a cross section of 20 × 0,5 mm and a fibre volume ratio of over 60%. Half for me and half for a friend. Ok, but what are the profiles for? It’s for the spar caps. So, we got a delivery for one of the most important pieces of the wing—and thus the airplane. A good reason to feel like having Christmas!

Depending on the mechanical stress, several layers of profiles will make the spar caps up. The profiles have perfectly oriented carbon fibers and a very high fibre volume ratio. Much higher and more reliable than a self made roving spar cap. Unidirectional pultruded profiles are predestined for caps, which are mostly subjected to axial stress.

The last two months we made some intensive investigations on the properties of the profiles:

 

Flexural testing of carbon reinforced beams can be quite intimidating:

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To be honest, I had not the courage to break the beam shown in the last photo. It provided some very good data, though:

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Having the data, I calculated the amount needed for the wing and we could place our order at R&G.