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Making a D-box

This section describes the processes you'll go through in making the carbon D-box shell. This, together with spar assembly, are the most complex processes involved in making carbon wings. They both involve making a number of jigs. That's the bad news. The good news is that many of the jigs are re- usable for any new wing and the rest are re-usable for making additional wings to the same design. If you are developing a new design and think you may want to increase its span in future, bear this in mind while you are making the jigs - it may be that you can make longer jigs at the outset and use them both for the current model and it's higher aspect ratio descendants.

Tools

I work on a sheet of 6 mm plate glass as its smooth, easy to clean and does not snag carbon. The tools you'll need are:

Preparing the carbon lay-up

Start by cutting out all the pieces of carbon you'll need for the D-box. If you are using unidirectional fabric make sure the fibres are running chordwise. Cut the cloth at least 10 mm oversize all round and treat the pieces carefully - especially the unidirectional material - to avoid disturbing the structure of the cloth. Make sure the vacuum setup is ready for use.

Wetting out 1

Mix the laminating epoxy well using the stippling brush. I use the small foil baking dishes that supermarkets sell for individual pies. From this point you must keep going as its essential that the layup is completed and in the vacuum bag well before the epoxy starts to go off. It takes me about 75 minutes from starting to cut the carbon until the assembly is under vacuum and curing. The SP 113 laminating epoxy I use remains workable for 30-40 minutes from the time its mixed.

Now put a sheet of release film, cut about 20 mm bigger all round than the carbon, on the glass. This is the inside of the D-box. Lay the first piece of carbon on it and wet it out with epoxy smeared on with the spatula.

Wetting out 2

If you are using unidirectional fabric this should be on the inside, i.e. the first layer to be put down. Blot and roll it with the kitchen towel and rolling pin until the paper comes off dry.

Wetting out 3

Roll hard but don't skid the rolling pin or you will move and distort the cloth. Add the next layer of cloth and repeat the wetting and blotting process until all the layers are in place. At this point the layup is almost complete. Allard van Wallene recommends putting more paper towel on the completed layup, covering it with vacuum bagging film and sucking it down for 5 minutes. This gets lots more epoxy out. Repeat until the paper remains dry.

Forming the D-box

Put the outer layer of release film on and drape the layup over the form, using masking tape to hold it roughly in place.

Draping layup 1

The assembly will now be covered with stiff mylar sheet, which helps to give a smooth, untextured surface to the outside of the D-box, and with breather felt. The felt is an open weave, resilient material that has two functions; it provides a path that allows air to be sucked out of the bag without getting trapped in bubbles and it assists in distributing atmospheric pressure evenly onto the carbon.

Draping layup 2

The carbon layup is covered in turn with 0.4 mm (0.014") Mylar sheet and breather felt. The mylar is important as it controls the quality of the outer surface. Make sure there are no wrinkles in the outer release film as or you will get them imprinted on the D-box for all to see! The whole assembly is placed in the vacuum bag, making sure that the breather felt provides a continuous vacuum path from the connection point to and all over the Mylar sheet and mould.

Draping layup 3

The bag is now sealed and vacuum is pulled. Pull the bag down slowly so that you have time to smooth the Mylar and breathing felt into place and to make sure that the bag fits right down over the mould. If you skimp on this step you may get bagging under the leading edge where there was insufficient pressure on the carbon to compact it onto the mould. This is only a problem at sharp curvatures and the sharpest is under the leading edge.

In vacuum bag

Leave the vacuum on for 24 hours. Then release the vacuum and remove the D-box from its mould.

Finishing up

Strip off the release film from both sides and roughly trim to shape with heavy scissors. If you did a good job spreading the epoxy the result should be a glass-like finish on the outside. Who said you got all that epoxy out? If the epoxy was less well spread you may find rough patches where there is less epoxy. As long as the cloth doesn't look too dry and the area isn't more than a sq. cm or so the D-box should still be usable. You can tip in cyano or add a little epoxy if you're really keen. In any case you'll probably only see this on your first few attempts.

Handle the D-box with care at this stage as the epoxy is still 'green' and can be deformed fairly easily. Pop it back on the mould for at least 24 hours to finish curing.

Green carbon

I find that I can comfortably make the four D-boxes needed for a wing in four evenings using a single vacuum bag. If your vacuum setup has a T-junction you can prepare and suck down another D-box alongside the first. Just remember to close off the first bag's vacuum line with a clip while you suck down the second bag. Once both are sucked down you can take the clip off.

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