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D-box options

The options available depend a little on the D-box material. Kevlar D-boxes can be made more simply than carbon ones because the formed D-box can be wrapped round the wing while a carbon one is more rigid and really needs to be formed to the final airfoil shape.

Kevlar
Usually this is formed on a 60 degree V-block with the point rounded to match the required LE radius. This rounding is essential. About 2 layers of 35 gsm Kevlar would be a good starting point for F1A. What comes off the form is a bit like wet cardboard and only gets stiff when its glued round a set of ribs and spar. This method needs a close rib spacing to maintain section shape and stiffness. Fitting a LE can help keep the ribs in place and aid assembly.
Carbon formed on a V-block
You can form carbon shell on a V-block, but it must be formed into a D-box within 24-36 hours of making it, while the epoxy is cured enough to handle but still 'green' and capable of holding a shape when its bent. The rib and LE requirements are essentially the same as for Kevlar formed on a V-block. The result will be stiffer than the Kevlar equivalent.
Carbon formed to the correct section under vacuum
This is my preferred method. Very few ribs are needed depending on the thickness of the shell. Another justification of this approach is that normal 1.5 mm balsa ribs in a D-box split the first time the wing gets hit and contribute little thereafter. My few ribs are made from balsa ply and so will not split but will still weigh less.

I use a variation on the method originally developed by Peter Allnutt. I use a Feathercut foam cutter from Tekoa to form the mould and an Autovac II vac-bagging system from ASP to form the D-box under vacuum. I find both tools are easy to use. The combination has made usable D-boxes from the start. The Feathercut also makes great wing building jigs. I cut the male form from Dow Roofmate RL, finger rub two coats of aliphatic into the surface for toughness and add a 1.5 mm ply riblet to each end of the foam to stop the vacuum from crushing the corners. The form needs to be 30 mm wider than the chord of your D-box and 30-60 mm longer than the panel. It is mounted LE up on a piece of wood to form an inverted T. The following picture should make this clear. It shows a D-fox form for one of my F1A tip panels with a D-box fresh off the mould in front of it.

D-box form

You are going to lay up your D-box material between two release films, drape this over the LE form and pop the lot into your vacuum bag and suck. Of course you will put heavy mylar outside the layup (to get a better finish on the outside of the D-box) and bleeder cloth over this to distribute the vacuum properly inside the bag. There are variations on this basic plot:

You can make the male form from anything that will hold its shape under vacuum. Even heavier foam (Floormate HD) would be better than the Roofmate RL but the minimum order quantities are very high. Carving the form from wood is also good; a use for Model Shop Balsa at last!
If you don't have access to a vac system you can make a wooden form and force it plus layup into a strong U-shaped plywood box lined with foam rubber. Pete Tribe does this.
Pete Allnutt makes a wooden T form somewhat longer than the layup so that he can tape bagging sheet onto the form all round the layup and seal it with silicone clay, modelling clay, or some such material. This way he only sucks vacuum on the form and does not need to use a vacuum bag.
Stefan Rumpp and Gerd Aringer use a strong epoxy female mould, which has the required section formed in it. They put the layup round a silicone rubber male insert and push the complete assembly into the female mould. The layup is compacted by clamping a metal or wood bar along the back of the form to close it and compress the rubber. The whole assembly is placed in an oven and cured at a raised temperature - the silicone rubber has a high temperature coefficient of expansion so you get lots of pressure.
Use lighter foam, omit the inside layer of mylar film, and you've got a foam-filled D-box. However if you do this you'd probably want to stick your spar (see below) to the back of the foam first and maybe add some 'runout' foam behind the spar. Less carbon should be needed because it is fully supported by the foam.

Everything you'll see in articles on D-box forming methods is just more elaborate descriptions of some variation on the basic theme. The critical parts are:

the form must be strong enough and accurate enough for the job. You want it to last through several sets of wings but not to be so costly that you can't just scrap it when you want to try another section or wing planform. Hence my enthusiasm for foam.
the carbon layup must be compacted during curing - hence the vacuum or clamping systems. If this isn't done you loose a lot of stiffness. 1/3 - 1/2 atmosphere of vacuum seems to be about right (and doesn't destroy the Roofmate RL form). If you're clamping instead, clamp it as hard as you can without breaking something or distorting the resulting D-box.

D-Box thickness

It's difficult to give good guidance here. You really need to make a test piece or two (this is good practise anyway) to get a handle on weights and stiffness. You will also need to know the thickness of the D-box in order to calculate the required spar depth.

You can then calculate weight; double the total cloth weight to allow for epoxy. Carbon cloth is relatively thick - its difficult to find anything under 80 gsm. Always use the cloth at 45 degrees - the D-box is there as a torsion member not for bending load. A typical F1A D-box is 2 - 3 layers of 80 gsm carbon or one of 190 gsm cloth. The cloth is always used with its tows at 45 degrees to the chord line and its important to use cloth with equal amounts of carbon in each direction. If this is not the case you will get warps.

I'm using 2 layers 83 gsm plus a single inside layer of Russian unidirectional material, which weighs 120 gsm, with fibres running chordwise as a sort of distributed rib for the inner panels. The tip panels have a single layer of 83 gsm cloth plus the inner layer of unidirectional Russian carbon. This way I only need a rib every 150 mm within the D-boxes.

The inner panel D-boxes come out at 0.38 mm thick and the tips at around 0.28 mm.

F1A tails (especially if they are the Woebbeking section) often use a single layer of 83 gsm carbon to make a narrow (10% chord) D-box. I haven't thought about F1J D-boxes yet, but would probably use 1 layer of 83 gsm cloth as a starting point. Have a close look at Randy Archer's Xenon design. Its light and strong and uses a carbon D-box. Also check out what the current F1B fliers are using as that should suit an F1J rather well.

Its very difficult to get the epoxy down to only 50%. You need to put on just barely enough to wet out the cloth and then squeegee and roll and blot with kitchen towels until you can't get any more out. Do this between layers. Weigh the cloth before you lay it up. Weigh the mylar if you're an accuracy fiend. Weigh the complete D-box. Calculate the epoxy percentage and you'll probably still find you've got 60% epoxy. Maximum strength results when the fibre:epoxy weight ratio is 50:50. Epoxy is heavy: excessive amounts of it add nothing to the D-box except weight.

D-Box finish

Carbon and Kevlar D-boxes are not waterproof and must be made so. Some folks use metallised mylar as the outside release foil and just leave it in place when they build the wing. This is easy, very waterproof, and keeps the D-box cool in the sun. Others just cover the D-box - with or without the Mylar outer foil - as part of normal wing covering. They typically use Polyspan (Victor Stamov. John Williams, myself), Ikarex 31 (Roy Summersby, John Cuthbert, myself), Lightspan or Micafilm (Chris Edge). Micafilm is popular on F1B.

D-box closure and strength

The maximum torsional rigidity isn't achieved until the back of the D-box is closed with something as strong as the D-box material itself. The typical carbon spar (0.8 mm carbon flanges, end-grain balsa between them, and wrapped in Kevlar thread or woven carbon sock) is not necessary for F1J, though its essential for F1A and F1C. It gives a good closure.

You need a full-depth carbon spar to close the D-box. In all models apart from gliders I'd suggest using a piece of medium 3 mm balsa (grain running spanwise), tapered in thickness with 0.2 to 0.4 mm pre-formed carbon flanges stuck on the top and bottom edges with thick cyano or epoxy. The flange thickness is dependent on the size and weight of the model. The spar needs at least one carbon web. For a light model this need be nothing more than a single layer of 80 gsm carbon cloth at 45 degrees compacted and laminated onto the rear surface. Heavier or faster models should have the entire spar enclosed in woven carbon sock or wrapped in Kevlar thread. See Appendix B for more information on the sock.

Cutting carbon cloth

This stuff has a fairly loose weave, so it distorts and snags really easily. While you can cut the sock with sharp workshop scissors this will not work for the cloth. I think the best way is to use a rotary cloth cutter and steel straight-edge and cut it on a self-healing cutting pad.

A sharp Swann-Morton or scalpel blade is almost as good as the rotary cutter, but the latter is particularly good because it rolls over the cloth it is cutting rather than dragging on the fibres. I think the cutting pad stops the cloth from distorting because the fibres get dragged down into it's surface and so are anchored before they are actually sliced apart. As a result you are less likely to snag or tear any delicate of loosely woven material if you cut it on a cutting pad.

Some people put masking tape on the cloth and cut through it. This is essential for Kevlar, but you need to take more care with carbon. If you try to peel dry masking tape off you'll make a horrible mess of the cloth. However, I've found out that if you dribble nitrate dope thinners onto the masking tape and wait 30 seconds it can be lifted off without disturbing the weave. This technique will even remove masking tape that has been left on the inside of a carbon D-box while it was wetted out with epoxy and cured in a vacuum bag. Don't ask how I know that....

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