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From Phil's XP-3 lay-up Notes:

Start by spraying a light mist of 3M77 on your lay-up bench and then dropping the Mylars in place. The 3M77 is needed to hold the Mylars in place during the lay-up process. 

Before starting the lay-up, wet out the leading edge of the core with epoxy. Also put all four tip doublers on the BENCH (or Mylar) and wet them out with epoxy. Roll a tiny bit of epoxy on the FOAM CORE in the area where the tip doublers go. Pick up the tip doublers and stick them to the CORE. They should land on the leading edge fabric but not extend beyond the leading edge and hence will not need trimming after bagging. Adding just a tiny bit of epoxy to the foam core was done because it causes the tip doublers to stick in place and stay put.

Add the .050 carbon rod servo hole reinforcements to the core now. Just coat them with epoxy and push them into the slots with a small screwdriver.

Start the lay-up by rolling epoxy onto the Mylars. It will bead up or "fisheye" but that is okay. So long as the epoxy is evenly distributed.

I mix a total of 65grams of epoxy to do an XP-3 lay-up by the way. Drop the carbon cloth in place on the Mylar. It should be about 1/4" back from the leading edge of the Mylar. You don't need a line on the Mylar for this, I just eyeball it. It will extend beyond the tip of the Mylar for now. Add the little carbon servo hole doublers on the bottom Mylar ( see picture) You should have lines on the Mylar showing hinge line and servo hole locations. Drop the bias 1.7oz. Kevlar in place butting it up to the carbon. Start at the root end and lower it into place towards the tip, it can be moved around a little after putting it in place but try to get it close on the first drop. It will extend beyond the tip and beyond the trailing edge of the Mylar in places.

Before adding any more epoxy or even rolling the fabric you should use Kevlar scissors to trim the excess carbon and Kevlar at the tip of the Mylar. Just lift the end of the fabric and cut it about 1/4" short of the tip of the Mylar. This is not a critical dimension. The tip doubler will cover this area. You want to be sure that the carbon and Kevlar is cut far enough back so as not to need trimming after bagging at the tip. There is no such thing as too short, as long as it is still covered by the tip doubler. Do not attempt to trim the trailing edge yet, that will be done later.

Roll epoxy into the lay-up now. Wet it out evenly and make it wetter than it needs to be. Drop the Flaperon doublers in place.
 

Go back to the Mylars now and roll the doublers until they are wet out. You may already have enough epoxy in the lay-up to do this or you may need to add a little more. Keep rolling the entire lay-up until the epoxy is evenly distributed.

The XP3 flaperon doubler is a partial span doubler. The intent is to have it extend a little bit inboard of the control horn and mostly outboard of the control horn. The exact spanwise location is not critical.

You must get the doubler as close as you dare to the hinge line without the chance of it extending forward of the hinge line. The doubler is on both sides. 

It is important to accurately place the flaperon doublers on a vac bagged DLG wing (see my recent posts). This can be difficult since it can be difficult to know exactly how the Mylar will sit in relation to the wing when the wing is bagged. On XP3 wings, I place the doublers on the lay-up, on the Mylars. This is done because it is fast and easy that way. I have done enough XP3s and the technique is so refined that I can do a pretty good job of hitting just at or just aft of the hinge line with the doubler.

Most guys don't do dozens of HL wings all the same and so they need a good method to locate the doubler which works for even the first wing. The solution is to place the doubler on the core rather than the Mylar. This is how I place the Kevlar tip doublers on my DLG wings. You simply wet out the doubler on the bench and then stick it to the core. To make the doubler stay in place you will need to either make it really wet or roll a tiny bit of epoxy onto the core first. My personal DLG wings have 2.4oz (80g) carbon doublers. This is the really expensive balanced carbon woven from 1K tow. It is one of the fabrics that looks a lot better and is probably thinner if you roll it with a hard roller after laying it up (1oz (35g) Kevlar is another of those fabrics). 

You can't roll the fabric with a hard roller after placing it on the core so this is what I do; I first make up a sheet of pre-cured carbon by just laying up the carbon by itself between two pieces of Mylar. I roll the heck out of it which closes up the little gaps between the carbon tows and hopefully makes the fabric a little thinner. After curing, I cut out the desired flaperon doubler shape. I then attach these doublers to the core during the bagging process. 

The pre-cured pieces are a whole lot easier to handle and can be positioned exactly where you want them. They also will stay in place if you just roll a little epoxy on the core first. It is simple to mark the exact hinge line location on the foam core. If you follow the procedures in my "nice trailing edges" document then you will have a nice straight cleanly defined foam core trailing edge as a reference.

You can use the plastic laminate flaperon template as a guide to mark the hinge line. Just place it with it's aft edge 1/16" (1.5mm) aft of the foam core TE and trace the hinge line with a fine point pencil. You will then place the bottom doubler just behind this line and place the top doubler one half of the hinge gap distance aft of the hinge line. Yes, they can be placed that accurately and no, you don't really have to be that accurate, they just look really cool that way. Another neat trick is to use softer foam ahead of the hinge line and denser foam aft of the hinge line. That way the hinge line is automatically marked for you.

Questions have made me pay a lot more attention to how much resin I leave in vs. soak out with paper towels. The more I look, the more I realize that I do not do dry lay-ups. I do the paper towel soaking only because I initially add way too much resin. This makes the lay-up go faster and makes the fabrics easier to position. The fabrics sort of float around in a pool of epoxy and are somewhat mobile. The carbon should not be moved much, if at all after initial placement but the Kevlar can be pushed up tight to the carbon as long as the lay-up is still real wet. As you soak more epoxy out of the lay-up with the paper towels, the fabrics become more locked in place. 

I only soak up the epoxy until the shine goes away. If you soak up more than that, you will first have cosmetic issues. The carbon will especially start to look porous and rough on the finished wing rather than smooth and glossy. Then if you continue to soak out more epoxy, you will have problems with "cosmetic" delamintions. By this I mean that the skin will tend to delaminate at the hinge line when you try to do the skin hinges. Or the skin will delaminate at the root end when you cut the wing in half with a table saw. I always add extra epoxy to the trailing edge area of the lay-up but if you didn't, then the TE would be a soft, mushy mess. I don't know if it would be possible to get it so dry that the wing would suffer structural failure. I've never gone that dry since these other things happen first and they are bad enough to turn me away from the super dry extreme. Mark did a super dry lay-up on his SuperGee2 but he did caution you to add extra epoxy to delamination prone areas and also he does not use a structural carbon skin.

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