Laminating the Wing Skins In the last step, we prepared the wing cores for skinning with an appropriate combination of composite materials. Last year's model used a full carbon fibre skin which was equal parts brutally strong and brutally heavy, so this year a compromise was reached. The leading edge up to the spar would be carbon, while the trailing portion of the wing would be skinned in Kevlar. That way, a good balance between strength and weight could be achieved. Composite construction isn't very hard to do acceptably (my first all-composite project was a heavy lift airplane), but the picky details will only come with practice. I'd encourage everyone to give it a go - there's nothing quite like the first time you pull a shiny new bagged wing out of its mylar shell. Plus, you'll find that this method is a lot quicker and more consistantly repeatable than built-up techniques. Lets get to it then!

Above Left: Here are some of the tools that you will need to work with composite skins. First, get a hold of some good laminating epoxy resin. I use West System 105 Resin with 206 hardener (slow, 1 h cure time), and find it to be light, strong and crystal-clear enough for my tastes. Your mileage may vary. Using the West System pumps makes measuring wasy, but does somewhat limit the size of the quantities that can be used. Also find some disposable gloves, mixing cups, and stir sticks. I like keep my rotary cutter on hand to trim oversize fabrics, but cover the handle with wax paper to avoid getting it too crusty with epoxy. Finally, find an old credit card or baggage tag to use as a spreading tool - preferably one that isn't too flexible or cracked. I end up using fairly significant amounts of pressure to squeeze excess resin out of the fabrics. Above Middle: Next, make up a pattern out of mylar that will conform to all the curves and areas of the core. The mylar is the white sheet just above the untrimmed tip panel pictured here. I'm using a 14 mil. mylar, since it seems to give the best balance between stiffness (surface finish) and flexibility (conformity to curves). Give it a few coats of a good mold release wax, buff it shiny, and set it aside overnight to dry. Since this wing is a fairly simple affair, I used a single piece of mylar to do the top and bottom skins of all three wing sections. I didn't find any need to wax the mylar between usages, since the mylar still released appropriately and the surface finish didn't change. Above Right: Without sounding like a commercial, go out and buy a can (or two) of 3M77 spray adhesive. It will be used to keep the various reinforcement patches from moving around on the core, and is also nice for adhering patterns to wood, tacking parts in place, and building EPP gliders. Essential for any well-stocked shop.

Above Left: You can see here all the various materials that will be used to skin the cores for the 2002 U of C model. The leading edge of the wing gets two layers of fabric: 1 layer of 4.7 oz 80/20 carbon cloth cut on the bias, and about 1.5" wide, and a slightly wider layer of 1.4 oz fibreglass, also cut on the bias. These are lightly adhered to the core with 3M77. The D-box section of the wing gets the carbon cloth (as well as the center panel center section) laid with the 80% direction spanwise, and the remainder of the wing is sheeted with bias-cut 1.7 oz Kevlar. Additional reinforcements will be added, but are shown later. Cut everything a hair oversized to allow for repositioning, and when dealing with cloth that is $22Am/yard, be careful about sizes! Above Middle: Of all the chores involved in sheeting this wing, cutting Kevlar has to be the worst one. I've tried a couple of different methods to cut the material, but it's resistance to abrasion means that this isn't an easy task. I eventually settled on outlining the part in masking tape, then cutting though that with Kevlar shears (available from ACP). If you're careful, the 1.7 oz cloth won't fray very much when you remove the tape, but it can be tricky at times. Luckily, all the seams will be hidden under the carbon cloth, so it doesn't matter either way. Do what you can. Above Right: Since I bag the bottom skins first, before I start I tape a sheet of wax paper to the top shuck to prevent the core from sticking. The core is laid into the top shuck, the completed mylar with wetted-out skins placed on top, then the mess in put into the vacuum bag together.

Above Left: Here are the reinforcement patches that are added to the skin just on the spar slot. They are intended to spread some of the load that comes as a result of the stress concentration inherent with the separate spar. This is a tip panel. The added double-taper patch is a result of the fact that the spar doesn't go the full length of the core. The same pattern is repeated on the top surface of the wing when that skin is added. The patches are lightly tacked in place with 3M77, then lightly wetted-out with epoxy leftover from the laminating process. Try not to get an resin on the part of the LE reinforcement that wraps to the back, that will be done with the top skin. Above Right: Same deal as the last photo, except this is the center panel. Since the spar is full-length, the double-taper patches aren't needed.

Above Left: Now comes the fun part: wetting-out the skins! This is the the center panel. First, a layer of 1.4 oz cloth, cut on the bias is layered onto the mylar. Squeeze out every last drop of epoxy possible! I've found that it's next-to-impossible to get a dry patch in the layup if the credit-card squeegee is used. I ended up using less than 2 oz of epoxy for the entire bottom skin, including the reinforcement patches and leading edge. Next the carbon leading edge strip is added out of 4.7 oz 80/20, with the 80-direction spanwise. The center reinforcement is the same material, except the 80-direction is now chordwise. Note that it overlaps the leading edge material. Finally, the remaining portions of the mylar are covered in bias cut 1.7 oz Kevlar, with about  0.5" overlaps with the carbon. Above Middle: The layup for the tips is a little different, with more Kevlar and no center patch. The tips are reinforced to prevent the ends of the ailerons from fraying, and to make the tips a little stiffer in general. Material for these patches runs chordwise. Above Right: In order to get excess resin out of the layup, after finishing with the credit card I lay paper towels over the material, and run a roller over the towel to get the last little bits. Make sure to wipe off the credit cards, cutting blades, and stir sticks to make sure that they can be used again with minimum fuss.

Above Left: With the skins and cores finished, they can be placed into the vacuum bag for curing.  For the bottom skin, both the core and top shuck go in the bag. The white material is a fabric breather cloth used to distribute the vacuum, and the blue seals on the end of the bagging tube are clamps available from ACP. Try to make sure that the core stays aligned in the shuck so it doesn't distort under pressure. Above Middle: For the pink foam that I'm using, I typically pull about 16-17" Hg of vacuum. Above Right: After the bag is sealed and a good vacuum has been established, the top shuck is placed over the core to prevent the whole thing from bowing. I then put bricks on top of the shuck, release the vacuum to allow things to settle again, and pull the vacuum one final time. I leave the cores under pressure for about 16-24 hours, depending on how patient I am. The pump cycles about once every 7-8 mins, running for no more than 20 seconds to top things off. Since this is a continuous duty air conditioner pump, it should last for about 300 years.

Above Left: Here's the tip panel right out of the bag. Note the excess stings of c/f that got stuck to the mylar. This is a good reason why both sides of the mylar should be waxed. Above Middle: Here's the center panel after the mylar is popped off. Just use gentle pressure, and it should give without any fuss. Now the excess material can be trimmed off, and you'll find that even the Kevlar cuts without a problem once the epoxy is cured. I run a strip of masking tape down the trailing edge, then cut to that edge. If you leave the strip on while the top skin is being done, you can cut the top skin flashing to the same tape line. I like to protect the entire perimeter of the bottom skin with masking tape to prevent resin leaks when the top skin is added later. Above Right: Now bring the core and the bottom shuck over to the drill press, and drill pilot holes through the center of the bolt blocks. Drill through the bottom skin.

Above Left: Now repeat the procedure for the top skin, adding reinforcement patches and wetting out the rest of the leading edge. Try to bag the top skin within 24 hours of finishing the bottom so that they cure as a single unit. Above Right: All done; ain't she purdy?

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