Dennis,
I searched here on the 'hanger and in my files and couldn't find a writeup on pressing wings--but I know I have described it before, and perhaps recently. (I'm getting the deja vu vibe here.)
Here are the basics:
--Prep your solid balsa wing until it is ready for glass (or carbon). Sanding finer than 220 grit is probably a waste of time?
--Clear off your bench to work over. Being FLAT is probably pretty important!
--If you have a flat bench and can leave your wing on it for a day to fully cure, you will only need one thick plywood sheet. I like 3/4" thick FLAT wood. If you don't have a flat bench or you need to move the wing somewhere else to cure you need two sheets of plywood and a bunch of long bolts (all-thread and nuts) that are evenly spaced around the entire periphery. This is what makes a press. The foam blocks fit just inside this bolted perimeter. If you can use your benchtop, then you can just use one plywood sheet on top and then load that down with a lot of weight to cause the pressing action.
--If the wing is semi-symmetrical, you will need two urethane foam blocks a couple of inches thick. One goes beneath the wing; one goes on top. This is the yellowish foam rubber used in furniture upholstery. Blocks with a smooth surface are needed, or the resulting wing won't be smooth.* If the wing is fully flat-bottomed (some of mine are) then you can skip the foam block on the bottom. In that case, I generally lay down a piece of window glass over the bench to improve the surface finish of the bottom of the wing. There are ways you can "fudge" the leading edge of a wing that has some bottom curvature but is otherwise flat. The two-foam-block approach doesn't have this problem.
--Go scrounge two pieces of Monokote backing material with no wrinkles to sandwich your wet layup be sure they are at least 3" bigger than your wing dimensions. If you have one bigger piece, so much the better, just fold it in the middle for extra insurance that the epoxy won't leak onto your (expensive) foam blocks
--Get your other materials and tools and go thru the sequence mentally to make sure you have everything ready. Cut the glass a couple of inches oversize in all directions. Don't try to fold the glass over any edges. If you want two layers of glass in the center for extra stiffness and strength, cut out all your pieces because they are all going on in one wet layup and cure cycle.
--Mix your EZ-Lam 30 minute laminating resin. You need enough to do both sides of the wing at once. Trying to do one side at a time is harder, takes longer and doesn't give as good a result. Don’t thin the EZ-Lam epoxy. It isn’t necessary and only bad things can happen. We’re not screwing with glue here, this is material that is designed to wet out on tight weave lightweight cloth. Anyone trying to use bonding epoxy that has a lot of fillers or thixotropic material is on their own. A little experience and good record-keeping will tell you about how much to mix for x-square inches of y-weight cloth. But don’t worry too much if you run out while wetting everything out. You can mix up another batch. The EZ-Lam is not going to go off on you while doing a small wing unless you really didn’t have things ready at the outset.
--Lay one foam block on the bench with the best side up. Lay a sheet of Monokote backing over that. Lay your glass for the bottom surface on the plastic. Now paint a complete coating of EZ-Lam on the bottom of the wood wing. I use a 1” china bristle throwaway brush. I brush it as thin as it will easily go on the wood. It still looks wet and shiny. Don’t remove any of this epoxy—you need it on there. It is almost the perfect amount for laying up .58 to 1 oz cloth. For heavier cloth I brush on a little heavier. Don’t agonize over the exact amount since the purpose of the next two steps are to force the cloth down against the wood, thereby avoiding cloth that is “floating” on a bed of resin and causing things to get heavy. Properly pressed, the excess epoxy will be squeezed out to the edges. Now lay the wing down on your pre-placed glass. And paint the other side with resin. Drape the top glass over the wing. Get it down straight and centered. I like to hold each “wingtip” of the glass and let it touch the wet wood in the middle and let it slowly roll out to the tips. No wrinkles means less fooling around with it. When it is down (all the layers) place your other sheet of plastic over it, or if you had one big sheet then fold it over. Now go wash your hands off. All the sticky stuff is inside the plastic and must stay there.
--Take a playing card of small piece of cereal box cardboard and squeegee as needed to get rid of bubbles and any wrinkles. You can move big lumps of excess resin around some, but if you painted the resin on evenly, you aren’t going to need to do much. Any real excess gets moved the TE or LE. Extra resin here is good. Do this to both sides. With everything encased in plastic it is easy to handle and work on. This is like a vacuum bag that you don’t have to seal up.
--With any air bubbles removed and the plastic laying down evenly (no rolling hills of epoxy), position it in the very center of the press (foam block) and add the top foam block. Add the plywood top plate. Either start screwing down the press, or if you are using just the top plate, add lots of weight to it. For a solid balsa wing that is laying down flat, you can add a lot and not worry about things breaking. I might end up with over a hundred pounds stacked on top. One thing to be careful of is that if your weight distribution isn’t even, over an hour or two the whole stack may tilt. And heavy expensive stuff (machinist knees), or messy stuff (full cans of housepaint) might go on the floor. You can help prevent that by using a set of equal-size stop blocks with one under each corner of the plywood. Give the foam an inch or so (YMMV) to compress and then hit the stop.
--Let it fully cure for a day; two if you have time. If you did everything right, the wing will come out as smooth as you plastic sheet and foam block. If you didn’t cheat yourself, the glass will be fully filled and either be ready to fly, or you can scuff sand it an paint on your trim.
I didn’t invent any of this. Racers and others have been doing this since probably the 1960’s. It is still around because it works.
* For an even better finish, you can use cauls outside the layup and avoid minor blemishes from slightly imperfect foam blocks. I have used cauls made from thin nylon sheet that will conform to the wing. This works on single-curvature shapes only.
Topic: .2 Carbon Veil or 3/4oz fiberglass cloth over sheet wing for stiffness (Read 2125 times)