[Courtesy of John Erik Larsson, joppe "at" sm.luth.se , March 1999]
I have glanced at the recent thread concerning sheeting of foam wings, and I would like to share some of my experiences with you. And I would of course appreciate any comments.
Below, I will describe a method that I have tried on a few foam wings, and I will soon apply this method onto the wing, stab and fin of my 3m span C130.
I was looking for an easy method to produce cheap, strong, light and straight wings (isn't everybody looking for such methods) when I got the idea to cover the wings with brown paper and epoxy. I haven't heard of this before, but maybe someone have used this method for eons, who knows? After a couple of experiments I came up with a method that seems to work. The result is a strong and light wing with an surface almost ready to paint. As the fibre orientation in the paper are almost random, the torsional strength of the resulting wing is better than if you use glass weave in only one direction, i.e. no 45 degree oriented layer (I've got the flame suit on).
The putty described under step 3. is very useful. I use it on white foam fuselages to get a really smooth surface before I cover them with glass and a clear polyurethane paint. The result is a strong, smooth and very light fuselage.
1. Cut the foam cores with your favorite technique and sand them to remove the angel hair. Don't worry about imperfections, they will be handled later.
2. Make slots for wing spars, the size, material, length and number of spars have to be adapted to the size and the load of the wing. Spars are necessary as the brown paper, obviously, don't have rigidity of balsa. I've tried carbon tows, spruce and thin plywood (0.4 mm for a 1.1m span wing), standing as a long web in the core, with good results. Here you can choose to join the wing halves, or you can do it when the cores are covered. Add LE and TE of your choice and sand them to shape, I put packing tape on the foam core to protect it. I have also cut cores with a leading edge, just adding a 3M brown paper based packing tape as reinforcement, to be done after the cores are filled and sanded.
The 3M brown paper based packing tape has the properties of brown paper with a really sticky glue on it, I think it would work fine to use this tape instead of brown paper and PVA glue in some smaller applications, the tape is 38mm or 50mm wide.
3. Now, when you have added the spars, you will probably have some dents and other imperfection AND you will certainly have all the small pits between the cells in the foam. If you don't fill them now you will get more work in the end and you will not be able to use the extremely light filler that can be used now. As the paper will be vacuumed (or hanged with weights) onto the core all these holes will be visible if they aren't filled. If you use ordinary putty directly on the foam core, you will probably end up with a heavy and hard to sand core. The putty i blend is based on a very creamy and fine grained putty I found in a paint store, I suppose it is some sort of latex putty. I mix the putty with water in a ca. 50/50 volume relation. There should be a notable feeling of increased viscosity compared to the water. Finally I add micro balloons to the mix (it is like mixing a cake mix) until the I get a creamy putty again. Beware not to add to much micro balloons, as that the resulting putty might get hard to apply. This putty is then applied to the whole wing and left to dry. As the binding substance from the original putty is greatly diluted, the sanding of the ultra light filler is very easy. I don't use anything coarser than 320 wet paper. Sand down the putty so that the foam cell surface appear, the glue has to attach to something. Use very light strokes as the foam cells might flex, which the putty won't, and cause the removal of putty between the cells.
4. Independent of which type of LE you have chosen, apply the 3M brown paper based packing tape to the LE, The reason to this is to avoid overlapping of the upper and lower sheet of paper which can cause a lot of unnecessary sanding. Cut brown paper sheets so that the wing is covered and make sure that the sheets only overlap the LE tape and that the the sheets extends and are joined behind the TE The mat surface of the brown paper should face the foam core.
5. Apply epoxy to the mat side of the brown paper, use a credit card or something to spread out the epoxy. As in the case of epoxy and balsa sheeting, it is very important to use as little glue as possible, the surface of the paper should only look darker. Use lamination epoxy, West system works fine for me. Lay the epoxied sheet on the core and put the lay-up into the vacuum bag, apply suction and remove any wrinkles as the bag is squeezing the core. Let the wing rest in the core cut outs during the curing. Cover simultaneously as many surfaces you feel comfortable with, for small wings I cover all surfaces at the same time. Let the epoxy cure completely before you do anything serious with the wing, if you test the strength to early you will certainly end up with a ruined wing (been there, done that). I use a refrigerator compressor as vacuum pump and I make the bags from plastic bought in rolls at the general store. Use a low vacuum pressure so that you don't compress the foam. If you don't have or don't want a vacuum system, you can probably cover one side at the time, with good results, by hanging the brown paper over the core and adding weight to the edges. I haven't tried it but I think i should work fine.
6. Trim of the TE with a knife and a ruler. Cut out for and add rudder surfaces, landing gear blocks and whatever you want.
I you used the lightweight putty and achieved a smooth surface before applying the epoxy and brown paper, there should be very little work left before you can paint the wing Be careful when you choose paint, some agressive paints may search its way through imperfections in the thin epoxy layer and attack the foam.
Good luck and best regards,
John E. Larsson