Built-up versus Foam - Paul Winter and his wings
Flying for some 40 years, 10 of them competitively, I have found that building a beautiful open structure in my wings to be part of the joy of modelling. I often ask myself what is the advantage of cutting all those wing-ribs, making a jig to put them on and then sheeting the front part of the wing. Once this is done and you remove the wing from the jig you have to very carefully cover it with silk, nylon, tissue or plastic. What’s the advantage?
I have built some 10 full-size stunters including Tsunami, Tempest (5 of them!), Voodoo, Miss Winter and so on. Tsunami had 47 ribs in the wing. OK it looked beautiful and boy, was it straight, but so much wood was wasted as I only got 5 ribs from one sheet of balsa. So much time is spent laying up the wing ribs in the jig, making sure that they taper correctly, that they are straight and most of all, light. So much work when you have to add all the cap-strips, sheet the leading-edge and trailing-edge, install the bellcrank, lead-outs and under-carriage blocks. Covering with nylon or silk is OK but this tends to be a bit heavy and in my view, very hard to work with. Plastic is the easiest but not the strongest and is not the best look for a Classic model. Tissue is my favourite – very light, very strong and easy to obtain, from PS Aeroproducts, of course! Before you have put on the nylon, tissue, silk or whatever, you have to prepare the wood with Dope, several coats, rubbing down, more coats, more rubbing down and so on. Once you have covered it, the coating and rubbing down is repeated. Don’t get me wrong – I love tissue rib finishes, but it takes so much time!
About 4 years ago when watching a video I was introduced to foam wings, to which some of my flying colleagues said “too heavy”. My answer to them was “rubbish”. Remembering that you have to use Epoxy or PVA glue with foam wings, not Zap, I have built 4 full size stunters with foam wings, and the wings themselves are superb. Available from Johnny Duncan in America, these foam wings and laser cut to perfection. At a cost of approximately £70 (including postage and packing), I was and still am really impressed. Johnny hollows out the wing, puts in spars and undercarriage riblets and they are all very light. The secret is to find the lightest you can with which to cover the wings. I cover mine with 0.8mm balsa, yes, that’s right, 0.8mm balsa. This is obviously where the weight saving comes in. My wood is obtained from overseas and a typical sheet of 0.8 x 100mm x 1070mm is only 5-8 grammes. Backtracking a bit, when you actually receive a set of wings from Johnny they are in covers. These covers are your jig for the wing coverings so don’t discard them and make sure that you leave the wings in the centre when skinning.
The foam cores are smooth as silk. Don’t sand them as when they are hot-wire cut the heat of the wire seals the outer layer of the foam and stops it from melting.
I put my skins on with West Products 2-pack Epoxy.
Lay out the skins on a sheet of newspaper. Spread out the Epoxy on the skins 2 at a time. Carefully scrape off all the excess and I do mean ALL of the excess until the skins look virtually dry. Lay one skin in the bottom wing cover, then put in the foam core, then the top skin and then the top wing cover so that you have a cover-skin-core/wing-skin-cover sandwich. As the glue takes about 8 hours to dry hard you have time to adjust the work until you are happy. Now you need to put loads of weight onto the sandwich, about 40-50 lbs is ideal, and leave the whole thing for about 24 hours.
PLEASE REMEMBER that if you want to stick balsa to foam, don’t use CA. Use a good glue / epoxy that is foam safe. Also, when painting, don’t let your dope make contact with the foam which will melt. Tape up any holes for pushrods, leadouts etc, especially when spraying, just to be on the safe side. When its all dry, remove the wing and the core centres, trim the leading edge and trailing edge, the wing tip and the centre area. If you have a spar in the wing mounting, the placement of the bellcrank and pushrod is easy. Cut a small swathe in the top and bottom of the spar at the centre of the wing. Feed in your leadouts through your inboard wing and stick the two halves together. The put in the top and bottom bellcrank mounting on the cut-out in the spar. Leave to dry. Once dry stick the leading edge and trailing edge on the wing tips etc and put our undercarriage blocks in place. Now this is where foam wings come into their own.
I cover my whole foam wing with 0.5oz fibreglass cloth, again put on with 2-pack and in the same way as the skin. Lay out the cloth, pour on some 2-pack and spread it around all over the area, again carefully squeezing off all excess until it looks almost dry. If you mix up 30cc of glue, and if you have done this correctly for one side of a 770 square inch wing, you should only use half of the mixture. Any more than that and you’ll need to go back and scrape off some more. Let it dry, again for 24 hours. Repeat the entire process for the other side. Do the entire job again for both sides of the wing. This really is where foam wings are better because as they have a total flat surface you won’t be risking broken ribs or going through the covering so easily. I also put a wing strap around the centre two halves of the wing to give it a little bit o extra strength. Now you can paint your wing or simply go out and fly it. This method of preparing and finishing foam wings results in a sealed and fuel-proofed wing, and you have the benefit of any finish going over 2-pack, which if you have used and applied it correctly, will be rock-hard and very light.
I can build a foam wing start to complete finish in 5 hours working time excluding drying etc.
When I go flying, I tend to be a bit heavy handed with my models and a foam wing is far more forgiving than its rib-based counterpart. Flyers from the old school like the sun reflecting on the rib bays and, yes, I agree that they can look superb, but for sheer strength and durability, foam wins the final contest.
My latest ‘Strega’ has a foam wing which I covered with tissue and finished with Brodak Butyrate Dope. I’ve done my time with both built up and foam wings and while there’s a place for them both, at the moment my money is definitely on the foam.
What do you think?