Bending Wing Skins
From "Construction Notes" Falco Builders Letter, March 1991
And finally, Stephen Friend says that "whatever I did, I didn't seem to be able to get the stabilizer, or fin, leading edge plywood to bend tightly enough. And then I remembered that I had some 16 gauge x 1" channel which, with clamps, was able to tighten the bend and still allow room for staples."
Al Dubiak said that he rigged up a nice steam jenny for bending the leading edge skins. For about $90, he bought a plumbers gas stove-one of those LP gas stoves that is used for melting lead for pipe joints-and put a five-gallon can on top of it with a half-inch hose coming out the top.
Al says that with only a few inches of water in the can and with the stove on full, you can really pump out steam and make the plywood let you 'have your way' with it. All woods become very pliable when heated to the boiling point of water, and one of the surprising things about this method is that it does not markedly change the moisture content of the wood.
Also on that subject Howard Benham writes, "One thing
we noted in bending the leading edge was to put a wet
towel along the bend line after soaking the skins for 3
or 4 days, then heat the skin with a steam iron. This works much
better than just using the iron on the wet plywood. Once the plywood
is hot, we clamp it in the jig and again apply the wet towel/iron
for a few minutes prior to placing bags of lead shot along the
bend line. We leave the skin in the jig for 24 hours, then remove
the skin and clamp on the wing. This gives you practice at clamping
the leading edge before the glue goes on and allows the skins
to dry in exactly the right shape. This might help some of the
new builders as the 2.5mm top skins can be rather imposing when
you first start working with them."
From "Construction Notes" Falco Builders Letter, June 1991
We seem to have unending variations on methods to bend the plywood for the wing leading edge. Most involve water, steam or a combination of both. Joel Shankle had prebent his skins by soaking the edge of the plywood in water and then clamped the plywood over a large broomstick. He kept wet towels on top of the bending section, and pulled the plywood around with weights and ropes.
George Barrett decided to pick up on that idea and go one better. He made long, tapered form blocks from Obege wood, a soft, easily-planed wood recommended by a lumber yard. He made a trough and soaked the leading edge part of the plywood for two days, then dumped the water out, boiled about 4 gallons of water and put this in the trough with the plywood. He left the plywood in the hot water for about 15 minutes, then clamped the skin over the form.
He said the key was working the alignment out, and he used an index line to accomplish this. The clamping was done with two boards. George said the system worked out beautifully and that the skins just dropped right on the wing. George has written up a full description of the method, and we'll include it in the next builders letter.
Cecil Rives bent his skins on the wing with steam. He tried using a pressure cooker with a hose to play the steam on the wood, but the pressure cooker kept turning over, wouldn't put out the volume of steam and kept burning his fingers. He finally went out and bought a Black & Decker 'Steam Works' wallpaper remover. This little devil costs about $36.00, and Cecil says it worked wonderfully for him.
Like everyone, he presoaked the skins in water until they were saturated. He just threw the plywood into his swimming pool until it sank-"The only use I've ever found for the pool"-in about two days. (Most builders only soak the part of the skin that will be bent, because the soaking can slightly distort the rest of the plywood.)
Then he used the Karl Hansen method of the one-by-one strip of wood with nails in it, but he bolted it in place rather than glued it, because he had a strip pull loose on him once. He used the normal long strip on the outside, and then put shorter backing strips on the inside. These were short so they would fit between the ribs, and he used four to five quarter-inch bolts at each section between the ribs.
With all this clamped in place on the wing, he wrapped the giant inner-tube rubber bands in place and then took the steamer and started playing steam over the leading edge. He said it worked fine and that the skin just slowly pulled around and settled into place as the steam hit the plywood.
From "Tool Talk" Falco Builders Letter, June 1986
For soaking the plywood for the leading edge bend-it is neither necessary nor desirable to soak the entire piece-most builders make a trough with lumber and plastic film. Steve Wilkinson found a neat alternative in the wallpaper department of his local hardware store. For soaking wallpaper, they sell plastic troughs that are about 6" wide, 6" deep and the length of a roll of wallpaper. Steve bought a couple of these dirt-cheap things, cut one end off each and epoxied them together.