Skinning the Wing

From "Construction Notes" Falco Builders Letter, March 1987

Karl Hansen sent Steve Wilkinson a letter with some tips on skinning the wing with Aerolite. Since all of you would benefit from this, we're including it below:

Dear Steve,
There seems to be many questions about using Aerolite glue, especially on the wing skins. I guess if the British could construct a Mosquito bomber with the stuff, it should be possible to make some simple small skin attachments. I admit I studied the problem a considerable amount before putting on the first skin, but as it turned out the dread was more in the contemplation than in the doing. I'm sure there is a better way, but this is the way I did it.

First of all, you should mark the rib and spar positions on both sides of each piece of wing skin, on the inside to locate the glue, and on the outside to know where to staple. I put the top skins on last because if I didn't have a perfect seal, I wanted it on the top side away from moisture. You have to seal the skins before installation. Only put the sealer up to about 1/4" from the glue joint as the sealer creeps in the wood, and you don't want any sealer in the joint. When the sealer is dry, you put the glue right out to the sealer and when it is catalyzed it becomes an excellent sealer.

You put the glue on the skin first and then the catalyst on the spar or other surface-a couple of coats so the wood is well-soaked. When you put the glue on the skin, extend outside the marks up to 1/4" to allow for misalignment. When the glue is catalyzed it will act as a sealer.

Don't do your gluing during the heat of the day. I usually prepared the day before and glued in the morning as it would get to 90-100° in the afternoons in the summer. There is quite a difference in catalyzing time from 65° to 75°. I never had any glue harden from proximity to the catalyst, only by contact.

The skin should be pre-indexed in several places to allow quick alignment and preferably some (2) nail holes on the first contact spot (spar) for "perfect" alignment. Be careful to only make contact at the initial spot, the main spar. Immediately staple down the main spar area. The skin will not touch the other catalyst as the skin is tangent to the curved surface. I stapled the rear of the main spar first including the nailing strips up to the main spar, then the ribs forward of the main spar, and start lacing the leading edge, then a row of staples just into the cap strip, then finish lacing down the leading edge. Not too tight-you can pull or break the leading edge plywood.

Stapling order for a typical wing skin.

The first skins are easy to check, but the glue joints and the skins are enclosed after installing the second skin, and you cannot check in very far except with a mirror. With the medium-speed catalyst and gluing when cool, you will find the joints are very good.

The main thing with using Aerolite is you have to be prepared. You have staple guns, plural. I used two electric and two manual in case of jams. I did some alone, but my brother helped with most-a big comfort when you are concerned with time. You'll want staple strips, nailing strips (pre-nailed), Saran Wrap laid out, extra staples-everything you will need when you put the skin down. You can't put the glue on and have a cup of tea while you lay out the tools. Actually you have plenty of time. The leading edge always got the final treatment and the glue really runs out even at the tail end of the project. Leave the leading edge clamped (laced) at least 24 hours to allow for complete cure and strength as there is a fair amount of tension on the curved plywood.

I used epoxy in a few places, the aft cover on the main spar and gluing the wing into the fuselage to allow adjusting time, and a few other minor places. But if I were to do it again, it would be Aerolite again. And I'd probably paint it jet black with white gold-edged strips. Good luck on your Falco. You'll like the way it flies.

Karl Hansen

As a builder, the most interesting thing to me was Karl's pointing out that Aerolite serves as a moisture-proof sealer just as varnish does, so you don't have to go to unusual effort to mask and varnish right up to the rib-and-structure line on the inside of the wing skins; just make sure you put Aerolite wherever you haven't varnished. Before he'd pointed it out, I'd always wondered whether each of my skins had an eighth-inch-wide line of untreated bare wood around each glue joint.

Another was his emphasis on getting all your ducks in a row-pre-nailed nailing strips, a spare stapling gun in case of jams, etc, etc. It's just common sense, but he has systematized it.

And finally, of course, his technique of gluing to the spar before concerning himself with rib-to-skin junctures. That's probably the way it would work for anybody using common sense, but he makes a point of the fact that you gain additional gluing "time" by preventing any glue/hardener interface until it's time for those surfaces to be joined.

Steve Wilkinson

From "Construction Notes" Falco Builders Letter, June 1987

Steve Wilkinson recently finished skinning his wings, and I asked him for his comments on what wisdom he could pass on that had not been included in the construction manual or Karl Hansen's notes published in the last builder letter. Steve's report:

There's really little I can add to the wing-building procedures outlined in the builders' manual. Certainly the vertical orientation of the wing is the way to go. I hadn't thought it would make much difference one way or the other in terms of ease of construction, but I sure would hate to have had to do much of that work leaning over the wide inboard portions of a horizontal wing (or worse, under one). Even carefully brushing glue on to a sheet of plywood in the horizontal position on the workbench can give you a backache after a few minutes of straining toward the far edge, so the benefits of sanding, stapling, gluing and doing everything else at normal height, as though you're working at a blackboard, can't be overestimated.

The only thing I had trouble with was bending the plywood over the leading edge. It looks so easy, and seems so neat in the builders' manual drawings: two scarfed sheets lapping over each other in a firm embrace and all that. In real life, I was just as likely to get the sheet lapped over 80 or 90 percent of the leading edge... and then, lo and behold, a yawning quarter-inch gap where there should be a glue line, when I cut away the one-by-one board glued to the excess plywood for use as an anchor for the inner-tube rubber bands that snug down the prebent plywood.

The problem is that the location of that one-by-one is relatively critical: leave too much excess plywood and the pull of the rubber bands (or even of bar-clamps, which I also used) tends to bow the sheet. Put the strip too close to the area where you're going to be scarfing the finished plywood and you'll have a messy job cutting it free, for it too will be glued to the wing. It's also a help to use cabinetmakers' clamps cinched over two opposing one-by-two clamping strips, one on each side of the leading edge, once everything is in place. And you must staple down the plywood all the way up to and onto the beginning of the leading edge radius before initiating the bending, else it will lift from the forward portions of the ribs.

When applying the lower skins, the anchor points for the rubber bands (and/or bar clamps) that do the bending can be the exposed main spar. When you apply the top skin, of course the main spar is covered, so you need to use a two-by-four that bears against some adequately strong members clamped to-and perpendicular to-the main spar where it still is exposed. When I got to the last sheet-the outermost upper wing skin-I found that I was still able to use a 2x4 of moderate length by using as one of my clamping points the padeye of the jackpoint already installed on the wing. (There's not that much clamping pressure on the two-by-four-certainly not enough to strain the padeye longitudinally.)

Another caveat: After wetting and prebending 2.5 and 2.0mm sheets for the inboard wing sections, when I got to the 1.5mm sheeting I thought, "Oh this stuff is so flexible compared to those obstinate thicker sheets that I won't have to wet or prebend it... I can just whip this into place by hand." A mistake, I quickly learned, for yes, you can bend it around by hand in a trice, but it'll crack. The soaking and prebending is not only to make it possible to bend plywood around a small radius, but to make it possible to bend it without cracking it.

Steve Wilkinson