Cozy

Chapter 19a - Wing jigs and foam

Making the jigs

I cut out the rectangles and stuck on the jig 1 through 5 templates. Plans say to cut out the airfoil shapes, cut each jig into four pieces, then mark the location of the 1 by 8 sticks to hold the jigs together. What about the thickness of the cuts? Surely this will make the jigs smaller by whatever thickness blade you use to cut the jigs into 4 pieces. I marked the crossmembers before cutting the jigs. Next issue was that the plans say to cut out the airfoil, then cut into four. I wanted to use my bandsaw to cut the airfoil shape, so I cut the jigs into four pieces first.

Wing templates

A fellow builder offered his aluminum wing templates on loan, but when I compared them to my plans I found that they were a long way off. His were first edition plans. All the templates were all uniformly and proportionately reduced in size. Either he'd worked from inadvertantly reduced photocopies, or the printing process for the second edition increased the size of the templates [Nat says this is not the case, so I suspect he worked from copies]. In the case of the bigger ones the difference equates to around 1/4 inch in the length, maybe 1/8 on the smaller ones. Of course, all the cut outs are out a little as well. Also, one of the big ones (68 I think) has a "bent" water line where the template parts join. This makes the trailing edge about 1/4 inch - 3/8 higher than it should be. Hmmm. I guess he'll have slightly smaller wings than plans.

I got some formica and made my own templates. You have to be VERY careful to ensure that the waterline is totally straight when joining the big templates - don't just line up the curves. A very small error here can raise or lower your trailing edge significantly. Using a band saw with a fine tooth blade and a lot of care it's possible to cut the shape accurately within 1/32 of the line, then sand for a really smooth hotwire edge. I learned while doing the canard [with 1/16 plywood] that you want absolutely NO joggles along the edge. Formica seems to be a much better material for this job. If I were doing this again I'd spend a few days right up front and cut ALL the templates for the airplane in formica. It would be a boring job, but once done it would be much appreciated at the beginning of every chapter.

Cutting the foam

I spent some time getting my head around the planforms and angles, then set up the first two blocks for the inboard right wing. I changed the wire on the hot wire saw because the old one had got rusty since I did the canard. I didnt like the look of arrangement of the "extra" block on top of FC1, but I went ahead anyway. The start tabs on the BL31 template caused me problems. At the BL65 end, Char could go straight down and start cutting the sheer web area. I had to go down and around the little tabs. The result was a totally f...ed up spar cap cut. Everything else went fine except the "extra" block on top. Hot wire cutting doesnt "feather" well. The extra piece was burned and curved at the edges. I'd recommend using a large block for this piece. In general our first airfoil cut was a bit of a disaster. When this happened on the canard I ordered more foam and did it again until it was perfect. That approach was expensive and time consuming. This time I decided I could fix the problems later and moved on. Next we hot wired the aileron holes. I accidentally got one of the templates on backwards and realized, as we were cutting, that Char was rotating her end clockwise while I was rotating anticlockwise. This is when we discovered that you can cut two long pointy spikes at the same time using a hotwire! I reversed the template and we cut again to get the remaining bits out. Our exit hole was a bit big, but otherwise we recovered from this, once we stopped laughing at ourselves. Hmmm. I wonder. Were we were laughing at me?

I'm planning to work my way through both wings at the same time, so we cut the FC1 for the left wing next. I reversed all the angles and templates, then walked around holding the first core up against the planned cut to be sure I was cutting all the right directions. Once everything made sense in my head (this took a while) we cut the second core without too many problems. Again, I'm pretty unhappy with the "extra bit", but I can see how this can be fixed with extra foam sanded to contour. Before sticking the pieces together and correcting the errors I wanted to hollow out the end pieces. I hotwired the wedges off the cores. I have a band saw, but there's no way that big core would go in it. Considered making a template to cut the 0.6 edge, but then decided to use a hacksaw blade. Since the torque tube hole was already cut, the top piece fell away. The "extra bits" were also missing, and they would be about 0.6 thick at the edge, so by the time I'd hacksawed the foam all the way around, I had a large gap on the top and about four pieces of foam. I did this on both cores, then microed everything back together. I added filler pieces on the top and sanded these to contour later. My sanding block for this kind of job is a 3 * 2 extruded alum square tube which Home Depot sell as a screen frame support. I got an 8 foot length of the stuff. Once the filler foam pieces were added and sanded to contour, and everything was microed together my inner core looked fine except for the spar caps. Fixing these was fairly easy with a straight edge and dry micro.

I replaced the wire on the saw again. Seemed to me that it was stretching and I wanted the tightest wire I could get. I've also noticed that there is usually some burned foam stuck to the wire after a cut. If you leave it on this makes the next cut a bit irregular, so I always sand the wire a little to clean it off. By the time you've got three 14 inch blocks standing edge to edge the foam looks huge. We cut the planforms without any trouble and proceeded to set up for the template cut. I did the small end and cut VERY slowly while Char did the big and and went as fast as the wire would allow. After the cut, the airfoil looked great, except for the spar caps once again. Wire lag had made these a bit curved and misshaped. No problem. I can fix this. We cut out the 1 inch holes for the wiring before microing the pieces together, because a hot wire wont cut micro. After cutting the first midsection core I noticed the plans indication to use the "thin" blocks left over from the canard for the outboard end. Makes sense, but I only have one of these left overs anyway.

Once all the cores were cut I laid out the left wing parts on the bench. Wow. The wing looks huge! Next I put the left wing away and laid out the right wing. Wow. The wing looks .... wrong! Did you notice that warning on page 10 "watch out for forward sweep". It was written for me. I could have sworn that I'd set up the angles correctly for the right wing outboard core, but apparantly I hadn't. I wouldnt mind, but the right wing is the one in the plans. I got the left [everything reversed] right. Damn! Damn! Damn! I guess one wing out of two ain't bad. Char tactfully told me it's actually only one mistake out of six. Hmmm.

I had to cut the outboard right core again. I searched around and located a 4 * 7 * 64 inch foam block which would do for the trailing edge. As I'd done each core I'd trimmed the offcuts using the hot wire and vertical templates. The new leading edge was made from five of these offcuts. They had the right angles already cut, so all I had to do was line them up and trim the 51.76 inch length. Yes. I know that number by heart now. This time I held the right midsection core up against the jigged blocks to make certain I had all the angles right. Now for the template cut of the new core. Seems my concentration was slipping. Almost everything that can be done wrong with a hotwire cut I did wrong with this one. First the nail holding the leading edge of the 118 template was angled just enough that it caught the hotwire. Jiggle #1. You have to deliberately angle this nail AWAY from the leading edge. Next I'd forgotten to add weights. The foam moved. Jiggle #2. I grabbed a weight and placed it as we were cutting. Jiggle #3. Finally, as we moved toward the trailing edge I noticed that the template was loose. I hadn't put any nails at the trailing edge. Duh! Jiggle # 4. I held the template with one hand while hot wiring with the other. The foam moved some more. Jiggles #5, 6, 7 and 8. After we'd done cutting we looked at the mess. No problem, I told Char. Its the right [her] wing.

All the jiggles were outward, away from the template, so spline sanding got rid of them. I ended up with a good core, but this was the worst cut I've ever done. After the cut I had lots of pieces, so I microed them all together and left the core standing to cure. After a few hours of curing I was browsing the plans when I reread the bit about hot wiring the leading edges off. What I hadn't realized was that my micro joins all crossed the sheer web. Hot wire don't cut micro. I had to cut the micro at the sheer web with a hacksaw blade before I could hot wire. Now for the final error.... Whatsamatter? You didnt think there were any more errors I could make? Well, there's one. I'm spline sanding away and thinking that I'm having trouble getting rid of that hump in the middle of the core. I have about 1/8 clearance at either end. Hmmm. I wonder. I turned the core over and checked with a straight edge. Yep. A 1/8 dip. My core is slightly warped. Probably because the main part was left over from the canard and has been standing for a couple of months. Its a good job I noticed, otherwise I'd have sanded the bottom, then had to fill the top. I left the core supported at either end and weighted in the middle to unwarp it.

Summary of wing foam cutting

This is my artist's rendition of white clouds in a blue sky using nothing but dry micro and my wing leading edge.

I had been somewhat dreading this stage. I had thought that the cuts had to be perfect, or my wings will be scrap. Not true. We [I] made lots of mistakes, many of them stupid ones, but the airfoil shapes came out exactly every time. Its real hard to screw up the curves. Its real easy to screw up the spar caps. If I did this again I might even leave out the spar caps, then cut these later like we do with the canard. Completely perfect cuts every time would have been wonderful and would have saved me a day of fiddling around fixing errors, but my net result is as close to perfect as I'll ever be able to get. I may have a couple of extra ounces of dry micro in the spar cap areas, but not enough to be significant. A few builders recommend cutting oversize, then spline sanding down to the plans templates. I considered this, then decided I didn't want to make another set of templates. It would have been a lot of work. In any case, cutting oversize would not have saved the errors I made. Also, I found spline sanding "along the numbers" a little difficult. I think I got better shaped and smoother airfoils hotwiring to size than I would have done by sanding. But, that's just my opinion. I could be wrong.

Did I mention the huge pile of blue foam which now litters my patio? There are a lot of cut offs and leftovers from all the wing and winglet cutting. Some nice wedge shaped ones which work well as wing supports later. I wish I could think of a good use for the rest.

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