ISON Tandem AirBike Builder's Log

May, 2000 -- Spars and more ribs

The first step in making spars for the ISON wing is to turn long pieces of spruce into longer pieces of spruce. The 3/4 x 3/4 spar caps, 199" long in my case, are built-up of two pieces each, joined by a 9" long tapered glue joint (scarf joint). Above is the table saw jig I made to cut the angled ends for the scarf joints. The jig has a runner on the bottom which rides in the table saw top groove seen at bottom left. The work pieces are clamped into the jig with the two bolts on top while the whole assembly is run past the saw blade.

This was very quick and produced amazingly good cuts (using a "hollow-ground planer" blade). Squareness is important in this cut, as any error will be doubled when the parts are mirrored and joined together.

Grady was here to help glue the spar cap scarf joints together. This was my first time at scarfing, and we happened upon a quick and very accurate method for gluing all 8 caps together at once. The dark brown strips are 3/4" pine strips cut straight and square and nailed to the table top with the brad gun. The strips are kept straight with the aluminum level, while being spaced apart just enough for the caps and two layers of waxed paper.

Once the scarf joints have been thoroughly saturated with epoxy (both parts, of course), they are pressed together with strips of waxed paper surrounding the joints. After applying sufficient pressure to verify appropriate glue squeeze-out throughout all of the joints, the caps are clamped to the table top at both ends to cure overnight.

After the joints were cured and removed from the fixture, the caps were laid together on the table and sanded to level the glue squeeze-out with the wood. I'm happy to say that the scarf joints came out beautifully.

Now we need to attach a perfectly straight strip of 3/4" x 2" wood onto the edge of the table to build the spars against. Wanna see a cool way to do this? Being an instrument maker came in handy here (you never know when your hobbies are gonna overlap):

The silver thing at bottom center is a zither-type tuning pin, screwed into a temporary plywood strip. A piece of .022 music wire (what I happened to have) is tensioned between this pin and another block of plywood on the other end of the table, about as tight as you think you can get away with. (If this wire were to break it could be dangerous. I once saw a guitar string break and then enter and exit the player's arm.) Don't have a zither pin? Improvise with a drilled AN bolt, lag screw or whatever.

Before the wood strip is attached, its bottom corner is chamfered slightly to give any trapped crud particles a place to go without interfering with the work piece (a good practice for all jigs and fixtures). With the wood strip clamped into place, a light wood shim (piece of a popsicle stick) is held by tension between the wire and the strip. The strip is tapped away slowly with a hammer, and when the shim falls the strip is nailed to the table top. Repeat at 10-12" intervals until the entire strip is attached. Easy, fast, and probably as straight as you're ever gonna get it.

Here's a set of spar cap doublers being glued to the long scarfed caps while being clamped against the new straightedge. I have a bag full of these walnut wedges which make excellent clamps for things like this. Simply nail backing blocks to the table top next to the wedges and then push or tap the wedges until the proper tension is applied.

If you don't have a brad gun (or "pin nailer") like the one on the right, take my word for it and go out and buy one. One of the best investments you'll ever make, and you'll know it after about 2 minutes of use. This type of brad gun shoots 18 ga. brads from 5/8" to 1 1/4" long.

Here's a close-up of the walnut wedges in use. At left is one of the special wedges I made to apply pressure to the tapered ends of the doublers during gluing. It simply taps in until proper pressure and glue squeeze-out are achieved, while a backing block keeps the spar cap straight.

Here's the inboard end of the first front spar being glued up. This shows how versatile the wedge clamps are. On small glue joints like this you really don't want much pressure or you'll squeeze too much of the epoxy out of the joint. Additionally, it is important to apply glue to all of the end-grain first, allow it time to soak in, and then reapply two or three times as needed -- before the joints are assembled -- until the thirsty pores are satisfied. Otherwise you may end up with weak "glue starved" joints. Some of the end grain on these spruce parts will suck in the glue for over 20 minutes, and giving this process some time will also allow the epoxy to thicken a little which will help discourage further soak-in after the joints are assembled.

The ISON wing has a striking resemblance to overgrown model airplane construction, and the waxed paper underneath the work pieces helps to reinforce the similarity. Just don't push large T-pins through the spar caps and into the table top, OK?

This process is completed for one front spar (middle of table) while the other is being glued up against the wood straightedge (right).

After the spar assemblies are leveled with a sanding block, the 1.5mm ply spar webs are fitted to the spar assembly and temporarily tacked in place with a staple on each end. Then the spar is flipped so that the locations of the wood members can be traced onto the web with a pencil. This leaves no doubt as to where the glue is applied to the web.

Now the spar is flipped web-side up, and the spar assembly is nestled into a length of waxed paper and lightly clamped against the straightedge with the wedge clamps. With glue applied to the spar assembly and the spar web, the parts are mated and stapled together. The two rear spars are now done (yes, I was very careful to make a right and a left!).

Grady spreads glue on the inboard web for the right front spar. The next day I glued up the left front spar by myself (it sure goes faster with four hands!). It's nice to have the spars almost done.

The root and tip ribs get sheeted with 1/8" plywood, and since the instructions don't offer much help on how to do this I'll show the steps I took. I stacked the four pieces of ply and pinned them together with an aircraft nail at each end so that all four ribs could be machined at once. Above, the aileron cutout has been located, cut out with the bandsaw, and is now sanded to final profile with a drum sander.

A temporary plywood template is fit to the aileron cutout, with the center point of its arc marked with an X. This locates the aileron hinge point. Why didn't you just lay out the whole rib before making the aileron cutout, you ask? Good question. (Actually, this turned out to be a good idea. See the June report.)

Now a sample rib can be located onto the rib ply, and the lightening hole cutouts are marked onto the ply. The plans say to only make these cutouts on the two root ribs, but Wayne told me that if fiberglass wing tips will be installed the tip ribs may be lightened in this manner also. So I'm making the cutouts in all four ply ribs.

The cutout corners were cut first with a 2" hole saw on the drill press, then the cuts were completed with the saber saw.

Here the cutouts are sanded to final profile with a drum sander on the drill press.

I needed a nose rib for locating the forward cutout in the ply ribs, so I went ahead and made the router template for the nose ribs. Aside from having these laser or water-jet cut, this is about the best way I know of to make a run of identical parts from thin plywood.

The template is 1/2" birch plywood (what I had handy), with three drywall screws protruding about 1/16" out the bottom. The 1/8" ply rib blanks are traced, roughly cut oversize on the bandsaw, and then pressed onto the template's screw points. Then the part is trimmed to exact profile with a laminating bit in a router table (above). The bearing on the laminating bit follows the template while the blades cut the part exactly to the template.

The slot at left for the 5/16" x 5/8" leading edge strip is traced onto the part with a pencil, and cut out later with the bandsaw. Alternatively, the nose ribs could be stacked together and dadoed on the table saw, but I bandsawed them individually on my last plane and thought that to be easier than setting up a fixture for the table saw.

Thank goodness tomorrow is June 1 -- this page has gotten much longer than one page really should! We'll be assembling the first wing panel real soon in the June page...

 

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