ISON Tandem AirBike Builder's Log

June, 2000 -- Spar & rib completion

Wade came over to help me rip the 10 1/2° angle on the top spar caps. The spars being 16 1/2' long, we had to move the table saw into the doorway...

...and run the spars out into the back yard. We sure had a pretty day for doing this. We used a featherboard to hold the spar against the rip fence, while carefully holding the spar down tight against the table saw top with our hands. With the saw blade at an angle, the width of cut will vary if the workpiece rises at all.

Mike was in the mood to make some parts, so I put him on nose ribs. Here he roughs out oversize blanks to be finished on the router table.

Robert came over to use my router setup to make nose ribs for his Tandem. Here he has pressed the blank onto the template's sharp points and is trimming to the template with the laminating bit.

"Before you do anything, you always have to do something else." I had to have wing strut and wing attach fittings to be able to locate and drill the holes in the spars, so I took a break from the woodworking and made the aluminum fittings. Now that the spars are drilled, we can start assembling the first wing panel. But first...

We interrupt this program for a special announcement. I had my first aerobatic flight (and second biplane ride) on Saturday, and in a Pitts no less! The Kansas Pilots Association was having a conference and mini-fly-in at the Lawrence airport, and there were several planes giving rides. Here I'm receiving an amazingly thorough lesson in how to jump out of the airplane and use the parachute.

The ride completely rocked my world -- I don't think even an experienced pilot can really know what to expect from their first flight like this! We did inverted flight, aileron rolls, 4-point rolls, loops, an immelman, a hammerhead, and the pilot even had me do a roll and a loop myself. Yikes. I loved it, to put it mildly, but one more maneuver and I may have had to use the sick-sack. He had me taxi the plane back to the ramp to help ease the queasiness, and it did help. But it took me hours to recover from the flight afterward! Fulfilling this dream was definitely worth it, though.

Here Grady uses a bandsaw for the first time, cutting off the excess wood on the rib fronts. Today Grady proclaimed, "I like power tools!"

Using a 90° fence on the edge sander to flush up the front edge of the ribs nice and square.

This is where it starts to get exciting for me -- the first few ribs are trial-fit to the spars. But I'm getting ahead of myself -- first I need to trim off the aft end of each rib and sand it square with the edge sander to match the trailing edge stock. Among other chores...

Remembering how awful it was to try and hand-brush the varnish onto the ribs of the last wing I built (I gave up and sprayed it), I decided to varnish the ribs on this wing before assembly while it is still relatively easy (although still very time-consuming). I varnished four ribs and got tired of it, Grady varnished two and hated it, and then another AirBike builder told me he thinks that varnishing is the hardest part about building a wood wing. I mentioned my fantasy about dipping the finished wing like they did in the old days:

This photo shows employees of the Travel Air company (Wichita, late 1920s) dipping an entire wing into a vat of varnish! I had considered dipping my ribs, but resisted the idea because of not wanting to have varnish on the glue contact areas. I finally decided that a little sanding in these areas would be much easier than varnishing all of the rib surfaces by hand...

So, I made this metal tray* and merrily dipped the remainder of my unvarnished ribs. A quick dip, then the ribs are hung to drip, then the major wet spots are wiped with a paper towel. The varnish needs to be quite thin for this to work best. I have no idea what varnish/thinner ratio I ended up with, but I'd guess that somewhere between 40/60 and 50/50 is the sweet spot.

Since dipping in varnish any thicker than this would probably turn into a colossal mess, this technique seems to be useful only for the first coat, which most builders seem to thin anyway for penetration. That's OK, though -- I'm happy to at least make it easier and faster to apply the first coat. And there's another benefit: dipping one coat will allow the varnish to get into all of those hidden areas where a brush can't reach anyway.

Here's how I glued the plywood skins to the root and tip ribs. With the aileron hinge point centered over the X on the plywood template in the hinge arc (arrow, see close-up from May), the parts are glued together and pinned to the table top with tiny aircraft nails until the glue is cured. I also used the straightedge on the table top to keep the rib straight on the bottom edge (with a 1/4 x 1/4 filler holding the rib away to prevent the rib from becoming a permanent fixture of my table top!).

You can see the 1/4" ply fillers in place around the aileron hinge area in the two photos above. These are installed in the root ribs, tip ribs, and ribs 5 & 9 (wherever an aileron hinge point is located). The step-by-step instructions in the builder's manual will not tell you to install these far enough in advance to be useful, so be sure to read and plan ahead here.

After I finish attaching more misc. spacers and blocks to these hinge ribs, I'll be more than ready to start assembling the first wing panel. Oh yeah -- I still have to hand-brush the second coat of varnish onto the ribs. Darn.

 

* Just for fun I calculated the size of the tank that would be required to dip my completed wing panels like Travel Air used to do. The 10" x 30" x 200" tank would require 260 gallons of varnish to fill, at a cost of around $5,000 for varnish alone, and would have to be strong enough to support the 1300 lbs. of varnish! This seems to support the practice of varnishing the ribs by themselves before assembly, which only requires a quart or so of varnish in the tray. No wait... Maybe I could get another AirBike builder to go in with me on the tank, and we could share the costs... yeah, that's the ticket... you listening, Robert? This tank would fit just fine in that big new garage of yours, doncha think?

 

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