Thursday, April 12, 2018

Restarting the Travel Air Project

Well, it's now spring of 2018 and the new shop/brewery/observatory is finally finished. Travel Air work began again in earnest a couple of months ago, but inertia has kept me from updating the blog. Now all two of you can start reading again.
Actually, it's just finished to the point that I can start working on projects again. I guess the work will never be done. In fact, I've stopped working on the wood parts being fabricated for the fuselage until the dust collection system could be completed. No more messy wood shop. Well, maybe.

The thing I've been anxious to install is the set of stringers down the fuselage. Two on each side and two on the bottom. These just form the shape of the fuselage for the fabric to follow. They are non-structural, but hold the taut fabric in it's shape for the life of the fabric, so the stress on the little wood parts is considerable considering the pressure on them is constant. I've seen one project that had little tabs welded to the fuselage tubing which held the stringers very firmly. My fuselage has already been blasted and primed, so I'm not too keen on messing up the very expensive Poly Fiber Epoxy Primer to install that type of "stringer mounts." A friend's T/A had little half-round plywood plates notched to receive the stringers which were then tied to the fuselage tubing with rib lacing cord. I wanted something more substantial without adding a lot of weight. Making these little plates was pretty easy. There are pictures in an earlier post showing the procedure.
What I came up with was just a home-made u-bolt, cut and bent from long #8 threaded rods. A small piece of rubber tubing was slipped over the middle before bending to protect the tubing after tightening. Each one was hand bent around a piece of 1" steel tubing as a form. Holes drilled in the plates receive the ends and metal stop nuts hold them firmly to the tubing.

I like the way they turned out. I just have to be careful when tightening the nuts since the only resistance is the (relatively) thin wood. Too tight and I'm sure it would break in half, or at least crack.

They are left loose while lining up the stringer slots, then carefully tightened. The fuselage has a natural curve (of course) and the upper and lower tubing converge toward the tail. Had to take all this into account while running the stringer "line" in order to assure a smooth, appealing surface to the fabric. Any mis-alignment,  or bumps on any of these stringers will result in an ugly protrusion. It should not look it has warts.

Another obstacle to progress has been the absence of wing bracing wires for the upper wings. The lower wings had a full set along with end forks and jam nuts.
These are the internal bracing wires that criss-cross in several "bays" within the wing. The actual name of these wires are drag and anti-drag wires. They prevent the wing from bending aft or forward during flight. Piano wire or braided cable is often used as are threaded rods. Mine will have rods. On the Travel Air drawings, wires are specified with rods as an option. Required tensile strength: 2100 lbs. with 10-32 threaded ends. Even 1/8" piano wire probably has tensile strength above 6000 lbs.
Now the big problem. There is only one manufacturer of flying and bracing wires left in the world: Bruntons of Scotland. And they are pricey! And I mean VERY pricey. The way Bruntons makes their rods is very cool. Steel rod, even cheap mild steel has tensile strength far in excess of anything required for this purpose. The weak part of a rod/wire in tension is the ends. On a rod there must be a threaded portion to accept a fork end that is held by a clevis pin. The threads are the weakest link. So Bruntons manufactures their rods with larger ends than the middle. Also, all threads are rolled, not cut. Cut threads result in a smaller diameter thread compared to rolled, and the cuts through the natural grain of the steel tend to weaken the rod compared to rolled threads. So all bracing and flying "wires" utilize rolled threads.
Here is a picture of an old MacWhyte rod along side (above) mine. As you can see, the middle of the rod is smaller than the end and there is a square section to hold a wrench for tightening. The lower rod is what I had made. It is a stainless steel rod with rolled right hand threads on one end and left hand threads on the other. A quote from Bruntons' supplier in the U.S. was $270 per rod. I need 16!! Yep, that's over $4000 for bracing wires. No way. I found a business in Wisconsin that rolls threads on various type rods and they quoted me a price for 16 rods of $388. SOLD!!!

 The only drawback is that the entire rod is approx. 3/16", so they will be slightly heavier than the smaller Bruntons rods. Both use stainless steel, which has slightly less tensile strength than carbon steel, but the resulting tensile strength is so far in excess of what is required, it is of no consequence. Now a builder of a standard certificate airplane would be stuck with finding originals or the hyper-expensive Bruntons rods. Experimental category is great sometimes.

Now the wings are braced and ready to be trammed.