So You Think You Understand Flying Wires!

March 2021 – Last month I mentioned sourcing some new flying wires for my DR-107 tail. I had found a source of flying wires out of New Zealand from a company called Old School Aero.

I was able to get the owner, Russ, to explain the process of making flying wires. It sounds like it’s as much an art and black magic as it is a science. As Russ mentioned to me, "It took me quite a few years to get the production processes right to produce an acceptable product. I sometimes joke that had I considered the IP (intellectual property) involved, I might have tackled something easier — like brain science or rocket surgery."

Fortunately for us, Russ chose this challenge for his spare time. Below is an abbreviated description that he shared with me on the process of making the wires.

Profiled or Streamlined (AN67X and AN68X Series) Flying Wires

Before we can profile (streamline) the central part of the wire, we have to thread the ends. You cannot successfully cut a thread in stainless steel; it will leave a stress riser at the end of the thread, which in time will fail. This is more important with tail wires, as there is no midwire support, as there is in wing flying and landing wires. This lack of midwire support sets up a harmonic, and the notch in the thread will cause the wire to snap. It’s just a matter of time before they fail.

A piece of steel is not that dissimilar to a piece of wood, in that it has grain running from end to end. Once you start removing metal, you have unsupported pieces of metal asking for help. If you are rolling the thread, you force the shank into the rollers and the grain runs around the rollers, keeping the grain contiguous and intact.

The shank to be threaded is first swaged down to the effective diameter, the midpoint between the major and minor diameters, and the thread is then rolled up to the nominal diameter.

Swaging block examples.

The thread-rolling process is swift. The surface temperature of the shank goes from room temperature to about 380 degrees Celsius in about 1/100 of a second, making the surface of the shank plastic, and the thread is formed around the rollers. Thread formation is at the rate of 35-40 mm per second, so a 2-inch thread takes about 1.5 seconds to form.

You have to thread the ends before you profile the streamline part for a couple of reasons. Firstly the blank is shorter, and secondly the blank is still round. Once the wire has been profiled, it is longer and no longer round. Try spinning a not-round piece of metal at 3500 rpm and the results are “unpredictable” (read: catastrophic). I don’t care how much concrete you have around the wire, the end result is not pretty; don’t ask me how I know.

Once we have a right-hand thread formed on the wire blank, it’s time to disassemble the rollers from the right-hand head and install them in the left-hand head. The thread-rolling process is repeated for the left-hand thread.

Swaging machine to prep the ends prior to threading.

The next step is to remove the now threaded wire from the threading machine and place it in the rolling mill, in the appropriate slot. The wire is rolled back and forth — just like a piece of pastry in a bakery — and the pressure is increased incrementally until the required wire length is achieved, and hopefully the width and thickness will also be in spec. The initial pinching pressure is in the order of 420 tons/square inch. If I put a piece of 1/4-inch stainless round bar and bring down the pressure, I can deform it 2 mm (0.080 inch).

Rolling mill to form the wires to the correct streamlined profile.

Next you remove the wire and check it for straightness. It is nowhere near straight in either width or thickness directions, so I built a device using a set of rollers to straighten the wires.

Once the wire is straight, the next process is polishing, which is a multistep process and can take time, as there is a bit of heat built up during the polishing phase and the wires may need to be tweaked a little to get them straight again.

Once the wires are finish-polished, they get wrapped in cling film (to keep fingerprints off them), packaged in a plastic tube, and sent off.

Round Tie Rods (AN70XX Series) Are a Different Animal

You start with a wild guess as to the length of the blank needed and select the swager blocks you need. For a ¼-inch tie rod, you will start off with a 9/32-inch block in the red swager. Place the blank in the machine, about 6 inches in so the swager is chomping away at the middle section of the blank. Move the blank back and forth to reduce the blank diameter. Stop the machine and adjust the blocks to close the gap, allowing about a 0.020-inch bite per action. After two diameter reductions with the 9/32-inch blocks, change the blocks to the 5.7 mm series and continue reducing the center section diameter until the final diameter of 0.180 inch is reached. It will take about five passes to get to that point.

Now you can force the blank through the blocks until you get the length of the swaged section you need.

You produce the swaged section first, and then proceed to thread the ends. After threading the ends, you stamp a square section or flat, using a 20-ton fly press.

Quite a few machines are required to make the wires. Russ has various sizes of swagers and presses, and none of them are small. It’s no surprise that there are so few options for sourcing flying wires. If you want to see how a swager works, the YouTube links below are a great introduction.

Two Hammer Swaging Machine Operation
Die-Closing Swager – Torrington Forming Machinery
Rotary Swager – Torrington Forming Machinery