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Control Cables - Connecting a pilot's actions to the airplanes reactions
By H.G. Frautschy (originally published in EAA Sport Aviation, August 2000)
Materials used to build airplanes seem to have contradictory qualities. Take control cable. It has to be flexible and strong. A length of 1/8-inch diameter, 7x19 galvanized cable has a breaking strength of 2,000 pounds-that’s enough to sustain the weight of a fully loaded Piper Cub, with a pretty good margin left over.
Control cable is wire rope. First created in the 19th century, it ranges from the cables used in suspension bridges to the finer strands that give shape to the brims of ladies hats. Control cable has been a staple since aviation’s earliest days because it’s light, reliable, cost-efficient, and does so many things well, like transmitting a control input around a corner or bracing a wooden wing.
Specs & Strands
Military specifications standardize the manufacture of the three major types used in aircraft. They define the cable’s three common uses and distinguish aircraft quality cable from the stuff you can get at the hardware store. All three are made of galvanized carbon steel stainless steel, and the strength differences between the two materials are minimal.
Its number of wires and strands classifies cable. For example, 7x19 cable, which measures 1/8-inch in diameter and is most often used to actuate rudders, elevators, and ailerons, has seven strands that wrap around each other, and each strand is composed of 19 fine wires. For a smaller cable such as the 3/32-inch diameter variety, you can imagine just how fine each of the 19 wires must be. Coated with a permanent lubricant, the fine wires give the cable exceptional flexibility, making it ideal for control cables that use a pulley or fairlead to change direction.
The second cable specification defines 7x7 control cable (seven strands of seven wires each), used where extra flexibility isn’t required, but abrasion is a problem. The third cable specification is the strongest but least flexible, and used with swage-type terminals. 1x19 cable’s most common use is general internal bracing and the external drag and anti-drag wires that brace a wing. In most cases, round and streamlined solid wires with threads on each end have replaced 1x19 for external bracing.
Don’t let dirt and grime build up in the spaces between the cable’s fine wires. The grime wears on the wires as it flexes, causing fraying over time. Never clean cable by soaking it in a solvent. This removes the lubricant that aids the cable’s flexibility. Instead, wipe it with a clean rag.
During preflight inspections and routine maintenance, inspect for fraying or broken strands visually and by running a gloved hand along the cable. A broken wire or two is not cause for immediate alarm; it means the cable is under stress, and you should check it frequently for signs of additional deterioration.
Special fittings or splices connect cables to the components they control. Those with nautical blood laboriously weave a five-tuck Navy splice. (About the only time you’ll see this splice today is on an accurately restored antique airplane.) Others wrap the end of the cable with soft steel wire and then dip it in solder.
On homebuilt and production aircraft, the most common cable terminal is the Nicopress sleeve, an oval-shaped, soft-metal sleeve invented and produced by the National Telephone Supply Company. Builders use a special tool to compress the sleeve, forcing the soft metal sleeve around the cable to grip it tightly.
Builders compress the Nicopress fitting three times. In order, they compress it in the sleeve’s center, then next to the thimble or bushing installed when making the cable, and, finally, at the outer end. At each compression, builders check the sleeve with a go/no-go gauge to ensure the compression has completely squashed the fitting into a circular cross section, resulting in a tight, strong cable end.
A thimble or bushing is always part of a finished cable end because it transmits the load on the cable to all the strands of wire equally without damaging individual strands. While the cable itself is strong, each tiny wire is susceptible to damage due to kinks or abrasion. Often, a shackle or turnbuckle rod end may be installed as the terminal end is fabricated.
The rotary swager is another special tool for finishing cable ends. Swaged terminal fittings create the neatest looking installation, and they’re often stronger than the cable itself. Their only drawback is that the process doesn’t allow a hand-tool installation, and the power swaging tool costs nearly $4,000. But, a number of companies, like Aircraft Spruce & Specialty, FAA-approved repair stations, and some FBOs, will swag cables to order.
The installation itself is simple. Builders slip a cable terminal, which has a hollow barrel on one end, over the end of a cable cut to length. The power swager, using high pressure and a set of dies, then squeezes the barrel of the terminal into the cable’s crevices. It’s a precise process, and builders check their work with a go/no-go gauge to ensure that the terminal is at least as strong as the cable itself.
One finishing challenge is cutting the cable, but handheld cable shears cut the cable cleanly so the end will slip easily into the barrel of a swaged terminal. Many builders wrap the cut end with a short piece of tape to help keep the fine wires from fraying and piercing a finger.
Some builders cut cable using a time-honored method that requires a deft touch with a hammer. After tightly wrapping plastic electrical tape around the cable where they will cut it, they put the cable on a hard surface, bisect the taped area with a sharp cold chisel, and give the chisel a good whack with a heavy hammer. With practice, the result can be a surprisingly clean cut. But a $25 to $50 handheld cable cutter provides more consistent results.
Next month we’ll address the rest of the cable story.