Building Basics: Control Cables

Nuts & Bolts
Building Basics
MATERIALS USED TO
ailerons, has seven
build airplanes seem
strands that wrap
to have contradicaround each other,
tory qualities. Take
and each strand is
Connecting a pilot's actions to the airplane's reactions
control cable. It has
composed of 19 fine
wires. For a smaller
to be flexible and
strong. A length of
cable such as the
TO/
H.G. FRAUTSCHY i:-:-.• • > :
1/8-inch diameter,
•'•-'
3/32-inch diameter
variety, you can imagine just how
7x19 galvanized cable has a breaking Specs & Strands
strength of 2,000 pounds—that's Military specifications standardize fine each of the 19 wires must be.
enough to sustain the weight of a the manufacture of the three major Coated with a permanent lubricant,
fully loaded Piper Cub, with a pretty types used in aircraft. They define the fine wires give the cable excepgood margin left over.
the cable's three common uses and tional flexibility, making it ideal for
Control cable is wire rope. First distinguish aircraft quality cable control cables that use a pulley or
created in the 19th century, it ranges from the stuff you can get at the fairlead to change direction.
The second cable specification defrom the cables used in suspension hardware store. All three are made of
bridges to the finer strands that give galvanized carbon steel stainless fines 7x7 control cable (seven
shape to the brims of ladies hats. steel, and the strength differences be- strands of seven wires each), used
Control cable has been a staple since tween the two materials are minimal. where extra flexibility isn't required,
Its number of wires and strands but abrasion is a problem. The third
aviation's earliest days because it's
light, reliable, cost-efficient, and classifies cable. For example, 7x19 cable specification is the strongest
does so many things well, like trans- cable, which measures 1/8-inch in but least flexible, and used with
mitting a control input around a diameter and is most often used to swage-type terminals. 1x19 cable's
corner or bracing a wooden wing.
actuate rudders, elevators, and most common use is general inter-
Control Cables
STANDARD NICOPRESS TERMINALSPUCE
1 STRAND OF
19 WIRES
1X 19 NONFLEXIBLE
USED FOR
BRACING
STRUCTURES
7 STRANDS
OF 7 WIRES
7 X 7 FLEXIBLE
WHERE EXTREME
FLEXIBILITY IS
NOT A FACTOR
RESISTS
ABRASION
88
AUGUST 2000
7X19 EXTRA
FLEXIBLE
7 STRANDS OF 19
WIRES
TOLERATES SEVERE
BENDS IN CONTRO
SYSTEMS.
SUSCEPTIBLE TO
ABRASION
DAMAGE
Tir'l.Oj
nal 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.
Cable Maintenance
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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.
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"••• r - •''••-•trr
Cable Ends
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 com-
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Building Basics
Resources
pletely 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 handtool installation, and the power
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90
AUGUST 2000
For more information on using
aircraft cable, check these
sources, which are available on
the EAA website at www.eaa.org
or by calling 800/564-6322.
• Homebuilder's Headquarters,
in the members-only section of
the EAA website.
• Sportplane Construction by
Tony Bingelis. $24.95—F01395
• Sportplane Builder by Tony
Bingelis. $24.95—F30140
• Aircraft Spruce & Specialty
Company catalog—800/824-1930
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.
•