On a
hang glider the side wires are critical bits of the
airframe assembly.
The
hang glider airframe consists of aircraft-grade
aluminum tubing
and flexible steel cables, designed so it can spread out to be a wing
some 30'+ wide and also fold into a compact cylindrical package for
transport and storage. The side wires connect the middles of the
leading edges to the
kingpost (top) and the
control triangle or
basetube (bottom), both mounted to the
central keel tube. The bottom side wires are particularly crucial
because they counter the forces trying to fold-up the wing like a cheap
lawn chair while in flight. They also transmit the roll inputs from the
pilot, through the basetube, which is several feet below the wing, to
the airframe for turning and such. Top side wires attach to the top of
the kingpost, several feet above the wing, to similarly support the
wings while the glider is on the ground and during the occasional
downward shove from turbulence in flight or during a bad landing.
There
are also wires from the kingpost and control frame to the nose and aft
points of the keel, for stability and pitch control. These and the side
wires are all typically 3/16" stainless steel multi-stranded cables with
attachment hardware fixed at the ends. Tandem gliders, for two people,
and some high performance gliders can see very high loads on the lower
side wires, so they will have 1/8" cables that are much stronger. These
wires are exposed to the airflow and produce parasitic
drag, which
depends on the cable diameter and increases exponentially with airspeed.
The cables are typically coated with flexible clear plastic tubing to
protect them, allow visual inspection, and to keep the wires from sawing
off
body parts or cutting emergency
parachute lines in the
rare
catastrophic structural failure. Lower side wires should be
inspected often and replaced at the sign of any damage, or once per
year, whichever comes first. In-flight failure of any cables is
extremely rare.
Competition pilots will strip the coating off the
cables so they produce less drag at racing speeds. Thinner lower side
wires of uncoated 5/16" or 2mm were used by racing competition pilots
for a time in the early 2000's for even lower drag, with the caveat that
they should be replaced every six months and never put to the high
loads produced by aerobatic maneuvers. After a number of broken side
wire incidents, mostly among pilots who ignored or were ignorant of the
warnings, these have fallen out of favor.
Wire assemblies on hang
gliders consist of a length of cable and
stainless steel tangs at the
ends. These thin tangs (think "tongues") are an inch or two long, half
an inch wide and have large holes in each end. A collar of soft metal (a
NiCo) is placed over the end of the cable and the cable is looped
through one hole of the tang, usually around a steel thimble to prevent
kinking and wear, and back through the NiCo, which is then compressed
between the jaws of a tool resembling
bolt cutters to secure the loop
in place permanently. NiCos must be compressed uniformly, all at once,
to certain diameters to provide consistent strength. A bolt goes through
the other hole of the tang and through the air frame, such as at the
leading edge and crossbar junction or the basetube/downtube junction.
Because of the high momentary loads that can be placed on a lower side
wire during a bad landing the wire holes of lower side wire tangs at the
basetube can become elongated; the cable assemblies should be replaced
in this case, though failure of even very stretched tangs is unheard of.
The
top side wires, and indeed all of the top and bottom front-to-rear
wires are under very little load in flight. The lower side wires are
extremely taut in flight and critical to a hang glider's
structural
integrity.
(for Side Quest 2024)