Variable Geometry (VG) is a feature of medium- and high-performance
hang gliders which allows the pilot to
adjust the wing's
performance and handling while
flying.
A hang glider's handling and performance are quite directly related to the stiffness of the
airframe and the
tension, or
lack thereof, of the
sail, the fabric that makes up the
wing surfaces. A stiff, tight wing generally has great
glide, especially at higher speeds, but
is very difficult to turn. A soft, loose wing is usually very
maneuverable but has poor glide which gets worse at speed. Before VG systems were
introduced pilots had to choose between easy to fly and land gliders that wouldn't "go" and hot bladewings that really "went", fed their egos, and
and taught them how to survive ugly landings. A variable geometry system allows the pilot to fly the glider "loose" for climbing in
thermals (circling),
flying near hillsides or other gliders, launching, landing, etc., and "tight" for making
efficient glides from point A to point B.
Hang gliders ca. 2001 have
nose angles of about 120 degrees, and the nose angle is determined by the
crossbars, which hold the
leading edges apart. The crossbars are fastened to the leading edges at some point near the middle of the leading edge, and are hinged
together where they meet above the
keel, which runs from the nose of the glider straight back; the crossbars form an angle of 130-140 degrees. Drawing the crossbars backwards spreads the leading
edges (this is the core of how a hang glider with a 30+ foot
span folds into an 18 foot, 1.5' diameter
cylindrical package). When the glider is assembled
the sail is tensioned by hauling the crossbar center back with a cable and attaching it to a point on the keel. During the flight testing phase of developing
a new glider design different haulback cable lengths are usually tried in order to fine-tune handling/performance. The flexible sail can accommodate a small range
of change without needing to be re-sewn.
In the 1970s and 80s, some gliders were made with 2 or 3 crossbar attachment points so the glider could be adjusted for conditions or pilot skill each time it was set up.
The first VG systems provided means to adjust sail tension in flight by using
block and tackle systems to haul the crossbar center backwards a few inches, spreading the leading edges
a little, increasing the nose angle a few degrees, tightening and flattening the sail. A couple of multi-pulley blocks are set up behind the crossbar and the
rope is threaded along or in one of the
downtubes to the
basetube, where the pilot's hands typically are during flight. These systems commonly use a
reduction between 7:1 and 14:1, which means
a lot of rope has to be pulled to move the crossbar a few inches, but without the reduction the pilot would either get worn out quickly and/or applying the needed force
would result in altering the glider's path, since the pilot's weight would shift, potentially wiping out any performance gains. The rope is set in a
cleat to maintain
the tension, and this allows the tension to be set at any amount desired. This is handy for weak lift situations where the extra bit of climb gained by having the sail a little tight is
worth the extra effort required to work the lift well. The ropes usually have index marks to indicate how much VG (1/4, 1/2, etc.) is "on". One manufacturer
made a system with an actuating lever and no pulleys, so it was either "on" or "off", in the belief that the middle settings weren't that useful, but
they eventually gave in to pilot demands and adopted the standard system.
The crossbar haulback system works well, except a drawback is that as the crossbar moves the
anhedral (angle between the wings in the horizontal plane) changes as well.
The anhedral plays a prominent role in the roll response, hence turn response, of a glider, and the interaction with sail tension effects is very complex and difficult to control.
Gliders with conventional VG systems might be great VG loose and VG tight but VG middle-or-so might have poor dive recovery or other problems. Additionally the
lower side wires of the glider, which are attached to the crossbars near the leading edge junction, have to be loose VG loose to keep them from being too tight VG tight.
Most pilots launch their gliders VG loose (for maximum maneuverability) but would prefer at least moderately tight side wires so they can feel the glider's roll forces at this
critical time. For these reasons, the cam VG system was conceived, with the first production system implemented by
Juan Corral of Moyes, California, in the late 1990s.
The cam VG system works by leaving the crossbar angle fixed and effectively lengthening the crossbars, eliminating the anhedral and side wire problems. At the
outboard
ends of the crossbars, pivoting plates are attached, with one side extending a couple of inches and the other about twice that. The shorter end is attached to the leading
edge and a cable from the keel area is attached to the long end, with the line from the leading edge attachment point to the cable attachment point roughly parallel to the keel. Again a block and tackle system is set up so that the cables are pulled inward when the rope is pulled, which pivots the short ends of the
plates outboard, spreading the leading edges, and so on. Since the long ends of the cam plates provide leverage, a less complex pulley system is needed,
which reduces weight and cost.
In the 1980s it was very typical for pilots to go from beginner to advanced skills only to leave the sport once they got high-performance
wings because they were so difficult to land that weekend flights stopped being fun (and got expensive, due to damaged tubing). Variable
geometry systems have been a boon to hang gliding because they allow gliders to be maneuverable and easy to launch and land, yet still offer glider performance only dreamed of
in years past.
(In the UK and other places variable geometry is known as variable billow, or VB, since "billow" is used to refer to the tension on the sail)