http://www.economist.com/blogs/babbage/2012/11/hang-gliding

Hang-gliding is quite the purest form of flying. Sailplanes
insulate the pilot too much from the passage of air. Motorised aircraft dull the
senses with noise and vibration, and isolate the pilot still further from his
surroundings. Helicopters move with all the grace of a washing machine. Hot-air
balloons and para-gliders are exemplary. But to lie prone in a hang-glider
harness, exposed to the elements and using only body motions to control the lift
and direction of flight, is at once to fulfill man’s oldest of dreams and to
experience the nearest thing to bird flight.

In a sense, wing-warping is what got modern hang-gliders back into the skies in
the 1960s. All credit goes to John Dickenson, an Australian who was trying to
develop a more controllable kite for hoisting water-skiers into the air from
behind a motor boat. By good fortune, he came across a delta-shaped flexible
wing invented in America by Francis Rogallo, and tested by NASA as a means for
recovering Gemini space capsules.

Mr Dickenson’s great achievement was to marry a billowing Rogallo wing to a
harness and control bar that supported the pilot while allowing him to shift his
weight fore and aft to affect the glider’s pitch, and from side to side to
affect its roll and yaw.

When this arrangement was scaled up, so that it could be launched by running
with it down a slope into a slight uphill breeze instead of being towed by a
motor boat, hang-gliding took off around the world. By 1974, a standard Rogallo
hang-glider could be had for as little as $400 ($2,000 in today’s money). By
then there were some 40 manufacturers of hang-gliders in the United States
alone.

With few safety aids, little experience and such a low entry-price, the
inevitable fatalities gave hang-gliding a bad name. Today, as the sport has
matured and become carefully regulated and more professional, there are
essentially only two manufacturers left in America, plus a handful elsewhere.
The biggest by far is Wills Wing of Orange, California. The company produces
around 650 gliders a year at prices ranging from $3,800 for an entry-level
Falcon 4 to over $8,500 for a competition-class T2C.

Wills Wing will celebrate its 40th anniversary next year. Having been a leading
light in the business since the beginning, the company has pushed the technology
further than most. Early Rogallo gliders, with their billowing sails, had a
lift/drag (L/D) ratio of around four-to-one, depending on the speed. Today, even
a trainer such as the Falcon 4 can have an L/D of ten-to-one, while a
hang-glider designed for cross-country competitions, like the Wills Wing T2C,
will have an L/D of over 15-to-one. That is less than a condor’s, but much the
same as a red-tailed hawk’s.

Such improvements have come mainly from taking the billow out of the Rogallo
wing, reducing its sweep, increasing its aspect ratio (span divided by
width)—and, above all, learning how to control the twist in the wing. A Rogallo
wing’s billowing fabric imparted too much twist—with the outer sections of the
wing attacking the air at a much lower angle than the inner sections. Most
wings, whether on gliders or airliners, have a little downward twist (or
“washout”) built into them deliberately, so that their inner sections stall
before their tips do. That helps the pilot maintain control in a stall,
especially when executing a roll.

But too much twist also hobbles performance. In contrast to the loose sails of
early hang-gliders, today’s craft rely on high-tech Mylar fabrics stretched over
thin aluminum tubing along the leading edges and shaped aluminum ribs that give
the wing its camber. The art has been in finding the right amount of twist to
stop the wing tips stalling, but not enough to stunt the glider’s ability to
soar and stay aloft.