Flying Wires Control Bar Ground Wires Figure 1-6.  Simple structure added to the Rogallo wing allows wires to hold up the wings on the ground and support the wing in flight. Figure 1-7.  An original Rogallo wing, 1975.  Hang Glider  Motorized Hang Gliders A Maturing Industry The WSC system and the good flying qualities of the Rogallo  wing and Dickenson wing, combined with its easy set-up  and portability, started the hang gliding craze in the early  1970s.  [Figure 1-7]  In 1967, the first powered aircraft based  on the flexible wing concept of Dr. Rogallo was registered  as amateur-built experimental. Flexible wing development  continued, and by the early 1970s several adventurous  entrepreneurs were manufacturing Rogallo wings for sport  use.  Activity in the hang gliding community increased throughout  the 1970s, which resulted in the proliferation and development  of stable, high-quality modern hang gliders like the one  shown in  Figure 1-9 .  Another significant step in wing design was an airfoil that  would change shape for optimum performance at slow and  fast speeds. It was the first Rogallo wing with a lower surface  that could enclose the structure that holds the wings out.  Enclosing this cross bar tube and providing a thicker airfoil  similar to the airplane wing provided a jump in high speed  performance. This double-surface wing was quickly adopted  by manufacturers as the high performance standard and is  used on faster WSC aircraft today.  [Figure 1-8] The propeller was moved lower for better takeoff and fl ight  characteristics, wheels were added, and the trike was born at  the end of the 1970s. A trike describes a Rogallo type wing  with a three wheeled carriage underneath (much like a tricycle  arrangement with one wheel in front and two in back). Trike  is the industry term to describe both ultralight vehicles and  Engines and airframe technology had made great advances  because the ultralight fixed wing evolution was providing lighter  weight, higher power, and more reliable propulsion systems. In the late 1970s, performance had increased enough to allow  motors to be added to hang gliders and flown practically. It  was not until the wings had become efficient and the engines  and propeller systems evolved that the first commercial motor  for a hang glider was introduced in 1977, the Soarmaster. It  used a two-stroke engine with a reduction system, clutch, and  long drive-shaft that bolted to the wing frame. It had a climb  rate as high as 200 feet per minute (fpm) which was acceptable  for practical flight. However, during takeoff the wing would  overtake the running pilot, and launching was very diffi cult.  Also while flying, if the pilot went weightless or stalled under  power, the glider would shoot forward and nose down into  a dive. Overall, with the propeller pushing the wing forward  during takeoffs and in some situations while flying, this was  unsafe for a wide application.  [Figure 1-10] 1-4