innovators gave success to the flexible-wing hang glider. First trikes In 1961, Engineer Thomas Purcell built a towable Rogallo-wing glider with an aluminum frame, wheels, a seat and basic control rods; soon he replaced the wheels for floats and motorized the aircraft. In 1964, Swiss inventor Pierre Aubert saw a photo of NASA's Fleep and completed construction of a similar trike. As with the Fleep, his Rogallo wing was fixed and did not allow for pendulum weight-shift control. In March 1967, aeronautical engineer Barry Palmer completed the earliest example of a true weight-shift powered trike: the Paraplane; it was controlled by a single vertical control bar as the Paresev experimental glider that inspired him. The Paraplane used two West Bend-Chrysler 820 engines of 8 hp at 6000 RPM, reduced to 4700 RPM for about 6.5 hp each, for a total of 13 hp. Each engine had a direct drive to a 27-inch-diameter two-blade propeller made of polyester and fiberglass. On March 24, 1967, Palmer registered the trike with the FAA as the Palmer Parawing D-6, serial number 1A, registration number N7144; no restrictions were noted. The second Palmer trike, Skyhook (FAA registration N4411), in spite of its early date of origin, had most of the attributes of a modern ultralight, except it used a single-cylinder snowmobile engine, as the two-stroke twin-cylinder engines were not available yet. It was powered by a 17 hp at 5000 RPM single-cylinder JLO L297 two-stroke engine, driving a composite propeller designed and built by Palmer himself and driven by a 2.1/1 reduction gearbox. The engine had electric start and the craft had fiberglass composite spring landing gear. Airframe construction was bolted 6061-T6 aluminum thin-wall tube, with 6061 T-6 extruded angle. The craft took off, flew, and landed at around 30 mph. The commercial availability of Dickenson's hang glider made the Rogallo wing very popular, and prompted several builders during the 1970s to attempt motorization of their flexible-wing aircraft but unlike Barry Palmer – who placed the center of gravity well below the keel – most builders were mounting the engine to the wing, where a fine balance existed between applying too much power, causing the aircraft to overtake the pilot, or not enough power for flight. It was not until Roland Magallon took a long look at the Motodelta ultralight (a hybrid Rogallo wing designed by Jean-Marc Geiser had a 'fuselage' and a rudder) that