From 1973 to 1976, a number of technical innovations rapidly transformed the basic flexible wing hang glider from a simple and frequently dangerous aircraft with mediocre performance into a glider that performed better and was safer to fly. Richard Eipper, an early hang-glider pioneer, contributed to this important transformation. His Eipper-Formance Company developed and marketed a highly successful series of flexible and rigid wing hang gliders during the 1970s. The National Air and Space Museum's Cumulus 10 is a flexible wing design from this period.
Foot-launched rigid wing gliders had existed since the 19th century but they were not popular until the flexible wing technology developed from the space program became publicly available. In 1962, the National Aeronautics and Space Administration (NASA) tested Francis Rogallo's flexible wing design as alternative to the parachute recovery system used in the Gemini space vehicle program. Several individuals adapted the flexible wing to water-ski kites towed by boats. After manufacturers began to sell these new ski-kites in 1970, people began to foot-launch them off from small hills. Eipper called his version of the Rogallo hang glider the Flexi Flier and it was the first commercially produced hang glider marketed specifically for foot launching.
The Flexi Flier and Rogallo wings like it were popular because they cost much less to buy and fly than rigid wing gliders. The key to low cost was simplicity. A simple nylon strap suspended the pilot beneath the keel tube of the wing. To control the glider, she shifted her weight against a control bar that also hung from the keel. After a flight, the pilot could fold the wing and carry it back to the hilltop. To manufacture the wing, or 'sail' in hang glider parlance, designers attached a single-layer of Dacron fabric to aluminum tubes, joined together at one end, to form the keel and wing leading edges. During flight, the relative airflow billowed the sail and gave it shape. The wing held its shape unless deformed by turbulence. If turbulence did deflate the sail, the glider could completely stall and enter an unrecoverable spin.
This deadly problem quickly led designers to make these 'standard' Rogallo designs more resistant to turbulence. The second generation of flexible wing gliders had less wing sweep to make the glider more responsive to weight shift control. Designers mounted bracing wires between the wing and control bar to help the hang glider to withstand the loads imposed by moderate aerobatics. They also inserted strips of fiberglass or metal, called battens, into pockets in the sail. The battens functioned exactly like ribs in a conventional wing. They made the sail stronger and more rigid in turbulence.
In September 1974, Eipper began developing a follow-up design to his Flexi Flier with help from fellow hang glider pioneer and Eipper-Formance chief designer, David Conk. They named this new improved Rogallo aircraft the Cumulus. The aircraft performed well and pilots won a number of competitions after Eipper began to sell the new glider in 1975. Cronk immediately began improving the design and by year's end, he had completed the Cumulus 5. Cronk made this model with expert pilots in mind and it proved very popular for competition flying. Eipper and Cronk strove to keep the design current. They soon released the Cumulus 5b and less than a year after introducing the '5, Cronk was working on the '9, a prototype for his Cumulus 10.
The Cumulus 10 was ready for production in early 1977 and the designers equipped it with a number of improvements over the Cumulus 5. Where Eipper and Cronk only used battens near the wing tips on the '5, they employed them along the entire span of the Cumulus 10 wing. The two men also increased the leading edge convergence angle from 110 to 114 degrees. They used a shifting cambered keel pocket to increase roll- and turn- rate on the '10. The keel pocket slackened the sail closest to the turn axis to decrease wing angle of attack, and reduce lift and adverse yaw. They also installed cables called 'deflexors' and strung them across the wing leading edges. Short tubes held the cables a few inches away from the wing. The deflexors stiffened the wing and reduced bending during high-G turns. Eipper and Cronk designed the Cumulus 10 wingtips to flex downward at low angles of attack, reducing drag and augmenting the keel pocket effect. The tips also flexed upwards at high angles of attack, increasing washout effect for better handling at low speeds.
All of these improvements in maneuverability allowed the designers to use a larger wing that increased the glider's thermaling and cross-country flying performance. The various modifications also greatly reduced the force needed to control the glider and less effort was required to fly long distances. The company considered this glider too hot for novices to handle and recommended that only pilots certified to the highest level of flight experience certification, the 'Hang-4' rating, could safely fly the Cumulus 10.
Eipper-Formance sold the Cumulus 10 in three sizes to accommodate different pilot weights. Prices ranged from $1,050 for the smallest glider to $1,250 for the largest. The aircraft proved popular among competition pilots through the end of the decade. By 1980, flexible wing hang gliders began to appear with large undersurface coverings, or 'double-surface' wings, and 'floating' cross-spars inside the wings. The Cumulus 10 quickly fell from favor and Eipper-Formance switched its focus from hang gliders to powered ultralight aircraft. The firm soon introduced one of the first successful ultralight aircraft, the Quicksilver design series.
In 1978, Steve Wilson, president of Eipper-Formance, donated a Cumulus 10 to the National Air and Space Museum.
