Unpowered aircraft derived from sailplane gliders and double-surfaced sport parachutes. Hang gliders are kitelike; paragliders are made of airfoil cells inflated by passing through the air.
Traditional gliders, progenitors of hang glider and paraglider flight, are unpowered, heavier-than-air craft that attain sustained flight via the aerodynamic forces acting on them. Gliders look like airplanes but are much lighter; they have low ratios of weight to wing area; and their wings are much longer and narrower than those of powered aircraft. Gliders include primary, secondary, and cargo types. Primary gliders have girder frameworks with attached wings, controls, stabilizers, and open seats at the framework’s front. Popular secondary gliders (sailplanes) have fuselages and cockpits, and look like airplanes with very long, narrow wings. Cargo gliders (CGs) are towed by powered planes and carry heavy commercial loads in tow, which they can land where powered craft cannot.
Modern sailplanes lose only a few feet of altitude per second and ascend air currents, rising just 2 to 3 miles per hour. Beginning in the 1870’s, pioneer aeronauts built successful gliders to define efficient wing and control system design. The best known is the German Otto Lilienthal, who studied air buoyancy and resistance, wing shape, and tail stabilization. In 1891, his crewed craft, capable of flying after a downhill run into the wind, made the first of thousands of flights. American glider pioneers Octave Chanute, the Wright brothers, and John Montgomery made many glider innovations.
Most modern glider design arose in post-World War I Germany. Its aircraft engineers, forbidden by treaty to build powered aircraft for military use, explored the great efficiency of light gliders with single, long wings and the weather conditions that optimized soaring flight. They found that strong ridge or thermal upcurrents provided the motive power gliders needed. Ridge currents form when steady winds blow against ridges or hills, but are limited to areas near their windward edges. Thermal currents (thermals) form by heat rising from the ground and are always present under cumulus clouds.
For traditional flight, a glider is accelerated to the speed needed to overcome gravity by means of a catapult or by being towed by a winch, automobile, or powered airplane. After launch, the craft disengages from the towline at a desired height and the pilot seeks thermals. The pilot turns into a thermal, and after reaching the maximum altitude possible seeks the next useful thermal. Good gliders move 20 miles horizontally for each mile of altitude attained and can stay aloft for many hours.
Although some hang gliders have small engines, most hang gliding is unpowered flight in a kitelike craft based on the structure of the sailplane. The name derives from the fact that early pilots hung onto the gliders. The designed evolved so that hang gliders have an aluminum frame, a fabric sail, and a comfortable harness for the pilot. In order to take off, the pilot dives from a hill, cliff, or mountain. The hang glider was born in the 1960’s, based on designs by Francis Rogallo of the National Aeronautics and Space Administration (NASA) for flex-wing parachutes for space vehicle reentry. Many design variations followed, and hang gliding, first merely a recreational activity, has become a competition sport, with flight-duration, distance, and altitude-gain events. Annual world championships are governed by the International Aeronautical Federation. The hang glider’s limited ability to maneuver and handle wind change, as well as the fact that pilots fly without protective body coverings, make the sport somewhat dangerous.
Hang glider construction begins with a light, strong airframe of aluminum tubes which support a sail (its airfoil) made of Dacron or another polymer, creating two joined wings. The airframe has five parts. First, a leading-edge tube (LET) runs along the front edge of the wings. The sail wraps around the LET and is secured to it. A crossbar tube connects the LET of each wing and the third part of the airframe, a keel. The keel runs from the glider nose to the center of its rear end, above the pilot. The fourth and fifth parts of the frame are the king post and control bar. They connect to the airframe where crossbar and keel intersect. The king post rises vertically from the keel and supports the glider when it is on the ground. The triangular control bar is used for support in flight, and is used by the pilot to guide the glider’s movement. Usually, ten stainless-steel cables tie the airframe tubes together, supporting the hang glider’s load. There are six “positive” wires running from the base of the control bar to the airframe and four “negative” wires running from the top of the king post to the airframe. They support the glider in its normal flight and in rare inverted flight. A third set of wing wires support the LET at all times.
Hang glider sails are made of polymer panels joined to create airfoils when stretched out on an airframe. Often a sail has riblike battens to help the airfoil keep its shape and to reduce drag. The hang glider is controlled by moving the pilot’s weight by means of the control bar. Forward pulls drop the glider nose and cause acceleration or dives. Back pulls raise the glider nose, slow it down, and can cause stall. Lateral pulls to right or left tilt the respective wing, causing right or left turns. These movements, when well choreographed, allow expert hang glider pilots to carry out complex aerial maneuvers.
Prolonged hang glider glide, especially in cross-country glides, uses ridge currents and thermals to provide lift. Whether in short flights or cross-country soars, hang glider pilots wear comfortable supine or prone harnesses fastened to the glider. Harnesses suspend the pilot near the point where keel, crossbar, control bar, and king post meet. The harness is made of padded Dacron cloth with a seat-beltlike support of nylon webbing. Harnesses allow for comfortable flight, weight movement in any direction needed for control, and the carrying of emergency parachutes.
Paragliding began in France in the 1980’s with canopies derived from double-surfaced Parafoil sport parachutes made of airfoil cells inflated by passing through the air. Paragliders are much longer than they are wide, and their wind-catching cells are inflated by gentle breezes. Their light weight and canopy softness make paragliding safer than hang gliding. The paraglider pilot, attached to a canopy by a seat harness, launches from a hill or another gentle slope using a canopy preinflated by the wind. The canopy behaves like an airfoil and can stay aloft for hours, a primary aim of paraglider enthusiasts. Paragliding is under control of the Fédération Aéronautique Internationale (FAI) hang-gliding commission.
A paraglider wing contains ten to seventy cells, joined side-by-side and closed along the entire trailing edge. The cells have inflation ports at the leading edge. Their walls also have interior ports that allow air to pass between them to maintain even internal pressure. Airfoil shape is maintained by this pressure, created by air entering the leading-edge ports kept open by stiff Mylar reinforcements. As long as the ports are clear, the airfoils keep their shape, and after full inflation, the internalized air stays put.
About 30 percent of paraglider lift is “plate lift,” created when passing air is met with the leading edge higher than the trailing edge, and 70 percent is “induced lift,” created by an airfoil shape which makes air pass over its top more slowly than underneath. The lines connecting airfoil and pilot are placed carefully to suit two requirements. First, many attachment points help keep the wing in an efficient shape when loaded. Second, as the lines cause drag, their number is minimized as much as possible. Also, paraglider operation depends on the match between pilot weight and aerodynamic forces. For optimization, the lines are joined to the harness by two to four pairs of webbing straps (risers).
As long as air flows evenly past the airfoil, lift keeps a paraglider moving upward, while gravity moves it forward and down. A paraglider descends slowly, due to the counterthrust of lift. Climb is only possible upon flight through thermal or ridge currents moving upward faster than a glider is dropping. Drag, due to pilot weight and the nonairfoil components, increases with the square of paraglider speed. Drag is also due to inequities of air passage around airfoils. Total drag is calculated by adding number values of the two drag types. The lift-to-drag ratio identifies the glide performance of the wing. A lift-to-drag ratio of five to one indicates that in still air, a paraglider moves forward 5 feet for every foot of descent.
The basic controls of paragliders are brakes. When they are not used, motion is straight forward at the best glide speed possible. Speed adjustment uses the brakes one-quarter on for minimum-sink speed and fully on to stall in a light-wind landing. Pulling the brakes causes the trailing edge to curve down, increasing camber and angle of attack. This increases lift but can cause stalls.
To steer, pilots pull down the brake on the side to they wish to turn into, increasing drag on that wing half. They can also assist steering by leaning in the direction of a turn. As to speed adjustment, most often it is useful to glide at less than the fastest glide speed. This is done by pulling the brakes down equally on both sides to increase the angle of attack. Speed increase is more difficult and uses a speed system, a foot stirrup connected to the front risers. Pushing on the stirrup shortens the front risers, reducing the angle of attack and increasing speed. Regardless, paragliders are slow aircraft and have only a small speed range.
Fair, Erik, Rod Stafford, and Rick Zimbelman. Right Stuff for New Hang Glider Pilots. Laguna Beach, Calif.: Publitec, 1987. A hang gliding manual full of important data on launching, soaring, wind effects, cross-country trips, forced and crash landings, and repairs. Pfeiffer, Rich, Maggie Rowe, and Rod Stafford. Hang Gliding According to Pfeiffer: Skills for the Advancing Pilot. Laguna Beach, Calif.: Publitec, 1984. Thoroughly covers useful aspects of hang glider flying, such as soaring, cross-country, and contest flying, equipment, and flight calculations. Poynter, Dan. Hang Gliding: The Basic Handbook of Skysurfing. Santa Barbara, Calif.: Parachuting Publications, 1977. Contains much useful information on hang gliding history, machine options, and flying. Sollom, David, and Matthew Cook. Paragliding from Beginner to Cross Country. Marlborough, England: Crowood Press, 1998. Contains information on paragliding equipment, techniques, and competitions. Whittall, Noel. Paragliding: The Complete Guide. New York: Lyons Press, 1995. Contains excellent information on paragliding theory, design, operation, and rules, as well as a useful glossary.
Forces of flight