Glider is an aircraft that resembles an airplane but has no engine. It flies in air currents as silently and gracefully as a bird. Gliders are sometimes called sailplanes. They are used chiefly for recreation and sport.
Airplanes and gliders remain aloft by flying fast enough for the air moving around their wings to produce an upward force. This force is called lift. An airplane’s engine or propellers give the plane enough speed to generate lift. A glider has no power, and so it flies by other means.
Gliders are usually launched by being towed into the air by an airplane and then released. Level flight cannot be sustained without a motor. Therefore, the glider glides downward through the air. In this way, the force of gravity produces the speed necessary for the glider to remain airborne.
Gliders can soar upward, gaining altitude by flying in air currents that rise faster than the plane sinks. In this way, glider pilots can remain aloft for long periods. Flights of more than 70 hours have been made. But most flights range from 1 to 5 hours.
Parts of a glider
Most gliders have three main parts—wings, a fuselage (body), and a tail. All these parts have a streamlined design that enables a glider to knife through the air with minimum drag (air resistance).
The reduction of drag gives a glider a high glide ratio. Glide ratio is the relationship between forward and downward motion. A typical glider used for recreation has a glide ratio of about 30. That means that for every mile or kilometer of altitude lost, it can fly 30 times as far forward. A high-performance glider has a glide ratio of more than 30. A glider made for competitive flying has a glide ratio of more than 40.
The wings
of a glider provide lift by the same principles as do the wings of an airplane (see Aerodynamics (Lift) ). But a glider’s wings are much narrower in relation to their length than the wings of an airplane. This narrowness reduces the drag at the glider’s wing tips.
As a glider or airplane flies, air tends to flow in opposite directions along the length of the wings. Air along the bottom of each wing tends to flow outward. Air along the top tends to flow inward. This opposite flow causes a swirling stream of air, called a vortex, to form behind each wing tip. The vortexes hold the aircraft back.
Long, narrow wings reduce the strength of the vortexes. As a result, they decrease drag. A typical glider used for recreation has wings about 50 feet (15 meters) long and about 3 feet (1 meter) wide. A sailplane made for competition may have wings 70 feet (21 meters) long and only 21/3 feet (71 centimeters) wide. Some competition gliders also have winglets. Winglets are vertical extensions at the wing tips that further reduce drag.
Each wing has a set of flight controls called ailerons. The wings of some gliders also have controls called flaps. Ailerons and flaps are hinged panels along the trailing (rear) edge of the wings. The ailerons are nearest the wing tips. The pilot moves them up or down to make the glider bank (tip) to the right or left for a turn. If one aileron is raised, the other is lowered automatically. The flaps are nearer the fuselage. The pilot can lower them slightly and increase lift at low speeds when flying in updrafts. Raising them slightly enables the glider to fly at a shallow angle at high speeds.
The fuselage
of a glider extends from the nose to the tail. It gradually narrows toward the rear. High-performance gliders have a slim fuselage that reduces drag. Such a design does not provide enough room for pilots to sit up. They must lie back while flying. In gliders used for flight training, the fuselage is less streamlined. The trainee pilot and instructor can sit up. Training gliders have two seats and dual controls. This arrangement enables the instructor to watch as the student pilots the plane. Some high-performance gliders also seat two people.
Gliders are made of materials with a smooth finish, such as aluminum, fiberglass, or wood. A glider also contains some steel parts. Most gliders have only one wheel. The wheel is on the undersurface of the aircraft, between the wings. The landing gear of most high-performance gliders can be retracted (folded up) into the body after take-off to reduce drag.
The tail
is also called the empennage. The tail of most gliders consists of a horizontal stabilizer and elevator and a vertical fin and rudder. The elevator is hinged to the stabilizer. The pilot raises or lowers it by means of a control stick in the cockpit. The position of the elevator tilts the glider to the angle desired by the pilot and helps control its speed. The rudder is hinged to the fin. The pilot moves the rudder to the right or left by means of pedals. The rudder helps control the glider during a turn.
The stabilizer of a sailplane may be attached to the top or the bottom of the fin. Or it may be connected to the fuselage. Some glider designers believe that two movable surfaces in a V-shaped tail assembly improve performance.
Flying a glider
A glider pilot uses four main instruments when launching, soaring, or landing. The Federal Aviation Administration (FAA) requires all gliders sold in the United States to be equipped with three of these devices. The required instruments are an air-speed indicator; an altimeter, which shows the altitude; and a compass. The fourth instrument, a variometer, shows the rate at which the plane rises or sinks. It helps the pilot detect lift. In addition, pilots of competition gliders use computers to calculate glide angles precisely.
Launching.
In the United States, most gliders are launched by an airplane that tows them into the air. This tow plane pulls the glider with a rope. One end of the rope is connected to a towhook near the tail wheel of the plane. The other end is fastened to a similar hook near the nose of the glider. A helper runs alongside the glider during the first moments of take-off and holds the wings level. Once airborne, the glider pilot releases the glider from the rope. The pilot generally cuts loose at an altitude of 2,000 to 3,000 feet (610 to 910 meters).
In Europe, gliders are usually pulled into the air like kites by an automobile. Or they may be launched by a hauling device called a winch that stands on the ground. Tow planes are also common in Europe. A few gliders have an engine-driven propeller that is used to take off. The pilot turns the engine off after the craft is airborne.
Soaring.
Most gliders fly with their best glide angle at 50 to 60 miles per hour (80 to 95 kilometers per hour). To gain altitude, the pilot searches for additional lift. There are three main sources of lift—slope winds, thermals, and wave lift.
Slope winds
blow against a hill and are deflected upward. A glider can fly back and forth along the windward side of a slope as long as a strong wind is present. Skilled pilots have used the slope winds of a mountain range to make flights of nearly 1,000 miles (1,600 kilometers). Birds drifting along a slope without flapping their wings indicate the presence of slope winds.
Thermals
consist of air that rises in a column or a bubble after being warmed by contact with heated areas of the ground. Dry surfaces that are dark or flat absorb heat from the sun. Heat-absorbing surfaces include blacktop roads, deserts, plowed fields, and roofs. . The air immediately above these surfaces becomes warm. The warm air rises until it mixes with cooler air high in the atmosphere. Thermals may be present from late morning to late afternoon on sunny days. As a result, this type of lift is the one that is most widely used by glider pilots. The heat given off by cities can also produce thermals. The development of a puffy, white cumulus cloud may indicate the top of a thermal. Other signs include rising dust and birds soaring upward without flapping their wings.
Wave lift
often occurs on the lee side—the side away from the wind—of steep mountains. Generally, the lee side of a mountain produces downward air currents. Certain weather conditions, however, may produce a powerful upward air current. The current may reach an altitude of 80,000 feet (24,000 meters) or more. Lenticular clouds, which curve upward on top and are flat on the bottom, often indicate the presence of mountain waves.
Another form of wave lift occurs along shear lines or convergence zones. These regions form when a mass of cool, heavy air moves into an area and forces warmer, lighter air upward. Some shear wave lifts can be flown for hundreds of miles or kilometers.
Landing.
The pilot gradually descends toward a landing area and levels off just before touching down. Many gliders have plates called spoilers that can be extended from the wings to control the angle of the glide during landing. If a glider has flaps instead of spoilers, the pilot approaches the landing area with the flaps tilted down sharply.
Gliders often land at a facility called a gliderport. A pilot who cannot find enough lift to get back to the gliderport must make an off-field landing. Gliders can land on any large, flat surface. The aircraft can then be dismantled, put on a trailer, and returned to the gliderport or taken home.
Gliding regulations
In the United States, the FAA regulates both glider pilots and gliders. A person must be at least 14 years old and in good health to be eligible for a glider pilot certificate. This certificate permits the individual to make solo flights in a glider. Other requirements include a course of study and flight training under an instructor certified by the FAA. The instruction may be given at a flight school or a gliding club. A pilot must be at least 16 and pass written and flying tests to obtain a private glider pilot certificate. A private certificate permits the person to carry nonpaying passengers. A pilot must be at least 18 to qualify for a commercial glider pilot certificate. A commercial certificate allows the pilot to fly paying passengers.
Gliders must be tested and approved by the FAA before they can be manufactured and sold. Some pilots build gliders from kits. This procedure lowers the cost of the aircraft by a half to two-thirds. Such gliders also must be approved by an FAA inspector.
History
Early days.
In 1804, Sir George Cayley, an English inventor, built the first successful model glider. In 1853, Cayley built a crude glider that carried his coachman across a small valley. Historians regard the coachman’s flight as the first human glider flight. However, he had no control over the aircraft. Otto Lilienthal, a German engineer, became the first person to make repeated, successful glider flights. From 1891 to 1896, he made more than 2,000 glider flights. In 1897, a Scottish engineer named Percy S. Pilcher first used a towing technique to launch a glider. Both Lilienthal and Pilcher died in glider crashes.
After World War I
ended in 1918, Germany was prohibited from developing powered aircraft. As a result, many German engineers turned to the study of glider flight. By the early 1920’s, they had developed gliders with advanced designs. German pilots began using the instrument called the variometer to detect lift. It was probably invented by balloonists during the 1800’s. In 1928, German pilots established the first U.S. gliding school at Cape Cod, Massachusetts. In 1930, the first U.S. gliding championship took place at Elmira, New York.
During World War II
(1939-1945), large gliders towed by airplanes were used to transport soldiers and equipment. In 1940, during its invasion of Belgium, Germany became the first country to use military transport gliders in combat.
Gliding today
is a competitive sport. The sport is supervised in the United States by the Soaring Society of America (SSA). The SSA oversees the awarding of badges for flights of specified distance, duration, and height. It also regulates national and regional gliding contests. The top pilots in the national contest represent the United States in world competition. The worldwide competition is held every two years.
The most common event in gliding contests consists of a race against time. In a race, the contestants follow a course around three or more predetermined markers. Contestants fly as far as 620 miles (1,000 kilometers). Gliders have crossed the finish line at speeds of more than 150 miles per hour (241 kilometers per hour).
Germany ranks as the chief producer of high-performance competition sailplanes. German aircraft have been used almost exclusively in national gliding contests in the United States.
In the late 1900’s, a sport called hang gliding became popular. A hang glider usually consists of a triangular sail of synthetic fabric on an aluminum frame. A pilot can launch it by running down the windward side of a hill until the glider is airborne. Or the glider can be towed by a vehicle or boat until it reaches a certain altitude.
See also Airplane ; Hang gliding ; Wright brothers .
