(A) HISTORY OF GLIDERS
The evolution of gliders originated from man’s fascination with bird flight. As early as the 1800’s, man observed, envied and began to imitate the free flight of birds. Sir George Cayley, a British scientist, was the first man to interpret flight through terms of mathematics (20). He also discovered the secret of flight through studying the bird. He demonstrated that a bird feather set at a certain angle could generate lift when it moved through the air or when air moved around it. Applying what he learned from this experiment, he was the first person to design a biplane glider that remained briefly in the air.
Jean Marie LeBris, a Breton sea captain, went as far as to kill an albatross to examine its wings. From his study of the albatross he built what became known as the LeBris glider (21). It was towed into the air like a kite via the power of a horse in 1857.
Otto Lilienthal, a German scientist, inventor and engineer, was the first great exponent of gliding (22). He made more than two thousand successful flights in his bird-like glider composed of peeled willow wands covered with waxed cotton cloth. Lilienthal kept records of each of his flights, constantly improving the design of his gliders. He finally started covering distances of about seventy-five feet above the ground. On August 9, 1896 he lost control of one of his aircrafts and fell to his death. Two of his followers, Percy Pilcher of England and Octave Chanute of America, carried on his works with a great deal of success.
John Montgomery was the first American to build a successful glider, and the first to be able to control his aircraft in the air. He started building and experimenting with gliders in 1883. Montgomery made his first public demonstration of his aircraft in 1905. As opposed to the other hang gliders jumping off dunes and cliffs, Montgomery used hot air balloons to lift his gliders into the air. Once the hot air balloon raised the glider to an appropriate height into the air, the balloon was released and Montgomery piloted the glider to earth.
Today, depending on good conditions, gliders can travel for several hundred miles. The longest recorded distance that a glider has traveled was in 1964, which was 647 miles (23). Unlike the late 1800’s when gliders were categorized as single or double winged, modern gliders are broadly categorized into two divisions—standard and open class. Standard class are gliders which are easy to fly, light weight, and cheaply made. They are strictly used for club or private use. Whereas, open class gliders are made for competitive flying and are very expensive to produce.
(B) PARTS OF GLIDERS
Most gliders have three main parts designed in a streamlined fashion which enables it to knife through the air with minimal amounts of air resistance (drag). The three main parts of a glider are the wings, body and tail assembly.
The wings of gliders are very narrow. The narrow design reduces the drag at the glider’s wing tip. As the glider flies through the atmosphere, air tends to flow in opposite directions along the length of the wings. Air along the tip of the wing tends to flow inward, whereas air along the bottom of the wings tends to flow outward. The opposite flow of air causes a swirling stream of air called a vortex to form behind each wing tip and hold the aircraft back.
The wings of a glider have a set of controls called ailerons. Some gliders are equipped with an additional set of controls called flaps. Ailerons and flaps are hinged panels located along the trailing edge (rear) of the wings. The ailerons are close to the tip of the wing, whereas, the flaps are nearer to the body of the glider. The glider pilot moves the ailerons up or down which causes the glider to turn either left or right. This maneuver is accomplished because as the pilot raises one aileron, the other is automatically lowered causing the glider to tilt in the direction that the air current is lifting the raised wing. If the glider is equipped with flaps, the pilot is able to increase lift at low speeds when flying in an updraft by lowering a flap which allows the air current to raise the glider upward. Maneuvering the flap allows the pilot to control the angle of the lift in relation to the speed of the air current.
The body of the glider is known as the fuselage. It extends from the nose of the glider to the tail end of the aircraft and gradually narrows towards the rear. The fuselage is made up of materials that can be sanded to a smooth finish. These materials include wood, aluminum or fiber glass. A glider may also include some parts made from steel. The landing gear of most gliders can be folded into the body after take-off which provides the aircraft with a smooth undersurface and eliminates extended parts which would increase drag.
The tail assembly is known as the empennage. The empennage consist of a vertical fin, rudder, a horizontal stabilizer and an elevator. The elevator is hinged to the stabilizer and is raised or lowered by the pilot by means of a control stick in the cockpit. By positioning the elevator, the pilot can tilt the plane to a desired angle to help control the glider’s speed. The rudder can be moved to the right or left via manipulation of pedals in the cockpit by the pilot. The rudder helps to control the glider during a turn.
(C) LAUNCHING A GLIDER
A glider can be launched into the air in one of three basic ways: towing it up behind an airplane; pulling it up like a kite by means of a cable and winch, or placing a small engine in the glider for unassisted take-off.
Most gliders use the method of being towed into the air via an airplane. The plane pulls the glider with a rope about two hundred feet long or sixty-one meters in length. The rope is connected to a tow hook near the tail wheel of the plane, and to a similar hook fastened to the nose of the glider. The rope can be released from the nose of the plane via a knob in the cockpit. The glider pilot generally releases the rope when the aircraft reaches an altitude between two and three thousand feet (six hundred and ten to nine hundred and ten meters).
For a winch launch, the glider is attached via a cable to a hauling device called a winch that stays on the ground. The winch pulls the glider along the ground until it reaches a speed of about sixty miles per hour. Once it reaches this speed, wing lift is generated for the aircraft to leave the ground and begin its steep climb into the air. When the glider reaches about two hundred feet, the pilot levels his plane out and releases the launch cable via a device in the cockpit.
Some gliders are equipped with an engine-driven propeller that is used for take-off. Once the craft is airborne, the pilot turns the engine off and begins the flight.
(D) HOW GLIDERS FLY
Although some gliders utilize engines for take-off, all gliders maintain flight through the manipulation of air currents. There are basically two ways that air currents are utilized to maintain a glider’s flight once takeoff is achieved. Hill lift and thermal convection are the two forms of upward moving air currents which aid a glider in maintaining flight. Hill lift was first discovered and utilized by Otto Lilienthal more than seventy five years ago (24). Lilienthal discovered that when wind blows against a hill, the air moves upward against the slope of a hill. This principle governing air movement became critical in the early 1920’s. It was during that time when pilots discovered that height could be maintained for longer periods of time by flying along a hillside as compared to flying away from a hill.
The use of thermal convection in gliding was discovered in 1928 by an Austrian scientist, Robert Kronfeld (25). First Kronfeld discovered that warm updrafts of air produced certain types of clouds called cumulus clouds. Warm air rises because it is lighter than cool air. This rising body of warm air is called a thermal.
Although thermals are invisible, they can be detected by pilots because of the type of cloud patterns they produce. Once a pilot detects cumulonimbus clouds, he uses it to lift the glider upward until it reaches the top of the warm current. The gilder will then descend in search for the next warm updraft or thermal. When the glider is raised by these thermals or warm updraft it does so in ascending spirals. (See Figure 3 at the end of this section). In addition to visually detecting thermals through cloud patterns, most pilots rely on two different types of instruments to detect thermals—a variometer and an altimer. A variometer is a very sensitive instrument which measures how fast a glider is gaining or losing height and the altimer records the height of the aircraft above the ground. These two instruments aid pilots in determining the thermals’ ascending effect upon the glider and descending effect when the glider has left the the thermal.
FIGUREIII How Gliders Fly
(figure available in print form)