GLIDING is a generic term for the art of flying a heavier than air craft similar to an aeroplane but not provided with an engine. In includes in its scope both gliding proper and soaring. In a gliding flight, the apparatus loses altitude continually through out its course, never rising above its starting point. In soaring flight, however, the machine is carried aloft by the rising air cur rents and is capable of completing manoeuvres high above the point of departure.
The history of American gliding is mainly a record of the scien tific achievement of the great pioneers, Chanute, Montgomery, Wilbur and Orville Wright. In the first half of the 19th century the English pioneers, Cayley, Henson and Stringfellow, made valuable theoretical investigations and model flight experiments. It is this group of workers who may be said to have "invented" the aeroplane. But their invention would never have been reduced to practice without the subsequent efforts of the early exponents of gliding.
Captain Le Bris, a French sailor, carried out the first significant glider work in the '7os, building gliders with wings shaped like those of an albatross and with a boat-shaped body. Le Bris made many remarkable glides, but he succeeded more by instinct than by scientific skill, and did not make any substantial contribution to the science of flight. The most famous exponent of gliding was undoubtedly Otto Lilienthal, who with his brother Gustav began to make experiments in 1867. Lilienthal realized that data were needed for success and accumulated much information from a study of the flight of birds. He was perhaps the first man to understand the superiority of the cambered or curved surface over the flat plate. In 1891 he built his first man-carrying glider, with a framework of peeled willow rods covered with tough cotton fabric. He attached himself to the glider by thrusting his arms through padded rubber tubes and holding on to a cross bar. His body hung in the air during flight and he attempted to control and stabilize the plane by moving his body. Percy Pilcher of England added a horizontal plane to the glider : his sole contribu tion. Both Lilienthal and Pilcher lost their lives in their experi ments.
In 1896, at the age of 64, Octave Chanute began to make gliding flights in America. He built a five-deck glider and followed this by a triplane and finally by a biplane. This was the famous Cha nute biplane, the wings being held together by vertical posts and diagonal wires forming a Pratt truss. This has been used frequently by biplane designers. Chanute discarded Lilienthal's method of securing control and substituted a rudder and articu lated wings. The wings could be swerved fore and 'aft to provide both longitudinal and lateral control although the pilot's body still hung beneath the glider. Chanute's biplane weighed 23 pounds and, with the pilot, 178 pounds. So stable did he make his gliders that his 2,000 flights were without a single accident.
John J. Montgomery, was the first American to use gliding as a means of aerodynamic study. Between 1883 and 1894 he was almost continuously engaged in fruitful experimentation. His first glider was a tandem wing affair united by a framework to which a seat was suspended, and provided with a horizontal tail which could be elevated or depressed by pulleys. The wings were cam bered or curved like those of a gull. Lateral balance was achieved by motion in the seat. In his first flight, he jumped into the air without a previous run and found himself launched upon an 8 to mile an hour wind, executing a 600-foot glide. This experiment led him to build a second glider using flat surfaces (probably to increase stability). An attempt at lateral stability was made by placing a diagonal hinge in the wing so that a portion of it might fold back against the restraining action of a spring to relieve undue pressure under the action of a gust. The control was im proved but the lifting power was poor because of the flat surfaces. In a third glider, the wings were made like those of a soaring vulture. The machine was perfectly controllable but the lift was inferior even to the second machine. In 1903 Montgomery under took experiments to determine the proper form of wing surface. He stretched a cable between two mountains and liberated various models. In 1904, at San Juan he extended his experiments on this principle with a man-carrying glider, and made observations on the effects of wind currents.
Montgomery's last achievement was to build his largest glider weighing 45 pounds and patterned after his first machine. This was launched on April 29, 1905 from a hot air balloon at an alti tude of 4000 feet. The flight lasted 20 minutes during which the pilot, Daniel Maloney, an experienced parachute jumper, per formed many complex manoeuvres.
The next great American exponents of gliding were the Wright brothers. Their first plan was to construct a glider which could be used as a kite in a steady breeze. For their flights they selected the Kill Devil sand hills near Kitty Hawk, N. C. which provided strong steady breezes. Through some errors in calculation, their man-carrying glider, tried out in 190o, proved a failure as a kite and they turned to gliding. The glider of 1900, though a biplane, differed in many respects from the Chanute glider. The pilot lay prone on the upper wing to reduce resistance, the vertical rudder was discarded and the horizontal rudder was placed forward. By warping the wings, they secured lateral control.
The Wrights' most successful glider was built in 1902. As a result of previous experiments, they now decided to use a vertical rudder, subsequently made adjustable. In September and October 1902, nearly moo flights were made, several of which covered distances of over 60o feet. The great glider achievement of the Wright brothers was in securing complete control by combining the horizontal rudder, with an adjustable vertical rudder, and warping the wings. It was this perfect control that made their gliding so safe, and which enabled them to proceed to the building of the first successful power plane. While the Wrights from 1903 onwards devoted the greater part of their energies to power-driven craft, they never lost their interest in gliding.
With more powerful controls, and the horizontal rudder in the rear, many long glides were made, the longest being of a duration of 9 minutes 45 seconds. This remained the record until Hentzen, a German, in August 1922, remained soaring aloft in the Rhine valley for 3 hours and 6 minutes.
From the time of the Wrights' endurance record in 1911 until Glenn Curtiss, whose interest was awakened by the German flights in 1922, began glider construction, no experiments were carried on in the United States. Curtiss built a flying boat hull with the tail surfaces carried on outriggers. The glider weighed 15o pounds and was launched by being towed by a motor boat. It performed admirably.
In Germany gliding reached a high state of popularity after the World War for two reasons. First, the Versailles Treaty pre vented the construction of large aeroplanes. Second, the meteor ological conditions in the Rhine valley, providing continuous upward currents of air, were most favourable to soaring flight.
In the United States, on the other hand, full sway was given to the construction of powered craft, and meteorologically suitable localities such as Kitty Hawk were found to be difficult of access from large centres of population. However, in the past decade, many gliding societies have sprung up whose members have offered the world keen competition.
The remarkable flights of 1922 in Germany were achieved by a radical departure from the past. That year witnessed the advent of the first true sailplane. The sailplane, unlike the heavier, cruder machines hitherto used, is a highly refined glider as light and as perfect aerodynamically as possible. Technically, it is a glider which has a sinking speed of o.8 meter per second which is equiva lent to saying that it is a machine that will soar in a wind that rises vertically upwards at a speed of 14 miles per hour. The first machine of this type was the German "Vampyr." At about the same time a new soaring technique was developed. Up to 1922 soaring was of a strictly topographical nature with the utilization of currents deflected upwards by local hills. Such flights were hazardous and limited by the extent of the range of hills. An increased knowledge of meteorology gave rise to several methods of soaring by which long distance flights could be made. Of these, thermal soaring and thunderstorm flight are most im portant. Thermal currents are formed by heat rising from the ground under certain conditions, as those existing on a hot summer afternoon. On reaching the cooler upper atmosphere, the moisture in the thermal current sometimes condenses, forming a cumulus cloud. A cumulus cloud therefore indicates thermal currents and soaring flights can be made by circling in this current. Thunder storm flight is accomplished by keeping near the boundary between two masses of air, one warm and the other cool, which is present during such a meteorological disturbance.
Since 1922, constant refinement in design and improved methods of flight have resulted in continuously better performance. Until the early thirties, all gliding records were held by Germany and Austria with little or no competition from other countries. Within the past few years, however, England, Russia and the United States have taken an active interest in gliding with the result that records have been flying back and forth between countries with amazing rapidity. In 1926, Kegel of Germany made a flight of 34 miles in a thunderstorm flight. By 193o this had been increased to 10 miles in a flight by Robert Kronfeld of Austria.
Richard C. Du Pont, on September 21, 1933, won the American distance record in his Bowlus sailplane by travelling 121.6 miles from Rock Fish Gap, Va., to Frederick, Md. At the Fifth Annual Soaring Contest held in June, 1934, Du Pont won the world's dis tance record by soaring 158 miles. He used an Albatross II sail plane built in 1934 by the Bowlus-Du Pont Sailplane Corp. This ship had gull-like wings having a span of 62 feet and a total wing area of 205 feet. The aspect ratio was 18.8:I. Although the weight, empty, was only 347 pounds, it was stressed according to the Department of Commerce Regulations.
That year Du Pont led all contestants in points won and was named national soaring champion for the year. The record has since, however, gone back to Germany, Rudolf Oeltzschner having soared a distance of 310 miles in July, 1935. Du Pont also holds the American altitude record of 6,223.7 feet. The flight was made with an earlier edition of the Du Pont-Bowlus sailplane, the Albatross I. But the world's record of 8,493•9 feet made by Robert Kronfeld in a Wien glider in 1929 still stands.
Jack O'Meara one of the foremost American exponents of acrobatic gliding flight made 46 consecutive loops at the National Air Pageant in October 1933. This record, though, was beaten by . the 300 loops performed by Simonov at Koktobel, Crimea in April 1935.
Russia, in recent years, has turned very seriously to gliding with the result that it has amassed many records. The duration record is now held by that country, Ivan Khartashev having flown for 38 hours and 20 minutes in June 1935. The American duration record of 21 hours and 34 minutes was made by Lieut. William A. Cocke, Jr. in a Cocke "Nighthawk" glider at Honolulu, in December, 1931.
A new use for gliders which has interesting possibilities, is the • glider train—several gliders towed in tandem by an airplane. The towing airplane continues non-stop to its destination while the gliders are released to land their loads at intermediate airports along the route. Again, it is the Russians that hold the world's distance record for glider trains. In September, 1935 a flight of 947 miles was made from Koktobel, Crimea. In the United States Jack O'Meara and E. P. Du Pont Jr. flew in a glider train from Key West, Florida to Havana, Cuba in less than two hours. This flight was made in May, 1935. This use for gliders may become commercially practicable for carrying cargo and express.
The National Soaring Society of America has done much to increase the popularity of gliding in this country. It was organ ized early in 1932 by a group of business men and soaring pilots deeply interested in the sport of soaring and gliding. The organiza tion served to coordinate the various clubs and societies scattered over the country by the sponsoring of annual competitions. Such contests have been successfully held at Elmira, New York, the sixth one in 1935. These annual meets are the first objective of the society. All its activities are conducted on a national scale.
Gliding has a definite place in the aeronautical development of a country for it gives in a relatively cheap and safe form, flying experience that is both exhilarating and scientific.
For elementary gliding, which means the more or less rapid descent either in circles or in a straight line from the top of a hill to its base, the requirements are simple. But where soaring is desired, the requirements are much more exacting. First there should be chosen a hill from I oo to 30o ft. high which slopes up from a level plain first slowly and then more abruptly to a rounded crest. The hill should face toward prevailing winds and, prefer ably, should be several miles long. The length is necessary in order to permit the glider to perform figure eights, so as to keep always on the top of the current of air flowing up and over the hill. There should be no fences, trees or shrubbery on the hillside or the plain at its foot, as these might cause disaster at time of landing.
Attached to the nose of the glider is a hook; to the hook is attached a ring with a rubber cable attached therefrom. This cable is grasped by two or more persons on each end, who run with it, stretching the cable to its limit. When the limit of the stretching is reached, the men holding the glider to the ground by the tail rope are commanded to let go by the pilot, and the glider is shot into the air similar to a slingshot.
In flat country, excellent results can be obtained by using a light wire cable five hundred feet or longer, attached to the back of an automobile, which drags the primary or secondary glider along the ground, until it obtains speed enough to rise. The accelerated speed of the automobile takes it to the full length of the cable, at which time the pilot must release. He then glides to earth, practicing his various manoeuvres. This is an excellent way to prepare for soaring.
There are two other interesting ways of motivating gliding— one, to use the wire cable as mentioned above, behind a speed boat. In this case pontoons are necessary on the glider. The other is to fasten the glider behind an aeroplane. This is very hazardous, requiring an expert and skilled glider pilot.
While using primary or secondary gliders, the pilot will learn a great deal about the glider controls, balance, and air currents, thus perfecting his technique in handling his plane, so as to main tain safe flight.
After his primary and secondary training, he should be equipped to start his training in the soaring type glider. This glider is launched exactly as the primary and secondary glider, but is capable of attaining higher altitudes, longer sustained flight, and greater distances, providing the terrain is hilly or mountainous where upward currents prevail. When he lands after any flight, he should always nose the glider into the wind; descend slowly and gently to the ground. (E. S. E.)