Having executed nearly two thousand flights with his monoplane, Lilienthal in 1895 built a two-surface glider. He found this still easier to control, and now thought he had sufficiently acquired the art of sailing to justify his undertaking the next and more difficult art of imitating the rowing flight of birds. He had constructed a ninety-pound engine, of two and a half horse power, to actuate the wings of his glider; but, before applying this motor, he went to the Rhinow Hills for a little further experience in sailing. Previously he had remained in the air twelve to fifteen seconds; but he wished to exceed this record.
On the 9th of August, 1896, he made a long glide to prove the effectiveness of the horizontal rudder, and then wished to undertake a second flight of the greatest duration feasible. No intimation had he that this sail would prove disastrous. Giving the timepiece to his assistant, he set forth on a level course, but suddenly dipped forward and plunged headlong to earth through a height of fifty feet. He was dragged out from the débris with a broken spine, from which he died the following day.
The machine on which the father of aërial gliding made his last flight is shown in [Plate XVI]. Of the hazardous nature of its construction Mr. Chanute thus writes: “The two surfaces were kept apart by two struts, or vertical posts, with a few guy wires, but the connecting joints were weak, and there was nothing like trussing. This eventually cost his most useful life. Two weeks before that distressing loss to science, Herr Wilhelm Kress, the distinguished and veteran aviator of Vienna, witnessed a number of glides by Lilienthal with his double-decked apparatus. He noticed that it was much wracked and wabbly, and wrote to me after the accident: ‘The connection of the wings and the steering arrangement were very bad and unreliable. I warned Herr Lilienthal very seriously. He promised me that he would soon put it in order, but I fear that he did not attend to it immediately.’”
It will be observed that Lilienthal gave fair attention to the merits of both the monoplane and the biplane, the two familiar types in lively competition at the present hour. The first he found in Nature; the second he could have found in England, as the developments principally of Wenham and of Phillips. His example and prestige did much to promote the biplane; but he seems to have had no very decided preference for either. Though he found his biplane very satisfactory, he thought of returning to the monoplane.
In April, 1896, he wrote:[27] “I am now engaged in constructing an apparatus in which the position of the wings can be changed during flight in such a way that the balancing is not effected by changing the position of the center of gravity of the body. In my opinion this means considerable progress, as it will increase the safety. This will probably cause me to give up again the double sailing surfaces, as it will do away with the necessity which led me to adopt them.” He thus seems to have studied the two types impartially, and to have invented a means for balancing the machine without shifting the center of mass.
Lilienthal had given a powerful and permanent impulse to aviation, both by his writings and by his practical experience in the air. He first showed quantitatively the advantage of arched wings, by carefully derived tables of wind pressure; then he mounted the wings himself and taught the world, by bold and frequent flight, the art of aërial gravity sailing. The two remaining achievements, dynamic and soaring flight, he was to undertake as promptly as possible. If his life had been spared, no doubt he would have contributed much to the advancement of these arts, both by example and by direct effort; for he was in the prime of life, full of energy and daring, highly equipped, and ardently devoted to his favorite science. He began his studies in aviation at the age of thirteen and died at the age of forty-eight years.
Among the admirable traits of the father of sailing flight must be mentioned his scientific liberality and esprit de corps. Though he patented his invention he did not conceal, or withhold, his discoveries when he could publish them properly. These discoveries were made at a great sacrifice of time and means, and must have appeared to him valuable trade secrets; yet he published all his scientific data, his theories, and observations; he encouraged his confrères in various countries to witness and emulate his experiments, to share intimately his laboriously developed knowledge of aviation, to join hands with him in hastening the advent of practical flight. Such is the esprit de corps which has ever prevailed among truly scientific men, as distinguished from the mercenary and commercial; such are the unselfish investigators whom the world delights to honor, both for their genius and for their liberal contributions to the common and permanent possessions of humanity.
Before his death Lilienthal had the pleasure of knowing that competent disciples were emulating him in doctrine and practice. One of the earliest and cleverest of these was Percy S. Pilcher, Assistant Lecturer in Naval Architecture and Marine Engineering at the University of Glasgow. In the summer of 1895 he built the glider shown in [Plate XVI]. This, like Lilienthal’s, was a double-tailed monoplane arched fore and aft; but, better than his for manual control, it was straight from tip to tip, like the designs of Henson, Penaud, and other predecessors. This improvement was introduced to prevent side gusts from rocking the craft so readily as they do the V-shaped gliders. His best sailer, the Hawk, shown in the figure, had wings curved one in twenty, about one third from their front edge.[28] Sometimes he sailed downhill; again he was towed or launched, like a kite, by means of a cord, running through five-fold multiplying gear, and drawn by running boys, or a horse. In both cases he controlled the machine to his own satisfaction, making in 1897 smooth downhill glides of 700 feet length, from an elevation of 70 feet.[29] He had also visited Lilienthal, but only after achieving success at home.
Having acquired some skill in sailing, Mr. Pilcher began work on a power machine. This was to be propelled by a screw actuated by an oil engine, and was to be mounted on wheels backed by stiff springs. Having observed his speed of descent in gliding, he computed that two tow-line horse power would float him and his machine, weighing together 220 pounds. A like result was obtained when he was flown as a kite. He was, therefore, on the straight road to achieving human flight on a screw-propelled, wheel-mounted monoplane. If he had been more cautious he might have been the first person to achieve human flight in a practicable type of dynamic machine; for he seems to have equaled, if not excelled, his German master in aëroplane design. But like the master he provided inadequately for the structural strength of his glider, and braved too far the dangers of gusty weather. One stormy day, September 30, 1899, wishing to please several persons who had come a long distance to see him, he made two trial flights in a gentleman’s park near Rugby. The second of these proved fatal. The spectators heard a cracking noise, saw the tail break, and the whole craft plunge headlong to the ground. Poor Pilcher was mortally hurt and died thirty-four hours later, without ever regaining consciousness. He was then in his thirty-third year.
Had this talented young Briton and his German tutor both lived, there would doubtless have been a pleasant race and rivalry between them; for the pupil was forming opinions and plans sufficiently divergent from those of his master and friend. He did not approve Lilienthal’s high wings and low center of gravity, nor his V-shape for lateral equilibrium, nor his flapping wing tips for propulsion, nor his method of launching the dynamic machine. Fortunately both published their ideas and experiments, leaving to their successors the task of judging the merits of their designs, and of adding any improvements that might still be required in order to achieve final success.