Flight, January 1, 1910
THE LILIENTHAL AND PILCHER GLIDERS COMPARED.
The Pilot's "Seat" and its Consequences.
Although Pilcher was doubtless inspired to his work by Lilienthal's example, it is apparent that he brought an original mind to bear in the constructive details of his task, whatever bias he may have been under to follow his leader's methods in their general lines. Constructionally the two gliders are as different as they can well be for machines of such similarity in size and purpose. But Pilcher and Lilienthal had this much in common, that they both employed the same method of riding their machines, by hanging upon them, and from such experience as has since been collected by the Brothers Wright, there is little doubt that the custom was at the bottom of many of their difficulties, if not actually the cause of Lilienthal's death. One consequence of deciding to use a glider as Lilienthal and Pilcher used theirs is the necessity of designing a machine of small weight, for the pilot has to be prepared to carry the load both when launching and landing. Pilcher, it is true, fitted a small chassis to his glider consisting of two spring-suspended wheels, but this addition was no more than a compromise, as its purpose was mainly to enable him to bring the machine more easily into position, and to take some of the initial shocks of landing. In flight Pilcher's body hung suspended beneath the wings, only his head and shoulders projecting above, his weight was borne upon two bolsters fitting under the armpits. Lilienthal was perhaps even more beneath his machine than Pilcher, for his arms rested in sockets beneath the wings, and the bolsters which supported his shoulders were about on a level with the upper surface.
Stability and Control.
The greater part of their weight being thus suspended beneath the supporting surfaces caused the centre of pressure to be considerably above the centre of gravity, and although this is often thought to be conducive to automatic stability by constituting a kind of natural pendulum, as a matter of fact, the inertia of such a system very naturally complicates the control, which in both cases was carried out by the shifting of the pilot's body in whatever direction might be required to restore balance. Pilcher had quite satisfied himself upon this point before he came to the end of his experiments, and another matter upon which he became convinced was that the upward slanting of the wings # or the principle of the dihedral angle # resulted in diminished stability in sidewings.
Of the two machines there is little to choose in weight, as they were both supposed to be in the order of 45 to 50 lbs. when actually in use. Lilienthal's glider has the smaller amount of supporting surface, and even that machine must have been one of the largest he constructed, for he was inclined at first to build gliders having barely 110 sq. feet of deck. The span of 22 feet for the Lilienthal machine is 2 ft. 8 ins. less than the Pilcher, and the overall length is 2 ft. shorter. The height is proportionally much less than that in the Pilcher glider, owing to a different system of staying the surfaces and to the absence of a chassis.
Both gliders were fitted with tails of similar type although different in shape, the tail in each case consisting of one vertical plane and one horizontal plane, the latter being set at a negative angle, and being also free to rise unrestricted if subjected to any pressure from beneath. It will be distinctly noticed from the accompanying drawings that the stay wires for the horizontal planes are only arranged so as to resist pressure from above. The vertical tail plane is nominally rigid, although, in the Pilcher glider, it happens to be mounted upon the horizontal member, and thus moves with it. In the Lilienthal machine the horizontal plane is hinged separately to the vertical plane, which is attached rigidly to the main frame. It was Pilcher's object in designing the tail of his machine as shown, to make it as compact as possible for storage, and with this end in view the tail was arranged to fold over on to the main planes.
(To be concluded.)
Flight, January 8, 1910
THE LILIENTHAL AND PILCHER GLIDERS COMPARED.
Willow and Bamboo.
THERE is a subtle difference in the construction of these two gliders, otherwise so much alike, which is directly attributable to the difference in the materials employed, for where Lilienthal used willow sticks, Pilcher adopted bamboo. Both had, it will be seen, sufficiently awkward materials to deal with, but Pilcher, it must be confessed, made a far more engineering-looking job of his work than Lilienthal, whose constructive methods are somehow reminiscent of bent-wood furniture. Both designers embodied the principle of folding-wings in their machines, and thus both were under the same necessity of adapting their construction to suit the special requirements imposed by this important condition. At the same time, however, both had to provide their wings when finished with an artificial camber, and it is particularly interesting to compare the two methods by which these details were accomplished.
The Folding of the Wings.
Pilcher, who was working with bamboo, fitted the inner extremities of the ribs of his wings with wooden plugs, to which he lashed a loop of iron wire so as to form a ring. These rings he subsequently strung over the central standard, around which each wing spread out like an open umbrella. In order to close the wings, it was only necessary to unlash the attachment between the front edge and the main frame, when the ribs would then all swing round one above the other into any convenient position.
Lilienthal obtained much the same effect, but in his case the ribs folded up side by side, being hinged to a central bracket into which their wedge extremities normally fitted like the spokes of a wheel.
In this machine the hinge centres are 6 ft. apart, and in the Pilcher machine the same points are separated by a distance of 7 ft. 6 ins. In both cases the hinges are joined by a wooden beam, but whereas in the Pilcher glider this member consists of a simple rectangular piece of wood, in the Lilienthal machine it is a built-up structure in the form of the letter X; which system was adopted in order to enable the staying of the wings to be conveniently accomplished. It would doubtless have been better had Pilcher attached more importance to the strains on the main beam of his machine, because it was that which gave way during his last glide.
Pilcher and His Wire.
Pilcher adopted a very different method of bracing his wings to that employed by Lilienthal, in fact, it is in this respect that the two machines are most unlike. He relied entirely upon the use of piano-wire for cambering the surfaces of his wings to the desired shape, and in order to get the effect sought he braced each to a central hinge pin consisting of a bamboo pole some 6 ft. in height, which stood up 3 ft. above the deck and projected 3 ft. beneath it. Radiating from the top and bottom ends of this pole are a number of wires, for the most part there are three wires attached above and beneath each rib, only the two front ribs in each wing having less than this number. There are nine ribs in each wing, and altogether 50 wires were required to stay them.
How Pilcher managed to retain all these wires properly taut is a matter for conjecture, but he must have been a man of great patience if he really gave proper attention to each of the 100 wires on which the camber of his surfaces depended. It is no easy matter to adjust wires used for such a purpose, as no sooner is a slack wire tightened up a little too much, than it relieves the strain upon two or three other wires, which become slack in turn. And Pilcher, it must be remembered, worked before the days of those very pretty little wire strainers which commonly form a feature in the bracing of modern flyers, and his method of adjustment must have been as tedious as it was effective. Each of his wires he fitted with a curtain-ring - Pilcher was very fond of curtain-rings - and on the rib he fastened a wire eye. The wires were cut to a fixed length in advance, and calculated to allow about an inch between the ring and the eye when in place. This gap was filled up by a piece of string, passed several times through each member to give the required strength, and by pulling on one end of the string a very minute adjustment could be made in the tension. More curtain-rings were employed for fastening the other ends of the wires to the vertical bamboo poles, so that the wings could be folded without twisting the wires.
Another little device for which Pilcher's construction is peculiar is his method of plugging his bamboo rods with wood. The wood plugs were glued into the hollow ends of the bamboo, which were then carefully lashed with string. Pilcher was very careful about lashing the ends of his bamboo poles as he evidently fully appreciated their liability to split. His use of wire eyes as lugs is another neat detail well worthy of attention, and these little fittings he would also lash in place.
Lilienthal's Stiffening Ribs.
Lilienthal, who very possibly had a natural objection to the use of a lot of wires, hit upon a very ingenious method of doing away with some of them, for he maintained the camber of his wings by the use of detachable supplementary ribs, of which there were four placed fore and aft above the deck. These ribs were strips of wood having an inverted T section, and they were cut to shape so as to serve as templates. As their permanence would have interfered with the folding of the wings, they were fitted in such a manner as to be detachable, and the method of doing this was to fasten small steel clamps on to the primary ribs of the wings, through which the flange of the template could slide. As the wings were naturally flexible there was no difficulty in sliding the curved templates in place from one end, and once in position these members were fastened by little clips. Their purpose was primarily to give the wings the desired camber, although they would of course tend to increase the rigidity of the structure; by their use, the necessity for employing a large number of wires was obviated, in fact, there were only two tie-wires radiating from each of the vertical posts mounted above the hinge plates. Beneath the wings, however, there was a wire to every rib, but even in this respect Lilienthal contrived to do without some 78 wires which Pilcher found necessary.
It may be remarked that the number of ribs in each of the two machines is identical, viz., 18, but the camber and arching of the wings themselves is distinctly different in the two cases. Pilcher employed an umbrella-like form, in which the camber may be described as being of a uniform character. Lilienthal, on the other hand, flattened the extremities of his wings to such an extent that the curvature of some of the ribs was virtually reversed. The maximum camber on the Lilienthal glider, as on the Pilcher machines, occurs at the hinge of the wing, which on the former is situated 18 ins., and on the latter 2 ft. 6 ins. behind the leading edge. At this point the camber is deeper on the Lilienthal than on the Pilcher glider, but on the Lilienthal machine the camber is almost confined to this point alone, and is far more concentrated, if the term may be used, than in Pilcher's construction.
It is known that Lilienthal experimented with cambers of considerable height in proportion to the chord, and he has stated that the maximum versine (camber) of the wing curvature should be less than 1/12 of the chord, and preferably only 1/18 to 1/15 for considerations of stability.
Both machines had the surface material attached above the ribs.
Having compared the wings and supplementary surfaces, it is of interest to consider the main frames of the two machines, by which we imply that central structure to which the whole is braced, and on which the pilot supports his weight. In the Lilienthal glider this member is of somewhat peculiar form, and is best observed by reference to the accompanying plan; it bears in a marked degree that bent-wood furniture appearance to which we have already drawn attention. One of the principal members is an approximately circular hoop of willow, across which pass in a fore and aft direction two willow rods that ultimately converge at a point where the bamboo tail rod emerges from the wing surface. Transversely across the hoop passes the main beam to which reference has already been made, and immediately behind this an orifice is provided in the surface to accommodate the upper part of the pilot's body. Two small bolsters attached to the frame rest under the pilot's shoulders, and the pilot's arms pass through rests provided for them in the corners of the X-shaped main beam. Just in front of this beam is another transverse rod which the pilot can grip with his hands, and in front of the main hoop member is another bent-wood construction lashed in place with wicker, the object of this device being to act as a fender in the event of collision.
On Pilcher's glider the main frame consists of two bamboo rods spaced 18 inches apart above the main transverse beam, to which they were lashed in a cradle. Fore and aft these members converge, and are joined together in metal sockets, that at the rear carrying a bamboo extension to the tail. The entire space occupied by the two frame members was left open in the Pilcher machine, in which respect it differs from Lilienthal's glider, where every possible square inch is covered in. The pilot on Pilcher's glider was supported by two small bolsters fitting under the armpits, which bolsters were attached to the ends of short rods projecting obliquely from the sides of the main frame. Thus supported, the pilot's forearms would rest along the frame-members, and flat pieces of wood are lashed in place at these points to give greater comfort. At the ends of these armrests two handles project for the pilot to grip.
Both machines are more or less similarly braced as regards the wings to the frame and the tail to the frame; but in the Pilcher glider a transverse wooden strut is provided between the bamboo uprights in addition to the diagonal tie-wires which alone sufficed in the Lilienthal machine. Underneath the frame on Pilcher's glider two bamboo rods project obliquely outwards and carry wooden extensions on which the chassis wheels were mounted. The hollow bamboo is here made use of to contain a spring, and the wooden extension, once more fitted in the form of a plug, in this case became a plunger.
Flight, July 5, 1913.
THIS is the tenth anniversary of the conquest of the air by the aeroplane, and it is appropriate that our special number should be made to commemorate the work of the pioneers, as well as other incidental facts, such as, for example, that FLIGHT itself is already in its fifth year of publication.
It was on December 17th, 1903, that the Wrights first achieved power-driven flight with their aeroplane (the first account of which in any English newspaper appeared in FLIGHT'S parent journal - the AUTO.), and prior to that day no man could claim properly to have flown at all. The most important work preceding this accomplishment of the Wrights was, unquestionably, the gliding experiments practised by themselves and by others, commencing with Lilienthal.
With Lilienthal's introduction of the glider the practical side of the art of aviation assumed its first real existence Otto Lilienthal was born at Anclam, in Pomerania, in 1848, and he died in 1896 as the result of a fall during one of his flights. His first work of note was the publication of a book entitled "Bird Flight as a Basis of Aviation," which has since been translated into English, and constitutes one of the classics that should find a place in the library of every student of flight.
In 1891, having come to the conclusion that it was possible to gain some practical experience in the air, in spite of the absence of any suitable engine - provided the experimenter could arrange to fly downhill under the propelling force of gravity - Lilienthal constructed a machine that he purposed using as an aerial toboggan.
His glider consisted of a pair of rigid outstretched wings measuring 23 ft. in span, and having, after various alterations, an area of 86 sq. ft. The wings were cambered, and they were surfaced with cotton twill stretched over a light framework of willow. The machine weighed only 40 lbs. complete.
In order to balance the glider in flight, Lilienthal relied upon his own dexterity in moving his body to and fro or from side to side whenever it was necessary to counteract a shifting of the centre of pressure under the wings. There can be no doubt that the use of his apparatus was fraught with considerable danger, but Lilienthal brought an immense enthusiasm to his work, which was not only an encouragement for himself, but a source of inspiration to others.
Among those who took great interest in Lilienthal's experiments was Pilcher, then a young English engineer whose name was associated with the firm of Wilson and Pilcher, at one time well known in the automobile industry before their cars were taken over by Messrs. Armstrong, Whitworth. He designed gliders of his own, more or less on Lilienthal lines, and at one time went over to Germany in order to discuss the subject with Lilienthal himself. If anything, he adopted an even more dangerous procedure than the German pioneer, for in order to avoid the necessity of finding a suitable hill as an aerodrome, he would attain the initial altitude necessary for a gliding flight by having his machine towed like a kite. An accident during one of these experiments resulted in his death, and thereby deprived England of a most promising student who, had he lived, might have done much to give this country pre-eminence from the first.
Instead, it was in America that the influence of Lilienthal's work took permanent root. Octave Chanute, a distinguished American engineer, but already an old man, was intensely interested in the prospect of flight, and extended his practical patronage to the early movement by financing an important series of experiments in which the central figure was the well-known pilot, A. M. Herring.
It was while reading the simple notice of Lilienthal's fatal accident that Wilbur Wright received his inspiration to study this branch of practical science, and from the first he obtained the whole-hearted co-operation of his brother, Orville. The story of their work has been told too often to need repeating. Lilienthal died in 1896, and thereupon the Wrights commenced their studies. They built their first glider in 1900; it was a biplane, constructed on the trussed-bridge principle introduced by Chanute, but it differed from the Chanute machine in several important respects. Among other things, it embodied the famous wing-warping control, and the pilot, being thereby relieved of the necessity for balancing the aeroplane by gymnastic exercise, was able to lie prone on the lower plane, and so reduce the resistance of his body to a minimum.