THE VICKERS WELLESLEY MONOPLANE is a machine built entirely on the geodetic principle. In geodetic wing construction there are no main spars or ribs, all the strength being supplied by outer members which also provide the shape of the wing. This aircraft has a retractable undercarriage. The structure below the wings which look like seaplane floats are streamlined bomb carriers. The Wellesley is the type of machine that was chosen at the end of 1937 for the attack by Great Britain on the world’s distance record in a straight line.
ALMOST every conceivable arrangement of wings and tail has been tried at some time or another in the history of aviation. Almost every form of construction has been used. Yet it is by no means certain that the fixed wing aeroplane, as it is today, is the final pattern to which all future types will conform.
Inventors have devised numerous unorthodox arrangements of the lifting and control surfaces, and it is the purpose of this chapter to deal with some of the more interesting examples. Moving wing types, however, have already been discussed, and will be excluded here. Moreover, the slotted wing machines such as the Handley Page Gugnunc, in which the fullest possible use was made of the slotted wing principle, will also be excluded.
One of the most ingenious and the best conceived structural devices ever seen in aircraft work is the geodetic construction invented by B. N. Wallis, of Vickers (Aviation), Ltd. The rather forbidding term geodetic construction is used because the main members in machines built to this pattern follow geodetic lines. A geodetic line is the shortest distance between two points on a curved surface.
The advantage of geodetic construction for aircraft purposes is that the main members which give the aeroplane structure its strength follow geodetic lines which conform to the surface of wing or fuselage. Normally the wing or fuselage is formed upon a framework of struts, spars, former ribs, compression members and tie rods, and on top of this framework is a secondary structure to make the correct cambered shape. In geodetic construction the members themselves take up the shape of the wing and they merely have to be covered with a thin sheet of fabric or metal for the wing or fuselage to be complete.
In brief, the geodetic wing or fuselage is hollow and has no internal struts and spars. Its strength is in or close to its external form. This simplification, however, is not its only advantage. Exceptional torsional strength is secured with the geodetic form. That is to say, that it is much more difficult to twist an equivalent geodetic wing about its centre line than a wing formed on a separate basis of spars and struts.
In the stressing of geodetic wings formidable mathematical difficulties presented themselves, but they were gradually overcome. Even then it was thought that the quantity production of geodetic type machines would be impossible. The inventor, however, devised a system whereby production could be as rapid with this type of construction as with any other. The geodetic form soon went into service in the Royal Air Force, the Wellesley single-engined bomber being the first type to be put into series production. Moreover, it was the type selected when it was decided to make an attempt to beat the world’s distance record in a straight line. A flight of Wellesleys was put on order for this purpose. Meanwhile, a twin-engined Vickers Wellington Mk.l was made. This also showed remarkably good performance characteristics and was ordered for the Royal Air Force. At this point it should be stressed that the term unorthodox aircraft does not necessarily mean freak aircraft. The unorthodox design and construction of today frequently becomes the conventional and generally-accepted aircraft of the immediate future.
CIRCULAR WINGS were used on the Tilghman Richards aircraft, a model of which is photographed on the left. Research and experiment were carried out on this type between 1910 and 1914. The chief advantages claimed for the annular wing arrangement was that the span of the aircraft could be about half that of an aircraft with orthodox wings of the same area. The Tilghman Richards machine attained a speed of 83 miles an hour with an engine of 80 horse-power.
A PTERODACTYL tailless in flight in the central picture. The Pterodactyl is a tailless machine designed by Captain G T R Hill, and the first model appeared in 1926. Its aim was to give improved stability in flight and to avoid the dangers of stalling. Several machines of this kind have been built, one of them as a high-powered Service aircraft.
THE TRICYCLE UNDERCARRIAGE, used on some of the earliest aeroplanes, has recently been revived. The Stearman-Hammond Model “Y” two-seater monoplane is a good example of the application of the tricycle undercarriage to modern machines. This American design has created considerable interest in Holland. The tail remains up when the machine is resting on the ground, and the nose wheel, as well as the two side wheels, plays a part in the landing of the machine. Landings are considerably easier than with the orthodox undercarriage, there being no tendency for the machine to bounce into the air if a bad landing is made.
Other unorthodox machines, of varying degrees of importance, have been built. In disposing the lifting area of aircraft, almost every formula has been tried at some time or another. Both triplane and biplane have had long periods of reasonable popularity, but today the monoplane is ousting all other types.
Wing shapes have varied from the parallel sided to the markedly tapered. There have been swept-back wings, straight wings and even swept-forward wings. There have been V arrangements, as in the early Dunne biplane, and rectangular arrangements, as in the old Avro. The tail plane has been put - as it were - in front, on several types, and in others it has been suppressed.
Several aircraft which have succeeded in flying have had circular wings with ailerons on the rear part. The tailless, or Pterodactyl formula is referred to below. Pusher aeroplanes, in which a single engine drove a propeller, were used extensively in the war of 1914-18, but have since gone out of fashion. The pusher propeller, however, may regain favour in multi-engined types. One of the latest military machines to be built in America has two pusher airscrews driven by liquid-cooled engines mounted in the wings.
Passing over some of the more fantastic creations of the distant past, we may refer to a few machines of historic interest.
Sir Hiram Maxim built in 1894 a machine with a steam engine as power unit. It was tested on a railway track and rose from the rails, only to crash immediately afterwards. It was half monoplane and half biplane and it had two airscrews. The aeroplane of the Brazilian inventor, Santos-Dumont, was a biplane with vertical panels dividing the wing structure into three cells on either side of the fuselage. Its span was 40 feet, and the engine was an Antoinette. In 1906 it made its officially observed flight of 25 metres (27·3 yards). In 1908 Santos-Dumont had changed from biplane to monoplane and had built his Demoiselle. This was a tiny tractor machine and brought him from the unorthodox box-kite type to the orthodox.
When Sir Alliott Verdon-Roe’s triplane was flown at Lea Marshes (Essex) during 1909 the aircraft was unorthodox, though this was not the first time that the triplane arrangement had been tried.
The Dunne biplane was the next unorthodox type in chronological order. It had its wings arranged with a marked sweep back and with vertical surfaces at their tips. The machine had no rudder or tail; control was effected by operating the vertical flaps and ailerons at the tips of the main planes.
WINGS WHICH SLOPED BACKWARDS considerably were one feature of the Dunne biplane, 1911-12. Another feature was the vertical surfaces between the extremities of the wings. These surfaces, with the ailerons, were used to control the machine, which had no tail. The ailerons were worked independently, and by lowering or raising them together, they carried out the function usually performed by the elevators on the tail of a machine.
Annular aeroplanes were developed by Tilghman Richards between 1910 and 1914 from a design evolved by G. J. A. Kitchen. Three models were built and some interesting results were obtained. The span was 22 feet, the wing area 280 square feet, and the speed claimed at ground level with an 80 horse-power Gnome and one man on board was 83 miles an hour. The annular wing arrangement was advocated on the grounds that the span was much less than that of a monoplane of similar area, that the speed range was wide and that the risks incurred by a sudden stall were minimized.
During the war of 1914-18 the unorthodox types were many, but none of them departed so profoundly from accepted practice as some earlier machines. The Morane Parasol was introduced in the early part of the war and extensively used. The object of the parasol arrangement, with the wings arranged well above the level of the pilot’s head, was to give to the crew an uninterrupted view of the ground.
The D.H.2 pusher scout was unorthodox and so, at the other end of the scale, was the Caproni triplane of 1915, with its hint of the large sizes that were to come. This machine had a span of 98·1 feet and a length of 43 feet. Still in the war period there was the Sopwith Salamander of 1918. This was a biplane and was heavily armoured and intended for low flying attacks on ground troops. The armour plate, which surrounded the pilot, weighed 690 lb. The machine flew well. Its wing loading - 9·7 lb. per square foot - seems low by modern standards. In those days, however, this was a rather high loading and so the undercarriage was scarcely able to take a bad landing and the axles curved up unless the pilot put the machine down with great care.
One of the biggest commercial triplanes ever built was the Bristol Pullman. This was one of the first large-size commercial machines with a totally enclosed cabin for the pilot. The cabin for the passengers was electrically lighted and heated and the power was supplied by four Liberty engines of 410 horse-power each.
In 1926 appeared the first of the Pterodactyl series of aeroplanes designed by Captain G. T. R. Hill. The idea of the tailless design was the same as in the early days of the Dunne. It was intended to give improved stability and to avoid the dangers of stalling. Afterwards other Pterodactyls were built, one of them as a high-powered Service machine.
In Italy some remarkable designs have been produced. One of these was for a racing seaplane - at the time of the Schneider Trophy contests - in which the fuselage formed a hull on which the machine floated when at rest. This hull was fitted with a propeller of motorboat type beneath the surface of the water; the normal airscrew in front had mechanism for starting it and stopping it in one position. Below the machine, deep in the water when it was at rest, were hydrovanes. The idea was that the engine should first of all drive the water propeller and, as the speed of the machine increased, it would rise up on the hydrovanes until it was hydroplaning on them with its body and wings well clear of the water. The pilot could then clutch in the airscrew and complete the take-off.
Another remarkable Italian machine was a “tunnel” machine, the Stipa-Caproni, in which the principle of the Venturi tube was adopted for the wing shape and intended to aid in securing specially high lift. This machine flew, but in recent years little has been heard of its further development.
Two other unorthodox aircraft demand mention, that fitted with the Besler steam engine - a re-echo of the remote past - and that fitted with a pressure cabin for stratosphere flying. The Besler steam engine has proved successful in flight, but so far it has not seemed capable of offering the performance that can be secured with a petrol engine. The development work on this unit has been done chiefly in Germany. The pressure cabin was first tried in France, where two makers did much experimental work with it. One machine, the Farman, was flown and appeared likely to produce results, but delays and difficulties have held up its progress. Pressure cabins have since then been fitted to American machines, and there seems no doubt that they will eventually come into general use for certain kinds of aeronautical work.
The tricycle undercarriage is merely a component and does not, strictly speaking, come within the purview of unorthodox aircraft. As, however, it modifies the whole aircraft in some important ways, it deserves a brief mention. It seems to offer enormous advantages for commercial and military aircraft, especially in the larger sizes.
The tricycle undercarriage was tried in the earliest aeroplanes but abandoned in favour of the two-wheeled undercarriage and tail skid. Later, the tail skid was superseded by the tail wheel; but the undercarriage remained, strictly speaking, a two-wheeled type because the main work was done by the two wheels and the tail wheel was a small wheel upon which little load came at the moment of landing.
The tricycle undercarriage abolishes the tail wheel and may use a nose wheel instead. The nose wheel is as important in the process of landing as the two side wheels, and it entirely modifies landing technique. The aeroplane remains with tail up when it is resting on the ground, and it lands in this position. The special advantage claimed is that, at the moment of touching down on the ground, the wings, because the machine tips slightly forward until the nose wheel comes down and bears its proportion of the weight, are rotated forward so that all lift is immediately taken out of them. Thus bouncing and floating are virtually prevented.
A TWIN-ENGINED GEODETIC MACHINE, the Vickers Wellington Mk. I long-range bomber monoplane. Two Bristol Pegasus engines, of 900 horse-power each, provide the motive power through three-bladed De Havilland controllable-pitch propellers. The fuselage and wings are built on the geodetic principle and are covered with fabric. A retractable undercarriage is used and the wings are tapered, with the trailing edges sweeping sharply forward.