THE Short-Mayo composite aircraft consists of two units, one the lower component, a flying boat, known as the Maia, and the other, the upper component, a twin-float seaplane, known as the Mercury. The upper machine is loaded with more fuel and cargo than it can lift from the surface under its own power ; but it is supported by a trestled device on the top of the more powerful but lightly-loaded Maia. The two components, with their eight engines working together, take the air, the Mercury—the upper component—thus obtaining the necessary lift that would normally be beyond its powers. When a suitable operating height and a safe flying speed have been obtained the units part company.
Originated by Major R. H. Mayo, Technical General Manager of Imperial Airways, the scheme was first worked out in detail in 1932—some two years before the Short Empire flying boats were designed for Imperial Airways.
The fact that the composite aircraft took five years to reach maturity meant that the designers could not take full advantage of aviation progress during that period. Despite this, some remarkable results have been obtained, results that would have been even more outstanding had the design been able to take cognizance of the latest advances and experience in long-range flying boats and their operation.
Unlike most ventures into new principles in aviation, the first design was worked out to the requirements of a commercial proposition, and not with the one object of testing and proving the principle. The object of the designers was to take an inventor's idea and to work it through all its various stages in the drawing office until they evolved a commercially practicable composite aircraft.
The engine power required by an aircraft and the amount of petrol that it must carry for a given range and speed are dependent almost entirely on the gross weight of the aircraft—that is, the weight of the machine itself, its crew, the necessary petrol and the payload in the form of goods, mail or passengers. Should the weight of the payload represent but a small portion of the gross weight, then the operation of the aircraft will be uneconomical. The problem is essentially one concerned with long-range flight, and the composite aircraft has been produced mainly with a transatlantic mail service in mind. In operation over short distances the amount, and therefore weight, of the petrol to be carried is considerably smaller and the weight difference is made up by greater payloads. Thus what is required is a means of increasing the payload of a machine without reducing its range. Major Mayo holds that his invention should double the range and the payload of an aircraft.
The crux of the whole problem is that an aircraft requires considerably more power to get it into the air than to maintain it in flight. The extra power needed for the take-off means larger engines than would otherwise be used, and that means added weight. More powerful engines require a stronger aircraft ; again weight is added. Also, the more powerful engines burn more petrol, so that weight is again increased by the extra fuel required for a given range. Even when a powerful engine is throttled back to a certain horse-power it requires more fuel than a smaller engine developing this horse-power at its normal full cruising revolutions.
Several ways of solving the problem have been suggested at various times. Of these, catapulting is the most likely to be useful, apart from the Mayo composite scheme. Already catapulting has proved its value by its constant use for the launching of normally loaded aircraft in a very small space such as the deck of a warship. It has yet to be proved, however, for the launching of large, heavily-loaded commercial aircraft ; and special strength, which adds weight, is needed for catapulted aircraft.
So far as land machines are concerned, if an exceptionally long run of, say, two or three miles could be provided, heavily-loaded aircraft could be got into the air. But the provision of aerodromes with such a run is a difficult matter, and in any event extra strong and therefore extra heavy undercarriages would be required.
Filling the aircraft with fuel while in the air has been suggested. There are certain difficulties in doing this in rough weather, and most methods have proved slow, so that valuable time might be wasted in this way. Another but untried proposal is the provision of a launching chassis running on rails and carrying the aircraft across the aerodrome at high speed.
So far as large commercial machines are concerned, the Short-Mayo composite aircraft appears to hold the secret of successful long-distance flying. The method, unlike catapulting, provides a smooth take-off—an important consideration for passenger services. Moreover there are no preliminary delays.
Adding Power and Lift
A MACHINE abnormally loaded could be made to take itself off if it were supplied with a larger wing area and more engine power. Major Mayo sought a means of adding wing area and engine power to a machine during the take-off and removing them when the machine was in flight and they became unnecessary. Thus he arrived at the idea of accommodating both in another machine which was joined to the long-range machine and which would return to its base as soon as the long-range machine had been launched.
Having decided that the machines should be fixed together one above the other, it became obvious that the assisted machine should be. the upper, for the lower one must stand all the taxiing strain for the weight of both machines. Therefore, the lower one must be particularly strong and seaworthy, factors that would increase unnecessarily the weight of the assisted machine were this the lower component.
In the Short-Mayo composite aircraft, the lower component carries the upper on its back, to a height at which the pilot of the upper machine will be able to deal with any emergency that may arise when the two components have parted. An incidental advantage of the system is that the upper component may be made small for the payload it carries when the payload is to be goods of high-weight density, such as mail. This reduction in size cheapens production of the aircraft and explains the apparently puzzling smallness of the upper machine of the Short-Mayo composite aircraft in relation to the lower. Only one lower component is required at each base to launch many of the upper components.
The two aircraft when joined are rigidly linked so that they may be considered as one. When thus linked, they become an eight-engined, biplane flying boat. All eight engines are used for the take-off, and the "surplus" wing area and engine power of the lower component make up for corresponding deficiencies in the upper component. When the suitable height and speed have been attained and the two components have separated, the upper continues to climb and the lower descends. There is no jerk on separation.
Aerodynamically, the reason for the successful separation of the two machines is, perhaps, the most interesting part of the design. The wings of the two machines had to be built so that the changes in their wing lift coefficients varied at different rates with changes in the angles of incidence.
The wings of the lower machine had to develop considerable lift at the low speeds of the take-off and at the steep climbing angle assumed during the initial climb, so that the upper machine should be resting on the lower. Then, when sufficient height had been obtained and the machines were to be parted, the wings of the upper machine had rapidly to increase their lift at the flatter angle of flight and greater speed, so that the upper machine was trying to lift the lower. At first it appeared that, to achieve this effect, the upper machine would have to pivot on its fixing to the lower machine, so that the angle of incidence of its wings could be varied.
A different and neater solution, however, presented itself. The effective camber of the wings of the lower machine is varied by means of flap extensions along their trailing edges. These roll out of the trailing edges of the wings in a slightly downwards direction, and are similar to those used with great success on the Empire flying boats. When extended they increase the lift of the wings at low speeds, and are used in this position for the take-off. As soon as sufficient speed has been developed for the wings of the upper aircraft to provide a useful lift the flaps are withdrawn.
At this stage of the flight—as the composite aircraft flattens out—the decreasing incidence of its lower wings causes their lift to fall off progressively while the lift of the upper wings progressively increases. Thus increasing strain is developed in trying to pull the two machines apart, and when the strain has become sufficiently great the automatic release gear comes into operation and the two aircraft become independently-flying machines. The first test flight of the two machines together was made on January 4, 1938.
The lower component is a four-engined flying boat with a single plane attached to the top of the hull ; along this plane are mounted the engines, two on either side. Attached to each wing are two floats. These assist in keeping the craft stable when it is on the water and they are fitted with shock absorbers.
Take-off in 12 seconds
IN general appearance the flying boat is remarkably similar to the Short Empire flying boats, but in its design there are considerable differences. The Maia, of which the identification letters are G-ADHK, was ready first it proved gratifyingly successful when tested by Mr. Lancaster Parker, Shorts' chief test pilot. The tests took place in August 1937.
The machine is remarkably steady on the water and in the air. Its engines are quiet, and it has the excellent take-off time of approximately twelve seconds. The handling of the Maia is sufficiently different from that of the Empire flying boats for it to be looked upon as an entirely different type of aircraft.
Structurally the main differences between the two machines are as follow. The Maia has a wing area larger by 250 square feet, its total area being 1,750 square feet. It also has tumble-home instead of vertical sides, and the hull has a greater beam. The necessity for the greater beam will be appreciated when the considerable weight of the composite aircraft and its high centre of gravity are taken into consideration. The engines are farther out from the hull to allow clearance for the floats of the upper machine. Finally, the control surfaces—ailerons, rudder and elevators—are particularly powerful, because during taxiing, take-off and all flying until the machines separate, these controls have to do all the work of manoeuvring both machines. Right up to the time of separation, the controls of the upper machine are locked, and the composite aircraft is flown as one machine by the pilot in the lower component.
The four engines of the Maia are of the Bristol Pegasus X type, and the gross weight of the Maia when carrying the fuel necessary for one launch and a reserve for emergencies, is approximately 25,000 lb.
The arrangement on top of the Maia for carrying the upper component consists of a central trestle with two supports, one on either side. The two machines are held together by a special hook fitting on to the bottom of the fuselage of the upper component. Fore-and-aft stability is obtained by means of two spigots and sockets on the central trestle, which supports the upper component's fuselage. Two similar spigots and sockets are provided for each of the floats, which rest on the two supports, one on either side of the central trestle. These float supports maintain lateral stability. By suitable adjustment of the hook fixing, the two aircraft are held rigidly together.
The upper component, Mercury, is also a top-winged, four-engined monoplane with the engines mounted in the wings. Its identification letters are G-ADHJ, and it was flight-tested by Mr. Lancaster Parker. The test flights were in themselves notable, because rarely has an aircraft proved so successful at its first test. No major variations in the design were necessary.
The first test flight of the Mercury was made on September 5, 1937. So satisfied was Mr. Lancaster Parker with the handling of the machine that he gave a demonstration of it only three days later. The performance of the machine was also remarkable in that the figures proved considerably better than the designers had anticipated.
The engines are Napier-Halford Rapier engines, having a total maximum horse-power of 1,360. These engines have a small frontal area which no doubt assists the high performance of which the machine is capable.
The Mercury, an all-metal aircraft, though appearing small in comparison with the lower component, is, however, a large machine. The overall length is 51 feet and the wing span 73 feet, with an area of 611 square feet. The overall height in level flight is just over 20 feet, and the width of the float track is 14-1/2 feet. The machine can carry a weight in petrol, payload and crew equal to its own weight, the gross weight being 21,000 lb. This weight is approximately 4,000 lb. less than that of the lower aircraft when normally loaded. A crew of two only is needed for the Mercury.
In certain circumstances the Mercury will have to take off under its own power, but. then it will be lightly loaded in comparison with the load when it is assisted by the Maia, for it will be carrying petrol for only a short flight, although it may have its full payload on board.
The normal cruising height for which the Mercury has been designed is 10,000 feet, and the pitch of the two-bladed, wooden propellers (airscrews) will provide fast cruising at this height. The maximum cruising speed is some 190 miles an hour, nearly 20 miles an hour more than the estimated figure worked out from the design of the aircraft. In still air the cruising range of the Mercury would be about 4,000 miles. The top speed is over 200 miles an hour.
An interesting feature of the design is the main fuel tank. This takes the form of a long tube running lengthwise through the wings, and so fixed to the main spar girder that it considerably strengthens the girder. This tank is fitted with baffles to prevent movement of the petrol from upsetting stability, and each baffle is fitted with a valve that permits petrol to pass towards the centre of the wing, but not outwards to the tips. This tubular tank reaches out along the wings to beyond the outer engines. The total petrol capacity is 1,200 gallons, none of which is carried inside the fuselage ; the fuselage is thus left free for crew and payload. The wing loading of the machine is slightly under 35 lb. per square foot.
The Napier-Halford Rapier engines are of the Mark V series and are supercharged to 13,000 feet, at which height they give a maximum power of 340 b.h.p. each at 4,000 revolutions a minute. The normal output, however. is 315 b.h.p. when running at 3,500 revolutions a minute at a height of 10,000 feet. A wireless aerial runs from a post at the forward end of the fuselage to the top of the rudder fin. The hook which holds the two components together is held in three ways. There are a lock controlled by the pilot in the upper machine, another lock controlled by the pilot of the lower aircraft and an automatic spring-controlled latch which will not release the, hook until the lift of the upper machine has increased sufficiently to produce a predetermined separating force. Until both pilots have released their locks the latch cannot come into operation.
The provision of separate locks for the pilots precludes all possibility of either being taken unawares, because he knows that, so long as his own lock is in position, the two machines cannot separate. The automatic latch ensures that the machines will not part until the upper machine is trying sufficiently to rise to guarantee safe divergence of the machines immediately on release.
When both pilots have released their locks, the separating pull from the hook moves the latch which is pivoted near the hook. This causes the other end of the latch to press against a spring-loaded lock which holds the latch in place. As soon as the pressure becomes great enough, the spring is overcome, and the latch is unlocked and flies over to free the hook. This automatic lock device thus makes it impossible for the pilots to separate the machines before the upper component has attained sufficient lift.
The pilot of the flying boat remains in full control until the machines separate, but the pilots are in telephonic communication the whole time, and an elaborate system of indicators, spring gauges and warning lights enables both pilots to know what is happening the whole time and what the other is doing.
Parted at 150 m.p.h.
WHEN the correct height and speed—approximately 150 miles an hour—have been attained, the pilots release their locks and then the automatic latch separates the machines. At this point the pilot of the upper aircraft takes over complete control of his machine. But he does not have to pull back on his control to raise the machine ; for, with the removal of the weight of the lower component, which his machine has been partly supporting, his machine rises. At the same time the flying boat below, having lost the support of the upper aircraft, immediately begins to descend. The lower aircraft, returns to its base, the upper component goes on its way.
Should the pilots decide not to separate for some reason, such as faulty running of the engines, the pilot of the flying boat is able to land the combined machines again as one aircraft.
The pilots' instruments are most elaborate, and even indicate whether the upper machine is tending to pull away from the lower on an even keel. This prevents the possibility of the upper aircraft "dropping a wing" at the moment of separation.
The first of the Short-Mayo composite aircraft is a notable achievement, but even greater possibilities are inherent in the idea. For instance, an even better payload may be achieved by increasing the wing loading of the assisted aircraft. The inventor considers that the wing loading may be increased to 45 lb. per square foot, which would add approximately 20 per cent to the payload. Finally, if the upper component were made a land machine with retractable undercarriage instead of a float plane, better ranges for a given supply of petrol would be possible.
Worked out to approximate, proportions, the Mercury is able to carry a payload of 1,000 lb. across the Atlantic on one-half as much petrol as is required by the long-range, version of the Empire flying boats when carrying no payload at all.