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Aviation History
1939
1939 - 1092.PDF
APRIL 13, 1939 FLIGHT. 383 would mean that the engines might be housed in the wing instead of mounted on pylons above it, and would make possible a redistribution of weight which is favourable from every point of view. A considerable advantage of the boat hull over the orthodox alighting geai is that it is less vulnerable to gunfire. Besides its utility in enabling the restrictions in design and operation due to the take-off condition to be avoided, assisted take-off brings many important benefits which are not obtain able under ordinary operating conditions. As the technique of water-alighting is simpler and safer at higher speeds than with land operation, higher wing loadings may be employed, with consequent gains in efficiency and load'capacity. For the same reason, wing-slots are no longer necessary, and flaps may be of the low-drag variety giving a high lift-coefficient for take-off. A strong and simple wing structure may thus be listed among the advantages of water operation with assisted take-off. As the extra thrust for take-off is supplied by means external to the aircraft, it is no longer necessary tor the airscrews to develop high thrust during this period. This means, of course, that except for certain military aircraft where the maxi mum rate of climb is essential, the variable-pitch airscrew will"be unnecessary. Fixed-blade airscrews of high pitch would suffice' for all ordinary requirements, as such airscrews give high efficiencies over the range between cruising and maximum speeds. This is a great saving, both in first cost and mainten ance charges, as the v.p. airscrew is a complex and expensive mechanism, not entirely free from trouble. Similarly, because it is no longer required to deliver excessive output of power for the take-off condition, the engine benefits from assisted take-off. The high rates of heat-flow and the detonation consequent upon full-throttle working at ground level are the principal factors limiting the period between overhauls. Also, it would appear possible to employ higher compression ratios if the engine could be run throttled at ground level, and thus to effect compensation for height and substantial economies in fuel consumption by this process alone, which increases neither the weight nor the cost of the engine. The considerations set forth above will have made it plain how much the design and performance of aircraft is bound up with the take-ofi and alighting conditions, and how a favourable variation of these conditions may bring about changes of quite a revolutionary nature in aircraft charac teristics. The combination of water operation with assisted take-off has been shown to constitute such a favourable varia tion ; it now remains to choose a method of assisting take-off best suited for the particular conditions. Assisting the Take-off Up to the present, the most commonly used method of assisting take-off is the catapult. This has been applied with complete success to both water- and land-operating aircraft. So far, it has only been employed for small machines with low minimum flying speed. Although there appears to be no insuperable difficulty in the construction of catapults to launch large machines of high take-off speed, such catapults would be of huge dimensions and of great cost. The drawback of the catapult method lies in the fact that the aircraft has to be specially strengthened at the points where the thrust is applied, and, in the case of water-operating aircraft, in the large amount of handling necessary to mount the aircraft in position for a launch. From the military viewpoint these catapults are open to the objection that a single hit with a shell or bomb, or an accident during a launch, might put the catapult out of commission for a long time. Proposals have been made for assisting take-off with the aid of motor-driven trolleys running on rails. This method is open to the same objections as the catapult in regard to the amount of handling necessary, and is considerably more vulner able to air attack and gunfire, as a number of railed tracks would be necessary in order that take-off might always take place into the wind; and these tracks, even with the most powerful motors fitted to the trolleys, would be of considerable length. The "composite aircraft" method of assisting take-off has operated successfully with specially designed components, but the method does not appear very suitable for general use owing to the large amount of handling involved and the slowness and expense of operation. A method of assisting take-off which the writer believes to be original and which appears to be free from the objections raised above is as follows : — The aircraft should be launched from the water by means of a launching gear which consists of a light platform sup ported by two hydroplane floats, the arrangement being similar to the under-structure of a twin-float seaplane, except that the floats should be longer and more widely spaced in relation <*> the aircraft. The aircraft is supported on this platform in flying attitude. Mounted on each float, immediately behind the wings of the aircraft, are two or more aero-engines and airscrews which form the means of propulsion of the launching gear. The aircraft is carried upon the launching gear until flying speed is reached, when it is released. The operation is similar to that of the Mayo Composite except that the lower component, the launching gear, never leaves the water. It should be appreciated that the launching gear may be provided with engines of many times the power of those oi the aircraft to be launched, and, as the floats and framing of the platform are of small weight, the power/weight ratio of the combination may be more favourable than is possible to any aircraft. This, of course, means a high rate of accelera tion and quick take-off. Owing to the fact that the airscrews of the launching gear are positioned immediately behind the wings of the aircraft, the in-draught causes a greater lift than that due to land speed, which enables the floats to get "on the step " earlier and also reduces the take-off run. The excess lift is also con verted into a gain of height immediately the aircraft is re leased, thus providing a margin of safety for errors of control or take-off speed. It will be obvious that the airscrews of the launching gear should be chosen to give high thrust iu the range between the static condition and take-off speed; whereas the airscrews of the aircraft should be of high pitch, giving maximum effi ciency in the cruising range. As the conditions of service are less exacting, the power units of the launching gear might conveniently be aero engines which had already completed their term of useful life in the air. Efficient Floats The floats of the launching gear, because they may be de signed for water performance only, may be more efficient in this condition than would be the floats of a similar seaplane. Also, as air resistance has not to be considered, the costly streamline form is unnecessary; a flat-bottomed, flat-sided punt with raised forepart is more efficient for skimming. That floats may be driven over the water at very high speeds was proved by a Schneider Trophy seaplane which took off at about no m.p.h. under conditions far less favourable. It will be apparent that this means for assisting take-off is far cheaper than any yet mentioned—using obsolete engines, as suggested, the whole launching gear should cost no more than an ordinary motor boat. The smaller sizes might be operated by a single attendant. There are economies to be effected in operation, too. The refuelling tender is unnecessary, as the floats provide an obvious storage place for fuel and oil which may be very simply transferred when the aircraft is mounted ready for a launch. By the simple device of constructing the launching platform so that it may be lowered in relation to the floats, no handling is necessary to mount the aircraft in position for a launch; it may be done from the water with out external assistance by passing beneath the hull while float ing on the water and then raising to the launching position. To avoid concentrated loads and the necessity for local strengthening, the means whereby the aircraft is held should operate over a large surface. Bearers, shaped to conform to the hull and so constructed that they may be exhausted of air, thus holding the aircraft by vacuum pressure, constitute a simple way of effecting this. This method has the advantage of requiring no precise alignment of fitting parts, and release is positive and simultaneous however many points are held. A Circular Canal? Enough has been said to indicate that difficulties of water- operation are not inherent; they may be surmounted with simple and inexpensive means, the provision of which involves no new problems. The advantages claimed may be realised with the designs and materials in use to-day. There are no obvious limits to the size of the hydroplane launching gear. For inland use, where no natural lake is available, it might be argued that the cost of constructing an artificial water- base would be unduly expensive. If the cost of soil drainage and surface preparation and continual maintenance costs in the case of the land aerodrome be set against the fact that no upkeep costs are incurred in the maintenance of a water surface, the water base may prove cheaper in the long run. Proposals have already been put forward for an artificial lake of" a mile in diameter to serve as a base for Imperial Airways Empire flying boats. In the opinion of the writer, such a large area of water is unnecessary. A circular canal of about 200 yards in width and the same diameter as the proposed lake "would suffice to operate much larger aircraft than the Empire boats. The water need not be deeper than is necessary to accommodate the alighting condition ; shallow water is an advantage to a hydroplane once it is planing. The island in the middle—which represents the saving on the original proposal—might house all the permanent buildings.
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