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Aviation History
1919
1919 - 1046.PDF
AUGUST 7, 1919 SOME DEVELOPMENTS IN AIRCRAFT DESIGN AND APPLICATION DURING THE WAR By the Right Hon. LORD WEIR OF EASTWOOD, P.C., Honorary Fellow of the N.E. Coast Institution of Engineers and Shipbuilders. Concluded from page 1016) PART III.—-Progress in Engine Design BEFORE 1914 it may be said that aero engine design had merely reached the stage of sufficient reduction of weight- power ratio to enable aeroplanes to fly. Beyond this, practically no special adaptation to aeroplane requirements had been attempted. The development of the aerial arm in the War, however, speedily emphasised the importance of other factors. The extension of the range of operations and the losses of machines, due to engine failure, raised insistent demands ior greater reliability, whilst the necessity for easier maintenance called for engines of greater all-round accessibility. The rapid progress of flying skill and the adoption of aerial acrobatics in fighting created a demand for engines of short length and quick controllability. The desirability of carrying out reconnaissance untroubled by enemy attack or anti-aircraft fire focussed attention on the necessity of increasing the ceiling or maximum height of operation. Coping with this most important requirement again produced an improvement of the weight power ratio, and the ceiling of approximately 7,000 ft. in 1914 was increased to nearly 30,000 ft. by the beginning of 1919. It must be remembered also that the diminished density at great height decreases the amount of oxygen taken into the engine, and therefore, the power which the engine can deliver. At 15,000 ft., for instance, a 300 h.p. engine can only deliver about 200 h.p. The extension of bombing activities at long ranges em phasised the importance of the study of the fuel consumption which, for a flight of six hours, amounts to a large weight per horse-power. The question of the thermal efficiency had, therefore, to be studied in conjunction with the weight/ power ratio, so that engine speeds increased whilst the aero dynamic requirements of the machine demanded reduced propeller speeds. Long before the end of the War the power requirements of some aeroplanes and seaplanes had far outstripped the possibilities of any one engine, so that machines possessing two, three, four, or even more engines were in service or being built. An important factor affecting aero engine development is the time which is required to produce a new design. Generally about eighteen months would have to elapse between the commencement of the design and a useful flow of reliable production engines. During most of this period no useful practical experience can be obtained as to the qualities or defects of the new design, and by the time bulk experience of its behaviour in service is available, it is necessary to supersede it by another more advanced type. Less than half the time is required for the development and trial of an aeroplane design, so that the aeroplane is generally well ahead of the engine for which it is designed. In 1914 our aercraft engine position was by no means satisfactory, and we depended for a large proportion of our supplies on other countries, principally on France, whose Gnome and Renault engines were pre-eminent. Great efforts were immediately made to extend our sources of design and supply, and by the end of the War British engines had gained foremost place in design, and were well up to requirements as regards supply. Many of the earlier designs were considerably influenced by previous automobile engine practice, but a wide divergence of design and detail soon took place due to the entirely different nature of the conditions to be faced. In automobile practice silence and good carburation over a wide variation of speeds and loads were the most important features, whereas those points are of small importance for an aircraft engine. The task of the aero engine designer was still further complicated by the fact that the order of importance of the various features of the engine is different according to the class of machine for which the engine is being designed. Certain entirely novel conditions had to be met and their attendant difficulties overcome. For example, an aero engine must have the ability to function in practically any position and, for a time at least, when completely inverted. This require ment has had a far-reaching effect on the lubrication system of aero engines, as it practically precludes the carrying of oil in the crankcase. The shape of the engine is a matter requiring careful consideration for aircraft, as head-resistance, accessibilit\ and small moment of inertia are all features of considerable importance dependant on the shape. The wide ranges of temperature and pressure through which an aeroplane may pass affect the carburation, the cooling system, the lubrica tion system, and even the ignition to a very serious degree. An aeroplane may undergo a very rapid change of as much as 75 deg.'F.in temperature, combined with the maximum difference in moisture content of the air. With water cooled engines, therefore, it has been found necessary to put a thermometer in the circuit and fit the radiator with blinds operated by the pilot, and even with such accessories the maintenance of the water at a constant temperature has often been a matter of great difficulty, and has thrown a heavy responsibility upon the pilot. At the same time evaporation losses must be reduced to a minimum, as the amount of water lost on long journeys is an important feature, so much so that it is true to say that the Atlantic could not be flown to-day with any of the water-cooling systems that were deemed sufficient at the outbreak of War. The intense vibration due to the conditions of high speed and lack of rigid support under which aero engines must work impose new and sever conditions mechanically on even part. The violent and varying slip stream from the pro peller also imposes a new problem as regards carburettor intake conditions, where again the seriousness of fire risk has to be taken into account and avoided. Probably no detail of the whole engine process has received more expert and prolonged attention than the ignition, and much of the increased reliability and efficiency of the modern engine undoubtedly results from this work. The very severe centrifugal and inertia effects which art- experienced in aerial fighting, coupled with the necessity under such conditions of immediate response to the throttle, have necessitated careful design of the petrol supply system to ensure a constant and adequate supply of fuel. In addition, the length and ramifications of the fuel system have increased considerably with the growth in the size of machines and in the number of engines. Fig. 39 shows a typical lay-out of petrol system for a large modern multi engined machine. The main tanks are numbered r, 2, 3, 4. 5 and 6, and have a combined capacity of 1,800 gallons ol petrol. These six tanks feed by gravity to the main collector which is situated in the engineer's cabin. In each of the pipes between the 'collector and tanks is fitted a n on-return SLJIL, IAK CCMTWiFUCAL fitmfOL PUMPS PfTffOL 3YS7TM Fi£. 39. valve to obviate any possibility of the petrol running from the top tanks into the bottom, as these six tanks are not all on the same level. From this collecting box the petrol is lifted by five centri fugal pumps, which are shown below, into the junction boxes, which ieed the engines. These centrifugal petroi pumps are driven by airscrews which are placed in the slip stream of the propellers in order that the petrol shall be pumped up whilst the machine is stationary. If these pumps should fail to act the petrol can still be lilted to the junction boxes by the hand pumps shown one on each side just above the collector. The two distributors or junction boxes feed the engines ; IO48
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