Modern Aero-Engine Production .sx A Detailed Description of the Processes of Manufacture Used in the Production of Jaguar Engines .sx By J. C. Briggs .sx FIRST Langley and then Maxim in 1895 produced a steam plant designed for aircraft , but no practical success attended these early efforts .sx Even in the light of present-day knowledge the practical limitations are so serious as to render the use of steam impracticable , for it must be borne in mind that the high degree of reliability attained in steam practice , has been , within limits , unrestricted by weight considerations , unhampered by vital factors in aircraft such as resistance , and the effects of the lower thermal efficiency on the endurance and load-carrying capacity .sx It was not until nearly ten years later that the possibilities of the internal-combustion engine were demonstrated as a motive power for aircraft , and with it came the realisation of mechanical flight .sx Research , experience and every phase of scientific production have since been concentrated in producing and developing this power unit which has been so intimately associated with the progress of aerial transport .sx The flight of the Wright Brothers , Bleriot's first Channel Flight in 1909 using an " Anzani " Air-cooled I. C. engine , and the introduction of the Gnome rotary type of I. C. engine are milestones in the history of the successful development of the power unit .sx The latter principle was extensively employed and represented by successive types for several years , but owing to limits imposed by inherent weakness of the type and the demand for higher powers , has been superseded by the static type .sx Modern Interpretation of Requirements .sx Developments have , therefore , been confined to the static type of internal-combustion engine , and the types in general use employ either air or water cooling , functioning on the well known constant volume cycle .sx If a comparison is made between power units constructed in 1910 and types representative of present-day established practice , it will be found that :sx Brake mean effective pressure has been increased by 75 per cent ; .sx Piston speed has been increased up to 100 per cent ; .sx Brake thermal efficiency has been increased by 33 per cent ; .sx The weight per b.h.p. has been decreased by 60 per cent at ground level , and if comparison is made , for example , at an altitude of 10,000 ft .sx the weight per b.h.p. has been decreased by approximately 80 per cent .sx These developments are the more interesting when it is considered that the standard of reliability has been more than doubled and the average period between overhaul increased by at least 350 per cent .sx These results have been achieved by :sx Improvements in design arising from intensive research and a clearer conception of the problems involved ; .sx The use of material of higher physical values , more uniform in structure ; .sx Improvements in manufacture .sx First in importance is the use of direct cooling , usually referred to as air-cooling .sx The advent of suitable and higher-power engines of this type comparable in weight/b .sx h.p. ratio and efficiency with the indirect or water-cooled type , may be classified as the most important development of recent years .sx To obtain the correct functioning of the water-circulating system under varying climatic conditions has always been a fruitful source of trouble , and is responsible for 45 per cent of the total cost of maintenance of this type of engine .sx When it is considered also that aircraft engines are called upon to function within a wide range of temperatures , the problems of this type can be appreciated .sx The necessity of carrying large radiators on aircraft in hot climates in order to take off , which are unnecessary when the aircraft is in the air , are at least as serious as those offered by the difficulties presented by the temperature at the other end of the scale , which may be as low as minas 50 deg .sx C. .sx Simplicity of the Air-cooled Type .sx Apart , therefore , from the reduction in the cost of maintenance , the reduced ground staff and the ability to put in the air under any climatic conditions a fleet of aircraft at a moment's notice , a factor of the greatest importance particularly for military service , the direct-cooled type has rendered the problems of the aircraft designer simpler , and restrictions in design have been removed by the deletion of the radiators and water circulatory system .sx Owing to the small moment of inertia of the propeller , and in order to reduce to a minimum the weight of mountings and the aircraft structure , the torque and the mechanical balance of the power unit must be as perfect as is practicable .sx The troubles arising from the neglect of these essentials do not frequently become apparent until after a period of service .sx Experience has shown that the careful selection of the number of cylinders in relation to the power output is attended by advantages not fully realised in the resultant life and freedom from trouble with the aircraft , particularly with the higher-power units .sx It is frequently suggested that this results in a large number of cylinders for the higher-power outputs , with consequent increase in cost of production and maintenance .sx Experience has proved that maintenance is actually reduced by the longer periods between overhauls of the aircraft and power unit , and the fewer parts required for replacement over a given period of service .sx A carefully considered policy , therefore , in this respect has advantages , the importance of which it is difficult to estimate .sx It is possible also to provide a series of power units , each of which has a number of cylinders , forming a multiple of the higher-power unit , which will possess the necessary degree of torque uniformity and good mechanical balance , enabling the more important detail component parts to be standardised , whilst providing power units each of which possesses a distinct and necessary application for aircraft purpose .sx The consequent reduction in service personnel , the reduced number and types of spares to be maintained with the resultant economy is an illustration of the practical benefits of such a policy .sx Consideration of Design and Development .sx The gradual evolution of the modern aircraft engine where speed and lightness of weight is important has called for a careful consideration of the general principles which influence design .sx For example :sx Cylinder temperature must be uniform and the temperature gradient gradual .sx Valve seats and valve guides most be adequately cooled .sx Valves must be of adequate dimensions with unrestricted port-areas .sx Valve mechanism must be capable of operating reliably at high speeds .sx Pistons must be designed to dissipate the heat with a gradual temperature gradient throughout their mass , and with a minimum of weight .sx Compression ratios must be provided which will allow fuels which are generally obtainable to be used .sx Emergency conditions have already shown the difficulties which arise from the adoption of unsuitable compression ratios .sx The importance of this factor , therefore , cannot be over-estimated , and it must be accepted that reasonable compression ratios based on conditions arising from emergency conditions must only be permitted .sx The problem of maintaining performance at operational altitudes by the use of superchargers also cannot be separated from the general policy affecting this question .sx Carburettors must have unrestricted passages and provide good atomisation over a fairly large range of throttle setting , whilst induction systems must be correctly designed to provide a uniform distribution of mixture .sx Connecting Rods must be light and constructionally rigid , as well as of sufficient strength , particularly the big-end , to avoid local flexure .sx Crankshafts and crankcases must be of adequate strength and stiffness to accommodate the varying degree of torsional resonance .sx Lubrication and bearings .sx The oil temperature must be kept within defined limits ; and a system employed which will allow the lubricant to be distributed freely and in sufficient quantity to each wearing part .sx Dynamic Balance and Torque .sx General .sx It is frequently ignored that bearings have clearances and that neither crankshafts , connecting rods nor crankcases are perfectly rigid .sx In the design of the aircraft engine , therefore , the best compromise must be effected between what may be termed theoretical principles and practical application .sx It is , however , essential to remember that good dynamic balance and a uniform torque are the basis of any aircraft engine design , and must be the result of careful consideration in relation to the power output and duty of the type .sx The possibilities of future development of a type in the light of modern requirements can be measured by a consideration of the degree of attainment of these essential principles .sx The aircraft designer demands that the speed of the airscrew shall not exceed 800 ft .sx per sec .sx : otherwise serious loss of efficiency will result .sx This demand has become , for all types of aircraft , an accepted principle .sx What influence , therefore , has this demand on engine design .sx To produce a simple and reliable airscrew reduction-gear , which is light in weight and efficient , the first essential is a smooth-running engine having as uniform a torque as it is practicable to provide with the reciprocating type of engine .sx In considering the problem of the reduction gear it must not only be simple in design , but capable of providing a ratio which , whilst permitting of efficient airscrew speeds , must not make the aircraft designer's task difficult by producing too large a diameter of airscrew , thus affecting the understructure and making the aircraft unwieldy , and an excessively heavy airscrew with its influence on the aircraft performance .sx At the same time due consideration to engine cooling is of equal importance .sx The only type of supercharger designed to improve performance at operational altitude which has been in successful use over a long period in large quantities is the centrifugal type of impeller directly driven through gearing from the engine crankshaft .sx It will , therefore , be apparent that the same principles are important in relation to this device , and to a large extent have been responsible for its successful operation .sx The ability to put an aircraft in the air within the shortest space of time demands a simple yet reliable form of starting equipment .sx Further , it must be capable of efficient use under all climatic and weather conditions .sx Various forms of equipment are now employed which have been proved to fulfil these demands .sx These may be divided into two categories :sx - .sx What is familiarly referred to as gas starting ; .sx Mechanical starting .sx Modern engines permit of these forms of starting equipment to be employed according to the particular use and application of the engine .sx In considering the improvement and development of aircraft engines a reference to certain of the most important parts may be of interest :sx Cylinders .sx Designs have been improved by the use of more suitable materials and the better distribution of material , resulting in lower and more uniform cylinder temperatures with the avoidance of hot spots .sx This has been particularly the case in cylinder-head design , where better provision has been made for cooling and conducting the heat from the valve-guide and seats .sx Valves .sx These have been improved by better cylinder-head design for dissipating the heat , and the use of improved material , the physical properties of which are better adapted for the duty .sx Valve Mechanism .sx It is interesting to record that with more reliable data affecting stresses of synchronous periods , and with greater uniformity in the material employed , a serious limitation in development has been removed .sx Pistons .sx Perhaps no other component has received such serious attention .sx With improvements in cylinder design , distribution and carburation , together with designs providing a more gradual temperature gradient better adapted to dissipate the heat , and with the use of improved materials , a considerable factor of safety for pistons of the maximum size now in use in aircraft engine practice has been provided .sx Crankshafts .sx Designs have been improved by the use of better materials , a more extended knowledge of stress distribution and synchronous periods .sx It is well known that the theory of the distribution of stress in an elastic body shows that the distribution of stress will not be uniform at any cross-section where the form or size is changing , and that when the change is rapid the concentration of stress will be of considerable magnitude .sx Experience has shown the importance of local concentration of stress at changes of section , and the effect of holes drilled at unsuitable points , sharp corners , tool marks and scratches , particularly when the load is alternating .sx