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Aviation History
1913
1913 - 0312.PDF
(/O GHT meter is 6 ft. 4 ins. and pitch 4 ft., revolutions r,ooo. The engine is a 4-cyl. air-cooled, ot 25-h.p. It should, however, be stati-d 111 l.ivoiii uf tin; propeller that although the machine repeatedly gut oft in run, ol lM$ than a hundred yards, the engine WHS never required to be run all out." MARCH 15, 1913. Two other curves of interest are those given in Figs. 8 and 9, reproduced on the previous page, the first showing the variation of slip stream velocity at different points in the disc area, and the second showing the variation of velocity with revolutions. ® ® ® ® AERONAUTICAL ENGINES. (Concluded j The whole matter is obscured by reason cf the fact that many of the moit kucceuful flights have been made with air-cooled engines of Hie rotary type ; indeed, it cannot be denied that but for the advent ol such engines aviation would never have progressed as far as it has, becUK watci-cooled engines at that time derived much of their lightness by a reduction of a factor of safety employed, and an increase in bearing pressures, with the result that the liability was impaired. Any engine, therefore, that could give equal results with a reduction in weight was welcomed with avidity. The author is of opinion, however, that the large amount 01 power alnorlwd in rolating the cylinders, the increased air restftance offered, the un-unifoim distribution of heat in the cylinder walls, and the variation in cooling effects at varying speeds rendeis the adoption of this method ot reducing weight a very doubtful expedient for obtaining a high effective horoe-power per unit of weight now that reliability has come to assume such an important aspect. With regard to the method indicated in (h), it must not be forgotten that the increase of power obtained by raising the speed of revolution is not an entire gain, for the weight of the gearing and its supports, possibly some reduction in the mean effective pressure, and certainly the loss of power through transmission by gearing will cause the ratio of weight to power to be somewhat greater than is indicated by the increase in the speed. Also, higher speeds of revolution naturally tend to increase the wear of the moving parts, and render the possibility of engine failure greater, since few engines are able to run for prolonged periods under such conditions. Hence such a system is not recommended (or general adoption. Turning to l he second aspect of reliability, namely, the absence of trouble fiom defective lubrication, cooling, carburation, and ignition, it will be found that the problem of cylinder lubrication is closely connected with that of cylinder cooling, and the water-cooled engine is superior in this respect for heavy and sustained loads to the air-cooled engine on account of the lower and more uniform temperatures employed. Furthermore, the vertical engine is superior to any other form of engine so far as cylinder lubrication is con cerned, because the supply of lubricant is under better control, and is more uniformly distributed, as the conditions are practically the same in all cylinders. With cylinders placed beneath the crank shaft, provision must always be made to prevent oil draining into them, and this may or may not be entirely effective, according to the quantity of oil in the crank-case. In any event the lower cylinders will receive a larger amount of oil than the upper cylinders, and this may be sufficient to cause flooding. In a modified degree these remarks will apply to Vee and semi-radial engines. To overcome the difficulty of insuring the uniform distribution of oil, the rotary engine may be adopted, but in addition to its defects in respect of cooling (it is hardly possible to use water-cooling for this type of motor) and air resistance, there arises another in that the oil consumption of such engines is excessive. This is only to be expected, since with ordinary engines there is quite enough difficulty in keeping the oil down from the cylinders, while in this case the oil is fed by centrifugal force, and carried by the fresh incoming mixture through the inlet valve into the cylinders. To reduce these troubles, which must always exist to a greater or lesser extent, forced lubrication to the gudgeon-pin and the cylinder seems to be the only remedy, as by this means the quantity of oil may be regulated by experiment to a nicety. With regard to the bearings, it is essential that there always should be a film of oil lietween the surfaces. The maintenance of this oil film depends upon the use of suitable bearing pressures, the elimination of distortion at the bearings, and the continuance of sufficient viscosity in the oil. Granted that the bearing pressures are not excessive, it will be clear that the shorter the distance between the bearings the more rigid will be the shaft, and, there fore, the less the liability to distortion. Hence, a bearing should lie provided between each ciank, and it is preferable not to attach two connecting-rods to one crank-pin, as is necessary with Vee engines, on account of the increase in the distance between the bearings which this leads to. The viscosity of any oil depends upon its temperature, and since the oil will reach very high temperatures when engines are run for prolonged periods at heavy loads, it is desirable to tit some cooling arrangement. This may take the form rom page 266.) already mentioned as provided in the Dorman and the Wolseley engines, but preferably a separate cooler should be included in the design. It is noteworthy in this connection that the Napier, the Sunbeam and the Green engines, which have successfully under gone severe trials of long duraticn, have fully forced lubrication systems and bearings between each crank. To ensure that the oil delivered to the bearings shall be free from any foreign substance, it is necessary that a filter should be inserted in the system. By the adoption of these means it is possible to obtain some immunity from trouble from over lubrication, as a supply of pure cool oil in sufficient quantities can be fed to every part continuously. It may be added that practically all these engines have forced lubrication to the main bearings, but it would appear that the advantage to be gained by the extension of the system throughout the engine mote than compensates for the increased cost of manufacture entailed. Carburation difficulties may arise in any petrol motor, since the mode of carburetting the air depends for its efficiency upon the particular design of carburettor employed, but on account of the comparatively small variations in power output required during flight, there should be little difficulty in satisfactory working. There is, however, one aspect of the question which should re ceive attention, namely, that of the effect of altitude. Assuming that the barometric pressure at the sea level is 30 in. of mercury and the temperature 15° C., at a height of about 5,000 ft. the barometric pressure will have fallen to 25 in. of mercury and the temperature to 8° C, assuming average atmospheric conditions. Since the weight of one cubic foot of air is 0-0807 lb. at o° C. at the sea level, the weight of one cubic foot will be 0-0807 x 273/288 = 0-0765 lb. at I5°C. at the sea level and 0-0807x273x25/281 x 30 = 0-0653 lb. at 8" C. at a height of 5,000 ft., that is, the weight of air will be in the ratio of I to 0-854. Further, the difference of pressure causing petrol to issue at the jet so far as velocity and static head are concerned will remain practically the same at any elevation, while the engine is running at a constant speed except for an increased viscosity of the fuel. Sorel gives a chart on p. 166 of his book on " Carburetting and Combustion in Alcohol Engines" from which it will be seen that the quantity of petrol discharged through a capillary tube under constant pressure at 8° C. is 0-965 of that at 150 C. Hence the mixture will tend to become richer in the proportion of 0*854 to 0-965 or I to I "13. Since most engine builders employ high tension magnetos for igniting the charge, and these have reached a high standard of perfection, it is not anticipated that trouble is likely to arise from this cause provided that extraordinary conditions do not prevail, that is, as long as the speeds at which they are driven are kept within reasonable limits. In general, these are satisfactory, but with the tendency to increase the number of cylinders or to run at high revolutions and gear down the propeller, it is preferable to fit two magnetos (as is sometimes done) rather than to risk failure in such an important part. The two magnetos will obviously require careful synchronising and adjustment, and will increase the weight slightly, but the advantage derived more than compensates for so doing. Economy In Fuel and Oil.—These will be the greater with engines having high mechanical and thermal efficiencies and em bodying very careful design, but although the air-cooled engine works at a higher temperature, the compression pressure that can be employed is limited, and hence the thermal efficiency is not greater than that obtainable with water-cooled engines, and on account of the greater frictional losses in the pistons, the mechanical efficiency is lower. Therefore, the fuel consumptions per b.h.p are, in practice, slightly less with the water-cooled than with the air- cooled engine. This is indicated in Table III, for, with the exception of the result recorded for the 60-h.p. Anzani, and which, it may be added, is open to considerable doubt, the average results for air-cooled motors are worse than for those using water- cooling. It should be noted, however, that practically all the results, excepting that for the Albatross, are excellent, and hardly likely to be improved upon. The same tendency is also exhibited in the table with regard to oil, and as this has been referred to previously, nothing further need be added. The Air Resistance offered bv air-cooled engines has been con sidered when dealing with the methods by which the weight may 318
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