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Aviation History
1926
1926 - 0130.PDF
SUPPLEMENT TO FLIGHT 24 THE AIRCRAFT ENGINEER FEBRUARY 25, 1926 (3) A cast-iron piston of normal design and of 100 mm. bore has a maximum temperature of about 440° C. under the same conditions of operation. At medium compression ratios (about 4-7) and at 1,800 r.p.m. it develops some (5 per cent, less power than a good aluminium piston and requires a greater petrol consumption (about 8 per cent.) per brake horse-power. The relative advantage is, however, likely to depend on circumstances. Thus, if the compression ratio is increased, detonation will first make its appearance in the cylinder fitted with the cast-iron piston, and a cylinder which would detonate violently with a cast-iron piston might work perfectly satisfactorily with an aluminium piston. Under these conditions the benefit of the aluminium piston would be much more pronounced. On the other hand, with a very low compression ratio the relative effect would not be expected to be at all pronounced. Recent experiments by the author on a 3J-in. water-cooled cylinder with a compression ratio of 4 -26 fitted with alternative pistons show that the improve- ment caused by the aluminium piston does not exceed some 2 per cent. (4) The design of the piston affects its maximum tempera- ture appreciably. The best piston examined has no ribs and a comparatively thin centre, the thickness of the crown being roughly proportional to the radius. This piston is some 20° cooler than one of the same weight but of a heavily- ribbed design. (5) Perforating the skirt of the piston increases its tem- perature. The saving in weight is so small that such perforations are not to be recommended. (6) The highest piston temperatures are attained with the weakest mixture capable of giving maximum power. (7) The temperature of the piston appears to be only very slightly affected by the compression, within the limits of the experiments, being slightly less with the higher compression ratios. (8) The effect of spark advance on piston temperature is not very pronounced, the highest temperatures, however, being obtained with the minimum spark advance. (9) In a piston of the slipper type, in which the gudgeon-pin bosses are earned bv higs joining the piston crown, a con- siderable amount of heat is transmitted down these lugs. and the temperature gradient across the crown is much less than in the normal type of skirt piston. At the same time, owing to the reduced bearing surface, the drop in temperature between the edge of the piston and the wall is greater than in a skirt piston. (10) In an air-caoled cylinder the hottest point of the piston is not at the centre, but at a point nearer the hottest side of the wall. Even in a water-cooled cylinder the sparking-plug may have a very marked heating effect on the wall in its vicinity, and consequently on the piston. In an extreme case the temperature at the edge of the piston nearest the sparking-plug may even be greater than that at the centre of the piston. (11) Assuming that the heat transference from a hot gas to a metal surface per second per unit area equals e62, where 6 is the difference of temperature in degrees Centigrade, the semi-amplitude of the cyclical fluctuation of surface temperature is given by— a = For the piston and combustion head surfaces, e has a value which varies from about 3-6 X 10~° in gas engines of 6 to 12 in. diameter at speeds in the neighbourhood of 200 r.p.m. to 11-0 X 10~6 in high-speed petrol engines, at about 2,000 r.p.m. These values are in C.G.S. units. The corresponding values expressed inC.H.U. persq. ft. per min. are 4 -4x 10~4, and 13-5 x 10-*. (12) In a high-speed petrol engine at 2,000 r.p.m. the fluctuation of temperature in the surface of the piston, if of aluminium, is of the order of ± 5° C. (13) In a high-speed petrol engine working on the weakest mixture capable of giving maximum power and with an air/petrol ratio in the neighbourhood of 13-5: 1, the heat given to the piston and flowing to the walls is approximately 3 -5 per cent, of the heat of combustion of the fuel. When account is taken of the heat dissipated from the under side of the piston, it would appear that the total heat given to the piston is sensibly the same fraction of the heat supply as that found in slow-speed gas-engine tests. (14) When burning occurs in an aluminium piston this is probably due ultimately to a local breakdown of lubrication, following overheating to a temperature which would not, however, in itself prove destructive. METALLURGY "It is impossible to predict the extent to which the use of alloy steels may develop and become of still further importance to the world. Research in this direction is showing that the possibilities of alloy steels are as yet exploited very incompletely ; the total field is of enormous extent, and by far the greater part is still unexplored." This paragraph, from the preface of Sir Robert Hadfield's book entitled, " Metallurgy and its Influence on Modern Progress," published by Chapman and Hall at 25s. net (postage 9d.), is particularly significant in its relation to aircraft manufacture at the present time, when nearly every aircraft firm in Great Britain is turning its attention to the problems of all-metal construction. The part played by Sir Robert Hadfield in evolving alloy steels is too well known to need enlarging upon here, and it has been truly said that Sir Robert holds a place in the " Age of Alloys " similar to that occupied by Bessemer in the " Age of Steel." Sir Robert Hadfield's book is not in any way a text-book on metallurgy, but it is a most fascinating history, to technical and non-technical alike, of human endeavour in the field of metallurgical science. In the space at our disposal it is useless to attempt adequately to review this extraordinarily interesting work, a more extensive review of which we hope to give in another issue ; but a very brief outline of its contents may serve to indicate the wide field covered, even if it cannot hope to convey the charm of the style in which the book is written. The first part of Sir Robert Hadfield's book is historical, and deals with the birth of science, the rise of steam, and the twentieth century. In Part II, which is entitled "' Metallurgy," the author treats of such subjects as the importance and antiquity of iron, carbon in simple and alloy steels, the rise and importance of alloy steels,manganese steel, silicon steel, heat treatment and micro-structure of steel, and application of special steels. A large section of the book is devoted to the subject of fuel economy, and another large portion, Part IV, to education and research ; while in the last section, Part V, the author takes a peep into the future. The work is profusely illustrated by photo- graphs, reproductions of engravings, and diagrams, and is one which all interested in metallurgy should make a point of obtaining. CORRESPONDENCE In order to leave as much space as possible for new articles in THE AIRCRAFT ENGINEER each month, letters dealing with the various points raised will be published in the corre- spondence columns of FLIGHT week by week. Such letters, referring to the publication of and articles appearing in the first of our monthlv technical supplements will be found in FLIGHT of February 11 (from Mr. T. (). M. Sopwith, Mr. W. S. Shackleton, Mr. Oswald Short, Capt. de Havilland, Mr. C. C. Walker, Mr. C. H. Dowty, Mr. H. P. Folland, Mr. F. Sigrist, Mr. W. O. Manning, and Capt. A. S. Keep) and of February 18 (Mr. C. C. Walker and Maj. F. M. Green). ERRATA Although a correction was published in the issue of FLIGHT of February 4 of an error which crept into the article by Mr. Handley Page in THE AIRCRAFT ENGINEER of January 28, it is thought that some readers may have missed this, and consequently the corrections are given again here. In Table 3 on p. 3 the last two divisions were headed flap angle —10° and flap angle — 20°. These should have read flap angle J- 10° and flap angle -f- 20° respectively. In the graph published in the top right-hand corner of p. 3 the positive signs were correctly given.—ED. 110/
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