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Aviation History
1930
UNTITLED0 - 0071.PDF
FLIGHT, JANUARY 3, 1930 TWENTY-ONE YEARS OF AIRCRAFT ENGINE DEVELOPMENT By MAJOR F. M. GREEN, O.B.E., M.Inst.C.E., F.R.Ae.S. Major F. M. Green was Engineer in Charge of Design atthe Royal Aircraft Factory at Farnborough from 1910 to 1917. Since ihat date he has been connected with Armstrong-SiddeleyMotors, Ltd., and Sir W. G. Armstrong Whitworth Aircraft, Ltd., of which firms he is now Chief Engineer. Previous to 1910,Major Green was, for six years, with the Daimler Company, so that his earliest engineering experience related to internal-combustion engines rather than to aircraft. THE history of the aircraft engine during the last 21years is concerned entirely with the development ofthe internal-combustion engine and, with very few exceptions, with the four-cycle engine with electric ignition. The use of the steam engine has been discussed from time to time, but it is generally agreed that its limitations are so great as to make it not worth while even trying an experimental installation. The aircraft engine of today is exactly similar in principle to that of 21 years ago, which, in its turn, did not differ from the motorcar engine of a much earlier date. The Wright brothers used a very ordinary looking 4-cylinder water-cooled petrol engine for their early flights, but this was already out of date 21 years ago and by far the most popular engine was the Gnome rotary 7-cylinder engine of nominal 50 h.p. While today there are in use some excep- tional engines giving 1,000 h.p. and more in a single unit, the most widely used engines range from 100 h.p. for light aero- planes up to 500 h.p. or 600 h.p. on military aircraft and large passenger aeroplanes. Curiously enough, the weight per horsepower of the engines is not so very different from the 50-h.p. Gnome engine. Air-cooled engines are generally one half the weight per horsepower, while water-cooled engines are about two-thirds if we include the weight of the radiator and the water. Progress has been made much more in the direction of increased reliability, accompanied by a decrease in the cost of upkeep and an improvement in thermal efficiency. It was generally necessary to take the early Gnome engines to bits after 15 to 20 hours' running. Nowadays, engines frequently run for S00 hour« err mnr? between overhauls. The "Father" of all aero Engines: Thefour-cylinder water-cooled engine used by the Wright Brothers in the first machines. (FLIGHT Photo.) The three chief factors which control the horsepower of an internal-combustion engine are the cubic capacity of the cylinders, the rate of revolution, and the average useful pressure in the cylinder. This latter is generally known as the brake mean effective pressure and is calculated directly from the brake horsepower. It differs from the average pressure taken from the indicator diagram, as it automatically includes an allowance for the mechanical efficiency of the engine. The weight per horsepower of a particular sort of engine is fairly closely proportional to its cylinder capacity, consequently weight per horsepower of the engine is reduced very nearly in proportion to an increase of speed and of brake mean effective pressure. The developments in the last 21 years have been largely to increase these two factors. The 50-h.p Gnome engine ran at 1,200 r.p.m. with a brake mean effective pressure of about 55 lbs. per sq. in. Present day engines generally run at round about 2,000 r.p.m., and their brake mean effective pressure is normally from 115 to 130 lbs. per sq. in From these figures one might expect that the weight per horsepower would have been reduced to something less than half that of the Gnome engine. The reason that this is not so is due to the relative increase of weight to ensure greater reliability and length of service. The increase of brake mean effective pressure is chiefly due to an increase of compression ratio and also to improvement in the design of valve gear and carburettors. I do not remember exactly what was the compression ratio of the Gnome engine, but I think it was about 3i to 1. Present-day engines have com- pression ratios between 5 and 5-| to 1. For special purposes this is often exceeded, but increase of compression ratio means a loss of reliability or else the use of anti-detonat- ing fuels. The increase of compression ratio automatically gives an increase of thermal efficiency, with a consequent decrease of fuel consumption. Modern engines generally have a consumption of between 0-53 to 0-58 pint per h.p./ hour from two-thirds to full power, while this figure can easily be improved by the use of higher compressions when the use of special fuels is permissible. The con- sumption of the Gnome engine was rather an uncertain The N.E.C. engine shown on the left) was a water-cooled two-stroke, four-cylinder vee type. It was used in some very early machines, and is here seen installed in a "Baby Wright " Racing machine. On the right is the E.N.V., a water-cooled vee type with copper water jackets. This engine was used considerably by early British experimenters. (FLIGHT Photos.) 71
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