so that the grain flow in the valve head swept continuously and uni- formly from the rim into the throat and stem, thus providing strength to resist sheer at all points of the head. The original practice, before bulk production warranted the use of stampings, had been to turn valves from the solid bar, a procedure which gave in the head a grain flow parallel to the stem.
For exhaust valves a steel having 14% tungsten and 3-5% chromium is necessary in certain of the " hotter " stationary-type engines. For the cooler-running engines a high-chromium stainless steel gives satisfaction. Either of such steels would be satisfactory for inlet valves, but, for economy of such high-grade materials, a plain nickel steel is used with great success. (R. K. B.-W.)
V. AERO ENGINES
Historical Resume. For many years mechanical flight was delayed for want of a light engine, and indeed from the first flight to the present day (1921) the aeroplane was ahead of its prime mover. Flight should have been possible in 1901 when Manley, in the United States, built for S. P. Langley a five- cylinder radial petrol engine developing 52 H.P. and weighing only 2-9 Ib. per H.P. By bad fortune this engine was, however, never used in flight until 1914, when it was mounted in the Langley aeroplane for which it was intended.
For their first flights in 1903, the brothers Wright built a four-cylinder car-type engine of 12 H.P. weighing 12-7 Ib. per H.P. By 1905 it was improved to 19 H.P., with a weight of 9-5 Ib. per H.P. and, as redesigned in 1908, gave 35 H.P. and weighed 5-5 Ib. per H.P.
The aero engine proper dates from about 1909, and the progress made is traceable reliably by the results of competitive tests held from time to time. Such tests were carried out in France, 1909-11-13, in cooperation with La Ligue Nationale Aerienne and the Auto Club de France; in England in 1909-12- 14; in Italy in 1913, and in Germany in 1912-4.
A certain section in England centred its hopes erroneously on the use of very small engines. A. V. Roe made the wonderful achievement of flying an aeroplane with only 9-10 H.P. in 1909. The Alexander prize of 1911 at first stipulated for engines of only 25 H.P. This was increased by the Advisory Committee at the request of the supt. of the Army Aircraft Factory to admit " 40 to 75 H.P." and was won by 24 hours' continuous running by a 50-60 H.P. Green sent in on Sept. n 1911. This engine weighed 296 Ib. complete, and developed an average of 53-5 H.P. The British Government competition of 1914, although won by a no H.P. Green engine, was chiefly useful in showing the merits of the 100 H.P. Gnome and the 90 H.P. RAF. Both of these did yeoman service in the war, but soon proved to be too small.
In Germany, the development of the airship led to the earlier study of larger aero engines, although the German competition of 1914 was won by a 100 H.P. Benz, weighing 4-2 Ib. per H.P. The importance of the aeroplane in war service gave an immense impetus to engine development along two main lines: (a) An extensive development of high tensile steels and aluminium alloys, and a more scientific use of the materials, led to a diminu- tion of the weight; (b) attention to detailed design, guided by scientific investigation, greatly increased the mean effective pressure developed in the cylinders and the thermal efficiency. The speed of rotation was also increased so that output was augmented, while at the same time fuel consumption was reduced.
Modern aero engines may be divided into two classes: (a) Engines which are developments of the motor-car type, i.e. all the water-cooled vertical, Vee, and broad-arrow engines; (b) types designed specially for aerial flight, i.e. the radial rotary engines and the air-cooled Vee engines.
The rotary air-cooled type, which was one of the earliest of these, was almost entirely due to the French; e.g. the Gnome, Le Rhone and Clerget engines. In this type minimum weight was the objective. The arrangement of the engine, with its cylinders radiating star fashion in one plane and operating on a single crank, afforded a crank -shaft and crank-case of minimum dimensions and accordingly gave a motor of extremely light weight. To increase the cooling by air draught, and save the weight of a fly-wheel, the cylinders were made to rotate round
the crank-shaft, which was fixed. Weight was economized by making the cylinders of steel, with very thin walls, and the difficulties due to distortion of such thin cylinders with heat were ingeniously met by using a brass obturator ring, as sub- stitute for the cast-iron piston rings which are universal in other engines.
In 1909 a number of rotary engines of powers ranging from 30 to too H.P. were available. Of these the 100 H.P. Gnome was the most powerful. In 1913 a i4-cylinder Gnome of 160 H.P. was launched, and on a British army aeroplane achieved the fastest flight up to that time, namely 130 m. per hour. At the outbreak of war in 1914, the 100 H.P. Monosoupape Gnome, and at a slightly later stage the no H.P. Clerget and the 100 H.P. Le Rhone came into current use, and the 160 H.P. Gnome was, unfortunately from the war fighter's point of view, discarded on the score of complication. In France in 1917 a higher-powered Monosoupape developing 150 H.P. was put into commission, while in Great Britain the BRi and the BR2 rotaries, developing respectively 150 and 220 H.P., were produced. Including the propeller boss the later Mono-Gnome weighed 2-03 Ib. per H.P. and the BR2 2-21 Ib. per H.P.
In 1914, and indeed at a later stage, none of the rotary en- gines were quite satisfactory; the type suffers from certain inherent disadvantages. It is liable to the distortion and over- heating of its cylinders; the earlier examples required special precautions against catching fire; its petrol and oil consumptions are high ; and it requires frequent dismantling and overhauling.
In spite of this the best of these rotaries formed the basis on which European air experience was founded, and as recently as 1912 the best aero engines (from the point of view, be it under- stood, of the aeroplane's performance, which is dominantly a matter of weight) were probably the Gnome rotaries weighing from 3-0 to 3-5 Ib. per H.P. At this time long-distance flights were exceptional and therefore their large fuel and oil con- sumption was not so serious. Throughout the war, and espe- cially in its earlier stages, they gave their best service in ma- chines of the single-seater high-speed class, in competition with the heavier water-cooled vertical engines on which the German air service relied almost entirely.
When the distance of flight was extended, the water-cooled car-type engine came to the front partly because the smaller weight of fuel to be carried compensated for the greater weight of the engine itself, and partly because it was at that time more reliable. The following table shows the total weights of engine, fuel and oil, for flights of different duration, in the case of a typ- ical air-cooled rotary engine weighing 2-25 Ib. per H.P. and consuming i-io Ib. of fuel and oil per H.P. hour, and of a water- cooled engine weighing 4-0 Ib. per H.P. and having a total consumption of 0-55 Ib. per H.P. hour.
Weight of engine, petrol, oil (Ib. per hr.).
Duration of flight (hrs.) . . . Rotary air-cooled engines . Water-cooled engines
For longer flights than 35 hours the water-cooled engine is here shown to involve a smaller gross weight.
It was largely emulation of the rotary which forced the pace of the progress on the car-type engine. This led to the replace- ment of cast iron by sheet metal for water-jackets; to the use of thin steel instead of cast iron for cylinder barrels and of alu- minium for cylinder-head castings; and to the use of two, and in some cases three, rows of cylinders operating on a single crank- shaft arid mounted on a common crank -case. The use of steel or aluminium alloy instead of cast iron for the pistons had been initiated in experiments for motor-cars. In some few cases air-cooling was adopted, e.g. in France the 70 H.P. eight- cylinder Vee Renault of 1912, and notably in England the 90 H.P. eight-cylinder Vee RAF of 1913-4, and the 140 H.P. twelve-cylinder Vee RAF4a, all of which had cast-iron L-headed cylinders. The last-named engine weighed 4-0 Ib. per H.P. and gave excellent service during the war.