Salmson 200 h.p. Radial Engine, Side View.
The 600 horse-power Salmson engine was designed with a view to fitting to airships, and was in reality two nine-cylindered engines, with a gear-box connecting them; double air-screws were fitted, and these were so arranged that either or both of them might be driven by either or both engines; in addition to this, the two engines were complete and separate engines as regards carburation and ignition, etc., so that they could be run independently of each other. The cylinders were exceptionally ‘long stroke,’ being 5·9 inches bore to 8·27 inches stroke, and the rated power was developed at 1,200 revolutions per minute, the weight of the complete engine being only 4·1 lbs. per horse-power at the normal rating.
A type of engine specially devised for airship propulsion is that in which the cylinders are arranged horizontally instead of vertically, the main advantages of this form being the reduction of head resistance and less obstruction to the view of the pilot. A casing, mounted on the top of the engine, supports the air-screw, which is driven through bevel gearing from the upper end of the crankshaft. With this type of engine a better rate of air-screw efficiency is obtained by gearing the screw down to half the rate of revolution of the engine, this giving a more even torque. The petrol consumption of the type is very low, being only 0·48 lbs. per horse-power per hour, and equal economy is claimed as regards lubricating oil, a consumption of as little as 0·04 lbs. per horse-power per hour being claimed.
Certain American radial engines were made previous to 1914, the principal being the Albatross six-cylinder engines of 50 and 100 horse-powers. Of these the smaller size was air-cooled, with cylinders of 4·5 inches bore and 5 inches stroke, developing the rated power at 1,230 revolutions per minute, with a weight of about 5 lbs. per horse-power. The 100 horse-power size had cylinders of 5·5 inches bore, developing its rated power at 1,230 revolutions per minute, and weighing only 2·75 lbs. per horse-power. This engine was markedly similar to the six-cylindered Anzani, having all the valves mechanically operated, and with auxiliary exhaust ports at the bottoms of the cylinders, overrun by long pistons. These Albatross engines had their cylinders arranged in two groups of three, with each group of three pistons operating on one of two crank pins, each 180 degrees apart.
The radial type of engine, thanks to Charles Manly, had the honour of being first in the field as regards aero work. Its many advantages, among which may be specially noted the very short crankshaft as compared with vertical, Vee, or ‘broad arrow’ type of engine, and consequent greater rigidity, ensure it consideration by designers of to-day, and render it certain that the type will endure. Enthusiasts claim that the ‘broad arrow’ type, or Vee with a third row of cylinders inset between the original two, is just as much a development from the radial engine as from the vertical and resulting Vee; however this may be, there is a place for the radial type in air-work for as long as the internal combustion engine remains as a power plant.
IV
THE ROTARY TYPE
M. Laurent Seguin, the inventor of the Gnome rotary aero engine, provided as great a stimulus to aviation as any that was given anterior to the war period, and brought about a great advance in mechanical flight, since these well-made engines gave a high-power output for their weight, and were extremely smooth in running. In the rotary design the crankshaft of the engine is stationary, and the cylinders, crank case, and all their adherent parts rotate; the working is thus exactly opposite in principle to that of the radial type of aero engine, and the advantage of the rotary lies in the considerable flywheel effect produced by the revolving cylinders, with consequent evenness of torque. Another advantage is that air-cooling, adopted in all the Gnome engines, is rendered much more effective by the rotation of the cylinders, though there is a tendency to distortion through the leading side of each cylinder being more efficiently cooled than the opposite side; advocates of other types are prone to claim that the air resistance to the revolving cylinders absorbs some 10 per cent of the power developed by the rotary engine, but that has not prevented the rotary from attaining to great popularity as a prime mover.
There were, in the list of aero engines compiled in 1910, five rotary engines included, all air-cooled. Three of these were Gnome engines, and two of the make known as ‘International.’ They ranged from 21·5 to 123 horse-power, the latter being rated at only 1·8 lbs. weight per brake horse-power, and having fourteen cylinders, 4·33 inches in diameter by 4·7 inches stroke. By 1914 forty-three different sizes and types of rotary engine were being constructed, and in 1913 five rotary type engines were entered for the series of aeroplane engine trials held in Germany. Minor defects ruled out four of these, and only the German Bayerischer Motoren Flugzeugwerke completed the seven-hour test prescribed for competing engines. Its large fuel consumption barred this engine from the final trials, the consumption being some 0·95 pints per horse-power per hour. The consumption of lubricating oil, also was excessive, standing at 0·123 pint per horse-power per hour. The engine gave 37·5 effective horse-power during its trial, and the loss due to air resistance was 4·6 horse-power, about 11 per cent. The accompanying drawing shows the construction of the engine, in which the seven cylinders are arranged radially on the crank case; the method of connecting the pistons to the crank pins can be seen. The mixture is drawn through the crank chamber, and to enter the cylinder it passes through the two automatic valves in the crown of the piston; the exhaust valves are situated in the tops of the cylinders, and are actuated by cams and push-rods. Cooling of the cylinder is assisted by the radial rings, and the diameter of these rings is increased round the hottest part of the cylinder. When long flights are undertaken the advantage of the light weight of this engine is more than counterbalanced by its high fuel and lubricating oil consumption, but there are other makes which are much better than this seven-cylinder German in respect of this.
