The Romance of Aircraft

by Laurence Yard Smith

Since by means of the explosion of the gases the force is produced which drives the airplane propeller, the violence and frequency of these explosions determine the power of the engine. Greater power can be obtained either by increasing the size of the cylinder so that it can hold more of the carbureted air, making a greater explosion possible; or else by causing more frequent explosions. The latter is the better method in an airplane engine, as larger cylinders mean more weight to be carried. In the average airplane engine from 1500 to 2000 explosions or revolutions occur per minute.

The combustion cylinder of an aircraft engine is usually built of steel, and the piston of cast iron or aluminum, which furnishes a very smooth gliding surface. The piston rod transmits the power to the crankshaft, a long rotating piece of steel. Every time the piston rod is thrust down by the explosion in the cylinder, its motion serves to turn the crankshaft and thus the vertical motion of the piston is transformed into the rotary motion which sends the propeller whirling through the air.

Wherever two surfaces of metal must rub against each other, as in the case of the piston and the cylinder, there is bound to be a great amount of friction. This friction causes the parts to heat and in time it wears away the surfaces and destroys the efficiency of the engine. In order to avoid this, the surfaces must be kept constantly well oiled or “lubricated.” In some engines all the parts are enclosed in one large box or “crank case” which is filled with oil. Small holes are bored through to the surfaces to be lubricated, and the oil is splashed upon them by the motions of the piston rod, the crankshaft, etc., as they plunge through the oil bath.

But overheating of the cylinder may cause this oil to decompose and in order to prevent this a “cooling system” is necessary. For only when the engine is kept cool and properly oiled can it be expected to run smoothly or give satisfactory service.

So now we come back to the problem of cooling, which caused so much anxiety and trouble to the early aviators. With their engines running at the great speed which was necessary to keep the airplane in the air, overheating and engine difficulties were sure to arise. Cooling of the cylinder is accomplished in one of two ways: either by water or by air. If water is used, a “jacket” in which the water circulates is placed around the cylinder,—the water as it becomes heated passing out of the jacket to the radiator, where it is cooled before it returns. The radiator, at the very front of the airplane body, is exposed to the swift current of the air as the machine drives forward, and this air current serves to reduce the temperature of the water.

This method was the one originally employed with the automobile engine, but in the early models the cooling system, though adequate for the motor car, was hopelessly insufficient when the same engine was installed in an airplane.

It was the Gnome manufacturers who first thought of a most ingenious scheme for cooling the cylinders of the internal combustion engine. Instead of having the piston and the crankshaft move, it was the cylinder itself which moved in the Gnome motor, while the crankshaft and piston were stationary. Thus cooling was very easily accomplished, for the cylinders, flying through the air, making as many as 1500 revolutions per minute, cooled themselves.

The crankshaft in the Gnome motor had been hollowed out to form a tube or pipe, through which the fuel or carbureted air passed to the cylinder by means of a valve in the head of the piston which worked automatically. The Gnome could be built up of any number of cylinders, according to the power required. Its cylinders were set in a circle about the crankshaft, so that the entire engine occupied a minimum of space in the airplane body. Scouted at first as a freak engine, it soon proved its superiority over all those in use and was rapidly adopted by builders of all types of airplanes.

To-day the stationary engine has been greatly improved, its provisions for cooling have been increased and it is once more looked on with favor by many manufacturers of aircraft.

The cylinders of an internal combustion engine can be grouped in one of three ways, and thus there are three main types of airplane engines we should be able to recognize. They are the straight-line engine, the V-type, and the radial. In the straight-line model four, six, or even a larger number of cylinders are placed in a row in one crank case. In the V-type of motor they are set instead in two lines, like a letter V; while in the radial type the cylinders form a circle around the central crankshaft. The radial motor may be stationary or its cylinders may revolve, in which case it becomes a rotary engine, as for instance, the Gnome.