Aeronautics - Methods of Propulsion

AERONAUTICS - METHODS OF PROPULSION The failure of the early attempts to fly was due in no small de gree to the want of a proper engine; there was little knowledge of the power necessary. Models were devised, worked by india rubber cords or clockwork, and in 1842 Phillips built a steam driven model which, according to his account, rose to a great altitude and flew across two fields. Steam was used by Henson (1843), and Stringfellow (1847), who, in 1868, obtained a prize of Ito° at an exhibition of the aeronautical society held at the Crystal Palace. The engine was estimated to give 3 horse power.

The weight of the complete model was under 12 lb. and the area of the wings and tail about .36 sq. feet.

The steam engine used by Langley (1903) was remarkable; it was designed by Manley and its weight was extremely small for the horse power produced.

In his earlier model the weight of the engine was 6o oz. and the power developed ih.p.; the four boilers weighed 7 lb. each.

Maxim, in his experiments of 1894, employed a pair of two cylinder compound engines, each weighing 600 lb. and giving 363h.p.

The whole problem was changed by the invention of the inter nal combustion engine burning petrol.

Santos Dumont employed a light and powerful petrol engine in 1906; about a year later Henry Farman used an eight-cylinder Antoinette petrol motor develop ing 4oh.p. at 'Jo° revolutions per minute.

In 1908, Wilbur Wright, in his notable flights in France, em ployed a four-cylinder petrol en gine of 24h.p., making 1,200 revolutions per minute and connected by chain gearing to two wooden propellors which revolved at 450 revolutions per minute. The weight of this motor was stated to be about 200 lb.

Engine Requirements.

From this time onward progress has been rapid and reference should be made to the article on AERO ENGINES for the details. The problem for the designer has been to reduce the ratio of weight to horse power ; and this implies not merely a reduction in the weight of the engine itself but also in that of the fuel consumed.

Modern engine weights vary from loo up to i,000 or 1,200 pounds and their power from 20 horse power up to 82o horse power, with a weight per horse power of 1.5 pounds, or even dis tinctly less, and a fuel consumption of under 0.5 pounds per horse power hour.

Again, in the design of an engine, attention must be paid to its head resistance; the aeroplane itself, with its wings, body and under carriage, offers considerable resistance to the motion, and this is increased in a marked degree, unless great care is taken in the design, by the resistance due to the engine itself, placed as it usually is in the extreme front of the machine. In designing a machine it is clearly of great importance to have regard to the engine with which it is to be fitted.

In the endeavour to combine lightness and power many prob lems await the designer; thermodynamic reasoning shows that the efficiency—the ratio of the energy given to the propeller to the heat, measured as energy, supplied to the engine—increases as the compression in the cylinder is increased, but too great a compres sion produces detonation—knocking—in the engine, and delicate experiments are required to investigate the cause of this. Some thing has been done recently by the introduction into the fuel of chemicals—dopes—which in some way reduce the tendency to detonate and permit a higher compression to be used. Further in vestigation now in progress should render this action, which is by no means completely understood, more clear.

Another difficulty arises from the high temperatures involved. A material is needed which will combine strength and the power to resist great heat, especially in the moving parts, the valves and valve spindles exposed to the outrush of the exhaust gases. Some of this difficulty may be met by sleeve valve construction.

The sparking plugs, again, are a source of difficulty; every mo torist knows of the trouble caused from time to time by a faulty plug. In a modern high-power aero engine the troubles are mul tiplied many times, and much care and thought are being spent in devising improvements.

Petrol is a source of danger, and for airships especially the use of a high compression engine of the Diesel type would have marked advantages. Such an engine, if light enough, would also be of undoubted value in an aeroplane.

Power and

energy fed into an engine comes from the combustion of the oxygen drawn into the cylinder at each alternate stroke of the piston, and the volume drawn in is the same whether the engine be on the ground or at an altitude. But the air density falls as the machine rises. The quantity of oxygen ready for burning in each stroke is thus reduced and the power of the engine falls off. The exact proportion in which this occurs varies with different engines, but approximately the power is pro portional to the density of the air drawn into the cylinder.

Supercharging.

To remedy this a device is added in certain cases whereby the density of the air supplied is maintained con stant ; this takes the form of a blower or pump which forces into the cylinder more air than would normally be drawn in ; the engine is said to be supercharged.

Carburation.

The petrol is supplied to the engine through the carburetter and the amount of petrol needed for most efficient working depends on the amount of air drawn in; the ratio of petrol to air should remain constant.

Thus, as the machine rises and the air density falls, it is desira ble to reduce the supply of petrol proportionately. Various devices have been proposed to do this automatically, and the R. A. E. automatic control had at the end of 1935 been satisfac torily in operation for some months.

Flow Meters.

The pilot has no ready means of knowing the rate at which petrol is being supplied, and for this a suitable form of meter to indicate the flow is highly desirable.

Modern Engines.

The article AERO ENGINE indicates how these numerous requirements are being met ; the splendid success achieved by the makers of the engine which won the race for the Schneider cup in 1927 is some measure of the advance that has been made.

Aero engines fall into two main classes: (I) water-cooled and (2) air-cooled; in the one the cylinders are cooled as in an ordi nary motor car engine by water which circulates round their jackets and is itself cooled by passing through the radiator, placed usually in the extreme front of the machine, but in the case of modern extra high speed aircraft in the wings.

For an air-cooled engine, the outside of each cylinder is fitted with a number of webs or flanges, thus increasing the cooling sur face, and the engine itself, placed so as to catch the full draught of air, is fitted with NACA cowls or Townend rings to reduce the resistance.

The air-cooled engine again falls into two classes; in the one the cylinders are fixed, the motion is conveyed to the airscrew by the shaft ; in the other the cylinders rotate about a fixed shaft and carry the propeller round. Details as to a few typical engines are given in Table III. on p.