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60 In some cases a reduction gear was incorporated in the engine itself. In the first ships of the N.S. class a length of shafting was used in order to give a better shape to the engine car and obtain better air- screw efficiency. This shafting had ultimately to be abandoned on account of torsional resonance, and the airscrew mounted direct on the engine. In the German rigid airships, however, where more weight was available, the reduction gear box and intermediate shaft- ing were employed.

Pre-war British airships and the first few rigids were fitted with swivelling propellers. The airscrews were carried at the ends of horizontal arms and driven through bevel gearing so that the axis of the airscrew could be rotated about a horizontal transverse axis, and the direction of thrust of the airscrew changed from ahead to astern, up or down. The ability to exert a vertical force independent of the headway of the ship was often very valuable to the then com- paratively inexperienced pilots under the bad landing facilities then existing.

Though engine failure has not the same consequences as in an aeroplane, the machinery must still be regarded as the part of the airship most frequently in need of overhaul. Experience shows that the engine cars must be easily detachable so that spare cars can be fitted and thorough overhaul made possible without excessive delay to the ship. They must be as the locomotive to the train, not as the machinery to a battleship.

Hydrogen as Fuel and Recovery of Exhaust Water as Ballast. Dur- ing a long flight the consumption of petrol so reduces the weight of the ship that, in order to restore her static equilibrium for landing or to avoid the increase of resistance if she is flown very light, it is neces- sary either to discharge a quantity of hydrogen or to acquire weight. The latter can be done by condensing the steam in the exhaust gas. Petrol produces steam equivalent to some 140 % of its weight, and the proportion of this which can be collected depends upon the tempera- ture and humidity of the issuing gas. The chief difficulty in the condensation is due to the fouling of the cooling surfaces with an oily deposit.

Attempts have been made to burn, as supplementary fuel, the hydrogen, which must otherwise be discharged. When burning hy- drogen alone in an engine with a compression ratio of about 5:1 it is not possible to develop more than 25 % of the engine's full power without serious detonation. When petrol and hydrogen are burnt together the proportion can be so adjusted that any fraction up to full power can be developed. A few of the smaller airships were fitted in this way but the system was abandoned on account of increased risk of fire.

Risk of Fire. Apart from hostile incendiary action the risk of fire in the air is small and is mainly due to the petrol. It is thought that the use of heavy oil fuel would give added safety. The heavy oil engine at present involves prohibitive weight, but a Diesel en- gine capable of burning only -38 Ib. of fuel per H.P.-hour would, on the basis of too hrs. flight, justify an increase of machinery weight of 12 Ib. per H.P. over the 5 Ib. per H.P. of the petrol machinery which burns -5 Ib. per H.P. hour.

Winches (for Kite-Balloons). The earliest form of winch used had a steam engine driving a single drum on which the wire was wound. It was mounted on a single chassis and was drawn by horses.

In 1915 the French adopted a steam winch of Col. Renard's design which was fitted with surge drums a pair of drums round which the cable makes a number of turns in grooves of correctly formed section. These drums transmitted the whole of 'the engine or brake torque to the cable and allowed it to be stowed on a separate storage drum under comparatively small tension and, therefore, less subject to damage. The winding unit of this type of winch, including the surge drums, liquid brake and storage drums, was adopted, with only modi- fications in detail, as the standard for all future winches.

The later winches were usually driven by petrol engines independ- ent of the motors driving the chassis which carried them.

After 1916 the German winches were made in two separate units, the motor on one and the winding unit on the other. These were treated like gun and limber and when in use were connected by a flexible shaft.

For naval purposes the standard winding unit was employed but driven by a steam engine in destroyers, an electric motor in light cruisers, and by hydraulic motor in capital ships, these being the most convenient forms of power available.

Gas for Airship Purposes. Hydrogen is almost invariably em- ployed for airships and balloons. Coal gas is cheaper and more uni- versally available. It is sometimes used for free ballooning, but has a lifting power of only about half that of hydrogen. Helium, al- though having only 93 % of the lifting power of hydrogen of equal purity, is totally non-flammable and has, therefore, signal advan- tages for airships exposed to attack with incendiary bullets.

Variation of Lift. The total upward force on the airship when at_rest is termed her "gross lift." If V be the volume of gas in the ship, ph its density and pa the density of the surrounding air: Lift = V(po-pA).

Variation with Height. The lift is constant as the ship ascends until a height termed " thepressure height " is reached at which the gas spaces have become full and further expansion involves the loss of gas. When descending, the lift will similarly remain constant,

because V varies directly and pa and ph inversely as the height, as- suming that the temperature of gas and air remain equal. As the ship rises above pressure height, V remains constant but pa and ph decrease.

Variation with Barometer is nil until the ship becomes full; after that it varies directly with the barometric reading.

Variation with Temperature. Provided the temperature of the gas exactly follows that of the surrounding air, there will be no change of lift until the ship becomes full. Then, after V has reached a maxi- mum the lift will decrease inversely as the absolute temperature rises. Radiant heat falling on the ship raises the gas temperature sometimes as much as 40 F. and often 20 F. above that of the air. The gas temperature changes comparatively slowly as the ship moves through air of varying temperature, hence there may be a consider- able difference between gas and air temperatures and this will sub- stantially influence the lift of the airship.

Variation with Gas Purity. Dilution of the hydrogen by ingress of air increases ph and decreases the jift.

Standard Basis of Airship Calculations. The variation of atmos- pheric density with height is a somewhat complex relation. The accepted relation is given in A.C.A. Reports, R.M. 509. The condi- tions at sea level are assumed to be: atmospheric density -0782 Ib./ ft. 3 ; temperature 282 A; pressure 1,014 millibars, i.e. 14-7 lb./in 2. As a standard basis of calculation of airship performance, the lift of hydrogen under these conditions at sea level is assumed to be 68 Ib. for each thousand cubic feet. This figure corresponds to a purity of 94 per cent.

Determination of Purity. The apparatus most usually employed measures the times taken by equal volumes of gas and air to escape through a small hole. The densities are inversely proportional to the squares of these times. An accuracy of =*= I % can be obtained with such an instrument.

The most accurate method is by chemical analysis.

Manufacture of Hydrogen. The choice of method is governed pri- marily by the transport facilities and the raw materials available in any district. Those most usually employed for airship purposes are:

The Water-Gas Process^ generally employed at large fixed bases where a supply of coke is available. It yields a steady supply of gas of about 99-0% purity. Calcined spathic iron ore is oxidized at about 800 C. by steam. Hydrogen is given off and the ore is then reduced by water-gas and the process repeated. In the Lane plant the ore is contained in iron retorts heated externally by coke or spent gas. In the Messerschmidt plant the heating gas is burnt actually in contact with the ore itself.

The Electrolytic Method is employed where cheap electric power is available or where the oxygen is valuable as a by-product. Dis- tilled water must be used and a yield of 5 to 7 cub. ft. of hydrogen per kilowatt hour with a purity of over 99 % can be obtained.

The Silicon Process is employed where a rapid yield is required and where transport of raw materials is difficult. Powdered ferro-silicon (90% Si) is fed into hot 40 % caustic-soda solution. One ton ferro- silicon and 2 tons of caustic give about 50,000 cub. ft. of gas of 99 % purity.

In cases where transport of materials is exceptionally difficult, hy- drolythe (calcium hydride made by passing hydrogen over strongly heated metallic calcium) is used with water. About 34,000 cub. ft. of hydrogen are given off per ton of hydrolythe.

Storage. Hydrogen is usually stored in gas-holders under a pres- sure of some 9 in. of water. It is transported in steel cylinders under a pressure of some 2,000 lb./in. 2 One ton of cylinders will carry some 2,600 ft. 3 of gas at N.T.P. In Germany special Kesselwagen (tank trucks) carried 2,600 cub. ft. for a weight of one ton of tank (see T. A. Monckton, Hydrogen Manual, Parts I and 2, H. M. Stationery Office).

Helium. Helium is present in the atmosphere as -0004%. It is present in certain natural gases in proportions up to 2^5 %. The main supplies are, however, in the natural gas in Texas, where the strength is about 1-8%, and in Canada, near Ontario, where the purity is -3 %. The process of collection is by liquefaction of the gas and by regenerative distillation. The cost, therefore, varies almost inversely as the proportion of helium present in the gas. The cost of production in a large plant working in America is about 12 per 1,000 cubic feet.

Such technical detail as has been published is contained in: Reports to the Advisory Committee for Aeronautics and Reports to the Aeronautical -Research Committee; lectures to the Royal Aeronautical Society, published in Aeronautical Journal; two lec- tures to the British Association in 1919 and 1920; lecture by Air Commodore Maitland to the Royal Society of Arts; T. A. Monck- ton, Hydrogen Manual, Parts I and 2 (H.M. Stationery Office); va- rious articles in the German aeronautical press, mostly in Illustrierte Flugwoche, Luftweg and Luftfahrt; in the Italian in L' A eronautica and Cazzetta del Aviazione, and in the French in L Aeronautique.

(C. B. C.) AEROPLANE: see AERONAUTICS. AEROTHERAPEUTICS. The term " aerotherapeutics," as a special branch of medicine, might convey the idea that there are special diseases due to aviation which require special treatment.