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Military


Airship Machinery

In the early days the machinery of airships and aeroplanes had to be extremely light. As development proceeded, the greater length of flight of the airship made fuel economy and some other characteristics of greater importance in the airship than in the aeroplane. In England neglect of airships before the war followed by difficulties of supply during the war caused the airships to use, not a special engine suitable for this requirement, but standard aeroplane engines. This general unsuitability of the engines used for airship work caused the machinery to be by far the most unreliable part of the airship as a patrol unit.

The advent of the commercial aeroplane for long flights is in turn bringing a requirement more nearly that of the airship. Even so, an aeroplane which flies 10 hrs. before refuelling must be compared with the airship which flies 100 hrs. on one load of fuel. A machinery installation which weighs, say, 5 lb. per H.P. burns 0-5 lb. of fuel per H.P. in one hour. An aeroplane in 10 hrs. will burn a weight of fuel equal to that of its machinery. In 100 hrs. an airship will burn ten times its machinery weight. The importance of saving fuel even at the expense of increased machinery weight is therefore much greater in the airship. During much of the airship's flight some engines are run at considerably less than their full power, thus introducing the need for good fuel economy at reduced power. In an airship repairs of some magnitude can be made in flight (a cylinder has been changed, cracked water-jackets patched, magnetos changed and retimed, etc., during long flights). The machinery must therefore be arranged so that advantage can be taken of this possibility.

The low speed of an airship renders desirable a larger airscrew than in the faster aeroplane. Moreover, airscrew size is not restricted by the consideration of landing as in the aeroplane. The large airscrew makes for fuel economy, and this being cardinal has been found to justify the use of reduction gearing. The most efficient arrangement for a rigid airship includes a fly-wheel fitted to the crank-shaft of the engine driving, through a friction clutch, a gear reduction box on which is mounted a large two-bladed airscrew. In R38 350 B.H.P. is transmitted through a 3-3:1 reduction gear to a I7i-ft. airscrew, turning at 600 revs, for a ship's air speed of 60 knots. There is usually, in addition, a dog clutch and an airscrew brake, so that the airscrew can be disconnected and locked horizontal when landing. The departure from aeroplane practice is here notable.

In early airships it was usually necessary to mount the engines in the car and to transmit the power to airscrews carried on outriggers. The weight available for this transmission was so small that there was frequent trouble, which could mostly be traced to resonance at some speed within the very wide range (often from 100% to 50% of the revolutions for full speed) over which the airship engine was driven.

Belts, chains, bevel-gear boxes with long lengths of shafting were used, but all gave trouble within a few hundred hours' flight.

German rigid airships derived great benefit from the Maybach engine, which was developed at the same time as the ship's designs progressed, and was devised primarily to be suitable for airship purposes. It departed from other aero-engine practice in many respects, and though it was not till quite late in the war that a modified type of a Maybach was used in aeroplanes, the German industry gained the earliest experience of large light-weight engines.

In the British airships constructed during the war there was no intermediate shafting, the airscrew being mounted on the engine. 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 airscrew 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 shafting 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 verticalforce independent of the headway of the ship was often very valuable to the then comparatively inexperienced pilots under the bad landing facilities then existing.

Though engine failure had 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.

The earliest form of winch used for Kite-Balloons 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 modifications in detail, as the standard for all future winches.

The later winches were usually driven by petrol engines independent 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 lik e 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.



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Page last modified: 11-07-2011 15:29:13 ZULU