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Aviation History
1920
1920 - 0598.PDF
JUNE 3, 1920 A certain number of heavy landings were made throughout thewar in Germany, but if the matter is looked into these heavy landings can be traced to war emergencies or to the inex-perienced captains of ships. In Germany, partly owing to its proximity to England, rigids were not used in a legitimateway, and their bombing raids produced such a high percentage of mprtality in their flying crews that much difficulty was Small German direct-driven power car experienced in maintaining highly trained and experiencedflying personnel. The question of car suspensions in general is one of con-siderable difficulty and complexity. The arrangement has naturally to be simple and of low head resistance. It has toprovide for the ship taking up a considerable angle of pitch some.35 to 45 degrees up or down). With the present largegeared-down pusher propeller, it is, besides, a matter of some difficulty to arrange a simple and reliable method of takingthe thrust. In the most recent power units this has to some extent been overcome by the increase in speed of rigids andgreat reduction in size of the power egg. This allows much smaller direct-drive propellers to be fitted without undue•blanketing. If the car is designed to have considerable overall height aft, it is possible to take the thrust by means ofa wire attached to the after end of the girder which runs along the top of the car. This wire, which just clears the propeller,is led aft and attached to the hull. At first sight this type of car would appear to be of some-what high head resistance, because, although well streamlined in plan, it is badly streamlined in elevation. It appears,however, that when fitting a small appendage to a large streamline body, the inner side of the appendage should notbe streamlined, but should lie parallel to the large streamlined body to which it is attached. This, it is seen, is compliedwith in the car shown in diagram, which is streamlined in plan and underneath, but the top is roughly parallel to theunderside of the ship, and consequently does not cause as much disturbance in passing through the air as would be thecase if it had been perfectly streamlimed. Power Units Naturally, the three main points to be achieved in designing the power units for a rigid airship are :— (1) Reliability. (2) Low petrol consumption per unit of thrust-horse-power. (3) Low head resistance. Assuming that heavy duty reliable engines are available of. variQ»s horse-powers, several points arise. In the first place, the greater the number of engines the greater the reliability, and, generally speaking, the greater the efficiency of the propellers, owing to the smaller horse-power of each unit. At the same time, the head resistance and weight of the installation will be materially increased, also. Each additional power unit requires at least two engineers, one on duty and one off, complete with their food, parachutes, sleeping ac- commodation, etc. It is seen, therefore, that there are important reasons for not fitting more power units than are absolutely necessary from the point of view of :— (a) Not concentrating too much weight at one point. (6) Not increasing the horse-power of each unit to such an extent that the propeller becomes too inefficient. (c) Maintaining the necessary degree of reliability, having regard to the probable reliability of each power unit as a whole. Where the horse-power required is not too great, a vertical engine is most suitable and easy to instal. Two clutches are at present required, one a dog clutch, the other a multiple disc. A reverse gear should be fitted to at least one:half of the power units for assisting in landing and mooring. Generally speaking, great efficiency is obtained with a large geared-down propeller. On the other hand, no very satisfactory form of gearing is available. Epicyclic gearing, although occupying little room and weighing much less than the ordinary spur gear, has not so far proved to be sufficiently reliable, and the ordinary tooth gearing takes up a good deal of room and is very heavy, the non-reversing gear and trans- mission in R 33 weighing as much as 54 per cent, of the weight of the engine, and for the reversing units 66 per cent. In practice, with a fast rigid of some 2,000,000 cubic ft. capacity, it is probably more efficient to fit very small stream- line power units, each unit developing from 250 to 300 h.p. and actuating a direct-drive two-bladed propeller, except in those cars where it is necessary to fit a reverse. A brake should be fitted to the transmission shaft between the pro- peller and the clutch, so that the propeller can be allowed to Si Si Si Friedrichshafen :General view from an airship SS Si Si Si Si Si 598
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