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Aviation History
1920
1920 - 0207.PDF
FEBRUARY IQ, 1920 that all fittings on the bag which are, in addition, attachedto the structure of the ship, are arranged to allow for this movement without causing excessive tension in the fabric.This is in general accomplished by fitting cones or sleeves of fabric between the gasbag and the fitting (Fig. 11). Outer Covey The outer cover consists of a number of panels of dopedfabric stretched over the outside of the ship for the purpose of providing a fair surface and to protect the girderworkand gasbags from weathering and exposure (Fig. 12). The fabric may be either cotton or linen, the dope is usuallya cellulose acetate dope. The total weight of doped fabric should not exceed about 120 grams per sq. metre. Therequirements for the dope in rigid airships are essentially different for those for aeroplanes. It is of the utmost im-portance that the weight of the dope applied should be as small as possible, but at the same time waterproofness mustbe assured. Tautening is not of material importance. If the dope is applied to the fabric before tha latter is made upinto the ship, it must be sufficiently flexible not to crack when the fabric is folded. The most satisfactory procedure calculated, and the nature of the forces which come upon thepart. For items subjected to vibratory or shock loads, such as car-suspension wires, a high factor of safety isnecessary. The worst case that can generally occur is when the ship is either climbing or diving at its maximum rate,and a factor of safety of at least four to six should be obtained in the car suspensoins under either of these conditions. Onthe other hand, on items which cannot have a load in them beyond a certain maximum or when the assumptions madeare the most extreme, such as for instance, the gasbags, and longitudinals in the first case, or the transverse framesin the second case, a large factor of safety beyond the cal- culated load is not necessary or indeed desirable, as theweight of the part is increased thereby. It is difficult, however, to generalise on what the factorof safety should be, and the choice of any particular figure usually depends on the individual designer and previousexperience. Handling and Mooring ArrangementsWhen the ship is in the shed and partially deflated, itjis desirable to support her as much as possible in order toprevent deformation of hull. This is done by fitting strops Fig. 12.—R 34, showing outer cover being laced into place before sealing strips are put on is to apply the preliminary coats of dope by machine beforethe fabric is made up, and the final coat when the fabric is in place on the ship.The colour of the dope depends upon the service to which the ship is being put. For tropical work zinc white oraluminium powder is preferred, owing to the necessity of avoiding superheating of the gas as much as possible, and alsoto protect the fabric against tendering. During the War the Germans coated the under portion of their ships with ablack dye dope, and the upper surface a mottled colour. This, of course, was for night work.When excessive pressure is likely to occur on the outer cover, as for instance forward owing to the air pressure onthe bow, or in the wake of the propellers, wire or girder stiffening should be provided for support. Formerly heaviercovers were fitted in these positions, but this has been found not to be necessary. Factors of SafetyThe factors of safety to be given to the various items of an airship naturally depend on the degree of reliability whichmay be placed upon the assumptions, the probability that the stress in the part concerned may become greater than that to the joints of the top longitudinals and main transverseframes, and slinging the ship from these to the top of the shed. Lugs are also provided at the inboard elevator pintles forthe same purpose. The cars are supported in cradles from the ground.For handling the ship on the ground, arrangements must be made for a trail rope forward and aft which can be droppedwhen the ship is within 500 ft. of the ground. These trail ropes, which should be as far forward and as far aft as possibleconsistent with the strength of the ship, must be capable of being released from the control car.J In addition, at frequentintervals along the ship, handling wires should be fitted about 25 ft. long with a bight on the end, to which toggles ropes tanbe attached. These arrangements should allow for the greatest restraining moment to be put on the ship with theleast force. If the ship is to be moored to a mast, special stiffening inaddition is necessary in the bow. As the greater pull comes on the ship at the forward trail rope, special care is necessaryto distribute this load as equally as possible from this point, throughout the structure. This is accomplished in Zeppelinships by attaching the mooring point to a wire system whereby 207 F 2
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