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gas cells is eliminated as far as possible. A certain amount of light is necessary in the keel, and this usually enters through the bottom two strakes of outer cover on which a transparent dope is used. The surface of the dope should be water-repellent in order to reduce the weight of water taken up in a rainstorm.

The fabric usually employed for the outer cover is linen weighing about 90 grms. sq. metre, although cotton, mercerised as thread before weaving, appears to have some advantages owing to its great uni- formity of contraction when doped.

Gasbag fabric must primarily have good gasholding properties for the minimum weight. The strength need only be sufficient to with- stand handling when the bags are being placed in the ship or are moving slightly with change of fullness.

Goldbeaters' skin a thin membrane from the caecum of the ox although easily permeable to moisture is extremely gaslight when in good condition. The skins vary in size, but, allowing for over- laps, each skin covers about 10 in. by 4 in. In English gasbags the skins are attached to the fabric by rubber solution, as this gives rather better gastightness for a given weight. The German method is to build up the skins into large sheets some 10 metres wide and of length equal to the circumference of the bag. Fabric is then stuck to these sheets with a form of gelatine adhesive. Skin contracts as it dries, whereas fabric contracts as it absorbs moisture; great care has, therefore, to be taken that the fabric is attached to the skin sheet under correct humidity condition. The fabric in which rubber is used as the adhesive is found to give trouble in hot climates, owing to the serious contraction of the skins and the softening of the adhe- sive just when good adhesion is most essential.

German experts are strongly of the opinion that the use of rubber in gasbags forms a non-conducting surface apt to become electrically charged by friction or in the vicinity of an electric storm. The use of rubber has, therefore, been abandoned in Germany since very early days.

Fabric made with glue adhesive appears satisfactory even under the most extreme tropical heat.

The envelope fabric of a non-rigid or semi-rigid ship, in addition to being gaslight, must have an outer surface capable of giving pro- leclion againsl light and heat. It is also called upon to take very considerable tensile stresses. These are due partly to local tensions in the neighbourhood of rigging attachments; partly to a bending of the envelope as a whole, but mainly to the internal pressure which is necessary in order to maintain the shape of this class of ship. When the ship takes up a steep angle of pitch there is considerable accumu- lation of pressure at the upper end, and if for any reason, such as a rapid rise, Ihe pilot allows the pressure lo become excessive Ihe len- sion in Ihe envelope is more likely to approach the safe maximum than from any other cause. The tension induced by internal pres- sure is, therefore, the main consideration and must be regarded as a load thai, although not very suddenly applied the interval between normal and maximum being at least 15 seconds cannot be expected to be maintained for long periods say, more than 15 minutes. The resistance of fabric to tension varies greatly with the rate at which the load is applied. For a high rate of loading say, 150 Ib./in./min. the load reached before failure is 10 to 20% higher than the load reached with the comparatively slow rate of 30 Ib./in./min. or less.

A load sustained for really long periods gives lower strength still. A load of only 50 to 60% of that which the material will stand for, say, 10 minutes will break it after a week.

Considerably more investigation on these points is still required, but they are probably due to the manner of failure of a woven mate- rial, being one of gradual slipping of the fibres of the twisted thread.

A small local cut produces considerable reduction of tensile strength of an ordinary fabric. This is due to the concentration of stress at the ends of the cut causing the failure of individual threads in succession. Provided the cut is more than J in. long across Ihe direclion of tension Ihe reduction of strength is to some 30 % to 40 % of the unwounded strength and is no greater until the size of the cut is such that it becomes an important proportion of the whole width of fabric in tension. In order to reduce this loss of strength fabric exposed to serious tension is usually made of 2 or 3 plies, of which one has its Ihreads at 45 to those of the other plies which lie along and normal to the direction of tension. The threads of the diagonal ply help to redistribute the concentration of stress at the ends of the cut. The extent of this reinforcemenl depends upon Ihe comparalive slrength of the diagonal ply and upon the nature of the material with which the plies are stuck together. The table shows with an accuracy of about 5 % the wounded and unwounded strengths of typical airship fabrics built up of one or more plies of the same cotton and expressed as percentages of that of single ply, the adhesive being in each case rubber.

Fabric

Slrength unwounded

Strength wounded

Single ply 2-ply parallel 2-ply diagonal. . . 3-ply parallel. 3-ply centre-ply diagonal

IOO 210

125

315 240

40 70 90 no 1 20

Rubber is particularly suilable as a doubling adhesive as it allows

the requisite movement of threads for the reinforcement to take place. Glue, being a much more rigid adhesive, will allow of prac- tically no reinforcing action by the diagonal ply.

Rubber is also a reasonably good gasproofing material and as it combines these two qualities it is almost universally employed in the construction of non-rigid airship envelopes. The fabric used for the envelopes of the N.S. airship was made of three plies of a cotton weighing 80 grms./sq. metre. The outer surface as a protection from light and heat was of 50 grms. of rubber containing a proportion of black litharge and a surface of aluminium powder. Between the outer and diagonal ply was 30 grms. of rubber and between the diagonal and inner ply 100 grms. of rubber as a gastight layer; some more recent experiments show that additional protection is given to the rubber by staining it with a suitable red dye.

Gastightness of most materials decreases considerably (4 or 5% per degree Centigrade) with increases of temperature.

A film of gelatine gives the greatest gastightness for a given weight, but its proteclion against the effects of moisture is a matter of con- siderable difficulty which has only recently been achieved with any degree of success in compound films now being developed.

Goldbeaters' skin is almost equally good, but is liable to small local defects caused in the process of preparation and building up.

An extract of the plum, cordia myxa or Turkish birdlime, has given satisfactory results in some respects, but its use has not been very fully developed.

It is important to realize that gastight fabric for airships must primarily stop the leakage of air into the gas. Loss of hydrogen is too small to be important, but the ingress of a weight of air definitely reduces the useful lift of the ship by an equal weight and this can only be partially got rid of even by the discharge of many times the volume of gas.

Airship Machinery. In the early days the machinery of airships and aeroplanes had to be extremely light. As development pro- ceeded, 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 difficullies of supply during Ihe war caused the airships to use, not a special engine suitable for this requirement, but stand- ard 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 com- pared with the airship which flies TOO hrs. on one load of fuel. A machinery installation which weighs, say, 5 Ib. per H.P. burns 0-5 Ib. 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 flighl 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, magne- tos changed and retimed, etc., during long flights). The machinery must therefore be arranged so that advantage can be taken of this possibility.

Arrangement of Power Units. 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 arrangemenl 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 mounled a large two-bladed airscrew. In R38 350 B.H.P. is transmitted through a 3-3:1 reduc- tion 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 praclice 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 1 00% 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 pur- poses. 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.

Jn the British airships constructed during the war there was no intermediate shafting, the airscrew being mounted on the engine.