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Military


Airship Fabrics

The outer cover of a rigid airship has to form a smooth fairing over the hull structure and gasbags. Unless it remains taut under all conditions the passage of air over it and more particularly the disturbed air in the vicinity of the airscrews gives rise to flapping, which not only increases the ship's resistance but may cause the cover to chafe and ultimately be torn. The tautness is produced and maintained by a dope, applied to the fabric partly before and partly after the sheets of fabric are laced to the null framework. The dope is generally similar to that used on aeroplane wings, but the unsupported expanses of fabric are so large- usually 3 metres by 5 metres-that the prevention of flapping is a much more difficult problem; indeed, these surfaces are so large that the maintenance of a correct difference of pressure between the inside and the outside of the ship is more effective than exactly correct tautness. The weight of the outer cover is such a large proportion of the total of the ship that very great care must be taken to apply only the minimum of dope necessary.

The outer surface must be made reflecting in order to reduce as far as possible the amount of radiant heat absorbed and transmitted to the gas in the cells and the air inside the hull. The pigment or dye employed in the dope must be such that the part of the light which most rapidly deteriorates the cellulose of the 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 90grms. sq. meter, although cotton, mercerised as thread before weaving, appears to have some advantages owing to its great uniformity of contraction when doped.

Gasbag fabric must primarily have good gasholding properties for the minimum weight. The strength need only be sufficient to withstand 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 gastight when in good condition. The skins vary in size, but, allowing for overlaps, 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 eastightness 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 adhesive just when good adhesion is most essential.

German experts were 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 appeared satisfactory even under the most extreme tropical heat.

The envelope fabric of a non-rigid or semi-rigid ship, in addition to being gastight, must have an outer surface capable of giving protection against 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 accumulation of pressure at the upper end, and if for any reason, such as a rapid rise, the pilot allows the pressure to become excessive the tension in the envelope is more likely to approach the safe maximum than from any other caase. The tension induced by internal pressure is, therefore, the main consideration and must be regarded as a load that, 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 lb./in./min. -the load reached before failure is 10 to 20% higher than the load reached with the comparatively slow rate of 30 lb./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 material, 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 i in. long across the direction of tension the 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 threads at 450 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 reinforcement depends upon the comparative strength of the diagonal ply and upon the nature of the material with which the plies are stuck together. Glue, being a much more rigid adhesive, will allow of practically 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 snow 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 protection against the effects of moisture is a matter of considerable difficulty which has only recently been achieved with any degree of success in compound films now being developed.

Goldbeaters' skin is almost equally good, out 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.





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Page last modified: 11-07-2011 15:29:10 ZULU