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Aviation History
1919
1919 - 1258.PDF
AIRSHIPS* By Wing-Commander T. R. CAVE-BROWNE-CAVE, CJB.E., R.AJ. Fabrics THE principal difficulty which is experienced with the use of airships for long periods, more particularly in tropical countries, is the deterioration of the fabric under the action of sunlight. This deterioration takes two forms—firstly, the strength of the textile material is decreased, and, secondly, the gas-holding properties of the proofing material are destroyed. The rigid airship contains a large number of gasbags formed of the lightest possible gastight material. This is usually a very light cotton cloth proofed with goldbeaters skin stuck to it by means of rubber solution. The skins are then varnished as a protection against moisture. The actual tensile strength of this fabric is only sufficient to prevent damage during the motion of the gasbags in the ship's frame work. The outer surface of the ship is formed by an outer cover of linen of much the same strength as that used for aeroplane wings. The duties which this outer cover has to perform are very important. It has to be stretched sufficiently tight to prevent any flapping or appreciable deformation during the passage of the ship through the air. It must be waterproof, and also water-repellant, in order to reduce the weight added to the ship by a shower of rain. A further function, and one very difficult to fulfil, is the protection that it has to afford to the internal gasbags. The outer surface of non-rigid ships is made with a shining aluminium finish so that as much as possible of the heat and light is reflected. The rubber immediately under the surface is made to contain considerable more litharge than otherwise necessary as this material absorbs a large proportion of that light which passes the reflecting surface. A similar system is being followed with the outer covers of rigid airships, and judging by the excellent results obtained from somewhat similar methods employed on aeroplane fabric, the protection both to the fabric of the cover and to the internal bag, should be very good. Experience with rigid outer covers is at present small, but it has been found that with non-rigid envelopes made by our latest methods of proofing, the deterioration of the outer surface becomes clearly visible before any serious loss of strength has taken place. The envelope of a non-rigid ship is formed of layers of fabric which have to be of considerable strength in order to resist the small internal pressure necessary to maintain the rigidity of the ship. Between these layers of fabric is placed one or more layers of gas-tight rubber. The Effect of Wounds The strength of a fabric envelope can be calculated with considerable accuracy if it is assumed that the fabric is undamaged. The reduction of strength caused by a small local wound is, however, very considerable, because such a wound causes concentration of stress at the edges of the hole, and there is a tendency for the fabric to rip. This dangerous concentration of stress can be very largely avoided in a two-ply fabric if the threads of one ply are laid at 45 deg. to those of the other. The threads of the diagonal ply then form an effective means of distributing the stress, and also when tearing actually begins tend to form together into a bunch which exerts considerable concentrated resistance to the extension of the tear. The following table shows the reduction of strength caused by a J-in. wound cut in a 6-in. strip in a direction at right angles to the direction of the stress :— Strength wounded. Fabric. Strength unwounded. Single-ply cotton .. .. .. .. 0-39 Single-ply linen .. .. .. ..0-52 2-ply parallel CC .. . . .. .. o -33 2-ply diagonal BD. .. .. .. o -57 2-ply diagonal CC. .. .. .. o -72 3-ply diagonal CCC. .. .. .. o-6g The strength of the fabric used in the envelope of the " N.S." airships is 1,770 kilos per metre, i.e., 100 lbs. per inch. The stress in an airship envelope is very largely determined by the internal pressure which it is necessafy to maintain in order to keep correct shape. The circumferential stress which this pressure produces in the fabric is, like that in an ordinary * Lecture delivered to Engineering Section " G " British Association. boiler shell, about twice that in the longitudinal direction. Ordinary fabric is usually of about equal strength in the warp and weft directions, and in order to resist the circumferential tension the envelope has to be made with a quite unnecessary longitudinal strength with,a consequent increased weight. It has been found possible to reinforce an envelope by means of flat bundles of string placed circumferentially round the envelope at suitable intervals, so that these strings con tribute the difference between the circumferential and longitudinal tensions. The fabric, therefore, need only be made sufficiently strong to take the longitudinal tension. This system of re-inforcement has not at present been tried on a full-size envelope, but was found to give very satisfactory results when adopted on 20-ft. model envelopes which were tested to destruction. In considering the best material for envelope reinforcement, very careful attention had to be directed to the tensile strength of a material compared with its weight for a given length. This was found to be most conveniently represented by the length of the strand which would hang vertically down with out breaking under its own weight. This was termed the breaking length of the material. Values for some usual materials are given in the table :— Material. Thread, 3-strand cotton Thread, 3-strand flax .. Thread, 3-strand hemp Pains light rocket cord. . Best Italian hemp cord Airship cotton fabric Aeroplane linen fabric .. Envelope fabric (proofed) Re-inforcing string tape Aluminium 10 tons/in. Duralumin 24 ton/in. .. Mild steel 30 tons/in. .. H/T steel 100 tons/in. .. Breaking length in metres. 12,600 .. 15,900 12,800 28,600 .. 27,500 10,000 14,000 4,000 12,600 6,400 14,400 . , . . 6,200 20,600 Gas Tightness The gastightness of fabrics and of seams formed in them is a matter which has been very thoroughly dealt with in papers to the Advisor ' Committee for Aeronautics, which it is under stood will be published very shortlj'. It may, however, be said that the amount of hydrogen which leaks out through the fabric of an airship is a matter of purely nominal import ance with fabrics of the excellence now available. The matter of real importance is the passage of air through the fabric into the gas space. This air reduces the purity of the gas, and unless very large quantities of gas are consumed by the ship by discharge through the valves, it is impossible to eliminate this air sufficient! ' to maintain the gas purity which is necessary to give the ship her required lift. An interesting point which has been clearly established is that the air which diffuses into the gas space tends to collect at the bottom of the envelope, more particularly in trilobe ships where the circulation of gas is less free than in circular envelopes. Samples of gas taken from within 12 in. or so of the bottom of the envelope show a hydrogen purity of some 1 per cent, lower than that in the rest of the envelope. Further, the ratio of oxygen to nitrogen in this impurity shows that the air has come in by diffusion and not by mechanical leakage. This puddle of impure gas only- collects while the ship is at rest in the shed, and is quickly dissipated as soon as flight is started. Some of the chemical and physical changes which accom pany the loss of gas-tightness of a rubber-proofed fabric are at present very obscure. Loss of gas-tightness appears to proceed very slowly for a considerable time, and then to become very rapid, almost in the nature of a sudden collapse. In some cases the gas-tightness is actually found to increase for a short time after the fabric has been made. Temperature Effects. The effect of the sun's radiation on the outer surface of the ship produces other important results. It raises the temperature of the gas and the air contained within the outer envelope to a point considerably above that of the surrounding air. This results in what is termed " false lift." Immediately the strength of the sun's rays decreases the superheat begins to disappear, and the false lift of the ship is reduced. The extent of this superheat sometimes amounts to 30 deg. F. to 35 deg. F., corresponding to a loss of some I26o
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