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Aviation History
1920
1920 - 0206.PDF
FEBRUARY 19, 1920 Fig. 10.—Gas-bags in place in R 34, showing " flat " formed at bottom of bag when partially deflated dependent upon the size of the bag and maximum pressure.The standard cotton fabrics in this country are as shown in Table IV. ; TABLE IV Weight. Strength, grins, sq. metre. kilograms/metre width.Quality.A B Bx € Ck DE 130 110 <¥> 80 80 65 45 1250 1100 goo 800 900 650 — IJ 3iUAn interesting fabric which was used by the Germans in L 70 was jap silk, having a weight of 30-35 grams/sq. metreand a strength of 435 kilograms per metre. The fabric used in R 33 class airships was Bx cotton weighing90 grams per sq. metre. The maximum tension in the fabric is assumed to be givenby the relationship T — pli kg./metre where p = pressure of gas in kilograms per sq. metre. R = radius of bag in metres. Each gasbag in the ship must be provided with an auto- matic valve to enable the gas to escape when the ship is . rising and so prevent excess pressure in the bag (Fig. 8). The position of this valve must be such that it is unaffected by changes in the air pressure on the outer cover and is also easily accessible. This is attained in recent ships by fitting the valve on the flat end of the bag just above the corridor, and carrying off the gas to the top of the ship by means of a ventilation trunk between the gasbags. In R 23 class, valves were fitted on the outside horizontal diameter, and trouble was experienced in these ships through the forward valves blowing off at other than the designed j^ressure. The initial setting of the automatic valve and the strength of spring and opening should be such that when the ship is rising at her maximum rate the pressure at the top of the bag does not become excessive. The valve, as fitted in the parallel body of R 33, was set to blow off initially at 14 mm. of water, the tension of the spring and opening being such that when the ship was rising vertically, at say 2,300 ft. per minute, the maximum blowing off pressure was approxi- mately 25 mm. of water, corresponding to a pressure at the top of the bag of approximately 46 kg. sq. metre. In addition to the automatic valve a manoeuvring or hand- operated valve (Fig. 9) is usually fitted at the top of some of the bags, chiefly for the purpose of enabling the pilot to valve gas in landing should the ship become too light. As a general rule these manoeuvring valves should not be fitted to the after bags, as it is desirable with the very fine tails now used always to retain as much lift as possible aft. When the ship rises into air of less density and pressure than as stated above, the gas in the bags expands and blows off through the automatic valves. If the ship subsequently descends, although the total weight of gas contained within the bags is unaltered, the volume which it occupies is neces- sarily decreased. The bag consequently is only partly full and the bottom of the bag rises. The shape of the bag in this condition is remarkable, as the bottom forms an absolute " flat " and gives a very good indication of the degree of fulness of the bag (Fig. 10). Care must be taken -..::' 0A3 BAC -HfCKTO CAS BAG CONE - FLAX CORD BINDING 9TORS HERE SPRING HOOK 'HOLE FOR -468 O/A.l ARRANGEMENT OF GLAND FITTIMCS FOR SHEAR WIRES AT UPPE.P END Fig. 11.—Showing method of fitting gas-bag sleeve on cone 206
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