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Aviation History
1931
1931 - 0522.PDF
FLIGHT, MAY 29, 1931 ships is in general about 1J in. H2O. This can easily bemaintained mechanically, but is also easily subjected to atmospheric variations.This small pressure corresponds to only 2° F. change in temperature and 0.11 in. of Hg. change in pressure,or 100 ft. change in altitude. This comparison shows the sensitiveness of pressure to outside atmospheric conditionsand the necessity for careful handling. Maintenance of Pressure Excess pressure is handled by automatic valves whichare designed for maximum rate of ascent and set so that air valves open first and the gas valves follow, preservinggas as much as possible. ' Manual control of the valve is also provided. In non-rigids the pressure is maintained by scoopslowered manually in the slipstream. The drop in pres- sure to or below atmospheric will cause the collapse ofenvelope and the necessity of abandoning the forward speed. The maintenance of too high a pressure produceshigh tensions in the envelope, causing rapid deterioration. The semi-rigid airship was evolved to relieve tensionload in the fabric, and this is achieved by introducing a rigid keel for carrying a high percentage of the bendingload. The opinion is often expressed that rigid airships areindependent of pressure. This opinion is erroneous, because the rigid airships are not, and a series of openingsare provided to keep internal pressure as close as possible to atmospheric. Some of the rigid airships tend toutilise the effect of pressure for the support of fabric. For instance, the R.101 was normally operated under slightpositive pressure to reduce napping of fabric covering, and openings in the envelope to satisfy these conditions wereprovided. Pressure as a Design Factor The pressure within a gas container increases upwarddue to the difference of the densities of air and gas. This property is called " gas head," and is similar to " waterhead," but of an opposite sign. If we call h the head of gas measured upward, and kthe unit lift of gas, the pressure due to gas head becomes kh.This pressure produces constant longitudinal force which in case of fully inflated circular airship is K-tr^h where r isthe radius of cross section. The pressure due to gas head increases toward the tonthe airship, therefore the resultant of this longitudinal force acts above the centre line, which usually coincVIwith the neutral axis of the airship. Consequently it wnitend to produce a hogging bending moment, which fn fully inflated ship is equal to knr3hj4. Whereas in non-rigids the transverse component of presure produces uniform transverse tension in the coverii]5 in rigid airships this transverse component ax:ts as a sideload on longitudinals, complicating their design by loading them with side load combined with direct stresses due tobending of the whole airship. This loading condition of longitudinals tends to explain why gas pressure is oftencalled a liability in the case of conventional rigid airships The diameter of modern airships has already reached132 ft. In this case the pressure due to gas head at the top of the hull at the maximum section becomes for ahelium filled ship: 820 lb. per sq. ft. or 1.57 in. H2O. The total longitudinal force becomes 57,000 lb. and thehogging moment 950,000 ft.-lb. These gas head pressures produce forces and moments ofsuch magnitudes that the designer should utilise them to his advantage. The longitudinal force is the most helpfulbecause it tends to produce a uniform tension throughout the structure, and all materials used in airships can earn,higher tensile loads than compression loads. It is of interest to study the help derived from the longitudinaltension on the hull. In an airship whose cross-sectional area of metal usedis A, which is distributed uniformly around the cross section, the moment of inertia of this cross section isAr2/2, and if this airship is subjected to a longitudinal force P and bending moment M, then the maximum com-pressive longitudinal stress will equal S = P/A — 2Mr/Ar2 Assuming, as in the case of the non-rigid, the outercovering incapable of carrying any compressive loads or limiting the value of stress, 5 = 0, then solving the aboveequation we obtain the critical value of bending moment in relation to longitudinal force M/P — r/2. If the non-rigid airship is full of gas and is subjected to pressure due to gas head only, then the relation of gas pressure bendingmoment to longitudinal force due to gas pressure M/P = ft^/Akr' = rjA ; or the value of hogging bending moment can be doubled before compressive loads will occur in the fabric, and the The ZMC.2 just before her fitst trial flight on August 19, 1929. The crew were Capt. W. E. Kepner, Carl B. Fntsche, E. J. Hill, A. G. Schlosser, I. Bisho p. The duration of the flight was 49 min. 55 sec. 484 •..•••• -•:;=£:;-;-
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