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Aviation History
1943
1943 - 0584.PDF
FLIGHT MARCH 4TH, High-altitude Flight Part 111—Shell Design of the Cabin : The Pressure Regulating Valve and Supercharger Control Unit By W. NICHOLS, A.R.Ae.S. IN the previous articles many of the problems connected with high-altitude flight, both from ^commercial and military con- siderations came under review. The observations on pressure cabin design are herein continued. LOADS on the cabin skin due to pressurisation have cer-tain peculiarities which distinguish them from flight' or landing loads. The magnitude of the imposed loads will be known fairly accurately, and providing suitable safety valves are fitted, they cannot be appreciably exceeded. The accompanying graph shows approximately the required thickness of the skin for various radii of curvature, and, in addition, various design pressure differentials. An inherent feature of pressuri- sation is a demand for considerable refinement of the skin joints, and provision for sealing the joints im- poses limits, on the rivet spacing and patterns to be used. If the loads to be transmitted by the scam are relatively large, it is generally necessary to resort to multiple-row spacing to ensure a high-efficiency joint. It has been found that the most efficient method is to place the rows rela- tively close together. By this arrangement difference of stretch between the skin panels is reduced to a minimum, and unit loading in W-O3 UJ I-O2 z 01 V) 30 4O Skin thicknesses corresponding to 56,000 lb.per sq. in. ultimate strength. Rivet seam efficiency assumed 75 per cent, for variousdesign pressure differentials. latches, or other suitable means, which require to be freely distributed along the sides. . The second and more difficult problem is that due to distribution of the longitudinal loads around a doorway. Material at the top and bottom of the opening must be supplemented to replace that cut away, and it must be' extended sufficiently far fore-and-aft of the opening to redistribute the loads without setting up unduly large local stresses. The door itself can be made a relatively simple panel carrying loads as a beam simply supported at its front and rear edges. A door that opens inwardly will require but a simply latching mechanism, since pressure loads are carried bv the door frame directly, but this arrangement is not always convenient in service. Fur- thermore, the substitution of slid- ing doors reduces considerably the advantages of the former type. If the doors are arranged opening out- wards they will require a consider- able number of latches sufficiently strong and reliable to carry the pressure loads. On the Boeinp Stratoliner the main entry dooi\ all hatches, and emergency exits for passengers and crew, open in- wardly for the reason stated above. In the design of window frames, if glass is to be used, it is impor- tant to avoid high stresses which might warp the frames. With transparent plastic materials, due to their inherent low modulus of . ^ —-—- —* —-* _-— 5O 6O 7O RADIUS Inches) 80 9O the outermost rivets is not appreciably higher than the elasticity, the problem is to prevent distortion of the pane average for the entire pattern •'-•" • '" '•• '•' • *-* • • p average for the entire pattern. In the sketch are shown four typical joints. The two- and three-row spacing as at (a) and (b) are usually critical in shear, or the skin is critical in bearing; thus the strength can be calculated closely. With the four-row spacing, as at (c), the joint will develop about 75 per cent, of the nett sheet strength in tension unless gauges are sufficiently high to cause shear failure of the rivets. The spacing at (d) compares well with the four-row pa'ttern, and on account of the greater "pitch" in each row it is a better arrangement where intersections with other joints may occur. To obtain higher efficiencies re- quires still further complexity of pattern, rapidly depart- ing from the practical. Sealing Skin Joints American practice is to seal the joint with 1 /64m. thick special tape inserted between the laps, a double-row of rivets diagonally spaced at approximately fin. pitch being used for the normal skin seams. An opening in a cylindrical shell, if extending to an appreciable arc, presents two problems of a different nature. The more simple of the two is that of carrying the circumferential loads. The loads considered as being interrupted by the opening will be carried as tension in the ring and skin elements immediately forward and rear- ward of the opening. Beam structures above and below- must, therefore, be provided to effect this redistribution of load. For most normal doorways these members arc of moderate size and present no great difficulty in design. The inward force of the ring and skin due to tension the rings through support flanges, itself when subjected to pressure. A Plexiglass panel 14111. by i6in. by £in. thickness, when tested for deflec- tions and ultimate strength, showed a deformation at the centre portion of o.8in. at 5 lb./sq. in. pressure. Failure occurred at 8.5 lb. /sq. in. The edges of this panel were provided with cover strips of Jin. width. Full considera- tion must be given to changes of temperature and their effects upon sealing and thermal deformations. These points have received attention in the method adopted by the junkers Co., for mounting a domed or cupola window to the pressure cabin. A sketch of this is shown on the next page. Rivet Patterns <» te™s of diameters). Examples covering reasonable range of joint efficiencies.
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