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Aviation History
1938
1938 - 1272.PDF
446 FLIGHT. MAY 5, 1938. HIGH-ALTITUDE transport aeroplanes designedwith pressure cabins for long-distance passengerservice in the sub-stratosphere (20,000ft. to about 30,000ft.) are under construction in practically all the major aircraft factories. Two of these sub-stratosphere liners, the Boeing Model 307 transport and the Douglas Model DC-4, will be flown this summer. The most in- teresting feature of these developments is that they are production jobs, and not entirely experimental. How- ever, the results of nearly twenty years of research, mostly by the Air Corps at Wright Field, culminating in the Air Corps' Lockheed XC-35 sub-stratosphere aeroplane, has been available as the basis for commercial design. The success of the XC-35, which has been flying regularly for nearly a year, as a practical passenger aeroplane for service at 30,000ft. altitude, ha% undoubtedly been the impetus to the present commercial design trend." This paragraph from the lecture delivered to the Royal Aero- nautical Society on April 21 by Prof. J. E. Younger, of the University of California, should give us something to think about. Not only has America two machines designed for high altitude flying which will be in the air this year, but nearly all . her main aircraft factories are producing such machines. The second important point is that the research which has made this progress possible was begun as long ago as 1920. Prof. Younger dealt with some 20 principal problems encountered in the design of aeroplanes for passenger service at heights of 20,000ft. to 30,000ft,, and although the main problems could be said to have been solved, he regarded the work done as only an " entering wedge," and pointed out that there was an enormous amount of work yet to be done before sub-stratosphere flying could become daily routine. For the purpose of his lecture, Prof. Younger classified the main engineering problems into the following:— 1. Structural problems; such as type of structure, strength, airtight joints, expansion, etc. 2. Mechanical problems; such as door and window design, control glands, de-icing, de-frosting, etc. 3. Air flow regulation; control and safety valves, free from noise and possibility of freezing, etc. 4. Physiological; such as volume and pressure required, pressure fluctuation permissible, etc. 5. Air supply; superchargers, mechanically or turbo- driven. 6. Fuel supply problems. Space does not, unfortunately, permit of giving in detail the lecturer's account of the history of the development of sub-stratosphere aeroplanes, but it is interesting to recall that the work was begun by the U.S. Army Air Corps at McCook Field in 1920 when an oval steel tank was fitted into the fuselage of a De Havilland biplane and a wind-driven gear supercharger provided pressure for the tank. Investigations were revived at Wright Field in 1935. This work was com- pleted in the spring of 1936, and the Lockheed XC^35 experi- mental aeroplane was completed by that company in the spring of 1937 The XC-35 has done a lot of flying since and has provided valuable data. In the meantime T.W.A., with Mr. D. \V. Tomlinson in <-FUSELAGE SKM icv\%\Avvvx'*r^T v^^ (A) FUSELAGE SKIN BULKHEAD SUB-STRATOSPHEK charge, were conducting flight tests with the object of choosing the best method of supercharging at 20,000ft. and above, to equip an engine for rated power operation at 30,000ft., to determine the actual increase of speed with altitude as a check against theory, and to determine by overweather cross-country flights the force and direction of winds. Structure.—Prof. Younger examined the different types of structure likely to be suitable for pressure cabins, and came to the conclusion that the standard type of semi-monocoque fuse- lage as used in most modern aircraft was the most logical form PATH OF' ESCAPING AIR Bulkheads designed to allow for expansion of the skin. SEA.LCO WALL Of OOOR VALVE SEAT - DURALUMIN ' OOOR HANDLE STEEL COP ATTACHED TO DOOR. HANDLE AIR HOLES IN STEEL CUP ROUND DISCHARGE HOLE STEEL CUP DISC OF SPONGE. RUBBER DISC OF OIL SOAKED LEATHER A release valve incorporated with door handle to relieve pressure before door is opened. of construction. The fuselage in this case should be of circular cross-section for minimum weight and should have ends as nearly hemispherical as possible. He thought little weight need be added to a fuselage of 6ft. to iof.t. in diameter for internal pressures giving 10,000ft. conditions at 30,000ft., a pressure difference of approximately 5 lb./sq. in. On the question of design factors, Prof. Younger said it seemed to him illogical to use a fixed design factor of two and a half times the differential pressure, as with safety valves it would be almost impossible to assume that a pressure in the cabin of two and a half, times the normal design pressure could ever occur at the same time that the maximum aero- dynamic design conditions were being realised. He recom- mended that the basic design pressure only should be used in connection with the usual load factors required in the struc- tural design of the aeroplane. For a commercial design he recommended a design pressure factor of one and a half to two times the basic differential pressure. For instance, in an aeroplane designed to fly at 25,ooof.t. with sea level pressure maintained inside the cabin, the differential pressure would be about 8.2 lb./sq. in., and for the design of the cabin this difference should have to be multiplied by a factor of 1.5 or 2. The problem of airtight joints in the shell plating of the fuselage was not difficult. The type of joint used in flying boat hulls was considered adequate. This joint, of course, is made by placing a strip of fabric soaked in marine glue or other sealing compound between the two edges o£ the metal sheets. Prof. Younger considered that if necessary the rivets could be divided into two classes, the larger for strength and the smaller for sealing. If a finished joint leaked slightly a coat or two of marine glue or other bitumastic non-dryiug paint would usually make the joint satisfactory. Expansion Problems.—Expansion of the skin under pres- sure was not serious and could be calculated quite accurately- Bulkheads produced the most serious problems, and the lec- turer suggested as an alternative to the hemispherical type, which was difficult to make, a conical type with flexible attach- ment to the fuselage skin. Forms of bulkhead attachments that had been found satisfactory are shown in one of. the illustrations. The question of the possibility of an explosion occurring should the fuselage skin be punctured for any reason naturally presented itself for consideration. As this could only happen when the stresses were near the ultimate strength, the lecturer
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