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Aviation History
1958
1958 - 0476.PDF
492 FLIGHT ORIGINS OF THE MODERN AIRLINER... construction adopted by the Americans for the "modern" airliner.In the early 1930s, the new monoplanes had wing structures which derived directly from Rohrbach's box spar. Further develop-ment of this form of construction was, however, to be consider- ably influenced by the work of another German—and associate ofRohrbach's—Herbert Wagner, who produced his tension field beam theory in 1929.The Boeing single-engine transport proposals gradually crystal- lized and emerged as the Model 200 which was known as theMonomail and first flew at Seattle on May 22, 1930. Only two Monomails were built, the first as a mailplane and the second(the Model 221) as a six-passenger transport. The loaded weight in each case was 8a000 lb. The two aircraft were later convertedto carry eight passengers and were used for a time on scheduled services by Boeing Air Transport. The type did not go into pro-duction, however, because its single engine was not attractive for passenger carrying and the full performance potential of thelow-drag design could not be realized because of the limitations of the fixed-pitch propeller fitted to the 575 h.p. Pratt and WhitneyHornet air-cooled radial engine. Even so, the cruising speed was 135 m.p.h., which was significantly better than the 100-120 m.p.h.of airliners then in service. Even faster was the first production transport with a retract-able undercarriage, the single-engine Lockheed Orion, which, although it was made of wood, demonstrated the advantages ofaerodynamically clean design, higher wing loadings and mono- coque construction. It was a logical evolution of the LockheedVega of 1927 (designed by Allan Lockheed and Northrop) which had first demonstrated these features. The Orion went into ser-vice, between New York and Washington, in the autumn of 1931. Retractable undercarriages had been first tried experimentallyto reduce drag on racing aircraft in the early 1920s. Wiencziers even tried one before the First World War. The new Americantransport designs also featured tailwheels and wheel brakes, in place of the tail skids which had previously provided the onlyground-braking. Wheel brakes also had been foreshadowed experi- mentally in the 1920s and earlier. They now became a necessitybecause of the increased take-off and landing speeds of the more heavily-loaded monoplanes and because of the gradual appearanceof paved runways, themselves made necessary by heavier wheel- loadings and by the more critical take-off performance of the newtypes. As a follow-up of their work on the Monomail, the BoeingCompany developed the B-9 bomber for the U.S. Army Air Corps. This aircraft made use of the same structural techniquesand layout as the Monomail, but its two engines were mounted in front of the leading edges, sufficiently far out from the fuselage oneach side to give adequate propeller clearance. The undercarriage retracted into spaces behind die engine nacelles. This layout ofa low-wing monoplane—but with the familiar corrugated metal- covered structure and fixed undercarriage—appears to have beenfirst: tried for landplanes by the Junkers Company with their S-36 mailplane and the related K-37 bomber, built in Sweden in 1928.In tie thirties, it was to become the standard multi-engined configuration. The B-9 first flew on April 29,1931, but, after being thoroughlytested by the Air Corps, was not selected for production. The contract went instead to the rather later Martin B-10 of similardesign. Meanwhile, in mid-1932, Boeing were giving serious thought (in conjunction with their associate company United AirLines, which had just been formed out of an amalgamation of a number of earlier operators, including Boeing Air Transport) toa. civil development of the B-9 configuration. The result was the famous Boeing 247 transport, which made its first flight onFebruary 8, 1933, and started airline service less than two months later. The scale of American production, even at this early date,is shown by the fact that thirty aircraft of this type had been delivered to United Airlines by the end of June 1933. Unitedeventually had a fleet of sixty 247s, bought at a cost of $50,000 (£10,000) each. The original Boeing 247 weighed 12,650 1b loaded and accom-modated ten passengers in single seats arranged along each side of the soundproofed fuselage. This capacity was similar to that of theFokker and Ford Trimotors which the 247 replaced on the routes. There were individual windows for each passenger and a centralaisle. The fuselage had a rounded cross-section 5ft wide by 5ft high, but the two spars of the wing caused obstructions across thefloor of the passenger cabin. The all-metal structure was similar to that of the Monomail, but made use of an improved aluminiumalloy (24S instead of 17S), while the trussed wing spars were of alloy and chrome-molybdenum steel tubes. The design of thestructure benefited from the fact that it was based on more detailed requirements, which came into force at about this time, for thecalculation of the strength of components. Less use was made of arbitrary fixed factors of safety. The two Pratt and Whitney Waspair-cooled radial engines were supercharged to 5,000ft and gave 550 h.p. each for take-off. In the first 247s, the engine nacelleswere not aerodynamically very clean. They had short-chord ring cowlings round the cylinders of the type which had beendeveloped in England by Dr. H. C. H. Townend in 1929. The 247 was designed to the American airworthiness require-ments of the period, which required the ability to maintain at least 2,000ft with full load after losing an engine. At first the aircrafthad fixed-pitch propellers and its performance, particularly on one engine, was inadequate. However, Hamilton Standard were pro-ducing their first production variable-pitch propellers at about this time and these were introduced on the 247 and on the improved247D, which also had more highly-supercharged geared Wasp engines cowled in the very much cleaner N.A.C.A. long-chordcowlings which had been developed at Langley Field, Virginia in 1930 and had, by this time, been used with great success onthe DC-1 and DC-2. The 247D's loaded weight was increased to 13,650 lb. The improved aircraft first entered service with Unitedin August 1934 and most of the sixty United 247s were later converted into 247Ds. They offered a coast-to-coast transcon-tinental schedule of 20 hours with seven stops. The Boeing 247D was notable as the first transport aeroplane to have constant-speedvariable-pitch propellers (adopted in 1935) and wing and tail unit de-icing, achieved by means of inflatable rubber overshoes on theleading edges. The appearance of the Boeing 247 on the air routes had, in fact,coincided with a big step forward in the all-weather operating capabilities of transport aircraft. Until the early 1930s by far thegreater part of all flying was done on a "contact" basis. The first effective turn-and-bank indicator had been demonstrated in flightby Lawrence Sperry as early as 1917 and these instruments came to be installed in military and airline aircraft in the 1920s. Themajority of pilots, however, avoided protracted flight on instru- ments as far as possible. This was as well, because even as late asthe early 1930s many experienced pilots still believed that safe flight without external reference was possible without instruments.Fortunately, they did not attempt it very often! The airlines in both Europe and America began to use radio fornavigational purposes in the 1920s and an increasing number of pilots started to fly on their turn indicators instead of trying tomaintain "contact" below the overcast. The 1930s, however, were to see a complete revolution in operating techniques. Not only didthe radio aids gradually improve out of all recognition but radio (and, in the 1940s, radar) began to solve the problem of landing in "Eyen faster was the first production transport with a retractable undercarriage, the single-engine Lockheed Orion"
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