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Aviation History
1943
1943 - 0441.PDF
FEBRUARY I8TH, 1943 FLIGHT 177 The Shape of Wings to Come American Experts Discuss Requirements in the Design of the PosUwar Transport Aircraft BRIEF reference to what the American journals termedan " air cargo engineering meeting," held at Chicagounder the auspices of the Society of Automotive Engineers, was made in last week's issue of Flight. In this it was recorded that, among a certain diversity of opinion as to just what characteristics should be embodied in the post-war transport aircraft, Mr. Carlos Wood, of the Douglas Aircraft Co., expressed the view that a mili- tary cargo aircraft of good design would produce an efficient type for peacetime purposes, whereas Mr. Charles Froesch, chief engineer of Eastern Airlines, thought that specially designed air freight carriers would be needed in the post-war period, and added that, in his opinion, the ideal cargo plane had yet to be built. The subject is of such interest at the present time, how- ever, that a more complete review of the ideas advanced by these two American experts is warranted, especially as they were made at a gatheruig (said to be unique in the history of American aviation) which included repre- sentatives of the U.S. Army and Navy, the Department of Commerce, the Civil Aviation Administration, manu- facturers and airline operators. Various other aspects of the subject, besides that of aircraft design, were, of course, dealt with by other speakers, whose own interests and experience lay in the direction of such matters as the packing and loading of airborne freight, the terminal handling of air cargo, and the economics of post-war car- riage of air cargo, but we are concerned at the moment only with the subject of design and such matters as directly affect it. And under this heading, Messrs. Wood and Froesch had a number of interesting things to say. Mr. Wood began by saying that it was not too late for the results of an analysis of the factors involved in the design of transport aircraft to be useful in the production of cargo planes which would go far towards winning the war. It was now time, also, to investigate the bases of economical operation of cargo aircraft that would go far towards winning the peace. Cargo Utility Factor Examining first the military cargo aircraft, Mr. Wood said that for any given unit of transportation, the utility could be expressed as. the ability to move a certain amount of pound-miles per hour of men or material; as a lesult the aircraft which could transport the largest number of pounds at the highest place-to-place speed would be the most useful. The utility of the aircraft was also affected by its landing and take-off distances, the former varying roughly with the square of the landing speed and the latter being a function of the landing speed and the power- loading. These values, again, had their effect on the man- hours necessary to build and maintain usable operating bases. Another consideration was availability in number of aircraft compared with the man-hours needed to produce them. This bore a direct relation to the weight empty of the aircraft, since it had been found that production man-hours were essentially a function of the unladen weight. All these items must be considered in deter- mining what he called the "cargo utility factor," which was a function of the range of the; aircraft at a given gross weight, a factor which was a product of a weight ratio and a speed ratio, and a more or less conventional means of assessing the efficiency of operation. .Since' these various factors governed the military air- craft, the comparison to the requirements of efficient com- mercial operation would at once be seen. • Man-hours could be expressed in production cost, weight governed operating cost, and landing speed was an index to the necessary ground facilities and the maintenance cost of the aircraft's running gear. Thus the factors affecting "cargo utility factor " represented a cost in man-hours, or in dollars per hour, so that the "c.u.f." was proportional to the pound- miles per dollar. In other words, the higher the value of "c.u.f." the lower the cost per pound-mile, and this led to "the belief that common-sense design of good military cargo planes (i.e., aircraft with a high " cargo utility factor") would also produce efficient types for future use in peacetime. Mr. Wood went on to point out that any good transport aircraft must be "built around the cargo compartment, " whether for military or civil use. That compartment must be of adequate size, suitably located in relation to the .test of the aircraft to permit of its fullest use, designed inter- nally to allow efficient handling and best use of available space, and to allow of easy and positive tying down of cargo when loaded. They could, he said, learn a lot from the humble freight car. Its floor was flat and level and at a convenient height from the ground. The car was about 40ft. long, 8ft. wide and 9ft. high, with side doors large enough to permit rapid loading, and it gave them an idea on the practical compartment size to work towards. Locating for Balance The freight compartment on a transport aircraft shduld be located in respect to the wings so that it held the centre of gravity, whether laden or empty, within the required limits of the aircraft. Gene/ally speaking, about one-third of the compartment's length should be ahead ol the 25 per cent, point of the mean aerodynamic chord. Comparison of current types showed that maximum "cargo utility factor" was not, apparently, a direct func- tion of the aircraft's size. On the other hand, the range at which this utility or economy was available did seem to be a function of size only; the larger the aircraft the longer the range at which the desired utility or economy could be obtained. This was due partly to a slowly in- creasing percentage of useful load with size, and partly due to increasing aerodynamic efficiency with increase in size. The increase in percentage of useful load was not automatic, but was primarily the result of unceasing developments in materials and design. In order to produce an efficient machine, it was necessary to know under what conditions it was to be used, since for a given size of aircraft the range for which maximum cargo utility was available was a function of the basic lay out. The basic factors to be considered for a cargo plane were: weight, drag, span, power, stalling speed and hand- ling time; and certain special conditions envisaged for the use of the transport aircraft would often affect its basic design. Usually, however, the design would be flexible enough not to make the aircraft useless except for its one special purpose. To produce a design of good genrral utility that would also meet—or come close to meeting— a number of special conditions of operation was a prolific source of headaches in the design room. Operation at higher altitudes would, he felt sure, bring circular-sectioned fuselages and supercharged cabins, for most aircraft of the future would carry' passengers and cargo, but with proper care many military aircraft would be convertible to civil use. "The military cargo airplanes," he con- cluded, "will be practically tailor-/nade for the job." As a representative of the operating side of air transport. Mr. Froesch did not express any opinion about the merits or otherwise of the military cargo plane for eventual civil use. His most recent experiences had been with the use
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