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Aviation History
1934
1934 - 0246.PDF
FLIGHT, MARCH 15, 1934 SPEED AND THE ECONOMICS OF AIR TRANSPORT " By MAJOR F. M. GREEN ~ (A resume of the discussion which followed the lecture delivered before the Royal Aeronautical Society on March I and published in an abridged form in FLIGHT for March 8). R. H. TIZARD (chairman of the Aeronautical Research Committee) presided at the lecture, and was supported by Mr. F. Handley Page (Vice-President of the Institute of Transport), the meeting being a joint one with the Institute of Transport. In opening the discussion, the chairman said that Major Green's conclusions were very provocative and warranted careful criticism and examination. He referred, in par- ticular, to the conclusion that, even doing as well as one could in the way of design, the economic speed of an aeroplane lay between 130 and 140 m.p.h, MR. G. E. WOODS HUMPHREY, Managing Director of Imperial Airways, felt that Major Green was to be con- gratulated for having solved so easily all the problems of air transport. At the same time there were many vital variables controlling the cost of air transportation which Major Green appeared to have dismissed in an easy manner. Mr. Woods Humphrey assured the lecturer—not from theoretical deduction or hypothetical assumptions, bHt from fact and experience—that the speed at which goods could be carried at the cheapest possible rate per ton mile was much below 130 m.p.h. The speed given by Major Green's formula, 130 m.p.h., was however some- where near that which would enable the operator to sell the largest amount of ton-miles. Mr. Woods Humphrey knew this from purely commercial experience, and was therefore at a loss to understand how Major Green arrived at it from aerodynamical calculations. He though the answer lay in the dismissal of vital matters like fuel and oil, and thus found himself unable to agree with Major Green's basic assumptions. MAJOR GREEN thought that hia paper had been mis- understood, and countered Mr. Woods Humphrey by point- ing out that the aircraft he used had a higher drag than 25 lb. per 1,000 lb. weight, at 100 m.p.h., and so their economic speed was not 130 m.p.h. He denied dismissal of the fuel and oil question, as horsepower was taken into account and 130 m.pJi. was the speed which would give the best petrol consumption figures. MR. WOODS HUMPHREY referred to the words of thepaper which stated that the cost of fuel was not of much importance. MAJOR GREEN replied that it was not of much import-ance in his analysis, as the amount of petrol used at 130 m.p.h. was about the minimum. MAJOR R. H. MAYO (Technical Advisor to Imperial Airways) offered serious criticism to the paper, but did not do so in any contentious or destructive spirit. He did not agree with many of the statements and assumptions in the, paper. The " Factor of Usefulness " was fundamentally incorrect, basically wrong and entirely mis- leading. It did not get within measurable distance of representing the ratio of earning capacity to running costs. Discussing the denominator of the fraction, which was made up of " standing charges," and " running costs," he agreed that standing charges depended largely on the gross weight of the aircraft, although both aero- drome charges and general office and traffic expenses depended on the number of journeys made, i.e., the cruising speed of the aircraft. The assumption that aircraft structure and aircraft engines cost the same per lb. was too rough an approximation. Major Green's con- clusion that running costs were proportional to the gross weight of the aircraft was amazing, and was reached by skating lightly over most of the major problems of air transport. Major Mayo took the six items used by Major Green in compiling the running costs, separately:—(1) Fuel and oil was not proportional to gross weight but to cruising power. (2) The upkeep of aircraft, being largely dependent on the wear and tear during taking off, landing and taxying, was also dependent on cruising speed and not only on the gross weight. (3) Upkeep of engines bore no relationship to the gross weight of the aircraft, but depended on cruising power and also on cruising speed, as the heaviest wear and tear occurred during take-off and initial climb. (4) Inspection was, like the last two items, equally dependent on cruising speed. (5) Wages of the crew were partly dependent on the gross weight and partly on the number of passengers carried. (6) Part of insurance he agreed was roughly proportional to gross weight. On the subject of fuel and oil, Major Mayo had a good deal to say, and basing his figures on those applicable to the British Imperial routes, he found that increasing cruising speed from 100 to 130 m.p.h. meant an average increase in total operating costs of approximately 20 per cent. Discussing the formula, and in particular that part of the speed range which, by virtue of Major Green's arbitrary assumption as to the power required for taking off, was most favourable to it, correcting this formula by taking into account the cost of fuel and oil and the items which were dependent on cruising speed ; Major Mayo said that the " Factor of Usefulness " was shown to be entirely misleading. He felt that there was no short cut to appraising the commercial value of a transport aircraft in the way Major Green suggested. The " Factor of Usefulness " might be of academic value, but did not represent the ratio of earning capacity to running costs, was not a criterion of the commercial value of an aircraft, and was not in reality a factor of usefulness at all. In no form of transport could the commercial usefulness of the vehicle be assessed by such a simple process. There was no such thing as an economical cruising speed in the sense that flying at any particular cruising speed could make air transport pay. The factor which most controlled the commercially economic speed was competition, and the transport operator must supply the public with the speed at which it was prepared to travel, and for which it was prepared to pay. Imperial Airways were often criticised for the comparatively modest cruising speed of their air- craft, but they were one of few important transport organisations in the world which had seriously faced up to the problem of making air transport pay. Major Green's optimum figure of 130 m.p.h. was an arbitrary one determined by the assumptions made in regard to power required for taking off. In conclusion, he believed that the commercially feasible cruising speeds would rise considerably above 130 m.p.h. in the future. MAJOR GREEN said he had hoped to get Imperial Air- ways to come out, and they had done so in a magnificent manner. Now that we knew that the cost of petrol repre- sented from 25 to 30 per cent, of the total cost of operation, it was a comparatively simple matter to find out the cost of operating an aircraft, and he was delighted it was so easy. He said that he was merely putting forward a method and not attempting to give a precise answer, and he did not think it was necessary for Imperial Airways to defend them- selves for not having flown their machines at 130 m.p.h., because having regard to the parasite drag of their machines they were flying at about the speed he would expect, and he was trying to persuade Imperial Airways, and other operators, of the importance of reducing parasite drag. He did not think the cost of upkeep varied appreci- ably with the number of take-offs, and, with regard to the horsepower per lb. required for take-off, there were very few aeroplanes flying with less than he had suggested. He intimated that he would reply to the other criticisms in greater detail in a written communication (Major Green refers to the full report which will appear in due course in the Journal of the Royal Aeronautical Society.—ED.), and no doubt he would be able to explain more clearly the precise purpose of the paper. The CHAIRMAN felt that Major Green had been rather misunderstood. His reasoning had real scientific interest to the Aeronautical Research Committee, and, while he did not pretend that the economics of air transport or any transport could be represented by a simple mathe- matical curve, the paper had done a lot of good in provoking some critical remarks. He hoped that Major Mayo would deal in detail with the conclusion that there must be a speed at which there would be the greatest difference between running costs and receipts if a definite amount were charged per ton mile, and also show how far costs were proportional to gross weight and how far to cruising speed. He appealed to all concerned to study the problem seriously, to find out whether it was possible 246
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