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Aviation History
1957
1957 - 0904.PDF
FLIGHT The Comparative Economics By R. M. Clarkson, Director, the de Havilland Aircraft Co., Ltd., WHEN a number of propeller-turbine and jet aircraft areexamined—and there is abundant published informationavailable to permit this—it is found that their economics all lie very close together. The very fact that the question of their comparative economics is so controversial at the present time is in itself a proof that the margin between the two types is small. This situation naturally raises the question: "Is this a fortuitous state of affairs arising from particular circumstances, or is it funda- mental?" We were therefore prompted to break down and dissect the relevant ingredients of the operating cost of both types of aircraft. Many comparisons have been published which are based on completely imaginary aircraft designed especially for the purpose. Some of these designs involve considerable extrapolation of the trends of aircraft and engine progress and some of the assumptions are questionable, particularly when the? result in an aircraft which is very much more efficient than present-day designs. In order to overcome these drawbacks, we have based our comparison on aircraft of the standard of those which are now being offered to enter service in the period 1959- 1962, and from these we have denned typical hypothetical aircraft for which we can fairly claim a factual basis. The operation is considered over stage lengths of 250, 500 and 2,500 statute miles. None of the large aircraft at present being offered has been designed specifically and solely for operation over stage lengths of around 500 miles; therefore, the typical hypothetical aircraft which are con- sidered here for operation on stage lengths of 250 and 500 miles are actually capable of flying distances of up to about 2,500 miles with capacity payload, and their operating costs on the shorter stages are not quite as low as those of more specialized aeroplanes would be. Similarly, the 2,500-mile stage has been selected as being representative of the routes on which the long-range aircraft will generally be used. For the above reasons the same typical propeller-turbine is con- sidered on all three stage lengths, while, in the case of the jet, the only difference is a small increase in thrust (for given payload) over the longest stage in order to maintain a reasonable take-off performance. Derivation of Typical Aircraft. It can be shown that the direct operating cost is dependent upon a number of factors denning the properties of the aircraft. These may be grouped as follows: —(A) Engineering quantities, which define the technical quality of the aeroplane : payload carried per pound of airframe weight and per poundof all-up weight; thrust or power loading at take-off; and ton-miles of payload per gallon of fuel.(B) Specific first costs: airframe cost per pound of weight; jet engine cost per pound of thrust; and propeller-turbine engine-plus-propellercost per e.h.p. (These quantities are dependent on production quantity, methods of financing, and so forth.)(C) Operational economic factors, which are dependent on the environ- ment in which the aeroplane is operated and the operator's accountingprocedures: fuel cost; obsolescence period and residual value; spares holding; utilization, etc., as specified in the operating cost methodemployed for the calculation. The quantities in Group (A) have been plotted for all the aircraft due to go into service in the period considered. The payloads used have been based on first-class layouts with a seat pitch of 38in to 40in, passengers being seated four-abreast in fuselages of under 12ft in diameter, and five-abreast in fuselages of 12ft or more in diameter. From these plottings, representative mean values for each of the engineering quantities have been obtained for each of the three TABLE 1 Aircraft Airframe weight (Ib) Gross wing area (sq ft) Take-off static thrust (Ib) ... Take-off power (e.h.p.) Cruising speed (m.p.h.) Airframe cost (£) Engine cost (£) Propeller cost (£) Total first cost (£) Stage length (miles) Take-off weight (Ib) Block speed (m.p.h.) Block time (hours) Take-off aerodrome length (ft) . Landing aerodrome length (ft) Fuel consumed (Ib) ... Jet Mediumrange 79,500 2,350 46,100 590 952,000 198,000 1,150,000 250 150,000 342 0.73 4,800 5,750 11,300 500 156,000 425 1.18 5,200 5,750 17,300 Longrange 79,500 2,350 48,300 ~550 952,000 208,000 1,160,000 2,500 191,000 500 5.00 7,800 5,750 52,300 Propeller-turbine 73,000 2,200 22,900 410 250 133,000 268 0.93 4,150 5,200 6,100 874,000 217,100 38,900 1,130,000 500 137,000 310 1.61 4,300 5,200 16,100 2,500 170,000 380 6.58 6,950 5,200 43,100 Take-off aerodrome lengths are in sea level I.S.A. +15 deg C. conditions; the landing aerodrome lengths are in sea level I.S.A. conditions to the U.S. 60 per cent rule. stage-lengths. Similarly, average figures of specific first costs have been selected, the airframe cost per lb of weight being the same for both jet and propeller-turbine types. It is assumed that each aircraft carries 100 passengers with an average proportion of freight. This gives a capacity payload of 29,000 lb, made up as follows: — 100 passengers with baggage, at 209 lbFreight ... 20,9001b... 8,1001b 29,000 lb The main characteristics of these hypothetical typical aircraft when operated over the various stage lengths are given in Table 1. On the 250 and 500 mile stages, both aircraft are assumed to be cruised at the speed and altitude for minimum block time rather than for best fuel economy. S00 MILES yOO MILES 250 MILES 500 MILES 2^00 MILES Direct operating costs •.-_"••:-.;„ Block speed 250 MILES 500MILES yOOMILES 250 MILES 500 MILES 2.500MILES Block time Work capacity
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