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Aviation History
1957
1957 - 0905.PDF
>5 July 1957 • .# of Jet and Propeller-Turbine Operations ind D. R. Newman, Chief Technician, Aerodynamics Department DURING the past few months our columns have dealt at length withcontributions to the jet v. turboprop economics controversy. Here, as foreshadowed in a leading article last week, is the case for the jettransport—presented by de Havilland. On the 2,500 mile stage it is assumed that the jet would becruised at a more economical speed—about 550 m.p.h. In the case of the propeller-turbine there is no appreciable change in operatingcost with speed, and it therefore seems likely that it would always employ the fastest cruise procedure unless range is limiting. Direct Operating Cost Comparison. Using operational factorsappropriate to a slightly modified S.B.A.C. method,* the relative direct operating costs per ton-mile of the above jet and propellerturbine aircraft are found to be as follows: — 250 miles stage, the jet is 6.9 per cent above the propeller-turbine;500 miles stage, the jet is 1.6 per cent below the propeller-turbine; 2,500 miles stage, the jet is 12.0 per cent below the propeller-turbine. There is considerable scatter of the basic engineering quantitiesamong the various aircraft on which the typical ones are based. In particular, the specific fuel consumption of propeller-turbineengines varies widely; this will have an effect on the block fuel consumption and take-off weight of these aircraft, but the operatingcost is not greatly affected, because the fuel cost is a comparatively small part of the total.Figures relating to individual aircraft will be found to be better or worse than those of the typical hypothetical aircraft—our pur-pose being to show the general comparison between the two types of propulsion.It is therefore believed that the comparison presented here (based on typical values) is perfectly objective and equally fair to bothtypes, and that the operating-cost differences stated above are correct to within about plus or minus 5 per cent.The very broadly equal direct operating costs are arrived at in spite of some substantial differences in the various items whichmake up the total cost, and these can be grouped together as follows: — Fuel cost: At the high-speed cruising condition assumed, the jet fuelcost per unit of work done is nearly double that of the propeller-turbine at short ranges; on the longest stage, where the jet is cruised at a moreeconomical speed, the difference is reduced to about 20 per cent. Engine cost: The relative simplicity of the jet engine and the absenceof a propeller result in its having a powerplant first cost and maintenance cost per unit of payload capacity which is only about 80 per cent of thatof the propeller-turbine. Airframe weight and cost: It appears that the airframe weight for agiven payload is rather higher for the jet than for the propeller-turbine, the effect of the greater strength and stiffness required by the higheroperating speed of the jet outweighing the longer undercarriage and larger tail areas necessitated by the latter's propellers and their destabi-lizing effects. There seems no reason, given equal production quantities in similar circumstances, why the airframe costs per pound of weightshould differ significantly. Speed: The obsolescence, insurance, maintenance and crew costsaccrue on an hourly basis; and hence the higher speed, and consequently greater work capacity or productivity of the jet, give it an advantage *The modifications consist of an increase in the depreciation rate from 80 percent of the first cost in eight years to 90 per cent in seven years, and a reduction in annual utilization from 3,000 hr to 2,500 hr; the modified figures are thoughtto be more appropriate to the type of operation under discussion. The reserve fuel basis which has been assumed is: 230 statute miles diversion;4S minutes' holding at 5,000ft; final reserve (1,000 Ib for the Comet 4B). The "chimneys" on the left show at a glance the relative charac- teristics of jets and propeller- turbine transports as defined by the authors. Black depicts the propeller-turbine, shaded chim- neys represent the jet. over the propeller-turbine in theconversion from cost per hour to cost per ton-mile. We have long been con-scious of this latter quality of the jet. In an early Cometreport written in 1945 the advantage of the jet relativeto the piston engine was summed up in the followingwords: — "Whilst the fuel cost andweight per ton-mile of payload is much higher in the jet thanon the other, this is largely off- set by the fact that the enginesdeliver at a great altitude a very high cruising thrust power in relation to their weight, cost, maintenance, drag, etc. The result of all this beingthat whilst the general characteristics of the aeroplane are fairly con- ventional, the speed is disproportionately high." This remains broadly true of the present situation relative tothe propeller-turbine. Table 2 summarises the make-up of the percentage differencesbetween the direct operating costs per ton mile of the jet and propeller-turbine. It shows that the more rapid increase in thecost of the jet as range decreases is due to fuel cost and to the fact that the difference in block speeds between jet and propeller-turbine decreases (the terminal time being a greater proportion of the block time). TABLE 2 Excess of jet coat over propeller-turbine cost, a> percentage of propeller- turbine cost. Stage length (miles) ... Due to fuel consumption Due to power unit cost Due to airframe weight and cost ... Due to block speed Due to higher take-off weight Net total 250 ^23.1 -6.2 • 2.9 13.6 + 0.7 4 6.9 500 + 19.1 6.6 -3.1 - 17.7 -' 0.5 - 1.6 2.500 + 6.1 - 5.4 4 3.5 -16.4 » 0.2 - 12.0 250 MILES 500 MILES 2500 MILES **»• Field length required The differences for the 2,500 mile stage are not comparablewith those for the other stages, as the cruising speed of the jet has been reduced to 550 m.p.h. The adverse effect of fuel con-sumption now becomes comparatively unimportant and the difference in block speed gives the jet a considerable advantagein cost. It is illuminating to note the effect on this comparison of theprice movements which have taken place in the last 10 to 12 years. It is found that airframe and engine costs have risen much moresharply than fuel costs. As the operating cost of the jet is more sensitive to fuel cost than that of the propeller-turbine, and lesssensitive to first cost, the movement of prices has been in favour of the jet. This partly explains why the jet is now more com-petitive, particularly on short stages, than it was just after the last war. It follows that the jet stands to improve its relative posi-tion if past trends continue in the future. There is also the question of the operating cost methodemployed (the constants in any method are necessarily somewhat arbitrary), and to check this point the calculation for a 500 milesstage length has been repeated using the unmodified S.B.A.C. method, the A.T.A. method, and alse the methods evolved byB.E.A. and B.O.A.C. to suit their own conditions. The relative cost of jet and propeller-turbine aircraft as dis-played by these various methods is as follows: — Jet D.O.C. as percentageMethed of propeller-turbine D.O.C. Modified S.B.A.C 98.4 Unmodified S.B.A.C 100.3 A.T.A 88.3 B.E.A 95.7 B.O.A.C 94.8 It is interesting to see that the relative situation is very
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