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Aviation History
1957
1957 - 0314.PDF
316 . , FLIGHT, 8 March 1957 THE MEDIUM-RANGE TRANSPORT Pure-jet Economics Pilloried By F. H. ROBERTSON, A.F.R.Ae.S. IN our February IS issue we published a synopsis of Lord Douglas'Brancker Memorial Lecture, in which he compared the economics of jet and turboprop transports. In the subsequent issue (February 22)we examined this paper through the eyes of the short- and medium- haul jet protagonists, in an effort to show that the disparity in operatingcost between the turboprop and the next generation of efficient, high- subsonic jet transports might not be so great as Lord Douglas showed.This has prompted a further contribution to the controversy. It is from a designer, and it comes down hard on the side of the turboprop. IN his recent Brancker Memorial Lecture, Lord Douglas con-cluded that the turbojet transport would prove to be 10 to15 per cent more expensive than the corresponding turbo- prop and that, although the bulk of medium range traffic wouldcontinue to be carried by turboprops, there was a case for a limited number of turbojets to carry first-class traffic and to compete withthe big intercontinental turbojets. In Flight's February 22 issue J. M. Ramsden restates LordDouglas' views and then goes on to look at the picture through the turbojet protagonist's spectacles.May I now present the other side of the case? First of all, I do not quarrel with Lord Douglas' findings.He has stuck to very firm ground, as a responsible airline operator should; but in fact he has leaned over backwards to avoid being"unfair to the jet." Let us, first, look at some figures: — TABLE 1 Aircraft Viscount 802 ... Britannia 300 ... Electra Caravelle Comet 4a Convair 880 ... Boeing 707 a.u.w. (Ib) 62.000 160.000 113.000 99.200 152,000 173,500 295,000 Wt. less fuel and payload (Ib) 39.200 88.500 61,000 46.500 70,000 85,000 128.000 Max. payload (Ib) 13,800 30,000 23,000 17.500 22,600 26,320 38,000 Max. Fuel (Ib) 14.930 54,200 34,800 33,750 70,200 70,000 158,000 Cruise Speed (let) 274 344 339 391 452 474 474 E.S.A. Range (Max.) n.m. 1705 4110 3040 2000 3780 2760 5125 Now I cannot vouch for the accuracy of all these figures, butthey have all been published from time to time and I think they are good enough for my argument. Let us now compare the direct operating costs of these aircraftover stages of 500 and 1,000 n.m. There are dozens of ways of comparing operating costs; most of them are reasonable as methodsof comparison provided you don't treat the absolute cost results with too much confidence. I am going to use my own formula*which covers first cost, insurance, maintenance and repair, and fuel. It assumes an annual utilization of 3,000 hours and a loadfactor of 67 per cent. My results are tabulated in Table 2: — TABLE 2 Aircraft Viscount 802 Britannia 300 Eleetra Caravelle Comet 4a Convair 880 Boeing 707 500 n.m. Stage E.S.A. Range (n.m.) 800 800 800 875 875 900 900 D.O.C. (pence/ton/ n.m.) 17.26 13.73 12.49 17.18 16.87 17.69 15.40 1000 n.m. Stage E.S.A. Range (n.m.) 1550 1550 1550 1700 1700 1750 1750 D.O.C. (pence/ton/ n.m.) 30.65 13.80 12.56 17.20 16.92 17.75 15.43 All sorts of conclusions can be drawn from these results; themost significant is that the best turbojet is 23 per cent more expensive than the best turboprop and, what is more, is twice thesize. (In passing, for the benefit of the non-technical reader, I should explain that the equivalent still air range for a given stagevaries because some engines need more fuel for the lower altitude and speeds necessary over some parts of the flight.) So much for existing aircraft. Let us now see what happensif we set out to design turboprop and a turbojet aircraft to carry "This simplified "short-cut" cost formula was published in the January 1957 issue oj the Journal of the Royal Aeronautical Society. For transport aircraft at 3,000 hr annual utilization and a 67 per cent load factor, total direct operating cost in pence per ton n.m. 1,000 W , 10,000 F . . = —yp 1 p£— where W — weight of mrcraft less fuel and pay- load in Ib; V — true mean cruising speed in kt; P = payload carried in Ib; R = equivalent still air range in n.m.; and F=fuel carried in Ib. The case for the turboprop, as made out in the accompanying article, is lent added weight by the economics of the Vanguard. According to the author's own cost formula, its operating cost is about six per cent lower than that of the Electra. capacity payload over a 1,000 n.m. stage. Several references havebeen made to a B.E.A. specification for a turbojet in which Vickers and Bristol are interested. Sticking my neck right out, I amgoing to write down what I think the parameters of such an aircraft might be: — Table 3: Imaginary data for "Brisvickjet" All-up weightWeight less fuel and payload Max. payloadMax. fuel Cruise speedE.S.A. range (max.) D.O.C. 500 n.mD.O.C. 1,000 n.m. It is seen that, in spite of increased efficiencies having beenassumed for both engine and airframe, the D.O.C. comes out higher than for the Boeing 707. This is the combined effect ofsheer size and the comparatively larger fuselage due to the larger percentage payload. Now let us look at the best possible turboprop which we candream up for B.E.A.'s European routes. Giving my crystal ball another rub, I get: — Table 4: Imaginary data for "Turbeuroprop" 118,00060,000 22,00044,000 480 2,100 16 Ib lb lb lb kt n.m.pence/ton/n.m. 16.06 pence/ton/n.m. All-up weightWeight less fuel and payload Max. payloadMax. fuel Cruise speedE.S.A. range (max.) D.O.C. 500 n.m. ..D.O.C. 1,000 n.m. 97,000 lb54,000 lb 26,000 lb26,000 lb 350 kt 2,600 n.m. 10 pence/ton/n.m. 10.06 pence/ton/n.m. We now see that for approximately the same total traffic-deliveryrate, the turbojet is 65 per cent dearer to operate than the turboprop. It is quite clear that, unless we are careful, we (the public) arenot going to get a fair deal from the airlines. It is all very well to talk about the public "demanding jet travel." This is rubbish.The only thing the public ever has demanded from transport is a lower fare. I agree with Lord Douglas that there may be a case for a fewhigh-speed first-class flights; but I would entreat him to take a far closer look at a turboprop design which is really aimed at thelowest possible fare. The sort of thing I have in mind is a sort of super Ambassadorwith a couple of big, low-consumption turbines driving 20ft propellers. Remember that any advance in turbojet design isalways reflected in the turboprop. You cannot close the gap; the turboprops will always have much the lower specific fuel con-sumption. In addition, for the shorter stages, where lower operating heights are desirable, the higher speed turbojet suffers fromhaving to meet a higher E.A.S. for structural stiffness and the empty weight advantage, which is apparent over long ranges,tends to disappear. In conclusion, I believe our trouble today is the fact that somany of us have tried to improve the turbojet, in order to meet the U.S. challenge, that we have forgotten to see how much wecould improve the turboprop.
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