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Aviation History
1957
1957 - 0174.PDF
FLIGHT Left, above: Another hypothetical transport tor Mach 7.9. Left below: A "just supersonic" (M = 1.2) design with an M (or alternatively, W) wing, with a 45-deg. sweep-angle. Th« aim is to establish a subsonic flow field and, at the some time, to minimize wave-drag. This the design does, with a low centre- T movement; but structure weight will be high. SUPERSONIC AIR TRANSPORT Having fixed these desiderata it is then possible to determinethe resultant relationship between the all-important factors of basic weight, fuel weight, and payload. The result is an aeroplanewith no payload at all, and which would fall into the Atlantic 800 miles short of New York. This is an unpromising start, and adjustments will have to bemade to the specification. First, the range sights could be lowered to single-stop (Gander or Keflavik) transatlantic operation—let ussay to 4,400 miles still air range (2,600 miles practical stage length). Specific fuel consumption could, at the most optimisticreckoning, be reduced to 1.2 lb/lb/hr. Drag could conceivably be reduced—possible to yield an L/D ratio as high as 7—by closeconformity to the area rule. All this presupposes immensely costly and lengthy research,much of it in fields which are only beginning to be explored. But—airily assuming that postulations are accomplished facts—wecan take a fresh look at the range equation to see what sort of payload emerges. Our assumptions are now a cruising speed ofMach 1.5, a still-air range of 4,400 miles, a specific fuel consump- tion of 1.2, an L/D of 7. A payload which works out at about8 per cent of gross weight emerges, provided that a basic weight of about 40 per cent can be achieved. This sort of payload, smallthough it would be, appears to present a reasonable proposition for further study, though its feasibility depends upon a most sweep-ingly optimistic set of assumptions—particularly the achievement of a cruising L/D ratio of the order of 7. Price of Technical Failure The proportion of payload would be very sensitive to variationsof L/D and consumption; if, for example, they came respectively to 6 and 1.3 through technical failures—and these are reasonablefigures in the light of aerodynamic and powerplant progress—our proportion of payload drops from 8 per cent to zero. The extentto which success depends upon great technical achievements in the aerodynamic and powerplant fields is clear: and even withsuch technical success die non-stop Atlantic dream would not have been attained. Nevertheless, assuming that an 8 per cent payload is achievable,it is worth comparing the economy of such a transport with that of the types it might replace—say the 707. It must of course beborne in mind that, by all the laws which should govern a perfect transport market, the operating cost of a new vehicle should beno higher, and ideally ten or more per cent cheaper, than that of its predecessor. We shall apply the costing formula as outlined in the paperpresented by Mr. G. F. Worley of Douglas at the I.A.S. meeting in San Diego in August 1956, omitting its details for the sake ofbrevity. The outcome is that the ratio of revenue-earning ability to operating cost for our Mach 1.5 transport of the stated specifica- "FligM" copyright sketches. tion is 3 15. The comparable revenue-cost ratio for a 707 alsooperating the Atlantic with one stop—which, of course, it will rarely do—is 5.2. It seems that though a Mach 1.5 transport couldbe made to work—assuming that some formidable technical hurdles can be surmounted in the time available—it would be limited tomedium-range operation and it would be very expensive to operate. There would, on the last count alone, be a very small market As soon as we begin to look at higher regimes of Mach we runinto kinetic-heating difficulties. But, ignoring these for the moment, let us do some more speed /range /cost sums, this timefor a transport with a cruising Mach number of 2.5, or 1,650 m.p.h. This is the higher supersonic realm wherein the jet engine, madewith the materials we know or can foresee, reaches its optimum efficiency, compensating considerably for the difficulty of raisingthe lift/drag ratio as Mach number increases. If we aim our Mach 2.5 transport at a 6,000 statute mile trans-atlantic still-air range, anticipate a specific fuel consumption of 1.5 and design for an L/D of 6—all reasonable aims—we find thatto provide for any payload at all we should have to achieve a basic weight of 30 per cent. This is probably out of the question: 40 percent would be optimistic, and 45 more likely with the low-speed lift devices which would have to be built in. More L/D Wanted Non-stop transatlantic operation at Mach 2.5 thus appears tobe utterly ruled out, unless L/D ratios of at least 10 and a Mach 2.5 specific consumption of the order of 1.1 can be attained. Thelikelihood that either—still less both—will be achieved in the next ten years seems extremely remote. An L/D of 10 at Mach 2.5appears to be so far out of reach—employing known principles— as to be laughable. L/D ratio can be increased at high Mach bylow aspect-ratios and ultra-thin wing sections; but these measures involve, respectively, poor low-speed characteristics and (assumingsections as thin as 3 per cent) formidable structural difficulties. Aspect ratios low enough to make any significant improvement inL/D would inevitably necessitate elaborate engine-assisted low- speed lift-increasing devices which, besides taking a long time toperfect, will assuredly eat heavily into basic weight and payload. The "ifs" and "buts" of this theme could easily occupy manypages. For example, if a low aspect-ratio is adopted, a radically different approach to stability and control will be required; forexample, downwash would render ineffective a conventionally situated horizontal tail. Continuing for the moment to ignore Mach 2.5 kinetic-heatingproblems, let us see how an aeroplane of this speed would work out on single-stop transatlantic ranges with "reasonably optimistic"assumptions for L/D ratio, fuel consumption and basic weight —say, respectively, 6, 1.3 and 40 per cent. Again, these figures presuppose aerodynamic, powerplant andengineering accomplishments of immense magnitude. But the payload which would be the reward of success would be of theorder of 10 per cent—sufficient to warrant more detailed brush- work. Operating cost would be high compared with that of the707; but there might be a market for any reasonably economic transport capable of flying from London to New York in fourhours, including an hour on the ground at Gander. A Mach 2.5 machine would show better economics on mediumstages: but it would still be extremely expensive to operate in comparison with long-range subsonic jet aeroplanes, and blockspeeds would be a relatively small percentage of cruising speed because of the large amount of trip-time devoted to "comfortable"axial accelerations and decelerations—i.e., not more than 0.2 g. But, of course, the dream of transporting infants and old ladiesat Mach 2.5 is melted away by kinetic heat—at any rate for 10 to 15 years, until materials are developed to permit safe and com-fortable flight at oven temperatures of about 400 deg F, and to withstand the effect of rapid heating and quenching during A Mach 2.5 airliner, made of steel or titanium-sandwich. Such a machine would operate at high powerplant efficiency (thermal and propulsive) but poses severe operational problems, not the least of which is heat-transfer. The intakes in this configuration might be favourably disposed in relation to the shock-wave on the wing trailing edge, but the maintenance of optimum conditions would be extremely critical.
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