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Aviation History
1957
1957 - 0020.PDF
2© FLIGHT Research, Development and Technical Issues . . . only an approximation to the result provided by the correct lineartheory. These ideas have led to attempts to design aircraft having a cross-sectional area at each section conforming to that of a bodyot revolution giving the minimum drag for a given thickness/ length ratio, such as the Sears-Haack body. This remains a potentially fruitful field of research directed toanswering at least three questions: (1) Is further progress possible in getting the best compromise between the shapes which emergefor subsonic, transonic and supersonic speeds? (2) How far in terms of forward supersonic speed can the onset of serious wave-making drag be delayed, carrying on the process that has already made so big an impression on transonic drag? (3) Are there con-figurations which can show a reduction of drag at medium-super- sonic speeds, so increasing the lift/drag ratio to a value greaterthan the figure of roughly 6 which is now attainable? The first question is relevant to the performance of medium-supersonic long-range aircraft in the climb and "through the speed of sound" phases; in such aircraft the installed thrust, whichbalances total drag in the medium-supersonic regime, inevitably exceeds by little the drag in the transonic condition through whichthe aircraft must fly. The second question is "How far in speed can the onset of seriouswave-making drag be postponed?" In the low-supersonic regime methods of the "area rule" type may have application to this end.The area rule approach does not specify the cross-sectional shape of the fuselage, but only the area, and it has been usual to waistthe fuselage with circular cross-sections; the Kuchemann approach specifies an indentation in the plane of the wing. It would seemthat an extension of both approaches would specify not only the area but the cross-sectional shape. Such a method has beensuggested by Dr. W. F. Hilton in his paper to the Institute (Journal of the Institute of the Aeronautical Sciences, Vol. 22,No. 3). In his approach the streamlines are determined not only at the surface but in the whole flow-field surrounding the profile.Hilton claims that for wings with sub-critical flow the shape produced is substantially independent of design Mach number,and is thus effective at subsonic, transonic and supersonic speeds. This is an interesting field for research which may produce, inthe low-supersonic regime, substantially better lift/drag ratios than are now current. The third question is "Can we do better than a lift/drag ratioof 6 in the medium-supersonic regime?" I think that this may prove to be quite difficult in practice but there are theoreticalshapes which offer possibilities, and these should be investigated vigorously because the reward for success is very high. I thinkit is likely that wing and body shapes which are successful in this way will not have good low-speed aerodynamic qualities, so thatthey may involve the use of special arrangements for take-off and landing. But my impression is that no easy way has yet emergedfor obtaining medium-supersonic lift/drag ratios greatly exceed- ing 6, and the virtues of new shapes may be found more in theirstructure weight than in their aerodynamics. Summarizing this discussion on lift/drag ratio, I think it comesto this. In the medium-supersonic regime one can attain something approaching 6 with practical configurations, and this is enoughto make medium-supersonic flight at ranges of 3,500-4,000 miles possible and attractive for some purposes. We should look hardfor higher L/D, but we will not find it easy to obtain; if we find it, it may involve configurations which require special provisionsfor landing and take-off (this is not, of course, without its own advantages). New configurations that can be built at a low struc-ture weight are also of great interest, even if their lift/drag ratios are not high. In the low-supersonic regime we may well attaina lift/drag ratio substantially better than 6, and research of the "extensions to the area rule" type may prove to be fruitful. Specific Fuel Consumption. Fig. 8 contains a plot of the effi-ciency likely to be attained by modern turbojet engines as a function of Mach number. The figure contains two envelope 100 curves, one for simple jet engines and one for ducted-fans ofby-pass ratio about 1.5; these curves display the efficiency likely to be reached by an engine specifically designed for best operationat the Mach number concerned. The figure also contains curves showing roughly how the overall efficiency of a simple jet enginedesigned for operation at Mach number 2.5 is likely to change with Mach number, and how this overall efficiency breaks down intothe two components, propulsive efficiency and thermal efficiency. The very high thermal efficiency that can be reached by jetengines at high forward speeds is notable and compares interest- ingly with the figure of about 31 per cent attained by modern landsteam plant. The rise of efficiency with Mach number offsets to some extent the drop of lift/drag ratio in the supersonic regime, so 1-0 20 MACH NUMBER that the movement of the factor ft) (?) in the range equation is substantially less adverse as the Mach number increases thanthe L/D figures alone might suggest. (In Fig. 9 this is plotted in Fig. 9. Typical variation ot product of L/D and overall efficiency with Mach number, for con- ventional configurations. 10 20 MACH NUMBER 30 the form (L/D)'] against M (?/=overall efficiency). (L/D),r/ isproportional to the range achieved for a given fuel weight ratio; see range equation, Appendix I. It is this effect that makes long-range flight at medium-supersonic speeds reasonably possible. For supersonic flight the combination of airframe and turbojetpowerplant is at its best at Mach numbers of 2.5 to 3, provided the problems of kinetic heating which will be encountered at thesespeeds do not cause an increase of the structure weight sufficient to reverse this favourable trend. It is interesting to compare the compression and expansioncycles for jet engines operating at subsonic and supersonic speeds; such a comparison is shown in Fig. 10. (This figure is plottedlogarithmically, so that the elements due, for example, to intake and compressor multiply to give the overall ratio.) The changebetween the two conditions is clear and accounts, of course, for the higher efficiencies achieved in the supersonic case. The simple jet engine is at an ideal state at Mach numbers near2.5, in the sense that it is then making the best use of its materials, at least as we know them now. Propulsive and thermal efficiency areinevitably intercoupled in the simple jet engine, since an increase in the temperature of the gas at the inlet to the turbine, whichwill improve thermal efficiency, must also increase the jet velocity with a consequent reduction of propulsive efficiency. Fig. 11ilustrates this point showing, for a particular Mach number, how the propulsive efficiency drops if the thermal efficiency is increased.When Mach numbers near 2.5 are reached, the two factors are no longer in serious conflict; the propulsive efficiency has risen toa high value and the materials of which the turbine is constructed can be worked to the peak of their properties, with benefit to thethermal efficiency and without so adverse an effect on the overall Fig. W Fig. 11 (left). Compression and expansion in jet engine systems: I, intake; C, compressor; T, turbine; N, nozzle, (right). Variation ot thermal and propulsive efficiencies with turbine-entry temperature in a simple turbojet. Fig. 8. Turbine-engine efficiency with Mach number: A, turbojet tor Mach 2.5; B, optimum turbojet; C, ducted tan; D, thermal efficiency of Mach 2.5 engine; E, propulsive efficiency of Mach 2.5 engine. •'• / JU 40. U730 o o20 o tr iio EXPA N i < 51 « en UJ c TOTA L c c I 1 T I N - T N 60 50 O jj i3 !t20 LU 10 0 PROPULSIVE P / f—7——' THERMAL - W00 1200 1300 TURBINE ENTRY TEMPERATURE(<ta)K)
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