FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1957
1957 - 0021.PDF
4 January 1957 21 efficiency as is illustrated in Fig. 11 for a lower Mach number.If this interaction between propulsive efficiency and thermal efficiency is to be avoided in the lower Mach number ranges, theintercoupling which is implicit in the simple jet engine must be broken in a way that permits greater independence of the pro-pulsive efficiency from the thermal efficiency. This is done in the propeller-turbine engine, which therefore permits of much higherpropulsive efficiencies in the lower subsonic regime than does the simple jet engine. The types of engine known as the by-pass andthe ducted-fan also effect such a decoupling; the extent of the decoupling is greatest in the ducted-fan of high by-pass ratio.I doubt whether there are theoretical differences in performance between simple jet engines and by-pass engines of low by-pass ratiothat exceed the variations in either type likely to be found due to degree of excellence of design. There is, however, a substantialpossibility that a ducted-fan working at a by-pass ratio of about 1.5 would give a better product of propulsive and thermal effi-ciency. The curve on Fig. 8 shows the extent, as it seems to me, that this possibility could be realized. At high subsonic Machnumbers the higher efficiency available from such an engine, as compared with a simple jet or a low by-pass ratio jet, is sufficientto give a significant improvement to long-range aircraft, after taking account of the higher weight of the ducted-fan. Thisadvantage disappears at early supersonic Mach numbers; the ducted-fan is an engine which offers advantage only over a re-stricted range of Mach number, but within this range is the speed that is of great interest to a most important section of civilaviation; we will return to this point later. Basic Weight: Influence of Configuration. For Mach numbersin the region of 2 the straight wing, the swept wing, and the delta wing are fairly equal contenders from the viewpoint of supersonicdrag. Undoubtedly all three can be made to work. We have had interesting experience in England recently with the tailless deltaconfiguration at supersonic speeds, particularly in the Fairey F.D.2 which raised the world's speed record from 822 to 1,132 m.p.h.This configuration has proved itself capable of providing good control and performance characteristics at supersonic speeds.Incidentally, the breaking of the speed record is a task that requires more than speed alone; the aircraft must be capable of verydelicate control in pitch to meet the altitude requirements, and the fuel capacity must be such that the take-off and climb, the firstrun, the turn, the second run and the return to base can be accomplished without the possibility of fuel lack limiting theaccelerating phase, particularly that before the second run. Above Mach number 2, the case for the straight wing steadilymounts, and there is much to be said for this configuration for aircraft starting their lives with a performance in the region ofMach 2.5. There will, no doubt, be the usual marginal gains in basicweight arising from development. The specific weights of engines will continue to fall, though probably slowly. The design of air-craft structures for Mach numbers exceeding 2 must take account of the effects of kinetic heating, and materials new to aviation,such as steel and titanium alloy, must therefore be used; but it is improbable that structure weights will be seriously affected. How-ever, it seems unlikely that we can expect any substantial fall in structure weight unless configurations different from those whichhave become conventional are adopted. The factor that can, and probably will, bring radical changes inaircraft configuration is the use of engine-generated lift to relieve the wings of any duty at take-off and landing. The developmentof vertical take-off can be viewed in several ways; it may be regarded as desirable in its own right (this could certainly applyto some classes of military aircraft); it may allow the use of a configuration with superior aerodynamic cruising characteristics,no longer compromised by the need for good low-speed properties; or—again because the low-speed characteristics are no longerimportant—it may allow the use of a configuration which can be built with a decidedly lower structure weight than conventionallayouts. Engine lift can be generated by a rotor (as in the helicopter), bya ducted-fan or by a gas jet. These methods vary downwards in their propulsive effciency and therefore upwards in their fuelconsumption per unit hovering time. The rotor is suitable only for use on aircraft of relatively low forward speeds. Fans requirea wing of fair plan area and aspect ratio to permit their installation, and they are therefore most suitable for aircraft of low to inter-mediate (say Mach 1) speed range. The jet engine in conventional form is most suited to a planform such as the narrow delta, whichcan provide the depth needed for installation whilst having char- acteristics suited to supersonic speeds. I expect that even moreradical integrations of the powerplant and structure will appear. The narrow-delta vertical-lift aircraft, first suggested by Dr.A. A. Griffith of Rolls-Royce, Ltd., is an interesting study in exposing the problems. Work in England has suggested that thedrag of narrow-delta configurations at supersonic speeds can be predicted with fair accuracy as an addition of the turbulent flat-plate skin friction plus a margin, the theoretical wing wave-drag as predicted by slender body theory, the drag of the fins alone, and base drag (if any). The reason why the friction componentseems to be greater than the flat-plate value is not fully under- stood and is an interesting subject for further research. On thepresent basis, and assuming a structure weight that is suggested by fairly detailed investigation, it emerges that a jet-lift narrow-delta configuration cruising at Mach 2.5 would have a range rather less than that of a conventional aircraft of similar weight but notso much as to make the concept of no interest. The dominant factor in this assessment is not that any marked improvement inlift/drag ratio emerges, but that the radically different configura- tion produces a structure weight distinctly lower than conven-tional arrangements, so offsetting the weight of the lifting engines and their fuel. There is a long way to go between an assessmentsuch as this and the engineering achievement embodying the principle. However, there is sufficient promise to make the positioninteresting, and to show that enough may be gained in structure weight by departing from conventional layouts to make, what isat first sight an uneconomical method of landing the vehicle, a reasonable proposition. Even more radical departures fromconventional configurations can be conceived and may well show greater gains; certainly this is a field with room for imagination.But hopes for city centre operation for such aircraft should not be raised too high for they will be very noisy. The runway is, and is likely to remain, an excellent method ofgetting into the air and returning to the ground. Arguments are mounted that runways are altogether too costly a luxury, but Ithink that there is little justification for such a view. The capital costs of base equipment for other transport systems such as rail-ways, which have a much more restricted command of distance, are also high; the case against aviation base costs can hardly longsurvive if the issue is put into perspective by considering the capi- tal put into railways a century ago. I think that the continueduse of long runways is economically justified. But this is not to say that improvement in runway performance may not be desirablefor some classes of aircraft, and reduction—or at least a curtailing of the increase—in approach speeds will be helpful to safety inbad weather. For marginal improvement in runway performance, "blowingover the flaps" is probably the best answer and, indeed, may be the only one (apart from conventional slots, slats and flaps, andreduction in wing loading). The emission of the propulsive jet along the trailing edge of the wing—an arrangement known asthe "jet-flap"—is a method suggested for improvement of a more radical kind, but short of full vertical take-off. The principle has, I think, been put forward separately fromwhat is now known as the jet flap, but in more radical forms, and I think such approaches may produce remarkable results in theend. But the following discussion is centred on the jet flap as at present publicized and a rough comparison follows betweenaircraft so equipped and conventional aircraft for the same duty. The jet-flap aircraft pays some penalty in weight for the distribu-tion of the propulsive jet and since the lift as well as the thrust depend on engine power, there is an adverse effect from the"engine cut" case as compared with conventional aircraft. How- ever, the wing of the jet-flap aircraft is smaller than that of theconventional aircraft with the same airfield requirement, and the gains in structure weight can make the jet-flap machine lighterif the wing loading of the conventional aircraft with which it is being compared is low enough—that is if the runway length speci-fied is below a certain value. In Fig. 12 are compared the direct operating costs of a jet-flap aircraft and a conventional propeller-turbine aircraft suitable for medium range operations (at the same range and payload) plotted as a function of the runway lengthspecified. These results are at the mercy of the assumptions made about weight penalties and the like, but variations to the extreme Fig. 12. Comparison between medium-range turboprop and jet-flap transports for same range and payload: A, allowance for ducts and structure; B, allowance for ducts only; C, no allowances. TURBOPROP JET FLAP WITH ALLOWANCE FOR DUCTS AND STRUCTURE TUR&OPHOP 1JDOO 2£00 1D00 RONWAY LENGTH (Hi 4P00 U»0 2fl00 5000 RUNWAY LENGTH (ft) 4J3O 7.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
