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Aviation History
1946
1946 - 1183.PDF
JUNE 13TH, 1946 FLIGHT 607 RECENT AERODYNAMIC DEVELOPMENTS flight, but one can well imagine that itwould present very considerable diffi- culties in attaining good control at lowspeeds. A little ordinary wind tunnel work on a design of this kind wouldgive a useful indication of the practic- ability of such a revolutionary layout. In summarising this part of my lectureI can safely say that the transonic region is at present the greatest unknown in thequest for higher and higher speeds of flight, and that it presents great diffi-culties of investigation owing to the fact that ordinary laboratory methods failand we are being forced to develop a new technique. Here, indeed, is analmost virgin field for the research worker of the immediate future. I make no apology for a few words onthe subject of " engines " in a lecture on aerodynamic developments, for thejet-propulsion type of power plant in- volves aerodynamic problems at least asmuch as engineering ones. Moreover, its progeny, especially when they indulgein supersonic flight, may well become almost purely aerodynamic. The efficiency of the combustion pro-cess in a gas turbine depends almost en- tirely on aerodynamic considerations.The compressor and turbine depend on aerodynamic reactions related to thoseof simple aerofoils, while the intake and ducting must be designed to minimizeloss of energy by any breakaway of flow in them; they must be "internallystreamlined," so to speak. Moreover, the air and gas speeds are high, so that theMach number of the internal flow may often approach unity. It follows thatgeneral theoretical and - experimental work on airflow at high Mach numbersnow has a very direct bearing on the design of the'engine as well as the air-craft. When we come to supersonic flightspeeds a curious position arises. The compressor of the jet engine becomes lessand less a necessity as the speed increases because the compression of the air at theintake due to the forward motion, the so-called ram effect, becomes increasinglyable to provide the pressure ratio re- quired for the subsequent processes ofcombustion and ejection. If, for in- stance, we assume that there is a planenormal shock wave across the entry duct, • Rockets, radio and radar will provide : supersonic data. the pressure ratio is 3.65 at a Mach num-ber of 1.5 and 7.85 at a Mach number of 2.0.It is quite likely that these pressure ratios can be improved by designing theintake so that the flow just inside it is still supersonic, the pressure recoverytaking place through one or more in- clined shocks followed by a weak normalshock. The " engine " to which these ideaslead is, in a sense, no engine at all. It is merely a duct in which aerodynamicand thermal changes occur, and it has been described as an aero-thermo-dynamic-duct, a somewhat lengthy word that some have contracted to " Athodyd " for convenience. Theprinciple is precisely the same as that of the normal gas turbine except thatthe initial compression is now entirely due to ram efiect, and so there is norotating part at all. Since the compres- sion increases with speed, so also doesthe thermal efficiency. The one difficulty lies in the fact thatthe action depends on the forward speed; the " static " thrust is zero and the" take-off " thrust is negligibly small. The use of the athodyd for supersonicflight will thus involve some other source of power, possibly rockets, to acceleratethe aircraft to a speed sufficient to allow the athodyd to begin work with reason-able efficiency. An alternative may be to carry normal gas turbines plusathodyds. As far as I am aware no complete testof this new propulsive device has yet been made, save for a very tentative ex-periment on an athodyd about one inch in diameter in the twelve-inch supersonictunnel of the N.P.L. The object of the test was to compare a measured thrustwith that calculated. There is no reason to doubt calculated values, provided thatthe intake conditions are known, so that the right shock-wave formation at theintake can be used in computing the pressure in the combustion chamber. The The bi-fuel rocket a possible useful way to study characteristics of supersonic flight. difficulty is to be sure of these intakeconditions, which are known to depend on the " effective blockage " in the ductdue to the processes of compression, combustion and subsequent expansion tothe outlet jet. Here, again, is a most interesting and fruitful field of researchfor .the future. Before concluding this brief referenceto new means of propulsion 1 must men- tion the bi-fuel rocket, of the kind usedby Germany in the V-2 weapon, and in a short-duration, high-parformancefighter. It has the inherent disadvan- tage of high fuel weight, since the oxy-gen as well as the combustible must be carried, but its interest lies in the factthat it may well provide an easy way to make supersonic fights in experimentalaircraft for the purpose of beginning to study, in the air, the new problems in-volved. This is because the high tem- perature that is possible in the shortcombustion chamber and exit nozzle allows the production of a far higherspecific thrust than can be obtained in the same space from a gas turbine, whileavoiding the difficulty of take-off associ- ated with the athodyd. It provides, inshort, a very high thrust which is almost independent of speed or altitude. I shall be accused of not being up todate if I do not at least mention atomic energy! Needless to say, the possi-bility of a propulsive agent which in- volves an almost negligible weight wouldrevolutionize all our ideas about high- speed aircraft. In fact, it would leaveonly one barrier to the attainment of almost any speed we wished: the barrierof temperature rise. This limitation may soon prove quite serious, even withmethods of propulsion that we can now foresee. As is well known, the surface of abody in motion is raised in temperature by very nearly the thermal equivalent ot the kinetic energy, that is, by anamount V2/2Cp or its equivalent MsT/5 tor air, where M is the Alach number andT the absolute air temperature. Thus for a Mach number of 1.4 the rise is 85 deg-Cin the stratosphere, and for M= \/5 the absolute temperature of the air is nearlydoubled. It does seem that if almost unlimited power ever becomes availablefor propulsion, the limit to man's achievement in speed may finally be wtby temperature. In this lecture I have deliberately notgone into any great detail; I have tried, and I hope I have succeeded in some Ultimate limit to man's achievement in flight speed is likely to be set by temperature. small degree, to indicate the nature of thenew ideas that have arisen recently in aerodynamic research, and to show whatbearing they have on the future In the past there have been periods inwhich the greatest advances came from improved engines, and others marked bygreat improvements in cleaner aero- dynamic design. It looks very much asthough the next period will be charac- terized by both, for we see glimmeringsof the possibilities of drags far lower than anyone could have hoped for a few yearsago, while on the engine side the recent marked successes of jet propulsion canonly be the first-fruits of the new forms of power plant. I do not believe there was ever a periodin aeronautical history so bright with the possibility of future progress. I can onlyhope that we shall be wise enough not to allow the inevitable retrenchment aftera long and costly war to hamper too seriously our research into the unknown.To my mind the one thing on*which we should not cut down expenditure is re-search, for it is only by research that we can be ready to meet the demands ofthe future. Indeed I would be so bold as to suggest that we ought greatly toincrease our research facilities for two very good reasons. One is that the warhas produced so many urgent problems that we have not had time for the lei-surely but very hard thinking, which constitutes the research method. The re-sult is a surfeit of ideas arising in all manner of ways, but not followed-upor thought-out in detail. We are,' there- fore, behind-hand with the implicationof these ideas, a process which will in- volve a lot of work by a lot of men. The second leason is even more im-portant. The researches arising from the ideas I have sketched in this lecture need,foi their proper study, equipment of a kind which we do not possess in thiscountry; equipment which, in some cases, is complex and costly. If we donot provide such apparatus, and that quickly, we shall lose a great oppor-tunity for British aeronautical science. Lastly, one other thing that I hopemost fervently is that we may be able in the future to advance hand-in-handwith our great wartime ally and friend, the United States of America. Surelythe continuance of the close co-operation which was so effectively established dur-ing the war years into the years of peace, would be the greatest tribute we couldpay to those two great men who did so much to inaugurate the aerial age, andin whose honour we hold this Wilbur Wright lecture every year.
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