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Aviation History
1946
1946 - 1176.PDF
FLIGHT JUNE 13TH, 1940 RECENT AERODYNAMIC DEVELOPMENTS tions and degrees of approximation ofthe simpler theories, such as that of Ackeret for thin aerofoils. Experimental verifications of the theo-retical predictions are rather scanty, especially with regard to the behaviourof aerofoils of finite span, mainly on ac- count of the lack of supersonic windtunnels of reasonable size. There are only two such tunnels in this countryand neither of them has been in use for very long. One is the eleven-inch con-tinuous-flow tunnel in the Engineering Division of the N.P.L., which has beenmainly used for work on projectiles, while the other is a one-foot injector-type tunnel in the Aerodynamics Divi- sion. This latter is the original high-speed tunnel running on the compressed air supply ot the N.P.L. compressed airtunnel, and provided with supersonic effusers in place of the original parallelintake. It has been used, in its super- sonic form, for a preliminary study ofthe characteristics of two-dimensional aerofoils, and an example of the resultsis given in Fig 10. This diagram exhibits the lift anddrag characteristics of a double-wedge aerofoil and a comparison with the de-ductions from the simple Ackeret theory, which is shown to be a very good ap-proximation when the Mach number ex- ceeds about 1.2. In the drag diagraman allowance has been made for skin friction, which is, of course, absent fromthe theoretical prediction. No serious experimental work has yetbeen done on three-dimensional models, such as those of complete aircraft. Alarger tunnel is badly needed for such work, and the development of the experi-mental technique will take some time, not the least difficult point being themethod of supporting the model without undue interference with the flow aroundit. The eleven-inch tunnel mentioned above has recently been used to deter-mine the pressure distribution on short aerofoils mounted on one wall and has soprovided some experimental data on the effects of aspect ratio. We have, in the supersonic field, awell-developed theory and we may be very thankful for this; it will help usalong the right path until we can build experimental equipment to study thoseproblems which arise in practice and are usually too complex for theoretical treat-ment in detail. One such is the whole question of skin friction in supersonicflow, or, in other words, the relative im- portance of Mach number and Reynoldsnumber in defining aerodynamic be- •Vtox Q with full control will settle super- sonic operating conditions. havioui. Even here we have alreadysome indications, and we shall not be far wrong if we assume the ordinary lowspeed skin-friction laws to be true and add the drag they predict to that arisingfrom the shock-wave system. What will the supersonic aircraft ofthe future look like? This is not an easy question to answer, but some trendsare fairly apparent. It will certainly have very thin wings, and if it has abody at all, in the conventional sense, that body will be as long and thin as itcan conveniently be made. In fact, thinness'' seems to be the key to lowsupersonic drag. The aspect ratio will probably bequite low, though whether the wings will be heavily swept-back or not is atpresent a moot point; we do not know enough to predict with any certainty. Ishall have more to say on the effect of sweep-back when I come to consider veryhigh supersonic speeds, where this effect seems likely to be definitely beneficial.The problem will be to obtain enough O-6 rv A r>. ? Cn i -O-2 O-O6 p*O4 CD I n J f i Ml 2- > r yyj K M1 46 s / / 2O 2 4 6 INCIDENCE (DECREES) . 1-1 I'Z MACH 1-3 NUMBER 1-4 1-5 Fig. 10. Lift and drag curves ofdouble-wedge supersonic aerofoil in comparison with Ackeret's theory stowage space without sacrificing theslim lines that are necessary for reason- ably low drag. Whatever form the supersonic aircraftultimately takes, it will almost certainly differ greatly from that of present-daymachines, and one problem which will arise is that of control at low speedsduring take-off and landing. I have already suggested that boundary layersuction may provide the key to the at- tainment of sufficient lift on thin wings,but the control problem may prove very difficult if the aspect ratio is very low. Fortunately we have the means tostudy this point in our present large low- speed wind tunnels, and it would be ofmuch interest now to carry out low-speed experiments on the unconventional formsthat appear likely to be suitable for supersonic flight in order to see if theirlow-speed control problems can be satis- factorily solved. The point is an im-portant one since the maximum lift co- efficient attainable with full control willsettle the supersonic operating conditions to a great extent. A little arithmeticshows that if the aircraft is to fly at an incidence near that at which its lift-drag ratio is a maximum, either the wing loading must be considerably higher thanpresent-day figures or else the operating altitude must be much greater. Theformer alternative makes the landing problem more difficult while the latterhas obvkras difficulties relating to pres- sure cabins and engine performance. Perhaps it is too early even to specu-late on such things; the first problem is to get off the ground and attain super-sonic speed. When that has been done, and we have experience of the new con-ditions of flight, we shall be more ready to face the detailed question of how todesign the best form of supersonic machine and how to specify its bestoperating conditions. This brings me naturally to what is probably the mostdifficult problem aerodynamicists have yet had to face: What happens in theso-called "transonic" region, in the im- mediate neighbourhood of the speed ofsound ? In recent years much information has Wind tunnels of little use for "transonic' region experiments. been accumulated on aerodynamic be-haviour at high subsonic speeds. The necessity for this was forced upon us bythe, ever-increasing speeds of aircraft, for alf*aough until very recently aircraftcould not fly horizontally at speeds high enough to make compressibility effectsimportant, they could and did dive at such speeds, and troubles that were met,particularly with the controls, made it imperative to study the phenomena in-volved. Most of the basic facts are now so wellknown that I need not describe them in any detail. Suffice it to mention thatat a given incidence the lift coefficient increases at first with the Mach number,but later falls abruptly; the drag co- efficient remains nearly constant at first,but above a certain Mach number, de- pendent mainly on "thickness," risesvery rapidly, while the pitching moment can exhibit a variety of behaviours,varying from almost complete insensi- tivity to very violent changes in eitherdirection, depending on the wing shape, wing camber and relative position ofwing and tailplane. Any of the more violent changes occur at a Mach numbersomewhat greater than the so-called critical Mach number, which is that atwhich the local speed of sound is first reached at some point in the field of flow,usually near the point of maximum sue-, tion on the wing. ^s This leads to the general conclusion,™well borne out in practice, that no trouble is likely to be experienced fromcompressibility effects as long as the speed of flight is below that correspond-ing to the critical Mach number. Since the increment of velocity when the airflows round a body is least when the body is thin, we should expect "thin-ness " to be good for high subsonic speeds, just as we have seen that it isat supersonic speeds. This expectation is realized in prac"tice; for example, the Spitfire is one oi the few aircraft that have given notrouble in fast dives, and it is a par- ticularly '' thin '' aircraft. But thematter cannot be dismissed as simply as this; the more nearly we approach thesonic speed the more likely we are to
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