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Aviation History
1939
1939 - 0204.PDF
d FLIGHT. JANUARY 26, 1939 An exterior view of one wing, showing the extent of the perforated surface through which the boundary layer is withdrawn. ing angle, and later of top speed, though this was not so noticeably affected. In measuring the rate of climb a series of tests were made with the engines throttled to 1,800 r.p.m. in order to prevent the overloading of the engines in a prolonged climb. During each flight the engines were being run under climbing conditions for practically an hour and a hall, and, in consequence, it will be appreciated that there was a tendency for the engines to overheat on full throttle. On the throttled climbs, averaged over about 20 flights, there was a 29 per cent, improvement for an extra expen diture of 8 h.p. in sucking away the boundary layer, at a blower efficiency of 60 per cent. To have obtained this extra increase in rate of climb by means of the aircraft engines themselves an extra 17 h.p. would have been required, so that you will see we got quite a sensible advantage by sucking away the boundary layer. Next we made a number of glides with the engines throttled back at the minimum rate of descent. Here the reduction in minimum gliding angle averaged ig per cent., or in other words, the CL/CD increased by approximately the same amount. This improvement was obtained for a suction h.p. of 6| at the same exhauster efficiency. Lastly, we made a few runs at top speed and found a 4-5 "per cent, increase in speed for an additional power expenditure of 13 h.p., compared with 27 h.p. which would have had to be developed by the engines in order to get the same improvement. Analysis has shown that at top speed the effect of removing the boundary layer was to reduce the total drag of the aeroplane by 8 per cent., or assuming that all the improvement was due to reducing the wing wake, this was equivalent to 22 per cent, decrease in the profile drag of the wings. Needless to say, the experiments are not yet sufficiently advanced to draw any general conclu sions, but we do think that there are possibilities of a practical nature in this device. Perhaps the idea of sucking over large areas will not be a thing of the near future, if it should ever be proved worth while, but very probably it will.be used shortly to eliminate unavoidable drag caused by interference such as at the junction of the fuselage and wings, or between the fuselage and tail unit. Moreover, in spite of the some what discouraging results obtained in the wind tunnel on quite small models, I am of the opinion that the quantity of air to be withdrawn for a reasonable aerodynamic- improvement is very much dependent on the Reynolds number. This is not merely a guess on my part, but has been substantiated by some theoretical work which we have done, which shows that if a means can be found by which the boundary layer can be removed in such a way that it never becomes turbulent, or in other words, the effect of the suction is to produce everywhere an extremely thin laminar layer such as one finds normally on the lead ing edge of a wing, then the quantity of air to be removed is an inverse function of the Reynolds number. Conse quently, whereas on the model scale the quantity is discouragingly large, at the higher Reynolds numbers encountered in flight the quantity falls to very much lower values. Although the theory cannot be checked at the higher Reynolds numbers, it is in reasonable agreement with Schrenk's model results. This probability of a marked scale effect is encouraging, more especially as I do not think that with the small amount of suction used in our experiments we could have got the improvements we found unless such a scale effect exists. The Real Problem It remains now to find out whether it is possible to suck away the boundary layer without affecting it adversely. Is the solution a relatively weak suction spread over the entire surface and carefully graduated to conform with the pressure distribution? Or does it consist of a number oi concentrated suction points placed on the surface where the flow is critical? Or is it a combination of the two? Or will some other technique be found, at present unsuspected? The success of a surgical operation depends very largely upon the skill and highly developed technique of the surgeon. So also in the particular problem we have been discussing crude and clumsy methods may be the bar to success. An intensive research over the range of Reynold= numbers from model to full scale is needed, I think, to find out whether in reality it is possible to operate on the laminar boundary layer present initially in such a way that its natural instability does not become sufficient, even in the presence of an adverse pressure gradient, to cause it to become turbulent. If we can find a suitable process which will enable us at any rate to approximate satisfac torily to the mathematicians' no doubt idealistic concep tions, we shall then be able to say quite justifiably that we have "covered a comfortable bit of ground." TOPICS of the DAY (Concluded the pilot in a hopeless position, since height cannot be held until the undercarriage is retracted—an operation that may take anything up to half a minute. With so much excess power, safety might be further considered by linking up the automatic pilot in such a way that a yaw beyond a certain limit would bring the rudder gyro strongly into action, even when the automatic pilot itself is not switched in. Engine failure, whether momentary or otherwise, during the first part of the climb would then be counteracted automatically while the pilot was busy holding the machine at its most effective single- engined climbing speed, getting the undercarriage retracted and, possibly, pumping up the pressure or otherwise recti fying the trouble. The latest problem with really large machines is that of providing controls which can be operated by a pilot of Irom page a.) normal physical powers. At least one recent design has a reputation of wearing out its crew in rough weather when the automatic pilot is not in operation, while in another it is rumoured that power assistance is now being incorporated. All manner of difficulties crop up in the design of control surfaces. If they are aerodynamical! v balanced for light operation they are liable to go out of control at certain speeds and, in particular, when the lead ing edges of the surfaces build up a formation of ice. One way out of the difficulty is to fit irreversible con trols, but there is a good deal of criticism to be contendd with before such a system becomes standard practice. On1 of the special advantages of irreversibility is in the fad that flutter is finally eliminated and aerodynamic balanc ing, therefore, can be carried to what may be called a logical conclusion.
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