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Aviation History
1942
1942 - 0665.PDF
MARCH 26TH, 194: FLIGHT 2S9 TOPICAL AIRCRAFT PROBLEMS Performance and Wing Loading : Boundary Layer Control : Direct 'Fuel Injection : Cooling Difficulties : Turbo-blowers and Ejector Exhaust INCREASES in perform ance and greater avail ability and reliability are the requirements which, in W^ie of war more than ever, By PROF. DR.-INC. GUNTHER BOCK THIS article, which is a translation of a paper read before the Lillienthal-Gesellschaft, Berlin, last December, surveys the problems confronting the aircraft designer, and outlines some solutions. tremely high wing loadings. High wing loadings may bring with them high landing speeds. In order to prevent this it is customary to pro- are demanded of flying equip- [ n j vide the wings with mechan- I ment. Improvements in per formance include greater speed, greater height and greater range. Improvements in availability and reliability are directed towards the production of aircraft which shall be ready for use in any sort of weather, and which shall be suitable for the rough handling received in air fighting as well as in operating from difficult landing grounds in the field. A measure of the progress in the development of per formance is afforded by the increase in the flying speed which has taken place between the world-war 1914-18 and the present war. While the maximum speed of fighters at the end of the world war was about 190 km/hr. (118 m.p.h.), that of to-day far exceeds600km/hr. (370m.p.h.). This increase in performance was brought about by im provements in the aerodynamic efficiency of the aircraft, by the employment of more powerful motors of lower weight and more favourable shape, and by going over to high-altitude flying. The following will examine the roads to further increases in performance which still exist, and how such increases are likely to react on flying qualities and safety. • Aerodynamic Improvement Let us begin with the possibility of aerodynamic im provement of the airframe. In present-day aircraft approxi mately 40 per cent, of the total air resistance is governed by the size of the wings. An obvious way, therefore, is to reduce the size of the wings, and thus the wing drag. That way has already been trodden in the past, and. since the world-war wing loadings have gone up from 40 kg/sq. m. (8 lb./sq. ft.) to 200 kg/sq. m. (41 lb./sq. ft.) and more. Very thorough investigations now show that only in excep tional cases can a further increase in wing loading be recom mended. In the case of high-altitude aircraft, the highest useful value of wing loading has already been closely approached, since flight in the thinner air at altitude demands greater wing area than flight near the ground. K^*. To this must be added the fact that, in addition to maximum speed, climb and manoeu vrability are .of great impor tance in the evaluation of the fighting power of an air craft ; and these two qualities are both wor sened bv ex- 0.O0 50 0.0040 0.0030 0.0020 00010 • 0 H FN*. UJ ^^ itl 1 RA G C U _ TUP LAI* JBULENT DRAG WAR DR AG 0.2 04 0.6 LOCATION OF TRANSITION riag 0.8 POINT 1.0 LAMINAR TURBULENT Fig. 1. Drag coefficient for laminar and tur bulent boundary layer. isms which raise the value of the maximum lift coefficient. Among these are all sorts of flaps and auxiliary wings which are adjusted by the pilot before landing. As experiments at the aerodynamic labora tory at Gottingen have shown, the effectiveness of such flaps can be increased by sucking away the boundary layer. This has been confirmed by flight tests. Work has also been done on reducing drag by improv ing the wing section. Until a few years ago there seemed to be little possibility of finding a wing section which would give an appreciable drag reduction. In recent years, how ever, a way has been indicated which appears to promise material progress in this direction. The Boundary Layer Wing drag is largely frictional -esistance. The air flow ing past the wing is so braked by friction that the particles of air in the immediate vicinity of the surface adhere to it, as it were, and only the particles some distance from the surface gradually attain the speed of the free air. The layer in which this speed gradient occurs has been named the boundary layer by Prandtl. It is represented, greatly magnified, in Fig. 1. Over the front portion of the wing section the air has an approximately parallel motion. The flow there is called laminar flow. Then suddenly the flow changes, and the air particles in the boundary layer have a violent movement at right angles to the direction of the main stream, which appears swirling. The turbulent boundary layer has a much greater frictional resistance than the laminar. The total profile drag is, therefore, very largely dependent upon the location of the transition point, i.e., the" point at which the laminar boundary layer flow changes into the turbulent flow. In Fig. 1 the transition point has been visualised as moved down the chord of the wing profile. This conception is based upon a wing of very small thickness, which has a chord of 2m. (6ft.), and which travels forwards at a speed of 600 km/hr. (370 m.p.h.) at a height of 6 km. (20,000ft.). If the boundary layer flow were turbulent over the whole chord, so that the transition point coincided with the leading edge, the profile drag coefficient would be 0.0054. It. now, we could succeed in moving the transition point to mid- chord, as shown in the illustration, the drag coefficient would drop to 0.0036 ; that is, a reduction of 33 per cent. Of this drag the laminar boundary layer would be respon sible for 13 per cent, only, while the remaining 87 per cent, would be due to the turbulent boundary layer. If the boundary layer flow were laminar over the whole chord, i.e., if the transition point lay on the trailing edge, the profile drag coefficient would be only 0.0007 '• that is. one-eighth Of the value for fully turbulent flow. From this it is clear how important it is to get the transition point as far back as possible towards the trailing edge. Both experimental and theoretical investigations point to a basic connection between transition point and pres sure distribution. It will be seen from the pressure distri bution shown in Fig. 2 that both on upper and lower surfaces of the profile the transition points lie near the beginning of the pressure gradient. Consequently, if we could succeed in finding wing profiles in.which the pressure gradient began as far aft as possible, there would be a
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