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Aviation History
1944
1944 - 0504.PDF
?.(>o FLIGHT MARCH 9TH, 1944 BOUNDARY LAYER CONTROL GREATLY IMPROVED PERFORMANCE AS A RESULT OF REDUCTION IN DRAG : INHERENTLY APPLICABLE TO JET-PROPELLED AIRCRAFT THE very simple principle of the turbine makes it a most desirable power unit for many forms of propul sion In addition to this inherent quality, there is immense contributory benefit to be derived from the employment of a type of power unit for aircraft which permits a design of excellent aerodynamic form, devoid of excrescences which create parasitic drag and air flow turbulence. The turbine-compressor is such a unit; it is compact, self-contained and of convenient cylindrical shape. in the consideration of an aircraft equipped with a turbo- unit, the necessity for providing an air intake for the tur bine can be turned to advantage by arranging the intake inlet to exert an influence on the boundary layer air flow. This may probably require amplification and so a brief description of what this layer consists may be found useful. the boundary layer is that region adjacent to the skin of the aircraft where particles of air are in a transitional stage of acceleration up to the speed of the air flow past the aircraft. This is illustrated diagrammatically in Figs, la and lb. The layer of air immediately next to the skin tends to adhere to it, the layer above that is less able to resist the general air flow, and so on successively until the outer layer of the boundary region is moving at the same speed as that portion of the main air mass flowing past the aircraft. It must be pointed out, however, that the phenomenon is by no means quite so simple of explanation as this; there are numerous factors which influence Ixmndary layer characteristics, and the composition of the layer itself is also subject to variation. The description above is, at best, very general, and anything further than this is both beyond the scope of, and out of place in, an article of this nature. Readers who desire to investigate this aspect of aerodynamics for themselves are referred to Warner—Airplane Design [Performance) \ and Dodge and MAIN MASS AIR STREAM Fig. 2. Boundary layer flow near the separation point on a circular cylinder. Thompson—Fluid Mechanics (from which Figs. 2, 3a and 3b are taken). The resistance to motion through the air for any air craft can be considered as being made up to two quanti ties, (a) the effect of viscous shearing stresses in the air, the resultant being known as skin-friction, and (b) the drag caused by the formation of a turbulent wake. The shape of the aircraft—fuselage, wings, tail unit, etc.—determines the character of pressure distribution over its entire sur face, and with finely streamlined shapes the changes of pressure are very gradual, so that separation of flow into turbulence is close to the rear end of the body, and the resulting wake is narrow. The resistance due to (b), in such cases, is only a very small part of the total drag, the remainder being due to (a) as the boundary layer covers almost the entire surface. Effect on Mass Flow Disregarding the physical properties of BOUNDARY LAYER REGENERATION OF BOUNDARY LAYER AFT OF INTAKE ORIFICE AIR INTAKE TO TURBO-COMPRESSOR Fig. ia. Diagrammatic illustration of boundary layer formation and relation to major air stream, showing abstraction into the turbo-compressor of air from boundary region in order to delay the separation point. MAIN MASS AIR STREAM PRESSURISED BOUNDARY LAYER Fig. ib. This shows the efflux from the turbo-compressor being ejected tan- gentially to the aircraft skin curvature in order to accelerate the boundary layer air and so prevent separation. the air, the depth and growth of the boundary layer is influenced in relation to the curvature of the shape, this curvatrn.'^ being mathematically constructed accord ing to the fineness ratio, i.e., the ratio of length to maximum cross-sectional dia meter. However, the break down into turbulence near the separation point in a boundary layer, as shown in Fig. 2, pro foundly affects the character of the major air stream flow, and does, in fact, induce a break down from laminar flow into general turbulence in the wake, so that the two quantities (a) and (b) which compose total drag are inter-related. Boundary layer control, with a view to reducing drag and thereby improving overall performance, has been the subject of a great deal of research and experiment for about 40 years, ever since Prandtl introduced, in 1904, what has become known as his boundary layer theory, iks a side issue to this it is of interest to record that the first explanatory theory covering the field of skin-friction was pro-
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