It has almost become a tradition in recent years to begin a paper on boundary layer by paying a tribute to the great Prandtl and his famous paper in which he introduced the conception of boundary layer into Fluid Dynamics.The year 1954 is, however, not only memorable to mark the passing of fifty years since Prandtl's classical demonstration of the effect of boundary layer suction on the flow pattern around a cylinder; it is also memorable because in 1954 most convincing demonstrations of practical applications of boundary layer control for aircraft have taken place. I am referring, in particular, to the demonstrations with the Attinello flap in the United States which mark the introduction of one form of boundary layer control as an engineering and practical reality.
It was a great revelation to me to read Lieut.-Comdr. Graham's study of the “Safety Devices on Wings of Birds,” reprinted in the Journal of the Aeronautical Society of January, 1932. I had always believed that the gaps formed by the tip feathers on a bird's wing were not strictly comparable to slots, because from a superficial observation of birds’ wings, it appeared that the slots were running in the wrong direction, namely, from the top to the lower surface of the wing. Lieut.-Comdr. Graham's outstanding contribution is his discovery of the reason for the emargination of the web of the slot forming wing tip feathers. There can be no doubt that this arrangement is aerodynamically equivalent to a multi-slotted wing.
During the last four or five years a remarkable evolution has taken place in the policy of aircraft design. Its ultimate aim is to reduce the aeroplane to the simplest possible form offering the least resistance to the air. This process of eliminating any structural members which are not necessary for the production of lift and the housing of passengers or military load has automatically led to the adaptation of the fully cantilever monoplane with retractable undercarriage (Fig. I). The final logical result of this evolution is, of course, the flying wing with the engines, passengers or military load housed inside, thus suppressing such components like the fuselage and the tail unit which at the moment are still causing a non-productive addition to the total drag. There are still certain difficulties which will prevent us from achieving the ideal of, say, a mere flying wing, and our present compromise solution consisting of a tapered cantilever wing, a streamlined fuselage and a cantilever tail unit will probably persist for some time to come. The next step in suppressing parasitic drag will, therefore, consist in a still greater refinement of fuselage design and in eliminating outside engine nacelles by using engines buried inside the wing.
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