In considering the principles of the design of low‐drag aerofoils, we saw that we should like the velocity outside the boundary layer to rise to a position as far back along the wing as possible, but that we are hindered by the danger of turbulent separation if the rate of velocity decrease at the back of the aerofoil is too great; the danger increases, roughly, when the thickness of the aero‐foil increases and when the position of maximum suction is moved further back. If, however, the whole of the pressure recovery, or velocity decrease, is concentrated over a very narrow interval along the chord, over which the boundary layer, or as much of it as necessary, is sucked away to stop separation, all danger of separation is avoided, and we can have, if we wish, a favourable velocity gradient over the whole of the rest of the chord. More specifically, if the upstream boundary layer, or part of it, is sucked in through a slot, there must be a streamline that divides the air crossing the slot from that entering it, and that streamline must meet the surface at a stagnation point (fig. 10). With the stagnation point behind the slot the flow in the boundary layer along the surface is reversed in direction between the stagnation point and the slot, and all danger of separation is avoided with a well‐designed slot if the design of the aerofoil is such that the pressure recovery takes place wholly between the upstream lip of the slot and the stagnation point. Up to now suction aerofoils have actually been designed with a discontinuous drop in velocity; the velocity rises to a given position on the chord, drops discontinuously, and thereafter rises, stays constant, or at any rate docs not decrease rapidly enough to produce any danger of separation, to the trailing edge; the velocity distribution has this character on both the top and bottom surfaces.
(1948), "Low‐Drag and Suction Aerofoils: The Eleventh Wright Brothers Lecture delivered by Dr Sydney Goldstein, Chairman of the Aeronautical Research Council, before the Institute of the Aeronautical Sciences on December 17th, 1947", Aircraft Engineering and Aerospace Technology, Vol. 20 No. 6, pp. 160-181. https://doi.org/10.1108/eb031641
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