The design of suction aerofoils with a very large CL-range

Abstract

The use of suction slots to remove the boundary layer at points where the air velocity has a discontinuity opens up wide new fields in aerofoil design. It becomes possible to envisage aerofoils which have laminar flow characteristics over the greater part of the surface throughout a Q-range so large as to completely cover the normal flight range, and which are also thick enough to provide ample room for the stowage of engines, passengers and other loads at much lower all-up weights than have hitherto been feasible. This paper considers four aerofoils designed on the basis of their velocity distributions in two-dimensional incompressible potential flow. The design method used was that of Lighthill's exact theory, set out in R. & M. 2112, which involves prescribing the velocity over the aerofoil surface as a function of position on the circle into which the aerofoil may be transformed. A few additional techniques to procure suitable velocity distributions were employed, and an exposition of these will be the subject of a later paper. The principal feature in the design is the replacement of the region of falling velocffy over the rear part of the aerofoll by a single discontinuity in velocity, at which point boundary-layer suction is applied. Thus adverse pressure gradients are completely eliminated throughout a wide range of incidence. The boundary layer remains thin and laminar flow may be achieved, even on aerofoils of very great thickness. At the discontinuity the mathematical shape is a logarithmic spiral, but this must be modified in practice to include the suction slot. In one aerofoil the spiral is avoided by having a steep fall of velocity over a short distance of the surface instead of a complete discontinuity, but this may detract from the performance. The paper discusses the relative merits of the aerofoils and considers possible improvements. Zero pitching moment is very desirable and can readily be achieved. A suction slot on the lower surface proves to be unnecessary for aerofoils cambered as these are so as to be efficient at high lifts, but it may be unavoidable if a less cambered design is required in an effort to get a higher critical Mach number. This is inevitably low with aerofoils of this thickness, and is the chief drawback of thick suction wings.