Effect on aerofoil drag of boundary-layer suction behind a shock wave

Abstract

To measure, at Mach numbers near the critical value, the reduction in drag due to boundarydayei suction on the upper surface of an aerofoil. To determine whether this reduction can be obtained with an economic use of power. The experiments were made on 2 in. chord aerofoils, NACA 0020 section, at 0 deg. and 4 deg. Drag was determined from pitot tube traverses at one chord behind the trailing edge of the model. Information on the flow over the upper surface was obtained from pitot tube traverses at 0.02 chord behind the trailing edge, from visual observation of shock waves, from surface tube observations just forward of the slot, and from normal pressure measurements. The cases considered are those for which shock waves cause boundary-layer separation and those for which shock waves are not present or are too weak to cause separation. Estimates of the power absorbed by the compressor, ignoring duct losses, are Obtained from (i) measurements of the mass of air sucked and the maximum stagnation pressure of the air issuing from the slot, and (ii) a boundary-layer relation which includes entry shock losses but not the losses in the slot. Suction has little effect on the critical Mach numbers, Mc = 0.65 for α = 0 deg. and 0.57 for α = 4 deg., but the minimum drags with suction on the upper surface are 40 per cent., α = 0 deg., and 50 per cent., α = 4 deg. lower than the aerofoil drags without suction, 0.45 < M < 0.735. The drag coefficients measured at the critical Mach numbers without suction are obtained with suction at Mach numbers which are 0.08, α = 0 deg., and 0.105, α = 4 deg., higher. The drag falls to its minimum value when about 0.6 of the mass of air in the boundary layer is sucked. In the present experiments, the power saved by the reduction in drag due to suction is about the same as the estimated power absorbed by the compressor. The experiments give promise that, at the Reynolds numbers of flight and for an efficient slot and suction system, the drag coefficient of a wing at the critical Mach number without suction can be maintained at the same value and. with an economic use of power to a higher Mach number, say 0.1 higher, by boundary-layer suction.