Tests of a griffith aerofoil in the 13 ft. x 9 ft. wind tunnel. Parts I. II, III and IV

Abstract

PART I. Wind-tunnel Technique and Interim Note. PART II. Effect of Concavity on Drag. PART III. The Effects of Wide Slots and of Premature Transition to Turbulence. PART IV. Lift, Drag, Pitching Moments and Velocity Distributions. This report describes tests carried out on a 16 per cent. thick Griffith suction aerofoil in the 13 ft. x 9 ft. wind tunnel. Prior to these tests being carried out, the principle involved in the design of these aerofoils had only been justified experimentally by tests on a very small scale in the National Physical Laboratory 4-ft. wind tunnel 1 ; the purpose of the present tests was to verify the feasibility of the Griffith 'discontinuity' principle on a satisfactory scale, and to obtain quantitative data on the aerofoil characteristics with and without suction, the amount of suction needed to prevent separation and to develop the optimum slot shape and width for maximum efficiency. Part I describes the technique used in the experiments, and the method of interpretation of the results to include in the drag a term to account for the power used to develop the necessary suction. The experiments show that separation of the flow on the surface can be fully prevented on this type of aerofoil by sucking less than half the air in the laminar boundary layer at the design position of the slot. If the flow is turbulent from the wing leading edge, the amount of air that must be sucked away is very little greater than that if the flow is laminar to the slot. In the experiments of Ref. 1, it was found that the flow to the rear of the suction slot remained laminar to the trailing edge of the aerofoil. In the present experiments this was not found to be so, transition to turbulence occurring some distance rear of the slot. Part II (page 7) of this report describes an investigation of this effect and shows that this instability results from the dynamic instability of the boundary layer along a concave surface, and that it is impossible to design any practicable aerofoil shape over which this instability can be prevented at the Reynolds numbers of flight. Part III (page 11) of the report extends the investigation of slot design to greater slot widths and less extreme shapes and includes the effect on suction mass flow of premature transition to turbulence forward. In Part IV (page 16) aerofoil characteristics are discussed both with and without suction, including the velocity distribution over the aeroIoil, lift coefficient, pitching moments and hinge moment variation with incidence. The effective drag coefficient variation is examined and extrapolation to full-scale Reynolds numbers carried out. It is shown that even with turbulent flow aft of the suction slot, a low-drag coefficient may be anticipated at the Reynolds numbers of flight. The effect of nacelles on suction wings is also examined.