Note on profile drag calculations for low-drag wings with cusped trailing edges

Abstract

In R. & M. 1838 calculations of profile drag were made based on wing sections of conventional design, and were later extended in an Addendum to "low-drag" wing sections with convex trailing edges. Further calculations were required for low-drag sections of more recent design with cusped trailing edges. Calculations were made on sections of the NACA 65-family of thickness 0.12c and 0.23c with maximum thickness at 0.4c from the leading edge, over a range of Reynolds number and position of the transition points. The results were found to differ considerably from those of Ref. 2 when the transition points were far back from the leading edge, the calculated values of the drag coefficient being in some cases as much as 25 per cent. less than the previous calculations. The results were in good agreement with wind-tunnel tests made at the National Physical Laboratory and the Royal Aircraft Establishment, but showed a large discrepancy with flight tests made at the Royal Aircraft Establishment. In these flight tests transition was fixed by means of tapes, but no account was taken of the possibility that transition may have occurred behind the tapes and not necessarily at the tapes themselves. In this way the drag for a supposed mean transition point may have been underestimated.