Results are presented for the aerodynamic characteristics of NACA 0012 aerofoil section at Reynolds numbers of 2-88 x 106 and 1.44 x 10 6 with some indications of scale effect at other Reynolds numbers. The measurement of CLmax at a Reynolds number of 2-88 x 10 6 is uncertain for two reasons. On the one hand, a laminar separation bubble disappears intermittently for reasons that are not fully understood. On the other, it was found that the flow broke down into a three-dimensional pattern when an appreciable extent of separation was present. This is a major problem. Boundary-layer control by suction in the vicinity of the wing/wall junction is shown to improve the two-dimensionality in the early stages of separation, but does not inhibit the appearance of three-dimensional flow at and beyond the stall. Distributed roughness was progressively applied from the trailing edge forwards over the upper surface of the aerofoil. Both sparse and dense distributions were used and they were intended to simulate the hoar frost deposit remaining after partial cleaning of the forward part of the aerofoil. Subject to the above qualifications, CLmax is not greatly reduced until the front edge of the roughness extends forward of 0.1 chord, at which stage also the drag increment due to the roughness rapidly becomes much larger.