Abstract:
Part I. The effect of isolated surface excrescences in a laminar boundary layer in producing disturbances which may lead to turbulent flow has been examined experimentally by several methods. Photographs of some of the flow patterns visualised by smoke and china-clay techniques are given. The critical heights of pimple which just give rise to spreading wedges of turbulent flow have been measured on a flat plate and on two aerofoils at several angles of incidence. The results are analysed and are presented in a form which enables approximate estimates to be made of the protuberances permissible on laminar-flow surfaces at full-scale flight Reynolds numbers. The estimates suggest that at an altitude of 30,000 It the critical pimple height is 0.004 in. for a speed of 850 m.p.h., whilst 0.002 in. may be permissible at all subsonic speeds. At sea-level, however, the tolerances are approximately halved. Part II. A method is given by Gregory and Walker in Part I for estimating the height of isolated surface excrescences that are just sufficient to cause a turbulent wedge in a laminar boundary layer. It is an empirical method based on wind-tunnel tests made at the National Physical Laboratory at wind speeds up to 200 ft/sec and aerofoils of 30-in. and 60-in. chord (maximum Reynolds number of approximately 3 x 10power6). To provide information at flight Reynolds numbers, two flights have been made on a Vampire aircraft indicating the effect of tiny paint pimples on the laminar boundary layer at a Reynolds number, based on wing chord, of 25 x l0 G near sea-level. It was found that the critical pimple height at 0.03-chord was 0.001 in. increasing to 0.003 in. at 0.20-chord, values which are within experimental error of those estimated by the method of Ref. 1. Although the pimples were of no specific shape, e.g., cylindrical or conical, it is suggested that, in view of the close agreement between estimated and observed results, no further flight tests are necessary.
