A recent study by digital computer of the theoretical characteristics of a family of low drag aerofoil sections was prompted some time ago by the need to redesign a particular NACA 6-series aerofoil of high camber (which was not producing the desired pressure distribution). Understandably, therefore, this new family of aerofoils—christened the GU series—was patterned on the NACA sections, and designed to provide the characteristic region of uniform velocity over the forward portion of one surface at the extreme of the low drag range, followed by a compression region of roughly constant adverse pressure gradient.