Abstract:

ESDU 99003 introduces a series of low-drag aerofoils designed at Glasgow University and a computer program for generating their geometry and their inviscid flow characteristics. The low-drag NACA 6- and 6A-series are considered and it is explained that they achieve their low drag through a favourable pressure distribution on their forward surfaces that inhibits transition. As a result, on a plot of drag coefficient versus lift coefficient, they show a drag bucket - a region of near-constant low drag over a range of lift coefficient. Because of the limits of computational technology at that time, the design of those sections relied on linearised theory, but the GUNACA sections rely on the full theory of Lighthill to design sections with the same objectives and so show similar characteristics to the NACA 6-series sections. By comparing their flow (and geometric) characteristics with those of the NACA sections in a series of sketches, their potential benefits are illustrated. The program is provided as ESDUpac B9903. The output includes the ordinates of the sections, the velocity distributions at both the lower and upper values of the design range of lift coefficient (approximately defining the extent of the drag bucket), the pressure coefficient distribution at any incidence, and the distribution of a parameter that allows the velocity distribution to be calculated at any incidence. In addition the characteristics of the chosen section are summarised including its geometric features, its lift-curve slope and zero-lift angle, its aerodynamic centre and zero-lift pitching moment, and the extent of the design range of lift coefficient.