Estimating the Oswald Factor from Basic Aircraft Geometrical Parameters

Abstract

This paper provides a sufficient accurate estimation method for the span efficiency factor, e, during the preliminary design stage. First, several approaches are identified from an exhaustive literature study and the accuracy and logic of these approaches are assessed. Second, a simple and logical general form is proposed as evaluation method for the Oswald factor for conventional configurations. The method starts with the calculation of a theoretical Oswald factor that accounts for the two parameters taper ratio and sweep - one depending on the other for a maximum Oswald factor. It then corrects the theoretical Oswald factor for the fuselage influence, zero lift drag influence and Mach number influence, making use of statistical aircraft data. Third, the paper delivers a method to calculate the Oswald factor for non-planar configurations, hence covering both conventional and unconventional designs. Out of the non-planar configurations, more attention is given to the wing with winglets, wing with dihedral and the box wing. For the box wing configuration an additional equation is proposed. The method proposed for estimating the Oswald factor is simple and accurate, with deviations under 4 %. It requires only few basic aircraft geometrical parameters and is useful for an efficient implementation into code for aircraft design optimization.