Aerodynamic Drag and Its Reduction

Unambiguous definitions and understanding drag terminology are essential for design and analysis of an aircraft. The extensive literature on drag and the abundance of terms featuring in it form a challenge to the advanced designer who is not a specialist in aerodynamics. This chapter presents the following schemes for decomposing the total drag of a closed lifting body in a single diagram: (1) pressure and friction drag; (2) viscous, vortex and wave drag; and (3) parasite and induced drag. This drag breakdown applies to an airplane for which lift and drag associated with engine operation are minor effects. The basic drag components are interrelated in a diagram which also incorporates major physical mechanisms and influential geometry on each component. The parasite drag coefficient of wings and fuselage bodies is related to the ratio of their frontal to wetted area. The induced drag coefficient of a planar lifting surface is derived from its aspect ratio and the lift distribution along the span. A comparison of several nonplanar lifting systems reveals that a significant induced drag reduction can be achieved relative to a planar wing with the same span and lift. Airplane drag polars are mostly represented by a two-term parabola consisting of zero-lift drag and drag due to lift. A simple method for estimating the maximum lift/drag ratio of a new design is derived from the skin friction drag of an equivalent flat plate, form drag and a statistical correction for distributed roughness. Promising aerodynamic technologies for drag reduction are discussed, such as laminar flow control and lateral wing camber.