Elements of Aerodynamic Wing Design

The main subject of this chapter is selection of the planform geometry which precedes the conception of the lateral airfoil section and twist distribution. The first case considered is the optimum planform geometry of straight-tapered wings with negligible sweep designed for subsonic flow. The second case deals with the aerodynamic design of transonic wings. Primary selection variables used are the design lift coefficient in cruising flight and the aspect ratio or the span loading. Optimization of a subsonic wing is based on the wing penalty function, consisting of the mission fuel and the weight of the power plant required to balance the wing drag and the wing structure weight. Closed form equations are derived for partial as well as global unconstrained optimum wing loading and aspect ratio. A design selection chart illustrates the results for a propeller-powered freighter design. Introduction of low speed performance constraints shows that, for this case, the unconstrained global optimizer is a feasible design. Optimization of a transonic airplane is complicated by compressibility effects. In this case, all wings are designed so that, for a specified cruise Mach number, they have the same wave drag coefficient. This requires adaption of the wing thickness ratio and sweep angle based on Korn's equation. This approach leads to a significant effect on the unconstrained optimum wing loading and aspect ratio. Requirements of the available tank volume, the buffet boundary and pitch-up avoidance are introduced to find the constrained optimum design. The chapter concludes with considerations of lateral variation of sections, tip devices and design for high lift.