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Flow control and wing. design
Preamble
This chapter deals with some of the fundamental principles of wing design for maximizing lift and minimizing drag. In this regard the behaviour of the boundary layer is critical and many design techniques and flow-control methods are available to counter adverse developments. Ways of identifying the most advantageous pressure distributions over the wing are given for low – and high-speed flows. Lift augmentation at low speeds by the use of multielement aerofoils and various types of flap is described, along with several methods of direct boundary-layer control. The chapter closes with descriptions of the methods for reducing drag in Its various forms (skin-friction, form, induced, and wave drag).
8.1 Introduction
Wing design is an exceedingly complex and multi-faceted subject. It is not possible to do justice to all that it involves in the present text. It is possible, however, to cover some of the fundamental principles that underly design for high lift and low drag.
For fixed air properties and freestream speed, lift can be augmented in four main ways, namely:
(i) Increase in wing area;
(ii) Rise in angle of incidence;
(iii) Increased camber; or
(iv) Increased circulation by the judicious application of high-momentum fluid.
The extent to which (ii) and (iii) can be exploited is governed by the behaviour of the boundary layer. A wing can only continue to generate lift successfully if boundary-layer separation is either avoided or closely controlled. Lift augmentation is usually accomplished by deploying various high-lift devices, such as Daps and multielement aerofoils. Such devices lead to increased drag, so they are generally used only at the low speeds encountered during take-off and landing. Nevertheless, it is instructive to examine the factors governing the maximum lift achievable with an unmodified single-element aerofoil before passing to a consideration of the various high-lift
devices. Accordingly, in what follows the maximization of lift for single-element aerofoils is considered in Section 8.2, followed by Section 8.3 on multi-element aerofoils and various types of flap, and Section 8.4 on other methods of boundary – layer control. Finally, the various methods used for drag reduction are described in Sections 8.5 to 8.8.
