ABSTRACT
To improve aircraft performance one can gainfully reduce drag, which will translate into better range and endurance, as well as economical operation. Drag breakdown of a subsonic transport airplane is shown in Fig. 9.1 [426], highlighting the importance of fuselage drag after the flow over the wing is rendered laminar by some passive means. This can bring down the wing drag from roughly 27% of total viscous drag to 15%. This seems feasible, as [428] has noted that the profile drag of 0.0085 for a fully turbulent airfoil at a Reynolds number of 107 comes down to 0.0010 if the flow is kept completely laminar. Thus, it is imperative to understand how such laminar flow can be achieved over an aircraft wing. To begin with, one must understand the airfoil drag contribution and how this can be pegged to a lower level. In this regard, a natural laminar flow (NLF) airfoil maintains extensive laminar flow solely by means of a favorable pressure gradient. This is a passive means to control the flow over an airfoil, as opposed to various possibilities investigated via active control. From the desired performance features of a regional aircraft, one can infer the desired aerodynamic characteristics of an NLF airfoil for such applications. Such an aircraft would have a chord Reynolds number in the range of about 4 to 17 million and a cruise Mach number in the range of 0.4 to 0.6.
