Configuring a Civil Aircraft Wing: Positioning and Layout
The first task for designing the wing is to select a suitable aerofoil. Aerofoil design is a protracted and complex process that is beyond the scope of this book. After an aerofoil is selected (it could vary spanwise), the next task is to configure a wing planform with reference area. It is not like the fuselage sizing determined by the passenger number; initially, it is from statistics for the aircraft class. At the conceptual stage of the project study, typical values of wing twist and other refinements are taken from the past experience of a designer. The values must be substantiated and, if required, modified through CFD analysis and wind-tunnel testing to a point when the flight test may require final local refinements (e. g., flap and aileron rigging). Initially, an isolated wing is analyzed to quickly arrive at a suitable geometry and then studied with the fuselage integrated. Subsequently, the wing is sized formally (see Chapter 11).
6.5.1 Aerofoil Selection
Section 3.7 outlines the strategy to search for an aerofoil that would provide a high Cbmax as well as a high lift-curve slope (dCL/da), a high L/D ratio for the prescribed cruise speed, a low pitching moment, and gentle stalling characteristics. While retaining these characteristics, consultation with structural designers should decide an aerofoil t/c ratio that would permit good structural integrity to increase the aspect ratio. This is an area in which designers should gain over the competition with a better aerofoil and material. Finally, for high-subsonic cruise speed, the aerofoil shape should minimize compressibility effects (i. e., wave drag). Typically, a supercritical aerofoil with a relatively flat upper-surface profile (i. e., Whitcomb) reduces the transonic effects. Figure 6.9 shows a typical flat upper-surface pressure distribution at cruise (i. e., supercritical aerofoil). Good aerofoil sections are proprietary information and mostly are not available in the public domain.
To minimize repeating work that is similar in nature, the chosen aerofoil section for worked-out examples is kept the same for both civil and military aircraft designs. For a relatively low cruise Mach number of 0.65 at the LRC and 0.74 at the HSC, the NACA 65-410 is chosen for both designs. It is not exactly a supercritical aerofoil but serves the learning process because it is a known aerofoil successfully applied to many aircraft (Appendix C gives the details of NACA 65-410 aerofoil).