Considerations in Configuring the Wing
Following are general considerations important for designing the wing:
(1) wing reference area, S^
(2) span and aspect ratio
(3) aerofoil section, t/c ratio
(4) sweep, twist, dihedral, taper
(5) position (for the CG)
(6) glove/yehudi, if any
Structure (affecting weight and external geometry)
(1) spar and rib positions
(2) stiffness, aeroelasticity, and torsion stability
(3) fuel volume
(4) undercarriage and nacelle, if any
The first task for wing design is to select a suitable aerofoil. This book does not undertake aerofoil design; rather, it uses established 2D aerofoil data from the public domain (the aerofoil data in Appendix C are sufficient for this book). Industry takes an arduous route to extract as much benefit from its in-house research that is kept commercial in confidence. It is an established technology in which there is a diminishing return on investment. However, the differences between the best designs and those in the public domain are enough to encourage industrial competition. The next task is to configure a wing planform with a reference area typically for the class of aircraft. It is not determined by the passenger number as in the fuselage; the initial wing size is determined from statistics. Subsequently, the preliminary wing reference area must be sized using the methodology described in Chapter 11.
Positioning of the wing relative to the fuselage is an important part of configuring an aircraft. It requires knowledge of the CG position and its range of movement with weight variation (i. e., fuel and payload). Because the aircraft weight distribution is not yet established, it is initially estimated based on experience and past statistics in the aircraft class. If nothing is known, then a designer may position the wing just behind the middle of the fuselage for rear-mounted engines or at the middle of the fuselage for wing-mounted engines. Subsequently, the wing position must be iterated after the aircraft component weights are known and the wing is sized. This may not be easy because moving the wing will alter the CG position – an inexperienced engineer could encounter what is called “wing chasing”; however, this is not a major concern. Here, the “zero reference plane” (typically at the nose of the fuselage) assists in tracking the aircraft-component positions.
A generous wing root fairing is used to reduce interference drag as well as vortex intensity at the aft-fuselage flow. A large aircraft BWB is an extreme example that eliminates wing root fairing problems. There is no analytical expression to specify the fairing curvature – a designer should judge the geometry from past experience and CFD analysis, considering the internal structural layout and the associated weight growth. In principle, a trade-off study between weight growth and drag reduction is needed to establish the fairing curvature. At this stage, visual approximation from past experience is sufficient: Observe the current designs and make decisions.