Figure 3.31. Wing geometry definition (Boeing 737 half wing)

as a rectangle (Figure 3.30b); that is, the edges are not straight up to centerline unless it is a rectangular wing normal to the centerline. Section 4.8 describes the various options available from which to choose a wing planform.

A typical subsonic commercial transport-type wing is shown in Figure 3.31. An extension at the LE of the wing root is called a glove and an extension at the trailing edge is called a yehudi (this is Boeing terminology). The yehudi’s low-sweep trailing edge offers better flap characteristics. These extensions can originate in the baseline design or on the existing platform to accommodate a larger wing area. A glove and/or a yehudi can be added later as modifications; however, this is not easy because the aerofoil geometry would be affected.

3.16.2 Wing Aspect Ratio

In the simplest rectangular wing planform area, the aspect ratio is defined as aspect ratio, AR = (span, b)/(chord, c). For a generalized trapezoidal wing planform area:

3.16.3 Wing Sweep Angle,

The wing quarter-chord line is the locus of one fourth of the chord of the reference wing planform area measured from the LE, as shown in Figure 3.31. The wing sweep is measured by the angle of the quarter-chord line extended from the line perpendicular to the centerline.

3.16.4 Wing Root (croot) and Tip (ctip) Chord

These are the aerofoil chords parallel to the aircraft at the centerline and the tip, respectively, of the trapezoidal reference area.

3.16.5 Wing Taper Ratio, λ

This is defined as the ratio of the wing tip chord to the wing root chord (ctip/croot). The best taper ratio is in the range from 0.3 to 0.6. The taper ratio improves the wing efficiency by giving a higher Oswald’s efficiency factor (see Section 3.10).

Figure 3.32. Wing twist

3.16.6 Wing Twist

The wing can be twisted by making the wing tip nose down (i.e., washout) relative to the wing root (Figure 3.32), which causes the wing root to stall earlier (i.e., retain aileron effectiveness). Typically, a 1- to 2-deg washout twist is sufficient. Twisting the wing tip upward is known as washin.

3.16.7 High/Low Wing

Depending on the design drivers, an aircraft configuration can place the wing anywhere from the top (i.e., high wing) to the bottom (i.e., low wing) of the fuselage or in between (i.e., midwing), as shown in Figure 3.33. Structural considerations of the wing attachment to the fuselage comprise a strong design driver, although in the civil aircraft market, the choice could be dictated by customer preference. The wing center section should not interfere with the cabin passage-height clearance – especially critical for smaller aircraft. A fairing is shown for low-wing aircraft (Figure 3.33a, Cessna Citation) or high-wing aircraft (Figure 3.33c, Dornier 328), where the wing passes under or over the fuselage, respectively. Both cases have a generous fairing that conceals the fuselage mould-line kink (i.e., drag-reduction measure), which would otherwise be visible. Midwing (or near-midwing) designs are more appropriate to larger aircraft with a passenger cabin floorboard high enough to allow the wing box positioned underneath it.

Aircraft with a high wing allow better ground clearance (see Figures 3.33c and 3.49) and the fuselage to be closer to the ground, which makes cargo-loading easier – especially with a rear-fuselage cargo door. Turboprops favor a high-wing configuration to allow sufficient ground clearance for the propeller. The main undercarriage is mounted on the fuselage sides with the bulbous fairing causing some additional drag. However, this configuration provides better aerodynamics (e.g., the BAe RJ100 and Dornier 328 are successful high-wing designs). The dominant configuration for civil transport aircraft has been a low wing, which provides a wider

Figure 3.33. Positioning of wing with respect to fuselage (all T-tail configurations)