Whereas the fuselage size is determined from the operator’s specified capacity, the wing size depends on many factors and requires a rigorous sizing exercise (see Chapter 11) to determine the planform reference area. The wing contributes to lift generation and the characteristics are based on the chosen aerofoil section in use. A given priority of wing design is selecting the aerofoil(s) that fits the purpose with the aim to improve existing designs. The aerofoil section is selected using the considerations described in Section 3.11; high-lift devices are described in Section 3.12. This section describes typical options for available wing planform shapes (generic options are listed in Section 4.17.1).
After obtaining the wing planform area, other geometrical details must be determined (e. g., aspect ratio, sweep, taper ratio, dihedral, and twist). The wing span is the result of the values of the aspect ratio, sweep, and taper ratio. Equation 3.18 defines the aerodynamic MAC parameter. The three-view diagrams in Figure 4.23 illustrate the fundamental planform-shape choices; each shape is discussed separately herein. For speeds exceeding Mach 0.5, sweeping the wing backward (Figure 4.23e) or forward (see Figure 4.40b) is necessary to delay the compressibility effects on the wing, as explained in Section 3.18.
1. Rectangular planform. This rectangular planform is used for low-speed (i. e., incompressible flow) aircraft below Mach 0.4. It is the most elementary shape with constant rib sections along the wingspan. Therefore, the cost to manufacture is lower because only one set of tooling for ribs is needed for the entire wing. However, this planform has the least efficient spanwise loading. This type of planform is well suited to small aircraft, typically for private ownership and homebuilt types. There are larger aircraft that have the rectangular wing (e. g., Shorts SD360 series and BN Islander).
2. Tapered (trapezoidal) planform. This is the most common planform shape in use because it offers good aerodynamic loading with a good spanwise load distribution. The taper ratio can vary – the delta-wing planform has an extreme value
of zero. In Figure 4.23b, the LE has a small backward sweep; other designs have a straight LE, and the Saab Safir has a forward sweep, which provides pilot visibility in a high-wing aircraft. With almost no sweep, this type of wing can be designed for a maximum speed of Mach 0.5. If it must go faster, then more wing sweep is required. The production costs of a tapered wing are higher than for a rectangular wing because the ribs are different spanwise. However, the tapered wing maintains straight lines at the leading and trailing edges, which provides some ease in jig and fixture designs.
3. Cranked-wing planform. The Beech 200 shown in Figure 4.23c is a good example of combining the available options. In this case, the center section is rectangular and the outboard wings are tapered. Other combinations are possible. A tapered wing can be modified with a crank incorporated (i. e., two tapered wings blended into one). The glove and yehudi can be used to extend areas at leading and trailing edges, respectively.
4. Elliptical planform. The Spitfire aircraft shown in Figure 4.23d is a fine example of an elliptical wing, which offers the best aerodynamic efficiency for having the best spanwise load distribution. However, it is the most expensive to manufacture and designers should avoid the elliptical planform because a good tapered planform approximates the elliptical load distribution, yet its manufacture is substantially less costly. Curved-wing leading and trailing edges would require relatively more expensive tooling. The Spitfire aircraft reached very high speeds for the time.
The Beriev 12 shown in Figure 4.23f has a gull-wing shape and the Junkers Stuka has the dihedral the other way for specific reasons. At the conceptual stage, the dihedral and the twist are taken from past experience and statistical data. Other wing parameters (e. g., aspect ratio and tapered ratio) are also available. Eventually, the wing design is fine-tuned with CFD analysis followed by wind-tunnel tests.