Maximum Takeoff Mass versus Wing Area
Whereas the fuselage size is determined from the specified passenger capacity, the wing must be sized to meet performance constraints through a matched engine (see Chapter 11). Figure 4.8 shows the relationships between the wing planform reference area, SW, and the wing-loading versus the MTOM. These graphs are useful for obtaining a starting value (i. e., preliminary sizing) for a new aircraft design that would be refined through the sizing analysis.
Wing-loading, W/Sw, is defined as the ratio of the MTOM to the wing planform reference area. (W/SW = MTOM/wing area, kg/m2, if expressed in terms of weight; then, the unit becomes N/m2 or lb/ft2.) This is a significant sizing parameter and has an important role in aircraft design.
The influence of wing-loading is illustrated in the graphs in Figure 4.8. The tendency is to have lower wing-loading for smaller aircraft and higher wing-loading for larger aircraft operating at high-subsonic speed. High wing-loading requires the assistance of better high-lift devices to operate at low speed; better high-lift devices are heavier and more expensive.
The growth of the wing area with aircraft mass is necessary to sustain flight. A large wing planform area is required for better low-speed field performance, which exceeds the cruise requirement. Therefore, wing-sizing (see Chapter 11) provides the minimum wing planform area to satisfy simultaneously both the takeoff and the cruise requirements. Determination of wing-loading is a result of the wing-sizing exercise.
Smaller aircraft operate in smaller airfields and, to keep the weight and cost down, simpler types of high-lift devices are used. This results in lower wing-loading (i. e., 200 to 500 kg/m2), as shown in Figure 4.8a. Aircraft with a range of more than 3,000 nm need more efficient high-lift devices. It was shown previously that aircraft size increases with increases in range, resulting in wing-loading increases (i. e., from 400 to 700 kg/m2 for midrange aircraft) when better high-lift devices are considered.
Here, the trends for variants in the family of aircraft design can be examined. The Airbus 320 baseline aircraft is in the middle of the family. The A320 family retains the wing to maintain component commonality, which substantially reduces manufacturing cost because not many new modifications are necessary for the variants. This resulted in large changes in wing-loading: The smallest in the family
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(c) Large twin-aisle aircraft
Figure 4.8. Wing area, , versus MTOM (A318) has low wing-loading with excellent field performance, and the largest in the family (A321) has high wing-loading that requires higher thrust-loading to keep field performance from degrading below the requirements. Conversely, the Boeing 737 baseline aircraft started with the smallest in the family and was forced into wing growth with increases in weight and cost; this keeps changes in wing-loading at a moderate level.
Larger aircraft have longer ranges; therefore, wing-loading is higher to keep the wing area low, thereby decreasing drag. For large twin-aisle, subsonic jet aircraft (see Figure 4.8c), the picture is similar to the midrange-sized, single-aisle aircraft but with higher wing-loadings (i. e., 500 to 900 kg/m2) to keep wing size relatively small (which counters the square-cube law discussed in Section 3.20.1). Large aircraft require advanced high-lift devices and longer runways.