examined in conjunction with Figures 4.4, 4.5, and 4.6, which show the range increase with the MTOM increase.

Fuel mass increases with aircraft size, reflecting today’s market demand for longer ranges. The long-range aircraft fuel load, including reserves, is less than half the MTOM. For the same passenger capacity, there is statistical dispersion at the low end. This indicates that for aircraft with a wider selection of comfort levels and choice of aerodynamic devices, the fuel content is determined by the varied market demand: from short ranges of around 1,400 nm to cross-country ranges of around 2,500 nm. At the higher end, the selection narrows, showing a linear trend. Figure 4.6 indicates that larger aircraft have better structural efficiency, offering a better OEMF; Figure 4.7 indicates that they also have a higher fuel fraction for longer ranges.

4.5.4 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

Figure 4.10. Empennage area versus wing area

failure of an engine amounts to a 50% loss of power, whereas for a four-engine aircraft, it amounts to a 25% loss of power. Therefore, the T/W for a two-engine aircraft would be higher than for a four-engine aircraft.

The constraints for engine matching are that it should simultaneously satisfy sufficient takeoff thrust to meet the (1) field length specifications, (2) initial climb requirements, and (3) initial high-speed cruise requirements from market specifications. An increase in engine thrust with aircraft mass is obvious for meeting takeoff performance. Engine matching depends on wing size, number of engines, and type of high-lift device used. Propeller-driven aircraft are rated in power P in kw (hp or shp), which in turn provides the thrust. Turboprops are rated in power loading, P/W, instead of T/W.

Smaller aircraft operate in smaller airfields and are generally configured with two engines and simpler flap types to keep costs down. Figure 4.9a shows thrust growth with size for small aircraft. Here, thrust-loading is from 0.35 to 0.45. Figure 4.9b shows midrange statistics, mostly for two-engine aircraft. Midrange aircraft operate in better and longer airfields than smaller aircraft; hence, the thrust-loading range is at a lower value, between 0.3 and 0.37. Figure 4.9c shows long-range statistics, with some twoand four-engine aircraft – the three-engine configuration is not currently in use. Long-range aircraft with superior high-lift devices and long runways ensure that thrust-loading can be maintained between 0.22 and 0.33; the lower values are for four-engine aircraft. Trends in family variants in each of the three classes are also shown in Figure 4.9.

4.5.6 Empennage Area versus Wing Area

Once the wing area is established along with fuselage length and matched engine size, the empennage areas (i.e., H-tail, SH, and V-tail, SV) can be estimated from the static stability requirements. Section 3.22 discusses the empennage tail-volume coefficients to determine empennage areas.

Figure 4.10 shows growth for H-tail and V-tail surface areas with the MTOM. The variants in the families do not show change in empennage areas to maintain component commonality.