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Graphical Method for Predicting Aircraft Component Weight: Civil Aircraft
The graphical method is based on regression analyses of an existing design. To put all the variables affecting weight in graphical form is difficult and may prove impractical because there will be separate trends based on choice of material, maneuver loads, fuselage layout (e. g., single or double aisle; single or double deck), type of engine integrated, wing shape, control architecture (e. g., FBW is lighter), and so forth. In principle, a graphical representation of these parameters can be accomplished at the expense of simplicity, thereby defeating the initial purpose. The simplest form, as presented in this section, obtains a preliminary estimate of component and aircraft weight. At the conceptual design stage – when only the technology level to be adopted and the three-view drawing are available to predict weights – the
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Table 8.1. Smaller aircraft mass fraction (fewer than or 19 passengers -2 abreast seating) Rapid mass estimation method: Summary of mass fraction of MTOM for smaller aircraft. A range of applicability is shown; add another ± 10% for extreme designs.
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Notes: Lighter/smaller aircraft would show a higher mass fraction.
A fuselage-mounted undercarriage is shorter and lighter for the same MTOM.
Turbofan aircraft with a higher speed would have a longer range as compared to turboprop aircraft and, therefore, would have a higher fuel fraction (typically, 2,000-nm range will have around 0.26).
prediction is approximate. However, with rigorous analyses using semi-empirical prediction, better accuracy can be achieved that captures the influence of various parameters, as listed previously.
Not much literature in the public domain entails graphical representation. An earlier work (1942; in FPS units) in [3] presents analytical and semi-empirical treatment that culminates in a graphical representation. It was published in the United States before the gas-turbine age, when high-speed aircraft were nonexistent; those graphs served the purpose at the time but are now no longer current. Given herein
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Table 8.2. Larger aircraft mass fraction (more than 19 passengers – abreast and above seating). Rapid Mass Estimation Method: Summary of mass fraction of MTOM for larger aircraft. A range of applicability is shown; add another ± 10% for extreme designs.
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Notes: Lighter aircraft would show higher mass fraction.
A fuselage-mounted undercarriage is shorter and lighter for the same MTOM.
Turbofan aircraft with a higher speed would have a longer range as compared to turboprop aircraft and, therefore, would have a higher fuel fraction.
Large turbofan aircraft have wing-mounted engines: 4-engine configurations are bigger.
are updated graphs based on the data in Table 8.3; they are surprisingly representative with values that are sufficient to start the sizing analysis in Chapter 11. Most of the weight data in the table are from Roskam [4] with additions by the author notated with an asterisk (these data are not from the manufacturers). The best data is obtained directly from manufacturers.
In all of the graphs, the MTOW is the independent variable. Aircraft – component weight depends on the MTOW; the heavier the MTOW, the heavier
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Table 8.3. Aircraft component weights data
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are the component weights (see Chapter 4). Strictly speaking, wing weight could have been presented as a function of the wing reference area, which in turn depends on the sized wing-loading (i. e., the MTOW) (see Chapter 11).
To use the graph, the MTOW must first be guesstimated from statistics (see Chapters 4 and 6). After the MTOW is worked out in this chapter, iterations are necessary to revise the estimation.
Figure 8.3 illustrates civil aircraft component weights in FPS units. The first provides the fuselage, undercarriage, and nacelle weights. Piston-engine-powered aircraft are low-speed aircraft and the fuselage group weight shows their lightness. There are no large piston-engine aircraft in comparison to the gas-turbine type.
Figure 8.3 Aircraft component weights in pounds
The lower end of the graph represents piston engines; piston-engine nacelles can be slightly lighter in weight.
The second graph in Figure 8.3 shows the wing and empennage group weights. The piston – and gas-turbine engine lines are not clearly separated. FBW-driven configurations have a smaller wing and empennage (see Chapter 13), as shown in separate lines with lighter weight (i. e., A320 and A380 class). The newer designs have composite structures that contribute to the light weight.
Figure 8.3 shows consistent trends but does not guarantee accuracy equal to semi-empirical relations, which are discussed in the next section.
