8.7 Aircraft Component Mass Estimation

8.7 Aircraft Component Mass Estimation

Mass estimation at the conceptual design stage must be predicted well in advance of detailed drawings of the parts being prepared. Statistical fitment of data from the past designs is the means to predict component mass at the conceptual design phase. The new designs strive for improvement; therefore, statistical estimation is the starting point. During the conceptual design stage, iterations are necessary when the configuration changes.

Typically, there are three ways to make mass (i.e., weight) estimations at the conceptual design stage:

1.Rapid Method. This method relies on the statistical average of mass one level below major aircraft components (i.e., in more detail). The mass is expressed in terms of percentage (alternatively, as a fraction) of the MTOM. All items should total 100% of the MTOM; this also can be expressed in terms of mass per wing area (i.e., component wing-loading). This rapid method is accomplished at the price of considerable approximation.

2.Graphical Method. This method consists of plotting component weights of various aircraft already manufactured to fit into a regression curve. Graphs are generated from analytical considerations (see [3]), superimposed by actual data. The graphical method does not provide fine resolution but it is the fastest method without the next level of mass estimation, as explained previously. It is difficult to capture the technology level (and types of material) used because there is considerable dispersion. Obtaining details of component mass for statistical analysis from various industries is difficult.

3.Semi-Empirical Method. This method is a considerable improvement, in that it uses semi-empirical relations derived from a theoretical foundation and backed by actual data that have been correlated statistically. The indices and factors in the semi-empirical method can be refined to incorporate the technology level and types of material used. The expressions can be represented graphically, with separate graphs for each class. When grouped together in a generalized manner, they are the graphs in the graphical method described previously.

The first two methods of component mass estimation provide a starting point for the design progression.

The state-of-the-art in weight prediction has room for improvement. The advent of solid modeling (i.e., CAD) of components improved the accuracy of the massprediction methodology; with CAD, weight change due to a change in material can be easily captured. As soon as the component drawing is completed, the results are instantaneous and carry on through subassembly to final assembly. CAD modeling of parts occurs after the conceptual design phase has been completed.

The design drivers for civil aircraft have always been safety and economy. Civilaircraft design developed in the wake of military aircraft evolution. Competition within these constraints kept civil aircraft designs similar to one another. Following are general comments relative to civil aircraft mass estimation:

1.For a single-engine, propeller-driven aircraft, the fuselage starts aft of the engine bulkhead because the engine nacelle is accounted for separately. These are mostly small aircraft; this is not the case for wing-mounted nacelles.

2.The fixed-undercarriage mass fraction is lower than the retractable type. The extent depends on the retraction type (typically 10% higher).

3.Neither three-engine aircraft nor fuselage-mounted, turboprop-powered aircraft are discussed in this book. Not many of these types of aircraft are manufactured. Sufficient information has been provided herein for readers to adjust mass accordingly for these aircraft classes.

The three methods are addressed in more detail in the following sections.

8.8 Rapid Mass Estimation Method: Civil Aircraft

A rapid mass estimation method is used to quickly determine the component weight of an aircraft by relating it in terms of a fraction given in the percentage of maximum takeoff mass (Mi/MTOM), where the subscript i represents the ith component. With a range of variation among aircraft, the tables in this section are not accurate and serve only as an estimate for a starting point of the initial configuration described in Chapter 6. Roskam [4] provides an exhaustive breakdown of weights for aircraft of relatively older designs. A newer designs show improvements, especially because of the newer materials used.

Because mass and weight are interchangeable, differing by the factor g, wingloading can be expressed in either kg/m2 or N/m2; this chapter uses the former to be consistent with mass estimation. To obtain the component mass per unit wing area (Mi/SW, kg/m2), the Mi/MTOM is multiplied by the wing-loading; that is, Mi/SW = (Mi/MTOM) × (MTOM/SW). Initially, the wing-loading is estimated (multiply 0.204816 to convert kg/m2 to lb/ft2).

Tables 8.1 and 8.2 summarize the component mass fractions, given in a percentage of the MTOM for quick results. The OEM fraction of the MTOM fits well with the graphs (see Figures 4.7 and 4.8). This rapid method is not accurate and only provides an estimate of the component mass involved at an early stage of the project. A variance of ±10% is allowed to accommodate the wide range of data.

It is better to use more accurate semi-empirical relations (see Section 8.10) to obtain the component mass at the conceptual design phase. The tables are useful for estimating fuel mass and engine mass, for example, which are required as a starting point for semi-empirical relations.

8.9 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

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.

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

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.

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. Aircraftcomponent weight depends on the MTOW; the heavier the MTOW, the heavier