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  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) x (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.