The weight of the different components can be classified into three principal groups:
- Those directly related to Maximum Takeoff Weight (MTOW), i.e., wing or fuselage.
- Fixed equipment, dependent only on passenger accommodation, which is held constant during the whole structural design process.
- Variable loads, such as fuel and payload.
Following this classification, the input set is restricted to external geometry data, such as span or fuselage length, and a few pieces of internal layout information, i.e., cabin length, fuel and payload, which includes passengers, crew and baggage (given as design specifications). The validity of the statistical formulation has to be carefully considered here, especially when novel designs are considered. However, the main target of the Weight and Balance module is to provide a first reasonable estimate that then can be refined by means of the GUESS sizing module. In detail, the weight and balance module calculates all aircraft component weights and their locations with respect to a coordinate system whose origin is conventionally fixed at the aircraft’s nose. Moments are determined assuming both external and internal geometry as volume entities with constant density for each component. Concentrated mass can also be added separately: if available, additional data such as the auxiliary power unit’s weight or auxiliary fuel tank capacity and position can be set manually, thus improving the prediction of the whole process.
Finally, a dedicated routine calculates the inertia matrix using either a coarse approximation that treats the whole aircraft as a solid with constant density. This is required to start the preliminary analysis of the flight mechanics performances and stability module. Finally, the SMARTCAD module, by means of GUESS meshing features, facilitates a more refined calculation of the mass matrix for the whole aircraft. This is carried out numerically for each element of the structural mesh on the basis of the real material distribution of the airframe and the nonstructural masses.