Military Aircraft (Combat Category)
This extended section of the book can be found on the Web site www. cambridge .org/Kundu and lists generic military aircraft-component mass as required in the conceptual design stage. The list covers aircraft components in the following groups. Structure Group Power Plant Group Systems Group Furnishing
Manufacture’s Empty Mass Operators Empty Mass Maximum Takeoff Mass Maximum Ramp Mass
8.3 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 ), 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 mass- prediction 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. Civil – aircraft 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.