Cabin Layout and Fuselage Geometry
The design of the fuselage is based on payload requirements, aerodynamics, and structures. The overall dimensions of the fuselage affect the drag through several factors. Fuselages with smaller fineness ratios have less wetted area to enclose a given volume, but more wetted area when the diameter and length of the cabin are fixed. The higher Reynolds number and increased tail length generally lead to improved aerodynamics for long, thin fuselages, at the expense of structural weight. Selection of the best layout requires a detailed study of these trade-offs, but to start the design process, something must be chosen. This is generally done by selecting a value not too different from existing aircraft with similar requirements, for which such a detailed study has presumably been done. In the absence of such guidance, one selects an initial layout that satisfies the payload requirements.
The following sections are divided into several parts: the selection of cabin cross-section dimensions, determination of fuselage length and shape, FAR's related to fuselage design and seating, and finally considerations related to supersonic aircraft.
For some examples of interior and exterior fuselage geometries see the drawings on Boeing's site (currently at):
http://www.boeing.com/commercial/airports/plan_manuals.html