= Fuselage
{wiki}
A <fuselage> is the main body of a conventional <aircraft>, accommodating payload or equipment and connecting major assemblies such as the <aircraft wings>. It is part of the load-carrying structure as well as the aerodynamic shape. Not every <aircraft> has a separate conventional fuselage, and not every fuselage is pressurized.<Fuselage by NASA Glenn Research Center>[Fuselage and Weight, accommodation, component connections and applied torques]{ref}
The structural role is more than enclosing a volume. <Forces> from attached assemblies and distributed <gravitational weight> must be transmitted through the body, producing bending, shear and axial loading. A pressurized body also carries loads from the pressure difference between its interior and the atmosphere. These are different loading mechanisms, even if they act on the same shell.
An original ideal model illustrates the pressure contribution. Approximate a closed cylindrical section as a <thin-walled pressure vessel> of radius $R$, wall thickness $t$ and internal excess <pressure> $\Delta p$. Force balance across a longitudinal cut of length $\ell$ gives $2R\ell\Delta p=2t\ell\sigma_\theta$. Balance of an end cap gives $\pi R^2\Delta p=2\pi Rt\sigma_z$. Thus
$$
\sigma_\theta=\frac{\Delta pR}{t},\qquad
\sigma_z=\frac{\Delta pR}{2t}.
$$
The first is <hoop stress> and the second <axial stress>. For $R=1.5\,\mathrm m$, $t=2\,\mathrm{mm}$ and $\Delta p=50\,\mathrm{kPa}$, they are $37.5$ and $18.75\,\mathrm{MPa}$. This stipulated membrane model excludes cutouts, joints, stiffeners and aerodynamic bending; it is not a complete fuselage stress analysis.
Shape also affects the combined flow around the body and wings. <Richard T. Whitcomb>'s <area rule> work examined the longitudinal area distribution of the entire combination, rather than judging the wing and fuselage as isolated streamlined parts. That connection explains why structural packaging and aerodynamic design cannot always be optimized independently.
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