 Area rule

The area rule is an aerodynamic design principle relating transonic wave drag to the longitudinal distribution of the total cross-sectional area of an aircraft, including its fuselage and aircraft wings. A wing-body combination can create a rapid area increase even when each part appears streamlined in isolation. Redistributing the fuselage area near the wing can smooth the combined distribution. This does not say that any reduction in frontal area guarantees lower total drag.^[ref]

An original geometric model makes the redistribution explicit. Let $x$ increase along the fuselage, and let $A_b(x)$ and $A_w(x)$ denote body and added-wing areas, counting their overlap only once. Over a stipulated interval of length $\ell$, takeIf the desired combined distribution on that interval is $A_0$, the bookkeeping choice $A_b(x)=A_0-A_w(x)$ gives $A_b+A_w=A_0$. For $A_0=3.0{\mathrm{m}}^2$, $a=0.6{\mathrm{m}}^2$ and $\ell =4\mathrm{m}$, the body's area is $2.4{\mathrm{m}}^2$ at the midpoint. Relative to a constant-area body, the removed volume isThis invented example exposes a design tradeoff: smoothing an area distribution can consume internal volume. It is not a calculation of wave drag, and the rest of the vehicle still needs smooth transitions.

Richard T. Whitcomb's motivating test used a specially built wing-body model in a wind tunnel, before applying the shape change to a fighter configuration. Comparing measured drag through the transonic range tests the proposed area-distribution effect; appearance alone does not. Viscous stress, lift-induced drag, loading and off-design conditions still matter to the complete aircraft.^[ref]