area rule

area rule, aircraft design principle formulated by American engineer Richard Whitcomb which stated that the drag on an airplane flying at high speed is a function of the aircraft’s entire cross-sectional area.

Bodies which pass through the so-called transonic zone—the zone separating speeds below from those above the speed of sound—suffer a great increase of drag coefficient as they approach the critical speed. During World War II the German aircraft designer Dietrich Küchemann advanced the theory that drag could be reduced by remolding such bodies (for example, fuselages) to follow the local streamlines. After the war, designers tackled these problems, and in April 1952, working in the transonic wind tunnel of the U.S. National Advisory Committee for Aeronautics, Whitcomb was able to express his “area rule” thus: “If a wing/body combination (including, in a combat aircraft, external stores, and other paraphernalia) be so designed that the axial distribution of cross-sectional area normal to the airflow is the same as that of a minimum-drag body, the design will have minimum drag.” In applying the area rule, additions to cross-sectional area (such as engine nacelles) are compensated for by subtractions from it elsewhere (e.g., by narrowing parts of the fuselage).