Reducing frontal area

Strongly unfavourable pressure gradients can be avoided by making all parts of the aircraft as thin as possible: in other words, by reducing the frontal area. In the case of the fuselage of an airliner, any reduction in cross-sectional area must be offset by an increase in length, if the same number of passengers is to be accommodated to an equal standard of comfort. The increase in length is accompanied by an increase in the surface area, and this in turn means that the surface friction drag will increase. There is always an optimum comprom­ise between decreased boundary layer normal pressure (form) drag resulting from reduced frontal area, and increased surface friction drag caused by the increased surface area.

In the case of wing sections, reducing the thickness will result in a reduction in the depth of the structural spars. The bending strength of a spar depends on its breadth, and on the cube of its depth. Any small reduction in depth must be offset by a large increase in breadth, and hence weight. Thin wing sections also have the disadvantage that they stall at relatively low angles of attack. The reasons for the use of thin sections on transonic and supersonic aircraft will be described later.