There is a large increase in drag near Mach 1, that is when aircraft are approaching the speed of sound. Mach number at which drag starts to rise rapidly is known as drag divergence Mach number. The large increase in drag is due to an extensive region of supersonic flow over the airfoil forming into shock waves. These shocks create severe flow separation downstream of the shock wave, which results in a large increase in drag.

These shocks create severe flow separation downstream of the shock wave, which results in a large increase in drag. Large drag rise near Mach 1 can be reduced by the use of thin airfoils and swept wings. Other is through the use of supercritical airfoil and area rule. The cross-sectional area of any aircraft has some abrupt changes with discontinues in the regions of the wing section. However to have a less drag variation of cross-sectional area should be smooth and there should not be any discontinuities. Therefore, in the regions of the wings and tails, the cross-sectional area of the fuselage should decrease for compensation of addition of wing and tail cross-sectional area. This leads to an area distribution within a relatively smooth variation.

Cross sectional area of fuselage is decreased at wings and tail to apply area rule

This design philosophy is known as area rule. The area rule helps to overcome the sound barrier. It is a technique to reduce an aircraft’s drag at transonic and supersonic speeds, particularly in between 0.75 to 1.2.