head drag, a force which a gas (for example, air) exerts on a body moving through it; this force is always directed opposite to the direction of velocity and is a component of the aerodynamic force. Knowledge of aerodynamic drag is necessary for aerodynamic calculations of aircraft, since, in particular, the speed of motion for given thrust characteristics of an engine installation depends on this knowledge.

Aerodynamic drag is the result of the irreversible transformation of a portion of the kinetic energy of a body into heat. Aerodynamic drag depends on the shape and dimensions of the body, on its angle of attack, on the magnitude of velocity, and also on the properties and conditions of the medium in which motion takes place. In actual mediums there is viscous friction in the boundary layer between the surface of the body and the medium, and there are losses in the formation of shock waves at transonic and supersonic velocities (wave drag) and in the formation of vortices. The predominant component of aerodynamic drag depends on the flight conditions and shape of the body. For example, for blunt bodies of rotation moving at a high supersonic velocity, aerodynamic drag is determined mainly by wave drag. For streamlined bodies moving at a low velocity, aerodynamic drag is determined by friction drag and by losses in the formation of vortices.

In aerodynamics, aerodynamic drag is described by a nondimensional aerodynamic drag coefficient C_D, through which the aerodynamic drag D is defined as

where ρ∞ is the density of the undisturbed medium, v∞ is the velocity of motion of the body relative to this medium, and 5 is the specific area of the body. The coefficient C_D for a body of a given shape at a particular orientation relative to the flow depends on the non-dimensional similarity criteria, which include Mach numbers and Reynolds numbers. The numerical value of C_D is usually determined experimentally, by measuring the aerodynamic drag in wind tunnels and in other devices used in aerodynamic experiments. Aerodynamic drag may be determined theoretically only for a limited class of the simplest bodies.