Page 52 - Aerodynamics for Engineering Students

Basic concepts and definitions 35

1.5.5 Types of drag
Attempts have been made to rationalize the definitions and terminology associated
with drag*. On the whole the new terms have not been widely adopted. Here we will use
the widely accepted traditional terms and indicate alternatives in parentheses.

Total drag
This is formally defined as the force corresponding to the rate of decrease in momen-
tum in the direction of the undisturbed external flow around the body, this decrease
being calculated between stations at infinite distances upstream and downstream of the
body. Thus it is the total force or drag in the direction of the undisturbed flow. It is also
the total force resisting the motion of the body through the surrounding fluid.
There are a number of separate contributions to total drag. As a first step it may be
divided into pressure drag and skin-friction drag.

Skin- friction drag (or surface- friction drag)
This is the drag that is generated by the resolved components of the traction due to the
shear stresses acting on the surface of the body. This traction is due directly to viscosity
and acts tangentially at all points on the surface of the body. At each point it has a
component aligned with but opposing the undisturbed flow (i.e. opposite to the direction
of flight). The total effect of these components, taken (i.e. integrated) over the whole
exposed surface of the body, is the skin-friction drag. It could not exist in an invisicid flow.

Pressure drag
This is the drag that is generated by the resolved components of the forces due to
pressure acting normal to the surface at all points. It may itself be considered as
consisting of several distinct contributions:
(i) Induced drag (sometimes known as vortex drag);
(ii) Wave drag; and
(iii) Form drag (sometimes known as boundary-layer pressure drag).

Induced drag (or vortex drag)
This is discussed in more detail in Sections 1.5.7 and 5.5. For now it may be noted
that induced drag depends on lift, does not depend directly on viscous effects, and
can be estimated by assuming inviscid flow.

Wave drag
This is the drag associated with the formation of shock waves in high-speed flight.
It is described in more detail in Chapter 6.

Form drag (or boundary-la yer pressure drag)
This can be defined as the difference between the profile drag and the skin-friction
drag where the former is defined as the drag due to the losses in total pressure and

*For example, the Aeronautical Research Committee Current Paper No. 369 which was also published in
the Journal of the Royal Aeronautical Society, November 1958.