Drag count

A drag count is a dimensionless unit used by aerospace engineers. 1 drag count is equal to a $C_{d}$ of 0.0001.

As the drag forces present on automotive vehicles are smaller than for aircraft, 1 drag count is commonly referred to as 0.0001 of $C_{d}$.

Definition

A drag count $\Delta C_{\mathrm {d} }\,$ is defined as:

$\Delta C_{\mathrm {d} }=10^{4}{\dfrac {2F_{\mathrm {d} }}{\rho v^{2}A}}\,,$

where:

$F_{\mathrm {d} }\,$ is the drag force, which is by definition the force component in the direction of the flow velocity,

$\rho \,$ is the mass density of the fluid,

$v\,$ is the speed of the object relative to the fluid, and

$A\,$ is the reference area.

The drag coefficient is used to compare the solutions of different geometries by means of a dimensionless number. A drag count is more user-friendly than the drag coefficient, as the latter is usually much less than 1. A drag count of 200 to 400 is typical for an airplane at cruise. A reduction of one drag count on a subsonic civil transport airplane means about 200 lb (91 kg) more in payload.

See also

• Drag coefficient
• Zero-lift drag coefficient