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:

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

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