Drag count

A drag count is a dimensionless unit used by aerospace engineers. 1 drag count is equal to a ${\displaystyle C_{d}}$ of 0.0001.^[1][2]

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

Definition

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

${\displaystyle \Delta C_{\mathrm {d} }=10^{4}{\dfrac {2F_{\mathrm {d} }}{\rho v^{2}A}}\,,}$ ^[3]

where:

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

${\displaystyle \rho \,}$ is the mass density of the fluid,^[b]

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

${\displaystyle A\,}$ 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.^[4] A reduction of one drag count on a subsonic civil transport airplane means about 200 lb (91 kg) more in payload.^[5]

Notes

1. See lift force and vortex induced vibration for a possible force components transverse to the flow direction.
2. Note that for the Earth's atmosphere, the air density can be found using the barometric formula. Air is 1.293 kg/m³ at 0 °C and 1 atmosphere