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:


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

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 lbs more in payload.