Drag count

Definition

A drag count Δ C d is defined as:

Δ C d = 10 4 2 F d ρ v 2 A ,

where:

F d is the drag force, which is by definition the force component in the direction of the flow velocity, ρ 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 used as a more user-friendly measurement as the coefficient of drag 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.