Drag count

A drag count is a dimensionless unit used by aerospace engineers. 1 drag count is equal to a <math>C_d</math> of 0.0001.<ref>Yechout, Thomas R. (2003). Introduction to Aircraft Flight Mechanics. AIAA. p. 51. ISBN 978-1-60086-078-2.</ref><ref>Basha, Wassim A.; Ghaly, Wahid S. (2007). "Drag Prediction in Transitional Flow over Airfoils". Journal of Aircraft. 44 (3): 824–832. doi:10.2514/1.22939.</ref>

As the drag forces present on automotive vehicles are smaller than for aircraft, 1 drag count is commonly referred to as 0.0001 of <math>C_d</math>.

Definition

A drag count <math>\Delta C_\mathrm d\,</math> is defined as:

<math>\Delta C_\mathrm d = 10^{4} \dfrac{2 F_\mathrm d}{\rho v^2 A}\, ,</math> <ref>Hucho, Wolf-Heinrich (2013). Aerodynamik des Automobils: Eine Brücke von der Strömungsmechanik zur Fahrzeugtechnik (in Deutsch). Springer-Verlag. ISBN 978-3-642-57903-5.</ref>

where:

<math>F_\mathrm d\,</math> is the drag force, which is by definition the force component in the direction of the flow velocity,<ref group="lower-alpha">See lift force and vortex induced vibration for a possible force components transverse to the flow direction.</ref>

<math>\rho\,</math> is the mass density of the fluid,<ref group="lower-alpha">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</ref>

<math>v\,</math> is the speed of the object relative to the fluid, and

<math>A\,</math> 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.<ref>"Ask Us – Drag Coefficient & Lifting Line Theory". Aerospaceweb.org. 2004-07-11. Retrieved 2019-10-26.</ref> A reduction of one drag count on a subsonic civil transport airplane means about 200 lb (91 kg) more in payload.<ref>van Dam, C. P. (2003). "Aircraft design and the importance of drag prediction". CFD-Based Aircraft Drag Prediction and Reduction. pp. 1–37. OCLC 53098762.</ref>

See also

• Drag coefficient
• Zero-lift drag coefficient