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.{{cite book|last=Yechout|first=Thomas R.|title=Introduction to Aircraft Flight Mechanics|url=https://books.google.com/books?id=a_c2V0zAFwcC&pg=PA51|year=2003|publisher=AIAA|isbn=978-1-60086-078-2|page=51}}{{cite journal | first1=Wassim A. | title=Drag Prediction in Transitional Flow over Airfoils | last2=Ghaly | first2=Wahid S. | journal=Journal of Aircraft | last1=Basha | year=2007 | volume=44 | issue=3 | pages=824–832 | doi=10.2514/1.22939}}

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:

: $\Delta C_\mathrm d = 10^{4} \dfrac{2 F_\mathrm d}{\rho v^2 A}\, ,$ {{cite book|last=Hucho|first=Wolf-Heinrich|title=Aerodynamik des Automobils: Eine Brücke von der Strömungsmechanik zur Fahrzeugtechnik|url=https://books.google.com/books?id=OQPOBgAAQBAJ|year=2013|publisher=Springer-Verlag|language=de|isbn=978-3-642-57903-5}}

where:

: $F_\mathrm d\,$ is the drag force, which is by definition the force component in the direction of the flow velocity,{{efn|See lift force and vortex induced vibration for a possible force components transverse to the flow direction.}}

: $\rho\,$ is the mass density of the fluid,{{efn|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}}

: $v\,$ 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.{{cite web | title=Ask Us – Drag Coefficient & Lifting Line Theory | website=Aerospaceweb.org | date=2004-07-11 | url=http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml | access-date=2019-10-26}} A reduction of one drag count on a subsonic civil transport airplane means about {{cvt|200|lbs}} more in payload.{{cite book | chapter=Aircraft design and the importance of drag prediction | title=CFD-Based Aircraft Drag Prediction and Reduction | date=2003 | last=van Dam | first=C. P. | pages=1–37 | oclc=53098762}}

drag count

Definition

Notes

References

See also