Drag count
A drag count is a dimensionless unit used by aerospace engineers. 1 drag count is equal to a [math]\displaystyle{ C_d }[/math] 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 0.001 of [math]\displaystyle{ C_d }[/math].
Definition
A drag count [math]\displaystyle{ \Delta C_\mathrm d\, }[/math] is defined as:
- [math]\displaystyle{ \Delta C_\mathrm d = 10^{4} \dfrac{2 F_\mathrm d}{\rho v^2 A}\, , }[/math] [3]
where:
- [math]\displaystyle{ F_\mathrm d\, }[/math] is the drag force, which is by definition the force component in the direction of the flow velocity,[lower-alpha 1]
- [math]\displaystyle{ \rho\, }[/math] is the mass density of the fluid,[lower-alpha 2]
- [math]\displaystyle{ v\, }[/math] is the speed of the object relative to the fluid, and
- [math]\displaystyle{ 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.[4] A reduction of one drag count on a subsonic civil transport airplane means about 200 lb (91 kg) more in payload.[5]
Notes
- ↑ See lift force and vortex induced vibration for a possible force components transverse to the flow direction.
- ↑ 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
References
- ↑ Yechout, Thomas R. (2003). Introduction to Aircraft Flight Mechanics. AIAA. p. 51. ISBN 978-1-60086-078-2. https://books.google.com/books?id=a_c2V0zAFwcC&pg=PA51.
- ↑ 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.
- ↑ Hucho, Wolf-Heinrich (2013) (in de). Aerodynamik des Automobils: Eine Brücke von der Strömungsmechanik zur Fahrzeugtechnik. Springer-Verlag. ISBN 978-3-642-57903-5. https://books.google.com/books?id=OQPOBgAAQBAJ.
- ↑ "Ask Us – Drag Coefficient & Lifting Line Theory". 2004-07-11. http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml.
- ↑ 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.
See also
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