Drag count

A drag count is a dimensionless unit used by aerospace engineers where 1 drag count is equal to a $C_{d}$ of 0.0001.^[1][2]

Definition

A drag count ${\displaystyle \Delta C_{\mathrm {d} }\,}$ is defined as:

${\displaystyle \Delta C_{\mathrm {d} }=10^{4}{\dfrac {2F_{\mathrm {d} }}{\rho v^{2}A}}\,,}$ ^[3]

where:

$F_{{\mathrm d}}\,$ is the drag force, which is by definition the force component in the direction of the flow velocity,^[4]

$\rho \,$ is the mass density of the fluid,^[5]

$v\,$ 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.^[6] A reduction of one drag count on a subsonic civil transport airplane means about 200 lbs more in payload.^[7]

References

1. T. Yechout. Introduction to Aircraft Flight Mechanics: Performance, static stability, dynamic stability, and classical feedback control. AIAA Education Series. 2003. http://books.google.ca/books?id=a_c2V0zAFwcC&pg=PA51&lpg=PA51&dq=drag+count&source=bl&ots=ZE7-7RqzBf&sig=_zGTQQWXAuPaYLlgd-tReHT1pa4&hl=en#v=onepage&q=drag%20count&f=false
2. Basha, W. A. and Ghaly, W. S., “Drag Prediction in Transitional Flow over Airfoils,” Journal of Aircraft, Vol. 44, 2007,p. 824–32.
3. Hucho, Aerodynamik des Automobils, ISBN 978-3-8348-1919-2
4. See lift force and vortex induced vibration for a possible force components transverse to the flow direction.
5. 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
6. "Ask Us - Drag Coefficient & Lifting Line Theory". Aerospaceweb.org. 2004-07-11. http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml. Retrieved 2010-12-07
7. Van Dam, C. P., Aircraft Design and the Importance of Drag Prediction, VKI Lecture Series 2003-2:CFD-Based Aircraft Drag. Prediction and Reduction, National Inst. of Aerospace, Hampton, VA,Nov. 2003.

See also

• Drag coefficient
• Zero-lift drag coefficient