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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.〔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〕〔Basha, W. A. and Ghaly, W. S., “Drag Prediction in Transitional Flow over Airfoils,” Journal of Aircraft, Vol. 44, 2007,p. 824–32.〕
==Definition==

A drag count $\Delta C_\mathrm d\,$ is defined as:
:$\Delta C_\mathrm d = 10^ \dfrac\, ,$
where:
:$F_\mathrm d\,$ is the drag force, which is by definition the force component in the direction of the flow velocity,〔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,〔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 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.〔"Ask Us - Drag Coefficient & Lifting Line Theory". Aerospaceweb.org. 2004-07-11. http://www.aerospaceweb.org/question/aerodynamics/q0184.shtml. Retrieved 2010-12-07〕 A reduction of one drag count on a subsonic civil transport airplane means about 200 lbs more in payload.〔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.〕

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