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General Forums => The O' Club => Topic started by: Nashwan on July 25, 2001, 12:32:00 PM

Title: Calculating Drag
Post by: Nashwan on July 25, 2001, 12:32:00 PM

Can somebody tell me the equations for calculating drag at different speeds.
Just the basic profile drag(?), eg if I stick my hand out of a car window, how many pounds of force at any speed.
I suppose the variables will be area, air density and speed, but beyond that I know nothing.
Any help will be greatly appreciated.

Title: Calculating Drag
Post by: funkedup on July 25, 2001, 12:42:00 PM

Simplest drag calculation is this:

D = 0.5 * CD * rho * A * V^2

D is drag in Newtons

CD is drag coefficient (dimensionless) which is related to the shape of the object and the conditions (Reynolds and Mach number) at which you are operating.  There is a table of values for different shapes here:  http://www.aerodyn.org/Drag/tables.html (http://www.aerodyn.org/Drag/tables.html)
Those will definitely get you "in the ballpark."

rho is density of air (1.225 kg/m^3 at sea level)

A is an area (usually cross sectional area of the item in question) in meters^2

V is airspeed in meters/second

[ 07-25-2001: Message edited by: funkedup ]

Title: Calculating Drag
Post by: funkedup on July 25, 2001, 12:44:00 PM

PS here is a great intro to aerodynamics:  http://www.lerc.nasa.gov/WWW/K-12/airplane/bga.html (http://www.lerc.nasa.gov/WWW/K-12/airplane/bga.html)

Title: Calculating Drag
Post by: mrfish on July 25, 2001, 01:14:00 PM

D=?! you newtonian blasphemer that's an F!!!!!

  :D

[ 07-25-2001: Message edited by: mrfish ]

Title: Calculating Drag
Post by: Nashwan on July 25, 2001, 03:44:00 PM

Thanks Funked.
Though my basic physics tells me I have to agree with Mr Fish.  :D

Title: Calculating Drag
Post by: funkedup on July 25, 2001, 05:13:00 PM

This ain't physics it's aerodynamics.  Drag is D, Dummy!  ;)

Title: Calculating Drag
Post by: mietla on July 25, 2001, 05:23:00 PM

Look at the drag coeffs for a human.

 http://www.aerodyn.org/Drag/tables.html#man (http://www.aerodyn.org/Drag/tables.html#man)

Why do people use parachutes at all  :D

Title: Calculating Drag
Post by: Vulcan on July 25, 2001, 06:34:00 PM

Hmmmm wonder if the WW2OL guys are trying to sort out the flight models <efg>

Title: Calculating Drag
Post by: mrfish on July 25, 2001, 07:26:00 PM

lol drag is D ...or R or hell i don't know x or even ghryutfuytrjh$it doesn't matter really so long as the relationship is accurate -

it's just a joke because the formula i learned in phys 1 was something like F = 1/2 (f sub k) A pv^2. or something like that. i haven't done any of those in a while and i'm too lazy to look it up. it seems like f sub k could have even been mu maybe i dont remember.   :)

the point is my professor was super retentive about labeling the result as a force. he claimed if we failed to even once we would forever forget it was the force in that direction and that our heads would collapse. just ribbin-

[ 07-25-2001: Message edited by: mrfish ]

Title: Calculating Drag
Post by: funkedup on July 25, 2001, 07:47:00 PM

Go f sub k yourself!

Title: Calculating Drag
Post by: mietla on July 25, 2001, 07:54:00 PM

:D

Title: Calculating Drag
Post by: mrfish on July 25, 2001, 09:27:00 PM

:D bast'd.....