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Understanding Cofficient of Drag

  1. Mar 20, 2008 #1
    Ok so i have looked and looked for a simple way to under stand how to find drag using the coefficient of drag.

    the formula i found is off of NASA website and it reads

    Drag = Coefficient * [ (Density * Velocity ^2)* .5 ] * reference area

    here is my question
    what units do i use.
    When i set it up with these numbers
    CD = 0.31 (unitless i know)
    Ref Area = 8 sq ft (ruff guess)
    Velocity = 75 fps
    density = 24 in Hg

    I end up with 167,400. but i know no clue as to what unit that is? should i be putting every thing in with different units... what is the end units?
  2. jcsd
  3. Mar 20, 2008 #2
    I would do dimensional analysis.
  4. Mar 20, 2008 #3
    At the risk of sounding like a moron..... HUH?
  5. Mar 20, 2008 #4

    Andy Resnick

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    Can you post a link to the site? Doing a quick dimensional analysis gives a consistent result:

    [drag] = ML/T^2 (force, M = mass, L = length, T = time)

    but you have odd units of density: pressure? It should be M/L^3
  6. Mar 20, 2008 #5
    http://www.grc.nasa.gov/WWW/K-12/airplane/drageq.html" [Broken]
    Last edited by a moderator: May 3, 2017
  7. Mar 20, 2008 #6
    Please google dimensional analysis.

    Hint: You can find this also in any chemistry book.
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