Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Drag coefficient equation

  1. Dec 3, 2017 #1
    So I did a project about a year ago and I cant remember one of the things I wrote in this equation.

    The equation apparently I got it from NASA's website.

    This is my equation

    CD = FL/ (0.5 * Air viscosity * V^2 * Area) = 0.4

    These are the numbers I used to plug in the equation

    Velocity = 13 m/s

    Diameter pipe = 0.000635 m

    Air viscosity = 0.00001599 kg/m*s

    FL = 1*10^-9 Kg <<< this is the problem

    Area of Dust = 1.96 * 10^-11 Microns

    Now in NASA's website instead of FL there's a "D" and it says the "D" represents the drag.

    Now I can't remember what (FL) means, is it truly a drag? and a drag of what exactly.

    My project was about compressed air moving inside a small tube, and the tube has pinched hols in it to pump the compressed air to clean a surface from dust.

    SO, can someone tell me what the "FL" that I wrote means? and what exactly does the number FL = 1*10^-9 Kg represent? Could it be "air drag??" I am not sure where I found this number.
  2. jcsd
  3. Dec 3, 2017 #2


    User Avatar
    Science Advisor
    Gold Member

    Are you sure that your equation used viscosity? That looks an awful lot like a situation where it should be density. With viscosity, your units don't work out. As for what "FL" means, I have no idea. Typically, if it was a lift coefficient, then "FL" would be the lift force and that would be density on the bottom.
  4. Dec 7, 2017 #3
    The equation I know for calculating air drag force is:
    F = 0.5 * rho * Cd *A * v^2

    rho = air DENSITY
    Cd = coefficient of drag
    A = cross section
    v = speed in m/s

    From this I get for Cd:
    Cd = F / (0.5 * rho * A * v^2)

    Comparing to your equation:
    CD = FL/ (0.5 * Air viscosity * V^2 * Area)

    So I think you used air density, no air viscosity.
  5. Dec 10, 2017 #4
    I suspect you were using a pipe equation, not an aircraft drag coefficient equation - I hope. That would be more applicable to a flow through a small tube. In pipe head loss equations, f is the friction coefficient, and L is the Length. At least a pipe head loss equation might have something to do with the flow through a small tube, which the drag coefficient calculation for an airplane really doesn't. I'm not sure how it reduces to anything like you have though.

    head loss = f L v^2 / 2 D g

    D = diameter
    L = length
    f = friction coefficient
    g = gravitational accleration

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted