# Drag coefficient equation

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1. Dec 3, 2017

### sam_smk

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.
https://www.grc.nasa.gov/www/k-12/airplane/dragco.html

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
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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. Dec 3, 2017

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.

3. Dec 7, 2017 at 9:27 AM

### jumpjack

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)

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

So I think you used air density, no air viscosity.

4. Dec 10, 2017 at 11:50 AM

### kcaldwel

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

http://www.pipeflowcalculations.com/pipe-valve-fitting-flow/flow-in-pipes.php