I'm trying to calculate skin friction drag on a truck so I can compare it to pressure drag. I can find the formula and typical coefficients for pressure drag easily enough but I've had a hard time finding it for skin friction.(adsbygoogle = window.adsbygoogle || []).push({});

So far I've found this:

https://www.physicsforums.com/showthread.php?t=100705"

Which gives the equation:

Rf = 0.5 x (rho) x V^2 x S x Cf

rho = density of fluid

V= velocity

S = surface area

Cf = coefficient of friction

I don't know what a plausible coefficient of friction would be, I found a calculator http://adg.stanford.edu/aa241/drag/SkinFrictionCalc.html" that I think is meant for aircraft, plugging in zero altitude and a plausible surface area (few hundred m^2) and speed (mach .09) I got .0027.

Trying that out I get:

.5(1.3)(29^2)(300)(.0027) = 443 at 65 mph

whereas for pressure drag I get

1/2(1.3)(29^2)(9)(.75) = 3690

Which gives skin drag being ~12% of the combined drag.

Is that right? I have no idea whether I'm entering a plausible Cf variable in the first equation, or doing it right. Can anybody help me with this?

Thanks.

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# Help me calculate skin friction drag on a vehicle please

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