Help me calculate skin friction drag on a vehicle please

[Somes J]

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.

So far I've found this:

http://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 here (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.

[jack action]

There are some typical values at the bottom of this page (http://hpwizard.com/aerodynamics.html) (Theory»»Drag coefficient) ranging from 0.025 to 0.05. The calculator gives a variation between 0.0285 to 0.076 according to typical body type.

[RandomGuy88]

I am not sure about the exact numbers but for a truck the pressure drag should be significantly higher than the skin friction drag. This is because the truck is going to have a large separated zone behind the truck. In this separated region the pressure will be significantly lower than the freestream. So this will create a large pressure difference between the front of the truck and the back. This is generally the opposite on an aircraft where the flow stays attached over most of the surface. So skin friction makes up a larger portion of the aircraft's drag.

The skin friction coefficient is very difficult to calculate for all but the simplest shapes because it depends on the thickness of the boundary layer and whether or not the boundary layer is turbulent or laminar. This means that you need to be able to predict the point where the boundary layer transitions from laminar to turbulent flow which is very difficult. The skin friction calculator that you used is calculating the skin friction for a flat plate. Without some sophisticated CFD this is probably the best estimate you will get. What did you specify as your transition location? For a truck I would imagine that the boundary layer is turbulent over most of the surface. Perhaps you could try several different transition locations to and get several different skin friction coefficients and use them to calculate a plausible range of the skin friction drag.

[boneh3ad]

This means that you need to be able to predict the point where the boundary layer transitions from laminar to turbulent flow which is generally impossible.

Fixed that for you.

[Travis_King]

Add 15-20%. With all the simplifications you would use trying to estimate the parasitic drag, you would still be, for all intents and purposes, guessing. Might as well save time and guess first. Unless you have to show the calculations...in which case, just do what you are doing.