Wake size and overall drag

[2TonJellyBean]

I've always wondered if drag factors for various vehicles were constants. To me it seems possible that as speeds get higher, certain laminar flows that don't interfere with each other at say 60 mph could possibly create havoc with one another at say 80 mph.

[oneinchsidehop]

Originally Posted by MetroMPG

Or as viewed from the rear, since it's projected area.

Would it be? I thought the purpose of a kamback or boat tail was to keep the flow attached as the projected area was reduced (approx a 5:1 or 7% slope), the reduction of the low pressure system statisticly diminishing when the reduction reached 50%, hence the abrupt chop off in a true kamback.

[trebuchet03]

Originally Posted by oneinchsidehop

Would it be? I thought the purpose of a kamback or boat tail was to keep the flow attached as the projected area was reduced (approx a 5:1 or 7% slope), the reduction of the low pressure system statisticly diminishing when the reduction reached 50%, hence the abrupt chop off in a true kamback.

That is the projected area at a specific region of the vehicle. To calculate aero resistance, you only need the max cross section. This is because the cD already takes into consideration all of the aero features (such as a kammback).

[lca13]

Originally Posted by MetroMPG

Or as viewed from the rear, since it's projected area.

You bring up an important point. The effective area is that which is orthogonal to the resultant wind vector. Since there always is some sort of crosswind, paying attention to to what the frontal area looks like, not only from straight on, but also from partial angles to the left and right, is also important.

.... gives me a whole new perspective on wheel skirts :-)

[brucepick]

Originally Posted by 2TonJellyBean

I've always wondered if drag factors for various vehicles were constants. To me it seems possible that as speeds get higher, certain laminar flows that don't interfere with each other at say 60 mph could possibly create havoc with one another at say 80 mph.

True that speed makes a difference.
From the little reading I did on aerodynamics (wikipedia mostly), I managed to learn that the formulas and calculations are slightly different for subsonic vs. supersonic speeds. Probably different for automotive speeds vs. small prop plane speeds.

If you're going to talk 60 mph vs. 80 mph, I don't know. I don't think the order of magnitude change is big enough to make much difference in the basic calculations and formulas used. But the simple fact of increasing from 60-80 mph alone is enough to increase the total drag considerably.