[Frank Lee]

Common sense? Take a cardboard folder, hold it at a 15 degree angle and move it through the air and note the resistance you feel. Then angle it to 45 degrees and fold it so that the frontal area is the same. Is the resistance more or less or the same??

Common sense tells me this does not apply here.

Car windshields- you've never noticed that economy cars have very slanted windshields? Prius? Heck, look at your own picture of the Beetle in the wind/water tunnel.

I've noticed that exotics have very slanted windshields, and their Cds are atrocious.

The fact that air piles up and forms a "bubble" proves that flow is not as easy as if the windshield were sloped back.

What is "easy" flow, and does it cause less, more, or the same drag?

I know you are into rear end aerodynamics. Did you ever ask yourself why airflow is better able to stay attached the shallower the rear surface angles are? It's because it takes less energy for the pressurized air to expand toward the surface and flow along it.

I'm not into rear ends any more than the next guy. Wait. That didn't come out right...

I know why flow stays attached at shallower departing angles- it's because on the aftbody, each molecule in the air mass that had been displaced by the forebody can only return to (roughly) it's original position at a certain rate in an orderly fashion. If that rate is exceeded turbulence ensues.

Just visualize-- use the spatial common sense that mother nature built into your brain which allows you predict where a ball will land depending on how hard and at what angle you throw it.

OK, I'm there now, in the field of dreams... this isn't helping.

And now, finally, on to the good stuff (thanks for linking that article! )

In #36 I said:

Originally Posted by frank #36

I can't tell enough from the Camaro pic to comment on it, but that region is known to not have high attached flow... Or, let's see data from the pros; either one will do.

...and the illustration and text you provided support that nicely.

Originally Posted by article

From all these data it can be concluded that the direct influence of windshield inclination on the drag is only moderate

Thank you. That's what I said. That's what Hot Rod Magazine found out too.

Originally Posted by article

Second, the deflection of flow at the transition from the windshield to the roof is smaller. The low pressure peak occurring there is therefore smaller and the positive pressure gradient in the remaining flow is less steep. Hence the momentum loss in the boundary layer is lower, allowing greater pressure recovery in the area of rear end. Therefore even if a strongly inclined windshield does not contribute to a local drag reduction, it helps to improve the flow over the rear part of the car and thus to reduce the overall drag.

Thanks again! See what they are talking about here? It ain't the bubble; it's the transition at the TOP- windshield/roof junction. When I said this:

Originally Posted by frank #38

So I'd wager hood blisters don't really affect the flow upstream and downstream... so then what?

... I meant if you replace the air bubble with a physical bubble, the flow ahead of it- over the hood- should be the same, and the flow after it- upper part of windshield, then windshield/roof transition, then aft- should be the same too. I see no reason for the faux bubble vs. a physical bubble in that location to make a material difference in drag.

Now you are off and running on something else besides the bubble: the windshield/roof transition.