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Old 03-27-2010, 03:13 PM   #22 (permalink)
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Originally Posted by COcyclist View Post
Here is the link for the AutoSpeed article where I believe the above drawing was lifted:

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And Julian Edgar's explanation for the terrible drag and lift coefficients for the New Beetle:

"So the problem must be at the back? And it is. But it's a different problem to that which we've seen before. Here the flow remains attached right down to the line of the guard/boot opening. And this results in a very small wake for the (it's larger than you'd think) size of the car. But it also means that the airflow wraps in one long curve from the base of the windscreen right around over the top of the car to nearly the rear bumper. Aeroplane wing, did you say? And not only will this shape have major lift (without an undercar ground-effects tunnel, anyway; and I looked under and didn't see that), but much of the force will be upwards and rearwards - creating that monster drag".

And farther down in the article, the VW factory wing designed fix, a spoiler to combat lift at high speeds.

Aerohead has suggested that the "birdbath spoiler" and the double wing on the Mercury Merkur also reduce lift and drag. But I'm no expert. I only know what I read on the internet and it can be rife with unsubstantiated opinions.
I would respectfully disagree with Mr. Edgar's appraisal of the New Beetle's aft-body flow.If what he says is correct,then the last 90 years of aerodynamic investigation goes right out the window.
Separation,and where it occurs causes the lift he is interested in.When the separation occurs directly behind the car it suffers rearward longitudinal lift( form drag) which cannot be remedied without the reduction of separation.
The separation cannot be remedied without the curvature of the roofline and side body being relaxed to that more like the template.( the whole basis of Kamm's research,fuselage design,submarines,etc.).
Both the Herrod Helper and Ernie's wing provide a structure for partial re-attachment of the stalled flow which moves the deformation of the outer flow field closer to that of the ideal pathway,as described in the template.
The 'attached'/'captured'/'locked'- vortex created above the horizontal shelf allows the adjacent flow to make it further back before separation and when it does separate,it's at a higher static pressure than upstream,which reduces the base pressure of the wake,consequently reducing the form drag.
As Frank has mentioned,if the flow was actually attached,all the way down as Mr.Edgar has stated,there'd be no need of Ernie's wing,nor would it ( could it ) be of any benefit.
In the jargon of AeroVironment,I believe Mr.Edgar is attempting to occillate at a level higher than his assigned frequency.If he wants to better serve the readers of AutoSpeed,he might invest a bit more energy into researching his topics.
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