Quote:
Originally Posted by daqcivic
I agree with (both of) your comments. I am not familiar with the idea of a"phantom" body, but it makes sense if attached flow breaks cleanly. I will perhaps try the trunklid extension first, since it will be much easier to fabricate, and also I can do a before/after tuft test on the rear window to gauge whether its helping. aerohead seemed to be saying that the roof extension is the better option (is that what Mercedes tested?), and I agree if done correctly, but it will be harder to make and I would have to get someone to snap pics from the outside with tuft testing it. Side extensions would definitely add to support and make more effective.
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The 'phantom' tail dates to Koenig von Fachsenfeld/Kamm with the 'K-car',or Kamm-back ( but Fachsenfeld deserves equal credit with Kamm).
Kamm considered the 'utility' of a full boat tail car and decided only a 'shortened' version would be practical for parking lots,parallel parking,etc..
So he essentially just started sawing away the tail of his car,as you would with a loaf of home-baked bread.
He discovered that you could remove quite a bit of length without degrading the Cd terrifically,and when examined in the wind tunnel under smoke,the air behind the car skipped over an invisible 'phantom' cone of air which mimiced the original long tail.
Bomber designers noticed the same effect when they chopped a fuselage off to add a gun turret at the rear.
This theme has been re-quantified over the decades with the likes of Walter Korff at Lockheed and Dr. Alberto Morelli at Pininfarina's wind tunnel in Turin,Italy.
The important thing is to fashion everything up to the chop as if you were going to continue to the full tail.
The Aerodynamic Streamlining Template Part-C,2nd image provides a preliminary guide to design from.It's purely amateur,but it's grounded in very good science of road vehicles operating in ground-effect which don't have the benefit of the axisymetrical flow jet pumping action of a free-flying structure.
The template can be used for a roof or side of a car.You'll notice amlost imperceptible curvature at the onset,building into steeper curvature the further back,up to a maximum tangent angle of 22-degrees which can't be obtained in less than one body height length behind the point of max body camber,whether top or sides.