Shelby fastback aerodynamics 1

[GT6Steve]

Greetings all,
Can someone explain the aerodynamics of the Shelby GT350 Mustang's fastback window treatment? I've been looking at them at the vintage races and wondering.
They use lexan rear windows with the front most edge dropped down into the interior of the car. Clearly shaped to exhaust air from the interior into the airstream over the roof. I'm told it provided a phenomenal downforce but I don't understand the dynamics. Can anyone provide an answer or point me to a Link?
Thanx in advance, Steve Smith

 

[NormPeterson]

If there's enough flow out through that gap the airflow over the roofline won't get dragged down as far as it passes over the rear window, and the upside-down diffuser-shape won't be as effective. I'd guess that the actuaL result was mainly one of less aero lift rather than the creation of actual downforce.

Norm

 

[patprimmer]

A little rule I always apply to peoples claims of down force from aero devices is to look at the strength of the device, especially the attachment points.

The down force can't be greater than the strength of the device, that is if the device never fails in use.

Regards
pat pprimmer@acay.com.au
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[GT6Steve]

Excellent comments both. I think Norm has the explanation and Pat has pointed out a missed but obvious point. Thanx to both. Steve

 

[FrankTHutchens]

It might be the same effect as boundary layer interrupt slots on airplane wings that vent a little just in front of the area of boundary layer separation. Disturbing that enough to keep the airflow attached rather than having it curl off and create lift.

Same as properly placed riblets, dimple tape, etc.

(did I get that right?)