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Old 05-13-2020, 01:10 PM   #19 (permalink)
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Originally Posted by JulianEdgar View Post
I was thinking about this - how could someone have developed a theory about lift which is so comprehensively wrong? But I think I now have it. The clue was in the mention of notchbacks having highest lift, and the rear spoiler on the BMW 2002.

Old cars, like that BMW 2002, had flow that separated at the end of the roof. Therefore, yes, lift was in part created by the separated flow and the low pressure associated with it acting on the trunk lid / rear window. And in that case, the spoiler did in fact work in the way stated.

But of course, major flow separation at the rear of the roof of a sedan hasn't occurred in any aerodynamic cars made since, about, 1990. So for the last 30 years, this mechanism of lift production has been irrelevant.

To attempt to apply it to the Cayenne (etc) is as absurd as I previously stated. In fact, it is easy to show how absurd it it. The measured lift pressures on the upper surfaces of these cars is lower than the wake pressure...

So, there's a good example of the perils of not keeping up with aerodynamic understanding, and applying to current cars a conceptual model that works only with old-shape cars.
*On the other hand, Mitsubishi Lancer and Subaru WRX notchback sedans became poster childs for high speed instability and rear lift, only mitigated by the addition of VGs and wings.
*Until you understand the implications of what the local pressure is at the separation line on top of the Cayenne,and what it would mean to move that separation line all the way back to the rear of the car,there would be no point discussing the degree of lift it can telegraph over the rear of the car.I recommend a brush up on Bernoulli.
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