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Originally Posted by JulianEdgar
I just saw this, and immediately thought: that must be from about the worst current textbook on car aero. And I was right - 'Theory and Applications of Aerodynamics for Ground Vehicles'.
That para is absolute garbage. To show the 'quality' of this book, note how reference 9, cited in the para, isn't even in the chapter references...
A spoiler, common in sports cars, is a negative lift device.
Yes.
It reduces the lift by slowing down the flow over its upper surface.
What surface? Not the spoiler, as this suggests.
A negative lift wing is the most common type of spoiler.
No, a wing is not a spoiler - how basic a mistake can be made?
When lifting devices are used, it is important to place them in the proper location at the rear of the vehicle, or they may turn out to negate the very effect for which they have been incorporated [9].
Ref 9 not included in chapter references (there are none for the book as a whole). It's highly unlikely that any spoiler will create lift on any modern car (where most lift comes from attached flow), unless something really weird is done so that the spoiler deflects air massively downwards.
For example, a spoiler on the vehicle rear roof only adds to the lift.
I don't think this is the case - in fact I think this is balderdash on any modern car with attached flow over the rear window. But OK, now where is the evidence for this? None is presented.
The book is full of mistakes - staggering that it was published by the SAE. If you want to learn about spoilers/wings/etc, Katz, Hucho or Scheutz are the gold standard - especially when compared to this book!
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Without specifics, I can imagine a common situation where the comment would be spot on.
If the spoiler is there to provide a surface of reattachment, then it IS addressing a situation in which the low pressure over the backlight/boot region is caused by the early 'fast' flow separation.
By providing for flow reattachment, a locked-vortex is established, over which the fast inviscid flow CAN decelerate, and by the time it reaches the top of the spoiler, is at a higher static pressure, plus the spoiler acting as a dam, sequestering the low pressure immediately over the boot, away from the base of the car, where it cannot effect the wake.
In this case, both drag and lift are reduced do to the 'slower' air.
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As to a 'wing' not being a 'spoiler', it may be a issue of semantics.
Anything that spoils lift is technically a spoiler, regardless of the actual device or technology. For example, a venturi is a 'spoiler.'
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Your comment about 'most lift comes from attached flow,' ought to include caveats, as there exists counterfactual evidence to your claim.
Streamlined bodies have completely-attached flow, yet generate zero lift.
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Your book contains mistakes about 'wrapped' flow and lift. ' He who sins not, cast the first stone.'