Quote:
Originally Posted by j-c-c
Simplistically:
A spoiler in my world "spoils" air flow on one side, upwind, the amount of spoiling is often adjusted by its angle to the air flow, and only requires air flow on one side
A wing is mostly horizontal and requires air flow on two sides, and creates force by the differential pressure of its two sides.
A flat horizontal aero surface/plate with air flow on both sides helps maintain air flow or allows air to on both sides to more orderly reattach and secondarily maintain existing pressure gradients that might be found useful. I'm having a hard time defining/incorporating a front "splitter" into this definition of a flat aero surface.
This is all based on my contention an aero device that is flat should not be called as a functional spoiler
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For road vehicle 'aerodynamics', what a 'spoiler' would be 'doing' is to 'spoil' flow separation which contributes to both drag, and induced lift.
Most non-high-performance cars exhibit aft-body flow separation due to their really bluff rooflines.
The OEM rear spoilers aren't there to provide 'direct downforce', so much as, rather to provide a means to reattach the separated flow onto the car.
The ideal 'kammback' solution is discounted on account of some vehicle specification which precludes 'aerodynamics' as a priority, and the aerodynamics engineer is tasked with providing some form of aerodynamic 'lipstick' for an aerodynamic 'pig', without infringing on the designer's 'design.'
As long as the 'tearing edge' of the spoiler reaches 'up' to the 'reversal point', where it's reaccelerated the flow just enough to kill the adverse positive pressure gradient, and get the boundary-layer reattached across a separation bubble, that's all it needs to do. And what it looks like is essentially meaningless.
If you're going to drive above 112-mph, an 'active' surface, which can reach even 'higher', will add some direct downforce for high-speed stability, although at a drag penalty ( induced drag ).
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If you overlay a transparency of the 1996 Porsche 911 GT1 on top of the 1972 Porsche 911 Carrera RS 2.7, with 'duckbill' rear spoiler, you'll see that the tearing edge of the duckbill coincides exactly with the 'surface' of the 911 GT1.
The 1996 911 'roofline' is what Dr.-Ing. Hermann Burst intended to fix the instability of the 1972 911, but was shot down by designer Harm Lagaai ( sp? ), because it didn't 'look like' what Ferdinand, Ferry 'Butzi' Porsche had drawn for the iconic 911.
It took 24-years before the Porsche big-wigs would allow an 'aerodynamic' roof on a Porsche 911.
And, the 'Ducktail' produces zero direct-downforce, only pressure-recovery due to re-attached flow.