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I wouldn't make a blanket statement like that; Hucho would probably say, "It depends on your goals and stipulations." Here's an excerpt from "The Aerodynamic Drag of Cars: Current Understanding, Unresolved Problems, and Future Prospects" (W-H Hucho, in Aerodynamic Drag Mechanisms of Bluff Bodies and Road Vehicles, Plenum Press 1978, pp. 32-34):
"A front spoiler reduces the velocity through the underbody region. Velocity profiles underneath a car which have been measured by Kramer et al. and which are reproduced in Fig. 28 clearly indicate this effect. By reducing the velocity adjacent to the extremely rough underside surface, the friction drag of the underbody region is reduced. If the front spoiler is properly matched to the car, this drag reduction can overcompensate for the additional drag originating from the spoiler itself.
Additionally, a front spoiler causes a reduction in pressure on the front underbody panel, see Fig. 29, and under the engine compartment's hood. The latter effect is welcome when the engine-cooling air flow has to be increased. In general, these reductions in pressure help to reduce the front-axle lift. In Fig. 30 the results from an actual spoiler optimization are compiled. The height of the spoiler is increased in steps. Very often, but not in the present example, if lowest drag is desired, a shorter spoiler results than when maximum lift reduction is the target."
I'll see if I can post the figures when I get home.
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