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Originally Posted by aerohead
1) We'd be compelled to accept Ford's data, as specific to this particular vehicle.
2) A review of my sources, of which presented drag tables derived for the specific test vehicle, all indicated for 'sweet-spots', where drag curves for both airdam drag, and underbody drag intersected, constituting the configuration of minimum total drag, as a function of airdam vertical 'size,' just as with streamline bodies of revolution vs fineness ratio.
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Yes, this is correct--but you are missing my point, which is that this "sweet spot" does not necessarily correlate with the height of underbody components, and thus that height should not be used as the determiner of air dam height;
testing should. Go back and re-read Hucho; if it's the same as later editions (I'm looking at the 4th and 5th editions right now, which have the same wording and graphs on front spoilers) you should notice that the overall drag curve for an air dam depends on "equivalent hydraulic roughness" of the underbody, and not the height of specific underbody components. Moreover, the drag reducing effect of a front spoiler derives not from its "shielding" the stuff hanging down under the body so that flow avoids it (or whatever people think is going on when they regurgitate the height rule), but in fact--
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The drag-reducing effect of a front spoiler is based on the fact that it diminishes the air speed under a vehicle, thus attenuating the contribution of the underbody airflow to overall drag.
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Where that sweet spot is cannot be intuited, and definitely not with a simple rule such as "no lower than the lowest-hanging component." And, as pointed out in the Ford paper, drag reduction from a front spoiler also depends on how it changes the flow through the cooling system and pressure at the back of the vehicle.
An example. Base:
4.5 inch dam: -6.7% change in drag
9 inch dam: -9.0% change in drag
The shorter dam sits well above the lowest hanging components under the truck and yet, you get 75% of the drag reduction as with the larger dam (which still sits an inch or so above the lowest part of the skid plate and front differential). That's a huge range of height to get similar drag reduction (2.4% is at the limit of what's measurable on the road).
We're in agreement about testing, but sadly I still don't see much of that going on around here. People want to build and then hope when they should go the other way: test and
then build.
