Quote:
Originally Posted by Vman455
There's been some published on air dam research, yes. The most recent article I have was published by Ford engineers in 2018, testing air dam depth and curvature on the 2015 F-150.
Nowhere in the literature, however, does the rule "air dams should extend as low as the lowest-hanging component under the car and no lower" appear (that I've read, anyway). The Ford paper points out that drag reduction depends on the pressure on the backside of the tailgate--which changes with air dam shape and size and cooling air inlet shape, size, and flow rate! It's tremendously more complicated than the "rule of thumb."
|
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.
3) Wolf Heinrich Hucho's 2nd-Edition textbook's very first drag curve ( Figure 4.80, page 166 ) demonstrates the phenomena.
4) Anyone interested, would be free to examine the particulars of each test vehicle represented in each SAE, or other sourced articles, and pay particular attention to the dimensional relationships of each.
5) The take-away is: as the airdam is introduced, drag is initially reduced up to a certain 'length', at which it nulls out, after which, drag begins to increase again, and at some 'length', the total drag exceeds the OEM drag for a non-airdam configuration.
6) The caveat is presented by Lieutenant Nathan A. Williams, in his September, 2003 Master's Thesis, in which he investigates airdam 'length' all the way to the road surface ( this would never be examined by an automaker, as they are constrained by SAE 'approach' angles of 16-degrees ).
7) The lowest total drag for his 1997 Dodge RAM 2500 airdam configuration occurred @ zero ground clearance.
8) Anyone interested, could take his data, plot it on a Cartesian X-Y grid, and statistically determine where the drag ' trend-reversal' occurred on the pickup.
9) This would constitute part of the vast amount of empirical data which establishes the ground rules for fluid mechanics, aerodynamic engineers must respect in seeking aerodynamic solutions with off-the-shelf technology, as Hucho admonished readers to take away from all data from all sources, across all time.
--------------------------------------------------------------------------------------
10) Wolf H. Hucho cautions us about 'adding frontal area', when pursuing any vehicle aerodynamic modification. I have deferred to his education and experience, plus empirical evidence within the rest of the literature I'm in possession of.
-------------------------------------------------------------------------------------
11) I agree with you 100% with respect to 'testing.' The qualifier would have to do with taking what's already 'known quantities' as a 'first-principle', 'first-approximation', which includes the caution about aggravating frontal area ( It's gotten me two speed records and multiple mpg records so far ).