Quote:
Originally Posted by windrider919
So sorry Frank Lee. I have not 'blown off' anybody. I just found your answer too brief and uninformative.
I have a library of over 20 books on aerodynamics and aircraft design <snip> a further dozen books on marine hull design and flow dynamics <snip> programs like Delftship, Michlet Water Resistance Calculator, ArchimedesMB Flowdesign. None of them give specific answers to my questions. <snip>
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The question may not be answerable without a lot more information on your particular application, but it's been my understanding (possibly incorrect and please folks, correct me if I'm wrong) that a 'spoiler' or lip at the upper aft of a road vehicle reduces drag by correcting for a too steep trailing angle in the roof line. The fastbacks of yesteryear were particularly apt to such improvement, since they produced large trailing vortecies as air spilled over the sides of the car, into the low pressure area created by the attached flow over the roof, rear window and trunk. A lip at the rear can detach (or at least raise) the flow, reducing the vortecies and reducing drag. As a clumsy rule of thumb (and I think Milliken and Hucho back this up) if your roofline droops more than 20 degrees below horizontal (but not so steeply that the airflow detaches from the body) a (properly sized) lip will reduce drag, and if the roofline droops 20 degrees or less, an unadorned Kamm cutoff provides the least drag for a given length of stern.