Do You Know? Help on drag question

[2000mc]

Originally Posted by Frank Lee

Seriously that little lip isn't worth all this obsessing.

maybe another way of saying that would be... if you fully tested this with and without the lip, your results would likely be inconclusive.

or as every other person says, try it and see....

or as several said, we cant tell you because there are too many variables...

for example from Car Aerodynamics - Top Speed - Hot Rod Magazine

station wagons with the deflector at the back of the car to keep road spray off the rear window. The deflector alone added drag, and a roof rack alone added drag, but when the roof rack was added to the deflector, drag was actually reduced. Of course, the combination was not as good overall as a smooth roof, but the process is a good example of nonobvious interactions.

[JackMcCornack]

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>

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.