Quote:
Originally Posted by ACEV
@niky,
Thanks for the good link. However, since the results are within any reasonable fudge factor, I would say that the differences were negligable.
However, that does raise questions about the dimples used. As a thin film, the dimples would have been quite shallow, and quite likely very equally spaced. Not so with the MythBusters application; leaving all sorts of variables. However, since there results were highly significant, their test is the one that needs further study, rather than dismissal.
Further, A-B-A testing is questionable because of such little difference between that and A-B. Why not A-B-A-B..... to infinity? Would that help? At what point do we say that the improvement is real? Or even good enough?
@MetroMPG,
Please note that the test results were very significant. They were not minimal. Therefore, the small details you mentioned would not likely make any significant difference. Actually, what we can really say is that they got real results that were significant.
9% is nothing to disregard. 
What still takes me by surprise is that no one seems to notice that the air on the surface of a moving object is what creates drag. Therefore it is reasonable that breaking that up would decrease the drag of attached air. Let's discuss the basics first. |
Read
Post #22
To follow up on that point, I found this nicely written piece from airliners.net user QantasA332:
Basically, there are three primary types of drag acting on an aircraft: induced drag, skin friction drag, and pressure drag. It is pressure drag that is the main factor involved in the dimple design's existence. Pressure drag is primarily the result of a moving body's wake. Depending on how soon the airflow separates as it passes over an object - that is, how far along the object the flow travels before no longer following the contour of the object - the size of the wake will be larger or smaller. A larger wake equates to more pressure drag (put simply, there is a larger region of stagnant air behind the body meaning the airflow pushing on the front of the body has less impeding its production of drag) and vice versa.
Now, imagine a sphere. Because its height/diameter is large in comparison with its length, it is what's known as a "bluff body." Bluff bodies such as a sphere have disproportionately large wakes, and as a result they have disproportionately high pressure drag. (This is compared to both their own skin friction drag and a not-bluff solid's pressure drag). Obviously, then, overall drag on a sphere (or other bluff body) can be dramatically reduced if pressure drag is reduced. That is, pressure drag is what you want to specifically target and minimize.
Enter dimples. Dimples turbulate the airflow over an object, thus increasing the flow's kinetic energy. This acts to delay flow separation, which then leads to a smaller wake, which in turn leads to less pressure drag. And this solves the bluff body problem! Because bluff bodies have such high pressure drag compared to their skin friction drag, what little extra of the latter drag is created by dimples is more than offset by the drastic reduction of the former drag. So a golf ball - the classic example of a bluff body - will travel farther with dimples than without, and that is of course why they have come to carry these dimples.
Now, to finally answer your question: 'normal' aircraft are very simply not bluff bodies. Dimples would create more skin friction drag than they would reduce pressure drag, defeating their purpose.
So looking at a car, only where you have flow separation, would these dimples or any kind of vortex generator help. On most sedans, the back glass is prone to flow separation, and so you may see some reduction in drag there if implemented right. The rest of the car should have attached flow and thus the dimples would only add skin friction drag.