Quote:
Originally Posted by Vman455
This 22-degree angle comes from Mair's work, as has been correctly stated on this site. But here's the thing: Mair was investigating flow over very long cylindrical bodies (14 times as long as their diameter) and very short, tapered tails (half as long as the diameter) to try and understand the mechanisms of flow attachment and the effect of taper on drag; these are bodies that look more like ICBMs than cars. He also reported on other researchers' work with the same types of cylindrical bodies who found drag minima at different trailing edge angles, as well as minima in side forces in yawed flow at different taper angles, and there are modern articles reporting on work with similar cylindrical bodies that find drag minima at differing taper angles depending on things like the transition between body and tail and other upstream features. 22-degrees was never presented as a hard-and-fast, "beyond this angle airflow detaches" rule because it isn't (except on this site, of course). And every aerodynamic textbook of which I'm aware gives a range of angles for rear taper within which minimum drag can be found. I'm not at home and don't have my books in front of me, but I believe Hucho even uses the word "broad" to describe the range of acceptable backlight angles for low drag, and Barnard writes about the effective angle between backlight upper edge and trunklid trailing edge, saying it should generally be between 10- and 30-degrees for lowest drag--a very wide range!
Contrary to what I believed in the past, the ideal angle for any car has to be found through testing. Depending on the body shape and details, an angle faster or slower than 22-degrees may be allowable or necessary to maintain flow attachment in the widest range of conditions. If you find yourself with a 40- or 50-degree angle, then there's a good chance the attached flow is due to vortex downwash, but the resulting high drag will show up in proper testing ( that's how this phenomenon was discovered in the first place). |
A well-supported summary. To put it another way:
The use of a template to guide rear extensions, judge the 'purity' of shapes, guide the positioning of rear spoilers on notchbacks, and assess whether a car will have attached or separated flows - they're all just rubbish.
I'm (again) putting it like that because I've noticed that lots of people here can't see the cognitive dissonance of agreeing with two completely contradictory ideas.
At least as a group we now seem to be moving away from invalid 'rules of thumb' to acknowledging that the real-world complexity of car aero modification requires testing, not guessing.