Autospeed Beetle Article

[Mighty Mira]

What do people make of this article here?

The relevant commentary from the author, Julian Edgar, is here:

So the problem must be at the back? And it is. But it's a different problem to that which we've seen before. Here the flow remains attached right down to the line of the guard/boot opening. And this results in a very small wake for the (it's larger than you'd think) size of the car. But it also means that the airflow wraps in one long curve from the base of the windscreen right around over the top of the car to nearly the rear bumper. Aeroplane wing, did you say? And not only will this shape have major lift (without an undercar ground-effects tunnel, anyway; and I looked under and didn't see that), but much of the force will be upwards and rearwards - creating that monster drag.

I'm not sure I buy his explanation. The way I understand the rear of a car to work, is that the top of the car is analogous to the top of a wing or airfoil. Give it too much angle of attack, and it stalls. See this page., which gives the angle at around 16 degrees, very much like what the maximum angle most low drag cars get away with. He also makes the point that the stall "speed" will vary with loading, but it is the angle of attack that stays constant wrt stalling. I've also measured the maximum rear angles of the UFE-II (15 degrees) UFE-III (18 degrees off very fuzzy picture, 17 degrees from another), EV1 (21 degrees), Prius (17-18 degrees) and Insight (15 degrees) using GIMP and google image search, looking for photos of these cars exactly side on. Then I find the steepest part and get the angle wrt the horizontal of a tangent to that region.

If the rear surface of a car will stall like an airplane wing, it seems as though this fits rather well. Especially if you consider that angle of attack is measured as the angle the chord of an airfoil makes with respect to the oncoming wind. Note that the chord is the line from the leading edge to the trailing edge of an airfoil, and so in a typical airfoil or wing there will usually be a more extreme angle to be found taking a tangent to the rear upper surface of the wing compared with the the chord line. This means that if a wing is just stalling at an angle of attack of 16 degrees, the most extremely angled tangent at the back of the wing (analogous to the boat tail of a car) will be higher, maybe 18 or 20 degrees or so.

This agrees well with the boat tail angles of very low drag concept cars and hybrids, which can be assumed to have pushed this to the limit through trial and error.

So if you have a look at the pictures of the rear of the beetle, there are also some tufts that aren't agreeing with Julian Edgar's explanation and appear to be in turbulence higher than the artist's interpretration of where turbulence is occuring. In fact, the third tuft from the top of the window is pointing back towards the front. Perhaps there weren't enough tufts to get a good idea of what is happening back there.

But maybe I'm wrong. It's where our mental models are challenged that we learn something new.