Info & video - Opel Calibra - Cd 0.26
 

Every once in a while, GM has shown it's capable of hitting one out of the park, aerodynamically speaking. Unfortunately, this one never saw North American roads.

Here's a pic of the Opel / Vauxhall / Holden Calibra, for those who don't know it:


Launched in 1989, it was designed by an American who headed GM design in Europe. At the time, it was apparently the most aerodynamic production car available anywhere. (Source: Wikipedia)

Obviously they started with a decent shape.

But the designers paid a lot of attention to optimizing the details as well, such as the grille openings: only the 8-valve version of the car had the Cd 0.26 figure - the 16v engine needed more cooling, which meant a bigger hole in the front, and a worse Cd of 0.29 (other contributors to the change were probably things like wider tires, chunky rims, etc).

Here's another line about the small and sometimes counter-intuitive details that the Calibra engineers dealt with:

This YouTube vid contains a bit more of a look into what makes the car slippery:

I find it very amazing that the little vent increases drag by over 10%.

Pretty sweet Cd on that thing, and it doesn't look half bad either.

It might help once you realize how big the flat plate area (i.e., the CdA) of the car is. It's not that big - 0.30 equates to 30% of the car's frontal area. Which is probably on the order of 6-8 square feet. Compare other vehicles here.

If you understand that drag coefficients for really, really, ugly things (aerodynamically) such as the Eiffel Tower are 1.8-2.0, and that routing air through the engine bay, under your car etc is probably just as ugly, what is 10% of the CdA and hence what is the actual area of grille that would be blocked to achieve the difference?

10% of 7 square feet is 0.7 square feet. If we take the drag coefficient of that particular area as 2 (not a bad assumption), you only need 0.35 actual square feet of grille to get the difference. That's the difference a "little vent" can make.

Very nice video. 0.26 is amazing for a car from nearly 20 years ago.

That's a really interesting way to emphasize that point.

Yes, it's a good way to think about it. Considering that an airfoil has a Cd of around 0.05, and good cars have a Cd of around 0.30, the airfoil has 1/6 the drag of the car and 1/20 the drag of a flat plate. It's as if the airfoil is almost invisible to the air.

Of course, being near the ground has issues, you can get to 0.11 or so but to get better you have to start approaching an airplane by having cowled wheels and hoisting a fuselage as high as you can without toppling in a corner. But still, a Cd of 0.11 is still 10 times better than a flat plate, which is what a motorcycle or upright cyclist is basically.

Another trick I learned from airplanes which the designers of the aptera seem to have learned is that you put the trailing edges of airfoils next to the leading edges of other aifoils. See this plane. If you google the AR-5 you can find more about it - total CdA of 0.88 square feet. I think what happens is that the leading edge of an airfoil creates high pressure, the trailing edge creates low pressure, and you minimize pressure difference by doing this, and hence minimize drag.

Daox - here's another bit of info that emphasizes the aero impact of the cooling system. On a study of an un-named car with Cd 0.29, it makes up the biggest fraction of total drag:

http://us1.webpublications.com.au/st...4/2455_7mg.jpg

This is from the Autospeed article, linked above. Worth a read: Modifying Under-Car Airflow, Part 1

I was practising archaeology throughout this aerodynamics forvm when much to my delight (I'm the lucky owner of not one but two Calibras :D ) I found this old post. I can't help but resucitate it :) .

Wayne Cherry, in collaboration with Opel Designstudios' then chief design engineer Erhard Schnell.

http://img37.picoodle.com/data/img37...cm_683dfdb.jpg

Correct, world record most aerodynamic production car in 1989, and for a decade afterwards until the Insight arrived.

Info on that towards the end of this pdf:

http://img34.picoodle.com/data/img34...cm_87f06ba.jpg

http://img37.picoodle.com/data/img37...cm_d19fd36.jpg

That's an incorrect statement I've seen everywhere in the net, probably the result of blind quoting by uninformed journalists without first hand experience on the car. All Calibra versions have identical front bumpers with equal size openings for cooling.

Tyres are the sole culprits (195/60-14 in the 2.0i versvs 205/55-15 or 205/50-16 in the 16V/V6/Turbo).

:D I guess the gentleman who wrote that was more used to SUVs and minivans than sports cars.

Yeah, he was definitely not in top form and suffered abnormally high perspiration :) . No such issues in the Calibra, the ventilation works as in any other car.

On the contrary, the Calibra is one of the cars with better all-around visibility I've driven (excluding top-down cabrios).

Well, it undoubtedly affects the Cd when open, but "destroys it" seems an exaggeration to me after reaching 235kmh / 146mph http://www.foren4all.de/images/smilies/mauer.gif in the Autobahn with sunroof deployed.

http://img26.picoodle.com/img/img26/...sm_bd2ba78.jpg

That's quite true :). Also the large sloped windshield fogs more easily than other cars.

http://img26.picoodle.com/img/img26/...sm_8a2c687.jpg





Some comparisons to get perspective (Cd x frontal area in square meters = SCd, the real value):

http://tbn0.google.com/images?q=tbn:...da_insight.jpg 2000 Honda Insight: 0.25 x 1.905 = 0.47625

http://tbn0.google.com/images?q=tbn:...dHoldenM_m.jpg 1989 Opel Calibra 2.0i: 0.26 x 1.93 = 0.5018

http://tbn0.google.com/images?q=tbn:...AudiA2/Ex1.jpg 1999 Audi A2 1.2: 0.25 x 2.18 = 0.545

http://tbn0.google.com/images?q=tbn:...yota_prius.jpg 1997 Toyota Prius: 0.26 x 2.16 = 0.5616





Pictures of my babies. The 2.0i (17 years & 223000km / 140000 miles and still in top form):

http://img98.imageshack.us/img98/228...evla018cc3.jpg

http://img34.picoodle.com/img/img34/...Hm_1a3d456.jpg


The Turbo AWD:

http://img26.picoodle.com/img/img26/...Lm_9fe0717.jpg

http://img26.picoodle.com/img/img26/...sm_384ba7b.jpg


(I know I know, guilty as charged, it's crazy to have two similar cars in the same colour :rolleyes: )

http://img82.imageshack.us/img82/369...elvm0011kt.gif


At your service for any quest for more information on these streamlined cars :) .

What happened to the video?