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:
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In the case of the Calibra, the centre section of the front spoiler was lifted to allow more (yes more!) air to flow under the middle of the car, so making the total flow more parallel with the car’s long axis. This reduced the amount of air being deflected outwards by the front wheels, reducing the size of the wake and so drag.
(Source: Modifying Under-Car Airflow, Part 1)
This YouTube vid contains a bit more of a look into what makes the car slippery:
Some points made in the vid:
obvious attention to boat tailing/tear drop shape
side skirts
continuous front contour, from hood, to lights, to bumper, to front undertray
And an interesting quote:
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One serious shortcoming is ventilation. Like many streamlined cars there's no through-flow of air, and the only way to get that is to have the fan on full almost all the time.
These sloping windscreens also allow an awful lot of sunlight in here and it gets very hot & stuffy.
The reviewer also notes that the sharply raked A pillars make ingress/egress more difficult. (My observation: they also affect outward visibility.)
One design issue that was out of place on the slick Calibra was an outward opening sunroof (the kind that slides on top of the roof, rather than inside). In the open position, it would destroy the car's Cd.
I find it very amazing that the little vent increases drag by over 10%.
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.
__________________ "Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed." - Isaac Newton
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.
__________________ "Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed." - Isaac Newton
I find it very amazing that the little vent increases drag by over 10%.
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:
I was practising archaeology throughout this aerodynamics forvm when much to my delight (I'm the lucky owner of not one but two Calibras ) I found this old post. I can't help but resucitate it .
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Originally Posted by MetroMPG
Launched in 1989, it was designed by an American who headed GM design in Europe.
Wayne Cherry, in collaboration with Opel Designstudios' then chief design engineer Erhard Schnell.
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At the time, it was apparently the most aerodynamic production car available anywhere.
Correct, world record most aerodynamic production car in 1989, and for a decade afterwards until the Insight arrived.
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... the designers paid a lot of attention to optimizing the details ...
Info on that towards the end of this pdf:
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... the 16v engine needed more cooling, which meant a bigger hole in the front ...
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.
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... and a worse Cd of 0.29 (other contributors to the change were probably things like wider tires, chunky rims, etc).
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).
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The reviewer also notes that the sharply raked A pillars make ingress/egress more difficult.
I guess the gentleman who wrote that was more used to SUVs and minivans than sports cars.
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One serious shortcoming is ventilation. Like many streamlined cars there's no through-flow of air, and the only way to get that is to have the fan on full almost all the time.
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.
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(My observation: they also affect outward visibility.)
On the contrary, the Calibra is one of the cars with better all-around visibility I've driven (excluding top-down cabrios).
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One design issue that was out of place on the slick Calibra was an outward opening sunroof (the kind that slides on top of the roof, rather than inside). In the open position, it would destroy the car's Cd.
Well, it undoubtedly affects the Cd when open, but "destroys it" seems an exaggeration to me after reaching 235kmh / 146mph in the Autobahn with sunroof deployed.
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These sloping windscreens also allow an awful lot of sunlight in here and it gets very hot & stuffy.
That's quite true . Also the large sloped windshield fogs more easily than other cars.
Some comparisons to get perspective (Cd x frontal area in square meters = SCd, the real value):
2000 Honda Insight: 0.25 x 1.905 = 0.47625
1989 Opel Calibra 2.0i: 0.26 x 1.93 = 0.5018
1999 Audi A2 1.2: 0.25 x 2.18 = 0.545
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):
The Turbo AWD:
(I know I know, guilty as charged, it's crazy to have two similar cars in the same colour )
At your service for any quest for more information on these streamlined cars .
Today
Other popular topics in this forum...
) I found this old post. I can't help but resucitate it 
.
in the Autobahn with sunroof deployed.
2000 Honda Insight: 0.25 x 1.905 = 0.47625
1989 Opel Calibra 2.0i: 0.26 x 1.93 = 0.5018
1999 Audi A2 1.2: 0.25 x 2.18 = 0.545
1997 Toyota Prius: 0.26 x 2.16 = 0.5616
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