There is a nice article in the San Jose Mercury news about Daimler Chrysler's new wind tunnel.

The article mentiones the Porsche GT2 with a low drag coefficient of 0.34, but does not mention that most Mercedes and Audis are better than 0.30.

Here is the link:
<a href="http://www.bayarea.com/mld/mercurynews/business/industries/automotive/5446472.htm" target="_blank">DaimlerChrysler's Biggest Fan</a>

 

Interesting that 30 % of the time they are testing race cars ! the state of the art wind tunnels are "rolling road" where the car is positioned on a large treadmill so the ground is moving under the vehicle to more accurately reflect how the air is "moving" under the car . Coefficient of drag is just part of the story and nearly meaningless until you multiply it by the frontal area ..... a 747 airplane and a 22 caliber bullet might have the same coefficient of drag . The Chrysler "Airflow" needs to be seen to be appreciated ...aero dynamic is relative but ugly is ugly ....

 

On CD, wasn't the 928 derided by the 911 crowd b/c it was a high CD, .34 sounds just fine in this context.

Further, I love how heavy cars have become today, the sl55 AMG is like 4300 lbs, our cars are relatively lightweight at 3500-3600. Who would have ever thought!

 

the more "comforts" ,conveniences ,insulation, sound deadening ,double windows ,big wheels ,and large brakes .....the more it weighs ; but weight has little to do with top speed . As evidenced by the DEVEK car in "street trim " over 3,700 lbs ( as I recall )plus two people still over 200 mph . Weight makes a car feel "solid" stabile , perfect for the Autobahn .

 

if you want downforce, its going to create drag.
remove the downforce, and watch the drag numbers go down!!
Mk

 

Aerodynamics are always some kind of compromise. However, I believe that there are multiple different ways of getting downforce, some of which create more and some of which create less drag. It probably depends a lot on the vehicle's overall shape.

The 928 started out with - if I remember correctly - a cd of 0.42. In fact, the car's cd was significantly better, if the car was measured backwards. However, then there was a lot of lift, too!

The "S" spoilers improved the cd to 0.385, and the S4 with it's electrically controlled cooling flaps achieved a very good 0.34 (the GTS with it's wider tires gave up some of that advancement - cd 0.35).

What might surprise you is that not only the big things you see (bumpers, rear spoiler, but also many little details such as the 20mm lower wiper arms only made possible the vast improvement over the S models.

In an article in the Christophorous magazine (September 1986), Porsche explicitly states that the belly pan improves high speed stability (a topic often discussed here).

As ugly as it is, I am fascinated by the aerodynamics of the new Ferrari Enzo, which varies downforce depending on speed. Basically, it adds downforce up to a certain speed, then retracts some of the spoilers in oder to reduce drag and increase top speed.

Ferrari has probably learned a lot of the tricks in their round-the-clock wind tunnel sessions for the formula 1 racers...

 

The question is:"Is there light at the end of the tunnel?"

 

Reinhardt:

These tunnels have no end!!!!!

Think about it: The air is constantly recirculated...

But there are lights where the cars are. Hope that makes you feel a little better. <img border="0" title="" alt="[Wink]" src="wink.gif" />

 

INDUCED drag and lift can be determined by similar formulas. For drag:

D = 1/2 r V[squared] s cd

Where D is drag, r is air density, V is velocity, s is surface area, and cd is coefficient of drag.

PARASITE drag on the other hand varies with the square of speed. That is, double the speed and you have four times the parasite drag.

-Induced drag is the drag that is produced in the production of lift. In automobiles, lift is indeed produced...on purpose. The difference between cars and airplanes with respect to generated lift is that airplanes generate lift upwards , while cars generate it downward...in order to press the tires against the ground.

NOW- an object travelling through the air has more drag with more surface area. A school bus has more frontal area than a 928 and of course it has more drag as well...but so does a Boeing 727... quite a bit more surface area obviously- but the drag per unit of surface area of the 727 is CONSIDERABLY less than that 'ol school bus and our sharks. Why? Well, the obvious attention to aerodynamics.

How aerodynamic is a 727? Well, I can tell you that if you wish to decend from altitude, even with the engines at complete idle...you need to plan your descent so that you allow 3 nautical miles for each 1000 feet of altitude. That works out to 18,228 feet of distance for each 1000 feet of height- and if you don't [and don't use the aerodynamic speed brakes] and just lower the nose-[similar to going down a very steep grade in your car] the aircraft will blow through the speed of sound in seconds and likely break apart. All because its coefficient of drag is so tiny.

[Most experienced airline pilots drool over two airplanes: the 757 for its power, and the 727 for its speeds-]

BASICALLY, a perfectly aerodynamic object's drag will be determined solely by its frontal area. Since objects like 727's and 928's are not perfectly aerodynamic, a system to mathematically model the effects of shape was devised...basically the Cd in the above formula is used to add a smidgen of "virtual" surface area in order to improve the accuracy.

Heh- Aerodynamics are formost on my mind sitting in traffic in Orlando. Since I usually drive with the windows down...stray breezes can mess up my hair~ <img border="0" title="" alt="[Eek!]" src="eek.gif" />

Normy!
'85 S2 5 Speed