ezC6

How is it that a car like the Infiniti G35 lists it Cd as:
Coefficient of Drag (Cd): 0.27/0.26 (w/ rear spoiler)

While a car that looks much more aerodynamic, namely the C6, is a 0.286? I would think the C6 would have a better Cd. Just curious...

Rocketblock

Where's Jinx or Scissors when you need em?

DigitalGriffin

There are several factors that greatly influence the cd of a car. Sometimes lowering the c/d decreases the stability of the car at higher speeds by redirecting the flow to places you don't want it. The C6 also has a shorter nose working against it. (More abrupt changes in air flow) Differing boundry layer air velocity deltas can create a a higher c/d also.

The main reason why golf ***** have demples in them, to make a greater boundry layer, and it easier for the slower air to flow around the back of the ball, so it has a higher pressure behind it.

Aerospace engineering is still a little black art type science. While simulations are getting better, they aren't perfect.

BTW: I do have an aerospace degree, but I haven't used it in years.

BigdogADM

Not sure where you got the above numbers, but the Infiniti website lists drag at .29 for the G coupe, or .28 with the aero package. This is much closer to the Vette's numbers.

CAVetteKerry

As with everything else in the design of a car, there are trade offs to optimizing the C/D. For sports cars it tends to be a trade off between reducing drag or reducing lift. If you reduce lift for stability at high speeds, you increase the drag.

It could be that the C6 is more stable at higher speeds than the G35 but had to pay for it with a slightly higher C/D...

As previously indicated, subsonic aerodynamics is a black art. Drag is measured in a wind tunnel and used to calculate C/D for a standard set of atomospheric conditions. Obviously designers have a way of estimating the drag but they never know for sure until a car is in a wind tunnel. Supersonic aerodynamics is actually easier to predict.

While it is interesting to compare C/D values of sports cars, the number by itself is very dependent on current climate conditions. The equation to calculate C/D is:

Cd = D / (A * .5 * r * V^2)
where D = drag, A = surface area, r = air density, V = velocity

The only variable not changing is the surface area of the car...

ezC6

Thanks for the responses guys.
I got the numbers on the G35 (sedan) from:
http://www.nissannews.com/infiniti/2...35/specs.shtml

Also, a friend gave me a useful link:
http://www.gmecca.com/byorc/dtipsaerodynamics.html

Here's some good info from the link:

Frontal Area
Drag coefficient, by itself is only useful in determining how "Slippery" a vehicle is. To understand the full picture, we need to take into account the frontal area of the vehicle. One of those new aerodynamic semi-trailer trucks may have a relatively low Cd, but when looked at directly from the front of the truck, you realize just how big the Frontal Area really is.
It is by combining the Cd with the Frontal area that we arrive at the actual drag induced by the vehicle.

Duck916

Another comment, if the Corvette is shorter and wider than the G35 (I know it's wider, but I'm not sure about length, offhand), it's much tougher to get a low Cd.

SWCDuke

Correct. It's tougher to get a low Cd with a short car, but Cd is not the whole story.

To compare TOTAL aero drag from one vehicle to another, multiply Cd times frontal area. I think one of the mags quotes this number.

Frontal area is difficult to measure, but a decent estimate is 80 percent of the total width times total height.

Motorcycles have typical drag coefficients of 0.6 to 0.8, but due to their small frontal area they have relatively low overall drag.

Duke

rambler323

I would think its probably more helpful to find Cd at some standard conditions and then use that to solve for the drag force at the condition you're interested in. That way you are solving for the actual resistance, not just some dimensionless number. But I don't really know anything about it...

raj

Good points by everyone regarding the car's length, width, etc. Couple of other reasons why the C6 has a higher Cd:

For all these reasons sports cars actually tend to have higher Cds than well designed sedans! Of course, as pointed out earlier the frontal area is quite key to estimating the total drag area (Cd*A) and Cd is only one part of that equation.

Duck916

For what it's worth, I just noticed that Car and Driver's data panels include estimated frontal area and Cd, and the resulting total aerodynamic drag figure.

rwd

So did GM.

Initially, they hyped the C6's .28 as being a marked improvement over the C5's .29. In reality, it is .286 versus .287 for the C5, and if they're that close, the .286 number has to be suspect.

To Euro-size the car, GM knowingly made choices that were bad for aero and racing. They have negatively impacted the C6R program and also affected the Z06 detrimentally. Doug Fehan, who manages the C5R/C6R programs touched on some of the issues like length and lack of overhang recently on Wind Tunnel.

The resulting problems won't necessarily be obvious, however, because we'll never see what the car that they didn't build might have achieved.