Aerodynamics - why the Cx doesn't tell everything

[idletask]

The air resistance is a force calculated as such:

F = 1/2 * rho * S * Cx * v^2

Where:

* F is the dragging force, in Newtons (N);
* S is the frontal surface of the object (the car in this case), in square meters (m²);
* Cx is the aerodynamic finesse, which is only dependent on the shape of the object;
* v is the relative speed of the object (the car) compared to the fluid (the air), in meters per second (m/s) - in fact it should be separated in vc (object speed) and va (air speed) and written (vc - va)²;
* rho is the density of the fluid, the air, in kilograms per cubic meters (kg/m^3), it's roughly equivalent to 1.55 kg/m^3.

What this tells is that the Cx is only part of the story when it comes to air resistance! For example, take my car, the VX220 Turbo, and a Rolls Phantom. Surely, you can guess that this pachyderm:



has more air resistance than this bathroom appliance:



Despite of the appearances, though, the Rolls has a Cx of 0.385, and the VX220 has 0.41! So the Cx doesn't tell everything, you must also take into account the frontal surface... Which, while it is relatively small on the VX (1.6 m²), approaches the one of a garage door in the case of the Rolls (2.8 m²)! Which gives us an SCx of 0.66 m² for the VX and 1.08 m² for the Rolls...

So, what really matters is the quantity S*Cx, which is the drag coefficient, and often referred to as the SCx...

Also, you can see that the air resistance increases with the square of speed, so you have 4 times more air resistance when you drive 100 than when you drive 50...

[Chade]

Very interesting post!!

They have really done o good job on the RR to get the Cx down, but isn't it a bit high on the opel?

How does the grille damper-"level" on the RR affect the Cx?

[andyhui01]

can someone explain what CX is?

[idletask]

Originally Posted by Chade

Very interesting post!!

Thanks 8)

They have really done o good job on the RR to get the Cx down, but isn't it a bit high on the opel?

How does the grille damper-"level" on the RR affect the Cx?

It is high on the Opel, but then when you design such a car you call for maximum downforce, or rather, minimum "upforce" - and bringing downforce always has a bad influence on the Cx. The big mouth in front is a great source of turbulence on the VX, but then it helps feeding the radiator, which is in front, with fresh air, and the scoops on the front is where the air "goes out", bringing downforce. More Cx, all this despite a flat underbody.

For the Rolls, you just have to see that the radiator is more of a grille actually, and all these vertical metal pieces help laminate the air fluid and reduce the drag.

[idletask]

Originally Posted by andyhui01

can someone explain what CX is?

The Cx is the aerodynamic finesse (I don't even know whether this is the correct word in English).

See the water droplets? Why do you think they have this shape? It's just because nature doesn't like waste, and tends to the best possible shape. A water droplet has a Cx of 0.05, and this is the best Cx you can find from any shape. Maybe now you see why the Insight (0.25 Cx) has vaguely the shape of a droplet, or even the Prius (0.26). In fact, drop any liquid, it will eventually get the shape of a droplet.

On the opposite, the sphere has a Cx of 1.

A F1 car, when it is configured for tracks such as Monaco (lots of corners) has a Cx which is more than 1. In fact, according to the track, its Cx varies from 0.8 to 1.2.

[andyhui01]

do they test the CX in the wind tunnel... like what you see in some car-ads?

[idletask]

Originally Posted by andyhui01

do they test the CX in the wind tunnel... like what you see in some car-ads?

Most manufacturers do. F1 teams do, of course, as it is hugely important to them to have "good" aerodynamics - but then of course, for a road car, the difference of "good" differs. F1 cars need downforce first, fluent airflow comes second. On a road car you want also to test for silence around the joints, ventilation inside the car, etc etc, all this while keeping a good airflow too. Downforce is accessory. Of course, there are exceptions (the Enzo comes to mind).

[v0od0o]

I'm sure they do
I was very surprised that this block of a thing has a lower Cx than a vx220, very weird.
But very informative post, thanks a bunch, wish we see more of those in here

[v0od0o]

@idletask both ur nick and signature tell me you're a computer science major, are you? It'd be good to find another "geek" with me around here

[idletask]

Originally Posted by v0od0o

@idletask both ur nick and signature tell me you're a computer science major, are you? It'd be good to find another "geek" with me around here

Well, computers are my job but most of what I know I've learned by myself... I've a diploma in economics computing but I'm into systems programming

[idletask]

OK, some more interesting tidbits, or how only a few modifications can greatly modify the Cx...

The pic above is the VX220 Turbo. Now here's the N/A VX220:



So, the mouth in front has an horizontal bar on it, there are no moustaches under the front bumper, no air scoops on the side and no spoiler on the rear. The tires are the same size. That's it.

It may not look much, but it's enough for the Cx to drop from 0.41 to 0.37, that's a good 10% LESS than the Turbo...

[andyhui01]

since we are talking about Aerodynamics... i've had this question in mind for ages... what are the aerodynamic effects of pop-up headlights and I don't see what so good about them in terms of aero-dynamics... but looks wise... its a different story

[idletask]

Originally Posted by andyhui01

since we are talking about Aerodynamics... i've had this question in mind for ages... what are the aerodynamic effects of pop-up headlights and I don't see what so good about them in terms of aero-dynamics... but looks wise... its a different story

AFAICS it's really only about looks, they're much more likely to increase drag than anything else... By how much I've no idea. But it does change the shape and therefore the Cx.

Completely unrelated but they're now banned because of new security norms for pedestrian safety... That's the supposed reason why the Corvette C6 doesn't have them anymore.

[andyhui01]

yup... I've noticed that.. cars like the NSX don't have popup headlights anymore... pop-up headlights are one of the coolest exterior part... i love seeing them when i'm in the rx7

[Chade]

Originally Posted by idletask

It is high on the Opel, but then when you design such a car you call for maximum downforce, or rather, minimum "upforce" - and bringing downforce always has a bad influence on the Cx. The big mouth in front is a great source of turbulence on the VX, but then it helps feeding the radiator, which is in front, with fresh air, and the scoops on the front is where the air "goes out", bringing downforce. More Cx, all this despite a flat underbody.

Yes, I figured those thing had to do with it... Imean - more air flow to engine, brakes, downforce and so on must increase the CX...
Interesting point there about the difference between the VX220 Turbo and the standrad one...

For the Rolls, you just have to see that the radiator is more of a grille actually, and all these vertical metal pieces help laminate the air fluid and reduce the drag.

Yes, byt they can open or close depending on how much cooling the engine needs... I wonder how much the Cx differs if they are totally open or totally closed... or if the Spirit of Ecstsasy is up or down...

I would think that one reason for not to have popup headlights is to eliminate one source of failure on the car...


Edit:
P.S. I'm a software engineer (no degree, but almost... ) and spends most of my wake time infront of the computer... Except when I'm in my car, of course...

[chaos386]

Good post! I came upon a similar revelation when I looked for the drag equation in my physics textbook. I thought that the Cd (Coefficient of drag, what idletask called Cx) was the whole story too, but it makes sense that it's a factor of S*Cd, since a larger object of the same shape would obviously produce more drag than a smaller object.

Does anyone know the Cd (or Cx, whatever) of an angled square by any chance?

[MXM]

About pop-up headlights:
In Finland we must use headlights ALL the time. In most countries you don't, so keeping them down at day time helps with aerodynamics. But now we have projector-type lights, so they will works just as fine and don't need much space. They can be placed at the same place instead of pop-up ones under glass, and don't reduce aerodynamics. That's why nobody makes cars with pop-up lights anymore... In older days that was a tool to encrease aerodynamics when you don't need them, but now same can be achieved with projectors.

[MXM]

Originally Posted by chaos386

Does anyone know the Cd (or Cx, whatever) of an angled square by any chance?

Cd of a cube is about 0.8, found here: http://www.engineeringtoolbox.com/21_627.html

[andyhui01]

Originally Posted by chade

I'm a software engineer (no degree, but almost... ) and spends most of my wake time infront of the computer... Except when I'm in my car, of course...

hmm... you and v0od0o would have alot to talk about

Originally Posted by chade

if the Spirit of Ecstsasy is up or down...

lol... never thought of that... we should send a mail in and ask JC to calculate how much drag would that cost over the period of driving in a year and how much petrol is wasted because of that (Another reason why Bentley's are better than rollers and why BMW's are better than Merc's)

[v0od0o]

[offtopic]Wow so there is 3 of us now, idletask, chade, and I I'm a software engineer myself with BSc degree in Computer science, very nice to meet u guys[/offtopic]