Why coupè have so much drag?

[sebbe]

I don't want to sound more stupid that I already have when posting my reply (specially now that we know that there are several F1 staff members reading our posts), but I found this at wikipedia.

The drag coefficient (Cd, Cx or Cw, depending on the country) is a dimensionless quantity that describes a characteristic amount of aerodynamic drag caused by fluid flow, used in the drag equation. Two objects of the same frontal area moving at the same speed through a fluid will experience a drag force proportional to their Cd numbers. Coefficients for rough unstreamlined objects can be 1 or more, for smooth object much less.
(...)
Cd in automobiles:
(..) Minimizing drag is done to improve fuel efficiency at highway speeds, where aerodynamic effects represent a substantial fraction of the energy needed to keep the car moving. Indeed, aerodynamic drag increases with the square of speed.

Now I'm really lost. I took Principles of Fluids Dynamics (or whatever you call it in English) when I was at the University and I don't get why frontal area and velocity wouldn't count when calculating Cd.

Here's more data: http://www.grc.nasa.gov/WWW/K-12/airplane/dragco.html

The drag equation Cd = D / (A * .5 * r * V^2)
and how it's measured can be found there.

Is there anybody to explain it to me?

[Carlos]

If you search "chassis" at the same site; there is a remark that frontal area does affect drag. In the equation, perhaps frontal area is assumed to be a constant, that is, for a Specified Frontal Area, the equation applies.

Your quote from Wikipedia, basically says this - " two objects of the same frontal area moving at the same speed ..."

It's never stupid to ask questions, perhaps only stupid to refrain from asking questions. Concerning professionals on the site - If F1technical had existed when they were at school; I am sure it would have been a valuable resource for both knowledge and the opportunity of discussion for them.

Just as you and I gain understanding from asking questions, professionals gain insight from explaination. The only professionals I regard with "awe" would be surgeons and aircraft controllers.

Edit - Because " my life" - depends on both. The rest are, perhaps, just doing their jobs, like most "working stiffs." Just like most of us.

[RH1300S]

Cd is a way to compare relative efficiency of SHAPES of whatever size (is the "r" in the equation the reynolds number anyone? - although I thought it was "Re"). I believe the Reynolds number is a dimensionless coefficient that lets shapes of varying size be compared accurately - for example scale models in a wind tunnel, you would need to use the Reynolds number to assess the results correctly (you don't just scale wind speed)

CdA gives total drag - so the same shape (same Cd) with less frontal area will have less overall drag.

Something else some (not all as some are very wide) coupes often have to their advantage - the Cd may not seem to compare well, but the CdA could be very good due to a small frontal area.

[sebbe]

"r" means density of the fluid. It should be the Greek word "rho" or something like that, although sometimes "delta" is used as well for density.

You can't use Reynolds (Re) in an equation, those dimensionless numbers are meant to characterize the flux of a fluid around an object, thus allowing you to compare two different situations. That's why you can't use Reynolds, Euler, Froude numbers in equations.
The most common situation is obtaining Re, with the aid of another magic number (or with a property of the system) you enter a chart and get another property, velocity being the most common information missing in an equation.

Hope it helps.

Thank you guys!, now I understand.

[Apex]

Just a quick opinion, F1 cars are aerodynamically efficient!

When you consider the fact that the designers are concerned with the ratio of downforce / drag. Also given the regulations the designers are constrained very tightly!

Id love to see a road car generating 700+ kg's downforce at 150km/h - not going to happen!

[Tom]

Watch the Le Mans cars, specailly the group C cars, they would generate so much downforce they would rip up the manhole covers round Montreal, even though they were weilded down.

[AeroGT3]

Whoever said the water droplet makes the most efficient shape is not totally correct.

For the thickness to chord ratio water makes, maybe. But the lowest drag shape is a flat plate. The most efficient shape can't just be that of the flow around water. A fluid of different viscosity (how about molasses) will form a different shape when falling through the air, etc. How do you know which fluid (water, oil, molasses) will produce the most efficient shape?

[Tom]

Surely the fluid with the lowest surface tension would form the most aerodynamic shape? Water is quite high, compared to things like petrol and oils, but I think even molasses will form a teardrop shap, although likely it'll be thinner than water. I don't think a flat plane is as aerodynamic as a teardrop but I'm no aerodynamicist (I am, however, a little tipsy) so I may be wrong.

[volarchico]

A bit of a side-note... it is a common myth that a falling raindrop is tear-drop shaped. In actuality, when a rain-drop falls, it is more of an ellipsoid (squashed sphere) due to surface tension.