Why do 250 mph (400 km/h) supercars have 0.40+ Cds?

[chillsworld]

OK, here's the plan...

One of you guys needs to buy one of the aforementioned "hypercars" and drop in a honda civic engine to test this heat exchange/CD relation

Then after we determine that, we can go about covering intake/vent holes to see how they impact the CD.

And we can do A-B-A testing with removing all downforce generating aero mods to see if it makes the CD better by getting rid of the drag associated with parts that create downforce.

Problem solved, and we all become a whole lot smarter


~C

[NHB]

Originally Posted by chillsworld

Knowing the lift generated with the AMG aero package does us no good without knowing what the amount of lift measured *before* the aero package was added was. If lift was higher before the aero package, then the package created negative lift (downforce). Right? That's why I wanted to read the article to see if they listed the lift prior to the addition of the aero package.

A-Klasse was tested only with aero package. They test cars in the same condition as they are delivered.

[serialk11r]

Originally Posted by Vman455

For the extreme eample, cf: XL1, which has as much greenhouse taper as the McLaren or Ferrari but a lower body taper to match which the latter cars lack. The trade-off is, it also has significantly narrower rear track, which would make a mid-engined RWD supercar so tail happy it would be nearly uncontrollable.

You're missing my point. Many MR supercars have carefully designed side ducting that vents straight into the wake, which reduces drag. Some of them also have tricked out C pillars as well (Ferrari 599 not MR but a good example, McLaren 12C). It's almost like if you took the VW XL1 and moved its wheels way out with streamlined side pods designed to minimize interference drag. If they didn't have this the greenhouse taper would be useless because you'd get air from the sides spilling over.

The problem is that this ducting usually has a radiator, ram air inlet, or some other cooling apparatus in its way. Remove these, the fat tires, and downforce adders, and I bet a supercar would have lower drag than at least a Civic or something of that sort.

[Xist]

Chillsworld,

I do not suppose that putting a Corolla engine into a Lotus Elise would count?

An Echo engine?

[serialk11r]

Originally Posted by Xist

Chillsworld,

I do not suppose that putting a Corolla engine into a Lotus Elise would count?

An Echo engine?

The Elise already has a Corolla engine though! lol.

The 2ZZ has the same dimensions as the 1ZZ and doesn't really need additional cooling since it has only marginally more surface area in the combustion chamber.

The Elise also like I said is not a good example. Does not have much downforce to show for its high drag, and its tires are not even that big. You wouldn't be able to test my side scoop theory out since the side scoops on that car are probably not optimized and they vent to the top of the engine lid, not out the rear bumper.

However the MR2 Spyder (which I have) is an example of an MR car with no rear plan taper. It's got something like a 0.35 Cd, but it's also a convertible that has so many things wrong about the aero I don't even want to get started. A Spyder with a cleaned up radiator vent and underbody plus a full fastback and some stuff moved around in the engine bay to promote flow from the side scoops into the wake would give you an idea of how low the drag can go on a short tailed MR car.

It would probably be easier to just find a hypercar with only front mounted radiators, block those off, and throw on narrower wheels and tires, if you want to test it. It's not like a hypercar would overheat rolling down the freeway barely above idle.

[OldSubi]

Is CD the difference between the drag of a flat plate and the actual vehicle or Wikipedia just told me "the projected frontal area of the vehicle" so maybe its got to do with lower frontal area and its harder to improve a small low area compared to a bigger area?
I think a kilograms of drag at a set speed would be a more meaningful number.

[freebeard]

Cd is a unitless number based on a square plate of unit area. A is the frontal area. CdA is the product of the two.

[chillsworld]

Originally Posted by NHB

A-Klasse was tested only with aero package. They test cars in the same condition as they are delivered.

Then the numbers don't "mean" anything... They are arbitrary with out lift numbers for the vehicle without the package.


~C

[serialk11r]

Originally Posted by OldSubi

Is CD the difference between the drag of a flat plate and the actual vehicle or Wikipedia just told me "the projected frontal area of the vehicle" so maybe its got to do with lower frontal area and its harder to improve a small low area compared to a bigger area?
I think a kilograms of drag at a set speed would be a more meaningful number.

A flat plate has a Cd higher than 1.

A Cd of exactly 1 is like if you had a plate with the exact frontal area of whatever object and you move it along a frictionless, sealed tube that is looped around and is the length of the distance you want to travel. Aka it corresponds to the energy input you'd need to accelerate exactly the amount of air you displace.

[aerohead]

Originally Posted by chillsworld

Then the numbers don't "mean" anything... They are arbitrary with out lift numbers for the vehicle without the package.


~C

The drag coefficient published for a particular vehicle will reflect any induced drag associated with the design.Like a Bugatti Veyron,with I think,three different Cds as a function of its rear wing orientation.
In a perfect world,we'd get front and rear lift coefficients,along with the Cd of a specific vehicle.Manufacturers may not want the average guy (and competitors) to have access to lift data.
When Darko measured my truck they provided me with front and rear lift forces at 130-mph.Based upon the 29-sq-ft frontal area and 130-mph velocity,it will be an easy matter to deduce the front/rear lift coefficients.
An LT1 Corvette at Bonneville,that was measured at Darko few days before me indicated front lift and at 220-mph got sideways,went vertical,did a reverse endo,then crashed onto its nose before getting its parachute out.The driver walked away but was visibly shaken.The SCTA and USFRA may want to discuss air fences further forward of the cowl.
If you want lift data on some cars you may have to pay for them.