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

[chillsworld]

Originally Posted by serialk11r

Also a little more on the original topic, the McLaren P1 has a 0.34? Cd, and that's with fat tires + cooling ducts everywhere, and the wing has that bend in it that can't be good for drag when in the down position. It's got much better downforce and stability at speed than a lot of the other supercars, so maybe this isn't so much a design goal problem as it is a problem with poor design.

The CD is different depending on whether it is in race or road settings... The ride height drops 50mm, the wing adjusts, and active aero in the front of the car adjusts as well. But I've never seen any data on how it imapcts CD... They say that the rear wing can reduce drag by 23 percent, but is that 23 percent bringing it lower than .34? Or is that 23 percent bringing drag back closer to .34 when in race mode?

I've also read, that they created active aerodynamic features to alleviate strain on suspension componets from the extreme downforce the car is capable of. Now THAT is crazy to me I think it makes like 1300 lbs of down force (the most of any production vehicle), so having that in mind... I think a .34 CD is simply amazing. And its probably due to the compact and super light body with inlet and vents wind tunnel tested/designed for optimal aerodynamic performance, especially in regards to interaction with the rear wing. Good read right here if you're interested: McLAREN P1 : McLaren Automotive Media Centre



~C

[NHB]

Originally Posted by chillsworld

I can't find an english version of that article, or a site that will translate correctly

But I found this about the A45 AMG, so I find the generation of lift you claim a little bit suspect:
"The car only comes with three options - black wheels ($490), stiffer suspension ($1990) or the AMG aero package which adds front winglets, front wing and a fully sick rear wing which sets it further apart from the A250 Sport but also adds 40kg extra downforce at 250km/h"
(2013 Mercedes Benz AMG A45 | Top Gear)

For what you need translation? It's quite simple data. I'm sure, that google translator can translate few simple phrases.

Top Gear is just car related entertainment. Their extra downforce can easily mean lift reduction. In my opinion everything here is just as it should be. 400 N @ 250 km/h means about 200 N @ 200 km/h. So the lift at rear end would be about 350 N @ 200 km/h without the spoiler. That sounds totally believable. SportAuto measured 380 N lift for Renault Megane RS.

Lets say the tires account for .025 and the KW increase accounts for .034, that's .059. What causes the other .049 increase in CD Even if the two combined account for .06 of the .1 increase on the CLA, where does the other .04 come from? It must be aerodynamics of some type right?

Just take a look at those pictures you posted. What do you see at the front end of CLA? There are huge holes for brake cooling and probably for charge air too.

[NHB]

Originally Posted by serialk11r

I'm skeptical about cooling causing so much drag. Twice the power does not mean twice the cooling drag necessarily. A bigger radiator would give you a lot more cooling capacity for the same air flow. You would think that with a bigger engine and a bigger turbo that they would add a bigger radiator as well.

These modern cars don't have that much free space anywhere. At the front end there are already multiple radiators. And it's not only the engine cooling. 45 AMG engine uses very high boost pressure, which needs big radiators for water cooling. A high power four wheel drive automatic gearbox most likely needs healthy size radiator, but low power manual transmission doesn't need any extra cooling. We also have brakes, which have totally different specs in speed oriented AMG model compared to eco racers.

I still say, the 0.22 is BS, they probably measured it without a "treadmill" which reduces drag from the wheels and underbody.

Mercedes Benz CLA 2013 Wind Tunnel Aerodynamic Test Part 2 - YouTube

[chillsworld]

Originally Posted by NHB

For what you need translation? It's quite simple data. I'm sure, that google translator can translate few simple phrases.

It could if I could find the actual article...

Top Gear is just car related entertainment. Their extra downforce can easily mean lift reduction. In my opinion everything here is just as it should be. 400 N @ 250 km/h means about 200 N @ 200 km/h. So the lift at rear end would be about 350 N @ 200 km/h without the spoiler. That sounds totally believable. SportAuto measured 380 N lift for Renault Megane RS.

Would the reduction of lift not come from the creation of negative lift? Is negative lift not "downforce"?

"Typically the term "lift" is used when talking about any kind of aerodynamically induced force acting on a surface. This is then given an indicator, either "positive lift" (up) or "negative lift" (down) as to its direction. In aerodynamics of ground racing (cars, bikes, etc.) the term "lift" is generally avoided as its meaning is almost always implied as positive, i.e., lifting the vehicle off the track. The term "downforce", therefore, should always be implied as negative force, i.e., pushing the vehicle to the road."
(Downforce)

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.

Just take a look at those pictures you posted. What do you see at the front end of CLA? There are huge holes for brake cooling and probably for charge air too.

You're right! The front end most likely creates more drag... I have never said that it doesn't.

[serialk11r]

Originally Posted by NHB

These modern cars don't have that much free space anywhere. At the front end there are already multiple radiators. And it's not only the engine cooling. 45 AMG engine uses very high boost pressure, which needs big radiators for water cooling. A high power four wheel drive automatic gearbox most likely needs healthy size radiator, but low power manual transmission doesn't need any extra cooling. We also have brakes, which have totally different specs in speed oriented AMG model compared to eco racers.

I don't mean increase the cross sectional area, you can increase the thickness of the radiator, that's how it's usually done.

[Vman455]

Originally Posted by Big time

They don't make THAT much downforce. At least not compared to all out racecars.
Are high Cd figures from cooling drag?

But compared to mainstream sports cars, let alone family cars, they do make a lot of downforce. A 2008-2010 Gen 4 Viper, for example, made a claimed 100 lbs downforce at 150mph, while the same car with ACR package (splitter, canards, and rear wing) made a claimed 1000 lbs at the same speed. It also lost 18mph on the top end, 202 mph for the regular coupe vs. 184 mph for the ACR.

The Veyron makes a claimed 733 lbs downforce in handling mode (up to 233 mph). The Venom GT makes a claimed 1050 lbs at 150 mph.

Originally Posted by aerohead

At these velocities,directional stability and safety are paramount to the manufacturers.They want their customers to live to buy another car.
They may do detail optimization to trim drag as long as it does not impact stability/safety.
Plan-views of these cars reveal drag-unfriendly profiles.

I think this gets at the real issue behind the higher drag numbers for exotic cars. Take a look at the plan views of various low-drag cars and their hypercar counterparts (especially the back halves):

Tesla Model S - Cd .24


Gen I Honda Insight - Cd .25


Gen III Toyota Prius - Cd .25


Chevrolet Volt - Cd .28


2014 Hyundai Sonata - Cd .25


VW XL1 - Cd .19


Bugatti Veyron - Cd .41/.36


Hennessey Venom GT - Cd .43


Koenigsegg Agera - Cd .37/.33


Ferrari Laferrari - Cd .31


McLaren P1 - Cd .34

[ERTW]

It's not so much that they need that much downforce at 250 mph. They want usable downforce at 100 mph and they happen to have enough hp to go 250. The faster you go, the lower the Lift coefficient you need. ie. 100 lb will keep the car on the ground regardless of speed.

The Colani Corvette has a Cd of 0.19, and went 254 mph with 770 bhp. The whole car is an inverted wing.

In many cases, hennessey in particular, they're not designers or aerodynamicists. They slap together a bunch of available parts. He copies existing designs and slaps on twin turbos and a wing.

[serialk11r]

Originally Posted by Vman455

I think this gets at the real issue behind the higher drag numbers for exotic cars. Take a look at the plan views of various low-drag cars and their hypercar counterparts (especially the back halves):

I don't think that explains it, at least not all of it. Almost all of the supercars have MORE plan taper at the greenhouse than the low-drag cars you mentioned. They also have side ducting that vents into the wake, which gets rid of some of the drag that comes from the wider rear.

The Veyron's rear has no taper to speak of, so that's probably bad. As someone else mentioned, the Hennessey Venom is just some Elise parts made a little bigger and slapped together, no surprise there (speaking of Lotus, the Exige comes from the factory with a ton of drag and not much downforce too). I also doubt that Koenigsegg does much careful aero optimization.

The areas I see on supercars that more drag comes from are:
1. Inlet scoops (fairly common, but are mostly pretty small and I don't imagine they add too much drag)
2. More radiators, other cooling ducts
3. Fat tires
4. Wings, canards, and other aero add-ons
5. Subtle things like the diffuser being angled higher to provide more downforce and more drag.

Without these things, I think the basic supercar shape (nearly perfectly streamlined greenhouse + nose) probably actually does very well, and the MP4-12C setup of having side scoops feed radiators that vent to the wake is probably a lot better than having radiators in the front.

[E.Roy]

All the aformentioned supercars have the engine (weight) in the back, and need large rear tires to balance the handling, not to mention to get traction at their high hp. That requires the "wrong" aero shape, but perfect aero takes a back seat to acceleration, braking and handling optimization. These cars aren't only about top speed, and they sure aren't about fuel economy. It is all a compromise for max performance. The 400mph land speed cars only need to do one thing, max top speed. They don't handle, don't brake, don't launch from a standing start.

[Vman455]

Originally Posted by serialk11r

I don't think that explains it, at least not all of it. Almost all of the supercars have MORE plan taper at the greenhouse than the low-drag cars you mentioned. They also have side ducting that vents into the wake, which gets rid of some of the drag that comes from the wider rear.

The Veyron's rear has no taper to speak of, so that's probably bad. As someone else mentioned, the Hennessey Venom is just some Elise parts made a little bigger and slapped together, no surprise there (speaking of Lotus, the Exige comes from the factory with a ton of drag and not much downforce too). I also doubt that Koenigsegg does much careful aero optimization.

The areas I see on supercars that more drag comes from are:
1. Inlet scoops (fairly common, but are mostly pretty small and I don't imagine they add too much drag)
2. More radiators, other cooling ducts
3. Fat tires
4. Wings, canards, and other aero add-ons
5. Subtle things like the diffuser being angled higher to provide more downforce and more drag.

Without these things, I think the basic supercar shape (nearly perfectly streamlined greenhouse + nose) probably actually does very well, and the MP4-12C setup of having side scoops feed radiators that vent to the wake is probably a lot better than having radiators in the front.

Sure, the lack of plan taper doesn't explain all of it, but I would bet it's a significant factor. A perfect greenhouse taper won't do much for the car if the majority of the body terminates in a large wake area, which all these hypercars do. The departing angles are radically different on the lower-drag cars than the higher-drag cars. 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.

I guess my argument is that the basic shape has to be right or the car will never achieve low drag. The CLA is a good example of this: with the extra cooling inlets, wider tires, etc. it has a much higher drag coefficient even though the basic shape hasn't changed, but without the good shape it would not be able to achieve Cd .22 even with all the other details tuned in, and the AMG version drag would likely be even higher than it is.

Also, the Venom GT isn't just a bigger Exige body--it saw at least CFD development (there are images from some of those runs on the web), and I would suspect a wind tunnel at some point. With the active rear wing, that would be a necessity to get it to work correctly.