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I was reading some information about aerodynamics in general and stumbled over this (at the end): http://www.autospeed.com.au/cms/A_107773/article.html
NSX aerodynamics
- Thread starter goldNSX
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shawn110975
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Wow nice find. great read.
but they def saved the best for last.
The NSX was the winner AGAIN....
but they def saved the best for last.
The NSX was the winner AGAIN....
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More articles of the writer, easily understandable: http://www.autospeed.com.au/cms/A_107773/article.html
Enjoy!
Enjoy!
Great read. It's interesting that the NSX is one of the best Aero designs.
At the time of the article, perhaps 0.32 drag coefficient was good, but technology (to a small extent), and manufacturers willingness (to a larger extent), has enabled a lot of commercial cars to get a lot lower. For example, the 3rd gen Prius, the Volt, etc are within 0.25 Cd.
Of course, as the article said, it is not just drag coefficient. It is the product of drag coefficient AND frontal area that determines wind resistance. Being a sports car, the NSX has a pretty small frontal area.
I've always liked the Autospeed articles. They are written well so most folks can understand them, and written by people with actual engineering knowledge - not just some "tuner" repeating what they've heard from other "tuners."
Autospeed also posted another aero article where they put small tufts of yarn over an RX7
FYI, here's a few more snapshots of the NSX in flowstream testing. You can get an idea where the flow separates, the boundary layer thicknesses, and where the flow reattaches. It's obvious the NSX-R wing is more effective after seeing these photos from the regular one.
Dave
Love the article! Great pictures as well.
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Cd=0.32 may was not superior for today's standard but is was far more superior at the time the car showed up in the showroom. Compare it to a Porsche 911, even one of today. The aerodynamic shape of a 911 is completely wrong IMO. Back in the day they produced a high amount of lift, fitting big proletarian rear spoilers to get wrid even part of it. They may thought of flying is even faster than driving. 
When Honda sold only few NSXs they went to the air-tunnel again and brought a facelift model (02+) and they were even refining it for the NSX-R NA2, producing downforce without big ass-wings.
There are plenty of (understandably written) articles on aerodynamics in this source. The most impressive ones are those who work on the underbody and test it on the road.
When Honda sold only few NSXs they went to the air-tunnel again and brought a facelift model (02+) and they were even refining it for the NSX-R NA2, producing downforce without big ass-wings.
There are plenty of (understandably written) articles on aerodynamics in this source. The most impressive ones are those who work on the underbody and test it on the road.
2000 Siena minivan is .32Cd
FD is .29 - .31 depending on the spoiler and lip option.
Holden Monaro is a .31Cd
FD is .29 - .31 depending on the spoiler and lip option.
Holden Monaro is a .31Cd
The 1991-2001 NSX have a Cd value of .32.
The 2002+ models have a Cd value of 0.30
If I remember correctly, the frontal area of a (1991-2001) is just 1.78 m2.
That is a LOT less than most other cars.
Besides that, I think a lot of people are missing some points.
The old model NSX produces almost NO lift, even at high speeds.
The NSX-R actually produces downforce at high speeds if I remember reading it correctly.
You cannot get downforce without loosing some aerodynamic efficiency AFAIK. Putting wing on a car to creat downforce will automatically increase wind resistance and lead to a higher Cd-number.
A Prius is just developed to have a low Cd-value because it is designed to be fuel efficient. You are not going to do any 120-140 mph sprints in a Prius.
I do wonder though what the Cd of a slighly modded (lowered) NSX might be with wheels that fit more flush with the body.
The 2002+ models have a Cd value of 0.30
If I remember correctly, the frontal area of a (1991-2001) is just 1.78 m2.
That is a LOT less than most other cars.
Besides that, I think a lot of people are missing some points.
The old model NSX produces almost NO lift, even at high speeds.
The NSX-R actually produces downforce at high speeds if I remember reading it correctly.
You cannot get downforce without loosing some aerodynamic efficiency AFAIK. Putting wing on a car to creat downforce will automatically increase wind resistance and lead to a higher Cd-number.
A Prius is just developed to have a low Cd-value because it is designed to be fuel efficient. You are not going to do any 120-140 mph sprints in a Prius.
I do wonder though what the Cd of a slighly modded (lowered) NSX might be with wheels that fit more flush with the body.
Autospeed has some excellent articles but the aerodynamic specifications of a 1988 Porsche 911 Carrera 4 aren’t shabby, either:
Porsche 964
- Cd: 0.32
- frontal area: 1.79 m2
- completely flat underbody with undertrays from front to rear
Edit: and I believe just about no lift front or rear with its retractable rear spoiler.
Porsche 964
- Cd: 0.32
- frontal area: 1.79 m2
- completely flat underbody with undertrays from front to rear
Edit: and I believe just about no lift front or rear with its retractable rear spoiler.
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Good point MvM that I neglected to mention in my post - a discussion of drag forces is irrelevant unless you know the frontal area, the drag coefficient, and what the vehicle is intended for (i.e. lift coefficients, etc).
P.S. My 3rd gen Prius is pretty stable at 110MPH :wink:
Dave
P.S. My 3rd gen Prius is pretty stable at 110MPH :wink:
Dave
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A german car magazine had measured some values far back. Anyone with the numbers?
Yes and no. If you use simple wings at the front or the rear then yes, downforce will increase drag. But if you work on the underbody and speed up the air with a diffusor (well more F360 like than the infinitesimal one on the NSX) you gain downforce for free. A lot of the NSX-R NA2 improvement was done on the underbody, esp. the front part, esp. compared to the 91 model. It helped the air under the car to the rear part of the car. It's said that it's more important to control the air flow in the front half of the underbody than in the rear one. Have a look at this article: http://autospeed.com/cms/A_110872/article.html where a guy tried to reduce the drag by convering the rear underbody. The front of this car is a brilliant work of aerodynamics but the rear 'looks' unfinished. The rear of the NSX underbody also looks 'unfinished' or like a 'compromise' due to therminal requirements (enigne overheating?).
Flushing the wheels with the fender is not so critical and could be a drawback, sep. if the wheel itself allows for a lot of turbulences within the fender which results in increased drag. Have a look at the wheels in the article, they're Pizza-like. This is aerodynamically optimal. From my memory I have in mind that with larger rims it took longer to reach 150 mph+.
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If I remember correctly from school...the Prius has one of the best aerodynamic shapes. Full wedge profile and a sliced off rear end.
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There was aerodynamic catastrophy design fault in the past. I don't know if you heard of it in the US, it was an Audi TT. The car was designed like the blue one, without a rear spoiler on the back. The turbulences around the soft rear produced so much lift at the rear that some of them crashed on the German Autobahn at high speed. Audi had to modify them during a recall and mounted a rear spoiler (silver car) to sharpen the edge over which the air had to flow.
I also like this article, never heard of Vortex before: http://www.autospeed.com/A_3059/cms/article.html
I also like this article, never heard of Vortex before: http://www.autospeed.com/A_3059/cms/article.html
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Yep. For the August 1997 issue of Sport Auto, they took a pre-facelift NSX to Mercedes-Benz's non-moving floor wind tunnel in Germany and measured a lift of 8 kg at 200 km/h over the front axle and 14 kg over the rear axle. Given a frontal area of 1.78 m2, the measured drag coefficient was 0.33.
For the August 2002 issue, Sport Auto took a 2002 NSX-R to the same wind tunnel. They measured 33 kg of downforce over the front axle at 200 km/h, 5 kg of downforce over the rear axle, and a drag coefficient of 0.34.
The figures measured for both cars vary a bit from what Honda claimed but that may have to do with the fixed floor of that Mercedes wind tunnel.
Creating flow seperation, ie spoiling the attached flow, wing effect and the high speed lift at the rear.
A couple great references:
Aerodynamics of Road Vehicles, Ahmed, Syed R., Et Al. - The bible
Race Car Aerodynamics, Katz, Joseph - The guru
Yep, non rolling road floor tunnels do not model true open road conditions. The results given would create a very unstable platform at speed with what a quick calc indicates as a negative static margin. Center of pressure in front of Center of gravity, not good for recovery of any slewing or high speed driving in a cross wind
.
A couple great references:
Aerodynamics of Road Vehicles, Ahmed, Syed R., Et Al. - The bible
Race Car Aerodynamics, Katz, Joseph - The guru
Yep, non rolling road floor tunnels do not model true open road conditions. The results given would create a very unstable platform at speed with what a quick calc indicates as a negative static margin. Center of pressure in front of Center of gravity, not good for recovery of any slewing or high speed driving in a cross wind
.To add to this for those that don't know what year the 964 appeared - 1989 1/2! So I'm not sure what Gold is talking about when he's down playing Porsche aerodynamics. So they were right at the forefront with the 964 and there wasn't a hell of a lot of difference between it and the 84 - 89s either. I'd leave comparos with Porsche in a good light if I were you....just sayin...
Wow - if the 964 was introduced in the States in 1989 1/2, that was a year after it was introduced in Europe. But its smooth underbody, rounded bumpers, etc., did reduce the 911's Cd from 0.38 to 0.32.
I got the data about the 964 from the auto, motor und sport Testjahrbuch 1989, page 266. To quote it:
"Luftwiderstandsbeiwert cw 0,32,
Stirnfläche A 1,79m2,
Luftwiderstandsindex cw x A 0,57."
Since a four wheel drive 964 can drive 260 km/h with 250 ps, those aerodynamic specifications sound correct.
"Luftwiderstandsbeiwert cw 0,32,
Stirnfläche A 1,79m2,
Luftwiderstandsindex cw x A 0,57."
Since a four wheel drive 964 can drive 260 km/h with 250 ps, those aerodynamic specifications sound correct.
Wow, you really dug up those numbers quickly.
Have spent the last weeks cleaning out my living room, removing all old Honda NSX parts lying around and finally put them away. But the NSX literature corner is still a bit of a mess though
Going from definite lift to some serious downforce on the 2002 NSX-R is a good example of the work done by Honda on the NSX-R model.
It's also nice to see that the Cd value of the NSX-R was measured at 0.34 whereas the standard 2002 model was given as 0.30.
I think that might be a very good example of how the increase in downforce results in a HIGHER Cd-value.
Now consider this: Look at the picture of the airflow over the NSX-R and see how high that car is. And then remember how basic the OEM Honda NSX-R front undercover is.
Now lower this car about 1", then add an undertray more like Greenberet made for his own NSX and then I wonder what the result would be.
Am also wondering how much difference a airsplitter might make, although I usually don't like their appearance very much.
An inch lower without any other changes will not improve the Cd. An inch lower, splitter, side skirts and an improved rear diffuser design would make an improvement. Yet, if you carelessly added increased rear pressure with the rear diffuser design, you would end up with extra downforce and additional drag.
Reducing wake turbulance is about all that is left on the table. Other than mirrors there isn't any decrease in area available and without messing with the body lines, it would be hard to reduce body drag. The trick would be flooring in the engine bay, but making sure the low pressure area would vent enough engine bay airflow to keep things cool and not create extra turbulence would require some effort.
The book on the Mclaren F1 included some interesting design sketches that Gordon Murray worked through to deal with airflow through the front mounted radiators, as well as information on the electric fans they used to deal with engine bay heat. I wish there was this type of design iteration information available on the NSX as they optimized the test forms.
Macattack, I'd have to disagree with you. The NSX-R flow is clearly diverging at the rear of the car and the streamline widens post wing. The NSX shows the airflow converging at the back of the car and the streamline has much more continuity after passing the wing. This to me explains where the NSX-R gains Cd compared to the NSX; more downforce too.
Reducing wake turbulance is about all that is left on the table. Other than mirrors there isn't any decrease in area available and without messing with the body lines, it would be hard to reduce body drag. The trick would be flooring in the engine bay, but making sure the low pressure area would vent enough engine bay airflow to keep things cool and not create extra turbulence would require some effort.
The book on the Mclaren F1 included some interesting design sketches that Gordon Murray worked through to deal with airflow through the front mounted radiators, as well as information on the electric fans they used to deal with engine bay heat. I wish there was this type of design iteration information available on the NSX as they optimized the test forms.
Macattack, I'd have to disagree with you. The NSX-R flow is clearly diverging at the rear of the car and the streamline widens post wing. The NSX shows the airflow converging at the back of the car and the streamline has much more continuity after passing the wing. This to me explains where the NSX-R gains Cd compared to the NSX; more downforce too.
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This picture looks fabricated (as in not a real picture of a smoke trail or CFD) and thus irrelevant. I think i've seen that picture before without the blue lines around the car.
As others have said, it's not just Cd that matters but Cd and frontal area which dictates the relevant drag # for the car. Most tests don't show the CdTOTAL (total drag) or "airdragindex" (cd x A)
If you look at this test (not all data is always accurate, especially those put out by a biased manufacturer, but its still interesting):
http://www.suzukaracing.com/GTR page/racetoroadgtr.pdf
Frontal area:
NSX-R - 1.78 sq.m (Announced) X 0.32cd = 0.57
Carrera GT - 1.9 sq.m (Calculated) X 0.39cd = 0.74
Porsche GT2 - 1.9 sq. m (Estimated) X 0.32cd = 0.61
360 Modena - 1.906 sq. m (Calculated) X 0.34cd = 0.65
Nissan GTR - 2.09 sq.m (Estimated) = X 0.27cd = 0.56
***Keep in mind all of these frontal areas (except for the NSX-R) are ESTIMATED. I'm not too sure the difference between the frontal area of a GTR and a 360 Modena is less than the difference between an NSX-R and the 360 Modena. I'm also a bit skeptical about the GTR's cd numbers, but a lot has gone into the body of the car but if its accurate, it's quite impressive.
Anyways, the NSX-R nets a decent amount of downforce (especially up front -moreso than a GT2). As with most articles like this, it should be taken with a grain of salt because there's a lot more to the whole picture than what is provided, but it's still interesting.
Billy
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I’d expect that a car’s Cd would improve by simply lowering it an inch. According to the Bosch Automotive Handbook, lowering a standard car by 30mm without any other changes should decrease the Cd by about 5%. The actual results will depend on the individual model of car being tested, but Mercedes, Volkswagen, etc. do lower all of their “efficiency” models to decrease wind resistance.
The bottom of a car will probably always have more wind resistance than the top simply because of the spinning wheels jutting down into the airflow. When you lower the car, some of the air that would have flowed past the rough underbody will flow over smoother top instead.
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