NSX aerodynamics

greenberet said:
Recommendations

For an NSX, I personally think the aerodynamically best thing to do would be to give it standard 2002+ bodywork, lower the car as far as is acceptable for you, put wheel spoilers in front of all four wheels, and build a completely smooth underbody with a diffuser that is angled upwards 3-5° and starts as far forward as possible. The car would still be perfectly practical but drag and lift would both be reduced. If you need more downforce over the front or rear axle, you will need to start sacrificing straight line speed with a steeper rear diffuser, a bigger (perhaps 2002+ NSX-R) rear wing, a front splitter, etc. All those things are likely to increase drag and make your car slower in a straight line but they may make it faster in turns.
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I agree.

Unless you understand aero theory and know the implications making haphazard aero mods can do, leave it alone.

I also don't condone making single drastic changes either to your vehicles' aerodynamics. It is a balancing act. The car is balanced very well right now for a drag/handling tradeoff.

For example, I removed my A/C condensors up front. As a result, I can feel some front end lift at higher speeds. Until I can compensate for this (brake duct intakes up there and auxilary driving lights), I'm hesitant to even get this up to 115MPH on the dragstrip.

Dave
 
Keep in mind there are other tricks that can be used to keep flow attached to diffusors.

The diffusor I'm designing will have a center section about 20" wide that will be at an 18degree angle. The sides will be around 13degrees.

To see if the flow remains attached or not, I'll be using my cheap $10 spy video camera taped underneath the car, with the diffusor covered in tufts of yarn. If it doesn't, my plan is to add vortex generators at the diffusor leading edge.

Also, my center section is steeper to take advantage of my exhaust-activated downforce design. The outlet is a center exit design, with four small 1.5" diameter exhaust tips at a 10degree angle that will poke through the diffusor.

Now that my exhaust is almost finished (still refining the "quiet mode" sound), I can get to work on my diffusor. I need to get this 4' x 8' sheet of aluminum out of my garage :wink:

Finally, as a diffusor is kinda useless without a flat area just upstream of it to apply the reduced pressure to (the NSX engine/tranny compartment is just too open to be of any good), I will enclose that area. That will require some temperature-instrumented tests to ensure I won't overheat anything. More drastic mods (cutting out the trunk and venting above the tailights, etc) will be required later for, hopefully, my big power plans.

Dave


I don't have any good pics of the exhaust or the angle, but here is a video that you can get an idea of the configuration:

<iframe src="http://player.vimeo.com/video/18223839" width="400" height="300" frameborder="0"></iframe><p><a href="http://vimeo.com/18223839">New Loud Mode NSX Muffler</a> from <a href="http://vimeo.com/user1744194">Dave</a> on <a href="http://vimeo.com">Vimeo</a>.</p>
 
Since we are all here talking about NSX aerodynamics...

Which front airdam would (probably) be the best available for the 1991-2001 NSX model when it comes to minimizing drag and/or improving downforce.

And the same question again if you had done the trouble of fitting a NSX-R front hood with an airduct behind the radiator and covered up the front underside of the car with the front bumper undercover and battery-tray.
 
Mac Attack – I look forward to following your progress!

MvM - The OEM 1997-2001 front spoiler is probably the best you can get to minimize drag for a pop-up headlight NSX. If you want to maximize front downforce at the cost of higher drag, get a deeper spoiler.

According to textbooks, if your car does not have a completely smooth underbody, you’ll minimize drag if the front spoiler is almost the lowest point of the body around the car’s middle and is as deep as you can live with in front of the wheels. Those are exactly the principles Honda followed when they redesigned the front spoiler in 1997. Even with a 2002+ NSX-R front undertray, an NSX still has a very rough underbody behind the passenger compartment so even then, the OEM 1997-2001 front spoiler is probably the best you can choose to minimize drag.

If Mac Attack builds a completely smooth underbody from bumper to bumper, it may be best for him to cut out the center section of the OEM 1997-2001 front spoiler lip. Not only might that decrease drag a bit but the additional mass flow along the car’s centerline should increase the airspeed beneath the car and may net him additional downforce.

That’s just textbook advice, though. Only computer simulations and physical testing would let you optimize all the details and let you know whether it actually holds in the case of an NSX.
 
greenberet said:
If Mac Attack builds a completely smooth underbody from bumper to bumper, it may be best for him to cut out the center section of the OEM 1997-2001 front spoiler lip. Not only might that decrease drag a bit but the additional mass flow along the car’s centerline should increase the airspeed beneath the car and may net him additional downforce.
Click to expand...



I believe this lip does that. But unless it is actually tested in wind tunnel, it is all speculation (theoretical or otherwise) and aesthetics - which is also important.
 

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greenberet said:
If Mac Attack builds a completely smooth underbody from bumper to bumper, it may be best for him to cut out the center section of the OEM 1997-2001 front spoiler lip. Not only might that decrease drag a bit but the additional mass flow along the car’s centerline should increase the airspeed beneath the car and may net him additional downforce.

That’s just textbook advice, though. Only computer simulations and physical testing would let you optimize all the details and let you know whether it actually holds in the case of an NSX.
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Good point on cutting out my front spoiler lip even more after doing the smooth underbody - Thanks!

I'm working on the back first, since that's the safest area place to start the aero mods. Before I would close off the front, I would add a vented and ducted hood, but I'm in the process of rethinking the front too to save some weight. Hope Danny doesn't mind what I did to his front just to get an idea of how I would like mine to look:

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I tend to believe more of the physical testing. I used to do CFD for a living at a Defense contractor. We had the best of the best - a team of dedicated PhD's writing our own Navier Stokes solvers, a slew of parallel-processing machines around the country to turn the crank, and testing facilities to validate selected results.

That being said, I know that garbage in = garbage out. Unless you're part of a world-class race team (F1) that has a lot of experience in CFD, can distinguish important inputs to consider, and can back it up with testing/experience, then I tend to just stick with the testing.

Know what CFD stands for? Colors For Dollars :wink:

That's why I like the tuft idea with a small, cheap, HD spy camera from China to record as you're driving. Then, quantifying downforce with LVDT's on your shocks, etc.

I also found greenberet's rear diffusor pictures interesting after it had gotten a little dirty - stuff like that gives me more trusted info than CFD does. My $0.02 of course.

Dave
 
I think MacAttack's approach is a good one. I went to an automotive aerodynamics seminar before where the presenter was telling us real world examples of in-field aero improvements he made to a drag car. After looking over the car, he actually taped rolled up magazines/pamphlets into some grooves and hollows around where the windshield was, and that resulted in measurable gains in their 1/4 mile times.

I think his point was that you don't need a wind tunnel and CFD, and you can actually do pretty good work with simple techniques like tufts, applying colored oil to see where the wind takes it, and doing coast down measurements for drag.
 
Not directly applicable to road cars, but interesting to think about nonetheless:

Audi’s 2010 Le Mans-winning R15 raised up the front to let air get to the downforce-generating underbody. The gap between the bodywork and the road is still less than that of an NSX though.



Porsche’s 2010 911 GT3 RSR has big splitters in front of the wheels but not in front of the center section of the car.



Red Bull’s 2010 F1-winning RB6 has a beautifully smooth underbody and uses strakes to keep the airflow attached in the steep diffuser section.

 
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Mac Attack said:
Keep in mind there are other tricks that can be used to keep flow attached to diffusors. An underbody wing to turn the airstream, so the transition into the diffuser isn't so large?

The diffusor I'm designing will have a center section about 20" wide that will be at an 18degree angle. The sides will be around 13degrees.
You will need vertical strakes to seperate flow or the flow will slide downhill, so to speak, to the lowest pressure area. Since the 2 zones are going to be at different pressures due to the 2 different transition angles.

To see if the flow remains attached or not, I'll be using my cheap $10 spy video camera taped underneath the car, with the diffusor covered in tufts of yarn. If it doesn't, my plan is to add vortex generators at the diffusor leading edge.
If laminar flow is detaching before equalizing pressure, I don't think turbulent air will stay attached any farther up the diffuser.

Also, my center section is steeper to take advantage of my exhaust-activated downforce design. The outlet is a center exit design, with four small 1.5" diameter exhaust tips at a 10degree angle that will poke through the diffusor.
If the exits protude past the diffuser wall you will have problems. If the exhaust exits are 10 degrees and the wall is at 18 degrees and you have 1.5 inch exits with high shear velocities, I think this is going to shed attached flow not promote it.

Now that my exhaust is almost finished (still refining the "quiet mode" sound), I can get to work on my diffusor. I need to get this 4' x 8' sheet of aluminum out of my garage :wink:

Finally, as a diffusor is kinda useless without a flat area just upstream of it to apply the reduced pressure to (the NSX engine/tranny compartment is just too open to be of any good), I will enclose that area. That will require some temperature-instrumented tests to ensure I won't overheat anything. More drastic mods (cutting out the trunk and venting above the tailights, etc) will be required later for, hopefully, my big power plans.
How does the Honda NSX-R diffuser work without an upstream flat area?

The air flow directed through the engine bay has to flow somewhere and blending heated air into general flow in a low pressure zone can be tricky. Have fun with your experimentations. Oh, and what happens if you lift in a high speed, decreasing radius turn?
Dave
Click to expand...


MvM said:
Since we are all here talking about NSX aerodynamics...

Which front airdam would (probably) be the best available for the 1991-2001 NSX model when it comes to minimizing drag and/or improving downforce.

And the same question again if you had done the trouble of fitting a NSX-R front hood with an airduct behind the radiator and covered up the front underside of the car with the front bumper undercover and battery-tray.
Click to expand...

I hope you don't mind if I add a little to your question; it's not that I'm dodging what you are asking, it's that I'm not seeing all the questions that I am asking myself. Side edit: the below information suggests the largest gain is enclosing the undertray. Yet, if you do that, you are kinda commited to venting radiator flow out the hood and over the car.

The NSX-R (2002) compared to the US NSX (1997):

Increased front downforce 79.6 lbs @108 mph
Increased rear downforce 55 lbs ""
To reach a targeted 40/60 balance front to rear at 108mph.
Was Honda concerned about the static pressure margin, or just matching the weight distribution?
From re-reading the honda nsx-r lit on the website, they did indeed tune out some static margin to take the car from high speed understeering to high speed "mild" understeering. Matching the weight distribution makes the car handle with the same characteristics at high speed that it does at low speed.

Increased front spring rate 197%
Increased rear spring rate 123%
Increased the spring bias to 120% from 86% front/rear (100% being neutral)
Why such a jump in front spring rate and why the significant change in spring bias?
The increased aero allowed honda to get more aggresive with the mechanical grip. The increased front downforce reduce tire slip and they went after the additional available mechanical grip.

Lowered the front of the car x.x inches
Lowered the back of the car x.x inches
changed the front roll center x.x inches
changed the rear roll center x.x inches
changed the roll axis by x.x degrees
Ran out of time to look up the figures and muse.


See what I'm getting at about looking at the whole package and not each individual piece, one at a time? The larger gain made on the front was:
The underbody panel Clf -0.040 and they were sure to point out the longitudinal fins?
The hood Clf -0.025
The radiator shroud Clf -0.010
Reducing the condensor openings Clf -0.025, interesting, hmm
The rear diffuser Clf -0.002 and Cd -0.003
The rear spoiler Clf -0.085, where's the drag value for that one:smile:.
 
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Ty B said:
How does the Honda NSX-R diffuser work without an upstream flat area?
...
The rear diffuser Clf -0.002 ...
The rear spoiler Clf -0.085
Click to expand...

I think the statistics pretty much answer that. For all practical intents and purposes, it doesn't work. The OEM "diffuser" only adds 2% as much downforce as the wing. It's main purpose is probably to look good.
 
Ty B said:
Oh, and what happens if you lift in a high speed, decreasing radius turn?
Click to expand...

That is the problem with exhaust-activation. But, good drivers don't lift in turns :wink:

Thanks for your insights, but at the rate it's taken me for my exhaust design, I'll probably never get the car aero the way I want it!

Yeah, as greenberet pointed out, that 0.002 lift coefficient difference for their diffusor was probably less than their measurement error :wink:

Dave
 
stuntman said:
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
Click to expand...
Some interesting wind tunnel finds that don't quite back up the data put out by the above Nissan article:

http://nzhondas.com/forum/pub/112100-interesting-wind-tunnel-testing-couple-road-cars.html

http://www.revscene.net/forums/wind-tunnel-testings-t602667.html

(from: http://www.fastestlaps.com/forum/viewtopic.php?t=699&postdays=0&postorder=asc&start=0)


Nissan GTR for example:

Suzuka Racing.com:

(A) 2.09 sq.m (Estimated)
(cd) 0.27
(cd x A) 0.56
at 245km/h
downforce front 24kg
downforce rear 24kg

Sportauto test:

Nissan GT-R
(A) 2,30 m²
(cd) 0,31
(cd × A) 0,71
at 200km/h
lift front 3kg
downforce rear 8kg


-Just goes to show to take information with a grain of salt. FYI the Ferrari 360 Modena was had a MUCH greater discrepancy than the GTR. The NSX-R remains fairly consistent.
 
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Interesting Billy, but keep in mind that the wind tunnel data is from a fixed floor tunnel. I've got to look up the wind tunnel design book for the specific data, but a rolling road makes a substantial difference when testing road vehicles.

So, the data they provide is skewed by a substantial amount compared to open road reality.
 
greenberet said:
I think the statistics pretty much answer that. For all practical intents and purposes, it doesn't work. The OEM "diffuser" only adds 2% as much downforce as the wing. It's main purpose is probably to look good.
Click to expand...

I agree with you on the idea that the Honda OEM NSX-R diffuser probably is more for looks than for any practical purposes.

However, I think this is caused by the fact that the diffusers more or lets sits 'alone' on the underside of the rear bumper.
What if you would attach and additional flap or (flexible) panel between the diffuser and the rear suspension arms?
This to cover up at least part of the area between the engine and the rear bumper?

Also, does anyone like me always wondered why Honda did not make a fuel tank undercover?
Does anyone know if the 2002 side skirts make any difference in the Cd or Cl values of the car?
 
MvM said:
...why Honda did not make a fuel tank undercover?
Click to expand...
They could have in the NSX-R if they would have found any positive effects. So I think the didn't. The aerodynamics in the front half is more important than in the rear half of the underbody.
 
On this web page, Honda stated that the 2002 redesign of the NSX's side sills reduced turbulence along the side of the car. That had to reduce drag and if you have a properly shaped underbody, lower sills should increase downforce as well.

Since Honda already made such significant changes to the front end of the 2002+ NSX-R to create enough downforce to balance out the big rear wing, maybe they didn’t add rear underbody panels because they couldn’t practically add even more downforce to the front to balance them out. Maybe. But only someone in the development team would really know.

In this series of tests, the undertrays at the rear of the car reduced drag more than those at the front:



However, that doesn’t hold for all cars. When Audi released the B6 generation of the A4 in 2000, they published some data regarding the aerodynamic improvements to the underbody. The front underbody panels look like they were at least as important as the rear panels. But the panels just in front of the rear wheels, where an NSX has its gas tank, were particularly important on that car. Note: the predecessor B5 generation already had a partial front undertray and wheel spoilers, so those aren't listed in the "improvements" to the B6 below.

 
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Ty B said:
Interesting Billy, but keep in mind that the wind tunnel data is from a fixed floor tunnel. I've got to look up the wind tunnel design book for the specific data, but a rolling road makes a substantial difference when testing road vehicles.

So, the data they provide is skewed by a substantial amount compared to open road reality.
Click to expand...
I agree and that could explain the differences especially in cars with diffusers/tunnels. Although I don't think the GTR gains a lot of downforce from the underbody so i'm not sure about the gains from a rolling road/reality. This also dosn't explain the frontal area or possibly the cd discrepancy for the GTR. Windshear is just under a mile away from work and A2 is less than 5 from where I live, pretty cool stuff.
 
stuntman - I agree. The more a car uses underbody aero to generate downforce, the more a fixed-floor wind tunnel will screw up the front and rear lift figures. The floor’s gradually thickening boundary layer is not a good simulation of the real world since there is no boundary layer on the road. For NASCARs with their deep front air dams and no underbody aero, that probably doesn’t make much difference. For Le Mans prototypes with their highly developed underbody aero, a fixed-floor wind tunnel may deliver pretty much worthless front and rear lift figures. Road cars traditionally use little underbody aero but recent Ferraris are starting to, so comparing the lift figures of a 360 Modena to an NSX-R in a fixed-floor wind tunnel can be problematic.

But the drag figures should still be pretty accurate. Wind tunnels measure the drag of a car and if you divide that by its frontal area, you can calculate the drag coefficient. If your estimate of the frontal area is wrong, you’ll get the wrong drag coefficient. But the overall drag index (Cd x A) is what was actually measured so that is not impacted.

Using the same wind tunnel, Sport Auto measured the drag indices (Cd x A) of various cars. Here’s a selection:

2005 Porsche 911 Carrera S (997) : 0.54
2003 Porsche 911 Carrera (996) : 0.56
2006 Porsche 911 GT3 (997) : 0.58
2007 Porsche 911 Turbo (997) : 0.59
2005 Corvette C6 : 0.59
1997 Honda NSX: 0.59
2002 Honda NSX-R: 0.60
1999 BMW M5 V8 : 0.62
2002 Mercedes SL 55 AMG : 0.63
2007 Corvette C6 Z06 : 0.65
2009 Corvette C6 ZR1 : 0.66
2003 Lamborghini Gallardo : 0.66
2008 Lamborghini Gallardo LP 560-4 : 0.67
2006 Ford GT : 0.67
2010 Ferrari 458 Italia: 0.67
2004 BMW M5 V10 : 0.68
2006 Ferrari 430 : 0.70
2007 Audi R8 V8: 0.70
2009 Nissan GT-R : 0.71
2004 Porsche Carrera GT: 0.72
2009 Audi R8 V10 : 0.73
2004 Dodge Viper : 0.75
2009 Aston Martin DBS : 0.76
2010 Lexus LFA: 0.78
2004 Pagani Zonda S: 0.84

Recent Porsche 911s have less drag than an NSX, but a Nissan GT-R has a lot more.
 
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greenberet said:
stuntman - I agree. The more a car uses underbody aero to generate downforce, the more a fixed-floor wind tunnel will screw up the front and rear lift figures. The floor’s gradually thickening boundary layer is not a good simulation of the real world since there is no boundary layer on the road. For NASCARs with their deep front air dams and no underbody aero, that probably doesn’t make much difference. For Le Mans prototypes with their highly developed underbody aero, a fixed-floor wind tunnel may deliver pretty much worthless front and rear lift figures. Road cars traditionally use little underbody aero but recent Ferraris are starting to, so comparing the lift figures of a 360 Modena to an NSX-R in a fixed-floor wind tunnel can be problematic.

But the drag figures should still be pretty accurate. Wind tunnels measure the drag of a car and if you divide that by its frontal area, you can calculate the drag coefficient. If your estimate of the frontal area is wrong, you’ll get the wrong drag coefficient. But the overall drag index (Cd x A) is what was actually measured so that is not impacted.

Using the same wind tunnel, Sport Auto measured the drag indices (Cd x A) of various cars. Here’s a selection:

2005 Porsche 911 Carrera S (997) : 0.54
2003 Porsche 911 Carrera (996) : 0.56
2006 Porsche 911 GT3 (997) : 0.58
2007 Porsche 911 Turbo (997) : 0.59
2005 Corvette C6 : 0.59
1997 Honda NSX: 0.59
2002 Honda NSX-R: 0.60
1999 BMW M5 V8 : 0.62
2002 Mercedes SL 55 AMG : 0.63
2007 Corvette C6 Z06 : 0.65
2009 Corvette C6 ZR1 : 0.66
2003 Lamborghini Gallardo : 0.66
2008 Lamborghini Gallardo LP 560-4 : 0.67
2006 Ford GT : 0.67
2010 Ferrari 458 Italia: 0.67
2004 BMW M5 V10 : 0.68
2006 Ferrari 430 : 0.70
2007 Audi R8 V8: 0.70
2009 Nissan GT-R : 0.71
2004 Porsche Carrera GT: 0.72
2009 Audi R8 V10 : 0.73
2004 Dodge Viper : 0.75
2009 Aston Martin DBS : 0.76
2010 Lexus LFA: 0.78
2004 Pagani Zonda S: 0.84

Recent Porsche 911s have less drag than an NSX, but a Nissan GT-R has a lot more.
Click to expand...

Thanks Andreas

That's VERY good information to have.

Looks to me that current day cars like the DBS, the R8 and 458 use the drag to create downforce.
(Or they are even bigger then the seam to be parked next to a NSX :smile: )
 
If you have lots of power and you need to turn corners it makes sense to go extreme in accepting drag to create downforce. There are not many cars that beat the SINOCO Porsche 917-30 Mark Donoghue used to race in the 70's (and maybe early 80's?). With the famous Porsche flat-12 turbocharged engines producing ridiculous power (> 1,200hp) they needed traction above all. Consequently their aero was all about downforce so really high Cd resulted, like around 0.6 or more. See piccy ...

If you've ever seen any Porsche 917 in the flesh, they are actually a very small vehicle, so the frontal area is minimal. (I guess around 1.6 sq. m. which would give Cd x A around 0.96)

At the other extreme a trick to reduce Cd is to make car longer, which is why the long tails in some of the Le Mans variants of 917's (see 2nd piccy). These cars had much lower Cd than the Sinoco shape, hence really high top speed on the Mulsanne straight ... (around 400 kph / 250 mph) but nowhere near the downforce (or power BTW) i.e. the best balance between downforce and drag depends a lot on the track you'll be racing and the power you have.
 

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