# Aerodynamics



## PZigouras (Jun 5, 2010)

I noticed a considerable amount of drag on a customer's car past 80 mph, in what I would consider to be a very aerodynamic car. Approaching 90 mph, the amps go through the roof.

The car is a 240Z with smoothed front end (no factory headlights), ground effects, and no real mirrors to speak of. Will lowing the car help?

Also, there is a big air vent built into the front air dam... but I didn't think that would make much of a difference.


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## palmer_md (Jul 22, 2011)

are you talking about a spike in the reading or that it is going up exponentially with speed and at 90 it is "through the roof"?


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## ken will (Dec 19, 2009)

It might not be just Aerodynamics coming into play.
That might be the point where the motor and/or the controller are becoming less efficient.
I am not a motor expert yet, but , IMHO, at higher RPMs the back EMF starts competing with battery voltage. If you have a SEPEX motor you can reduce current in the field windings which will reduce back EMF.

If I am wrong will a more knowledged person please let me know.
I am always trying to learn more.


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## PZigouras (Jun 5, 2010)

palmer_md said:


> are you talking about a spike in the reading or that it is going up exponentially with speed and at 90 it is "through the roof"?


It seems to go up exponentially, so it's not a mechanical issue.

I'm starting to think that the front air dam is acting almost like a parachute. Are front dams supposed to me completely closed up?


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## TomA (Mar 26, 2009)

PZigouras said:


> It seems to go up exponentially, so it's not a mechanical issue.
> 
> I'm starting to think that the front air dam is acting almost like a parachute. Are front dams supposed to me completely closed up?


Well, you could close up and smooth out the whole front, but it isn't going to change the drag curve. Aerodynamic drag goes up with the _square_ of speed, and the power required to overcome it goes up with the_ cube_ of speed. Yes, the amps required to indeed go through the roof as speed climbs. A car that needs 15kW to go 40mph might easily take 40kW to go 60, and over 100kW+ to go 80mph. 90? That's movin'...

Obviously, this is a huge area of study and expertise. Go over to Ecomodder and look through those forums- those guys (some of whom are here, too) really have the information you need to get smart on the matter.

The short answer is there's little you can do about your drag by changing the surface features of the body envelope you have. The drag curve seems as expected from your description. Yes, making the intake opening flat will help a lot, but paradoxically, the big improvements in reducing aero drag are to be found at the back of the vehicle, not the front, and real gains take real mods, like an underbody pan, wheel pants and tail cone. I haven't seen a 240Z on a lift in 25 years, but there's probably as much or even more to be gained aerodynamically from cleaning up the _rear_ bumper of your Z-car, which is likely shaped very much like a parachute, than the front.

Relax, 90mph is very, very fast for an EV. That's max everything. Your motor and controller are going to need serious cooling at that speed for more than a minute, and where do you plan on doing that anyway...


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## PZigouras (Jun 5, 2010)

You're right, it may be the overall shape of the car. 

This is what it looks like now:

http://www.youtube.com/user/EPCcorp?blend=2&ob=5#p/u/0/7f7eu9WBwlM

If you forward to 1:56, you can see the large hole in the air dam (almost the width of the bumper).

But the rest of the ground effects seem to smooth out the car... at least to the untrained naked eye (mine). I saw some of the same modifications on Ecomodders, but obviously, those cars have a lot more work done to them. 

I could also lose the hood scoop, but I think the motor needs it for cooling. I could drop the car down another inch or so... do you think that would be of any help?


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## Bowser330 (Jun 15, 2008)

I am pretty sure lowering will help your aerodynamics.

Also close any and all openings on the front of your car, try to make it as smooth as possible for the oncoming air.

under body tray as previously mentioned

a lot of ground effects are used/sold purely for aesthetics and have no real functional form...


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## PZigouras (Jun 5, 2010)

Bowser330 said:


> a lot of ground effects are used/sold purely for aesthetics and have no real functional form...


What do you think of the ground effects on this car? Strictly cosmetic?


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## Bowser330 (Jun 15, 2008)

Those side skirts look fine they will definitely keep the air less turbulent while running along the side of the 240z.

The hood scoop needs to be closed.

The front bumper needs to be blocked, closed up.

There is much less heat generated in an electric motor vs. an ICE so there is no need to have openings on the front for radiator cooling.


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## TomA (Mar 26, 2009)

PZigouras said:


> What do you think of the ground effects on this car? Strictly cosmetic?


The term "ground effects" as used in automotivedom refers to the period of Formula One racing in the late 1970s and early 1980s (started by Colin Chapman with the Lotus 78) when polycarbonate longitudinal skirts were used to keep air from being sucked into the sidepods, which were fitted with upside down airfoils on their bottoms. These "wings-in-ground-effect" were just the opposite of the WIG designs the Soviets developed for high-lift ultra-low altitude aircraft (Ekranoplanes,) and in instead of lift the reversed F1 ground-effect units made downforce. They also made quite a lot of drag. That's a great trade for cornering on the racetrack, but a disaster for efficiency. "Ground effects" do not reduce drag, and were never intended to improve efficiency.

Ever since then, all manner of side trim, rocker extensions, and lower panels are called "ground effects" but in almost every single case, they neither increase efficiency, nor actually produce downforce. They are styling, pure and simple.

Yours are no different. The fender spats and rocker extensions are almost certainly just making turbulence, slightly increasing your frontal area, and being extra weight. The chin spoiler is helping keep air out from under the car, but at the expense of more frontal area. The rear bumper extension and plane are worse than having nothing there, almost certainly trapping air like a parachute, and actually creating lots more drag than the stock body and rear bumper would. The wheel/tire package is also no friend to the air...

Frankly, you'd be better off removing the whole body package, smoothing the stock nose apron into a full belly pan, which you would then extend, incorporating a couple of subtle venturies aft of the rear axle, all the way to the rear panel. Smooth, smooth, smooth, which is far easier on an EV without exhaust or cooling to deal with. Lowering the front 3" and the rear 2" would help, as would narrower tires on smooth wheels, and putting the stock hood back, with a polycarbonate filler panel over the lights and former grille opening. This is all very basic aero stuff, which most people don't do because they don't like the look of narrow tires, moon discs, etc. and prefer the "ground effect" styling.

The 240Z also has some very busy 70s era details, like an upright windshield, protruberant wipers, greenhouse trim and a forward-leaning grill, all working against it. Here's some pretty interesting wind tunnel study work done on it:

http://forums.hybridz.org/index.php?/topic/55944-windtunnel-test-data/

Sort of astonishing how high the Cd is- from .46 to over .50 with a front spoiler like your car, then carefully worked down to under .40 Its really a much higher drag car than I expected from looking at it.


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## PZigouras (Jun 5, 2010)

TomA said:


> The chin spoiler is helping keep air out from under the car, but at the expense of more frontal area. QUOTE]
> 
> Ahhh, I think I know what the problem is.... I don't have a wind tunnel with sensors in my back yard like these people do.
> 
> ...


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## JRP3 (Mar 7, 2008)

Teardrop shape is very efficient. Wider in the front smaller in the rear.


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## piotrsko (Dec 9, 2007)

PZigouras said:


> Ahhh, I think I know what the problem is.... I don't have a wind tunnel with sensors in my back yard like these people do.


Well us glider pilots tuft EVERYTHING with red yarn and go out to see where the tufts wander. any that don't go exactly in the same direction as you are pointing to a drag problem


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## Dink (Jun 3, 2010)

JRP3 said:


> Teardrop shape is very efficient. Wider in the front smaller in the rear.


Wind tunnel tests do show that. Watch a drop of water falling in high speed. Nothing is affecting it other than gravity and wind drag/resistance. That form is achived when it reaches terminal velocity.


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## Dink (Jun 3, 2010)

piotrsko said:


> Well us glider pilots tuft EVERYTHING with red yarn and go out to see where the tufts wander. any that don't go exactly in the same direction as you are pointing to a drag problem


Nice! A very simple effective real world live test. Maybe having someone video the car at speed while following it from different angles would help spot the problem areas.


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## BrownBageV (Nov 10, 2011)

Tons of aerodynamic tests have been done on z-cars in a windtunnel.

http://forums.hybridz.org/index.php/topic/55944-windtunnel-test-data/

Very good stuff that probably applies to all older Ev's.

The lessons are pretty clear:
Get a front end and side skirt body kit 
Dam up the front vents as much as possible 
Fix front minor aerodynamic issues with lights, wipers, mirrors, etc
Small spoilers

-David


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## TomA (Mar 26, 2009)

BrownBageV said:


> Tons of aerodynamic tests have been done on z-cars in a windtunnel.
> 
> The lessons are pretty clear:
> Get a front end and side skirt body kit
> ...


I'm not sure these lessons are all that clear, or that there are tons of things in the spreadsheets to consider:

The side skirts are worth a reduction in Cd of .004, pretty small potatoes. The big air dams are better when they're blocked off, but they add frontal area, and since an EV doesn't need any air going through the nose of the car for cooling, you could probably get away with a really small one, and actually a bluntish, pointier nose might be far better.

Untested in the 28 trials is a full belly pan, which is easily the best mod to reduce drag, and one of the easiest on an EV with no exhaust system or cooling airflow to deal with. It will be easier, tidier and less frontal area to do the belly pan without side skirts. They also didn't try skinny tires and Moon discs.

The place to look at mods that would help Pzig is ecomodder.com, but since he's sort of personna non grata around here due to the build quality of, and his evasiveness surrounding the controllers he sells, methinks this thread be dead...


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## PZigouras (Jun 5, 2010)

I've been looking on ecomodder for the past few weeks and I'm not too sure how good some of those mods really are. 

Some are obvious, such as deleting mirrors, lowering the car, etc. But some people told me that the aerodynamic mods have questionable results -- unless you have real-world testing to back it up. And by that I mean actually wind-testing it, not just keeping track of fuel economy. That can change day-to-day, depending on how you drive.


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## JRP3 (Mar 7, 2008)

A,B,A testing on the same route on the same day should give pretty good indications of what you've achieved. I've been toying with the idea of using window heat shrink film over a light frame to mock up and test some aero mods. 
http://ecomodder.com/forum/showthread.php/heat-shrink-window-film-body-mods-18584.html


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## piotrsko (Dec 9, 2007)

if you get the tear proof window stuff, I used it for RC combat gliders that were clocked above 100 mph. not sure how to do the structure though.


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## JRP3 (Mar 7, 2008)

I was thinking of some combination of wire, rods, wooden dowels, and duct tape might do it.


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## piotrsko (Dec 9, 2007)

Ahh yes, duct tape is mandatory for structures.


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## BrownBageV (Nov 10, 2011)

I'm pretty new to the ev community, but not the z community. Take another look at those windtunnel data. They did not modify the assumed frontal area when they put on the air dam kits, the actual drag forces decreased significantly. That said, I agree that the easiest mods are a belly pan and simply closing off the front vents entirely, two things you don't get to do easily with ICEs. Plus, for his z there are relatively cheap add on kits available that do it all.
What I was surprised about is how little change the light covers and "smoothing" mods make.


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## gor (Nov 25, 2009)

BrownBageV said:


> I'm pretty new to the ev community, but not the z community. Take another look at those windtunnel data. They did not modify the assumed frontal area when they put on the air dam kits, the actual drag forces decreased significantly. That said, I agree that the easiest mods are a belly pan and simply closing off the front vents entirely, two things you don't get to do easily with ICEs. Plus, for his z there are relatively cheap add on kits available that do it all.
> *What I was surprised about is how little change the light covers and "smoothing" mods make*.


reminds Mithbusters episode about golfball-scaled dimples on the car: "Soon all Nascar racers will be like that!"


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## PZigouras (Jun 5, 2010)

gor said:


> reminds Mithbusters episode about golfball-scaled dimples on the car: "Soon all Nascar racers will be like that!"


I heard about that episode but I never saw it and it's not available on demand... did it make a difference?


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## gor (Nov 25, 2009)

PZigouras said:


> I heard about that episode but I never saw it and it's not available on demand... did it make a difference?


yes, surprisingly significant (don't remember number (10>??) fuel economy; tested with vs.witout)
just looked silly (funny), poked with those dimples; they been laughing - soon all nascars would look like that : ))


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## gor (Nov 25, 2009)

http://dsc.discovery.com/videos/mythbusters-dimpled-car-minimyth.html
+
here + here2


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## TomA (Mar 26, 2009)

IIRC, they did have a significant improvement. Actually, it was TOO significant, and suggested to me there were a number of problems with the tests and their methodology. 

Beyond that, the whole idea of dimples on something like a car is a little silly. The reason you see dimples on golf balls is because they're omnidirectional, and present the same face to the airflow no matter how the ball spins in relation to its path through the air. The round shape of the dimple is determined by the application- on the surface of a sphere.

You just don't need that with a car. The airflow is unidirectional, and can be dealt with much more efficiently on the surface with specific aerodynamic devices, like trip wires, vortex generators, spoilers, splitters, etc. The dimple works well on a sphere, but it isn't the best shape to prevent separation on a surface that's fixed in relation to basically unidirectional airflow. 

If it wasn't a gimmick, every airplane in the world would be covered with dimples, or most would at least have them in the "special spot" where the big efficiency gain is supposed to be found using them, or whatever. 

In this case, I would declare it "myth perpetuated." Makes for good TV, I guess...


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## gor (Nov 25, 2009)

TomA said:


> IIRC, they did have a significant improvement. Actually, it was TOO significant, and suggested to me there were a number of problems with the tests and their methodology.
> 
> ...


IIRC... you probably mean Reversed porsh episode, where fuel consumption tests showed inconsistency and been discarded.
Anyway - it is an entertainment show about ideas, not test facility with exhaustive test run; 

what important - ideas and what they (mb) are doing, results - can, should and will be questioned and interpreted differently

btw - Porsche, Reverse Engineering episode: http://www.youtube.com/watch?v=fXMgc4xaiwg
old idea - some cars can have better aerodynamics in reverse than in forward (teardrop shape)
so, they cut and turned car body around to check it... : ))))))


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## JRP3 (Mar 7, 2008)

They never compared the car's rolling resistances which could be quite different, and the zero to sixty times don't mean much since only part of the run would even be affected by wind resistance. Also the reversed car seemed to be sitting much lower in the front and looked nose heavy, which probably messed up the alignment. They should have just taken the car and towed it in reverse at 60 with a strain gauge on the tow line and then do the same thing going forward. Less strain less resistance.


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## palmer_md (Jul 22, 2011)

JRP3 said:


> They should have just taken the car and towed it in reverse at 60 with a strain gauge on the tow line and then do the same thing going forward. Less strain less resistance.



that would not make a very entertaining show.


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## tomofreno (Mar 3, 2009)

I've been thinking of just bending some 1/16" plastic around my plastic front bumper and bolting it in place to create a smooth, rounded, no-holes front end, and adding a full belly pan. Any feel for the approximate change in Cd for those mod's? I'm wondering if even that is worth doing. 

The teardrop rear end some eccomoders added seems by far the most effective, but I'm not about to stick something like that on the car.


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## JRP3 (Mar 7, 2008)

Bellypan seems to be the basic mod the ecomodders use, when you look at how many nooks and crannies most bottoms have it certainly looks like a good mod. I haven't done any A/B testing with mine so I can't provide any numbers. I'd think a smooth front has to help as well, I've been planning to try one as well.


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## PZigouras (Jun 5, 2010)

TomA said:


> The reason you see dimples on golf balls is because they're omnidirectional, and present the same face to the airflow no matter how the ball spins in relation to its path through the air. The round shape of the dimple is determined by the application- on the surface of a sphere..


What about side wind? I live on the coast, where 30 to 45mph winds are common place, and since Route 3 goes up the coast, it runs perpendicular to the wind most of the time. Cars going up and down the highway almost always get hit on the side. 

But my guess is, because of the shape of the car, it won't work very well (after all, golf ball is round).


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## TomA (Mar 26, 2009)

gor said:


> IIRC... you probably mean Reversed porsh episode, where fuel consumption tests showed inconsistency and been discarded.
> Anyway - it is an entertainment show about ideas, not test facility with exhaustive test run;
> 
> what important - ideas and what they (mb) are doing, results - can, should and will be questioned and interpreted differently
> ...


Nope, I wasn't talking about the reverse aero episode.

The dimpled car, with completely slipshod testing methods, showed a whopping 10+% efficiency gain, which would require something like 20% reduction in drag. That data is in error. One look at the test conditions (eyeballing the speedometer, foot on the gas, a bumpy surface, but incredibly precise fuel measurements) and the gig is up...

BTW, its no surprise that some cars have better airflow in reverse. The cooling system is managing airflow, and the windshield needs to be easier to see out of than the rear window, and the cabin airflow also needs to take and vent some of that outside air. Moreover, the car has to combat lift and aerodynamic instability at speed. The idea that the _shape_ might produce less drag going backwards is meaningless, unless you want to try _operating _it that way (and not just once around with nothing really working until it overheats...)

I think this rather silly point was first made as a criticism about the Chrysler airflow, but it was proven to be incorrect in that case as well.


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## Rational (Nov 26, 2011)

If you need to run tests of the sort "type A aero profile is better than type B aero profile to better than a 95% confidence level" I can probably help with the data reduction. 

A 40 hour course at a NASA facility taught me methods and provided lookup tables which proved to be very useful at turning 'matters of opinion' into 'matters of fact'. And almost no one, and certainly no politician, wants to know what the numbers say!


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## JRP3 (Mar 7, 2008)

tomofreno said:


> I've been thinking of just bending some 1/16" plastic around my plastic front bumper and bolting it in place to create a smooth, rounded, no-holes front end, and adding a full belly pan. Any feel for the approximate change in Cd for those mod's? I'm wondering if even that is worth doing.


I have a couple of data points for you, I put the belly pan back on and did the same test route I've done before without the belly pan.

First run, no belly pan:
20 mile trip 2nd gear
50mph moving average
60mph top speed
55mph steady target speed
48 ah used
65 degree starting pack temp
65 degree air temp
No headlights

Second run, with belly pan:
20 mile trip 2nd gear
50mph moving average
60mph top speed
55mph steady target speed
48 ah used
60 degree starting pack temp
40 degree air temp
Headlights on entire trip

The headlights on the Fiero are popups so they affect the aerodynamics, along with the increased power draw. So the fact that I used the same amp hours on a 25 degree colder day, with some strong cross winds, and the headlights popped up, suggests the belly pan does help some. Obviously more runs would be needed to have much meaning. It's a real pain to take the pan on and off so don't expect anything more this year 
I should note that I still need to seal off the bottom of the rear bumper as the pan ends before it and it's a bit of a scoop, causing drag.


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## tomofreno (Mar 3, 2009)

> I have a couple of data points for you...


 Yeah, looks like it does help. I think I will try adding a pan - though maybe next spring when the garage is warmer.


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## Rational (Nov 26, 2011)

Assuming you only do two runs for each of two aero configs and you want 95% confidence that there is 1 mph difference between the two, here is one way to achieve this - numbers are mph.
config 1: 60, 70
config 2: 30, 40

One problem is the very high certainty required of a very small sample size [2 each]. 
With a sample size of three runs each, you do considerably better. 
You'd also do better if the samples had a smaller scatter than +/-5 mph and if you were looking for a difference bigger than 1 mph.


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## Salty9 (Jul 13, 2009)

Something to consider: http://www.autospeed.com.au/cms/A_108677/article.html?popularArticle


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## dtbaker (Jan 5, 2008)

JRP3 said:


> Bellypan seems to be the basic mod the ecomodders use, when you look at how many nooks and crannies most bottoms have it certainly looks like a good mod. I haven't done any A/B testing with mine so I can't provide any numbers. I'd think a smooth front has to help as well, I've been planning to try one as well.



I just added an abbrev belly under motor back to the rear of the torsion bar.... mostly for protection from dirt, mud, snow. Probably will close the grill as well, but can't get there for a couple weeks. I might have aero benefit, but both were things I was going to do to protect engine bay from elements anyway.

Rear bumper pan will come later from rear battery box to bumper might be a good thing, OEM is a rear air scoop affair that can't be good. Then perhaps rear wheel covers.... but most of my driving is 'low' speed (25-45mph) so this is mostly academic.

I just sent my CycleAnalyst back for repair/replacement, so I can't do any real ABA testing, but will do that after I get metering re-installed. My CA was an early one that never dealt well with 120v nominal.... stuck at 127.3 for some reason. I am hoping they can fix or replace with current product; or I'll switch to evDisplay most likely.


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## Scuderia Elettrica (Sep 10, 2011)

TomA said:


> IIRC, they did have a significant improvement. Actually, it was TOO significant, and suggested to me there were a number of problems with the tests and their methodology.
> 
> Beyond that, the whole idea of dimples on something like a car is a little silly. The reason you see dimples on golf balls is because they're omnidirectional, and present the same face to the airflow no matter how the ball spins in relation to its path through the air. The round shape of the dimple is determined by the application- on the surface of a sphere.
> 
> ...


A slightly different application of dimples http://www.racecar-engineering.com/articles/technology/can-dimpled-aerodynamic-surfaces-reduce-drag/


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## Third_degree (Jan 11, 2012)

A couple of comments per the originial thread of extra power at speed:

Power required goes up with the cube of the velocity. Or W proportional to V^3. So that's normal regardless of car,

As to improvements:

1) Want the the grill opening as small as possible. New focus for example closes off the the front vent relative to cooling requirements in order to optimize aero.

2) Skirts are good for ground-effects (down force) more than drag. A smooth floor pan is better for drag

3) Lowering the car... again best for downforce (changing the relative pressure above and below car). Ideally car has both small cross section, and higher ground clearance. This reduces the relative volocity of the air next to the body, so skin friction is reduced.

4) Lose the hood scoop.

5) Dimples. The only good version of this would be 3M riblet film. The car is kept as smooth as possible, but any areas that taper too quickly (Sharper than 15 degrees) that have flow separation can be improved by adding riblet film. Puts a series of reversed NACA ducts or dragon scales to make tiny vortexes that reduce drag. This surface ripples are TINY...

6) other forms of vortex generators can be useful, but on a case by case basis.


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## TigerNut (Dec 18, 2009)

I've had a set of homemade vortex generators on the rear of the roof on my (ICE) Echo for a couple of years now. Under just the right conditions (temp/humidity) you can see vapor streamers go down the back window, and they do seem to improve the adhesion of the airflow around the back window, which should improve the overall Cd. From what I recall, I noted about a 3% range improvement, on average, after doing the installation.

The design is based on this paper by Mitsubishi:
http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf

and I made them from pieces of 1/4" thick FR4 circuit board material, epoxied to mounting plates made from 1/32" FR4. I then painted them and used double sided trim tape to attach them to the car roof.


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## Third_degree (Jan 11, 2012)

I agree, that style of vortex generator is excellent. It is needed because the angle of the rear windshield is too great, and those help to generate localized turbulence that keeps the flow attached. So not as good as a shallow teat angle (Prius / Insight). But better than if no VG,s are used on that angle

Completely agree with this case


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