Best approach for aero improvement on sedan roof?

[Xist]

It is amazing how often I forget about Autospeed. They always have good stuff!

[California98Civic]

Originally Posted by oldtamiyaphile

Yes, but Autospeed's tuft testing showed attached airflow on the rear window of a sedan after installing VG's.

Yeah, that is true. BUT this is the conclusion of that study: "On the basis of the scientific evidence that is available, and the results of this test, we very much doubt whether vortex generators fitted to the trailing edges of vehicles will reduce drag."

They argue that VGs energize the boundry layer and creates more attached flow, but they do not conclude that VGs reduce drag at the steeper angle that the trailing edge offered. And the car they used, a GEN 1 Prius, arguably had a much better angle on the rear glass than this 2007 Malibu. Considering the steeper edge, I'd suggest avoiding the VG tuing for this application.

AutoSpeed - Blowing the Vortex, Part 4

[mcrews]

your going to REALLY need to lower the rpm to get 40 at 80. there is a new guy here with the old big body caprice getting some crazy numbers but hes running a 2. 7 (??) rear ration and taller tires.
Since you just bought tires, you'll have to wait to go up size.
And thinking that your 'cutting the corners faster' is gaining anything is false.
how often are your 'turning'??? maybe 10% of the time?
if you're turning more than that, you'll NEVER get better mpg.
you would be better off with taller tires because if you are on the freeway, it's all about low rpms.
Do taller tires and nylon bushings on the sway bar and suspension points if you want to race thru corners.

[19bonestock88]

Well, my car has the 4T45E transaxle(still despise it's being an automatic but no manual was offered on this car) with the 3.63 final drive ratio, and I turn around 2600 at an indicated 80mph... Other final drives were offered (the 4T45 behind the 3.5V6 was 3.08, and the Malibu Maxx had like a 3.23 ratio to compensate for weight), but I don't know how well my L61 (2.2 L4)PCM would play with a taller transmission... Not that it has power to turn taller gears anyway... The main issues with getting great highway mpg is the interstate on which I do much of my highway driving(most of it is two lane at 55-60mph), it has lots of 5 and 6% grades, on which the engine struggles, and the trans kicks down, and then I'm turning 4000+RPM getting an instant reading of about 12... Figuring out how to get the trans to stay in OD and still maintain speed is just a matter of reducing resistances(drag, rolling resistance, weight), but I'm just getting started...

[19bonestock88]

... And I read the whole article on the VGs, and they claimed that they achieved a .007Cd reduction on the Evo they developed them on, and the result of installing them was a smaller wake(in the second part of the above posted article by AutoSpeed), even with the wing considered... The makers of the AirTabs claim as much as a 7% fuel savings at a constant 120km/h... That alone, if I gained the full claimed benefit, would get me up to 32.8mpg at 80mph(30x1.07)...

[California98Civic]

Originally Posted by 19bonestock88

... And I read the whole article on the VGs, and they claimed that they achieved a .007Cd reduction on the Evo they developed them on, and the result of installing them was a smaller wake(in the second part of the above posted article by AutoSpeed), even with the wing considered... The makers of the AirTabs claim as much as a 7% fuel savings at a constant 120km/h... That alone, if I gained the full claimed benefit, would get me up to 32.8mpg at 80mph(30x1.07)...

Test the manufacturer's own claims for FE at the risk of your own $$. It might be fun, and perhaps there is an off chance that it'll work.

The AutoSpeed story does claim a small reduction in drag for the AirTabs. I see that. But 0.006 is not going to produce a 7% gain. A kamm or boattail is going to be better. I just think those are your better proven options. But if you are curious, then experiment. Share your tuft testing here.

On third thought, maybe your Malibu's rear window is close enough to the angle of the Gen 1 Prius that AutoSpeed tested, though the interaction with the trunklid is harsher:





Good luck.

[19bonestock88]

Originally Posted by California98Civic

Test the manufacturer's own claims for FE at the risk of your own $$. It might be fun, and perhaps there is an off chance that it'll work.

But if you are curious, then experiment. Share your tuft testing here.
.

I don't plan on spending anything to make the VG array... I can get scraps of coroplast(trapezoid shape 19x15) for free at work, I plan on using that to make the VGs... All I need are nine 1"x2" right triangles and a 1" wide strip the width of the roof, and tape to hold it on...

I do need to figure out how to photograph my tuft tests, as I usually take photos with my iPad, and I'm not about to hang that outside the car for photography...

Anyway, the roof is one of the last places I'm gonna look for a gain, because that is harder to decide on a path... I need some sheets of coroplast to make a belly pan and to make wheel skirts, and maybe buy some cheap hubcaps to smooth out...

[kach22i]

Try something like this?



Aerodynamics by George Kachadoorian | Photobucket

[aerohead]

Hucho states emphatically,that the lowest drag is always achieved with the fastback roofline.And it needs to follow closely to the Mair/Buchheim contour.

Here,GM has used it on their Citation concept,which has returned Cds below 0.14.

A recent Buick Riviera concept

Here is Mair

Here is Buchheim

The Aerodynamic Streamlining tool at the top of the forum page respects Mair's contour.If you wanted to consider a Kamm extension,that's what it's for.

[kach22i]

Originally Posted by aerohead

Hucho states emphatically,that the lowest drag is always achieved with the fastback roofline.And it needs to follow closely to the Mair/Buchheim contour.

Don't leading edge air slots/slats alter the story line?

Wing Design - Level 3 - OTHER WING ADDITIONS


NB1 BMW ACS Style Roof Spoiler - BMW E90 3 Series Coupe


Porsche


Just trying to be practical here.

Theory is great for a clean sheet design, but we are reto-fitting, right?

http://zenithch801buildlog.blogspot....dge-slats.html

And in this picture again as above with the air path moving further rearward on the wing. The purpose of the leading edge slat is to accelerate the air across the top surface of the wing hold the boundary layer against the wings top surface in high angle of attack manouvres which prevents the stall allowing a much greater critical angle than a standard wing section (without the slat).

http://www.homebuiltairplanes.com/fo...dge-slots.html