Interesting aero design on WRX

[thehelix112]

I was walking down Lincoln Blvd in Santa Monica to get myself a coffee when I noticed this WRX.

 

The small flap is designed, I believe, to redirect air down the rear windscreen, doing two things. 1, reducing the drag caused by the rear windscreen (at the expensive of drag caused by the flap itself), and 2, to increase the amount of air that flows over the rear wing.

 

I'd been mulling over this idea on and off, but in my case, there is no reason why I wouldn't just move the wing up into a clearer stream.

 

My questions on the flap are as follows:

 

1. Is it likely(possible?) that the additional downforce created by having more flow to the rear wing would negate the additional lift caused by the flap? Is this likely because the wing is further back and will hence have a larger moment for the same pressure than the flap?

 

2. Is it likely(possible?) that the additional drag caused by the flap would be less than the drag reduction caused by seperation of flow from the rear windscreen?

 

Thanks,

 

Dave

[JMortensen]

Read this: http://forums.hybridz.org/showthread.php?t=115294 then donate to the wind tunnel testing and send bjhines a deflector and you'll get a correct answer to your questions. I think that this is a different idea that serves the same purpose as the vortex generators on the new EVO.

[thehelix112]

Jon,

 

I've read the thread (and also contributed).

 

As I said, this is not a serious concern for me, and definitely not worth my time (or anybody elses most likely) to real-world test.

 

What I asked, I did so from a theoretical design perspective.

 

I agree it most likely serves the same purpose as the vortex generators, but achieves the desired effect in a very different way. I can't help but feel that unless you spent a long time tuning such a flap and a very short time tuning the vortex generators, that the VGs provide a neater/cleaner/more efficient solution.

 

Dave

[proxlamus©]

Once Subaru noticed that Mitsubishi was implementating Vortex Generators on the roof of their cars.. Subaru stepped in and developed this spoiler

[proxlamus©]

In addition..

 

note the Subaru WRX WRC car.. the wing is much larger and has diffusers built into the wing

 

[thehelix112]

Diffusers built into the wing? Where? I see lots of vertical strakes which appear to be a lot more focussed towards the spoiler surface than the wing.

 

Dave

[RedFive]

Dave,

 

The idea is not the same as vortex generators. Completely opposite. The vortex generators create non-laminar flow, whereas the mission of the wing you posted a picture of is to prevent the flow from separating. Your hypothesis on the function of that upper wing is generally correct, but who knows if it's placed correctly.

 

If it's arbitrarily placed, the free stream might be separated before it even gets to the upper wing, thereby rendering it completely and utterly useless.

 

We just did some research a couple weeks ago on pickup trucks. We found that it makes virtually no difference if the tailgate is up or down. Why? Because the flow is already separated off the rear of the cab, so it doesn't matter what happens after that. In the same series of tests we also added a camper shell and found that the drag increased, mostly due to skin friction. With the camper shell, the separation still occurred (obviously), but just simply delayed until further down the truck. Adding a wing like the one in the photo you have would help reduce the separation, much like the vanes used in the corners of a wind tunnel, or more commonly, the "wings" that some manufacturers mount on the rear of SUVs. Older blazers/jimmys have this...but who knows how effective they are. I could probably run a wind tunnel test in the spring sometime if you wanted.

 

 

Logan

[proxlamus©]

Ok .. let me re-word "diffuser"

 

The Subaru has "vertical stabilizers"

[tannji]

In addition..

 

note the Subaru WRX WRC car.. the wing is much larger and has diffusers built into the wing

 

 

 

If you look closely, I believe the Subaru also has the spoiler at the top of the hatch, like Mitsubishi.

[proxlamus©]

That is correct tannji .. Subaru calls it the "roof vane" while Mitsubishi has vortex generators.. yes RedFive they are different

 

I found this article online

AERODYNAMICS

First, when referring to the evolution of chassis, the improvement of the aerodynamic performance from the change of the body shell must be mentioned. To decrease the air resistance caused at the front edge of the based body, the front had been slanted to repress the flow of air on the surface of the bonnet hood.

Also, this had allowed the efficient use of air to be induced to the cooling system and to draw out air from the engine bay. On the most notable exterior feature, the rear wing, a vertical rectifying plate was inserted in the space between the large double wings in order to improve the stability in steering. The garny on the rear end of the wing has adjusted its height in order to gain more down force. Along with the four vertical plates, it is now able to capture efficiently the flow of air from the side and stabilize the aerodynamics feature in the mid-low speed range when the tails slide. The shape of the front bumper and front side air splitter is designed primarily to suit the down force in the high-speed rally. The lower front chin spoiler has been changed to flexible soft carbon from last year's rubber. These changes are made in order to prevent the deforming, breakage and dropping while catching the air.

 

The shape of the door mirror has been changed also to maximize the aerodynamic performance. It has always been a pursuit to reduce the size, but this time, the air is allowed to flow through the side of the body as the mirror is detached from the body.

 

In recent years, WRC has become a sport of speed, and the evolution of the aerodynamics performance is constantly requested. The aerodynamic set-up has become much essential factor just like the closed circuit racing.

 

 

Also

The Impreza WRX STI is equipped with a rally-inspired spoiler that does a lot more than just draw admiring glances. The large, WRC box-type rear spoiler helps to provide added down-force to keep the car stable at high speeds. For 2006, a new roof vane helps direct airflow towards the spoiler for even better aerodynamic performance.

 

While you might not be able to see it, the underside of the 2006 Impreza WRX STI is just as aerodynamically advanced as its body, thanks to its engine and floor aerodynamic covers as well as a new-for-2006 rear diffuser, which helps provide exceptional stability at speed.

[Armand]

RedFive, have you seen the MythBusters episode about the tail gate up/down theory for better fuel economy?

[cygnusx1]

There was a certain generation of Supra that had the black wing at the top of its hatch. Early 80's.

 

Anyone old enough to remember these?

[RedFive]

Armand, no I have not. But I heard that they think you get better mpg with it up. We noticed this too, but difference in drag was so small that I consider it being negligible. But of course every truck is different...

 

Proxlamus, it sounds like the wing next to the roof is a stock option? In that case it's probably placed correctly. But doing it on another vehicle would take some testing.

[Sven]

I've found some goodies that pertain:

Now, before the panty-bunching begins: I'm aware that an Evo and WRX are different cars. Overlay their basic shapes over each other, and it's apparent this rally rat race b/w Subura and Mitsubishi has produced nearly identically shaped(ly) cars with different front ends and taillights. The side shots show the extreme similarity in windshield and C-pillar angles, which should make roof/spoiler aero comparisons somewhat worthwhile.

 

I'm still making sense of this, Cd appears to be a draw.

...lil spoiler adding downforce?(negative lift) Undertrays and diffusers may come into play here also.

 

It also smoothes airflow onto that grab handle, I mean wing

 

It would be really nice to have the same wind tunnel pics of a WRX w/o the funny lil spoiler. That would take this from conjecture to a fair comparison.

[thehelix112]

The idea is not the same as vortex generators. Completely opposite. The vortex generators create non-laminar flow, whereas the mission of the wing you posted a picture of is to prevent the flow from separating. Your hypothesis on the function of that upper wing is generally correct, but who knows if it's placed correctly.

 

Correct me if I am wrong (obviously I don't think I am), but vortex generators are used to delay flow separation, just as that flap will.

 

If it's arbitrarily placed, the free stream might be separated before it even gets to the upper wing, thereby rendering it completely and utterly useless.

 

True, but that would seem to defeat the purpose of putting it on there.

 

We just did some research a couple weeks ago on pickup trucks. We found that it makes virtually no difference if the tailgate is up or down. Why? Because the flow is already separated off the rear of the cab, so it doesn't matter what happens after that. In the same series of tests we also added a camper shell and found that the drag increased, mostly due to skin friction. With the camper shell, the separation still occurred (obviously), but just simply delayed until further down the truck. Adding a wing like the one in the photo you have would help reduce the separation, much like the vanes used in the corners of a wind tunnel, or more commonly, the "wings" that some manufacturers mount on the rear of SUVs. Older blazers/jimmys have this...but who knows how effective they are. I could probably run a wind tunnel test in the spring sometime if you wanted.

 

Logan

 

My main question to this is, wanna test my race car in your wind tunnel after I've built it?

 

Dave

[thehelix112]

Very interesting pics there Sven.

 

Dave

[Mike kZ]

There was a certain generation of Supra that had the black wing at the top of its hatch. Early 80's.

 

Anyone old enough to remember these?

 

I have always wondered about that wing also. I have a wing like that, that is made for a Z car. Not sure if it helps with anything

 

[proxlamus©]

thanks for those pics Sven!! awsome!! now time to find out how to read them! lol

 

Helix if you check on the aerodynamics thread on this site.. there is a PDF file from mitsubishi which shows that the vortex generators delay the airflow seperation.. so the air stream hits the wing and creates more downforce.

[veritech-z]

Here are some close ups I took of the spoiler on the Subie WRC WRX at the tAmpa international auto show last month:

[JMortensen]

I always thought the flap on the back of a Blazer was to prevent the rear window from getting dirty.