FLASH! Porsche Engineers Defy Nature, Physics (and Bernoulli)!

RaceProEngineer · 06-22-2006, 06:29 PM

Didn't anyone else pick up on this?

June 2006 issue of Panorama Magazine; Page 5, referring to the brand new 911 Turbos:

" Speed actuated rear wing produces downforce without added drag."

The floor is open . . . . .

Ed

sketchers356 · 06-22-2006, 06:59 PM

I hear that those new fancy turbos are really magnetic monopoles.

ZOA NOM · ZOA NOM's Garage

Isn't the operative word "added"? It doesn't say "no" drag.

450knotOffice · 450knotOffice's Garage

Yea. I'm guessing the intent of the statement was to say that they increaed the efficiency of the wing, which added downforce but created no more drag than the old one.

masraum · 06-22-2006, 08:03 PM

Don't forget the early tails did almost the same thing. THey reduced lift to nearly 0 while actually decreasing drag.

304065 · 06-23-2006, 07:42 AM

We've rehashed this multiple times but here's how I think about it:

1. The design of the car generates lift, resulting in induced drag. There is also parasite drag present.

2. By disrupting the airflow, the ducktail (a spoiler) reduces the formation of lift and its associated induced drag. It also reduces parasite drag.

I think what the Pano writer means is no NET drag increase.

Wavey · 06-23-2006, 08:12 AM

OR they meant that because it's speed-actuated, there's no added drag when it's not deployed? Doesn't really say that but the point could be argued.

RaceProEngineer · 06-23-2006, 08:23 AM

All good points, gentlemen. I had not considered the various "interpretations", based on history, but was focusing on the action:

" Speed actuated rear wing produces downforce without added drag."

1. The car increases speed.
2. The rear wing "changes" in some manner (persumably like its predecessor.)
3. That change(s) produce(s) downforce.
4. That additional downforce does not carry with it an increased drag penalty.

Point 4 was my puzzlement!
Ed

Netspeed · 06-23-2006, 08:49 AM

With all the hype of vortex generators a few years back (and I think the Lancer Evo uses them now), why don't more manufacturers use them?

Vortex generators

randywebb · 09:44 AM

Even some fixed devices can create additional downforce and still reduce drag.

There is nothing novel about it.
Nor does anything about it "Defy Nature, Physics." Bernoulli's equation is one of the most misunderstood things in physics (after E = mc2). This often surprises students or others who haven't either
a. gone thru the equations carefully, or
b. looked eddy effects from flow visualizations.

It's even been discussed in old threads on this bbs. I forget which device - maybe the ducktail did this.

Grady Clay · 06-23-2006, 09:43 AM

Leonard is a pretty good writer (and attorney). I have been reading his work for 40 years. Anyone who has authored this kind of article knows the challenge of simply having the caption associated with the correct image. I suspect this caption was written or edited by someone else and it lost something in the process.

How would it be better written without becoming overly verbose (as I’m inclined to do)?

Best,
Grady

randywebb · 06-23-2006, 09:45 AM

To make it more clear one could say adds downforce...

People are getting hung up perhaps on definitions. Distinguish net and gross downforce...

RaceProEngineer · 06-23-2006, 11:36 AM

Hey Guys, just trying to have a little fun here . . .

Our understanding of current thinking (without reading NetSpeed's link above) is that vortex generators reduce FORM DRAG. They do this by tripping the boundry layer of air before it can become so turbulent that it creates "back pressure". While reducing FORM DRAG, these little wicker bills create some of John's PARACITIC DRAG on their own. So that is the lesser of evils.

I believe the short answer to why other manufacturers do not use them is that they do not know how exactly!

As to downforce without added drag, this MIGHT be achievable in managing the air UNDER the car correctly (the jury is still out), but I can think of no circumstances whatsoever where any downforce generator on the upper surfaces of the vehicle does not bring with it a penalty in additional drag. If any of you can, there are 11 Formula One teams waiting to hire you!

Like most of this racing stuff, all about making the most of the compromise!

Ed

rusnak · 06-23-2006, 02:14 PM

They are making similar statements about the 997 GT3.

The LMP2 car also is supposed to be aero efficient. Porsche must have built a new wind tunnel.

randywebb · 06-23-2006, 02:44 PM

profile drag will be much greater than induced drag at low Re

at higher Re, the profile drag will be reduced

randywebb · 06-23-2006, 02:52 PM

pic of their tunnel from Christophorus - completed a few years ago...

rusnak · 06-23-2006, 02:54 PM

that dude is about to become German pate

randywebb · 06-23-2006, 03:16 PM

No, actually he is the power to spin the blades. The gerbils used in the old tunnel just weren't up to the new, bigger fan...

304065 · 06-23-2006, 03:50 PM

Wow, that is a very cool photo.

I have the Pano right here. It says,

Quote:

In spite of the more muscular look, Porsche engineers have for the first time been able to generate rear axle downforce (60 pounds at top speed) without adding air drag, and the 997 Turbo's Cd is the same as its predecessor at .31.

I didn't know Leonard Turner was a lawyer (but I should have guessed it from his excellent writing style

) Lawyers are not inclined to be imprecise in their language, therefore, I would say that what he means is they have found a way to not merely kill lift but generate NET download without increasing total drag.

But I'll email him and ask him. :0

RaceProEngineer · 06-23-2006, 04:29 PM

Hi John,

I am quoting the caption above the photograph at the top of the page.

Ed