Porsche 911 Ducktail - Study/Alternate

[kach22i]

Waiting for the snow to melt so I can take some field measurements of a building, so I cut up a business card and compared a Ducktail to an alternate location a bit higher.

Would be made of clear Polycarbonate or Acrylic.

Not sure yet what would happen with the flow/vortex and pressure level above the engine cooling inlet.








This is a side-by-side comparison, would not double Duck, although I guess one could.

As I understand it, the Ducktail, Teatray, and Whaletail all try to correct the approximately 30 degree roof angle to mimic an approximate 20 degree slope (almost like a louver - steps).


Image of wind tunnel smoke in link below.
https://www.gettyimages.com/detail/n...re-id653712864

[freebeard]

This is a side-by-side comparison, would not double Duck, although I guess one could.

And pass up that naming opportunity?

Compare to sides that wrap forward more horizontally like the 4th Gen Pontiac Firebird. I think it would add a downward component to the vortexes generated and also capture more of the flow. Or not. The ducktail works by being opposd to the flow. I'm probably thinking about the tea tray.

You could start with a Gurney flap at the top of the glass. That would simulate multiple base plates. I can't find a pic right now.

[kach22i]

Originally Posted by freebeard

And pass up that naming opportunity?

Compare to sides that wrap forward more horizontally like the 4th Gen Pontiac Firebird. I think it would add a downward component to the vortexes generated and also capture more of the flow. Or not. The ducktail works by being opposd to the flow. I'm probably thinking about the tea tray.

You could start with a Gurney flap at the top of the glass. That would simulate multiple base plates. I can't find a pic right now.

I guessing that someone at Porsche (pre-1973) thought of this upper duck solution at some point, but doing something not applicable to the coupe version wasn't going to fly marketing wise.

The Porsche 924 was introduced in 1976 for example, this rear spoiler might even fit - who knows?

https://www.porsche.com/usa/accessor...924/924-turbo/


As far as doing a double duck and staging the airflow in a series of steps or ladders Merkur XR4i Bi-Wing style, that's like trying to fix something that isn't broken in my opinion.

I'm exploring this alternate in part because I might be removing my rear Targa window to repair some rust likely hiding under the old rubber seals. Then replace the glass with lightweight scratch protectant coated Polycarbonate.

Old thread - somebody did Targa bubble in Polycarb (page 2)
http://forums.pelicanparts.com/porsc...rga-glass.html

[kach22i]

I retook the model pictures after removing the Ducktail and then played around with the "paint" program a bit to produce a marked-up composite.


And I remembered where I've seen this before, a Saab Turbo.

Sourced the original photos from here:
https://petrolicious.com/articles/th...f-swedens-eden

Tried my hand at creating a composite photo with mark-up.

https://petrolicious.com/articles/th...f-swedens-eden


Oh, and I have a name for it, UPPER-DUCK, not sure what it's really called.

[aerohead]

With flow separation 'over' the aft-body,the 911 generates lift.
The whale-tail provides a surface for flow re-attachment to spoil the lift,while incorporating an embedded heat exchanger.
If you look at the 1996 911 GT1 'silhouette' race car,you can see how Porsche corrected the too-steep backlight area to kill the rear lift,then double-downed with a spoiler for extra 210-mph,Mulsanne Straight stability.
Cd 0.28 vs 0.40 if I remember correctly.

[aerohead]

I would caution against the upper spoiler.I'd be concerned that you'd kill any downwash that is currently available to the engine bay inlet.

[Vman455]

Originally Posted by aerohead

If you look at the 1996 911 GT1 'silhouette' race car,you can see how Porsche corrected the too-steep backlight area to kill the rear lift,then double-downed with a spoiler for extra 210-mph,Mulsanne Straight stability.
Cd 0.28 vs 0.40 if I remember correctly.

In early 2000, Audi had to recall the TT and fit it with a rear spoiler to reduce lift after 5 people died in high-speed accidents.

[kach22i]

I found an image of a Porsche 924 without the rear spoiler while looking for Audi TT with and with-out rear spoiler.

https://rennlist.com/forums/924-931-...ictures-4.html


As Maxwell Smart would say.............."Missed it by that much".

I can see why the later 924's and 944 had the rear spoiler.

A much greater discrepancy on the older 911 design, but with a Ducktail it's a different story. I know it's not the favored "attached flow", but it's a step in the right direction.

Porsche Says These Are the Best Wings Porsche Has Ever Made
https://www.roadandtrack.com/car-cul...nd-rear-wings/

From ducktails to fiberglass to fold-out power, these are Porsche's five most interesting aerodynamic designs.

This Audi TT below is a few years later than the 2001 model, I think this retractable one allows air to sneak between it's bottom lip and the trunk lid. The result appears to be some smoke gets under there, perhaps a good thing, I'm not sure.

aerodynamics? - VW GTI Forum / VW Rabbit Forum / VW R32 Forum / VW Golf Forum - Golfmkv.com


The thing I worry about is water getting trapped in the joint of the proposed "Upper Duck". If vented or weeped at the bottom some flow similar to the Audi above could feed the cooling air intake vent. That's a pretty big "could be" at the moment.

Didn't someone in the forum build a scale model wind tunnel tester a few years ago?

Originally Posted by aerohead

I would caution against the upper spoiler.I'd be concerned that you'd kill any downwash that is currently available to the engine bay inlet.

It's a pretty strong fan, even the five bladed one I have. Discussions on replacing it with an electric fan were deemed unfeasible in the Pelican forum as the motor would be more powerful than a Prius electric motor. Updating my 5 bladed fan to a better cooling one would rob me of 5 hp, similar to lugging around an extra 50 lbs - no thanks.

My point is, as much as I've looked into the fan draw issue, I've never found a VW Beetle or Porsche 911 wind tunnel smoke study that included air being drawn into the engine bay. A great appointment for me, and even greater now that I'm pondering this proposal.

[freebeard]

IIRC the Audi Quattro Spider Concept introduce the DLO as a conceptual arch. The New Beetle and the TT coupe over-formalized it. Then they bowed to 'time's aero'.


http://www.topspeed.com/cars/audi/19...ure179268.html

Didn't someone in the forum build a scale model wind tunnel tester a few years ago?

If they did it wasn't as good as Graysgarage's inverted water tunnel.

This isn't a photo, but a conceptualization based on ??? It suggests the intake air is sucked off the top of the detached turbulence.



Possibly a solution like the Mercedes retractable spoiler/Gurney flap?

[kach22i]

Originally Posted by freebeard

IIRC the Audi Quattro Spider Concept.........

I see this working much in the same way as the original Honda Ridgeline.

Meaning the side arches flow the air past the best they can, and the air over the top steps from roof to deck in an unattached but non-uplifting way (a giant louver).

The deck or bed has to be just long enough, and the Aero-Template we use often in the forum has proven useful in showing us just where that is.

EDIT:
I found something interesting and related to what happens after a Gurney flap or wickerbill rear spoiler. I tend not to trust drawings and CFD as much as wind tunnel images but they have their uses.

http://www.firebirdnation.com/forums...of-a-trans-am/

The Trans Am was different. Pontiac's engineers as well as the design team put together a package that managed the air flow. The made the air work for the car. While the assortment of spoilers, spats, and shakers made a stunning visual impact, the fact that they were designed to work a well as they looked is a tribute to those involved.

Looks like there will be (-) negative air pressure, that is air moving away from the car body after the rear spoiler. Meaning my cooling fan will be fighting these forces, not working with them - I suspect.

The Trans-Am spoiler is a smaller version of the original Porsche Ducktail, the (+) plus signs indicate positive pressure, meaning an opening on top of the deck/trunk there would allow air in (downward). Hey, that's how these things create down-force, imagine that.