major effect of the Ducktail is to reduce drag!.

[Mike Kast]

major effect of the Ducktail is to reduce drag!.
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This was from the other tail thread

Could anyone explain this please? What exactly is drag? A duck tail won't keep the car planted?
If I ever do a tail the duck is the only one I could see doing regardless.

[Ho Hum 74]

If my memory serves me correctly. There is a fundamental law of aerodynamics that says: Whenever you produce lift you produce drag as well. This is called "lift induced drag". There are three major drags: Frontal area, surface friction and lift induced drag. If the ducktail is producing downforce (lift) then it is producing drag as well.

[ruf-porsche]

There are various definition for Drag

1. That when a man put on women clothes and and pretends to be a women!!!!!

2. That when you're at a stop light with the p-car and the guy next to you look over the car and rev his engine a couple of time and you reply by reving your engine and as soon as the light turns green you mash the accelerator and pop the clutch so that you can out "DRAG" him to the next intersection.

As for the Duck tail, I think that was a hair style that the Greaser Kids use to wear in the late fifties, early sixties.

Hey Mike couldn't help myself with these replies.

[Mike Kast]

No! Mr. Ruff! That was very informative... and greatly appreciated

[silverc4s]

The design purpose of the ducktail on the early cars was to reduce LIFT at the rear of the car at speed. I would suppose that any device that reduces the lift would have a positive drag component to it.
However, the overall drag at the rear of the body could be reduced by the addition of the tail if the resulting shape has less overall drag than the body without the tail. Interesting ...;-)

[silverc4s]

After some reflection, I recall that the high speed lift at the rear on the std body was caused by high pressure buildup UNDER the rear of the body. The duck tail deflected the airflow upward sufficient to reduce the high pressure / lifting force under the rear by some hundreds of pounds. This would certainly be a reduction in DRAG as well, I believe.

[MotoSook]

Here we go again....

run a search on drag or lift ...

[island911]

A spoiler spoils the lift. (on the back end)
Less lift = Less Drag
(except, of course, for short gay men . . they usually wear heels to gain lift with their drag)

[nostatic]

Originally posted by island911

(except, of course, for short gay men . . they usually wear heels to gain lift with their drag)

ummm....island....you seem to know a bit *too* much about this particular topic.

[Elombard]

I think you are wrong Island on the lift /drag thing, I believe you have it backwards. If you look at the indy cars and the "super speedway" trim they run less wing so they will have less drag and higher speeds. In F1 on a tight course they run more wing for more down force around the corners but they sacrifice high speed because of the higher drag.

That said the Duck Tail may cause less Drag because of Air flow (as mentioned above) but if my examples are correct and you assume the Duck tail causes an increase in down force, then it should cause more drag. Aerodynamics is a black art and either statement could be true. I have not read anything about the duck tail to help either way.

Cheers

[Kurt B]

from www.howstuffworks.com
[/i]
When you sum up all the pressures acting on the wing (all the way around), you end up with a net force on the wing. A portion of this lift goes into lifting the wing (lift component), and the rest goes into slowing the wing down (drag component). As the amount of airflow turned by a given wing is increased, the speed and pressure differences between the top and bottom surfaces become more pronounced, and this increases the lift. There are many ways to increase the lift of a wing, such as increasing the angle of attack or increasing the speed of the airflow. These methods and others are discussed in more detail later in this article. [/i]

Reducing drag has nothing to do with lift by definition. If the ductail's purpose was to reduce drag, then it would simply make the car move through the atmosphere at higher speeds for the same work.

[island911]

Originally posted by Elombard
I think you are wrong Island on the lift /drag thing, I believe you have it backwards. . . .

Geeze, you know I may have my share of problems, but being wrong doesn't seem to be one of them.

You see a rear spoiler "spoils" the lift; rather than a rear (upside-down) wing, which produces down-force.

A real spoiler is, in a way, like dimples on a golf-ball. By messing-up, an otherwise nice and tidy, low-pressure air pocket, the amount of drag (and lift) are diminished.

quote:
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Originally posted by island911

(except, of course, for short gay men . . they usually wear heels to gain lift with their drag)

Originally posted by nostatic
ummm....island....you seem to know a bit *too* much about this particular topic.

I've met Island and I can say with absolute certainty that he is not short.....................


Oh yeah, he also has a beautiful wife....

And another thing, ducks rule.

[Groesbeck Hurricane]

But Island, if I am reading your examples correctly, this would not work for men with high heels. They would have already increased lift and drag with the same function?

[Early_S_Man]

If you want the definitive engineering information on ducktail and other aerodynamic issues over the lifetime of the 911, including factory wind tunnel tests ... get Paul Frere's Porsche 911 Story! It is the only source of such factory information in print ...

[Bill Verburg]

Originally posted by island911
A spoiler spoils the lift. (on the back end)
Less lift = Less Drag
(except, of course, for short gay men . . they usually wear heels to gain lift with their drag)

Island is closest to the truth, there are only minor nomenclature or usages that I would quibble w/. They can easily be interpreted differently, unfortunately we are all stuck w/ a set of common usages which are not quite correct in the strictly technical sense.

there are 3 different aero aids that are often confused w/ each other.

• spoiler - decreases lift and increases drag, only has flow on 1 side
• wing - decreases lift and increases drag, has flow on 2 sides
• appendage - decrease lift, does not increase drag, and can reduce drag

The aero aids on the early 911, like the ducktail, are more properly called appendages.

Another good source of aero info besides Frere is Dr. Robert White's article in the Nov '77 issue of Pano. He is an engineer that worked at Weissach, and was often a presenter at various PCA tech sessions. A very interesting guy.

this is one of the graphs he presented

it was from the original research at Weissach where it was discovered by accident that as the lid is raised both lift and drag are reduced.

[Randy Webb]

Ok, drag is the force vector that points backwards rel. to the movement of a body thru a fluid and is caused _by_ the rel. movement of body and fluid.

There is no _necessary_ relation between lift and drag. It depends on the particular shapes involved, surface roughness and many other factors. I will leave it at that. If anybody wants a list of readings on fluid dynamics, let me know and I'll work one up. Unfortunately, the DUmmies Guide people have not seen fit to publish one, despite the great need. There are some very interesting examples tho.

[oldE]

Aerodynamic drag results when air passing over an object (or being displaced by an object passing through it), cannot flow smoothly over the surfaces and creates turbulance. It takes energy to create turbulance. That energy load comes from the available power. If you can reduce turbulance, you can put that power to better use, either top speed or downforce.(sometimes both!)
Porsche, and other, engineers have approached this problem in a variety of ways.
At one time the idea was to create an envelope body over the vehicle, smoothing the airflow as much as possible. This is good for top speed and has been demonstrated to produce undesired lift (think long tail 908s and early 917s) or managed downforce as in modern LeMans cars. That lovely shape of the rear half of our 911s invites the air passing over the car to drop down to join those parts of the airflow passing over the sides (more turbulent) or underneath (very dirty). Pulling that air down creates lift and some drag.
It has been demonstrated you can reduce the effects of drag by acheiving separation of the air flow by using a device such as a duck tail a spoiler or a Kamm tail. As mentioned above, Paul Frere quotes research figures gained from testing. The duck tail added to a Carrera RS with a standard air dam reduced the drag co-efficient from .409 to .397 (a small amount to be sure) but had the more noteworthy effect of reducing lift from320 lbs to 93 lbs at 152mph. (Quoted from Porsche 911 story, p 227)
Les

[Yargk]

It seems to me that the ducktail acts in a similar way to those very small "lip" spoilers (or appendages by Bill's definition) on the back of sports sedans. If I recall correctly this would be like the back of an audi RS6 sedan versus a regular A6. The point is that a rounded down turned edge like a 911's rear and a rounded trunk of a sedan produce both lift and drag. A ducktail or lip appendage will reduce both lift and drag. Why does the rear end of a corvette just stop? That car looks like a wedge with a flat rear for a reason. Check out the rear of the honda insight too.

Maybe I'll get back to you guys after I've taken a course in fluid dynamics. Although it seems we already have some people on the board who are more qualified than I will ever be.