major effect of the Ducktail is to reduce drag!.
 

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.

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.

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.

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

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 ...;-)

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.

Here we go again....:D

run a search on drag or lift ...

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)

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

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

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.

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)


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.

Lifting drag???
 

SmileWavy 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? :D

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 <b><i>Porsche 911 Story!</b></i> It is the only source of such factory information in print ...

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
http://forums.pelicanparts.com/uploa...1058619705.jpg
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.

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.

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

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.

HooBoy. The engineers are excited now. Hide the womenfolk.

Isn't it interesting that an air-grabbing device like a ducktail, and placed in the REAR of the car where the air has already bounced off the car anyway, can actually make the car more slippery. Better gas mileage, for instance.

According to legend, someone left the deck lid ajar on a 911 that was the subject of wind tunnel testing. The car suddenly had less drag. And less lift.

It disrupts eddies that would otherwise casue parastic drag -- like a Kamm tail.

Before experiments were run (1930s) people thought a tapering rear would be best as it matched the streamlines. But, not so.

I believe you need to add velocity to this discussion. A tapered car is the ideal shape for low velocity. Example: GM's Sunracer.

If one could accelerate a rain drop the shape would change. The rain drop shape is ideal for the rate of fall. When it would be stretched out of shape due to a higher velocity, it would need an apendage to compensate.

The issue with cars is that they are half a rain drop. New rules apply when the air can't flow around the shape equally. There may be two lifts, high pressure under the car and higher air velocity at the rear top.

While I agree that there is not necessarily a realationship between lift and drag, eventaually one has to deal with both.

I'm an amatuer here, I just didn't see anything about velocity in the theories presented.

i'm sort of afraid to post after all thathttp://www.pelicanparts.com/support/smileys/wat5.gif
Thanks guys.

:D
You beat me to the punch, I was going to say something about parasitic drag, but I wanted to check my book before I did, in case I was remembering wrong!

Being guilty of fueling this thread:
One point that has not come up yet is-
If you descide to put a turbo or Carrera tail, it is a general opinion that a rubber lip spoiler should be installed on the front valence to balance the car.
Also I believe that the Turbo and Carrera tails reduce lift but the major effect of the Ducktail is to reduce drag!.

As for my opinion I think all of the tails look good!

But a Porsche without any front spoiler does not quite look right (unless on a pre impact bumper Porsche!)

rgds`Ben
_______________
1971 911T Targa - twin plugged 2.7
2002 VW GTI 1.8T - daily driver

- Does that mean I get a prize ! :D
Especially as know one has effectively proved me wrong!
Rgds Ben

I think I'm the person orginally quoted at the beginning of this thread. The point I was trying to make in the previous thread was that the ducktail produced less drag than subsequent (Carrera, Turbo) tails. It didn't reduce lift quite as much as the later ones, but its virtue was that it didn't generate as much drag.

Thats not entirely true.

The slipperiest tail is body style dependent. (the early carrera tail is the slipperiest on the big bumbered cars . .per Paul Frere, anyway)

Yeah, but it's not the ducktail's fault that those ugly crash bumpers stick out so far. ;)





I disagree that the major effect of the ducktail was to reduce drag. I think that it is an equal effect with lift reduction.

In fact, drag reduction is just a very fortunate byproduct of the lift reduction. I gaurantee you that Porsche still would've used the ducktail even if it increased drag, due to it's considerable benefit to high speed handling and stability.

Well, when Paul Frere and I disagree...

...I think it's safe to go with Paul. ;)

Agreed, Ben, the effects at the front and rear of the car must be in balance... if you add a bigger wing with a rubber lip, you need to make a corresponding change to the front (rubber lip, bigger spoiler, rake or ride height, different vents, etc) so the mechanical systems of the car (suspensions, tires) are "squeezed" equally onto the road...

Any change to the aero aids will have a proportional change to both components of lift and drag... Like E. Lombard and Jack point out it's a balance between speed (less drag) and "squeeze" (more downforce equals more drag) for a given circuit or car setup.

For Porsche the advantage of the bigger wings (more downforce equals faster corner speed) outweighed the ducktails less drag and resulted in faster lap times...

Just out of curiosity, anyone know the drag coefficents for a '73 RS with a ducktail, and without?

I thought about the RS versus, say, an S but the rear flares of the RS would throw off the comparison.

Okay now its bugging me . ..not the bumper-snob comments, but the "there is not necessarily a realationship between lift and drag".

THERE IS ALWAYS A RELATIONSHIP BETWEEN LIFT AND DRAG .
(. ..unless of course you have one of those errr-lee cars that rusts .. er. .rests in the garage. :p ie: no velocity=no lift )

I'm not saying that if you have drag, you have lift. I'm saying, when you have lift, you have induced drag. . . .if you reduce the lift, you will reduce the component of induced drag.
The trick is to do this w/o inducing parasitic drag.

That is what (as commonly refered to as) rear spoiler will do for ya.

. . .are these aero threads the best, or what?:rolleyes:

I found it interesting that without a ducktail on my 65 911, I could only push about 90mph through the back turns (I think it's 8 and 9) at Willow Springs, and the back was skipping the whole way and I could not go all out. When I used a ducktail, I could go all out at about 105 and the rear was planted! It was not skipping at all and the car felt so solid. I could floor it the whole way around!

I was amazed what a small change in aerodynamics could do. It made a huge difference in my lap times.

It was great! Although, I am back to a non-ducktail appearance! I like the look without it better!

my "faster" 911 with the old wing:
http://forums.pelicanparts.com/uploa...1063079403.jpg

the current "slower" setup:

http://forums.pelicanparts.com/uploa...1063078831.jpg

Island: No. You need to go dig thru a fluid dynamics text. OTOH, you may be correct to the extent that in the limited universe of car body designs, all of them alter both lift and drag as a F() of each other - but this not the general result.

Zeke: 2 reasons why raindrops aren't good models for cars:
1. they aren't trying to stay on the road. i.e. they are in free fall.
2. they are not rigid bodies like a car or airplane. On that note, a colleague of mine at another univ. was studying how the fat under a dolphin's skin altered the drag (a lot it turned out) -- distributed layer boundary layer damping or some such title for one publ. He was getting a lot of USN bucks too. One year he didn;t show at a meeting. never saw him again. Apparently his work was _really_ interesting to the Navy & he went black (as we all know, you never go back). See how close you can get to a sub nowdays. If the divers doing port security don't kill you first, you will find that the skin is not rigid.

So are you trying to say I'm full of Bumper Snobbery? :D

Well, at least you and I agree on the aero, if not on the bumpers. :)

So, Randy, should we patent that idea for a onroad transportation ? Have a wetsuit like skin on cars and trucks with hydraulic fluid between two flexible membranes? Pressured by a pump and proportional to speed. Would the surface end up dimpled like a golf ball but rippley ?

Or is the effect only valuable at higher speeds (Salt Flats) or pressures (underwater) ?

Garrett - what a beautiful car - it looks great with and w/o the tail!

According to Frere's book, they added the duck tail solely to reduce lift. If I remember correctly, he didn't specify whether the wind tunnel test was on a narrow body or RS-flared car. Either way, the lift at the rear of the car without the tail was approx. 300+ lbs., wheras with the duck tail it was lowered to about 27+ lbs.

JSDSKI, There would be a wt. penalty -- I've always thought that bird feathers probably do something like that... They defintiely make a sort of flexible "shell" around the bird's body as wind velocity increases -- maybe growing feathers on a car would help.

But I will settle for someone making a suspension strut as strong asnd light as a bird's wing bone. Way down on my list of things to do is to find a SEM photo in a textbook on Ornithology that shows how the bone is designed and post it. YOu give evolution a 100 million years or so to work and ya got sumthin'.

"No" to what?
And sheesh, I need to dig thru a fluid dynamics text. . .are my basics that far off? Here I wa thinking I should dig thru all my aerodynamics books.

So where am I wrong in what I posted. . . what am I missing? Keep in mind, that explainations like my golf-ball example, I am tring to post in the spirit of Aerodynamics for Dummies :) (that was a good one, btw) . . .rather than throwing out big pedantic words like; parasitic drag, vortex shedding, etc.

People know(have first hand experience) that golf ball dimples can help the ball 'fly' faster, further and staighter (less drag) . .and yet this is counter intuitive, as people are told "smooth" is more 'aero.'

Similarly, the duck spoiler is a not-so smooth lump, that make the car 'fly' faster.

If the idea of zero drag is the topic we could discuss D'Alembert's "paradox" . . and get a whole bunch of virtual stares . .. or just pointing and laughinghttp://www.pelicanparts.com/support/smileys/eek3.gif