major effect of the Ducktail is to reduce drag!.

[Superman]

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.

[Randy Webb]

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.

[Zeke]

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.

[Mike Kast]

i'm sort of afraid to post after all that
Thanks guys.

[Yargk]

Originally posted by Randy Webb
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.

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!

[twin plugged targa]

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 !
Especially as know one has effectively proved me wrong!
Rgds Ben

[Jack Olsen]

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.

[island911]

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)

[Tyson Schmidt]

Originally posted by island911
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.

[Jack Olsen]

Well, when Paul Frere and I disagree...

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

[JSDSKI]

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...

[Tyson Schmidt]

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.

[island911]

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. 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?

[Garrett]

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:


the current "slower" setup:

[Randy Webb]

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.

[Tyson Schmidt]

Originally posted by island911
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. 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?

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

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

[JSDSKI]

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!

[}{arlequin]

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.

[Randy Webb]

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'.

[island911]

Originally posted by Randy Webb
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.
. . .

"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 laughing