Ducktails

[tasdrouille]

On a sedan, you have to trace a tangent between the back of the roof and the rearmost part of the trunklid (including any kind of spoiler). If the angle of the imaginary line is in the vicinity of 13 degrees you have a winner. If the angle without spoiler/ducktail/extension is already 15 degrees or less, any appendage is probably superfluous. I do not believe anything significantly pointing up past 10 degrees could do any good for drag, except maybe just a little sharp "tripwire" bump (as they did on the volt) for the flow to leave cleanly as it tends to stick (there's a small section of reverse flow) a bit on rounded corners.

Big dave, my guess is that if the trunklid is long enough the flow will to reattach to it anyway, so that ducktail would not serve any purpose. On a short trunklid for which the angle is greater than 15 degrees and the flow would never reattach anyway, then it would be benifical (as for a hatchback, the volvo c30 with the downward pointing ducktail comes to mind).

Most drag reducing aero attachments are bandaids for poor design or stylistic constraints, when they even work.

If I remember right, Hucho reduced the Cd of the vw sirocco by 0.01 with a well designed lip spoiler.

[aerohead]

AMG did that on a fast Mercedes in combo with a deck wing spoiler.It was effective for both downforce and drag reduction.I think Peugeot has also done this.

[Christ]

This version of spoiler as shown on the back of the Honda Civic Hatchback (as posted by thebrad)

Is actually more commonly referred to as a "brow" spoiler... the lower counterpart to it is referred to as a "duckbill" Mugen makes them for those cars. (that's the stock one pictured, there is another one that goes just above the tails and wraps around the rear at the lower edge of the glass. (these cars have a tailgate, and a hatch window.. the metal part folds down, the glass goes up.))

They're essentially worthless for aero, they help to keep the car attached to the road... not so much to keep airflow attached to the car.

[red91sit]

Originally Posted by TELVM

What you call 'ducktail' is a trip spoiler. It's used to keep at bay aerodynamic lift at the rear of the car, which can be dangerous when cornering at high speeds if the stern loses grip.

The 'ducktail' trips the boundary layer prematurely, assuring no aerodynamic lift would be generated.

This can be critical at high speeds. The original Audi TT had an aerodynamically poorly designed very rounded tail that in itself generated lots of lift at high speed. Audi screwed big time on this. The first production models, without rear spoiler ...



... suffered some fatal crashes while cornering at high speed (180kmh/112mph +) in the Autobahne. Even a known race driver was killed. Audi was forced to recall and fit an inprovised spoiler ...



In the 2nd generation TT they solved the problem with a retractable spoiler, which deploys at high speed:




Unless you need them to corner at high speeds, these devices just increase drag (by enlarging the wake area) and fuel consumption. At 55mph they are perfectly useless.

This is one car that actually benefits from having a spoiler in several ways. It creates a cleaner break in the airflow (think kamback) . It is true, most vehicles have sharp enough of a break at the trunk lid to not require this, but if you do have a generous radius here, it may be something to look into.

When you look at other round cars ( think the bubble taurus, rx7, contour, and similar shaped vehicles) you'll notice they all have a very sharp trunk edge and/or lip spoiler. NOT to be confused with a wing, usually BAD.

So to try to answer the poster question, if your trunk doesn't look like this \|/ you could benefit from a little spoiler.

[trebuchet03]

Little upwards pointing lips allow for the styling appreciated by the masses while keeping round edges...

You may be interested in reading about the Coanda Effect
Coandă effect - Wikipedia, the free encyclopedia

Which explains why rounded trailing edges are bad.... By having a square or raised trailing edge, you eliminate local suction and effectively fix the separation point (what is loosely called "clean separation")

Why do we see lots of lip spoilers like this in modern design?
1. It looks good according to today's styling preferences
2. It shaves off a bit of cD
3. Low Cost (compared to say a complete undertray and such).

[lunarhighway]

i wonder if this Coanda Effect could be used to make boxy cars more aerodynamic. a lot of cars wich don't have a boat tail design (bad) have rounded edges at the sides as well (bad) but perhaps a vertical lip spoiler or even a thick strip could cause the air to separate cleanly just as it went beyond the bend so there's a clean separation, but not parallel to the body panels but angeled inwards as if there would have been a boattail

maybe that happenes natualally to a certain extend why round edges are still used, but i think it's got more to do with safety regulations and resign language than good aero

[some_other_dave]

One note: The original "ducktail" spoiler, as used on the early-70s Porsche 911s, did in fact decrease drag as well as decreasing lift. The later "whale tail" (think late-70s 911 Turbo) was better at both.

This might be due specifically to the rather unique shape of the 911, but evidently extending the effective height of the rear of the car and making the end a sharp corner rather than a gradual slope helped both to improve stability (particularly when combined with a front spoiler) and to reduce drag.

-soD

[aerohead]

Originally Posted by some_other_dave

One note: The original "ducktail" spoiler, as used on the early-70s Porsche 911s, did in fact decrease drag as well as decreasing lift. The later "whale tail" (think late-70s 911 Turbo) was better at both.

This might be due specifically to the rather unique shape of the 911, but evidently extending the effective height of the rear of the car and making the end a sharp corner rather than a gradual slope helped both to improve stability (particularly when combined with a front spoiler) and to reduce drag.

-soD

Hucho's book has complete tables showing lift/drag/velocity relationships for the 911 rear spoilers.

[carousel]

Originally Posted by some_other_dave

One note: The original magnefine "ducktail" spoiler, as used on the early-70s Porsche 911s, did in fact decrease drag as well as decreasing lift. The later "whale tail" (think late-70s 911 Turbo) was better at both.

This might be due specifically to the rather unique shape of the 911, but evidently extending the effective height of the rear of the car and making the end a sharp corner rather than a gradual slope helped both to improve stability (particularly when combined with a front spoiler) and to reduce drag.

-soD

Ducktails only cause drag for the most part. Unless you plan to go speeding/racing, this is not necessary.

[some_other_dave]

Good point that they only cause drag for the most part. The 911, though, was an exception. Porsche found that they had better high-speed stability and better top speed due to reduced drag, which made it a real "win-win".

The high-speed stability is not of interest to us (most cars are quite stable enough at sub-60 MPH speeds anyway), but the reduction of drag certainly is of interest.

Mostly I'm pointing out that there are a lot of factors that go into deciding if a spoiler would help or hurt fuel efficiency, and that in at least one case it helped. Even though chances are pretty decent that it will actually reduce FE on most cars.

-soD