Chin/Front Spoiler first, then....

[Swoboda]

I have read with great interest the discussions here about chin/front spoilers and tails.

Aesthetics aside, it's clear there's a reason for having nothing (no front, no rear) or all (a front with a rear)--and little room in between.

That all sounds nice until we have to talk about budget....

I'd like a black chin/front spoiler and a ducktail on my 1985 Targa. At present, it has no front and nothing on the back end. I don't have the funds to jump all in and do both at the same time.

Can I start with the chin spoiler and add the ducktail later?

If I start my project with a chin/front spoiler and do the ducktail some time in the future, am I really putting myself and my car in jeopardy?

I'm a casual driver with an occasional three digit run on the highway to make sure the pipes stay clear. No racing or DE (yet).

Thanks for your thoughts.

Lawrence

[5:04]

The black magic of aerodynamics is a fun one and it's important to take most of what you see online lightly. You can get many answers for one question and all can be right and wrong. This is mainly due to how difficult it is to measure and how expensive the tools that do are. Most of it is an educated guess and check on the track. It's one of my personal favorite parts of tuning a car. Aero is something that has to be looked at like a package just like any other part of your car. Like building an engine you wouldn't go through and change just your pistons and not touch anything else. On a street car this it is much less important however.

To answer your question, No. You won't have to worry about flying off of the road if you add a chin spoiler to your car without a duck/whale tail. If you were on track you'd experience a bit of oversteer with the front spoiler. Would I add just one? No.

[moneymanager]

None of this matters much below 75mph or so. At 130, having one and not the other isn't so much fun.

[TT Oversteer]

Your car will be noticably more stable over 80 with front and rear spoilers working together. If you must do them one at a time start with the front first. Adding a rear spoiler without a front one will make your already light front end lighter at speed.

[RSWiser]

I have a front spoiler, not sure I have seen a rear spoiler. Can someone post a picture.

[fastfredracing]

When I bought my SC it had a turbo tea tray tail on the rear, and no front spoiler. I really did not think the car handled that bad, and this car regularly saw speeds well above the legal limit. I did finally add the front lip, and did notice the car seemed a little bit more planted at speed. But if I was going to do both, I would add the front first, unless you are not planning on any high speed jaunts , then I dont think it will really matter.

[DARISC]

Originally Posted by RSWiser

I have a front spoiler, not sure I have seen a rear spoiler. Can someone post a picture.

Whale tails & ducks are spoilers. Neither are wings and neither produce downforce; they just spoil the smooth airflow over the airfoil-shaped roof, thus reducing lift.

Wings aren't spoilers. They're mounted upside down compared to an airplane wing. Like airplane wings, they produce lift, but the lift is upside down also and is called downforce.

Do a search for Jack Olsen's fascinating thread wherein he documents his experiments with a wing on his 911.

[RoninLB]

I installed Carrera front and C2 electric rear spoilers.

Best off track entertainment is with late night commuters between 9-11pm doing 70-85 on a 20 mile twisty parkway designed to be toured at 45 mph that now has a 55 limit. Two parkways run parallel about 20 miles from each other. One's a 6 lane and the other a 4 lane.

I never raise rear C2 unless it's very windy and the front only is noticeably better than nothing up front. I imagine that with a stock suspension the improvement would be even better.

[Wil Ferch]

(Oh man....here we go again....)

Quote...." Whale tails & ducks are spoilers. Neither are wings and neither produce downforce...""

Ahhh.....yes, these tails and ducktails....these "devices" themselves ....indeed produce downforce as installed. The question is "how much?". Not enough to result in a "net" overall downforce on the rear of the body at substantial speed. So in one sense.... true enough,.... they "reduce lift" ( net effect on the rear of the body)....but the spoilers themselves in the installed location produce *some* amount of downforce.

[DARISC]

Originally Posted by Wil Ferch

(Oh man....here we go again....)

Quote...." Whale tails & ducks are spoilers. Neither are wings and neither produce downforce...""

Ahhh.....yes, these tails and ducktails....these "devices" themselves ....indeed produce downforce as installed. The question is "how much?". Not enough to result in a "net" overall downforce on the rear of the body at substantial speed. So in one sense.... true enough,.... they "reduce lift" ( net effect on the rear of the body)....but the spoilers themselves in the installed location produce *some* amount of downforce.

True. And a rectangular slab mounted like a wing would also produce "some" downforce also, no? (the air over the roof slamming into it and forcing the rear end down - just as it does when it slams into a whale tail or a duck) And it would, to "some" degree, "spoil" the airflow as well. But it would not be functioning as a wing - because, it ain't a wing, right?

As I understand it, the intended function of whale tails and ducks is to disturb (spoil) the smooth flow of air over the airfoil-shaped roof line, the goal being to reduce the lift that would otherwise be present because the air flowing over the top is at a lower pressure than the air just above it (air pressure decreases with increase of velocity), which creates lift, no?

And, as I understand it, the intended function of a wing (airfoil) is to produce lift in the opposite direction of the lift created by the air-foil shape of the top. If the wing produces lift that equals the lift created by the roof, there is no signicant downforce.

There is only downforce when the wing produces more lift than the opposite lift that the top produces.

Aerodynamics is indeed complex (I know nothing about it), but this is very basic aerodynamics, basic physics really. No?

Could be that I'm missing something, but if I modify my statement to read, "Whale tails & ducks are spoilers. Neither are wings and neither produce significant downforce.", would we be in agreement then?

[Flieger]

I think he just meant that the increase in static pressure on the ducktail do to slowing the flow makes downforce but there is still net lift from the rest of the car's surfaces. The ducktail will also spoil the air and detach the flow over the sloping rear end, decreasing the lift and possibly the drag (at least skin friction drag).

Splitters are free downforce. Very little drag. They work by putting the pre-existing area of high static pressure in front of the car (air dam effet) to use. they just put a horizontal area below the pressure for it to push down on.

[RSTarga]

You didn't say what car you had, but Porsche only had a front spoiler until the ducktail came on the carrera so you'll be ok. The ducktail does reduce rear end lift at speed and make the car less sensitive to crosswinds. It makes it feel more secure at high speeds.
Paul Frere did a great job in covering all the different effects of spoilers and tails in his book. The spoilers front and rear also reduce drag in varying degrees.

[Flieger]

See my post #4 here: http://www.early911sregistry.org/forums/showthread.php?61489-Aero-advantage-of-early-S-bumper-over-standard&highlight=front+spoiler

[Flieger]

[DARISC]

Originally Posted by Flieger

I think he just meant that the increase in static pressure on the ducktail due to slowing the flow makes downforce but there is still net lift from the rest of the car's surfaces.

That seems to me to be consistent with my layman's way of putting it; slamming that air into the ductail surely slows it down.

Originally Posted by Flieger

...The ducktail will also spoil the air and detach the flow over the sloping rear end, decreasing the lift and possibly the drag (at least skin friction drag).

True, but you say also, whereas it's my understanding that spoiling the airflow is the primary job of the tail. No?

Originally Posted by Flieger

...Splitters are free downforce. Very little drag. They work by putting the pre-existing area of high static pressure in front of the car (air dam effet) to use. they just put a horizontal area below the pressure for it to push down on.

That enlightens me, re splitters, which I never really understood.

I know that dams function to reduce airflow beneath the vehicle, which reduces lift. What I can't seem to intuit is exactly how that works. On the one hand it seems self evident to me that a large volume of air flowing beneath the vehicle would tend to destabilize it more than a smaller volume. On the other hand I wonder why a larger volume of fast moving air wouldn't be more beneficial than a smaller volume, because the pressure of the larger volume would be less than the smaller volume, producing more downforce.

I'm thinking that it doesn't because that airflow isn't directly creating a differential between the air directly above it, because, unlike airflow over the top, airflow beneath the car is seperated from air directly above it by the car itself.

So then my thinking starts to get wonky and I wonder, maybe the air under the car gits all bunched up an' pushes da car up wid it? Can you enlighten me on this?

Edit: you posted while I was typing. Am about to read it. If I study that info, will it answer my above questions?

[Flieger]

The thing with keeping air from flowing under the car is mostly just that if the car pitches up a little bit, there will be significant pressure on the underside and the car wants to be blown over, for lack of a better description. The car is unstable that way- like firing an arrow backwards.

If you have a diffuser like an F1 car or 956, that is a whole other matter.

Aerodynamics gets much more complex when you move away from free stream flow and have the road surface (stationary) and the car bottom (moving) involved, then add pitch and yaw angles, crosswinds, etc. If you can accelerate the flow under the car, the pressure drops. You can do this by adding exhaust energy into a diffuser or using a fan to suck the air out. The diffuser shape is just the other side of the proper airfoil shape for generating downforce. The top would be flat and the bottom curved up.

[DARISC]

The two charts pertain to reduction of lift. Am I confusing myself from distinguishing lift from downforce?

[Flieger]

Oh, a small spoiler is mainly there to spoil the airflow- detach flow to lessen drag and the lift effects of the downstream surfaces. If you had a small Guerney flap at the front/top of the rear window, it may do the same thing. The ducktail being so tall also adds more of a downforce element by piling up air. The Guerney does this to when placed on the trailing edge of a wing. It is inefficient in adding downforce to a wing as far as the drag penalty goes.

The ducktail also gets more air to the engine cooling intake, so that is one reason it is in the place that it is.

[Flieger]

Originally Posted by DARISC

The two charts pertain to reduction of lift. Am I confusing myself from distinguishing lift from downforce?

Reduction in lift = more downforce

911s did not get downforce until things like Cup Car wings and GT2 tails and the banana wings favored by racers.

[DARISC]

Originally Posted by Flieger

The thing with keeping air from flowing under the car is mostly just that if the car pitches up a little bit, there will be significant pressure on the underside and the car wants to be blown over, for lack of a better description. The car is unstable that way- like firing an arrow backwards.

If you have a diffuser like an F1 car or 956, that is a whole other matter.

AH HA! So the air does bunch up under there an' cause problemas.

How 'bout a layman's explanation on how diffusers work? My understanding is that they utilize the Venturi effect whereby the exiting air is accelerated, producing a low pressure area at the rear, i.e., downforce. Am I close?