Poor Man's Early Car Aero Idea

Jack Olsen · 05-17-2006, 01:36 PM

At the track, I run a big 3.8RS rear decklid with extensions and a 50-odd inch wing. It does a great job at reducing my car's high-speed lift, but I'll make no secret of the fact that I don't like the way it looks.

So, I'd like to replace it with something else. The problem is, none of the pre-1989 tails can match its capabilities.

Oddly, modern 996's and 997's have adopted something close to the ducktail in back, and the newer track models add a simple extension and wing. I know the new cars have totally different aerodynamic footprints, but seeing the recent cars has got me thinking about adding something similar to a ducktail.

Here's what I mean about the newer cars:

Now, my gut reaction to the new configuration would be that the lower element would seem to put a higher-pressure zone right where the upper element is relying on lower pressure -- i.e., the 'ducktail' is pushing air up toward the low-pressure underside of the elevated wing. Obviously, Porsche wouldn't be doing this if my gut-driven analysis were correct. Can anyone fill me in on what the air is doing with the two elements on the 996/997 cars?

And also, does anyone have input on the viability of adding a wing in a similar way to an early car?

Imagine a set of curved uprights and an elevated wing being added to a silhouette like this:

Would I want the wing forward of the ducktail, or behind it, or right above it?

I know I'll be giving up some of the cooling advantages of the 3.8RS decklid, and I know it still won't look period-correct. But at least it won't be as huge.

I assume the higher the wing is elevated, the better (for the sake of clean airflow) -- but then, my assumptions are based on precious little real knowledge.

Any input?

kach22i · kach22i's Garage

I think the 996/997's act as a true "fastbacks" with a receeding incline roof rake angle of 20 degrees. This allows the air to cling to the rear glass and not break off or roll off in turbulance.

To catch the clean airflow you will have to ride higher (postion the wing higher) than the 996/997 models, an ugly fact of aerodynamics.

Far be it for me to preach to you, sounds like you have something which works called "engineering". If you want something that "looks" you will have "marketing".

Tyson Schmidt · 05-17-2006, 01:46 PM

I'd like to know the answer to this also.

Unfortunately, the people who really know, aren't likely to want to share this kind of info, and to really know for sure requires expensive wind-tunnel testing.

Personally, I love to see you guinea pig this for us. I like the idea of less weight, and ease of removal. You could attach captivated nuts on the inside so that it would be simple to remove before the drive home from the track.

It'd also be a good reason to ditch your current wing, since it has that irregularity that is no doubt hurting it's effectiveness.

Bill Verburg · Bill Verburg's Garage

There's a venturi(sometimes called a slot effect) between the 2 foils. Any sailor is very familiar w/ this effect. It can be quite powerful.

My solution for track only was a 993TS biplane wing, you get a lot of the effect w/ not too much visual damage

oldE · 05-17-2006, 01:58 PM

Jack,

I think the ducktail would be too disruptive to the airflow of a wing mounted over it. In my understanding, when two wing elements are operating in close proximity, they achieve an effect greater than the sum of the two. (I suppose a parallel would be overlapping jibs on a sailboat.) The 'duck' tumbles a lot of air over its trailing edge.

Having said that, I would be inclined to try mounting your upper wing element lower and closer to your lower element, (temporary, adjustable brackets) to see if there's a 'sweet spot' to be had with the right arrangement of the two.

Les

304065 · 05-17-2006, 02:04 PM

Forget everything you know about bernoulli lift: you could stick an ironing board up there with a negative angle of attack and generate lift (in the direction of the track, let's not go there again!).

There's nothing magic about the 3.8 wing with the endplates BTW, half the time the untold story is it was designed to meet German TUV regulations with regard to pedestrian safety or something. So why not fab a pair of endplates and screw them to the sides of your duck? It will require some extensions because the outer edges of the ducktail are not parallel to airflow, but something light that won't break could be fabbed that would easily extend down past the edges of the lid into the engine compartment for additional rigidity would work.

One thing is certain: you want the wing as high and in as clear air as possible to make it work. If the wing is in dirty air, such as that coming off the upper edge of the roof where the flow separates, it's not going to be as efficient. Therefore, make the wing supports very, very tall. The further aft you mount it, the greater mechanical advantage you will have and greater likelihood of keeping it out of the crap coming off the roof. Maybe put some vortex generators on the roof to keep the flow adhered as long as possible and to keep your OTC competition guessing.

Porsche Doc · Porsche Doc's Garage

Hey Jack,
Hank at GT Racing has a Crawford look set up for the early cars.
It is part # 017.
I think it would do nicely.

Check it out.
http://www.gt-racing.com/catalog/default.htm

cegerer · cegerer's Garage

I know where there's a slightly used wing in the Burbank area

It'll bolt right on and it seemed to work fairly well for Brumos.

Bill Verburg · Bill Verburg's Garage

Quote:

One thing is certain: you want the wing as high and in as clear air as possible to make it work. If the wing is in dirty air, such as that coming off the upper edge of the roof where the flow separates, it's not going to be as efficient. Therefore, make the wing supports very, very tall. The further aft you mount it, the greater mechanical advantage you will have and greater likelihood of keeping it out of the crap coming off the roof.

All true, but doesn't diminish the visual impact of the thing

defcon65 · 05-17-2006, 02:34 PM

I'm no aerodynamic engineer but I can recall reading how scale mock-ups can be used to produce approximate full-size results. Maybe a mini-wind tunnel and a close-to scale model of your car could be set up to give you some idea of what the airflow is doing when it hits the tail and wing? At the very least, it could be a really cool science fair experiment for some Pelican and their youngster. Just an idea...

Joe

Jack Olsen · 05-17-2006, 03:10 PM

Okay, it seems clear that the dirty aero characteristics of the old-style back end and the ducktail means the idea of making the two components work together wouldn't pay off.

But I like the simplicity of the idea, and there's nothing stopping the two pieces from working individually, right?

I'll grant that my engineering and testing capability is vastly inferior to Porsche's, or most guys with a 10th-grade science education. I don't have a wind tunnel or any engineers lying around here at the house. But I think I do have a way I could test this. I have a data logger with a number of unused inputs on it. If I could source a pair of cheap rotary-style potentiometers with 45 degrees or less of range, I could wire up ride height sensors front and rear and test different wing heights (and angles) at speed until I found a kind of trial-and-error 'sweet spot.' The logger could clean up the data from, say, repeated tests on a one-mile stretch of smooth freeway on a windless day at a set speed (the higher the better, points on my driving record notwithstanding).

It seems like uprights alongside the ducktail could only help its function, by preventing some of the airflow from rolling off the sides. The question is where the best location would be, fore to aft. I could fab up uprights with multiple height options for the testing.

Am I way off to start with something like this, moving the wing up and down in steps for testing?

304065 · 05-17-2006, 03:17 PM

I like that wing on Hank's site, #17, that's probably what you are looking for, Jack. Those aerodynamicists among us may want to take a grain of salt with the claim that the "predator" wing at 0 Degrees AOA produces lift. Well, ALMOST zero anyway.

Eric P · 05-17-2006, 03:29 PM

I thought the ducktail only reduced LIFT on the rear (by spoiling the speed of the air over the backside of the car). I think I've seen smoke trail pics of a ducktail 911 and no air went UP from the ducktail (up "into" where you want to put the wing) so your proposed idea would be fine (unless the wing was like 1" from the uppermost part of the ducktail). The wing would then be left to produce downforce.

I wish I remembered where I've seen those pics...

Walter_Middie · Walter_Middie's Garage

Jack,

I am an aerodynamisist with over 30 years of wind tunnel experience - working on wings, not cars. I can tell you what I think, not necessarily what I know. You have been given some good info here by the people that have responded. Lets start with the basics. The duck tail creates downforce by separating the airflow from the rear window. This creates a high pressure area (downforce) and drag. Mounting a wing above this separated flow makes no sense (aerodynamically). You want to create clean flow over the back of the car, and mount a wing in this clean flow. You can tape some strips of knitting wool to the rear window to check how well the airflow is staying attached back there. Maybe you don't have to mount your wing as high as you have it now, but I would definitely keep with a wing for low drag.

The idea of two wings mounted closely together has been around since the Wright Brothers - but you don't see airplanes built that way anymore. That's because it creates a lot of drag. The new wings from Porsche may (as John_Cramer points out) be designed for something other than just aerodynamics in mind. I work with a guy that just transfered from one of the major US car manufactures. He transfered because he got tired of aerodynamics always taking a back seat to aesthetic design. I will ask him to take a look at your car a suggest some things.

Take a look at the wings used in F1 - those guys do a lot of wind tunnel testing. Obviously the cars are much different, but the aerodynamics of the rear wings are the same. I know they are pretty radical, but if there is something in F1 that fits what you are looking for, maybe there is a way to aesthetically incorporate it into your cars design.

DARISC · 05-17-2006, 03:45 PM

As I understand it, the duck tail is a "spoiler" which spoils the smoothness of the air flow over the top of the 911's airfoil profile, thus reducing lift - however, it's not creating any downforce.

The wing, (an upside down airplane wing, as it were) doesn't want spoiled, but rather smooth air running under it, as spoiled air flow would reduce lift (downforce).

It seems then, in that sense, that a duck & wing are operating more contra to one another the closer in proximity they become.

Take a look at some of the old Chapparals of Jim Hall that had super high wings mounted at the extreme rear, and no air spoiler on the rear deck.

I love your car, Jack.

Maybe to go fastest, sometimes ya gotta look a little silly?

cegerer · cegerer's Garage

Did someone say 'leverage'?

Jack Olsen · 05-17-2006, 04:51 PM

Thanks, everyone. The more input the better, and I'll be moving slowly on this because of other obligations. But I think a way of testing effects on ride height is essential. I've got different tails, underbody pieces, side skirts and front end pieces cluttering my garage, and it would be great to be able to quantify the effects of each piece (if for no other reason, to free up garage space).

Rex, I'd appreciate any and all input from you or anyone you know who's been involved in this sort of thing professionally. This is one of the things that makes this board so great.

I'm going to start a separate thread on the ride height sensor and try to get the last piece of that project in place as a first step.

I like the GT Racing piece, although I'm not sure the length of their extension pieces makes sense from an aerodynamic point of view. They might make them that way only for the sake of resembling the 996 and 997 cars.

Ideally, I'd like to do a wing and extensions that can bolt off easily, as Tyson describes. Changing decklids is still doable, but it's a step I'd be happy to eliminate from my track day routine.

atlporsche · 05-17-2006, 05:03 PM

IMO, keep the duck, and try roof mounted shark fins like on an EVO MR or what's aftermarketly available for a Subaru STI. they work. by disturbing the airflow as it separates when it comes off the roof, more is directed down the rear of the car. I think that's why the IROC wings (as well as most other porsche wings are really just huge platters catching the air as it tries to reattach off the rear of the car.

sjd

Jack Olsen · 05-17-2006, 05:23 PM

Quote:

Originally posted by cegerer
Did someone say 'leverage'?

Now there's a guy after my own heart.

randywebb · 05-17-2006, 05:28 PM

As you do it, I think you want to do some attempts at tracing the flows -- you want to find out where the streamlines are, and where the eddies are. Ideally, you'd want to know pressures and gradients (how close streamlines lie to each other), but I don't see that happening.

How serious are you? Want to build a water tunnel and make some scale models?