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aerodynamics and airflow

Tonight I got to thinking about the aerodynamics of this old 69 porsche. So I got the wifey to blow air via the comprosser over the top line of the car to see where what would happen. I told her to hold the air in front of the roof line at root line level.

I stood at the rear of the car and tried to find the airstream. At roof line level, there was absolutely no airflow. Instead, the air flowed along the roof and engine hood of the car and the air ended up at the bumper. Not surprising since the profile of the porsche is simlar to a wing of an airplane and we all know that that's how air flows over curved objects, old school knowledge.

It's surprising how efficient the air blew over this old car. This is a greatly designed car for the age.

Anyway, I'd like to ask you experts simple questions because I'm not in the that engineering field. At speed, does the air push down on the rear of the car? Does it force it's way into the engine grill? The aircooled engines required the engine bay be sealed so that the fan could suck the air through the engine bay, but I can't imagine that that little fan could pull enough air without some kind of help from the physics of the airflow.

Also, the air behind the bumper creates a sucking effect, correct? I would assume that on the racetrack, a porsche would actually have the advantage over other cars (such as a Toyota Scion Xb, damn box!) cause it would have a very small airstream since the air blow down and over rather than pulling a larger air pocket.

Any information about airflow around, over, through engine, is appreciated. Invoking expert knowledge and experience.
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Old 10-14-2004, 06:40 PM
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I'll give a partial answer as it is beddy-bye time -

The sloping rear of a 911 actually kinds stinks, aero-wise.

The slope falls away, rather than going in a straight line, causing significant lift from behind the engine grill area/on back (partially cured by the assorted tail bandaids)

ZzzzZz goodnight
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Old 10-14-2004, 07:09 PM
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Quote:
Originally posted by Craig911
The slope falls away, rather than going in a straight line, causing significant lift from behind the engine grill area/on back (partially cured by the assorted tail bandaids)
What does that mean? What is the best way to get good airflow thru the grill and into the engine bay?
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Last edited by ishmeal; 10-14-2004 at 07:34 PM..
Old 10-14-2004, 07:14 PM
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When Porsche tested the aerodynamics of the 911 in the 70s they discovered that at ~140mph the car generated about 450lbs of lift. When the front spoiler was added with a whale tail (Early) the total lift at ~140mph was about 35lbs. The stability of a 911 with spoilers is dramatic compared to without. The spoilers also cause the total drag of the car flowing through the air to be lower.
One of the added benefits of using a tail is that it turns the air intake part of the engine lid from a low pressure zone to a high pressure zone. This causes more air to be forced into the engine compartment. I always notice that my car runs cooler with the tail than without. Check out the "porsche 911 Story" by Paul Frere for more info.

THe bottom line was that the shape of the 911 was really not a great aerodynamic design without aerodynamic aids.
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Old 10-14-2004, 07:42 PM
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Does air naturally flow into the grill via the design or does air flow into the grill because of the sealed engine in combination with the sucking fan?

Will a tail force more air into the grill?
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Old 10-14-2004, 07:45 PM
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The 911 was a good design for getting a low Cd for its day. PAG neglected lift and focused only on drag. They know more now.

There are tons of posts on this - do a search.
Old 10-14-2004, 09:10 PM
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air flows into the grill because of the sucking of the fan, otherwise at speed the back of the car will be a low pressure area affectively sucking air away from that section of the car and trying to lift the back end off of the ground.

The shape of a 911 is very similar to a wing. What happens to a wing when it goes fast enough, it lifts off, right. That's bad for a car. To make it worse the engine fan blows air out the bottom of the engine making it even worse.

Adding a tail and front lip improves lots of things as noel said. It reduces the lift to near 0, it reduces the drag, and it helps force air into the engine compartment.

You may also want to read these threads, or maybe not

major effect of the Ducktail is to reduce drag!.
911 rake front/rear
Downforce
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Old 10-14-2004, 09:24 PM
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Quote:
Originally posted by ishmeal
Does air naturally flow into the grill via the design or does air flow into the grill because of the sealed engine in combination with the sucking fan?

Will a tail force more air into the grill?
"Will a tail force more air into the grill?" I think that's what Noel said.

The first part is tough to answer w/o stating the perameters. But since you indicate a running engine installed correctly with seal, then, yes, all three factors contribute.

I say this because you could conceivably do a wind tunnel test w/o the engine running and get different results.
Old 10-14-2004, 09:26 PM
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ishmeal,

Where to begin? Perhaps with the thoughts leading to your "wind tunnel" experiment: As you rightly point out, the shape of our 911es, viewed from the side, is strikingly like the cord of an airplane wing. Herein lies the first problem - like the airplane wing, our Porsches generate aerodynamic LIFT! The faster the speed (apparent wind), the greater the lift. And that effecient flow of "attached" air over the roofline of which you speak plays hell with stability on the track!

To be truly telling, your experiment needs a large "front" of "colored" air blowing at the entire frontal area of your car at > 70 MPH. Without going into extensive detail, you would notice trememdous swirrling in front of the front bumper, in front of the windshield, and as Craig points out, behind the rear window. The two in front of the car are "high pressure", acting to hold the car back from penetrating the air. The one at the rear is "low pressure", also holding the car back! Not so apparent would be the air being forced (pressurized) UNDER the car. These areas are turbulent, and as such, make for tremendous drag and lift.

Noel describes the improvements Porsche made to two of these trouble spots: The front spoiler or "air dam" limits the free-flow of air under the car. The rear spoiler (whale tail, etc.) limits the separation of air flow off the back of the car. It actually "delays" that trailing low pressure area, so it is farther behind the car at speed, where it will have minimum detrimental effect.

Your question about the tail channeling more air into the engine bay through the grill is an interesting one. Noel suggests that the addition of a tail changes the pressure above the engine lid from minus to plus. That might have some effect, but most of the air blowing past the engine comes from under the car. Indeed, many 911 racers believe the grill is there as an EXHAUST rather than an INTAKE. (Where is some nice colored smoke when you need it?)

Noel's last sentence sums it up nicely: the 911 was not designed with aero in mind, although the newer models are better. Indeed, each time we go to Pocono we are rudely reminded that the boys at Porsche never consulted Bernoulli.

Ed LoPresti
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Old 10-14-2004, 09:30 PM
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RaceProEngineer, I applaud you. I'm dropping a watercooled engine in and will no longer seal the engine bay. I wanted to know where I could get a good source of air to flow over the radiator. I want to mount it behind the engine (engine is boxer 4cyl), but I don't know where to pull turbulent air to get effective cooling. Alternatively, I was going to mount it under the engine grill, hoping the air would flow enough in.

Maybe, I should just scoop the "pressurized" air from under the car up and thru the radiator.

Any thoughts?
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Old 10-15-2004, 05:13 AM
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Ed;
Actually I'd suggest that when they designed the 911, they did have aero in mind (limited by what little they knew at the time), just like the 917 long-tail design incorporated aero thought. The early 911's actually had a pretty good drag coefficient, certainly better then just about any contemporary car excepting the Citreon DS. The flaw was that they didn't consider the rest of the package -- specifically lift and cooling flow. Keep in mind that the cooling flow was adequate for the 2.0's, 2.2's and even 2.4's. They were certainly not oblivous to the concept of pulling air into the low pressure area behind the car. Take a look at where the cockpit air exhaust is located -- right above the rear window where it will tend to pull air out of the passenger compartment. Compared to other cars ventelation systems (especially from the 60's), the 911's passenger ventelation is great. BTW, Remember all the stories of Piech ignoring driver complaints about the 917LH's handling and instead taping up the vent holes to get a few more MPH of top speed. This gives you an idea of the mind-set.

Nowadays designers know that it is better to get the best compromise of lift and drag (l/d ratio), engine cooling and passenger comfort.
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Old 10-15-2004, 07:18 AM
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What IS the CD of a 1965-1989 911??
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Old 10-15-2004, 07:24 AM
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Hi, John,

It is rare to find myself disagreeing with ANYTHING you post, but this time may be an exception. Please bear in mind that my critique of Porsche's aero design process is purely subjective.

In 1973, I purchased my first Alfa Romeo Spyder, after testing a 911T and a 911E. Not only were the Porsches much more expensive, but the Alfa was faster than the "T" and had much better high speed stability than either. Indeed, my Spyder suffered none of the maladies you mention above. I am certain there was some lift, but it was well compensated-for by the wonderful penetration of the Pininfarina body.

Now we race a 1983 911SC, with all the PCA G-Stock class restrictions, and continually puzzle over the thinking which went into the shape! From those early "T" and "E" (and "S") models, a full ten years of development later, we are still contending with an almost UPRIGHT windshield, and mirrors the size of small mail boxes, plus, plus!

I agree that Porsche put much more thought into their purpose-built racers, but even then, in the case of the 917 and 935, most of the stability and aero development was accomplished by their CUSTOMERS, then retrofitted.

Our 911es have MANY, MANY compelling and endearing (addicting?) qualities, but aerodynamics certainly is not one.

Ed LoPresti
Old 10-15-2004, 07:57 AM
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Aerodynamic knowledge of the day came directly from the aircraft industry, some of it copied without testing or understanding and was just styling/marketing. Looking at the cars of the day, I must assume they they assumed that ALL drag was bad, and the perfect shape was a teardrop. It's been said that the Stingray shape would of worked as an upside down airplane wing, had the underbelly been kept smooth as the designer wanted.

356 1940 prototype..................


Citroen DS 19............................


The two stroke Saab prototype................


More images of the porsche........................
http://www.conceptcarz.com/folder/vehicle.asp?car_id=1424

The Stingray, just voted the coolest car ever (not by me).
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Old 10-15-2004, 08:10 AM
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Ed (RaceProEngr) IMHO has hit er right on the head. The 911 shape, while great to look at, is pretty much 'all wrong' aerodynamically.
They ahd aero in mind, but only in philosophy - not in practice.

Sloping wedge front end? Bzzt, not good.
Tapered rear which falls away in a lif-inducing arc? Not here either.
That windshield?? Tall, too upright.

So many tweaks were made over the years to compensate for these problems......but it sure makes for fun discussion and adding those tweaks.

Samll price to pay for a TOUGH, well built car!
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Old 10-15-2004, 08:46 AM
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I can toss in on this...I'm an ME, but by no means an aerodynamicist...I'm not even sure if I spelled it correctly.

basically the separation line exists starting at the top of the rear window. (that's why the little vents are there.) below and behind that nothing but low pressure exists...

ishmeal and I have talked greatly about the watercooling project....I have one (subaru power), and put my radiator behind the car (without sealing the engine compartment) for the specific reason that there's a high pressure area in front of the engine, and a low pressure area behind the rear valance...it seems to work well without help....I have some more testing to do...and it's less risky now that it's cooler.

when the original engine was smoking in my car it was very interesting to watch the contrail effect as the air rolled off the back of the car...from a rear view, the contrail turned clockwise on the drivers side, and anti-clockwise on the passenger side....it's quite possible that as the air rolls into the lowest pressure area on the center line of the car, some of that air finds it's way down into the grille...

sjd
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Old 10-15-2004, 09:47 AM
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altporsche, do you think it'll work with the valence in place? I'm eager to see some test results. If this does work, we're golden.
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Old 10-15-2004, 10:00 AM
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The Cd , as I recall....for the 65-89 911's was about 0.39 to 0.40+ range.

Factoid....even as late as the later 1970's.....it was found that the 928 shape worked better ( aero wise) backwards !

So Porsche even then had a lot of extra learning to do.

Boy ...I'm with Ed LoPresti !! It is simply amazing for us Porsche historians to realize that Porsche's only mantra in the early days was drag....with no regard to lift. In the 50's,....the Swiss May ( of later Turbo fame) used a huge center mounted wing like on the later Chaparral...on his 550..and blew the factory cars out of the water. What did Porsche do? Did they say "aha" and embrace this? No!...good ole Hutscke got the organizers to ban the May car!. In fall of '69, it was the Wyer crew that came up with the upturned rear 917 to make the car handle. Surprisingly( for Porsche) it was faster too even though straight line speed was limited. Later yet, how did the 936's look? Hmmm...not like the later 917's...but more like the early ( unstable) 917's....Porsche was back again looking for slipperiness on long straights....and forgot all the Wyer "learnings"....
Many of the things ( at least aero..but also mechanical / turbo like the Penske/Donahue years)...were inspired by or actually done by the racing customers....not the factory.

Wil
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Old 10-15-2004, 10:23 AM
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To Tail or not to Tail...

I've read the posts on this thread twice and still can't figure out if a tail will help with engine cooling.

I am thinking about removing the (non-functional) air conditionong unit to increase air flow and improve engine cooling (and backdating the heating- to improve the appearance of the engine compartment).

Will a tail (I am considering a Duck Tail) improve air flow in the engine compartment???


Thanks,
Alan
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Old 10-15-2004, 10:47 AM
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A tail (Duck, Early carrera, late carrera or tea tray) will help keep you engine cooler. I have first hand experience with this, as I switch lids from time to time.
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Old 10-15-2004, 11:05 AM
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