Let's talk aerodynamics
06-13-2003, 06:16 PM
#1
Drifting
Thread Starter
Let's talk aerodynamics
How much downforce, if any, does the 951 body generate? How fast do our little beasts have to go before we notice a difference in handling?
I have noticed that it gets REALLY LOUD in the car really quickly with the windows open and sunroof off... maybe it was just the speed I was going <img border="0" title="" alt="[Eek!]" src="eek.gif" />
Justin
I have noticed that it gets REALLY LOUD in the car really quickly with the windows open and sunroof off... maybe it was just the speed I was going <img border="0" title="" alt="[Eek!]" src="eek.gif" />
Justin
06-13-2003, 07:04 PM
#2
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Tough question. I would guess almost no negative lift from the car's body, since it's flat on the bottom and curved on top, just like an airplane's wing. The question is, how much lift is at any given axle at, say, 100 mph.
By adding a splitter, you could start to generate downforce, but then perhaps not. And you could start to upset the front/rear aerodynamic balance designed in by the factory. Likewise, bolting a rear wing a la 968 Turbo S or even 968 could change things enough to notice.
Unfortunately, time in a wind tunnel is brutally expensive. There was an article in Autocar recently about how they got blown away at last year's Nuerburgring 24 hr race, due to the aerodynamics of their Caterham 7. So they fabricated bunches of different things to add onto their car to make it smoother, then rented the wind tunnel somewhere in England for a day and tested. They were able to find a lot of top speed, and tune the car aerodynamically, but it cost a lot to do it -- at least several thousand dollars as I recall, since you also need someone to run the tests for you as well as an engineer to interpret the results.
If you go that route, please heed Alain Prost's advice about setting up a car: First, you must get the optimum setup from a mechanical point of view. That means, springs, dampers, anti roll bars, suspension alignment, tire pressures. These are things that change how the car handles at low speeds, where aerodynamics don't really matter, and high speeds. THEN you can modify the aerodynamics to get the car to handle better at high speeds.
I'd love to change the aerodynamics of my 944 Turbo, perhaps by adding some sort of simple splitter to force more air into the brake cooling ducts and of course the intercooler, etc. But I'm concerned that I could really change the high speed handling balance of the car, so that the car works fine in a 50 mph corner, but a 90-100 mph sweeper at Lime Rock becomes a completely different situation, with a car suffering from aerodynamic-induced oversteer due to new found front end grip through downforce.
It would also be cool to ditch the factory rear undertray in favor of some sort of F1-style diffuser, perhaps to balance the effect of a front splitter.
The first step might be to find a 1:18 or 1:24 scale model 944 Turbo and build your own wind tunnel at home, then test what happens as you add stuff to it. Don't laugh, it's a reasonable approach, albeit not always with predictable results in the real world. I considered doing this for my son's pinewood derby car, but the aerodynamic drag at those speeds is negligible.
I'm sure this doesn't help much, but it's a fun subject to think about.
Hunter
By adding a splitter, you could start to generate downforce, but then perhaps not. And you could start to upset the front/rear aerodynamic balance designed in by the factory. Likewise, bolting a rear wing a la 968 Turbo S or even 968 could change things enough to notice.
Unfortunately, time in a wind tunnel is brutally expensive. There was an article in Autocar recently about how they got blown away at last year's Nuerburgring 24 hr race, due to the aerodynamics of their Caterham 7. So they fabricated bunches of different things to add onto their car to make it smoother, then rented the wind tunnel somewhere in England for a day and tested. They were able to find a lot of top speed, and tune the car aerodynamically, but it cost a lot to do it -- at least several thousand dollars as I recall, since you also need someone to run the tests for you as well as an engineer to interpret the results.
If you go that route, please heed Alain Prost's advice about setting up a car: First, you must get the optimum setup from a mechanical point of view. That means, springs, dampers, anti roll bars, suspension alignment, tire pressures. These are things that change how the car handles at low speeds, where aerodynamics don't really matter, and high speeds. THEN you can modify the aerodynamics to get the car to handle better at high speeds.
I'd love to change the aerodynamics of my 944 Turbo, perhaps by adding some sort of simple splitter to force more air into the brake cooling ducts and of course the intercooler, etc. But I'm concerned that I could really change the high speed handling balance of the car, so that the car works fine in a 50 mph corner, but a 90-100 mph sweeper at Lime Rock becomes a completely different situation, with a car suffering from aerodynamic-induced oversteer due to new found front end grip through downforce.
It would also be cool to ditch the factory rear undertray in favor of some sort of F1-style diffuser, perhaps to balance the effect of a front splitter.
The first step might be to find a 1:18 or 1:24 scale model 944 Turbo and build your own wind tunnel at home, then test what happens as you add stuff to it. Don't laugh, it's a reasonable approach, albeit not always with predictable results in the real world. I considered doing this for my son's pinewood derby car, but the aerodynamic drag at those speeds is negligible.
I'm sure this doesn't help much, but it's a fun subject to think about.
Hunter
06-14-2003, 08:30 PM
#3
Nordschleife Master
let me put it this way:
I didn't properly latch the sun roof once, I mean only one side of the two pushbars latched correctly. I didn't think much of it.
A while later, just after hitting 165mph, I heard a loud POP! It was the sunroof, it came loose.
Dat's a lotta negative pressure right there!
I didn't properly latch the sun roof once, I mean only one side of the two pushbars latched correctly. I didn't think much of it.
A while later, just after hitting 165mph, I heard a loud POP! It was the sunroof, it came loose.
Dat's a lotta negative pressure right there!
06-14-2003, 08:45 PM
#4
Three Wheelin'
Negative pressure?? Um you mean vacuum or suction, or LIFT?
I once read in Flying magazine that the 911 (pre 993) produced 400lbs of LIFT at 60mph.
Downforce must be produced, most cars produce lift, which is why spoilers, wings, splitters etc are needed.
I once read in Flying magazine that the 911 (pre 993) produced 400lbs of LIFT at 60mph.
Downforce must be produced, most cars produce lift, which is why spoilers, wings, splitters etc are needed.
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06-15-2003, 04:22 AM
#8
Race Director
There's very, very expensive CFD software available that will allow you to model air-flow similar to a wind-tunnel. I've seen sample model from a package that actually computed realistic loading on the tires of a NASCAR and actually showed effects of being close to the wall on one side. I suspect that we actually have lift at speed and the car gets lighter and lighter on its wheels as speed increases.
I know that the hood lifts up at speeds over 60mph when I left off one of the two bolts holding the hinge to the hood. And the remaining one was loose as well. From 60mph+, the rear of the hood started coming up higher and higher.
I know that the hood lifts up at speeds over 60mph when I left off one of the two bolts holding the hinge to the hood. And the remaining one was loose as well. From 60mph+, the rear of the hood started coming up higher and higher.
06-15-2003, 05:33 AM
#9
Nordschleife Master
I suspect that we actually have lift at speed and the car gets lighter and lighter on its wheels as speed increases.
You suspect right Danno. Ever see a cross section of a wing?
<img src="http://www.dynawing.com/AirFlow.jpg" alt=" - " />
Look at the cross section of a 993:
<img src="http://flat-6.net/photos_new/993/993_Carrera/11.JPG" alt=" - " />
Niether drawing is correct in how the air flows, but they are pretty close, for our purposes, anyways.
As speed increases, more air is displaced, the pressure differential increases, and thus, LIFT increases. (But, DRAG also increases exponentially with speed)
Air always flows from High pressure to Low pressure.
You suspect right Danno. Ever see a cross section of a wing?
<img src="http://www.dynawing.com/AirFlow.jpg" alt=" - " />
Look at the cross section of a 993:
<img src="http://flat-6.net/photos_new/993/993_Carrera/11.JPG" alt=" - " />
Niether drawing is correct in how the air flows, but they are pretty close, for our purposes, anyways.
As speed increases, more air is displaced, the pressure differential increases, and thus, LIFT increases. (But, DRAG also increases exponentially with speed)
Air always flows from High pressure to Low pressure.
06-15-2003, 06:17 AM
#10
Burning Brakes
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951 does not generate any downforce. It is generating lift at speed so it has spoilers which are designed to reduce lift. The most important one and the one which gives you the feel of our cars being glued to the road at high speed is the rear undertray spoiler. Take that off and go for a high speed run - the diference is huge. Any car will generate lift at speed as to Sandor's drawing. On road cars passive reduction in lift is implemented - spoilers, in F1 cars they use active managment and obtain true downforce.
06-15-2003, 09:18 AM
#11
Nordschleife Master
The most important one and the one which gives you the feel of our cars being glued to the road at high speed is the rear undertray spoiler. Take that off and go for a high speed run - the diference is huge
I don't think so. The 3 undertray sections do not act as a "spoiler" in fact, it has three purposes. It smoothes the airflow below the car, and secondly, diverts a small amount of air upwards to cool the engine bay, and third, it protects the engine bay from road dirt/salts/etc.
The main purpose of the undertray is to reduce the underbody drag of the car, helping with top speed.
The funny thing is, as you smooth the airflow under the car, you help create more "Lift" and whenever you create Lift, you create drag. Drag caused by "Lift" is called "Induced Drag" Drag caused by the shape of the body is "Form Drag" and drag caused by the mirrors or antennae is called "parasite drag"
Induced drag doesn't kick in till about 150mph, that's when it's hard to overcome all that air pulling you back.
If you didn't have the undertray, the air gets crasy turbulant under the engine, and has a much bigger impact on drag, much earlier. I've run without it and the biggest difference is in the highspeed areas. I'm sure that wihtout it, you'd have quite a bit more oversteer because the car is not being "lifted" as high due to the smoother underbody airflow. (When the car "lifts" up, it has more understeer because there is less weight on the wheels and suspension.)
If you check out the underbody on the Modena and McLarenF1, you can see how they tried to make the underbody and "upside down wing" to reverse the "lift effect"
I don't think so. The 3 undertray sections do not act as a "spoiler" in fact, it has three purposes. It smoothes the airflow below the car, and secondly, diverts a small amount of air upwards to cool the engine bay, and third, it protects the engine bay from road dirt/salts/etc.
The main purpose of the undertray is to reduce the underbody drag of the car, helping with top speed.
The funny thing is, as you smooth the airflow under the car, you help create more "Lift" and whenever you create Lift, you create drag. Drag caused by "Lift" is called "Induced Drag" Drag caused by the shape of the body is "Form Drag" and drag caused by the mirrors or antennae is called "parasite drag"
Induced drag doesn't kick in till about 150mph, that's when it's hard to overcome all that air pulling you back.
If you didn't have the undertray, the air gets crasy turbulant under the engine, and has a much bigger impact on drag, much earlier. I've run without it and the biggest difference is in the highspeed areas. I'm sure that wihtout it, you'd have quite a bit more oversteer because the car is not being "lifted" as high due to the smoother underbody airflow. (When the car "lifts" up, it has more understeer because there is less weight on the wheels and suspension.)
If you check out the underbody on the Modena and McLarenF1, you can see how they tried to make the underbody and "upside down wing" to reverse the "lift effect"
06-15-2003, 10:28 AM
#13
Nordschleife Master
I wouldn't call the rear valence an undertray, but anyway,
It does direct the underbody flow of the car and jet it upwards a bit, to create some downforce on the rear end as you speed up.
It does direct the underbody flow of the car and jet it upwards a bit, to create some downforce on the rear end as you speed up.
06-15-2003, 12:40 PM
#14
One of our members works in this field and has done a little <a href="http://www.club944.net/nuke/html/modules.php?name=News&file=article&sid=34&mode=&order=0&thold=0" target="_blank">report</a>.
(You may find interesting to notice that <a href="http://www.club944.net/images/articles/aero08.png" target="_blank">the front pressure distribution</a> proves that an aditionnal opening on the front panel between the pop-ups (for the intercooler) is useless)
(You may find interesting to notice that <a href="http://www.club944.net/images/articles/aero08.png" target="_blank">the front pressure distribution</a> proves that an aditionnal opening on the front panel between the pop-ups (for the intercooler) is useless)
06-15-2003, 12:52 PM
#15
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Justin,
I recall some yrs. back reading how Dunlop had designed in there SP-8000 tires a pattern of tread that served to cancel out some of the tire noise that the rest of the tire produced. No doubt this type of technology is widespread. Yes, w/sunroof off and windows down our cars are noisy at certain speeds. Here is the best application of the above mentioned technology for 951's. To cancel out all that noise, keep your right foot planted and eventualy the noise goes away. For some reason and I don't know why, passengers stop hearing the noise long before the driver. Then they become the noise. Then, more than ever, do not lift. It's blissful. Where would we be without this type of technology?
I recall some yrs. back reading how Dunlop had designed in there SP-8000 tires a pattern of tread that served to cancel out some of the tire noise that the rest of the tire produced. No doubt this type of technology is widespread. Yes, w/sunroof off and windows down our cars are noisy at certain speeds. Here is the best application of the above mentioned technology for 951's. To cancel out all that noise, keep your right foot planted and eventualy the noise goes away. For some reason and I don't know why, passengers stop hearing the noise long before the driver. Then they become the noise. Then, more than ever, do not lift. It's blissful. Where would we be without this type of technology?