911 rear spoiler - anyone here know about aerodynamics?
Discussion
Wozy68 said:
Orangecurry said:
how many times have I heard that advice....Anyway - no just the spoiler on its own weighs 2.5kgs.... I want to get rid of the metal engine-lid+spoiler motor+ inner grill+ wires and the spoiler, and replace with a lightweight engine-lid and something to spoil the air, or not bother.
I'm thinking 'not bother', and put a featherweight mesh over the lightweight engine-lid hole = better cooling as well. Same as the older 911.
TDT said:
I thought it would be something like this?
Crudely - The pop-up spoiler achieves the following...
Raising of the spoiler creates a greater upper surface area to keep smooth fast airflow attached to the car, and also creates a low pressure area under it and cleaning up the trailing edge.
The air on top of the spoiler is flowing faster creating a higher pressure and because high pressure alway acts on low pressure the spoiler is pressed down.
The balance of this interaction determines how much negative lift/downforce you have.
Dependant on the desired outcomes..the size and placement of the wing also contributes as this has a lever type function so dependant on its placement, the front has to also have some treatment in order to stop the front lifting/lightening....
Happy to be corrected..
Not really. 'Faster flowing air' creates lower pressure not higher pressure. This is the basic principle of winged flight.Crudely - The pop-up spoiler achieves the following...
Raising of the spoiler creates a greater upper surface area to keep smooth fast airflow attached to the car, and also creates a low pressure area under it and cleaning up the trailing edge.
The air on top of the spoiler is flowing faster creating a higher pressure and because high pressure alway acts on low pressure the spoiler is pressed down.
The balance of this interaction determines how much negative lift/downforce you have.
Dependant on the desired outcomes..the size and placement of the wing also contributes as this has a lever type function so dependant on its placement, the front has to also have some treatment in order to stop the front lifting/lightening....
Happy to be corrected..
Edited by TDT on Monday 8th February 18:14
https://en.wikipedia.org/wiki/Bernoulli%27s_princi...
https://www.nasa.gov/sites/default/files/atoms/fil...
Anyway, you're not going to get force downwards on a spoiler like this. The spoiler is simply 'spoiling' the low pressure, not in any way generating downforce. There will still be low pressure above and behind the spoiler, but it will be less.
Orangecurry said:
Fantastic - thank you very much.
It's good to understand stuff - spoiling airflow is not size/shape critical. Perhaps I can stick the wife on the back?
Reading comprehension failure. But never mind. As per the quote below the graph posted above and describing the manner in which the RS duck was 'discovered' / developed, this isn't as complex as implied in the OP. Whether it's the duck or the pop-up, you've got something of roughly the same size (rough width dictated by then engine cover aperture etc) and it will have roughly the same effect. Something significantly larger like the Turbo spoiler (again as per the graph) will have a greater effect on reducing the area of low pressure. As per the the graph (again cubed) you've still got lift with the Turbo spoiler, just less of it.It's good to understand stuff - spoiling airflow is not size/shape critical. Perhaps I can stick the wife on the back?
If you're going to have something sticking out of the engine lid area, if it's roughly the same size as the duck / pop-out, you'll get roughly the same results. Which answers the original question. Or not if you'd rather it didn't!
F6C said:
Anyway, you're not going to get force downwards on a spoiler like this. The spoiler is simply 'spoiling' the low pressure, not in any way generating downforce. There will still be low pressure above and behind the spoiler, but it will be less.
If you're going to have something sticking out of the engine lid area, if it's roughly the same size as the duck / pop-out, you'll get roughly the same results. Which answers the original question. Or not if you'd rather it didn't!
Isn't that what I said? Emphasis on critical? Genuine question.Orangecurry said:
Fantastic - thank you very much.
It's good to understand stuff - spoiling airflow is not size/shape critical. Perhaps I can stick the wife on the back?
Reading comprehension failure. But never mind. As per the quote below the graph posted above and describing the manner in which the RS duck was 'discovered' / developed, this isn't as complex as implied in the OP. Whether it's the duck or the pop-up, you've got something of roughly the same size (rough width dictated by then engine cover aperture etc) and it will have roughly the same effect. Something significantly larger like the Turbo spoiler (again as per the graph) will have a greater effect on reducing the area of low pressure. As per the the graph (again cubed) you've still got lift with the Turbo spoiler, just less of it.It's good to understand stuff - spoiling airflow is not size/shape critical. Perhaps I can stick the wife on the back?
If you're going to have something sticking out of the engine lid area, if it's roughly the same size as the duck / pop-out, you'll get roughly the same results. Which answers the original question. Or not if you'd rather it didn't!
So the RS M002 spoiler, which is in the same 'place' but wider with more volume than both pop-out or ducktail, removes more of the low pressure area, and so you have less lift on the back, same as the various Turbo spoilers?
As I've got you here, and I'm interested in balance.....
What are they trying to achieve with front corner spoilers/splitters? on the RS please?
Edited by Orangecurry on Tuesday 9th February 19:37
F6C said:
....and describing the manner in which the RS duck was 'discovered' / developed, this isn't as complex as implied in the OP.
It seems we owe Mr Tilman Brodbeck an apology Courtesy of https://www.porscheroadandrace.com/tilman-brodbeck...
Tilman’s background meant that after he joined Porsche he was promoted to body project engineer. “One of my tasks was on the 924 (Porsche’s first front engined model) where getting sufficient cooling air to the engine was quite a challenge within the overall shape we wanted for the car.”
Prior to this though and before even being assigned a title, Brodbeck had had to prove himself: he was handed the daunting responsibility of resolving the 911’s tendency to lift its front end at high speed. Simply adding weight to the front fender did not solve the problem. “We used to drive up to Ehra-Lessien (VW’s extensive proving ground), and after Kassel, the autobahn straightens out and you were expected to drive that section flat out. I always dreaded having to take the wheel on this stretch, because the 911 wandered about so much.” Experiments in the wind tunnel at Stuttgart university showed how air was lifting the front end of the 911, but at that time, no one was using spoilers or aerodynamic aids on production cars. Flaps and attempts at ducting airflow had however indicated some improvement when tried on race cars and inspired by this Tilman, developed a lip for the 911 front valance. The result was a 50% reduction in lift. “Ferdinand Piëch was delighted, but then he really piled on the pressure giving us three weeks to prepare a lightened 911 for the motor sport division.”
Glass fibre bumpers replaced the originals and thinner glass was specially ordered. With Brodbeck’s ‘lip’ the 911’s drag coefficient was improved, but the rear wheels now lacked traction. “This was a real headache,” recalls Tilman, “I literally couldn’t sleep.” Then he remembered his first car, a rear engined Fiat 850 coupé Spider and how its successor, the 900 turned out to have a much higher top speed than the 5bhp increase should have made. But that 900 had a lip, a sort of spoiler on the engine cover…Brodbeck hastily fabricated a rear spoiler and took a 911 so modified to the wind tunnel: the results were sensational. Rear lift went down by 60% and drag and top speed were also improved. The modification was presented to Tony Lapine’s design studio and Wolfgang Möbius produced the ‘Bürzel’ or ducktail. Proving runs at Weissach and Ehra-Lessien by Günter Steckkönig, in Brodbeck’s view as fine a driver in his day as Walter Röhrl, confirmed the improvement seen in the wind tunnel. Homologation would require a production run of 250 units.
The sales department baulked at this just as it had with the 911 R five years earlier and briefly it seemed the latest lightened 911 would remain a prototype too. However, newly appointed CEO Ernst Fuhrmann was keen to make his mark and in a dramatic scene which Tilman Brodbeck witnessed by chance, Fuhrmann asserted himself, telling his sales chief he would either sell 250 or none at all. “So that was how the Carrera RS 2.7 came to be,” said Tilman, “an entirely accidental success! I’m not an old car fan (a view shared by former design director Harm Lagaaij who believes logically that newer is always better) but that RS is one old Porsche I would love to own!”
Traditionally an engineering led company, in the early 1970s, Porsche had over 200 engineers, vastly out numbering the 25 designers employed by the styling department. When Butzi Porsche left the firm, his deputy Tony Lapine took over and in asserting the role of the Porsche design studio, at times made it appear like a separate company, sharpening divisions between engineering and design. Famously, he used to say to his cohorts that ‘if marketing likes it, ignore them and if engineering likes it, start again.’ Tilman Brodbeck has not forgotten what it was like to be caught in the crossfire between the machine shop and the easels. “Tony Lapine could be quite arrogant. He had great difficulty in accepting that you need studio engineers.” Part of Brodbeck’s role was liaising between engineers and designers. Sometimes this worked well from the outset, as for example with the whale tail. The German highway authorities objected to the ducktail on grounds of pedestrian safety. So, it was redesigned, largely by Wolfgang Möbius, with polyurethane edges to soften it; not only did this resolve the problem, but it expanded the spoiler sufficiently for it to incorporate the intercooler of the 3.3 version of the 930 Turbo in 1977.
This is interesting. Snetterton in a 964 without the rear wing deployed. The car was slightly loose due to the Weissach effect on the rear axle. It wasnt too bad but slightly vaigue with occasional oversteer when fully loaded. That was fixed by fitting a set of rose jointedTarrett rear sping plates.
I would definately reccommend a set to anyone tracking a 964. The OP's 993 wont suffer with this problem as the rear suspension hardware and geometry is more sophisticated.
I noticed that the oil temps were lower with the spoiler deployed so i cracked it open manually after that run to help dissipate some heat from the engine bay. I subsequently replaced the spoiler motor and it is now functional. The Tarrett spring plates make a substantial difference to the lap time and help with rear tyre lifespan. The spoiler makes no real difference to performance on the lap but it does help cooling
https://www.youtube.com/watch?v=qXzMkKmdUZY&fe...
I would definately reccommend a set to anyone tracking a 964. The OP's 993 wont suffer with this problem as the rear suspension hardware and geometry is more sophisticated.
I noticed that the oil temps were lower with the spoiler deployed so i cracked it open manually after that run to help dissipate some heat from the engine bay. I subsequently replaced the spoiler motor and it is now functional. The Tarrett spring plates make a substantial difference to the lap time and help with rear tyre lifespan. The spoiler makes no real difference to performance on the lap but it does help cooling
https://www.youtube.com/watch?v=qXzMkKmdUZY&fe...
"Remembered his first car"
"Hastily fabricated a rear spoiler"
"Rear lift went down by 60%"
Q.E.D.
Infinitesimally tuned with a petaflop of CFD power it wasn't.
As before, stick something of roughly the same size in that spot and you'll take a similar chunk out of the lift / low pressure area. Wouldn't sweat the details, personally. They won't make a material difference to the dynamics.
"Hastily fabricated a rear spoiler"
"Rear lift went down by 60%"
Q.E.D.
Infinitesimally tuned with a petaflop of CFD power it wasn't.
As before, stick something of roughly the same size in that spot and you'll take a similar chunk out of the lift / low pressure area. Wouldn't sweat the details, personally. They won't make a material difference to the dynamics.
Orangecurry said:
Yes. Thanks. Think I will have to come up with my own design, as I don't like the current offerings, :
Whilst your there beavering away in the shed building yourself a new spolier with copious amounts of fibreglass you could also uprate your wheels like this guy 
OC .... First trip out after the upgrades
https://www.youtube.com/watch?v=nqqZ28m8uCo
Free country.
Getting rid of some weight is one thing, but I actually think the rear spoiler on the 993 when 'out' is an ugly thing, and (from real-world angles) so is every other version, RS M002 included.
There - I've said it.
Looks great from some angles, but from the back it's a wall of plastic. Same with the Turbo and aerokit spoilers; all look pants.
Try and find a simple rear-view of any of these - you can't. It's not nice, that's why.
1970s Ducktail is slightly better, but IMHO the 1970s look doesn't suit the 993.
An updated ducktail is much better. As per the Gunther Werks 2019.
But my creative genius is telling me someone should come up with something better AND lightweight, and you could have a 964 version.
Everybody wins.
No
No
No
No
Guther Werks - tapers in to the top, sharper, not 1970s...
Getting rid of some weight is one thing, but I actually think the rear spoiler on the 993 when 'out' is an ugly thing, and (from real-world angles) so is every other version, RS M002 included.
There - I've said it.
Looks great from some angles, but from the back it's a wall of plastic. Same with the Turbo and aerokit spoilers; all look pants.
Try and find a simple rear-view of any of these - you can't. It's not nice, that's why.
1970s Ducktail is slightly better, but IMHO the 1970s look doesn't suit the 993.
An updated ducktail is much better. As per the Gunther Werks 2019.
But my creative genius is telling me someone should come up with something better AND lightweight, and you could have a 964 version.
Everybody wins.
No
No
No
No
Guther Werks - tapers in to the top, sharper, not 1970s...
....excellent points both - I prefer the non-spoiler look as well, and don't fret how it looks, as I never see it out.
BUT if I replace the rear lid with something lighter, I'd prefer the anti-lift aspects of the spoiler, and therefore it'll be a fixed spoiler, and I'll have to look at it all the time.
First world problems.
And the factory spoiler deploys at 55mph? so are the anti-lift aspects useful at UK motorway speeds?
BUT if I replace the rear lid with something lighter, I'd prefer the anti-lift aspects of the spoiler, and therefore it'll be a fixed spoiler, and I'll have to look at it all the time.
First world problems.
And the factory spoiler deploys at 55mph? so are the anti-lift aspects useful at UK motorway speeds?
Orangecurry said:
....excellent points both - I prefer the non-spoiler look as well, and don't fret how it looks, as I never see it out.
BUT if I replace the rear lid with something lighter, I'd prefer the anti-lift aspects of the spoiler, and therefore it'll be a fixed spoiler, and I'll have to look at it all the time.
First world problems.
And the factory spoiler deploys at 55mph? so are the anti-lift aspects useful at UK motorway speeds?
I have to admit I’m in the same camp as you .... far prefer the 993 shape with the spoiler down..... or on any other 911. BUT if I replace the rear lid with something lighter, I'd prefer the anti-lift aspects of the spoiler, and therefore it'll be a fixed spoiler, and I'll have to look at it all the time.
First world problems.
And the factory spoiler deploys at 55mph? so are the anti-lift aspects useful at UK motorway speeds?
If mem serves the reason originally for the front lip that appeared in the early Seventies was to help reduce the 911 moving about in side winds .... and the rear one became about to help balance everything out in later models because of it.
The SC and 3.2 could be specd as a non sport model without them and yet the cars top speed wasn’t limited.
Maybe you just disconnect the motor and drive around with the lid shut? Save you a heck of potential outlay.
After all where could you actually get above 70mph (let alone 120) on a motorway in the U.K. this side of a 6am start or north of somewhere like Cumbria?
If you could, no doubt there’d be a scamera van on the next bridge focusing down on you.
I’d have no issues driving my 911 down the autobahn at silly speeds with the lid down. I always thought the deployed spoiler on the 993 (with all it’s time getting it’s aerodynamics ,under trays etc etc sorted in the wind tunnel) a bit of a gimmick for real world driving.
Edited by Wozy68 on Friday 12th February 19:01
Surprising lift numbers without a spoiler, you would certainly feel less stable at motorway speeds.
https://forums.pelicanparts.com/porsche-911-techni...
https://forums.pelicanparts.com/porsche-911-techni...
https://forums.pelicanparts.com/porsche-911-techni...
https://forums.pelicanparts.com/porsche-911-techni...
Wozy68 said:
Maybe you just disconnect the motor and drive around with the lid shut? Save you a heck of potential outlay.
Yes but I want to get rid of the weight.Wozy68 said:
After all where could you actually get above 70mph (let alone 120) on a motorway in the U.K. this side of a 6am start or north of somewhere like Cumbria?
If you could, no doubt there’d be a scamera van on the next bridge focusing down on you.
Ahhh you don't appreciate the backwardness of West Sussex. It's like going back in time. All of the Police are in Crawley.If you could, no doubt there’d be a scamera van on the next bridge focusing down on you.
Wozy68 said:
If mem serves the reason originally for the front lip that appeared in the early Seventies was to help reduce the 911 moving about in side winds .... and the rear one became about to help balance everything out in later models because of it.
It's 'lift'.... lift - didn't you read any of the really interesting stuff above I quoted from the Porsche designer/engineer? 
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