Shaved rain gutters and high speed aerodynamics
10-10-2006, 06:44 PM
#1
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Shaved rain gutters and high speed aerodynamics
Here is another of my goofy hypothetical questions.
Ruf, and a few others will "shave off" the rain gutters on our 911s... presumably for an improvement in aerodynamics. This seems like a good idea... at first blush. Then I began to wonder if there isn't a negative effect of the roof air spilling down the sides of the car rather than being forced to stay on top and exert a bit of downforce on the rear wing.
The rain gutters on the a-piller are a sure drag inducer, and taping over them (or using the nifty, Ruf a-piller fairings) seems like a logical good idea. Removing the rain-gutters entirely from the a-piller would have the additional benefit of reducing the total cross-section of the car by a small bit. This would certainly have a positive benefit to high speed aerodynamics.
Has anybody ever heard of any actual studies on this? Any opinions? Jokes at my expense?
Ruf, and a few others will "shave off" the rain gutters on our 911s... presumably for an improvement in aerodynamics. This seems like a good idea... at first blush. Then I began to wonder if there isn't a negative effect of the roof air spilling down the sides of the car rather than being forced to stay on top and exert a bit of downforce on the rear wing.
The rain gutters on the a-piller are a sure drag inducer, and taping over them (or using the nifty, Ruf a-piller fairings) seems like a logical good idea. Removing the rain-gutters entirely from the a-piller would have the additional benefit of reducing the total cross-section of the car by a small bit. This would certainly have a positive benefit to high speed aerodynamics.
Has anybody ever heard of any actual studies on this? Any opinions? Jokes at my expense?
10-10-2006, 07:07 PM
#3
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Steve - did you do the A-pillar fairings? I looked at the pic on the Ruf Dallas site. It showed two fairings and another item. Any idea what that other item is?
10-10-2006, 07:31 PM
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ruf roland I think said something like +3mph in open road racing.
That would also be my guess.
It could be more though if you have limited stretch to accelerate like on V-Max Event or on a race track.
That would also be my guess.
It could be more though if you have limited stretch to accelerate like on V-Max Event or on a race track.
10-10-2006, 08:23 PM
#6
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Doug,
Yes I have the Ruf a-piller fairings. The other item you see is a tube of glue, I think.
Anybody have any thoughts about the aero advantage of removing the entire rain gutters?
Yes I have the Ruf a-piller fairings. The other item you see is a tube of glue, I think.
Anybody have any thoughts about the aero advantage of removing the entire rain gutters?
10-10-2006, 08:27 PM
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I remember someone(Kevin?) saying that the rain gutters were also where the welds are that hold the top on. What would the aero effect of the top peeling off be? Hiding those welds without rain gutters sounds expensive!
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10-10-2006, 08:43 PM
#8
Nordschleife Master
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Duct tape can hold the roof on... no problem.
Yes, the top is "rolled" into the sides via the rain gutters... a common construction technique on ancient old sleds like our 911s. A friend of mine shaved off his rain gutters on an early 911. He would grind about two inches, weld, then grind some more. He was VERY sorry he started that project. It sure does look clean with them off, though.
OK... back to the original question... does it help (or hurt) to have the entire gutters shaved off?
Yes, the top is "rolled" into the sides via the rain gutters... a common construction technique on ancient old sleds like our 911s. A friend of mine shaved off his rain gutters on an early 911. He would grind about two inches, weld, then grind some more. He was VERY sorry he started that project. It sure does look clean with them off, though.
OK... back to the original question... does it help (or hurt) to have the entire gutters shaved off?
10-10-2006, 09:19 PM
#11
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I can give you a VERY basic aero lesson. It won’t answer your question but may help you draw some conclusions. I think you would need a wind tunnel for any definitive answers.
The way a wing works is by making the air above the wing (the curved part) move faster than the air below the wing (the flat part). It does this by making the air travel farther along the top of the wing than the bottom since the air must travel a greater distance along the top of the wing. This creates a pressure differential (higher pressure below and lower pressure above). This effectively creates a vacuum that sucks the wing upwards. If you look at your car in profile, you will notice it is basically a wing. You can defeat the pressure effect in a couple of ways. You can prevent air from entering the bottom of the car or better still create an upside down wing underneath the car. Lotus did the latter in their F1 cars a few decades (has it been that long Mario?) ago and called it Ground Effects. It basically sucked the car down onto the track. Another way is by adding anti lift devises such as the wing we have on the back of our cars. The rear wing is not a spoiler like you would see on an old 911 duck tail. That only destroys lift by creating turbulence and “parasitic” drag. BTW any time you create lift, be it positive (up) or negative (down), you create drag. Not only drag by the device itself fighting being moved through the air, but induced drag. The more lift you create, by lowering Flaps for example the more induced drag you create.
Now your assumption that the gutters will keep air flowing over the rear wing is correct. There is no way of knowing how effective they are without that wind tunnel though. You may have noticed that many of the airplanes out there now have vertical wing tips that stick up from the wing. The ones on the MD-11 are 6 feet tall! These “winglets” attempt to keep the air from spilling off of the top of the wing. With a swept wing, there are aerodynamic losses that occur because the air from the top of the wing spills off of the tip. With the winglets, you increase efficiency and reduce drag.
Now if you took the gutters off, you would spill a bit more air down the sides of the car, but where it would end up is a guess. You wouldn’t lose enough to prevent the wing effect but you may reduce the induced drag by some.
My head hurts.
The way a wing works is by making the air above the wing (the curved part) move faster than the air below the wing (the flat part). It does this by making the air travel farther along the top of the wing than the bottom since the air must travel a greater distance along the top of the wing. This creates a pressure differential (higher pressure below and lower pressure above). This effectively creates a vacuum that sucks the wing upwards. If you look at your car in profile, you will notice it is basically a wing. You can defeat the pressure effect in a couple of ways. You can prevent air from entering the bottom of the car or better still create an upside down wing underneath the car. Lotus did the latter in their F1 cars a few decades (has it been that long Mario?) ago and called it Ground Effects. It basically sucked the car down onto the track. Another way is by adding anti lift devises such as the wing we have on the back of our cars. The rear wing is not a spoiler like you would see on an old 911 duck tail. That only destroys lift by creating turbulence and “parasitic” drag. BTW any time you create lift, be it positive (up) or negative (down), you create drag. Not only drag by the device itself fighting being moved through the air, but induced drag. The more lift you create, by lowering Flaps for example the more induced drag you create.
Now your assumption that the gutters will keep air flowing over the rear wing is correct. There is no way of knowing how effective they are without that wind tunnel though. You may have noticed that many of the airplanes out there now have vertical wing tips that stick up from the wing. The ones on the MD-11 are 6 feet tall! These “winglets” attempt to keep the air from spilling off of the top of the wing. With a swept wing, there are aerodynamic losses that occur because the air from the top of the wing spills off of the tip. With the winglets, you increase efficiency and reduce drag.
Now if you took the gutters off, you would spill a bit more air down the sides of the car, but where it would end up is a guess. You wouldn’t lose enough to prevent the wing effect but you may reduce the induced drag by some.
My head hurts.
10-10-2006, 09:26 PM
#12
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Md11,
I love your "geeky" answer. I think your explanation helps to understand why a Turbo "S" tail (with a small inverted wing) is more efficient than the stock Turbo spoiler. Less drag and perhaps more downforce. Maybe.
On the gutters... I agree, and wonder if there is any actual data to support that shaving the gutters gives an aero advantage... other than the obvious one I mentioned up front in making the a-piller skinnier, thus reducing the cross section.
I love the size of those Md11 "winglets". Are you one of those two guys? Thnaks for the response.
I love your "geeky" answer. I think your explanation helps to understand why a Turbo "S" tail (with a small inverted wing) is more efficient than the stock Turbo spoiler. Less drag and perhaps more downforce. Maybe.
On the gutters... I agree, and wonder if there is any actual data to support that shaving the gutters gives an aero advantage... other than the obvious one I mentioned up front in making the a-piller skinnier, thus reducing the cross section.
I love the size of those Md11 "winglets". Are you one of those two guys? Thnaks for the response.
10-10-2006, 10:21 PM
#14
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I have the wind tunnel, provided Steve comes up with a 40-50% scale 993 model
Its a rolling road as well! Full belt tracking, vacuum platen to negate belt lift etc. Here are the specifications:
Dimensions:
Test section: 9’W x 8’H x 22’ L (2.7mW x 2.4mH x 6.7mL)
Rolling road: 8’W x 16’ L (2.4mW x 4.9mL)
Contraction ratio: 5.28 to 1
Main fan: 5-bladed, 15’ diameter (4.6m)
Main fan power rating: 500 hp (370 kW)
Flow variables:
Speed: 40 – 140 mph (17 – 63 m/s)
Temperature range: 70F – 75F (21C – 24C)
Static pressure: ambient
Dynamic pressure: 4 – 50 psf (20 – 245 kg/m2)
Reynolds Number range: 0.37E6 – 1.3E6 per foot
Boundary layer profile: less than 0.2% deviation from free stream
Model:
Scale: 35% to 50%
Motion: Heave, pitch, roll, & yaw
Wheel base: 35 – 63 in. (89 – 160 cm)
Wheel track: 18 – 36 in. (46 – 91 cm)
Wheel mounting: on or off model
Data Acquisition:
Control system: Industrialized HP 745i
Force measurement: 6- and 3-component internal strain gauge balances
Wheel drag: on or off model strain gauge
Pressure measurement: 48 port mechanical or 64 port electronic modules
But even ground effect race cars dont always stick to the ground
Its a rolling road as well! Full belt tracking, vacuum platen to negate belt lift etc. Here are the specifications:Dimensions:
Test section: 9’W x 8’H x 22’ L (2.7mW x 2.4mH x 6.7mL)
Rolling road: 8’W x 16’ L (2.4mW x 4.9mL)
Contraction ratio: 5.28 to 1
Main fan: 5-bladed, 15’ diameter (4.6m)
Main fan power rating: 500 hp (370 kW)
Flow variables:
Speed: 40 – 140 mph (17 – 63 m/s)
Temperature range: 70F – 75F (21C – 24C)
Static pressure: ambient
Dynamic pressure: 4 – 50 psf (20 – 245 kg/m2)
Reynolds Number range: 0.37E6 – 1.3E6 per foot
Boundary layer profile: less than 0.2% deviation from free stream
Model:
Scale: 35% to 50%
Motion: Heave, pitch, roll, & yaw
Wheel base: 35 – 63 in. (89 – 160 cm)
Wheel track: 18 – 36 in. (46 – 91 cm)
Wheel mounting: on or off model
Data Acquisition:
Control system: Industrialized HP 745i
Force measurement: 6- and 3-component internal strain gauge balances
Wheel drag: on or off model strain gauge
Pressure measurement: 48 port mechanical or 64 port electronic modules
But even ground effect race cars dont always stick to the ground
10-10-2006, 10:43 PM
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When the 935s were racing the teams would fill in the front of the a pillar like the ruf inserts do but they would leave the rest of the rain gutter on. Why? They said the front was the only part that created real drag and shaving the whole thing had a negative effect on high speed stability at Le Mans. So they just filled in the front (with silicon mostly LOL) and left the rest of the gutter.