Master Thread - Aerodynamics
04-09-2009, 03:11 PM
#1
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Thread Starter
Master Thread - Aerodynamics
It's about time for this isn't it?
Original Drag Coefficients provided by Jim M. some time ago:
928 0.42
928 S/S2/S3: 0.39
928 S4/SE/GT/GTS: 0.34 with flaps shut
928 S4/SE/GT/GTS: 0.36 with flaps open
Original Drag Coefficients provided by Jim M. some time ago:
928 0.42
928 S/S2/S3: 0.39
928 S4/SE/GT/GTS: 0.34 with flaps shut
928 S4/SE/GT/GTS: 0.36 with flaps open
04-09-2009, 03:14 PM
#3
Administrator - "Tyson"
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Does anyone know what the frontal area is of each model?
04-09-2009, 03:21 PM
#4
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Thread Starter
I have a question about exhausting the high pressure under the hood.
Mark K has cut a duct out of the hood, Colin is talking about ducting out the back of the car.
I asked my engineer father-in-law, who would be the first to admit he knows nothing about race car aerodynamics (although he does know a thing or two about making aircraft invisible to radar
) what he would do given the wind tunnel chart that's been floated around, and he said exhausting the air with a very low profile but wide duct at the rear of the roof line made the most sense to him. This is where Carl's car has the vortex generators.
Any comment?
Mark K has cut a duct out of the hood, Colin is talking about ducting out the back of the car.
I asked my engineer father-in-law, who would be the first to admit he knows nothing about race car aerodynamics (although he does know a thing or two about making aircraft invisible to radar
) what he would do given the wind tunnel chart that's been floated around, and he said exhausting the air with a very low profile but wide duct at the rear of the roof line made the most sense to him. This is where Carl's car has the vortex generators.Any comment?
04-09-2009, 03:25 PM
#5
Rennlist Member
Thread Starter
I feel like I've just given birth to a still-born thread.
C'mon guys, let's keep the rimshots to a minimum - at least until the conversation gets off the ground.
C'mon guys, let's keep the rimshots to a minimum - at least until the conversation gets off the ground.
04-09-2009, 03:31 PM
#6
Nordschleife Master
Adam, I will be ducting the air which gets sucked out from under the car out the back.
The air which comes in through the front will be vented through the hood.
As to the air venting for the cabin, I was thinking about finning my quarter panel windows :P
The air which comes in through the front will be vented through the hood.
As to the air venting for the cabin, I was thinking about finning my quarter panel windows :P
04-09-2009, 03:37 PM
#7
Rennlist Member
Pass it by your dad again, with the wind tunnel charts and plots.
the lowest pressure area is just after the nose, THATs why the pros have all started venting the incoming air to the hood vents. venting incoming air to the roof rear area is interesting, but how do you get it there efficiently? any losses along the way with tubing or other, will decrease the differential pressure and defeat the purpose. remember the purpose is to get the air from going under the car to over the car or around the car (ie splitters).
carls vortex generators are designed to keep air attached to the surface so it is not deflected upward to give more air to the rear wing. This is used in airplanes so that during near stall conditions, the allerons and other control surfaces still have air moving around them for control. Autos use them to keep the air lower to hit the wing if one is mounted low.
His vents are useless as they vent into a higher pressure zone compared to ambient outside and in the car. However, if his car was pressurized, they would act as vents, but that is not what I think is happening to his. he could loose those vents and seal them up and probably be better off. a simple tufts test would be ideal for testing that, so dont take my word for it. (i could be wrong there) I did it with scots car, as I cut holes like that in my old rear window . when I found stuff coming in those holes rather than coming out, I sealed them up and then just gave the window to scot.
What was the question again????
mk
the lowest pressure area is just after the nose, THATs why the pros have all started venting the incoming air to the hood vents. venting incoming air to the roof rear area is interesting, but how do you get it there efficiently? any losses along the way with tubing or other, will decrease the differential pressure and defeat the purpose. remember the purpose is to get the air from going under the car to over the car or around the car (ie splitters).
carls vortex generators are designed to keep air attached to the surface so it is not deflected upward to give more air to the rear wing. This is used in airplanes so that during near stall conditions, the allerons and other control surfaces still have air moving around them for control. Autos use them to keep the air lower to hit the wing if one is mounted low.
His vents are useless as they vent into a higher pressure zone compared to ambient outside and in the car. However, if his car was pressurized, they would act as vents, but that is not what I think is happening to his. he could loose those vents and seal them up and probably be better off. a simple tufts test would be ideal for testing that, so dont take my word for it. (i could be wrong there) I did it with scots car, as I cut holes like that in my old rear window . when I found stuff coming in those holes rather than coming out, I sealed them up and then just gave the window to scot.
What was the question again????
mk
I have a question about exhausting the high pressure under the hood.
Mark K has cut a duct out of the hood, Colin is talking about ducting out the back of the car.
I asked my engineer father-in-law, who would be the first to admit he knows nothing about race car aerodynamics (although he does know a thing or two about making aircraft invisible to radar ) what he would do given the wind tunnel chart that's been floated around, and he said exhausting the air with a very low profile but wide duct at the rear of the roof line made the most sense to him. This is where Carl's car has the vortex generators.
Any comment?
Mark K has cut a duct out of the hood, Colin is talking about ducting out the back of the car.
I asked my engineer father-in-law, who would be the first to admit he knows nothing about race car aerodynamics (although he does know a thing or two about making aircraft invisible to radar
) what he would do given the wind tunnel chart that's been floated around, and he said exhausting the air with a very low profile but wide duct at the rear of the roof line made the most sense to him. This is where Carl's car has the vortex generators.Any comment?
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04-09-2009, 03:38 PM
#8
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04-09-2009, 04:15 PM
#9
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OB-S about 21.09sqft frontal area. Had to find it once to figure out HP loss at speed on the G-Tech. Came out about a 18-23 HP loss to wind at 70-75mph. S4 should be close in frontal area?
Here's where I found it.
http://www.mayfco.com/dragcd~1.htm
Finally here is a great drag calculator:
http://www.gtechprosupport.com/support/AeroDragCalc.php
Here's where I found it.
http://www.mayfco.com/dragcd~1.htm
Finally here is a great drag calculator:
http://www.gtechprosupport.com/support/AeroDragCalc.php
04-09-2009, 05:12 PM
#11
Rennlist Member
get a total drag in lbs at 100mph, and you can figure out the hp to drive it. each 5 lbs of drag is about 1hp at 100mph. 6000rpm in 3rd, 4.5:1 gear ratio.
my wing at 250lbs of downforce at 100mph is about 25lbs of drag or near 5ft-lbs of torque to drive it (near 6hp)
mk
my wing at 250lbs of downforce at 100mph is about 25lbs of drag or near 5ft-lbs of torque to drive it (near 6hp)
mk
04-09-2009, 07:01 PM
#12
Three Wheelin'
Hi Adam,
You might check out a couple books.
"Race car Aerodynamics: Designing for speed" by Joseph Kaltz
A little bit on vehicle aerodynamics is in "Engineer to win" by Carroll Smith
Mark Kibort has it figured out.
You might check out a couple books.
"Race car Aerodynamics: Designing for speed" by Joseph Kaltz
A little bit on vehicle aerodynamics is in "Engineer to win" by Carroll Smith
Mark Kibort has it figured out.
04-09-2009, 07:09 PM
#13
Rennlist Member
Thats really cool!
100mph for our car, costs 47hp!! (if the drag coef. is .34) put on some wings and other stuff, and things get worse in a hurry.
172mph WR for the 928. 242hp just for aero drag, plus any rolling friction.
200mph.................380rwhp! just for aero drag.
I'm thinking rolling friction is fairy constant. figure 20-30lbs to roll the car on a flat surface? at 100mph, thats near 6-7hp, at 200, more like 18hp. There you go, 400rwhp or so to go 200mph!
mk
100mph for our car, costs 47hp!! (if the drag coef. is .34) put on some wings and other stuff, and things get worse in a hurry.
172mph WR for the 928. 242hp just for aero drag, plus any rolling friction.
200mph.................380rwhp! just for aero drag.
I'm thinking rolling friction is fairy constant. figure 20-30lbs to roll the car on a flat surface? at 100mph, thats near 6-7hp, at 200, more like 18hp. There you go, 400rwhp or so to go 200mph!
mk
OB-S about 21.09sqft frontal area. Had to find it once to figure out HP loss at speed on the G-Tech. Came out about a 18-23 HP loss to wind at 70-75mph. S4 should be close in frontal area?
Here's where I found it.
http://www.mayfco.com/dragcd~1.htm
Finally here is a great drag calculator:
http://www.gtechprosupport.com/support/AeroDragCalc.php
Here's where I found it.
http://www.mayfco.com/dragcd~1.htm
Finally here is a great drag calculator:
http://www.gtechprosupport.com/support/AeroDragCalc.php
04-09-2009, 07:19 PM
#14
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Some things that a lot of people don't usually seem to take into consideration as far as that goes are wider tires, different styles of wheels, and fender flairs. They may improve looks or cornering, but they'll cost you when it comes to drag.