Master Thread - Aerodynamics
04-11-2009, 03:38 PM
#61
Three Wheelin'
[QUOTE=mark kibort;6470780]an analogy. you have a wing.(our cars are wings basicaly). if you cut holes in the front or bottom of the wing and routed it to the rear of the wing exiting up, what would happen? you would still have lift. if you vented the nose of the wing to just after the nose of the wing, you would reduce more lift as more air would be traveling over the wing now, (vs prior) so, you would be raisng the pressure along the top of the entire wing. venting to the rear, wouldnt have the same effect . why do airplanes have to be so concerned with icing? just a thin layer of turblulence over the wing reduces the differntial pressure vs the air running under the wing. result, lift is reduced (ideal for what we want to do), but drag is increased as well. We want to reduce the lift, as well as keep the flow as laminar as possible. letting the air vent from the hood area does this best and most efficiently, with the least amount of drag.
its been awhile since Ive looked at this stuff, but thats the general idea.
Mark,
Apparently you were trying to respond to a statement I made regarding the importance of attempting to match the outflow of ducted air to both the direction and velocity of the air flow in the low pressure area. I couldn't make much sense of what you said as it related to my comment.
I'll attach an illustration from "Race Car Aerodynamics" to better show what I was trying to say. There are four examples of outlet openings, A, B, C, D. Notice that there are no examples of just a hole. That's because it is so bad it isn't considered. In an earlier post, you mentioned your vent showed it was working by the tufts pointing straight up. I was trying to give you a tip that the straight up tufts would be indicating a big drag increase and that there is a better way of accomplishing the vent. While example "D" is most efficient for an outlet, it is the most complex. Sometimes a compromise is reached for the vent outlet by using simple louvers over the opening to direct the air rearward and increase the velocity at the outlet.
https://rennlist.com/forums/attachme...1&d=1239474850
its been awhile since Ive looked at this stuff, but thats the general idea.
Quote:
Originally Posted by Louie928 View Post
When you add vents to direct unwanted air from a high pressure zone to a lower pressure zone, you should also provide some ductwork, inside the body outside of the airstream, to accelerate the vented air and direct it in the same direction as the slipstream air. That is, do not simply cut a hole in a low pressure zone to vent the higher pressure air. That air will exit normal to the direction of the airstream and greatly increase the drag. Have the air exit at a small angle, less than 30 deg, to the surface and near the same velocity as the slipstream. Use yarn tufts to check out the flow direction and turbulence at the exit.
[/QUOTE
mk
Originally Posted by Louie928 View Post
When you add vents to direct unwanted air from a high pressure zone to a lower pressure zone, you should also provide some ductwork, inside the body outside of the airstream, to accelerate the vented air and direct it in the same direction as the slipstream air. That is, do not simply cut a hole in a low pressure zone to vent the higher pressure air. That air will exit normal to the direction of the airstream and greatly increase the drag. Have the air exit at a small angle, less than 30 deg, to the surface and near the same velocity as the slipstream. Use yarn tufts to check out the flow direction and turbulence at the exit.
[/QUOTE
mk
Apparently you were trying to respond to a statement I made regarding the importance of attempting to match the outflow of ducted air to both the direction and velocity of the air flow in the low pressure area. I couldn't make much sense of what you said as it related to my comment.
I'll attach an illustration from "Race Car Aerodynamics" to better show what I was trying to say. There are four examples of outlet openings, A, B, C, D. Notice that there are no examples of just a hole. That's because it is so bad it isn't considered. In an earlier post, you mentioned your vent showed it was working by the tufts pointing straight up. I was trying to give you a tip that the straight up tufts would be indicating a big drag increase and that there is a better way of accomplishing the vent. While example "D" is most efficient for an outlet, it is the most complex. Sometimes a compromise is reached for the vent outlet by using simple louvers over the opening to direct the air rearward and increase the velocity at the outlet.
https://rennlist.com/forums/attachme...1&d=1239474850
Last edited by Louie928; 06-13-2013 at 05:13 PM.
04-11-2009, 10:56 PM
#62
Racer
Join Date: Sep 2003
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Generally, venting or ducting air though the vehicle doesn’t help much with reducing drag, though it can certainly help with cooling and downforce generation. That’s why the drag is less with the flaps closed. Look at the land speed record machines. Very few vents.
04-12-2009, 01:35 AM
#63
Rennlist Member
You got the main message. I dont really care too much about the drag factor of that air exiting the vent. sure, I would LOVE to have a GT40 vent , but thats not really possible. The real point was you dont want the air exiting the sunroof via tubes and duct work, for the reasons I listed.
By the way, a flush exit is about a factor of 10 lower drag coefficient than exit guide tubes, and thats what I have. The tufts going straigth up was an exageration, they were at a 45degree angle inidicating the exit air angle.
mk
[QUOTE=Louie928;6472913]
By the way, a flush exit is about a factor of 10 lower drag coefficient than exit guide tubes, and thats what I have. The tufts going straigth up was an exageration, they were at a 45degree angle inidicating the exit air angle.
mk
[QUOTE=Louie928;6472913]
an analogy. you have a wing.(our cars are wings basicaly). if you cut holes in the front or bottom of the wing and routed it to the rear of the wing exiting up, what would happen? you would still have lift. if you vented the nose of the wing to just after the nose of the wing, you would reduce more lift as more air would be traveling over the wing now, (vs prior) so, you would be raisng the pressure along the top of the entire wing. venting to the rear, wouldnt have the same effect . why do airplanes have to be so concerned with icing? just a thin layer of turblulence over the wing reduces the differntial pressure vs the air running under the wing. result, lift is reduced (ideal for what we want to do), but drag is increased as well. We want to reduce the lift, as well as keep the flow as laminar as possible. letting the air vent from the hood area does this best and most efficiently, with the least amount of drag.
its been awhile since Ive looked at this stuff, but thats the general idea.
Mark,
Apparently you were trying to respond to a statement I made regarding the importance of attempting to match the outflow of ducted air to both the direction and velocity of the air flow in the low pressure area. I couldn't make much sense of what you said as it related to my comment.
I'll attach an illustration from "Race Car Aerodynamics" to better show what I was trying to say. There are four examples of outlet openings, A, B, C, D. Notice that there are no examples of just a hole. That's because it is so bad it isn't considered. In an earlier post, you mentioned your vent showed it was working by the tufts pointing straight up. I was trying to give you a tip that the straight up tufts would be indicating a big drag increase and that there is a better way of accomplishing the vent. While example "D" is most efficient for an outlet, it is the most complex. Sometimes a compromise is reached for the vent outlet by using simple louvers over the opening to direct the air rearward and increase the velocity at the outlet.
https://rennlist.com/forums/attachme...1&d=1239474850
its been awhile since Ive looked at this stuff, but thats the general idea.
Mark,
Apparently you were trying to respond to a statement I made regarding the importance of attempting to match the outflow of ducted air to both the direction and velocity of the air flow in the low pressure area. I couldn't make much sense of what you said as it related to my comment.
I'll attach an illustration from "Race Car Aerodynamics" to better show what I was trying to say. There are four examples of outlet openings, A, B, C, D. Notice that there are no examples of just a hole. That's because it is so bad it isn't considered. In an earlier post, you mentioned your vent showed it was working by the tufts pointing straight up. I was trying to give you a tip that the straight up tufts would be indicating a big drag increase and that there is a better way of accomplishing the vent. While example "D" is most efficient for an outlet, it is the most complex. Sometimes a compromise is reached for the vent outlet by using simple louvers over the opening to direct the air rearward and increase the velocity at the outlet.
https://rennlist.com/forums/attachme...1&d=1239474850
04-12-2009, 01:39 AM
#64
Rennlist Member
This is true. Also downforce does come at the expense of drag.
04-26-2009, 12:26 PM
#65
Rennlist Member
Thread Starter
I'd like some input on whether the top foil of this GT2 wing would be functional as the rear wing on an OB racer.
My understanding is that the S4 wing is more to clean up air flow than create downforce, and I'm curious if anyone has an opinion if this is a downforce wing (which it appear to be to me) or if it is just a spoiler. I'm talking about putting this wing on tall uprights like what the other racers are running.
I have the oppurtunity to pick this up today from a fellow R-lister.
Thanks,
My understanding is that the S4 wing is more to clean up air flow than create downforce, and I'm curious if anyone has an opinion if this is a downforce wing (which it appear to be to me) or if it is just a spoiler. I'm talking about putting this wing on tall uprights like what the other racers are running.
I have the oppurtunity to pick this up today from a fellow R-lister.
Thanks,
04-26-2009, 01:44 PM
#66
Three Wheelin'
I'd like some input on whether the top foil of this GT2 wing would be functional as the rear wing on an OB racer.
My understanding is that the S4 wing is more to clean up air flow than create downforce, and I'm curious if anyone has an opinion if this is a downforce wing (which it appear to be to me) or if it is just a spoiler. I'm talking about putting this wing on tall uprights like what the other racers are running.
I have the oppurtunity to pick this up today from a fellow R-lister.
Thanks,
My understanding is that the S4 wing is more to clean up air flow than create downforce, and I'm curious if anyone has an opinion if this is a downforce wing (which it appear to be to me) or if it is just a spoiler. I'm talking about putting this wing on tall uprights like what the other racers are running.
I have the oppurtunity to pick this up today from a fellow R-lister.
Thanks,
That wing should generate downforce for you. It depends on the mounting angle. Even the S4 wing will make downforce when it is mounted with the trailing edge raised from the normal position. Some of the ORR folks do that. I'm saying if you like the way this wing looks, get it. If you want to stay with a more stockish look, you can get downforce with the wing you have.
04-26-2009, 02:39 PM
#68
Three Wheelin'
04-26-2009, 09:49 PM
#69
Nordschleife Master
Join Date: Sep 2004
Location: Not close enough to VIR.
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It is a wing, not a spoiler. A spoiler is a vertical extension from the car. If you connected the leading edge of a wing to the car, it would become a spoiler.
04-27-2009, 01:11 AM
#70
Race Director
In an ideal world.....it would be VERY interesting to get some well known race 928's in a windtunnel and do some tests......the amount of downforce on the racers like Kibort-Anderson-Fan-Carl and the low drag of the ORR guys like Tim and George..... It would be very interesting to see the differences....
For example....the 600hp Dodge Viper has a top speed of 202mph....the ACR package with all its wings that make claimed 1000+lbs of downforce will top out at 177mph.....
For example....the 600hp Dodge Viper has a top speed of 202mph....the ACR package with all its wings that make claimed 1000+lbs of downforce will top out at 177mph.....
04-27-2009, 02:15 AM
#71
In order for a wing to produce lift, the air path over the top of the wing must be longer than the air flow path on the bottom. The air must travel faster over the top and thus will have less pressure. Less pressure on the top of the wing, and thus higher pressure on the bottom produces lift. It will be just the opposite for a "wing" to produce downforce. Depending on where the wing sits in the air stream..simply the wing should be flat on the top and have a curved bottom part... (all this ignores turbulent or non laminar flow)... I can't tell from the pic, but it seems to have a simple flat upper surface..and probably has a bulge on the leading bottom edge...which..according to theory should produce downforce. Of course angle of attack as Louie mentioned changes things too..
04-27-2009, 05:21 AM
#72
Rennlist Member
Thread Starter
The good news is that I did buy it. It's in perfect shape, looks like a good design, and will look good on the car. I'll try to set up a mock fit so everyone can see what it looks like on the car.
Thanks for the input everyone.
04-27-2009, 09:12 AM
#73
Owns the Streets
Needs Camber
Lifetime Rennlist
Member
Needs Camber
Lifetime Rennlist
Member
How high off the rear hatch will you mount the wing?
Think Kibort used the factory S4 wing on Aluminum spacers that raised the wing and added more wing angle to add some downforce with a little extra drag.
04-27-2009, 11:35 AM
#74
Parts Specialist
Rennlist Member
Rennlist Member
I am just stopping by to say this is a great group of guys and I cant wait to see that wing on Adam's car... GL with it !!
Keep the sharks alive, they are awsome!!! - love what you guys are doing and the performance you are achieving!
Keep the sharks alive, they are awsome!!! - love what you guys are doing and the performance you are achieving!
04-27-2009, 12:16 PM
#75
Rennlist Member
Yes, the S4 wing is a wing, just not as efficient at a "real" wing like the GT3 wing I have now. Remember my on road wind tunnel tests??? 
80lbs of down force at 10 degrees angle of attack at 100mph vs near 50% more down force than the GT3 wing.
the stand offs I used were the same shape as the mounting points, but made from a foam, plastic composite sandwitch and painted red.
I raised the wing about 3" from stock. it was very effective.
mk
80lbs of down force at 10 degrees angle of attack at 100mph vs near 50% more down force than the GT3 wing.
the stand offs I used were the same shape as the mounting points, but made from a foam, plastic composite sandwitch and painted red.
I raised the wing about 3" from stock. it was very effective.
mk