Most Effective Wing?
03-22-2011, 12:11 PM
#166
Three Wheelin'
the wheel openings vent air rearward as well. remember,we also have to look at drag that is created from air moving over the car too. but the main reason the wheel pods are rear ward, is because in that area, its in a spot where venting straight up will be venting to a high pressure zone. you have to realize that the low pressure zone is in a narrow area just passed the front bumper to about mid hood on most cars. after mid hood , the pressure rises significantly. almost as much as the nose pressure, and then it goes to the low pressure again as the air travels over the windshield and roof line. Its all a balacing act of flow and pressure control.
on my car for example, if i vented upward, at the rear of the wheel well, air would be forced in and wouldnt relieve its pressure as great as the sides would.
this is for drag reduction,not downforce obviously.
as far as Katz. I havent seen that study, so I cant comment on what he is talking about, but more air, even warm would be better and more effective for downforce and rear wing effect if it came originally from the under the car and through the hood. its adding to the mass air flow going over the car, AND, its releaving the pressure of the air over the car as well creating downforce. It could have been mentioned in a different context I would imagine. Plus, a lot of the concepts of downforce and air flow in regards to race cars is still a little black art. unless you are testing everything with sensors everywhere, or have a wind tunnel or incredible computer simulators.
mk
on my car for example, if i vented upward, at the rear of the wheel well, air would be forced in and wouldnt relieve its pressure as great as the sides would.
this is for drag reduction,not downforce obviously.
as far as Katz. I havent seen that study, so I cant comment on what he is talking about, but more air, even warm would be better and more effective for downforce and rear wing effect if it came originally from the under the car and through the hood. its adding to the mass air flow going over the car, AND, its releaving the pressure of the air over the car as well creating downforce. It could have been mentioned in a different context I would imagine. Plus, a lot of the concepts of downforce and air flow in regards to race cars is still a little black art. unless you are testing everything with sensors everywhere, or have a wind tunnel or incredible computer simulators.
mk
A couple things in response ...
As for warm air going over the car, I believe Katz was discussing it in a similar context. Re: whether we vent over the car vs. to the side, keep in mind that if we vent to the side, there is still lots of incident air that will travel over top of the car to create the differential pressure over the car vs. under, to create downforce. Venting air out the hood doesn't make more air go over the car, get to the rear wing, etc. It just causes warm from under the hood to displace other air that would have been there anyway (incident air), to flow over the car. Either way, there will be lots of air going over the car.
That video does show lots of hood movement. I guess I shouldn't be surprised. So venting that clearly will be an advantage, both in reducing drag and also relieving some lift that occurs from air being trapped in there (so essentially adding some downforce by venting it).
Scott
Last edited by stownsen914; 03-22-2011 at 02:07 PM. Reason: clarified the last sentence
03-22-2011, 02:11 PM
#167
Rennlist Member
what do you mean by "incident air"? are you talking "stagnation"?
Actually, the differential pressure that is created from top of the car (front , top and rear) are given by the shape and aero their aero effects. generally, the air is higher presure at the nose , lowe pressure front hood, higher pressure back hood (near cowl for cowl ram air) and lower windshield, roof, and then higher in the back, mostly due to a wing. Lower or higher, meaning compared to ambient pressure underneath of the car. (call all these pressures vs ambient for sake of arguement)
Like a venturi tube, you have a pressure drop across it (drag) and a lowering of pressure in the area where it speeds up. however, if you add more air molecules to the flow, you still raise the pressure overall. (like if you sped up what is feeding the air, or used denser air.) the formula is PV=nRT
so, in the case of our cars where air is flowing over it, if you divert air that would normally go under the car rolling off the nose, and route it to the sides, you keep the ambient pressure underneath the car. (i.e. splitter use). this also focuses a high pressure zone in front of the engine bay inlet mouth. in a cup car, same thing, but no engine.
now, the air has high pressure and it wants to go to low pressure. it goes into the engine bay and easily leaves the engine bay and goes under the car. (bad for downforce)
if you have a hood vent, where this air can actually more easily enter the car , go through the radiator, and leave out the vfent, you add to the flow over the car. two things happen. the air that would normally go under the car, goes over the car because air moves to differential pressure. there is a lower pressure point at the hood vent range, which the air wants to go, as under the car is ambient. (call it 0psi) hood vent might be -.2psi while nose pressure and base of windshield and rear of hood, might be +.2psi.
The additional air moving over the car, vs where it could have gone, under the car, adds to this differential pressure. in essence, you are trying to neutralize the "vaccum" (bad choise of terms , but you get the point) or lower pressure found at the top of the hood. this lowers the differential pressure and reduces lift of the car in that area. added benefit, is that the air now is more dense because of this and when it hits the wing, it is more effective. this is why the air should be routed over the car vs under the car and not to the sides.
As I mentioned earlier the reason that there are vents over the wheel wells, is for the same reason. the high pressure under the fenders is causing lift, usually fed by brake duct work and under body flow. there is also vents to the rear of the wheel well, to release even more pressure rearward, as in that area, above it is a higher pressure zone, so it cant flow upward ,so it flows rearward.
so , the reason that the hood is moving so much in the video , is because there is a differential pressure under the hood vs over the hood. most of it is going under the car, to the 0 pressure zone, but it wants to escape to the low pressure zone on top. absolutely, when you vent the hood, you releave this pressure and it adds to the air flow over the car, which is all good for downforce and rear wing effects. in essesnce, the hood vent is kind of turning the front of the car into a wing (albeit , much less efficient than a traditional airfoil, the same kind of things are happening) But like the venturi tube, we are forcing more air over the top of the car, raising its pressure vs without hood vents, and the under the car air flow is reduced lowing its pressure as well.
I would always want to add any air that would normally go under the car to over the car, as it would help, no mater its tempurature.
Actually, the differential pressure that is created from top of the car (front , top and rear) are given by the shape and aero their aero effects. generally, the air is higher presure at the nose , lowe pressure front hood, higher pressure back hood (near cowl for cowl ram air) and lower windshield, roof, and then higher in the back, mostly due to a wing. Lower or higher, meaning compared to ambient pressure underneath of the car. (call all these pressures vs ambient for sake of arguement)
Like a venturi tube, you have a pressure drop across it (drag) and a lowering of pressure in the area where it speeds up. however, if you add more air molecules to the flow, you still raise the pressure overall. (like if you sped up what is feeding the air, or used denser air.) the formula is PV=nRT
so, in the case of our cars where air is flowing over it, if you divert air that would normally go under the car rolling off the nose, and route it to the sides, you keep the ambient pressure underneath the car. (i.e. splitter use). this also focuses a high pressure zone in front of the engine bay inlet mouth. in a cup car, same thing, but no engine.
now, the air has high pressure and it wants to go to low pressure. it goes into the engine bay and easily leaves the engine bay and goes under the car. (bad for downforce)if you have a hood vent, where this air can actually more easily enter the car , go through the radiator, and leave out the vfent, you add to the flow over the car. two things happen. the air that would normally go under the car, goes over the car because air moves to differential pressure. there is a lower pressure point at the hood vent range, which the air wants to go, as under the car is ambient. (call it 0psi) hood vent might be -.2psi while nose pressure and base of windshield and rear of hood, might be +.2psi.
The additional air moving over the car, vs where it could have gone, under the car, adds to this differential pressure. in essence, you are trying to neutralize the "vaccum" (bad choise of terms , but you get the point) or lower pressure found at the top of the hood. this lowers the differential pressure and reduces lift of the car in that area. added benefit, is that the air now is more dense because of this and when it hits the wing, it is more effective. this is why the air should be routed over the car vs under the car and not to the sides.
As I mentioned earlier the reason that there are vents over the wheel wells, is for the same reason. the high pressure under the fenders is causing lift, usually fed by brake duct work and under body flow. there is also vents to the rear of the wheel well, to release even more pressure rearward, as in that area, above it is a higher pressure zone, so it cant flow upward ,so it flows rearward.
so , the reason that the hood is moving so much in the video , is because there is a differential pressure under the hood vs over the hood. most of it is going under the car, to the 0 pressure zone, but it wants to escape to the low pressure zone on top. absolutely, when you vent the hood, you releave this pressure and it adds to the air flow over the car, which is all good for downforce and rear wing effects. in essesnce, the hood vent is kind of turning the front of the car into a wing (albeit , much less efficient than a traditional airfoil, the same kind of things are happening) But like the venturi tube, we are forcing more air over the top of the car, raising its pressure vs without hood vents, and the under the car air flow is reduced lowing its pressure as well.
I would always want to add any air that would normally go under the car to over the car, as it would help, no mater its tempurature.
A couple things in response ...
As for warm air going over the car, I believe Katz was discussing it in a similar context. Re: whether we vent over the car vs. to the side, keep in mind that if we vent to the side, there is still lots of incident air that will travel over top of the car to create the differential pressure over the car vs. under, to create downforce. Venting air out the hood doesn't make more air go over the car, get to the rear wing, etc. It just causes warm from under the hood to displace other air that would have been there anyway (incident air), to flow over the car. Either way, there will be lots of air going over the car.
That video does show lots of hood movement. I guess I shouldn't be surprised. So venting that clearly will be an advantage, both in reducing drag and also relieving some lift that occurs from air being trapped in there (so essentially adding some downforce).
Scott
As for warm air going over the car, I believe Katz was discussing it in a similar context. Re: whether we vent over the car vs. to the side, keep in mind that if we vent to the side, there is still lots of incident air that will travel over top of the car to create the differential pressure over the car vs. under, to create downforce. Venting air out the hood doesn't make more air go over the car, get to the rear wing, etc. It just causes warm from under the hood to displace other air that would have been there anyway (incident air), to flow over the car. Either way, there will be lots of air going over the car.
That video does show lots of hood movement. I guess I shouldn't be surprised. So venting that clearly will be an advantage, both in reducing drag and also relieving some lift that occurs from air being trapped in there (so essentially adding some downforce).
Scott
Last edited by mark kibort; 03-22-2011 at 03:14 PM.
03-22-2011, 02:26 PM
#168
Rennlist Member
Sunday, I really dont see what is so offensive to you on my response.
you say below that you agree with hood vents. that you acknowledge that there is greater pressure under the hood than on top.
However, you make the critical error in thinking that the increased flow to the hood through a vent , that will go over the car, will lower the pressure ? (backed up with the thought you had that going to the sides would be preferable). if there is high pressure under the hood, and lower pressure on top of the hood, venting to the lower pressure will not further lower pressure, but RAISE pressure , which is good for downforce. you are confusing flow with pressures. actually, this raising of the pressure will certianly increase flow over the car AND raise those already very low pressures creating downforce. (and help with rear wing effectiveness). just because you add flow to the top of the car vs the bottom of the car, doesnt mean you increase lift. again, PV=nRT. we are increasing the n and not changing V, so P will go up. It would act like a wing and create lift, as it does normally on an unmodified car, as the air flow over and under the car, if the V over the car is traveling faster, then P would have to go down because nRT would be the same (well reletively the same) this is how we get lift on the race car which is bad. we short circuit the flow with hood vents.
you say below that you agree with hood vents. that you acknowledge that there is greater pressure under the hood than on top.
However, you make the critical error in thinking that the increased flow to the hood through a vent , that will go over the car, will lower the pressure ? (backed up with the thought you had that going to the sides would be preferable). if there is high pressure under the hood, and lower pressure on top of the hood, venting to the lower pressure will not further lower pressure, but RAISE pressure , which is good for downforce. you are confusing flow with pressures. actually, this raising of the pressure will certianly increase flow over the car AND raise those already very low pressures creating downforce. (and help with rear wing effectiveness). just because you add flow to the top of the car vs the bottom of the car, doesnt mean you increase lift. again, PV=nRT. we are increasing the n and not changing V, so P will go up. It would act like a wing and create lift, as it does normally on an unmodified car, as the air flow over and under the car, if the V over the car is traveling faster, then P would have to go down because nRT would be the same (well reletively the same) this is how we get lift on the race car which is bad. we short circuit the flow with hood vents.
Exactly! There is high pressure under the hood, creating lift. The option is to vent the high pressure which reduces the lift on the underside of the hood, but the compromise is that it lowers the pressure on top of the hood with increased flow and creates lift. Which is better?
Clearly venting the high pressure is better and results in a net reduction in lift. This has been proven over and over with fender vents and the fact that it is being widely used by production car teams, with wind tunnel access, tells us that is true for hood vents too.
One of my points is that the best is if it can be vented out the side - then you do not increase lift on top of the hood. But production car rules limit what you can do with the body, so it appears this is really not possible to properly vent to the sides.
Clearly venting the high pressure is better and results in a net reduction in lift. This has been proven over and over with fender vents and the fact that it is being widely used by production car teams, with wind tunnel access, tells us that is true for hood vents too.
One of my points is that the best is if it can be vented out the side - then you do not increase lift on top of the hood. But production car rules limit what you can do with the body, so it appears this is really not possible to properly vent to the sides.
Last edited by mark kibort; 03-22-2011 at 03:13 PM.
03-22-2011, 05:01 PM
#171
Three Wheelin'
By incident air, I mean air through which the car moves as it drives, the air moving toward the front of the car. This air will flow over the car regardless of any air coming out of hood vents, fender louvers, etc. In fact, this air should dwarf the volume of air coming out of any vents in hood/fenders. I would think the air coming out of a hood vent would be "dirty"/non-laminar in comparison, so would not contribute much to clean air flow over the car, at least for downforce purposes.
I've certainly no special training or knowledge to support that venting out the side is better than through the hood. Just what I've seen and read. But for what it's worth, it seems designers of a number of contemporary prototypes prefer the side vent method. GT cars more often use hood vents, which must do a good job or they wouldn't have them. But keep in mind that properly designed side vents may not be an option for them due to rules constraints (but no, I haven't read the rules).
Scott
03-22-2011, 05:45 PM
#172
Mr. Excitement
Rennlist Member
Rennlist Member
Mark we need to find some kind of solvent that dissolves snark and soak your posts in it. At times you have interesting things to say but for Dog’s sake you are one truly irritating person. Why is it you pop up and post so much in almost every thread I find interesting? I find that I agree with some of what you post but following up would risk contracting kibortpostosis.
Small item. most people put the “reply with quote” quote at the top then reply under it. Considering that your posts tend to run well past an un-scrolled down screens worth of space I recommend you follow convention. This way we can at least see what it is you are talking about.
BTW the 911 has an larger better laid out hood vent than pictured. This was added after I did some digging around and the info I found suported hood vents as a good thing.
Small item. most people put the “reply with quote” quote at the top then reply under it. Considering that your posts tend to run well past an un-scrolled down screens worth of space I recommend you follow convention. This way we can at least see what it is you are talking about.
BTW the 911 has an larger better laid out hood vent than pictured. This was added after I did some digging around and the info I found suported hood vents as a good thing.
03-22-2011, 05:51 PM
#173
Rennlist Member
I see.
Im no expert either, past what I have learned and experiemented with in the field. However there are certainly characteristics of air flow that still remain consistant, and that are true to things flying through the air, even if we dont want them to fly!
Actually, the air coming out of the well designed hood vents, is not dwarfed by the air moving out of the car. keep in mind, the air is facing the car and hits the nose and below. the frontal area there hits the air and it moves either over or under the car. half of that air can enter the engine compartment, and flow through the hood vent. think GT40, or FordGT. this high pressure air from the nose, is routed out the lowest pressure of the hood, which is front/middle. it exits and adds to the flow. sure, it can disturb laminar flow, and create drag, but all downforce will create lift. Drag due to lift (or downforce) is a the common denominator. however we are dealing with differntial pressure of over and under the car. the more air that would go over the car, the higher the over the car pressures will be. if you vent to the sides, you dont get the benefit of raising the pressures over the top of the car, that is an important point, PLUS, you get more air flow over the aero surfaces at the rear of the car (i.e wing). People often think that more flow over the car will increase lift, it doesnt, unless an equal amount of flow was going under the car. if you divert the flow to OVER the car, you raise the over the car pressrue, and that directly effects the downforce levels to be greater. (not adding lift, but raising pressures and creating downforce)
The reason venting to the side is not as effective from many perspectives, is that the air venting to the side, is being fed to a higher pressure, so it will be a lower pressure differential, which means LESS FLOW. also, the low pressures over the car will be unchanged , not changing the downforce. certainly you would not be feeding the rear wing either with this flow. warmer air or not, you want to add to the flow going over the car, because you are not adding to the flow going under the car. this adds downforce.
Splitters do what you are talking about. it takes air, that normally would go under the car and routes it around the car. (along with air dams) the problem is that you cant stop all of it from going under the car, because it still is a lower pressure zone compared to the frontal air impact area. so the splitter becomes closer and closer to the ground. it has limits for obvious reasons. so, the hood vent was used to take the air that needed (or didnt need by rear engine cars), to take all the air that was piled up by the splitter at the nose area and vent it all out the hood, raising the pressure over the hood and feeding the high pressure at the base of the windshield, as well as the rear wing.
do you have any links to the contray for side release of incoming air?
if you look at daytona prototypes, all this theory is put in to action.
I just noticed a section of the hood vent pointing to the outside after coming out of the hood for downforce. maybe its done its job at this point, there is plenty of flow over the car at the rear for wing, so no need to increase the air flow over the cabin and roof area. interesting!
Im no expert either, past what I have learned and experiemented with in the field. However there are certainly characteristics of air flow that still remain consistant, and that are true to things flying through the air, even if we dont want them to fly!
Actually, the air coming out of the well designed hood vents, is not dwarfed by the air moving out of the car. keep in mind, the air is facing the car and hits the nose and below. the frontal area there hits the air and it moves either over or under the car. half of that air can enter the engine compartment, and flow through the hood vent. think GT40, or FordGT. this high pressure air from the nose, is routed out the lowest pressure of the hood, which is front/middle. it exits and adds to the flow. sure, it can disturb laminar flow, and create drag, but all downforce will create lift. Drag due to lift (or downforce) is a the common denominator. however we are dealing with differntial pressure of over and under the car. the more air that would go over the car, the higher the over the car pressures will be. if you vent to the sides, you dont get the benefit of raising the pressures over the top of the car, that is an important point, PLUS, you get more air flow over the aero surfaces at the rear of the car (i.e wing). People often think that more flow over the car will increase lift, it doesnt, unless an equal amount of flow was going under the car. if you divert the flow to OVER the car, you raise the over the car pressrue, and that directly effects the downforce levels to be greater. (not adding lift, but raising pressures and creating downforce)
The reason venting to the side is not as effective from many perspectives, is that the air venting to the side, is being fed to a higher pressure, so it will be a lower pressure differential, which means LESS FLOW. also, the low pressures over the car will be unchanged , not changing the downforce. certainly you would not be feeding the rear wing either with this flow. warmer air or not, you want to add to the flow going over the car, because you are not adding to the flow going under the car. this adds downforce.
Splitters do what you are talking about. it takes air, that normally would go under the car and routes it around the car. (along with air dams) the problem is that you cant stop all of it from going under the car, because it still is a lower pressure zone compared to the frontal air impact area. so the splitter becomes closer and closer to the ground. it has limits for obvious reasons. so, the hood vent was used to take the air that needed (or didnt need by rear engine cars), to take all the air that was piled up by the splitter at the nose area and vent it all out the hood, raising the pressure over the hood and feeding the high pressure at the base of the windshield, as well as the rear wing.
do you have any links to the contray for side release of incoming air?
if you look at daytona prototypes, all this theory is put in to action.
I just noticed a section of the hood vent pointing to the outside after coming out of the hood for downforce. maybe its done its job at this point, there is plenty of flow over the car at the rear for wing, so no need to increase the air flow over the cabin and roof area. interesting!
By incident air, I mean air through which the car moves as it drives, the air moving toward the front of the car. This air will flow over the car regardless of any air coming out of hood vents, fender louvers, etc. In fact, this air should dwarf the volume of air coming out of any vents in hood/fenders. I would think the air coming out of a hood vent would be "dirty"/non-laminar in comparison, so would not contribute much to clean air flow over the car, at least for downforce purposes.
I've certainly no special training or knowledge to support that venting out the side is better than through the hood. Just what I've seen and read. But for what it's worth, it seems designers of a number of contemporary prototypes prefer the side vent method. GT cars more often use hood vents, which must do a good job or they wouldn't have them. But keep in mind that properly designed side vents may not be an option for them due to rules constraints (but no, I haven't read the rules).
Scott
I've certainly no special training or knowledge to support that venting out the side is better than through the hood. Just what I've seen and read. But for what it's worth, it seems designers of a number of contemporary prototypes prefer the side vent method. GT cars more often use hood vents, which must do a good job or they wouldn't have them. But keep in mind that properly designed side vents may not be an option for them due to rules constraints (but no, I haven't read the rules).
Scott
03-22-2011, 06:09 PM
#174
Rennlist Member
Hey Kurt.... thanks, I think . 
I understand what you are saying. surely I can easily change that reply with quote orientation to be more easily read.
I pop up so much when multiple posters are saying similar things, that I feel the thread might be helped with some additional information to see another way of thinking about the answer or situation. This is a great example of it. the hood vent. another great topic. thats why im posting about it. people thinking that more flow over the top would lower pressures not raise them. It is some complicated principles, so I try to use some analogies and descriptions that might be a little unclear due to the fact it has been years and years since Ive really studied this stuff, but the basics still remain clear. Its a discussion, so i would expect those with a different opinion to chime in and make corrections or add additional information to make their case in another direction, and I certainly would welcome that!
Now, if someone chimes in an posts a contrarian view to the masses, thats irritating? well, then, be prepared for more irritation. Its called a discussion. Im not always going to be right, but I will learn from if I am not.
I understand what you are saying. surely I can easily change that reply with quote orientation to be more easily read.
I pop up so much when multiple posters are saying similar things, that I feel the thread might be helped with some additional information to see another way of thinking about the answer or situation. This is a great example of it. the hood vent. another great topic. thats why im posting about it. people thinking that more flow over the top would lower pressures not raise them. It is some complicated principles, so I try to use some analogies and descriptions that might be a little unclear due to the fact it has been years and years since Ive really studied this stuff, but the basics still remain clear. Its a discussion, so i would expect those with a different opinion to chime in and make corrections or add additional information to make their case in another direction, and I certainly would welcome that!
Now, if someone chimes in an posts a contrarian view to the masses, thats irritating? well, then, be prepared for more irritation. Its called a discussion. Im not always going to be right, but I will learn from if I am not.
Mark we need to find some kind of solvent that dissolves snark and soak your posts in it. At times you have interesting things to say but for Dog’s sake you are one truly irritating person. Why is it you pop up and post so much in almost every thread I find interesting? I find that I agree with some of what you post but following up would risk ....
Small item. most people put the “reply with quote” quote at the top then reply under it. Considering that your posts tend to run well past an un-scrolled down screens worth of space I recommend you follow convention. This way we can at least see what it is you are talking about.
BTW the 911 has an larger better laid out hood vent than pictured. This was added after I did some digging around and the info I found suported hood vents as a good thing.
Small item. most people put the “reply with quote” quote at the top then reply under it. Considering that your posts tend to run well past an un-scrolled down screens worth of space I recommend you follow convention. This way we can at least see what it is you are talking about.
BTW the 911 has an larger better laid out hood vent than pictured. This was added after I did some digging around and the info I found suported hood vents as a good thing.
03-22-2011, 06:44 PM
#175
Race Car
Thread Starter
Seriously, this would be a better place if you learned to play nice with others or just stopped posting.
Scott
03-22-2011, 11:03 PM
#176
Rennlist Member
The hood vents on all three cars in the pics are using a NACA low pressure duct profile ( the narrow front widened rear curved delta) which is typically used to create low pressure area to encourage venting through an aperture. Which suggests that the ram air flow through the radiators is getting stalled at the face.
Splitters are interesting in that the primary downforce is created by redirecting the airstream down on the top surface of the splitters planform, almost like standing on it. Too much venting in that area and you will loose one of the largest contributors to downforce on the front of the car no matter where the outlet is...
Splitters are interesting in that the primary downforce is created by redirecting the airstream down on the top surface of the splitters planform, almost like standing on it. Too much venting in that area and you will loose one of the largest contributors to downforce on the front of the car no matter where the outlet is...
03-23-2011, 12:47 PM
#177
Rennlist Member
Richard,
This is not true. the hood vents, expecially on the GT40 are like a NACA duct shape, but are not adding any significant pressure drop to pull air out. remember differential pressure! the nose pressure is very high. highest than any point on the car, then at the hood, there will be lack of pressure there, and it will move with great vengence to the areas of lower pressure.
why do you think the ram flow is getting stalled at the face?
also, the vents they are using in the cars pictured above are for venting at the same making the flow efficient. if they wanted to create an even lower pressure, they would have a large bump before the exit of the duct. none of them have this. The NACA shape you usually see, and might be the shape of these ducts , are used to creat high pressure zones to push air in with minimal drag. You are quoting quite the opposite of the purpose of most NACA ducts.
to the splitter comment, thats not true at all either. this is why you dont need much support on a splitter as its only pressure downward is the frontal pressure of the nose ram air effect. the downforce effect comes primarily from the air not going under the car, but being diverted around the car. it increases substantially when diverted to the hood vents for the reasons I pointed out. ideally, if you could get all the flow entering the engine compartment, you would have more downforce than a splitter could create. thats why datona prototypes have no spltters . their inlests are at the splitter area and are forcing the air in the inlet duct and out the low pressure point of the hood.
Do you have any information that can back up your statements?
EDIT :
Here is a drawing/chart of where the pressure areas are of a 928. they are not that different than most any other sports car, but the point is, you can see the pressure zones clearly (on other cars these zones will shift slightly,but they still remain for most any car)
take a look at the pressure at the nose, going to vacuum on the hood, and then pressure at the base of othe windshield., back to vacuum on the roof, and then neutral at the rear. this profile is what is being exploited with hood vents near the front of the front wheel as far as position. and ram air from the nose.
keep in mind, these pressures are near .2psi at 120mph generally, and -.15psi in the vacuum (or negative pressure) areas. if you want to do the math, take the splitter for example and figure out the differential pressure or downforce on the plate itself. 50" across x 3" wide only produces about 22lbs of downforce on the plate itself, but its effect can be enough to offset a 500lb downforce rear wing. It works for other reasons than direct plate forces.
the chart also clearly shows why the position of the hood vents are where they are. Its all about differential pressure.
This is not true. the hood vents, expecially on the GT40 are like a NACA duct shape, but are not adding any significant pressure drop to pull air out. remember differential pressure! the nose pressure is very high. highest than any point on the car, then at the hood, there will be lack of pressure there, and it will move with great vengence to the areas of lower pressure.
why do you think the ram flow is getting stalled at the face?
also, the vents they are using in the cars pictured above are for venting at the same making the flow efficient. if they wanted to create an even lower pressure, they would have a large bump before the exit of the duct. none of them have this. The NACA shape you usually see, and might be the shape of these ducts , are used to creat high pressure zones to push air in with minimal drag. You are quoting quite the opposite of the purpose of most NACA ducts.
to the splitter comment, thats not true at all either. this is why you dont need much support on a splitter as its only pressure downward is the frontal pressure of the nose ram air effect. the downforce effect comes primarily from the air not going under the car, but being diverted around the car. it increases substantially when diverted to the hood vents for the reasons I pointed out. ideally, if you could get all the flow entering the engine compartment, you would have more downforce than a splitter could create. thats why datona prototypes have no spltters . their inlests are at the splitter area and are forcing the air in the inlet duct and out the low pressure point of the hood.
Do you have any information that can back up your statements?
EDIT :
Here is a drawing/chart of where the pressure areas are of a 928. they are not that different than most any other sports car, but the point is, you can see the pressure zones clearly (on other cars these zones will shift slightly,but they still remain for most any car)
take a look at the pressure at the nose, going to vacuum on the hood, and then pressure at the base of othe windshield., back to vacuum on the roof, and then neutral at the rear. this profile is what is being exploited with hood vents near the front of the front wheel as far as position. and ram air from the nose.
keep in mind, these pressures are near .2psi at 120mph generally, and -.15psi in the vacuum (or negative pressure) areas. if you want to do the math, take the splitter for example and figure out the differential pressure or downforce on the plate itself. 50" across x 3" wide only produces about 22lbs of downforce on the plate itself, but its effect can be enough to offset a 500lb downforce rear wing. It works for other reasons than direct plate forces.
the chart also clearly shows why the position of the hood vents are where they are. Its all about differential pressure.
The hood vents on all three cars in the pics are using a NACA low pressure duct profile ( the narrow front widened rear curved delta) which is typically used to create low pressure area to encourage venting through an aperture. Which suggests that the ram air flow through the radiators is getting stalled at the face.
Splitters are interesting in that the primary downforce is created by redirecting the airstream down on the top surface of the splitters planform, almost like standing on it. Too much venting in that area and you will loose one of the largest contributors to downforce on the front of the car no matter where the outlet is...
Splitters are interesting in that the primary downforce is created by redirecting the airstream down on the top surface of the splitters planform, almost like standing on it. Too much venting in that area and you will loose one of the largest contributors to downforce on the front of the car no matter where the outlet is...
Last edited by mark kibort; 03-23-2011 at 02:01 PM.
03-23-2011, 12:53 PM
#178
Rennlist Member
Please help me understand how you would like me to present my seemingly contrarian point of view as to not upset you. Everyone gets a little too personal with their thoughts being proved wrong or contradicted.
Im trying to post facts.
for example, how would you approach J Richard that seems to completely disagree with all the concepts I am referring to here? He is saying the hood vents are being used to create low pressure and air is stalling at the nose. things Ive proved even with my own sensors and flow meters to not be true. He even classifies the ducts as "NACA low pressure ducts". NACA ducts are for high pressure zone creation , at reduced drag for inlets, NOT exhaust flow. their shape may resemble a NACA duct, but thats where the simularity ends.
He is also contray to popular knowledge of how splitters work. (force is not really on the spliter itself, but in the air being redirected around or through the car.) Give me an example, maybe PM me, how you would respond!
thanks
Mark
Im trying to post facts.
for example, how would you approach J Richard that seems to completely disagree with all the concepts I am referring to here? He is saying the hood vents are being used to create low pressure and air is stalling at the nose. things Ive proved even with my own sensors and flow meters to not be true. He even classifies the ducts as "NACA low pressure ducts". NACA ducts are for high pressure zone creation , at reduced drag for inlets, NOT exhaust flow. their shape may resemble a NACA duct, but thats where the simularity ends.
He is also contray to popular knowledge of how splitters work. (force is not really on the spliter itself, but in the air being redirected around or through the car.) Give me an example, maybe PM me, how you would respond!
thanks
Mark
No, Mark, you just don't get it. The problem is not that you have a different opinion. It's your presentation of the opinion that makes you so damn obnoxious and irritating.
Seriously, this would be a better place if you learned to play nice with others or just stopped posting.
Scott
Seriously, this would be a better place if you learned to play nice with others or just stopped posting.
Scott
Last edited by mark kibort; 03-23-2011 at 01:28 PM.
03-23-2011, 02:07 PM
#179
Still plays with cars.
Lifetime Rennlist
Member
Mark, my pal who races a C6 Z06 with a lot of aero mods has a carbon fiber splitter. It is so well braced that you can stand on it - jump on it even. Why? Because a similar splitter in glass fiber broke because it could not support the downforce. Maximum speed on our home track is around 260 KPH in that car. The redesigned splitter and internal bracing was done by John Powell - he of Corvette racing fame. The car also had the benefit of wind tunnel testing in North Carolina under John's supervision. Downforce is on the order of 1,000 lbs and is matched front and rear. Not much downward force on the splitter you say? Wind tunnel says BZZZZT.
03-23-2011, 02:08 PM
#180
Rennlist Member
I have a question. based on pressure curves, i wonder why someone hasnt put a huge air duct from the bottom of the car venting through the cockpit, up to the roof . with up to 8x the negative presssure vs under the car, it seems that might be the biggest potential for downforce. sunroof open?