Landracing Forum
Tech Information => Technical Discussion => Topic started by: Sumner on October 12, 2004, 02:08:00 PM
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I've been going to B'ville for the past 14 years, but don't remember ever seeing NASCAR style roof flaps on any cars there.
I'm wondering if we installed roof flaps on a '53 stude we run in competition coupe if that would be acceptable under the rules? Does anyone have any thoughts on this?
NASCAR orientes one at a 140-degree angle from the centerline of the car on the right side since they feel most cars spin in that direction (not maybe the case at b'ville). Once this flap opens, it disrupts the airflow over the roof, killing all of the lift. An area of high pressure forms in front of the flap. This high-pressure air blows through a tube that connects to the pocket holding the second flap, causing the second flap to deploy. The second flap, which is oriented at 180 degrees, makes sure that the car continues to kill the lift as it rotates. After the car has spun around once, it has usually slowed to the point that it no longer produces lift.
The above was taken off of a site explaining the flaps. I'm wondering at b'ville if it might be better to have 3 flaps. One pointed towards each side and one facing back.
After seeing some cars become airborn at b'ville over the years it looks like this could maybe improve safety there. I'm not saying I would like to make it mandatory, but optional if the crew desired to do it. Maybe even in the production classes if the roof flap installation showed no improvement in aero over a stock roof.
c ya, Sum
<small>[ October 12, 2004, 02:23 PM: Message edited by: 1FATGMC ]</small>
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John Rains has them on his super fast fire bird set up on an angle like the nascars
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Thanks, you don't by any chance have pictures of the car? Did he angle both sides or just one side?
Anyone know how I could contact John?
c ya, Sum
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My car has two roof flaps and I would be interested in reading more about about the design and layout of nascar flaps. Could you give me the site that you pulled the information above off. Thanks.
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Here are two sites:
http://auto.howstuffworks.com/nascar-safety1.htm (http://auto.howstuffworks.com/nascar-safety1.htm)
and,
http://www.motorage.com/motorage/article/articleDetail.jsp?id=88747 (http://www.motorage.com/motorage/article/articleDetail.jsp?id=88747)
These aren't great articles and don't go into much detail. I've considered trying to contact a NASCAR team and see if I can get a few more details.
I would be interested in any photos of what you have done and your ideas on the subject.
Thanks,
Sum
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Whatever became of this subject? Any conclusions?
.....Rip Van Winkle
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Thanks, you don't by any chance have pictures of the car? Did he angle both sides or just one side?
I have some pictures of John's car, but won't be able to post them until morning when I get home from work.
Okay here is a roof picture from the rear of his car at 2007 speedweeks impound.
The NASCAR entry that spun also was as expected running roof flaps and the depoyed during the spin and it stayed on the ground at about 200 mph. (although that did not keep the windshield in the car)
I posted pictures of the spin in the NASCAR thread.
http://www.landracing.com/forum/index.php/topic,2936.0.html
Larry
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Lucky,
Believe they put a flap or flaps on Hooley's Stude, just checked my pics from this year's speedweek and couldn't determine if they are still on it or in what configuration.
Believe Sumner is at the big shootout this week so will prob be awhile before he gets back to his 'puter and gives us the latest. Meantime you might click on his link above in his post,"my Bonneville pages", and check around, should be some info there.
Good question, I have been wondering lately how they might work for other body styles, liners, lakesters, etc.
Ed
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If someone really is interested in reading a technical paper about this subject. Here is the SAE paper on the development of the roof flap ($14). A2's very own Gary Eaker was heading up the GM wind tunnel when they came up with the idea and was one of the authors of the paper.
http://www.sae.org/technical/papers/942522 (http://www.sae.org/technical/papers/942522)
Author(s):
Gary Nelson - National Association of Stock Car Auto Racing
Jack Roush - Roush Industries, Inc.
Gary Eaker - General Motors Corp.
Stan Wallis - Ford Motor Co.
Abstract:
This paper describes the effort initiated by the National Association of Stock Car Auto Racing (NASCAR), and joined by General Motors Corporations (GM), Ford Motor Company (FORD) and Roush Industries, Inc. (Roush) to develop and manufacture a roof- mounted, aero flap system (referred hereafter as "roof flaps") for race car applications. This system addresses the problem of how to greatly reduce the severity of accidens involving spinning race cars by means of preventing aerodynamic lift-off. There were several NASCAR race car accidents in 1993 that involved cars which became airborne.
A variety of tests and procedures, both inside and outside the wind tunnel environment, were conducted to evaluate and consider possible solutions to this problem. Ultimately, a roof flap system was produced that has significantly reduced extreme yaw lift in race cars. The proof of success of this system has been the virtual elimination of aero lift-off in NASCAR race cars since the introduction of roof flaps, when coupled with previously mandated anti-lift devices, in February 1994.
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How does having the left side windows open in NASCAR translate to LSR cars?
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If you are talking to me. A left side window open would not, but roof flaps is the subject. I was simply putting info out there so people could go to the source of the exact science behind the flaps, how they were tested/developed, and then put to use. There might be someone interested in the technology. Instead of people asking the question... how can we find out more about the roof flaps? I am putting the info at their finger tips. Relevant to LS or not.. there is an SAE paper on roof flaps for anyone to read and learn about the technology.
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I guess what I was trying to ask is -- Do they work on an enclosed cockpit?
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They should work on a fully enclosed passenger car body, or the typical open window application as used in NASCAR.
If you look closely at John Rains car you will see two small air ports on the sides of the vehicle behind the doors. These provide slight suction to the roof flap boxes when the car is at speed in normal attitude, but when the car yaws excessively they would pressurized the roof flap box to help open the flap into the air stream.
I would think the same design could be made workable on open cockpit cars like the roadsters and cars similar to the Bocar that blew over this year when it got sideways. Likewise even a streamliner. You would need to do some model tests to find best placement for those applications.
I think it would be a good idea to allow applying the NASCAR roof rail concept to other body styles by allowing builders to put 2 four to five foot long, by 1/2 - 3/4 inch high strakes on the largest top surface of the car to spoil lift if the car gets sideways and induce down force if it is in significant yaw angle to the air flow.
This would be no issue on the special construction cars since their body design is unlimited but for the open cockpit modified sports etc. it is not clear to me if it would be legal to put a NASCAR roof rail like strake down the top of the hood to spoil lift of the car if it spins.
Larry
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They should work fine on an enclosed cockpit. However, there are several things to think about.
-time + money to install the flaps. It could be very difficult and expensive to install correctly. (It is hard to put a price on safety)
-They are configured for a stock car/truck turning left and spinning left, so how do you configure them for a LS car without development?
-testing the roof flaps. There is a good chance that they will not work the same on all type/styles of cars. If you wanted to test the flaps you would need a wind tunnel with high yaw capability. Lockheed, Langley or the Manufactures are $2000+/hr. Or, go out at high speed and spin your car to see if they deploy and work. :? (I’m sure there are not too many people crazy about that idea)
-Not all cars become airborne in a spin at Bonneville, so witch cars install them?
-It might be a better use of time and money to work on the aerodynamic balance of your car to help the stability at high speeds so you don’t spin out.
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-They are configured for a stock car/truck turning left and spinning left, so how do you configure them for a LS car without development?
Good point, John canted both roof flap boxes symmetrically unlike the typical NASCAR install that places the right hand roof flap at an angle and the left flap in line with the car. When I was talking to him he said he had never needed them but thought it better to have them and not need them than need them and not have them. As you mentioned in an untested install you mostly have a "good faith" safety measure that you trust is more likely to help than it is to hurt the cars spin behavior.
Although untested is a gray area, wing spoilers and the principles for spoiling lift are pretty well understood, and even very simple measures to spoil flow over a wing like structure will disrupt the airflow, killing lift and building pressure on the surface in front of the obstruction. The FAA also has some guidance on low tech wing spoilers you can strap to a planes wing while it it is parked on the ramp to kill lift in high wind situations so the plane does not try to fly on its tie downs. Living in a high wind area I have actually seen light planes hovering 3 inches off the ground on the ramp at the airport down the road during chinook wind events where wind speeds were 90-110 mph.
It does not take much of an obstruction (like the NASCAR roof rails) to cause total flow separation and turbulent flow over the upper surface.
If I were doing it, I would certainly spend some time with some car models to get at least some basic testing data to see what the air flow is like over that car body style at 30*, 60* 90* and 120* yaw angles so you had some clue where to best place the devices.
As mentioned in the FAA circular AC 20-35C (page 14) all it takes to spoil lift on a wing is a 2x2 board on the front 25% of the cord of the wings top surface. Applying that rational to a car roof or hood should be a fairly simple task looking at the car body as a wing section and placing an obstruction that would deploy at high yaw angles in that first 25% of the cars roof or upper body, to spoil the airflow and build induced pressure on the roof section on the upwind side of the obstruction.
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/3121C979AF8A048C862569D60074B3B3?OpenDocument
Larry
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Hey Larry, do you live in Alaska? Or do they have chinook winds in the lower 48?
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Usually south of Alaska:
http://en.wikipedia.org/wiki/Chinook_wind (http://en.wikipedia.org/wiki/Chinook_wind)
Mike
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Hey Larry, do you live in Alaska? Or do they have chinook winds in the lower 48?
No I live in Colorado. The entire eastern front range of the Rockies are subject to them, usually during the dry winter months of December, January and February (had my fence blown down 2 years ago by 110 mph gusts -- took the 4x4's and broke them off at the ground).
I live Northwest of Denver. That area near Boulder/Broomfield is notorious for these wind storms, sort of like the Santa Anna winds in CA. We have more hurricane force wind days than most of the gulf coast does.
There is one scenic overlook near my house that I can park my car at during those storms and have a free open air wind tunnel with steady wind speeds of 80 mph with gusts to over 110 mph. I have done some tuft tests up there and you can park the car at any yaw angle you want to the wind.
http://www.ucar.edu/communications/factsheets/winds.html
http://www.bcna.org/winds.html
Larry
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Believe they put a flap or flaps on Hooley's Stude, just checked my pics from this year's speedweek and couldn't determine if they are still on it or in what configuration
(http://purplesagetradingpost.com/sumner/hooley/2006-Const-15.jpg)
Yes we have one roof flap and it is still on the car and the car has never gotten in the air on it's partial spins....
2007 -- http://www.youtube.com/watch?v=VyWUfWaWEDQ&eurl=
2006 -- http://www.youtube.com/watch?v=CU59SUocJoA
........ but I can't tell you for sure if the roof flap helped or not, but it did open all the way.
More on the construction here....
http://purplesagetradingpost.com/sumner/hooley/hooley-construction-2006-1.html
c ya,
Sum
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We are putting one roof flap in because that's all that will fit with the narrow roof. All the current flaps are made the same way and are the same size with two used on circle track cars, Cup Cars? The two come as a pair with the air passage molded between them but we cut our's apart to use a single.
I like the spring pop up Hooley has on his car and we will be doing the same. I cannot find a good place to pick up air for pressurizing the underside of the flap to automatically pop the flap open if we go sideways like the NASCAR boys.
Perhaps if I replace the glass quarter windows I might place a duct there. Oh, yes another test to do in the wind tunnel! :-D
I have some extra flaps and will post them in the for sale section.
Geo
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I cannot find a good place to pick up air for pressurizing the underside of the flap to automatically pop the flap open if we go sideways like the NASCAR boys.
I don't understand, all you need is space for a small hole. See my post on page 1 that shows a close up picture of the side port on John Rains Pontiac. He has on on each side of the car so if it goes to a wide yaw angle that port gets pressurized and passes the pressure to the roof flap box. You need very little space to make that sort of option work.
Larry
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It is great that this thread has been restarted after a long hiatus. A few years ago I was looking into roof flaps and had a chance to look over an old NASCAR display car. I remember replaying repeatedly a video of a car going airborne to see at what yaw angle initiated flight. Another video showed a similar car yawing at the same angle, when the flaps deployed instantly and the car stayed on the ground. I think NASCAR has added an additional flap, and angles may have changed slightly since then.
I may have to download the SAE paper but I think the principal is that the roof shape causes high velocity, low pressure air, resulting in lift. The flaps cause turbulence and spoil lift. In LSR the roof rails do essentially the same thing when the car is sideways to the course (or to the relative wind). I say relative wind because if you have a crosswind (almost always at Bonneville) you do not have to yaw very far for lift. As the car continues in the spin the rear end faces directly into the wind. The roof rails become useless and flight takes place. I have seen a modern sedan fly at least 6 ft into the air at Bonneville. It lifted off when going backwards down course, not when it was sideways. Flaps are allowed as long as there is no aero advantage.
A couple of points are (1) not all body shapes create lift. It is doubtful a '27 modified roadster body produces much lift when going backward. The real problem is the aero shape of sedans and (2) flap deployment should be automatic. On the early NASCAR I studied, the flaps deployed automatically. As the low pressure built up that was all that was needed to deploy the flaps. The pressure differential (inside the car vs the top of the roof) worked just fine. While current designs use interconnected tubing I have not looked into these designs.
Serious thought should be given to automatic deployment. All the NASCAR vehicles work that way. The pressure differential is all that is needed to deploy the flaps. The driver is just too busy to react in enough time to unlatch them. I can't tell you how many times driver reaction time at Bonneville has been too slow to deploy a chute in enough time to stop a spin. Sometimes a spin occurs so fast no human could react correctly. Other times the symptons are there but the driver delays pulling the chute thinking he can save it. I have seen hundereds of chutes come out too late, when the car was going backwards! If the flaps are unlatched, and automatic they will always deploy at the right time due to pure physics. If your flaps are spring loaded but latched sooner or later you will get your "wings".
YMMV and this is not meant to disparage any existing vehicle or design. It is offered only as additional thoughts for those studying the idea.
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Hello
I am going to install one roof flap, it is all I see room for and it should work.
Are there any hoses hooked up from the air ports on the side of the car to the roof flap box? My roof flap has one inlet on one side should I add an inlet to the other side? Is there a best place to put the air inlets in the body?
Thanks
Rocket
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I am going to install one roof flap, it is all I see room for and it should work.
Are there any hoses hooked up from the air ports on the side of the car to the roof flap box? My roof flap has one inlet on one side should I add an inlet to the other side? Is there a best place to put the air inlets in the body?
I am going through the same thought process right now so YMMV :wink:. Since you are considering a flap I assume you will have roof rails which are required over 200mph. The rails will help when the wind is sideways. But as the car continues to rotate in the spin there is a point when the wind is neither perpendicular nor parallel to the longitudinal axis. This "transition" zone is where the wind starts to produce lift and lift will continue to increase when the wind is directly down the axis of the car going backward. If you notice NASCAR cars there are two flaps and one is always at an angle to the longitude of the car. You might want to consider two flaps positioned like the chevrons on a military insignia. That way they will actuate regardless of the direction of spin and they will both be deployed when going backward. (I know you said you don't have the space for two flaps).
A single flap, positioned across the roof, may not spoil lift until you are almost 180 around and it may be too late. You might want flap actuation to start when you are 135 degrees to the course. I don't have an answer as to the air ports. I'm sure you don't want NACA style intakes though because going backwards they are ineffective.
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Thanks SaltFever
It seems the same as a lot of things. They are designed for another type of racing and we try to make them work and they are not always just right. Am I right to ASSume that a smaller flap may work since it is said that the roof rails help. I also built two flaps at the rear of our car right by the push bar & chute these should also come down in a spin blocking air from going under the car but these are untested. Any low financial risk way to test flaps. :-D
Thanks Rocket
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Sometimes it lands at your feet. :-D
I stopped to see a good racing friend and he just happened to have bought a cup car! I ran over to look at the roof flap install. They are automatically operated and I will do this to our car.
The way it works is a Helical Torsion Spring is wound around the hinge bar to counter the weight of the roof flap reducing it to nothing. You can open the flap with a piece of paper! If you press down on the flap, limited by the rubber stop, it will pop back up about 1/8 inch. Any reduction of pressure outside the car near the flap will cause it to open helped by the spring and normal or higher pressure inside the car leaking into the flap box and under the flap.
No need for ram air or pressure tubes feeding the flap box. Rocket, the tubes are stops for the limit cables that hold the flap open and prevent fold over. Large washers stop the cable travel at the end of the tube. Any openings into the interior of the car allow air to flow to the underside of the flap helping to push it open.
Easy peasey pie!
Geo
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The spring sounds like a good idea let me know where you find the right spring.
Still think the inlets on the body are a good idea. I am still considering installing only one.
Later Rocket 8-)
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(snip . . . ) Any low financial risk way to test flaps. :-D
I'm not sure if your question was in jest because of the smiley or if real. But I will offer an idea.
I doubt putting the car backwards on an open trailer and pulling at highway speeds will give you the speed you want. But if you are serious you could build a tubing structure to be bolted to your car in some manner (either front or back) that could hold a flap assembly. If everything was secure and safe, SCTA might let you run TO for testing. If not, USFRA most likely would let you run the 125 class. I'm not sure at what speeds flaps are supposed to deploy. You would probably want a controlled rotating assembly (think a lazy susan on the yaw axis) to see how much yaw it takes to get actuation. The cost of tubing and your time is about as cheap a test rig as I can think of.
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Folks, I am not reviewing, criticizing, or suggesting change to any existing car or fabrication. As far as I am concerned you are the test pilot and we can all learn from your experience. My comments are offered only for additional thought by others that may be studying the idea but have not posted to this thread. Secondly,I am not an engineer or a physicist so YMMV.
Some thought should be given to the relative wind. Relative wind is defined as any wind direction other than straight down the track. An example would be a left front crosswind coming from 45 degrees. Other than slowing you down, the effect on the car is far more complicated. Most people figure the car will start to fly when you are going perfectly backwards straight down-track. While true, it is a least likely scenario because lift can be generated long before you go around to 180 degrees.
If you are going 200 mph into a left front crosswind you do not have a 200 MPH wind straight down the car but a lower velocity component of that wind flowing over the car. (Pilots whip out your trusty E6-B here) However, many times a vehicle enters a spin and deviates 15-30 degrees from the course which can be either into or away from the wind. Depending on direction, this course deviation further amplifies the relative wind effect. So even though aero is different for every car, and ambient conditions vary widely, the possibility remains that under the right conditions, a vehicle could lift off with much less than 180 degrees of rotation.
May all your winds be at your six! :-)
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Rocket123,
The only way to really test something like this is buy some time in Lockheed Martin Wind Tunnel in Marietta, GA. They have a yaw table capable of 360deg of rotation and 200mph windspeed for $2000/hr. Even at that cost, if you are well prepared you could get away with 1 hour of testing and find your results. However, I'm not sure if they have a minimum # of hours you must buy, and if they did then they would not be very cost effective. Building a type of "rig" that Saltfeaver suggested, would end up costing you as much or more (time & materials), and with a pull vehicle in front of the test vehicle you will never have good air flow to learn what you want because you will be in the wake of the first vehicle. Just a thought.
Dave
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I have thought about trying to test on a trailer the problem I see is that then the car is backwards no revolution. I cant say that I have seen a hundred wrecks that happen this way nor can I say I haven't and as you are saying SaltFever the car almost always starts to lift at one rear corner before it is completely backwards. also would the roof flaps deploy going backwards (or do the deploy when you are sideways then stay deployed when you are backwards) or would the pressure be the same as going forward? Is all the lift coming from underneath the car if so would side skirts help from say 0 to 135 degrees. The closest I will get to wind tunnel time is copying peoples ideas who have been in one LOL but true.
Thanks Later Rocket
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I'm not sure at what speeds flaps are supposed to deploy.
Speed has nothing to do with it. A pressure differential between the inside of the car and the roof is what deploys the flap. A NASCAR spin at low speed deploys the flap. NASCAR has determined the angle and placement through testing. The correct location on your car may be different.
From The Physics of NASCAR
Stock cars have negative lift coefficients, but there is plenty of evidence that they can – under certain conditions – leave the ground. Although most of the air travels over the car, some air can get beneath the car. One of the reasons that stock cars have what is known as rake – that is, the back end of the car is higher than the front end – is to help air escape from under the car. If the pressure under the car is greater than the pressure on top of the car, the car will have lift.
The yaw angle is the angle between the direction the car is heading and the direction of the air flow. A car pointing in the same direction as the air flow has zero degree yaw. During normal racing conditions, a car usually has a few degrees of yaw; however, if the car spins, its yaw angle can reach 90 degrees or more. If you look at a race car from the side, its shape isn’t all that different from a wing.
Roof strips were added to disrupt airflow and decrease lift. The roof strips helped, but didn’t solve the problem completely.
The first test of the roof flap was conducted at Darlington airport and used the NASCAR corporate jet to generate 200 mph winds.
They measured the pressure every ten inches over the entire surface of the car. The results made it clear where to locate the flaps.
The air under the flap is normally at the same pressure as the air over the flap. If the air pressure over the roof starts to decrease, the air pressure under the flap becomes greater than the air pressure over the flap and the flap starts to rise. The wind catches the now exposed edges of the roof flaps and pulls them upright.
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I'm not sure at what speeds flaps are supposed to deploy.
Speed has nothing to do with it. A pressure differential between the inside of the car and the roof is what deploys the flap. A NASCAR spin at low speed deploys the flap. NASCAR has determined the angle and placement through testing. The correct location on your car may be different.
Excuse me, but speed has everything to do with it! What do you think causes the pressure differential! The roof shape is an airfoil. As wind accelerates over the top it speeds up relative to other locations on the car. As our friend Bernoulli has indicated higher velocity means lower pressure. The low pressure is directly related to wind velocity. You are missing the fundamental principle that works the flaps. The Bernoulli principle states there is a an inverse relationship between velocity and pressure. Re-read what you posted :wink:
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Roof flaps are in an enclosed pocket located at the low pressure area of the roof near the rear window. When the car rotates to 140 degrees off center the flaps deploy.
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Rocket123,
The only way to really test something like this is buy some time in Lockheed Martin Wind Tunnel in Marietta, GA. They have a yaw table capable of 360deg of rotation and 200mph windspeed for $2000/hr. Even at that cost, if you are well prepared you could get away with 1 hour of testing and find your results. However, I'm not sure if they have a minimum # of hours you must buy, and if they did then they would not be very cost effective. Building a type of "rig" that Saltfeaver suggested, would end up costing you as much or more (time & materials), and with a pull vehicle in front of the test vehicle you will never have good air flow to learn what you want because you will be in the wake of the first vehicle. Just a thought.
Dave, Keith Turk has his car tested in a new Tunnel started by a private party and it might be cheaper than $2,000/hr. I was thinking of less than $50 of tubing mounted to the front of the car. We are only talking about testing a device 14" x 5" x 4" that weights a couple of pounds. It only needs to be stuck out in front a couple of feet (in front of the bow wave) of the car. Burt Rutan used the same technique when testing Space Ship One. They attached various shapes out in front of a flat bed truck and ran up and down the runway at Mojave.
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Roof flaps are in an enclosed pocket located at the low pressure area of the roof near the rear window. When the car rotates to 140 degrees off center the flaps deploy.
That is true of a NASCAR car with tightly controlled specifications (Templates, build drawings, inspections, etc). It is easy (through testing) to locate the correct location which will be the same for all the spec cars. However, LSR is wide open on car type, configuration, and even speeds. You can't say, with absolute certainty, a specific location or angle is perfect for a car that we don't even know exists. Look at the NASCAR pic. Notice how the new flaps are an integral unit with a fixed angle. It might be a good unit for LSR but location is a "best guess". I think anything is better than nothing but testing will optimize your location. YMMV
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Dave, Keith Turk has his car tested in a new Tunnel started by a private party and it might be cheaper than $2,000/hr. I was thinking of less than $50 of tubing mounted to the front of the car. We are only talking about testing a device 14" x 5" x 4" that weights a couple of pounds. It only needs to be stuck out in front a couple of feet (in front of the bow wave) of the car. Burt Rutan used the same technique when testing Space Ship One. They attached various shapes out in front of a flat bed truck and ran up and down the runway at Mojave.
Our tunnel (A2 Wind Tunnel www.A2WT.com) is the tunnel Keith tested, but very few full size wind tunnels are capable of 360deg of yaw which is where you would need to develop the flaps for a particular car. I would say that most all cars have different aero characteristics and there is no one model that will tell you what every car would do in a spin. You can either try to give it your best educated guess in the placement or test in a wind tunnel that can yaw the car 360deg to see
1) if they deploy in the configuration you install
2) what kind of lift numbers your car has as a function of Yaw angle.
With the data you can see at which speed and yaw angle you will produce enough lift for your car to become airborne.
I appreciate alternative testing methods and thinking out side the box, I also worked at a company in Phoenix that built UAV's, and we did the same type of testing as Burt Rutain because we did not have a wind tunnel readily available. But I’m not quite sure what testing the flaps alone in front of a vehicle would tell you? The roof flap is part of a complete system (the car) and without the cars interactions to the flap you are just guessing at how they would react in a spin with your car. Look at all the different roof shapes, windshields, window edges, qtr glass, A-B-C pillars etc… out there. All of those interact with the air flow and pressure over a car and a sharp edge will change the air flow different then that of a smooth edge. There are so many variables with aero that when it comes down to it, there is no easy cost effective way to test the flaps.
Speed has nothing to do with it. A pressure differential between the inside of the car and the roof is what deploys the flap. A NASCAR spin at low speed deploys the flap. NASCAR has determined the angle and placement through testing. The correct location on your car may be different.
The pressure difference is how the flap deploys, but speed has everything to do with a car becoming airborne. As I'm sure you all know, the aerodynamic forces (lift or downforce) will increase with the square of the speed (speed^2). This means every time the speed doubles, the forces increase by 4 times the amount. If you spin at 100 mph, chances are you are not going to become airborne, but if you spin at 200+ the chance is increased significantly. An example: I will keep the #'s and static balance simple for illustration. Say you have a car that is 4000# static (2000# front 2000# rear) and you test in a wind tunnel to find that at 120 deg yaw and 125 mph you have 600# lift on the rear. This will not make the car airborne because 2000# is greater then 600#. BUT, at 250mph (double the speed) you now have 2400# (4 times the force as you double the speed) on the rear and this will lift the car off the ground making it airborne.
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........ As I'm sure you all know, the aerodynamic forces (lift or downforce) will increase with the square of the speed (speed^2). This means every time the speed doubles, the forces increase by 4 times the amount. If you spin at 100 mph, chances are you are not going to become airborne, but if you spin at 200+ the chance is increased significantly. An example: I will keep the #'s and static balance simple for illustration. Say you have a car that is 4000# static (2000# front 2000# rear) and you test in a wind tunnel to find that at 120 deg yaw and 125 mph you have 600# lift on the rear. This will not make the car airborne because 2000# is greater then 600#. BUT, at 250mph (double the speed) you now have 2400# (4 times the force as you double the speed) on the rear and this will lift the car off the ground making it airborne.
Thanks for all the good info and I hope you keep it coming,
Sum
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The only way to really test something like this is buy some time in Lockheed Martin Wind Tunnel in Marietta, GA.
There are other options to get ball park information. Like Burt Rutan, some pretty simple low tech setups can give you very good basic information. Park your car behind a light plane on an airport and have them run the engine up. You won't get 200 mph winds but you will surely get enough air speed to determine if the flaps will deploy.
All you need is a friendly local pilot and a cooperative airport authority to run some base line tests to find out what yaw to the relative wind begins to lift the flaps. Add some tufts to indicate airflow over the body and a video camera positioned above the level of the car roof and if you want to get fancy one of those hand held wind speed gauges.
Larry
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I should be a little more specific. There are always alternative methods for any type of testing, but my point was, the quickest way to know a definite answer is to get in a wind tunnel. That way, you would know at what yaw angle your car will become airborne and at what speed. It's hard to put a price on safety, and it's hard to determine when you have tested something enough to know that it will work properly no matter what the safety device. Not only could you get data for your car in various yaw, but you can see exactly what the flaps do once they open, and if they spoil the air enough to work. i.e. at the angle determined to produce enough lift to get your car airborne, now what happens to the aerodynamic forces when the flaps are open at that yaw angle vs. no flaps (or flaps taped down). How does it affect the lift, and is it enough to keep it on the ground. It would quantify if what you are doing will really work. What if your car could become airborne at 120 deg of yaw and your flaps don't deploy until 140 deg? Yeah, it's only 20deg, but it's too late because you would be on your lid before they open.
Look at the two pictures below. One is the streamlines off a propeller (it's a boat prop but should be similar to a plane and is just illustrative) and the other is CFD for a car. Now, I agree that if you put a car behind a plane you could see if the roof flaps open, but what does that really mean, and what would you walk away knowing? Answer: The flaps open in a very turbulent condition? Your tufts will be going every direction due to the turbulent air so how would you interoperate any tufting pictures? That is not the same air your car will see (second image) where the stream lines are smooth and consistent.
Now don't get me wrong. I think that someone can install roof flaps with out wind tunnel testing and it probably wouldn't hurt to have them on rather than not have them. And you could do some type of testing like put your car behind a plane etc… to see what happens and interoperate the data however you see fit, but I am just throwing out some ideas to get you guys thinking about the question. If your going to make a safety improvement to your car, make sure you have done what you think is “enough” to make your self satisfied with the change/equipment. After all… it’s your butt in the seat and you make the call on what is good enough for you.
(http://www.nautica.it/superyacht/533/tecnica/ottimizzazione-5.jpg)
(http://www.digitalmedics.de/projects/mfes/_img/cfd2.png)
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Hello windtunnel Good info thanks. Awesome picture is that computer generated (by a program) or did you make it.
Now don't get me wrong. I think that someone can install roof flaps with out wind tunnel testing and it probably wouldn't hurt to have them on rather than not have them.
My choices seem to be install, without a wind tunnel or don't install.
What if your car could become airborne at 120 deg of yaw and your flaps don't deploy until 140 deg? Yeah, it's only 20deg, but it's too late because you would be on your lid before they open.
I have seen flaps open late and the car come back down on its wheels instead of its roof and if the were not there I think it would have been on its lid, Do we need a different way to deploy them put a switch on the chute or brake pedal or a better way. Then make it so the flap can push its way up could be deployed before 90 degrees.
Would the angle of the roof flap matter as much if it was manually forced open or would it work mounted towards the rear. I understand the best way would be to test in a wind tunnel it is just not an option so all info is appreciated. Also thanks Larry may be able to find an airport not to far away but we already scared all the ones close when we asked about testing.
Thanks Rocket
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So, maybe actuated roof flaps with a weather vane switch that trips the at 30 degrees from straight. Could be connected to the chute as well. Car gets 30 degrees out of sorts, flaps pop up and chute pops out....
Just a thought to help make someones build harder, but possibly safer.
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As A2 has indicated the best and most controlled conditions will be in a wind tunnel and alternative testing may not reproduce conditions correctly and could lead to false assumptions with a possibility of incorrect installation.
With that thought I would like to point out that testing with a propeller plane may not be best. The air stream flowing back from a prop is not laminar flow. It flows in a helical manner like the picture of the boat prop above. It is so significant that it is a concern in all aircraft design because of possible affects on the vertical stabilizer and rudder in particular. As mentioned above NASCAR used the corporate jet for initial testing of flaps. At first I thought they needed it to get the high speed velocity of the cars. Now I realize it was more than that. The flow from a jet is laminar in nature. Wind tunnels have vane straighteners and go to great lengths to provide laminar flow. A jet's exhaust is probably better than yea old Cessna 172 tied down on the ramp if you don't melt all your fiberglass work :-)
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Hello windtunnel Good info thanks. Awesome picture is that computer generated (by a program) or did you make it.Thanks Rocket
A2WindTunnel said the pic is CFD for the car. Computational Fluid Dynamics is a computer program invented by NASA along with Finite Element Analysis (FEA), in the 1960s for the Apollo program. CFD is a very expensive program costing as much as $37,000 for a one year license. It was used in the development of the JBC LSR Diesel streamliner (driven by Andy Green) at Bonnevlle in '07.
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I was just wondering if the air went over where the headlights are that smooth.
LAter
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I was just wondering if the air went over where the headlights are that smooth.
It really depends on the CFD program and how fine you mesh the model as to how fine your results and flow visualization. The finer the mesh the longer it will take to model and compute. As Rocket123 said, CFD can cost up to $37,000 for a 1 year license and you still have to spend a heap of money on a computer that is capable to run the CFD software. Plus a PhD. to run the program because it can be very complex to model and analyze.
To answer your question better, I would expect more separation off the rear and a wake that you don't see in that particular CFD model. Here is CFD for a stock car and you can notice the huge wake and turbulent air off the back and a high pressure off the front where the air on the ground ahead of the car is affected. A car without a spoiler would not be as pronounced on the rear, but would not be as smooth as the previous CFD model I posted. I will also post several CFD models that have finer meshing and illustrate how complex the flow can become in some models.
(http://www.eng.fea.ru/spaw2/uploads/images/STAR-CCM-race-CFD2.jpg)
(http://www.heggemann-autosport.com/images/cfd/cfd06.jpg)
(http://www.biketechreview.com/aerodynamics/images/cfd_side.jpg)
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very cool thanks
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In the Book "The Physics of NASCAR" By Diandra Leslie-Pelecky, Ray Evernham (2008) Chapter 6: The Wizard & The Flying Car Problem (pages 91-104) is an over view of how Gary Eaker worked to solve the lift off problem in NASCAR spins. It lists the challenges and some of the thought process of what was done to overcome this problem. Leading him to the idea of the roof flaps and their development. Pretty interesting reading for those who would like to know more about the history of the roof flap.
Dave
(page 91-104)
http://books.google.com/books?id=OAK3yFlHoTAC&pg=PA93&lpg=PA93&dq=A2+Wind+Tunnel&source=web&ots=UclDdtRpZK&sig=SNLrztXGd5ikWkDttt5uvb5XEnA&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA91,M1 (http://books.google.com/books?id=OAK3yFlHoTAC&pg=PA93&lpg=PA93&dq=A2+Wind+Tunnel&source=web&ots=UclDdtRpZK&sig=SNLrztXGd5ikWkDttt5uvb5XEnA&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA91,M1)
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PS: There is a link at the bottom of that if anyone missed it. I was not telling you to go out and buy the book, but to click on the link and read. :-)
Link:(pg 91-104) http://books.google.com/books?id=OAK3yFlHoTAC&pg=PA93&lpg=PA93&dq=A2+Wind+Tunnel&source=web&ots=UclDdtRpZK&sig=SNLrztXGd5ikWkDttt5uvb5XEnA&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA91,M1 (http://books.google.com/books?id=OAK3yFlHoTAC&pg=PA93&lpg=PA93&dq=A2+Wind+Tunnel&source=web&ots=UclDdtRpZK&sig=SNLrztXGd5ikWkDttt5uvb5XEnA&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA91,M1)
Dave
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I was just wondering if the air went over where the headlights are that smooth.
LAter
Yea, and I was going to comment on that o so smooth wake.[pic pg. 3] Maybe nasa ought to go to El Mirage and learn a few things.
Well maybe they can tell us how come there is no vacuum at the rear of the car.[no drag] Talk about energy savings [no more mideast oil.
JL222 :?
Bvillrcr post a video of the real world.
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The video skips a little, but you can see the spoiler better here than my other video. :cheers:
http://www.youtube.com/watch?v=NJZbf4_IUeo
Second video a little more dramatic.
http://www.youtube.com/watch?v=mHUEA-pScY8
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Yea, and I was going to comment on that o so smooth wake.[pic pg. 3] Maybe nasa ought to go to El Mirage and learn a few things.
Well maybe they can tell us how come there is no vacuum at the rear of the car.[no drag] Talk about energy savings [no more mideast oil.
FYI: I think that CFD model on pg. 3 was from a university... not from nasa. A university is not going to have an advanced CFD program or modeler to have a very accurate representation. I think that is from a student project and would not read into it very much as what the bounding streamlines are really doing on a car like that. The dust in the youtube videos is an excellent representation of the wake of the car.
:mrgreen:
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Yea, and I was going to comment on that o so smooth wake.[pic pg. 3] Maybe nasa ought to go to El Mirage and learn a few things.
Well maybe they can tell us how come there is no vacuum at the rear of the car.[no drag] Talk about energy savings [no more mideast oil.
FYI: I think that CFD model on pg. 3 was from a university... not from nasa. A university is not going to have an advanced CFD program or modeler to have a very accurate representation. I think that is from a student project and would not read into it very much as what the bounding streamlines are really doing on a car like that. The dust in the youtube videos is an excellent representation of the wake of the car.
:mrgreen:
The 222 car vacuums up more dust because of the rear spoiler and lower pressure under it [ AR I know you know this but some readers might not] any how have you ever tested a rear overhanging spoiler like a prostock car [15''] or 10'' in our case to measure the pressure difference between top and bottom? Our spoiler measures 10 by 53=530 sq. in". 1 lb less pressure = 530 lbs downforce but have no idea what it really is. I'm sure the prostock guys know.
JL222 :?
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I really had a hard time with the above post. It kept posting on its own before i was done!
JL222 :?
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Yes we have and majority of our customers in A2 are race teams that range from NHRA pro-stock to NASCAR which all run spoilers. I'm not sure if anyone from the teams would be willing to share any data to help answer your question. All information learned at each test is proprietary to the team and I cannot discuss/post any of their data or findings. I can generally discuss aero topics and general information to help you guys. We do have 18 channels of data acquisition available for anyone to run pressure taps on their car. This could range from incrementing a spoiler left to right positive (+) and negative (-) side, longitudinal or lateral on the car to see what the pressures are doing on the surface at different locations. All this can prove to be very useful information.
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:roll:
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I'm interested in getting more into building and modifying cars, do you have any place that you recommend for a beginner?
Possible ten year later spam post :? :? :?
Sumner
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Sum, you're the second one to question Mr. Neat's purpose on Forum, so maybe I'll just watch for a little while more and then, as necessary, swoop in with my cursor of death!
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He's a roofer and looking for roof flap repairs? :? :roll: :? :roll: