Roof Flaps

[Rocket123]

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

[Dean Los Angeles]

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.

[saltfever]

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 

[hotschue]

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.

[saltfever]

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.

[saltfever]

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

[A2WindTunnel]

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.

[Sumner]

........  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

[hotrod]

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

[A2WindTunnel]

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.

[Rocket123]

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

[Stainless1]

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.

[saltfever]

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 

[saltfever]

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.

[Rocket123]

I was just wondering if the air went over where the headlights are that smooth.
LAter