Roof Flaps

[fredvance]

Hey Larry, do you live in Alaska? Or do they have chinook winds in the lower 48?

[4-barrel Mike]

Usually south of Alaska:

http://en.wikipedia.org/wiki/Chinook_wind

Mike

[hotrod]

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

[Sumner]

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

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

[Geo]

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! 

I have some extra flaps and will post them in the for sale section.

Geo

[hotrod]

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

[saltfever]

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. 

[Rocket123]

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

[saltfever]

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  . 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. 

[Rocket123]

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. 

Thanks Rocket

[Geo]

Sometimes it lands at your feet.   

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

[Rocket123]

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 

[saltfever]

(snip . . . )  Any low financial risk way to test flaps. 

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.

[saltfever]

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!
 

[A2WindTunnel]

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