Landracing Forum

Introductions => Formulas => Topic started by: nebulous on July 31, 2010, 05:49:20 PM

Title: Flat bottom /tunneling
Post by: nebulous on July 31, 2010, 05:49:20 PM
Gentlemen
I would like to findout about tunnel design ,and anti lift resulting from their use.
 I agree with blue about departure criteria,and effort to study it is important!
Thanks Jack Costella
Title: Re: Flat bottom /tunneling
Post by: Glen on July 31, 2010, 06:54:10 PM
Jack, look at the rear engine modified roadster that Chavin Emmons built I believe it has a tunnel and two years ago it spun for over 1/2 mile and stayed on the ground. It should be at Speed Week. The front engine roadster had some ground effects as well. Both cars very fast. See ya in a few days.
 :cheers:
Title: Re: Flat bottom /tunneling
Post by: mike mendoza on July 31, 2010, 10:42:17 PM
Gentlemen
I would like to findout about tunnel design ,and anti lift resulting from their use.
 I agree with blue about departure criteria,and effort to study it is important!
Thanks Jack Costella
Title: Re: Flat bottom /tunneling
Post by: dw230 on August 01, 2010, 01:13:10 PM
Thanks Mike :lol:

DW
Title: Re: Flat bottom /tunneling
Post by: mike mendoza on August 01, 2010, 09:47:39 PM
saw the underside of the elec. machine at Bville from the college: looked like an hourglass
Title: Re: Flat bottom /tunneling
Post by: Rex Schimmer on August 02, 2010, 12:16:41 AM
Jack,
Get a book called "Race Car Aerodynamics" by Joseph Katz and a similar named book by Simon McBeth, both provide good information related to tunnel design. Down force generated by tunnels is efficient but not free, it does require horse power.

Rex
Title: Re: Flat bottom /tunneling
Post by: Jerry O on August 02, 2010, 12:19:04 AM
Jack;   I worked about 15 years on several Indy car teams and IMSA GTP teams that use tunnels. These devices can create a lot of down force if designed proper. I think tunnels would work very good on your cars, because your cars ride very close to the ground and the lower the car the greater the down force.
We would set the ride height on a Indy car at the speedway to be at zero when it was at speed. We used skid plates on the bottom of the cars to keep from grinding the body away. The car would just skim the track surface. As any device used to add down force, aero balance was very important. I once built a oil pan using a similar design as a tunnel for a NASCAR team. As a test to show the team owner how effective it was, I placed the pan on a flat surface at the ride height the car would race at. I then had a large fan blow air under the pan and then ask the owner to try and lift the pan off the surface. He could not get the pan to move off the surface. So tunnels in the bottom of the car can be a great way to create down force. Feel free to contact me any time if you have any questions.  

Jerry O
Title: Re: Flat bottom /tunneling
Post by: The wonderful One on August 02, 2010, 10:19:00 AM
I know this is the same Jerry O. VERY SMART BUILDER. I remember when he got his liner picked apart last year by someone who did not like the way he mounted the cage. That is why he quit posting. To bad. He has a lot to offer. I hope he is back. He will be a help. The Wonderful One
Title: Re: Flat bottom /tunneling
Post by: SPARKY on August 02, 2010, 10:50:08 AM
I also hope he continues to share.

It isn't like we don't have a lot of opinions---  With all of the opinions and comments I encountered with my "UGLY Betty"  Ratical 

I learned to seek out the fair and level headed  SCTA inspectors and board members
--most are--get their opinions, then proceed.  Please remember the reason so many of us are attracted to LSR the CHALLANGES  (on and off the track)  and the FREEDOM to persue our ideas. 

A lot are like me, don't have a lifetime of being around race cars--we don't have the knowledge base pull from we make simple choice mistakes about the way we do things that can have big consequences

Title: Re: Flat bottom /tunneling
Post by: ironwigwam on August 18, 2010, 08:42:59 PM
Might be interesting to look at a Porsche 962? not sure if I remember the correct number, car model that shows the interuptors over the rear wheels with the diffuse angles at exit?
Title: Re: Flat bottom /tunneling
Post by: interested bystander on August 18, 2010, 10:18:27 PM
Anybody bothered to check out the Mormon Missle underbody, or from a couple yeas back, the Haas entry (BGS?) or the Eyebal Engineering Electric and the later Dempsey Electric from ten years ago?.

Those designs, by Gerald Arivett, had FRONT and REAR tunnels, and the cars run relatively low but not flat. The electric designs were tested in the now defunct Galcit wind tunnel.

 Bill Scott's quick and dirty built lakester had a large tunnel out the underside and when originally built by the Arivett Bros., had the exhaust dumping into it. (Like today's Formula One). BILL DIDN'T LIKE IT BECAUSE HE COULDN'T HEAR THE ENGINE!!???! There's a Harnick picture from a few years back at Elmo of Bill in the traps and a HUGE dirt cloud pouring outward and upward from the tunnel. Look it up!

The Missle used the Arivett/Haas molds slightly modified.

Don Ferguson owns the molds for all but the Eyeball car, which is an Arivett/Scott Knight aluminum body.

Besides the excellent book by Dr Katz, people overlook Forbes Aird's (interseting name!) book Aerodynamics for Sports and Racing cars.  Not math heavy and Aird explains aero   phenomena in understandable terms.
Read the dissertation on Drag Race parachutes - surprising in the conclusions drawn! He has an example or two and opinions on LandSpeed "chutes.


Incidentally Gerald has two 1/4 scale models in development for  wind tunnel testing currently.
Title: Re: Flat bottom /tunneling
Post by: Jonny Hotnuts on September 22, 2010, 05:11:36 PM
My concerns with using DF is that it works all fine and well with intended forward motion but as soon as the vehicle gets a little out of shape the forces that once held the vehicle down now radically change*.
(*resulting in just a loss in drive traction or other?)

A tunnel also creates an area under the vehicle for pressure to build under the vehicle with no ablity to bleed laterally and will act in reverse (as originally intended) if the vehicle was to spin....and very likely causing the vehicle to take flight.

Obviously this would be dependent on the vehicle and application.
Title: Re: Flat bottom /tunneling
Post by: akk on September 22, 2010, 08:00:18 PM
Once a very serious guy asked my opinion of his ground effects tunnel...it was not pretty...I thought it had very high potential drag...my comment was that they do not award down-force trophy's out here...lead works real good and is dragless...dragless down force....unless you are going over 350..

Akk 
Title: Re: Flat bottom /tunneling
Post by: Anvil* on September 22, 2010, 11:00:11 PM
My concerns with using DF is that it works all fine and well with intended forward motion but as soon as the vehicle gets a little out of shape the forces that once held the vehicle down now radically change*.
(*resulting in just a loss in drive traction or other?)

A tunnel also creates an area under the vehicle for pressure to build under the vehicle with no ability to bleed laterally and will act in reverse (as originally intended) if the vehicle was to spin....and very likely causing the vehicle to take flight.

Obviously this would be dependent on the vehicle and application.

Unlike a wing, a tunnel can work while traveling backwards. It would drop off dramatically then return and drop off (repeat) through a spin. If allowed a tunnel would be preferable to a wing, but like others have commented, after investigating options for adding ballast. And I agree the best compromise would be vehicle/application specific.
Title: Re: Flat bottom /tunneling
Post by: debgeo on October 26, 2010, 08:46:14 AM
spam :evil: :evil: :evil:
Title: Re: Flat bottom /tunneling
Post by: Richard 2 on March 03, 2014, 12:22:41 PM
Gentlemen
I would like to findout about tunnel design ,and anti lift resulting from their use.
 I agree with blue about departure criteria,and effort to study it is important!
Thanks Jack Costella

Anyone have any new or updated info on tunnels or Strakes?
Richard 2
Title: Re: Flat bottom /tunneling
Post by: John Burk on March 03, 2014, 10:47:02 PM
Hard to argue with Speed Demon's closed on front open in back tunnel . The skirts need to hang far enough below the belly to form an actual tunnel so the whole body sees the low pressure created in the back .
Title: Re: Flat bottom /tunneling
Post by: tauruck on March 04, 2014, 05:15:27 PM
Gentlemen
I would like to findout about tunnel design ,and anti lift resulting from their use.
 I agree with blue about departure criteria,and effort to study it is important!
Thanks Jack Costella

Anyone have any new or updated info on tunnels or Strakes?
Richard 2

Richard, for what style car is it?.
Title: Re: Flat bottom /tunneling
Post by: Richard 2 on March 04, 2014, 10:01:58 PM
mod. Roadster flat bottom 175" wb
Title: Re: Flat bottom /tunneling
Post by: WOODY@DDLLC on March 05, 2014, 11:08:14 AM
Here are some basic diffuser test results to ponder based on published wind tunnel tests and various anecdotal comments. Trouble is when you put any of these or other schemes on a real vehicle (with wheels & such) it all changes! Remember what was done on an F1 or Indy car was based on a different vehicle, operating conditions and rule book!

Diffusers are typically more efficient than tunnels. Strakes in the diffuser may or may not make a difference. It all depends on a lot of interacting variables.  :-o

This is THE basis for virtually all formal diffuser studies: SAE980030 The Aerodynamic Performance of Automotive Underbody Diffusers, Kevin Cooper

It's not real hard with the right tools [CFD or wind tunnel] but it is real complicated!

But if all else fails there is always monkey-see monkey-do!  :-D
Title: Re: Flat bottom /tunneling
Post by: JimL on March 05, 2014, 04:34:36 PM
Richard, we set up our Mod Roadster with skirts that pinched together under the front axle point.  There was an inlet area allowed by angling the skirts.  The skirts went straight down along the sides, followng the line of our body/chassis all the way to the back of the car.  I set it up with a gradual outward taper, as viewed in topview planform, for about 166".  That was in 1998.  There were sure a lot of folks asking questions about it, but we didnt know much yet, when we first got there with the new car.

The car had a full bellypan, with a step down under the engine/bellhousing area.  We had about an inch of front suspension, none rear.  The first time the car ran to about 175 it sucked the engine portion of the bellypan into ground contact.  I had to run tie down straps through the edge bracing to pull it up and keep it off the salt.  Note that I had done a lot of work to keep that car from sucking up dirt at Muroc and ElMirage, which may have contributed to the belly pan sucking down.

Gosh, that was a while ago, I realize! :-P

Hope this helps.

JimL
Title: Re: Flat bottom /tunneling
Post by: Richard 2 on March 06, 2014, 10:09:09 PM
Thanks a lot for the reply's this helps reinforce my plans for the car.

JimL,  Did you ever go over 200 mph with the tunnel? And do you think the tunnel may of worked to well. By that I mean did it rob more hp than needed to go straight.

Woody, To make sure I'm correct about how to read the chart. Red is Fast and Blue is stalled air.
If there was a wind tunnel close to me I would be there quick. But as luck would have it I found a car much like mine that works well over 250 mph with some strakes in the rear. So, I guess, I fit the Monkey See Monkey Do category. :-D
I also know from my real Job that what works for one may not work for another.

John Burk, I have to agree, but.

Thanks Richard 2
 
Title: Re: Flat bottom /tunneling
Post by: JimL on March 06, 2014, 11:00:21 PM
Our best time slip was 186, most runs were 175-183 area.  This was a 2.0 liter 4-wheel-drive Celica engine mildly modified.  I always thought keeping the air out from under was helping our speed, but I dont really know.

All I am convinced of, is that you dont want any bellypan inside a tunnel that is wimpy or poorly mounted!  I forgot to mention; the last year we ran front suspension on the car it ground the front area of the skirts down at speed.  They had about 1/2" ground clearance at rest.  I guess our simple tunnel did something.

JimL
Title: Re: Flat bottom /tunneling
Post by: WOODY@DDLLC on March 07, 2014, 09:25:14 PM
Woody, To make sure I'm correct about how to read the chart. Red is Fast and Blue is stalled air.
If there was a wind tunnel close to me I would be there quick. But as luck would have it I found a car much like mine that works well over 250 mph with some strakes in the rear. So, I guess, I fit the Monkey See Monkey Do category. :-D
I also know from my real Job that what works for one may not work for another.

Thanks Richard 2

R2, yes the red is fast and blue is really slow or maybe even dead! :-P

The overall results for this series of tests showed a drag variation of 34% and a lift variation of 46%. Also a pitch variation of 25%. Pitch is nose up or down which loads or unloads the front and rear wheels depending on where the CG is located. These objects are much simpler than a real vehicle but they show what relatively minor changes can do. Keep in mind that what you get away with at 200 mph might not work at 225 mph or 250 mph. The forces go up 27% from 200~225 and 56% from 200~250. HP goes up 42% & 95% respectively. Imagine you are on a compound see-saw with wheels going really fast - because you are - so balance is even more important now than on the playground! Traction can hide a lot of sins and the lack of it can make you seek redemption! Or a new pair of shorts!  :-o

The A2 is not that far away and I think everyone who has been there will tell you it's time & money well spent! See you on the salt! :cheers:

Title: Re: Flat bottom /tunneling
Post by: Richard 2 on March 07, 2014, 11:08:27 PM
Thanks, to every one again.
Richard 2
Title: Re: Flat bottom /tunneling
Post by: tauruck on March 07, 2014, 11:46:40 PM
Our best time slip was 186, most runs were 175-183 area.  This was a 2.0 liter 4-wheel-drive Celica engine mildly modified.  I always thought keeping the air out from under was helping our speed, but I dont really know.

All I am convinced of, is that you dont want any bellypan inside a tunnel that is wimpy or poorly mounted!  I forgot to mention; the last year we ran front suspension on the car it ground the front area of the skirts down at speed.  They had about 1/2" ground clearance at rest.  I guess our simple tunnel did something.

JimL

You think??? :-D :-D
Title: Re: Flat bottom /tunneling
Post by: Bratfink on July 10, 2014, 03:55:22 PM

The A2 is not that far away and I think everyone who has been there will tell you it's time & money well spent! See you on the salt! :cheers:


I've not been in A2, does it run the same boundary suction and wheel rotation systems as A1? If so I would question validity of results specifically in the areas of underfloor flow. Not saying the results are wrong, I have met Gary Romberg before and he is a very smart and cool guy, and I know the tunnels get good correlation for NASCAR, plus I know for what we are doing few people can afford Windshear.

But if you are specifically looking at small interactions under the car then I would suggest that directly sucking the airflow through the floor in the test section is not going to give a good representation of how air moves under a car. You really need a moving belt with the boundary layer taken care of before it gets to the vehicle.

This is something I have worked on, on a number of different race (and road) cars and it does not have a simple answer. CFD is a good tool for it but struggles with accurately modeling splitter separations that occur on real cars, thus you often gain too much confidence in a CAD design only to find you are short on front downforce on the real car. Or even worse the pitch sensitivity is through the roof. The famous flying Merc at LeMans was caused by a pitch change of about 1/10 of a degree!

I have actually been thinking of writing a paper for the LSR community on this very subject to aid teams in designing safer race cars, as of yet I have not found a backer to help with the testing and my boss isn't very keen on lending me the wind tunnel and letting me make a model free of charge.     
Title: Re: Flat bottom /tunneling
Post by: manta22 on July 10, 2014, 10:11:59 PM
Bratfink;

Interesting comments- thanks. "Pitch change" can do odd things. A friend who was an engineer with John Wyer Automotive Engineering in the '60 & '70s told me a story about wind tunnel testing a sports racing car in the MIRA facility. They were investigating the effects of ride height & attitude on downforce and found that under certain conditions, the nose of the car would start oscillating up and down above a critical speed.  :-o

The explanation was that as the speed increased, the body shape generated increasing downforce in the front, which compressed the front springs and dropped the nose down. At a critical speed, the nose dropped down so far that the airflow under the car dropped dramatically and the downforce also dropped, allowing the front springs to raise the nose back up and then the whole cycle repeated itself. Stiffer front springs solved the problem.

Regards, Neil  Tucson, AZ
Title: Re: Flat bottom /tunneling
Post by: Bratfink on July 11, 2014, 08:21:41 AM
Bratfink;
They were investigating the effects of ride height & attitude on downforce and found that under certain conditions, the nose of the car would start oscillating up and down above a critical speed.  :-o

The explanation was that as the speed increased, the body shape generated increasing downforce in the front, which compressed the front springs and dropped the nose down. At a critical speed, the nose dropped down so far that the airflow under the car dropped dramatically and the downforce also dropped, allowing the front springs to raise the nose back up and then the whole cycle repeated itself. Stiffer front springs solved the problem.

Sports prototypes still do it! You watch footage of cars coming down the Mulsanne and see them bobbing along, people think it's the bumps in the track, and some of it might be, but the effect is way too sinusoidal for my liking. and it mirrors what I see in the wind tunnel with this sort of car. I have seen models get so out of shape they would end up grinding the front splitter on the belt!

My boss came from Mira (now he's a guy I'd love to get out on the salt, he'd have a field day helping people out), it's a fixed ground tunnel. The effect could have been worse had the "road" been moving. You can think of a front splitter as "a diffuser for the front of the car", the same air flow interactions that make diffusers work also make splitters work. And if you choke the flow to them they also stop working the same way.   

Title: Re: Flat bottom /tunneling
Post by: tauruck on July 17, 2014, 12:21:42 AM
In the old wing car era they called that phenomena "porpoising" and it would drive you nuts. :-D
Title: Re: Flat bottom /tunneling
Post by: Bratfink on July 29, 2014, 11:23:50 AM
In the old wing car era they called that phenomena "porpoising" and it would drive you nuts. :-D

Still is ;-)
Title: Re: Flat bottom /tunneling
Post by: Slide on November 12, 2016, 01:29:55 AM
Anyone have access to that sae paper:

SAE980030 The Aerodynamic Performance of Automotive Underbody Diffusers, Kevin Cooper

I can only find abstracts for it. Being on a limited income now, I difnt even look at what it would cost to be an sae member to be able to then down load it...

Just trying to learn more and more!
Title: Re: Flat bottom /tunneling
Post by: mtkawboy on November 12, 2016, 02:57:02 PM
sinusoidal , I learned a new word at 72 years old