The wing hanging out past the trunk looks like a winner in overall downforce as well - it's surprising how the wing over the trunk increased lift over the rear glass, and how moving the wing past the trunk reduced lift over the roof and boosted pressure over the hood. It seems as if the low pressure area between the wing and the trunk in the on-trunk setup is accelerating airflow over the entire top surface of the car.
Me an Elvis Say 'A Little less Conversation , A little more Action Please '
Saul Goode Came by and Noticed my Lack Of Progress On the Vette, I must Agree.
In reply to GameboyRMH :
Because we are dealing with subsonic flows, everything you do at the back makes some impact at the front, and of course a change at the front will affect everything behind it.
It is all just pressure waves traveling at the speed of sound (~700mph) and until you get really close to that or exceed it the pressure can/will propagate forward.
I Lost My place posting But Will hunt up the Flat Wing and Post That Too. Basicly The same Deminsions but top Surface is Flat And Angle Back To the Tail Is not Curved But straight to the tail Behind the Chord That Is also 9" deep About 16" behind the Leading Edge. This is current Dirt Car Stuff. the pic I lost saying front wing is same shape as the large wing but 3.6 "deep and 24" Wide Front to Back,Where the drawing show's 1/2" at 6 from rear the small wing would be 3/8 at 3 1/2"/ 3 1/2
I’ve got nothing constructive to add but this thread is an awesome read. Keep up the work guys 
sleepyhead said:I thought about making a joke... or generating my own "k car" with a saw tooth bottom to "better represent a car's underbody"... or generating a "hatchback" version, since I expect that would be helpful for Matt and Robbie...
but, I need to back off a bit from running analysis... and devote more time for the next 4 weeks to getting some sim/video time to be ready for OneLap. So, I'm going to check out for now, and just keep my eye on this. I do appreciate the pointers you've given me... and the motivation to dig in to javafoil... I can see that that will pay some dividends, which hopefully I can plow back in to be helpful to others here
I LOVE K-Cars!! maybe i'll own one again one day, its been a while...no, i won't
i was cracking busy yesterday...but the aero balance is a thing i need to get a handle on so thanks to Steve for posting that, i didn't think it was that big a difference, so now it makes a bit more sense why rules makers specify limits for aero devices referenced to the envelope of the cars body.
is a Diffuser a topic i can bring up here or will i be shunned? i have been taking some dims and taping up cardboard in a terribly reckless fashion...but my BNDL mentality will probably do more harm than good unless i understand the aero forces ive already thrust upon my poor Camaro!
I've been working on this on and off the past few days but I think I just crashed javafoil... oops.
we should definitely discuss diffusers too. Here or in another thread doesn't matter to me.
stafford1500 - I sent you a PM last week, did you get it?
GTXVette said:Me an Elvis Say 'A Little less Conversation , A little more Action Please '
Don't worry!
Robbie said:I've been working on this on and off the past few days but I think I just crashed javafoil... oops.
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we should definitely discuss diffusers too. Here or in another thread doesn't matter to me.
bwahahaha... well, now we know not to do that. Make the floor flat in Javafoil and then go do it on the car 
initial thought: the diffuser is like the rear bottom back edge of the foil GTX posted. It makes the path air follows easier to “get back to normal” / “makes it turn up”... as such it creates downforce... the wing position a low pressure from the bottom side can help make the diffuser “flow more” by encouraging the flow from under the car to exit and/or turn up... which will then make your splitter more effective because it will see higher flow speed on its underside. Iirc, ymmv, etc etc
In reply to Robbie :
I did not get your message.
I will send you one to see if it goes thru.
Robbie said:I've been working on this on and off the past few days but I think I just crashed javafoil... oops.
we should definitely discuss diffusers too. Here or in another thread doesn't matter to me.
I think you have definitely crashed it...
The software is not really meant for complex shapes (like cars), but can be used if approached cautiously. YOu might try increasing the number of cells in the flow field (entries at the top/left). This will increase resolution on the computation and increase compute time, but should get better results. Also add some points along the bottom of the car, even if they are on the same line. That will help define the shape for the solver. My 'car' model has similar issues on the rear glass and trunk...
Been following this thread. If there's a splitter/diffuser discussion I'm all in! Here or a separate thread.
Here's what I've been working on. For you aero expert types, yes I know the diffuser ratio is too much but I made the cardboard mock up so I can trim to fit under the car then make one for testing with another material. This whole contraption is suspended from the car, built in sections, and can be installed/removed from the car while the wheels are on the ground. It's height adjustable and pitch adjustable so the flat floor section doesn't have to be the same pitch as the chassis and it can be raised or lowered relative to the track surface. It is NOT for a challenge car project.
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I may have to employ some 3D CFD to put together a good starting point for a thread on splitters and diffusers. The main reason is they generally have zero gap to the rest of the car and the flows can be quite complex. Wings are a little easier since they are USUALLY physically remote to the body.
There is a piece of software on one of my machines that will convert a picture directly to a section profile for use in something like JavaFoil or CAD system. I may have to dig that up and post a link. It would help with defining an unknown section or one that is built without a guiding profile.
In reply to NOT A TA :
I am pretty sure I sent you some thoughts on this floor a while back, but I may be mistaken.
In reply to sleepyhead :
Then you can achieve greater efficacy with Gurney flaps.
They are basically pieces of angle aluminum stuck in the trailing edges of the wing. Dan Gurney used them to put Added or less down force on his Indy cars.
He could add or subtract rear downforce. During a pit stop. By lifting an end plate( tab) he could slide out a 1/4 inch piece and slide in a 1/2 inch piece. Or whatever was called for. It took him less than 15 seconds to do.
I'd like a link to that section software
re:3D...
are we talking OpenFoam? or something else?
SleepyHead:
Home use is openfoam
Work is a commercial product
Frenchy:
Gurney flaps/wickers/Gurneys whatever you want to call them are good at making downforce/lift, but are really not very drag efficient. If designing a wing from the start, you can design it to the top of your performance window and adjust back from there to match your needs. Basically try to design to the most drag efficient downforce needed and then give yourself some drag benefit as you reduce downforce. A great example for you is Road America: I would not want anymore drag than absolutely necessary for those long straights (especially that uphill run to the start finish), and the corners are fast enough that the downforce should still be pretty good. For a street race, like Long Beach I would give up drag for the straights to get as much downforce as possible in the slow/tight corners.
I’ll chime in to say that possibly we should take splitters/diffusers to a separate thread.
All the stuff works together, as a system, to create the performance benefit you need/want... but splitters/diffusers are different enough... and complex enough that they might warrant splitting off.
I’m also happy to move this to “Tech Tips”... if it’ll be easier to find/reference?
sleepyhead said:I’ll chime in to say that possibly we should take splitters/diffusers to a separate thread.
All the stuff works together, as a system, to create the performance benefit you need/want... but splitters/diffusers are different enough... and complex enough that they might warrant splitting off.
I’m also happy to move this to “Tech Tips”... if it’ll be easier to find/reference?
Tech Tips might be a good location for this.
I agree a splitter and/or diffuser discussion should be a separate thread (or two). Once the components are discussed, they could be linked to a thread that discusses how to combine the various bits.
I like this thread, hoping to use some of it on the Funderbird. Will also follow along with any splitter/diffuser threads as we scrapped our bumpers and I need to put something in the empty spaces we made. Carry on.
I’ll wait to move this for Robbie to chime in, as OP
sleepyhead said:I’ll wait to move this for Robbie to chime in, as OP
oh sorry! yes please move to tech tips. I'm hoping to build the "first gen" this weekend so I will keep updating this thread.
stafford1500 said:SleepyHead:
Home use is openfoam
Work is a commercial product
Frenchy:
Gurney flaps/wickers/Gurneys whatever you want to call them are good at making downforce/lift, but are really not very drag efficient. If designing a wing from the start, you can design it to the top of your performance window and adjust back from there to match your needs. Basically try to design to the most drag efficient downforce needed and then give yourself some drag benefit as you reduce downforce. A great example for you is Road America: I would not want anymore drag than absolutely necessary for those long straights (especially that uphill run to the start finish), and the corners are fast enough that the downforce should still be pretty good. For a street race, like Long Beach I would give up drag for the straights to get as much downforce as possible in the slow/tight corners.
You are absolutely correct about design versus adapting, but the reality here is none of us have access to a wind tunnel or any other way of getting things perfect.
What Gurney Flaps do allow is extremely quick and easy way of altering the wing to achieve a goal.
I do agree that if you have the facts and data you can design better. But that’s not realistic.
Forget all the theory and math, get what you can and adjust to better. The tiny bit of additional drag over theoretically perfect won’t matter as much as your seat time.
Mind you my Black Jack was faster at Elkhart Lake than a real DType even though it was no where near as aero
what effect do the endplates have on the wing? is there a design rule for them?
stafford1500 said:Robbie said:I've been working on this on and off the past few days but I think I just crashed javafoil... oops.
we should definitely discuss diffusers too. Here or in another thread doesn't matter to me.
I think you have definitely crashed it...
The software is not really meant for complex shapes (like cars), but can be used if approached cautiously. YOu might try increasing the number of cells in the flow field (entries at the top/left). This will increase resolution on the computation and increase compute time, but should get better results. Also add some points along the bottom of the car, even if they are on the same line. That will help define the shape for the solver. My 'car' model has similar issues on the rear glass and trunk...
This reminds me that there's a bit of "foil coordinates" that I've learned along the way, that might be useful to those that are dropping in here, and thinking about analyzing their own...
120points is kind of the minimum xfoil likes to work with, especially in "Low RE" flows. There's a command ("PANE") that will automatically re-populate/interpolate the shape to have 160 points... which it is much happier with. Along the same lines, it'll give an error on loading a foil that has a "panel-to-panel" angle that exceeds 10deg... i.e. that three points connected by two flat lines between them will result in an angle between those two panels >10deg. It'll recommend you run "CADD" in "GDES" where it'll popp in more points where it's found those large angles, try to hold the "shape" while putting in more points and "rounding over the corner".
A similar method would probably be helpful for entering shapes into Javafoil... although it seems a good bit less sensitive to these geometries than xfoil. similarly, you want some points along a straight line (like on the back window) so it can have an easier time figuring out where the flow will reattach, if it does
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