I'm thinking about doing something dumb and building a fully custom front wing/splitter for my car. I have done research into airfoil designs and such for building my own rear wing in the past, but I never actually got around to it and ended up just purchasing something premade instead. So now I've decided to try and do something 10 times more complicated instead.
Essentially what I am considering doing is creating a splitter where the outboard edges (2 feet or so on either end) are airfoil shaped to attempt to make as much downforce as possible (similar to Dusold's Camaro here - https://www.instagram.com/dusolddesigns/p/C3yTLVfrIN7/). The center could end up being airfoil shaped as well, but at a much lower AoA due to radiator clearances. I've looked into a lot of the off the shelf splitter tunnel designs, and none of them would really fit within my packaging constraints. Also they require significant reinforcements to the splitter to not allow it to become floppy when added together into a single giant tunnel. I've just never been happy with the idea of splitter tunnels any way that I look at it.
The other option would be to pay an aerodynamics company to design and build a one off part for me. Unfortunately that isn't even close to aligning with my budget. Neither is paying for them to make it a second time after it's first lawn mowing excursion at the track.
So I like to believe that I grasp a little bit of the concepts of aerodynamics, and fully understand that no part that you slap on your car is going to perform the same as it would in free air. And that is sort of where my question stems from.
So my question - does an airfoil with no airflow across the "high pressure" side of it, behave similarly to one in free air (I have no expectation of similar downforce levels here). Asked another way, if I go pick an airfoil shape off of airfoiltools, I build it, and then the top of it gets sealed to the bottom of the front bumper with an airdam, is there any expectation that it will have similar stall and other such characteristics to the same airfoil if it were mounted as a rear wing?
I know that the fender, tire, and wheel well are all going to effect actual downforce numbers and stall angle to some degree because "everything affects everything else", but I'm mostly just wondering how I should go about picking an airfoil shape for this application since I'm not an aerodynamicist?
(Did I say airfoil enough to sound like I know what I'm talking about? 
)
I think you might want to spend some time reading the splitter-related articles at occamsracers.com. Lots of great info there that goes far beyond "make it stick out".
You are allowed one more "airfoil" to get to your per-post quota.
Using the suction/low-pressure side of an airfoil for the lower side of you splitter/nose assembly is a decent way to start. The biggest thing that will affect the underside is the proximity to the ground. The other items you listed and others: fender, wheel/tire, suspension, chassis, etc, will have som impact on the performance but not as much as the ground clearance. Aircraft begin to "feel" ground effect at about half-span (so a 30 ft wide wing would feel ground effect at about 15ft from the ground). Since your wing is on the order of 6ft long (width of a generic car) you will also be in ground effect.
As for the performance using tools like airfoiltools the net free-air performance will not help you predict the ground effect AND suction surface only performance. Just know it will be much better for downforce (maybe as much as 3*) and roughly equal on drag. The splitter tunnel you mention are effectively what you have described, but you may or may not have the outside wall to help the efficiency. That is one of your design decisions.
If you really want to get your head wrapped around the general idea of airfoils on and around car shapes, but keep it simple, you could sketch up the wing/nose shape you have in mind along with the rest of the outer body of the car thru the gap between wheel and chassis (right thru the suspension). What you will start to see is that the car body itself can be considered a wing and you are talking about adding leading edge slats/flaps. The same goes for adding rear wings and diffusers. Treat the car as the main plane for the wing and consideered the add-ons as slats/flaps.
You can get some of the typical airfoils to work in ground effect using JavaFoil (free-ware) and the potential for ground effect can be explored with a little more comfort. JavaFoil is a modernized version of an OLD NASA panel code (CFD speak).
The other thing to keep in mind is you do want a healthy leading edge radius on the your splitter to get the best results if you are running it closer than about 2 inches to the ground AND the loads generated by an airfoil shaped bottom surface can have MORE load than a flat splitter so the structure you need is still going to be significant thanks to that increased ground effect efficiency.
In reply to stafford1500 :
This is exactly the kind of information I was interested in. I have no idea how you put together an understandable response in 20 minutes, but I definitely appreciate it. Glad to know I was on the right track looking at the low pressure side of existing airfoils.
Also, I was aware of ground effect, but I had no idea that it had that large of an affect, and began to occur at such large distances from the ground. That's definitely good information.
stafford1500 said:
Just know it will be much better for downforce (maybe as much as 3*) and roughly equal on drag.
I'm assuming that 3* meant "3 times" here? And when you say "roughly equal on drag", are you saying 3x drag also? Or was that saying it will make similar drag figures to free stream without ground effect?
I'm definitely gonna check out JavaFoil at some point. I've seen you mention it before. I only have so much time and brain capacity to learn new things with though 
In reply to Keith Tanner :
I've definitely spent some time over there. I've read most of his free articles (it's been a while on some of them). Maybe it's time I donate to his coffee fund to get access to the rest
I just wanted to follow up on this thread. I decided to pick up Javafoil because I figured I should probably learn more about ground effect, and boy am I glad I did.
I had intended on using the FX74-CL5-140, which is an airfoil shape I found from Occam's Racer a while back. It has pretty good all around characteristics with a very good lift-to-drag characteristic.
After playing around with Javafoil and finding the controls that I was interested in, I imported the FX74 airfoil shape and hit go. Come to find out that when put around splitter height (11% of chord above ground level with the chord length I planned on using), that the FX74 had a much higher Coefficient of Lift, just as Stafford had mentioned (iirc it was in the 2.5x range). Unfortunately, the Coefficient of Drag increased way more dramatically than anticipated (from ~0.03 to almost 0.6, which is almost 20x the drag). Using the same AoA, the lift-to-drag ratio went from a very impressive ~145:1 to an abysmal ~10:1 when used at splitter height.
My assumption is that this massive drag increase is due to the highly cambered airfoil shape is almost trying to compress the air into the ground. But I'm no aerodynamicist. If someone can explain further, I would love to learn.
So now I'm off trying to find a new airfoil shape that doesn't have nearly as much drag in ground effect. Messing around with some 4 digit NACA airfoils, it seems like the low camber airfoil shapes seem to work pretty well. I was able to get one of them into the 40-50:1 lift-to-drag range, which I'm happy with, but want to get higher numbers if possible.
If anyone has any recommendations for airfoils to try out, I just don't have the time to sift through every single shape in airfoiltools.
