No problem JHN, I am expecting lots of input, as a matter of fact, its the challenging of my ideas and assumptions that will help refine the design.
The reason that not all of the internal components are installed is because I am still working out the CG. I am trying to engineer the CG into a forward location so that the vehicle remains stable without having to add weight or come up with some other post-build fix. I think it is worth the extra time up front to plan on having the car set-up with CG and CP where they are optimum, that way we aren't forced to put some inefficient aerodynamic band-aide on or load lead into the car to correct some issue.
I am a little less concerned with the frontal area than I am with the wetted area and the ability to retain a highly laminar flow. You are spot on in your statements, the driver and the engine dictate that it must be a certain size. I've thought about everything from an in-line 8 and having the driver lay on his side to reduce the width, but none of it seems safe or practical. Oddly enough, you only hear about frontal area in automotive circles. I've seen some pretty large frontal areas on many aircraft that zip right along at some rather high speeds (see reference photo). (I've also seen what their gas bill is to accomplish it, of course, weighing 1.3M pounds has a pretty big effect on fuel consumption too.) I think it is more important to use highly efficient overall shapes and limit any disturbance of the air. Any air that is disturbed, needs to be controlled or shaped as much as possible to eliminate drag.
(Ref pic 1.0)
The outboard position of the rear wheels came from trying to add stability to the design and to incorporate a wing for down force. Theoretically, we haven't added twice as much drag (x2 wheel fairings), but rather only one. I was going to blade the aft portion of the fuselage, but I don't think we need to. The whole idea of putting a blade tail on the rear is to provide stability in yaw, much like the vertical stabilizer on aircraft, and it moves the CP aft. In this proposed configuration, we have the mechanical stability of a wider stance and the aero stability of two vertical stabilizers. One of the questions this raises for me is what happens when the CP is in the same plane as the CG in the z-axis? When a vertical stabilizer is used, it moves the CP aft, it also raises the CP higher than the CG, I am sure that has some effect I just don't know how much yet.
Down force - flat vs round bottom: This debate will probably rage forever, I haven't yet pitched my tent in a camp, but I am leaning a bit. I get the aero advantage of preventing airflow under a car. There are all kinds of proven examples out there from street cars to Indy cars and even LSR cars. The problem that I see with it is that you really have to be diligent to maintain a lack of flow under the car to maintain down force. Any change in the flow under the vehicle will affect its ability to generate down force. I also get concerned about cross winds and heaven forbid the car get a little sideways. The flow under the car from being sideways (not even 90 degrees, but some amount of sideways) will negate it's ability to generate down force. Expect to not have flow under the car and maintain that, and then what do you do when the unexpected happens?
My approach is that rather than control the variables, just eliminate or reduce the number of variables. I know that a wing works to generate lift, we've all seen it many times. I also know that you can introduce some really wild angle of incidence and still maintain good lift, so if the car gets a bit sideways, a wing is still effective. The other benefit to a wing for down force vs. flat bottom generated down force is that it is easier to control. Let's say that a flat bottom generates 5,000 pounds of down force at 400 mph and I have a wing that can generate the same down force at 400. Now, lets assume that I only need 2,000 pounds of down force to prevent wheel slip, any more than 2,000# is just generating useless drag. How do you control dumping the excess down force from a flat bottom? With a wing, you either decrease the angle of attack or retract the flaps and you can easily lower the down force in a controlled and precise manner.
I am not planning on using just physical weight to provide traction to the rear wheels, but rather aerodynamically generated down force to control traction. I want to put the CG where it needs to be for stability, then use aero to push the tires into the Earth. This will give us the benefit of having a lighter vehicle, a more stable vehicle because we have the CG where it should be and controllable traction for the rear wheels.
I would love to see 1/4 mile speeds in the 350+ range. To accomplish this, I would like to build a light car, very light. A light car can accelerate much quicker than a heavy one (I know, that was obvious), lack of weight obviously reduces the traction as well. There is going to be fine line that needs followed as aero down force can only be generated at certain speeds whereas weight is weight (for traction purposes). I would rather make the car light and use the aero to provide down force over the rear axles rather than move the CG aft to provide traction. I don't yet have enough data available to provide definitive proof of this concept, once we get more finalized in the design, I can determine the accuracy of this assumption.
Don't hesitate to challenge the concepts, I look forward to defending my ideas. Just remember, I have the right to disagree with others as much as they disagree with me! Challenges don't have to be adversarial, hurtful or mean, they can be spirited and respectful and at the same time very beneficial to a successful outcome.
