Let us discuss a little further, I think IO and Rob are on to something in their dialog.
Fa in and of itself is not a useful number, it doesn't convey any meaning or measurement other than ft2. It doesn't describe a shape or how it interacts with the air, it only conveys a number.
Here is an example that illustrate the issue:
Let us assume we have two objects that have an Fa of 1937.5 ft2, which one of them is more aerodynamic?
You can't tell from these numbers alone...or can you? (<-Legit question, I don't think you can, but maybe I missed something)
The two objects I have used here is a 747 and a building where the dimensions are 20ft wide and 100ft tall. Both of these can fly, but only one can do so efficiently!
It is not until you introduce the Cd with the Fa that you get something useful. The Cd describes how the air is moved around the object with an Fa(x), and from that you can determine the general aerodynamic properties of the shape. (FYI, the drag coefficient of that 747 I used above is 0.03 (
http://www.withouthotair.com/cC/page_273.shtml).
IO is correct in his statement about FPE as well: "the difficulty is in determining each of their drags and how they arise out of the different modes of drag creation--shape, skin friction, etc.". It may be a completely cost prohibitive endeavor to remove and measure each component of a standard vehicle to measure it's contribution to drag. Think about the challenge of measuring the frontal area and the drag coefficient of a fender, hood, bumper, windshield, door handle, tires, hub caps, antenna, mirror... not to mention the fact that you are missing the aerodynamic interaction of one part against another.
Here I think is where the real differentiators are: In the world of aviation, you are not incumbered with certain details like, will it fit under existing bridges, how well will it parallel park, will it fit in the average parking garage, I don't need a rear view mirror, tires only need to be sticking out when I am on the ground, all other times they are out of the airstream....
The world aviation engineers have to work in have different requirements because they have different goals. You don't see many passenger cars with a ~2000 ft2 frontal area that seat 400+ and drive cross country in 4 hours. Conversely, you don't see many airplanes that you can hop in and go 3 miles from home and run through the drive-thru to get your kids french fries either!
Back to IO's comment. I think in a door-slammer situation, you are not likely going to do an entire buildup using the FPE method to determine total drag, it is not economically feasible. I do think though that if you are talking about "Specialty construction" or building adaptations for your door-slammer, there is great value in knowing the FPE, if for no other reason, you can get your drag, Reynolds, wetted... if you base your build on known shapes.
If you are like me, looking at this link
http://en.wikipedia.org/wiki/Drag_coefficient will yield nothing but an aggravation -- unless the subject interests me, then I'll devour it as best as I can. You can skip reading the whole thing, just look at the top right side and the pictures/shapes they have. I am not sure the accuracy of these numbers, nor am I implying they are reliable, I am only illustrating that your basic shapes can be estimated, some work has already been done, there may be more accurate sources for Cd's as well.
I know for sure that the numbers presented in that link are not precise, because there are many "streamlined" shapes (foils) to choose from, velocity, size, AOA... all contribute to the drag. Some foils are more effect at different velocities and AOAs and produce less drag than represented in that table. If you are talking about a foil, get a copy of "The Theory of Wing Sections", it has what you need to calculate foils, lift, drag, by AOA, by velocity...