My $0.02 is this:
Using the frontal area calculation that the automotive world lives by is a good rough measure or attempt at comparing two shapes against each other. I'll use reductio ad absurdum to present examples.
Let us assume we are running three objects through the Fa formula and calculate the HP required to move each of these objects 100 MPH. The results of which are as follows:
Object 1: .4 HP needed
Object 2: 78 HP needed
Object 3: 8,000 HP needed
What did we learn from this? Well, Object 1 is the most aerodynamic since it needed the least amount of HP. Object 3 is the least aerodynamic because it needed 8,000 HP to achieve 100 MPH. But what are these objects? What shapes do they represent?
This formula is good for a rough comparison of both known and unknown shapes to each other, but it doesn't tell you how to make them more aerodynamic. The reductio ad absurdum portion of this exercise is that object 1 is a sparrow, object 2 is a car and object 3 is a locomotive.
Now, if we look at the method that uses flat plate equivalent alone, it is not that far from what the Fa formula tells us. The difference is that the resulting numbers are compared not to each other, but to a 1 square foot flat plate placed perpendicular to the airflow. It really just changes the anchor point of reference from object-to-object to object-to-flat plate. Neither of these methods alone gives you the ability to identify and quantify explicit areas of drag.
The second part of the method Rob makes reference to is the build-up calculation. How I believe it works: If you take each discrete shape that is interfacing with the air and calculate the flat plate equivalent, THEN add all of those together, you have a total drag number for the object. The advantage to this is now that you have these details and an anchor of FPE, you can start to analyze for improvements. You can now compare the wheels to the windshield to the spoiler to determine which areas of the shape are yielding the most drag so that you can focus on those areas. If the fuselage is 12% of the total drag, but the parachute tubes are also 12% of the drag, you may want to look at the parachute tubes first.
Here's the other part of Rob's statement, what kind of drag is it? If the parachute is a cube bolted to the top of the car, you have both stagnation and separation drag. Placing a more aerodynamic shape on the leading portion of the cube will help, maybe not ideal, but you can reduce the stagnation drag. Maybe because of packaging or money, that is all that can be done, but at least you know where the drag is and what it costs. You can also measure the improvement from 12% to %12 - x when you reduce the stagnation drag.
Rob references tires and the stag/sep drag they cause. Not much you can do about the shape of a tire, it will be what it will be, but you can look at the shape of fairings and ways to present the tires to the air. These shapes can all be calculated and added together to give you a reference of what the air should be doing with the shapes you have chosen. It will also give you some reference within your own design to know where your problems are and where to focus your efforts.
This isn't a new concept or a new idea that is being proffered, it is how the aviation world has worked for years. It is just being highlighted that it can also work in areas outside of aircraft design.
Sparky - I like your concept of pools(buckets, classes...) of drag, knowing which kind and how much is in each bucket as well as what you can do to correct AND MEASURE THE RESULTS of your changes is how you make your shape better.
