With the theoretical airflow template curvature worked out, now it's time to attempt to determine the shape of the tail extension bodywork.
To do this, some measurements from the car are taken. But how does one measure angles on the car? What reference point does one use?
One way I could think of to solve this, was to attach a "rod" to a step ladder and place the step ladder directly behind the car in the following manner:
1) Position the car to that it faces straight on the driveway.
a) this was verified by opening the driver door and aligning the step with the edge of the driveway. I had to move the car back and forth manually to accomplish this.
2) Position the car so that the tail was right over a perpendicular slab edge in the driveway.
3) Use this edge to align the step ladder also perpendicular to the back of the car
4) Align an aluminum beam such that is parallel with the ground
5) Use this reference beam to ensure that all photographs taken can use the beam as a reference point for angular measurement
6) Any picture taken, must have the beam also in the picture to allow measurement from "straight" of the surface of the trailing edge of the car.
This first picture shows the step ladder, along with parallel aluminum beam, reaching over the tail of the car. This is our movable data point for later measuring surface angles of the rear portion of the body.
This is the measured angle at position #1
Position #2
Position #3
Position #4
Position #5
Position #6
Position #7
Position #8
Position #9
Position #10
Position #11
Position #12
Lastly, this shows the locations where each angle measurement was made.
Robert had mentioned that his tuft testing showed some of the yarn not adhering to the glass at the outer edges. This makes sense, as you can see that the effective angle of departure in these locations is actually steeper than in the middle portion of the hatchback.
Also note that no attempt was made to "smooth" the transitions between various angles. These angles all can reveal the measurement uncertainty of my process. When sliding the step ladder along the ground, it is quite easy to get the legs slightly twisted, and thus adversely affect the angular measurements on the side of the car.
The author attempted to minimize this effect by sliding the ladder carefully, although it's easy to see there was still some non-parallelism of the ladder legs present in these measurements especially for measurements #1, #2, and #3.
EDIT @ 8:53pm After checking my numbers, it seems that the horizontal angles are steep by about 2.5 degrees, and this is most likely due to the step ladder resting on the sidewalk portion of the driveway, which has a slightly different angle than the slab that the car sat on. Whoops!! I have adjusted the angles with this in mind.
Jim.