I hope he doesn't mind my excerpting his book here, but this quote, from Chapter 4 "The Aerodynamic Design of Family Cars" in
Road Vehicle Aerodynamic Design: An Introduction, is salient to the discussion surrounding the 'template' on this site in recent months:
Quote:
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As was explained in Chapter 2, due to the proximity of the ground, the 'ideal' form for a road vehicle is neither an axisymmetric teardrop nor a half teardrop, but a cambered version, slightly flattened on the underside, with the optimum geometry being dependent on the ground clearance. A further complication arises from the fact that the cross-section will need to contain at least two people seated side by side, so a circular or semicircular section is unsuitable. Also, if excessive frontal area is to be avoided, the surrounding shell needs to fit the occupants quite closely. With these conditions imposed, there is no longer a single 'optimum', and the best shape for low drag will depend on the ground clearance and the degree of tailoring required to accommodate the occupants and internal components.
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A few comments:
We know from the literature on car aerodynamics and direct measurement that the range of variation of shapes that can support attached flow (that is, that are streamlined) is large; I posted a quote from a 2010 paper by Hucho on another thread here yesterday regarding the leeway available to designers to change fastback backlight angle without increasing drag, for example, and tuft test images posted here by a handful of people show attached flow over a range of shapes.
This excerpt considers the impact of practical factors, such as occupant room, and physical parameters, such as ground clearance, on the 'ideal' form. Even if we were building cars from scratch--which most of us aren't--slavishly following a 'template' will not by default obtain the best results.
Now, consider that most of us are modifying already-existing production cars, cars that are not half-bodies of revolution, cars that may have air dams or limited underbody paneling or extensive underbody paneling from the factory, cars with narrow tires or wide tires, cars with high ground clearance or low ground clearance, cars with varying rear-body shapes that translate to varying flow fields and pressure profiles, cars with all sorts of differently-optimized (or not-optimized!) aerodynamic and styling details.
I used to think, when I didn't know any better, that extending along a 'template' was the solution--based on what I read on this site. I now realize that this an incredibly simplistic view, and one that does not take into account the facts that 1) there is no single optimum 'template', and 2) the variation in shape and thus flow over the cars we modify is in no way consistent, so a "one size fits all" approach simply cannot produce the best results possible.
For example, a few weeks ago I measured surface panel pressures and found that the flow speed down the center of the roof of a Prius is faster than at the outside edges, and that over the rear window the opposite is true. The 'template' assumes uniform pressure and thus uniform flow speed over its circular section. Given that this is not the case on a Prius, why would a person assume that extending it with a half-circular tail at a specific angle is automatically the best solution to decrease drag? Maybe it isn't. Perhaps the optimum solution doesn't involve fitting a tail at all, if doing so would increase difficulty of parking or not being able to fit in a standard garage, for instance, but looking at other areas of the car to decrease drag. Perhaps a tail that does not have a half-circular cross section would reduce drag more than one that does.
How on earth would one know without testing? Without measuring, I would never have known that the flow speed over the rear body is not constant side-to-side in the first place!
'Templates' are like security blankets: someone guarantees results with a minimum of thought or effort. But after trying it, testing is infinitely more rewarding, not very expensive, and not that difficult; the hardest part is really just finding the time to do it, and that's not a huge hurdle. More importantly, testing will reveal what the air is actually doing over
your car--no guessing or predicting necessary. Go try it!