Fitting a boattail to a car is what will really bring a car's drag down to bare minimum, after the wheels, undertray, grille and rear view mirrors are taken care of. Since minimizing drag is what will ultimately enable an EV with high range with existing battery technology, I have been thinking about what sort of vehicles to use as a base.
Now, ideally we want a boattail that is as short as possible while still being functional (i.e. reducing drag, being used for storage). We need to know the maximum taper that the boattail needs. Then it is just a matter of finding a car shape that when you overlay the rear segment of an airfoil over the car in such a way that it only intercepts but not intersects the furthest points of the car, the point of the boattail is a minimum of distance from the rear of the car.
I was thinking about this earlier, and came to the conclusion that the Australian version of the Ford Capri would work great:
I suppose any other similar convertable would be ideal. The most ideal convertibles are cars where the rear of the last hard piece is relatively close to the front of the car.
If you look at the rear of a car, the cross section is a rough rectangle. Minimizing the length of the boat tail really comes down to finding the minimum side length of this rectangle. And thus something like the capri shows a lot of promise. A full boattail might only converge to 4 feet behind the car.
I thought that convertibles had horrible drag coefficients though... is this not correct?
Sure they do.
But as a base to put a boattail on...
I suspect it would be a LOT easier to accomplish with a convertible.
Of course, if you were willing to totally hack into the mira, remove or cut the rear window in order to start tapering early...
But the difference between the capri and the mira is 7 inches, and with the capri one could start probably a foot or two further into the car. Meaning that you can get away with a still smaller boattail with the capri.
I thought this was useful to understand drag minimization.
In the first two pictures, drag is minimal. As the angle of attack increases, more of the boundary layer separates, and the wing stalls. Essentially the rear of the car can be viewed as "stalling", and hence generating lots of drag.
This behavior is a function of angle of attack and reynolds number (i.e. speed for a given car). I suspect that my angle as drawn is a little generous, however, it could be easily tested with coroplast (corflute), duct tape and an instantaneous FE meter.
What good is this to someone who wants to design an FE car? Well, start with a good car, and then experimentally compute Cd for different configurations. Start with a fairly steep boattail, and make it shallower until the drag drops off at speeds that the car will be driven at.
A strong wind according to the Beaufort scale is 50kph. If we assume that gale force winds are rare, then the maximum that the car's angle of attack should be is 26 degrees. Which should be enough to seriously impact the car's drag, especially if we design it so that it's just on the verge of stalling in regular conditions (no wind).
Mighty Mira: Since your driving a "box" shaped vehicle, I am going to share some background/experiences I've had with you, which I found very fascinating and curious. Maybe you will be able to use it with your car, maybe not.
I had a Plymouth Reliant Station Wagon, with a 2.4 L engine and an automatic transmission. We used it to go camping quite a lot, so I built a car top for it, the length of the top and about 20" tall. I built the front to match the angle of the windshield and just squared off the back.
In terms of packing and hauling it worked really well. I had to put air shocks on the car to get it back to level, but after that it worked great.
The only problem was that with the heavy load of people and equipment filling the entire car and car top, I would get up to about 56-58mph and then it would be like a hit a wall. If I had a really long run, I might have been able to get it up to 65mph.
One year, on a annual trip to Death Valley, in December, I wanted to take some fire wood with us, since you can't get any their. However, we were out of room, so I put a tarp on the top of the car top, stacked a pile of wood about 10" high and about 3 ' long, at the back end of the car top.
In previous years I would get about 17-18mpg, with the car top on. My gas mileage went up to about 22mpg. Additionally, when I got up to about 57mph, I didn't seem to hit the wall, like I had previously.
So'o I got some cardboard and fabricated a shape to put on the top of the car-top/carrier. The shape was a reverse airfoil, upper half, placed across the top of the car-top/carrier. With this shape, I was able to drive at a higher speed, I could pass other car's at 65 and so forth. I also was able to get 22mpg-25mpg.
I gave the car and car top to my parents. They made a trip with it, with 5 full size adult's and with the car packed with everything from the kitchen sink to who know's what. I asked my dad how he thought they had done on mileage and he said he had only checked the first tank of gas, which didn't make any sense, because he was always checking mileage, on trips. He said that on the first tank they got 25mgh and he was dumbfounded. He said he didn't think anyone could ever get that kind of mileage, with that kind of load, in a car with that size of engine. So'o he quit checking it because he didn't want to try to figure out how it could be getting that, in case it was because of some etheral experience or cause he might jinx or upset the balance of, if he tried to figure it out, to closely.
I believe what was happening is that the huge box shape on the back side was basically creating a huge vacume behind the car and that the reverse airfoil shape caused the air to be accelerated from the top of the car back down behind the car, breaking up or releasing the vacume effect's on the car. In either case, their was definitely a force beyond just frontal surface area, which was really dragging the car down, as the speed's went up.
Hope this information might be insightful or helpful.
Note how in a crosswind, the car actually might get more drag than in a direct headwind, especially with a boattail! And since it is not going to be feasible to put boattails on the side to make the vehicle look like a manta ray (it will take up both lanes of traffic), it probably doesn't pay to taper the sides of the vehicle, only the top and bottom (since vertical wind is rather rare). Because if you taper the sides, a crosswind will develop boundary layer separation that much easier on the lee side of the boattail.
Thanks to Greg Locock for pointing me in the right direction.
Thus, if crosswinds are a factor, the flounder or stingray points the direction to a low drag design. It makes sense, since both of these animals need to be able to hover over an area when currents are changing direction. A shark or fish doesn't need to be able to do this, because it is more like an aircraft than a plane - it orients itself so that it is always hitting the air head on. The only way this could be accomplished with a car is if it had four wheel steering and could somehow point itself in the right direction. Which wouldn't work in gusty conditions.
Hence, the ideal shape for an EV will be flat as flat as possible.
Of course, knowing the average wind conditions will surely help. If crosswinds are a factor, then a more shark-like design will be better. Suffice to say, something like the Capri will naturally be better than a Mira as a base for an EV. And the above example car is going to be impractical. But tall vehicles, in general, we be penalized, hard. It also means that side skirts are going to hurt with a crosswind.
A practical car is therefore going to be somewhat elliptical in cross section, including the boattail. Hence the ev-1. Looks like those engineers really had it figured out. If you wanted to "roll your own" EV, the most logical starting point would be something like a capri. Ah well, looks like I may be selling the Mira down the track. I mainly bought it for the engine. It would be interesting to stick the Mira engine in a Capri, but probably better to convert the Capri to an EV. Oh well.
Mira: another approach to the crosswind issue, perhaps not practical, but certainly very creative, is 4-wheel steering: angle the car while underway to make best use of its "straight ahead" aerodynamics.
our vehicle ECUs could calculate, like the Synergy Simulator does now, what the 'Corrected Wind Angle' composite is, what the 'TAS' or 'True Airspeed' is and then appropriately steer the rear wheels & turn our headlamps so that the body of the vehicle approaches the Corrected Wind Angle head on. Calculations show that this can increase vehicle mileage by as much as another 15%. It might look a little bit like a dog running sideways down the road but, as soon as folks knew that meant fuel savings; it would likely be considered cool technology!