I was thinking to myself about crosswinds the other day, how to design a bodyshell that would maximize range. With a high enough Cd, it might even be able to be driven by lead acid batteries.
I figure that there has to be a market for something that:
a) Can go 300 highway miles with lead acid batteries.
b) Can do it in a strong crosswind.
c) Can protect from weather.
d) Can carry perhaps up to 4-5 people.
The shell should be built with plastic panels to minimize cost.
This doesn't leave me with many choices. Naturally, if we are to minimize drag, the airfoil or a corruption of it for the sake of convenience is the only real way to go.
Since we are talking about highway miles, I assume 60mph as a base speed.
Now, our vehicle design will depend on the crosswinds we expect.
Here is the
average wind speed for the USA. To convert m/s to kph, multiply by 3.6.
According to the Beaufort
scale:
Near Gale: 62
Gale: 75
Strong Gale: 87
Now, we are going at 100kph, or 60mph.
air speed = (highway speed^2 + high wind speed^2)^0.5
theta = arctan (high wind speed/vehicle speed)
So, different theta will be:
Near Gale: 31 degrees
Gale: 37 degrees
Strong Gale: 41 degrees
And air speed will be:
Near gale: 80kph
Gale: <80kph...
Conclusions from that are that if we take maximum crosswinds into effect, we don't need to take into account as much theta as if we just looked at theta alone. That is because the largest theta comes from a slight tailwind, which has a partial effect of slowing the true airspeed and hence the drag on the aircraft.
The driver of the vehicle could also be advised to SPEED UP to reduce drag in an exceptionally high wind. Counterintuitive, but it means that the car is facing more straight on and is so better equipped to take the drag.
However, we WILL need to take into account the crosswind. That means that at some maximum angle facing the front of the car, the car needs to be producing as little drag as if it were facing headwind.
The only way to do that is to imagine a maximum rear width of the car. Now, start drawing an airfoil section towards the front, such that it is drawn parallel to the maximum expected angle that the wind will be facing the car at highway speed.
Basically, at any angle between these two maximum angles, the wind should see nothing but airfoil (corrupted or otherwise). And when I say airfoil, I mean a neutral airfoil, not a wing. If anything, the rear should be sloped slightly upwards to add some downforce for stability.
The wheels should be faired as well.
This shape seemed so logical to me, I went out and bought some plasticene right away and molded it, since I don't know how to do the same thing on computer.
The resulting plan view of the car looks like one of those ghosts from pacman.
front
rear
rear side
side
top - (pacman)
side
Note how small the rear area is. If necessary, the car can be extended some more so that there is no turbulent rear area, no "stall" region, however, this will not be necessary IMO.
I show how small the rear vacuum area of my prototype is (dark brown shading) with the equivalent section of an opel calibra sized rear end of roughly the same volume (dark blue). We know that the best case Opel Calibra with wheel skirts and undertray is approximately Cd 1.7, of which most will be derived from the rear vacuum area. Thus we can estimate that if we have half that area, then the drag of my prototype will be approximately 0.75. AND, that drag will be irrespective of whether there is a crosswind below galeforce. If the Opel Calibra is in a crosswind, note that the rear area where there is a vacuum baloons out to the largest purple area, because it includes both the back AND the side.
Since this shape will get a Cd of approximately 0.75, which is less than a third of most cars, it stands to reason that the same battery load will take it three times as far. Which puts it certainly within the ballpark of 300miles with regular lead acid batteries.
Sure, it looks whacky by conventional standards (kind of like one of those Nasa experimental rocketships), but I see no reason why it couldn't:
a) carry 5 passengers
b) be electric powered
c) go 300 miles with conventional lead acid batteries
d) be immune to most crosswinds.
e) be stable at speed and in all manner of wind
f) handle reasonably well - all batteries will be placed between the wheels below the floor.
g) be built reasonably cheaply - needs a chassis, wheels, electric motor, lead acid batteries, windscreens, lights, plastic shell and some sort of spaceframe to protect/fit the batteries.
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