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Depending on the kingpin position on the trailer it can have quite a bit of headswing. In a full turn the entirity of the accordian would be suspended on the corner of the trailer, and only at the top of the accordian. It would need to be designed in such a way that the lower part was never extended past the bar that runs along the top. This would produce gaps and reduce effectiveness. Further reduction would be caused by the compressed folds when a trailer is hooked up (I'm not even sure if it would be more aerodynamic than having open space while compressed). Lastly, those air lines (and power cord) usually connect to the middle of the front of the trailer which wouldn't be so bad to work around with the accordian thing, but in some cases the connection point is near one of the corners (I know a very bad design as it causes the most possible stretch during full turns). In those trailers especially, and to a lesser degree in the trailers with the connection in the center, during and after a turn the lines must have unimpeded travel. This is quickly complicated when you take into account that turns do not always happen on level ground, if a truck is turning and cresting a hill for example, the air lines will be stretching across a much different area of the accordion occupied space than on level ground. The only way I think you could be assured of safety would be to cut fairly large sections of the bottom rear accordion out (and since there is going to be a gap there anyway it likely won't have too much of a detrimental effect).
It's a good idea, and would work great for trucks while bobtailing, but in this day and age a bobtailing truck is losing money for everyone involved so I doesn't happen all that much. The 20 miles to go pick up a trailer is a dop in the bucket compared to the 1000+ miles the truck spends to deliver the load after pickup.
To be effective I think we need a smooth, rigid surface that doesn't interfere with the air lines. I have proposed an idea to a manufacturer, but haven't received a bid yet for a prototype yet:
first is a top piece that spans the distance between the back of the sleeper and the front of the trailer. It connects via a hing on the sleeper and rests upon some sort of low friction, low vibration connection in the center of the trailer (that would accommodate x and y axis motion, the small amount of z axis motion should be handled by the hinge), the center mount is a pivot point and with the small amount of motion accommodation mentioned previously this top piece will remain stable during turns of all sorts. While bobtailing it can be set to an optimal downward angle off the back of the sleeper, and could even be built with a slight curve to help in that capacity.
next is a pair of side pieces which are connected to the existing side fairings and extend back past the front of the trailer. They ride on small wheels, or balls, or a low friction material. When a trailer is in a turn they continue to follow the sides of the trailer either with springs holding them in or a natural curve in their material.
Problems: during the resolution of a full turn the trailing edge of one side wing will be riding along the front of the trailer and be forced to negotiate the curve of the trailers corner. This will require some sort of guide materials on that edge of the wing and will create a lot of stress there, this in turn will create a gap and possibly some unaerodynamic shapes. I expect that this will still result in a net gain (or loss in terms of drag rating) in aerodynamics. Alternatively we are thinking of mounting the guide materials onto the corners of the trailer instead, it isn't something we have been able to think our way around, we need to get some models going and see effects.
We would have a very similar problem as the accordion in terms of air lines, but the trailers I plan to use these on have center mount airlines and I was intending to put a block on the trailer front to force the wing to bend outwards to and never interact with air lines.
The biggest drawback to all this: It must be somewhat custom built for the application. I do not think a one-size fits all solution can be made, and the system on the truck and trailer have to be matched up. This works for my use as these trucks have dedicated trailers and mostly they are all identical anyway. In cases where non-matched units have to work together they can take the wings off the truck and fold the top piece down against the back of the sleeper.
I apologize for how long winded this is, I hadn't intended it.
By the way, This isn't merely a thought experiment. I own a small fleet of trucks and trailers. Fuel is our biggest expenditure. Any amount saved means more profit. If this system could net 5% less fuel consumption it would mean aprox. $120 per week per truck in savings. So even if it ends up costing $3,000 per unit, it has paid for itself within half a year, and I expect we can get it down to less than that, and with more than 5% savings eventually. Although I expect a kamback style air dam on the rear of the trailer will have a larger impact and cost less. That is a different project.
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