"Wind Vane" boattail
 

So, it seems one big issue with broadtails are cross wind effects.
Haven't read it discussed here, how would a wind vane style damped limited hinged boat tail improve negative cross wind effects but still reduce drag? I suspect hinge would be at or near mid-point in length,

So.... not a rudder?

There is an example from the 1930s I can't find but here is Berton's BAT 7 with slotted fins:

https://photo-voiture.motorlegend.co...t-7-130424.jpg
https://photo-voiture.motorlegend.co...t-7-130424.jpg

IMO, not a rudder, as it would not resist or redirect any air flow, but more likely follow the preponderance of the existing air, which existing fixed boattails are likely to resist, causing the negative effects encountered during cross winds.

Maybe I don't know the negative cross wind effects.

For a rigid body, constrained by the tires' contact patch, I thought it's all about having the center of gravity forward of the center of pressure. A steady side wind would be countered by 'tacking' the steering wheels. Buffeting from wind gusts would happen at a variety of frequencies.

Needs testing?

'big issue'
 

I'm wanting to participate, but need help a bit with the nomenclature.
* When you say 'broadtail', are we talking about the overall 'width' of the vehicle at its trailing edges?
* Or the 'area' of the aft cross-section?
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Also, I'm unaware that issues with crosswind stability, or gust stability is being reported.
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And, are we talking about an articulated tail, which would have one-degree of freedom, which would 'wag' if presented with crosswind, shedding yaw-moment?

https://cdn.myminifactory.com/assets...33441280-n.jpg
https://cdn.myminifactory.com/assets...33441280-n.jpg

Why just one hinge?

Sorry, "broadtail" is spell checker for "boattail".
And in my experience's crosswinds are seldom "steady", why I feel the need for improvement when resorting to substantial boattail solutions.
Multiple hinges might with a passive damping system be very hard to control vs one hinge, ie KISS vs bang for buck with this solution.
And one degree of freedom

'Substantial' boat-tail'
 

The DOT will allow up to five-feet of elongation with an inflatable tail, four-feet, if 'fixed.'
Since there's virtually zero 'overhang' at the back of a trailer van, and since a proper tail would have diminishing aerodynamic 'mass' as it extends rearwards with its reducing cross-section; and since research promotes ample edge radii for all boat-tail surface intersections, for vortex mitigation, yaw moments, even in strong gust, might not be an issue.
If is was deemed prudent to engineer in, a means to 'erase' an inflated 5-footer,in an 'emergency', off-the-shelf technology would offer us a means to automatically 'defeat' the tail, should accelerometers ever experience longitudinal 'excursions' which exceeded some pre-determined threshold, programmed into the controlling electronics. Blow a rupture panel to collapse the envelope.
On a 'rigid' tail, the same sensor/ actuation could 'unlock' your hinge, allowing the tail to 'unload.' You'd want to limit how far the tail could displace, side-to-side though, as if you happen to be near another rig at that moment, you'd have very little clearance in between.
I'm already seeing 'Trailer Tails' torn up, probably from 'jerk' experienced from repeatedly hitting potholes and such.
Light-weighting the construction as much as possible would minimize the inertia responsible for 'spontaneous disassembly'.
I'd first want to verify the threat posed by crosswind and gust.

I think your supposition is valid in say in large mass/low rear overhang semi-trailer applications. My concern and focus was geared more towards typical RV two axle, large relatively rear overhang applications with "average" drivers in cross wind situations. In Florida 'approved' aero devices are not limited in length, but what requirements are needed for approval seem to be rather vague, including any listing of what has been previously approved.

A two-axle big-*ass RV isn't a good test case. What other options?

• Plan taper abaft the rear axle with length added to preserve interior space
• Twin-tail flaps rather than a boat tail
• Active Coanda nozzles.