As a complete layman, the design of the Stratolaunch airplane, especially when reminding myself of its size, seems too flimsy, in that the only connection between the two fuselages is the center wing. Apart from apparently trusting their materials and connections, what might the reasons have been for Dynetics engineers to not also connect the tailplanes? Also, are there no aerodynamic disadvantages to having that many more trailing edges (if that's the terminology used)?
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$\begingroup$ While I don't know, I'd guess that part of the answer is that it's supposed to carry a rocket vehicle underneath, and drop it to launch. The separation between tails would allow for something like Sierra Nevada's Dreamchaser with its own vertical tail... $\endgroup$– jamesqfCommented Apr 14, 2019 at 17:33
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$\begingroup$ Interesting that neither of the two answers bring that up. Perhaps there'd be enough space even with a connection. $\endgroup$– SixtyfiveCommented Apr 15, 2019 at 11:57
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3 Answers
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There is indeed an Eigenmode possible on the Stratolaunch which does not exist on regular airplanes: If the two fuselages pitch in opposite direction, thereby twisting the center wing. If that happens at the torsional eigenfrequency of the center wing, flutter might be possible.
Stratolaunch in flight (picture source)
Given the pitch damping of the horizontal tails, however, this scenario is rather unlikely. I bet the elevators have plenty of mass balance to avoid this flutter case. Underbalanced, their dynamic response would support flutter, but overbalancing would dampen this flutter mode. Connecting the tails would not make much of a difference and would even reduce pitch damping somewhat in the above mentioned case.
Similar configurations used both, a connection horizontal tail (F-82 Twin Mustang) or individual tails (He-111 Zwilling).
North American F-82 Twin Mustang (image source)
Heinkel 111 Z (image source)
answered Apr 14, 2019 at 14:49
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$\begingroup$ I'm going to accept this for its use of comparison with other, past aircraft that illustrates the difference between the two design types real well. Also, I'd never seen either the F-82 or the 111 Z and had thought the White Knight to have been the first aircraft of this kind, so that puts things into perspective, too. Thank you, Peter! $\endgroup$ Commented Apr 15, 2019 at 12:01
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$\begingroup$ The more I look at the Mustang, the more elegant it looks. And the Heinkel, while not as elegant, must have been quite the feat back in the day. According to Wikipedia, pilots apparently liked the Zwilling quite a lot. Do you have any idea how they achieved the structural stability necessary? I mean, what if there's a wind shear pulling on one tailplane and pushing the other - the torsional forces in the wing must get immense? $\endgroup$ Commented Apr 15, 2019 at 12:06
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$\begingroup$ Wind shear is not so small-scale - this is not worse than with any other large aircraft. By giving the center wing a stiff skin, its torsional rigidity is sufficient for safe operation. $\endgroup$ Commented Apr 15, 2019 at 13:34
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That's not how airplanes work. An airplane is the wing first, everything else just hangs on the wing in flight.
The correct way to look at an airplane is to imagine the wing without all the attachments, then add them one by one as weights. The engines are a semi-exception in that they also pull forward at their attachment points. The fuselages, on the other hand, are just fuel tanks, and also carry control forces from the tail surfaces.
Placing two heavy fuel tanks (fuselages) a considerable distance apart is much more structurally efficient for the wing than only placing one in the center. That's the reason it's done that way.
It's inefficient from the payload perspective (the payload, be it live or cargo, prefers one big tube), but the Stratolaunch carries its payload slung between the twin fuselages.
As for why the tailplanes aren't connected, the right question is "why should they be". There are multiple possible reasons for such a decisions, and a number of twin-boom aircraft where they are connected, often with a lifting surface that acts as a second wing. But the Stratolaunch is a twin-fuselage aircraft with conventional tails, and in this case it would cost a lot of extra weight to connect the tailplanes.
It's only on the ground that the aircraft rests on its wheels and the wing rests on the two fuselages. And indeed, a twin-fuselage arrangement does make ground handling more complicated, and on the ground the wing is subject to some forces arising from out-of-sync loads on the fuselages. But given this aircraft's mission, that's a minor concern.
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$\begingroup$ I would be terrified if ailerons failed on this aircraft, and the only way to roll would be using elevators asymmetrially, with potentential crazy roll inversion behaviour depending on airspeed. $\endgroup$– user21228Commented Apr 14, 2019 at 11:11
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$\begingroup$ Connecting the tails might help with managing twisting moments in case one of the rudder/elevator actuators fail in flight. But presumably they've modeled that and concluded the fuselage/wing connections are stiff enough to handle that load. $\endgroup$ Commented Apr 14, 2019 at 11:58
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$\begingroup$ @qq, I bet they could fly it with rudders only for roll, and avoid asymmetric elevators. $\endgroup$– MikeYCommented Apr 14, 2019 at 13:04
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$\begingroup$ Unfortunately one cannot accept two answers. Thank you as well, Therac, especially for approaching the question in a bottom-up, engineering, kind-of-way. FWIW, having this here adds to the one from Peter! $\endgroup$ Commented Apr 15, 2019 at 12:02
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It’s mainly because the load is shared between two fuselages like in mechanics the load of a bridge is shared among its towers sharing the bending moments
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$\begingroup$ Hi and welcome to the site! This doesn't seem to address the question, which is about the tail planes as opposed to the wing. The airframe would, at first glance, gain stiffness if the tail planes were to be coupled. $\endgroup$ Commented Jun 8, 2019 at 13:17