Why does the zeppelin NT tail use 3 fins? I don't if I use the correct vocabulary. By "fin", I mean the control surface at the tail of the airship. As a control surface, it contains a moving part.

As airship used to have at least 4 fins at the tail: an elevator (1 fin on the right side, another one the left side) and a rudder (one fin on the bottom and one on the top, I highly suspect both are useful to be sure at least one is in undisturbed airflow while cruising at any positive or negative angle of attack). This seems reasonable to ensure both redundancy and simplicity (not mixing pitch and yaw controls). Yet, aircraft with V-tail prove that we can reduce the number of mobile control surfaces located on the tail, but it does not seems to be reasonnable in all cases.

For the Zeppelin NT, only 3 fins are used. I clearly imagine the vertical one is used for yaw control only. The 2 others are shaped as an inverted V, thus I imagine they mix both yaw and pitch control as V-tail does.

What is the rational behind the choice of number of control surfaces? Why do the Zeppelin NT uses 3 of them?

Wikipedia provides few reasons I fail to understand:


it states less fins allow weight gains. for me, less fins implies bigger fins to provide the same authority, thus mitigating this point
it states the loss of one fin can be compensated by the 2 other. For me, the loss of one fin would provide a rolling moment when trying to compensate, and there is no other surfaces to mitigate it (on an aircraft, coupling is possible because it has wings)
The likely reason would be ground clearance.

While it is hard to guess the reason for a relatively minor design decision like this, it seems that using four fins would complicate the tail design. Specifically, the airship would need to have a lower gondola or an X configuration for the tail, which may or may not impose a weight penalty on the tail structure.

As you note V control surfaces tend to be a net weight penalty once you factor in the higher loads placed on fewer surfaces. However, when the choice is between four small surfaces and three larger ones, the latter is likely to be more efficient.

You also note that the loss of one lateral surface would create a rolling moment, which is true, but this is the one situation where an actual "pendulum effect" (sigh) can be observed: the buoyant lift of the airship is directed against gravity, so it has a stable equilibrium point with the gondola pointed down, which simplifies the control problem.

Without knowing the critical load cases for that tail, it is hard to give a definitive answer, but this stands out as a plausible one.