Longitudinal stability of airships: How is the critical speed defined?
On page 43 of the Technical Manual of Airship Aerodynamics I found the following image:


Here $F_e$ is the resultant airforce on the hull, or the dynamical lift of the hull. As this is before the CoG, it is destabilizing.
This source mentions Munk's formula being:
$$ M_e = Volume \cdot 0.5 \rho V^2 (k_2-k_1) \sin{2\theta} $$
Where $k_2$ and $k_1$ indicate the effect of gas movement inside the airship.
I tried to trace the origin of this formula in Munk's paper, available here, but I can't immediately find a clear explanation.

The critical speed is reached when this resultant airforce equals the moment induced by the weight:
$$Wh\sin{(\alpha + \theta)}=Volume \cdot 0.5 \rho V^2 (k_2-k_1) \sin{2\theta}$$  

In this sense, the critical speed is the maximum speed at which the gravity moment is able to compensate the moment induced by dynamical lift. As such, one should stay below the critical speed.