Torsional spring constant duct

To obtain a torsional spring constant (torsional stiffness, rotational spring stiffness) for a duct it can be modelled as a torsion bar/shaft. Then $k_{\theta}$ is the torsional stiffness, the torque per angle of twist.

Symbol
$k_{\theta,d}$
Unit
N.mm/rad
Formulas
$\frac{G_d J_d}{L_{span}}$
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