Time evolution of the pitch angle, α = tan⁻¹(−m/p), of the filaments in the runs with ⟨Q⟩ = 1 and varying ℳ_c. The upper panel shows the ℳ_c = 14 and 21 runs, and the lower shows ℳ_c = 29 and 36. All panels show results after the onset of the instability, and all times are normalised by the rotation period of the galaxy at R = 3 kpc. Circular points represent the mean pitch angle and error bars show the standard deviation. The dashed line shows α = 90°, that marks the transition between the trailing spirals with a positive pitch angles to leading spiral with negative ones. We see that all the runs show a decreasing trend in the pitch angle as the feathers get sheared due to differential rotation of the galaxy.