A rotary propulsion system for a tiltrotor aircraft operable to transition between rotary and non rotary flight modes, the rotary propulsion system comprising: an engine (26a, 26b) having an engine rotating speed in the non rotary flight mode; a freewheeling unit (42a, 42b) coupled to the engine (26a, 26b); a gear system having a torque path coupling assembly between first and second gear assemblies (44, 48), the first gear assembly coupled to the freewheeling unit (42a, 42b) and having an output, the second gear assembly having an input; and a proprotor assembly (20a, 20b) coupled to the second gear assembly, the proprotor assembly (20a, 20b) having a plurality of proprotor blades with a radially extended orientation and a proprotor rotating speed in the non rotary flight mode; wherein, the torque path coupling assembly has an engaged position wherein the output of the first gear assembly is coupled to the input of the second gear assembly thereby providing a torque path between the engine (26a, 26b) and the proprotor assembly (20a, 20b) and a disengaged position wherein the output of the first gear assembly is independent of the input of the second gear assembly thereby interrupting the torque path between the engine (20a, 20b) and the proprotor assembly (20a, 20b) characterised in that the torque path coupling assembly includes a coupling sleeve and a synchronizing ring, the coupling sleeve having internal splines that are in mesh with the input of the second gear assembly and in mesh with the output of the first gear assembly when the torque path coupling assembly is in the engaged position, the synchronizing ring having outer splines and an inner friction cone, the outer splines operable to selectively align with the internal splines of the coupling sleeve; wherein, the output of the first gear assembly has an outer conical face; and wherein, the inner friction cone of the synchronizing ring is shifted into friction contact with the outer conical face of the output of the first gear assembly responsive to shifting the coupling sleeve to synchronize a rotating speed of the output of the first gear assembly and a rotating speed of the input of the second gear assembly. an engine (26a, 26b) having an engine rotating speed in the non rotary flight mode; a freewheeling unit (42a, 42b) coupled to the engine (26a, 26b); a gear system having a torque path coupling assembly between first and second gear assemblies (44, 48), the first gear assembly coupled to the freewheeling unit (42a, 42b) and having an output, the second gear assembly having an input; and a proprotor assembly (20a, 20b) coupled to the second gear assembly, the proprotor assembly (20a, 20b) having a plurality of proprotor blades with a radially extended orientation and a proprotor rotating speed in the non rotary flight mode; wherein, the torque path coupling assembly has an engaged position wherein the output of the first gear assembly is coupled to the input of the second gear assembly thereby providing a torque path between the engine (26a, 26b) and the proprotor assembly (20a, 20b) and a disengaged position wherein the output of the first gear assembly is independent of the input of the second gear assembly thereby interrupting the torque path between the engine (20a, 20b) and the proprotor assembly (20a, 20b) characterised in that the torque path coupling assembly includes a coupling sleeve and a synchronizing ring, the coupling sleeve having internal splines that are in mesh with the input of the second gear assembly and in mesh with the output of the first gear assembly when the torque path coupling assembly is in the engaged position, the synchronizing ring having outer splines and an inner friction cone, the outer splines operable to selectively align with the internal splines of the coupling sleeve; wherein, the output of the first gear assembly has an outer conical face; and wherein, the inner friction cone of the synchronizing ring is shifted into friction contact with the outer conical face of the output of the first gear assembly responsive to shifting the coupling sleeve to synchronize a rotating speed of the output of the first gear assembly and a rotating speed of the input of the second gear assembly.