A propulsion system, comprising: a first propulsion unit configured for being fixedly mounted to an airframe; a second propulsion unit; a rotor having a hub and a rotational axis, the rotor being configured for being pivotably mounted with respect to the first propulsion unit to allow selectively pivoting of the rotor about a pivot axis from a horizontal mode to a vertical mode; the rotor having a first longitudinal side and an opposed-facing second longitudinal side along said rotational axis; a first coupling for selectively coupling and decoupling the rotor with respect to the first propulsion unit; a second coupling for selectively coupling and decoupling the rotor with respect to the second propulsion unit, independently of the first coupling; wherein said first propulsion unit is reversibly coupled to the first longitudinal side of the rotor, and wherein said second propulsion unit is reversibly coupled to the second longitudinal side of the rotor, such that, in said horizontal mode, the first propulsion unit is reversibly coupled to the first side of the rotor facing forward in use, while the second propulsion unit is reversibly coupled to the second side of the rotor facing aft in use; wherein the first propulsion unit is reversibly coupled to the rotor via a first driveshaft; wherein said first driveshaft comprises a first portion (221, 221') driven by the first propulsion unit and a second portion (222, 222') for driving the rotor when the first driveshaft is coupled to the rotor via the first coupling; wherein said second portion (222, 222') is operatively connected to said hub; characterized in that in said vertical mode said first portion (221, 221') is at a first angular disposition with respect to said second portion (222, 222'), wherein in said horizontal mode said first portion is at a second angular disposition with respect to said second portion; and further characterized in that said first portion (221, 221') and said second portion (222, 222') pivot with respect to one another about said pivot axis between said first angular disposition and said second angular disposition, wherein the first angular disposition is less than the second angular disposition. a first propulsion unit configured for being fixedly mounted to an airframe; a second propulsion unit; a rotor having a hub and a rotational axis, the rotor being configured for being pivotably mounted with respect to the first propulsion unit to allow selectively pivoting of the rotor about a pivot axis from a horizontal mode to a vertical mode; the rotor having a first longitudinal side and an opposed-facing second longitudinal side along said rotational axis; a first coupling for selectively coupling and decoupling the rotor with respect to the first propulsion unit; a second coupling for selectively coupling and decoupling the rotor with respect to the second propulsion unit, independently of the first coupling; wherein said first propulsion unit is reversibly coupled to the first longitudinal side of the rotor, and wherein said second propulsion unit is reversibly coupled to the second longitudinal side of the rotor, such that, in said horizontal mode, the first propulsion unit is reversibly coupled to the first side of the rotor facing forward in use, while the second propulsion unit is reversibly coupled to the second side of the rotor facing aft in use; wherein the first propulsion unit is reversibly coupled to the rotor via a first driveshaft; wherein said first driveshaft comprises a first portion (221, 221') driven by the first propulsion unit and a second portion (222, 222') for driving the rotor when the first driveshaft is coupled to the rotor via the first coupling; wherein said second portion (222, 222') is operatively connected to said hub; characterized in that in said vertical mode said first portion (221, 221') is at a first angular disposition with respect to said second portion (222, 222'), wherein in said horizontal mode said first portion is at a second angular disposition with respect to said second portion; and further characterized in that said first portion (221, 221') and said second portion (222, 222') pivot with respect to one another about said pivot axis between said first angular disposition and said second angular disposition, wherein the first angular disposition is less than the second angular disposition.