The time-varying dynamics of a tiltrotor during its transition from the helicopter mode to the fixed wing aircraft mode are investigated from a flight dynamics perspective. The time scales over which the nacelle angle variation occurs are considered to be of the same order as the flight dynamics of the tiltrotor, due to which the time-varying aspects of the dynamics become significant. A characterization of the influence of different time trajectories of the nacelle angle on the flight dynamics is performed. The concept of a time-varying pole is used and the flight dynamic properties of the time-varying modes are determined using this concept. This time-varying pole is used to determine time-varying short period and phugoid modes, which are generalization of the conventional short period and phugoid modes. An analytical tool that evaluates the sensitivity of these time-varying modes to different nacelle angle trajectories is demonstrated, along with the degree of controllability of the tiltrotors flight dynamics for each such trajectory. An H-infinity controller is designed to influence the time-varying dynamics and to stabilize the system when it is hit by a wind gust during transition from both- the hover regime to airplane regime and airplane regime to hover regime.