In this research, a novel numerical technique for modeling the unsteady aerodynamics of rotorcraft flows has been developed. The aim of this research is to formulate a momentum source based model for discrete rotor blades that modifies the time-averaged steady momentum source approach and captures the unsteady loading on discrete blades as well as the time-accurate flowfield representing the rotor wake. In this approach, the rotor blades are represented as a distribution of momentum sources in the flow, the strengths of which are determined from the flowfield properties, rotor geometry and the aerodynamic characteristics of the blade cross-section. The instantaneous position of each rotor blade is taken into account while determining the influence of the rotor on the flow. The discrete blade unsteady rotor model is verified by first implementing it in a well-validated structured solver, where the unsteady, incompressible, laminar Navier-Stokes equations are solved in a Cartesian coordinate system using a finite volume based primitive variable algorithm. Flows over isolated helicopter rotors in hover, both in- and out-of-ground effect, are simulated and the time-accurate rotor flowfield and performance characteristics are presented. A three-dimensional, unstructured Navier-Stokes flow solver is also developed as part of this research. In the unstructured flow solver, the physical domain is discretized into tetrahedrons, with median-dual control volumes constructed around the vertices and a collocated approach is implemented for the primitive variables, which are solved for sequentially using the SIMPLER solution algorithm. The unsteady rotor model is integrated with the unstructured flow solver and validated for model helicopter rotors in hover. This research, thus, develops a momentum source based rotor model for discrete rotor blades and shows the coupling of the rotor model with different types of flow solvers, resulting in a time-accurate solution methodology for rotorcraft flows.