Dynamic stall of helicopter rotor blades occurs on the retreating blade in forward flight, where the angle of attack and freestream speed change quickly causing increased aerodynamic loads, causing blade vibration that limits helicopter forward speed. Difficulties in modelling variable freestreams lead current dynamic stall models to neglect this parameter. This thesis examined the influence of an unsteady freestream on the mechanism of dynamic stall, using a 2D URANS CFD simulation. The inlet velocity boundary condition was changed as a function of time. The angle of attack was 15 ± 10
degrees and the Mach number was 0.18 – 0.78. The variable freestream caused shock-induced flow separation at low angles of attack, delaying stall and enhancing the influence of the trailing-edge vortex. An increase in aerodynamic loads was observed. The results suggest that the variable nature of the freestream is an important factor in modelling dynamic stall.