The present paper contains a concise literature review on helicopter flow analyses and presents some truly unsteady flows past helicopter rotors obtained using low and high order CFD models. The low order model is based on a panel method, coupled with a viscous boundary layer approach and compressibility correction, for which the USAERO code is used for the computations. The high order model is based on the Euler and Navier-Stokes equations. For the high order models, a truly unsteady scheme, using the CFD-FASTRAN code with the Euler equations, is considered for hovering rotor flows. On the other hand, a quasi-steady approach, using the WIND code with the Navier-Stokes equations and the SST turbulence model, is also considered to assess the validity of the approach in forwarding helicopter flows simulation. In the high order models, a Chimera grid technique is used to describe the blade motions within the parent stationary grid. Comparisons with experimental data are performed and the truly unsteady simulations provide a reasonable agreement with the available experimental data. The panel method and the quasi-steady approach are found to overestimate the loads on the helicopter rotors. The USAERO panel code is found to produce more thrust owing to some error sources in the computations when a wake-surface collision occurs, as the blades interact with their own wakes. The automatic cutting of wake sheets as they approach the model surface is not working properly at each time step.