Under transient conditions, a helicopter rotor generates a complex, time-dependent pattern of shed and
trailed vorticity in its wake that has profound eects on its loading. To examine these eects, the response
of a two-bladed hovering rotor to a ramp change in collective pitch is investigated using three dierent
computational approaches. Solutions obtained using a Compressible Reynolds Averaged Navier{Stokes ap-
proach are compared to results obtained from lifting-line theory coupled to an Eulerian Vorticity Transport
Model, and from a simple single-state dynamic in
ow model. The dierent numerical approaches yield
very similar predictions of the thrust response of the rotor to ramp changes in collective pitch, as long as
the ramp rates are small. This suggests that the basic underlying
ow physics is properly represented by all
the approaches. For more rapid ramp rates, an additional delay in the aerodynamic response of the rotor,
that is related to the nite extent of the wake during its early history, is predicted by the Navier{Stokes
and Vorticity Transport approaches. Even though the evolution of the wake of the rotor is strongly three
dimensional and highly unsteady, the predictions of the Navier{Stokes and lifting-line models agree very
closely as long as the blades of the rotor do not stall. In the pre-stall regime, a quasi two-dimensional
representation of the blade aerodynamics thus appears adequate for predicting the performance of such
systems even under highly transient conditions. When
ow separation occurs, the resulting three dimen-
sionality of the blade aerodynamics forces the predictions of the Navier{Stokes and lifting-line approaches
to diverge, however. The characterization of the wake interactions and stall propagation mechanisms that
are presented in this study oers some insight into the fundamental
uid dynamic mechanisms that govern
the transient aerodynamic response of a rotor to control inputs, and provides some quantication of the
limits of applicability of some popular current approaches to rotor aerodynamic analysis