Investigation of the dynamic wake of a model rotor.
In this study the dynamic induced velocity field of a model helicopter rotor - excited in
collective and cyclic pitch at frequencies extending to 1.5 times the nominal shaft speed - is
investigated using mainly hot-wire and laser Doppler anemometry.
The dynamic induced velocities are found to vary significantly with radial station and
frequency. For cyclic excitations, azimuthal variations are also observed. The results point to
the dynamic induced flow being influenced by the distribution of shed vorticity in the wake and
cannot be explained using simple momentum theory. Vertical variations of the measured
inflow response are also observed, with phase changes possibly partly due to transmission type
delays. At frequencies above shaft speed a change in character of the induced flow is seen and
around shaft speed an increase in the general level of turbulence is found. The available data
on the dynamic induced velocity field of a rotor under controlled excitation, are substantially
The measured induced flow response was compared to that predicted using the Pitt and Peters
dynamic inflow model. In the 'collective' case good agreement was found, suggesting that the
primary inflow model parameters such as the inflow gain and apparent mass are correct with
some evidence that a higher order inflow representation might be desirable.
A novel method is used to infer the aerodynamic hub loading, which could not be directly
measured, from the blade flapping data. This is used to isolate the inflow response using the
Pitt and Peters dynamic inflow model and the results are compared with experimental
measurements. The method shows the Pitt and Peters dynamic inflow representation to be
adequate in the 'collective' case. In the 'cyclic' case, the inferred hub loads were very sensitive
to the blade model and hence conclusions for this case are limited.
A literature survey and review of the Pitt and Peters dynamic inflow model are also given