In recent years, various strategies for the concurrent operation of fixed and rotary wing aircraft have been proposed as means of increasing airport capacity. In response, the research community has focused attention on assessing the impact of encounters with the wakes of nearby large transport aircraft on the safety of helicopter operations. This paper reviews the current literature and proposes an objective and general way of assessing the severity of such encounters. The sensitivity of the predicted interaction severity to some traditional modelling assumptions is then examined by presenting calculations, using various techniques, of the response of an isolated helicopter rotor to an interaction with an isolated vortex. Traditionally two different approaches to modelling the rotor wake have been used: inflow-type models, where the flow through the rotor is represented as a velocity distribution defined only on the rotor disc, and flow-type models where the dynamics of the entire rotor wake is captured in the simulation. In most previous studies the interacting vortex has been modelled as a distribution of vorticity in space which is time invariant in the frame of reference attached to the vortex. This frozen-vortex assumption is essential for consistency with inflow-type formulations but neglects the mutually-induced distortion of the interacting vortex and the rotor wake. We show that calculated interaction severities can be impacted significantly by these assumptions when they are embedded within current simulation techniques. Rotor responses calculated using both inflow-type and flow-type models are compared, and, for flow-type models, predictions obtained using the frozen-vortex assumption are contrasted with calculations where this assumption has been relaxed.
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