A new appreciation of inflow modelling for autorotative rotors.
PhD thesis, University of Glasgow.
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A dynamic inflow model is a powerful tool for predicting the induced velocity distribution over a rotor disc. On account of its closed form and simplicity, the model is highly practical especially for studying flight mechanics and designing control systems for helicopters. However, scant attention has been so far paid to applying this model to analyse autorotative rotors (i.e. rotors in the windmill-brake state), which differ from powered helicopter rotors (i.e. rotors in the normal working state) in that the geometric relation between the inflow and the rotor disc.
The principal aim of this research is to theoretically investigate the applicability of existing dynamic inflow models for autorotative rotors, and if necessary, to provide a new dynamic inflow model for autorotative rotors.
The contemporary dynamic inflow modelling is reviewed in detail from first principles in this thesis, and this identifies a modification to the mass-flow parameter for autorotative rotors. A qualitative assessment of this change indicates that it is likely to have a negligible impact on the trim state of rotorcraft in autorotation, but a significant effect on the dynamic inflow models in certain flight conditions.
In addition, this thesis includes a discussion about the small wake skew angle assumption, which is invariably used in the derivation of Peters and He model. The mathematical validity of the assumption is cast doubt, despite the resultant model has experimentally been fully validated. The author discusses on a theoretical ground the possible reason why the Peters and He model works well in spite of its inconsistent derivation
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