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MODERN HELICOPTER AERODYNAMICS phần 4

Tham khảo tài liệu 'modern helicopter aerodynamics phần 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | HELICOPTER AERODYNAMICS 545 in time and the position of the vortex and hence the blade loads can therefore be calculated fairly accurately. For example Caradonna et al 1988 solve the three-dimensional full potential equations good agreement with experimental data was obtained for both parallel and oblique vortex interactions when the core size of the vortex is fixed in time. However when the structure of the vortex is calculated along with the balance of the solution of the rotor wake the results are not nearly as good. Moreover a number of blade-vortex interactions are expected to occur over a single blade cycle. Lorber 1991 has experimentally investigated BVI in the forward flightdescent regime for a four-bladed rotor. He was able to map regions where BVI is expected both retreating and advancing BVI events were found. He was also able to develop a model using a simple wake distortion model and the agreement with experiment is encouraging although some BVI events in the experiments were not predicted in the computations. See also Gorton et al 1995a for additional experimental results. A detailed study of multiple self-generated BVI encounters has been given by Hassan et al 1992 . They solve the Euler equations to produce results for a number of BVI encounters during a single rotor revolution. The solutions are found to be extremely sensitive to the value of the vortex core radius chosen. A typical result at two inboard stations is depicted in Figure 14. In these results BVI commences at around ý 60 . Note that agreement with the experimental values is good until about ý 90 note the differences between vortices of different core radius. These results are typical of results at other inboard stations as well and reflect the fact that the surface pressure is extremely sensitive to the position and structure of the vortex system relative to each blade. The strength of the tip-vortex as measured by its circulation is a strong function of the blade tip shape from which