tailieunhanh - MODERN HELICOPTER AERODYNAMICS phần 5

Đầu làm việc trên sự tương tác thức tỉnh thân máy bay được thành lập là một giải phóng mặt bằng nhỏ hơn giữa các cánh quạt và khung máy bay sẽ dẫn đến airloads gây ra cao hơn và độ rung trên khung máy bay do sự tương tác bề mặt xoáy mạnh (Wilson và Mineck năm 1975, Landgrebe et al 1977, Smith và Betzina 1986). | HELICOPTER AERODYNAMICS 555 Early work on the wake-fuselage interaction established that a smaller clearance between the rotor and the airframe will lead to higher induced airloads and vibration levels on the airframe due to the strong vortex-surface interaction Wilson and Mineck 1975 Landgrebe et al 1977 Smith and Betzina 1986 . Smith 1979 found that the presence of the fuselage distorts the wake which in turn generates enhanced blade-wake interactions the interaction can trigger a torsional aeroelastic response by the rotor that may lead to a premature retreating blade stall. The work described just above identified important phenomena and suggested the development of computational models of these interactions. In the mid-1980s there were no computer codes capable of predicting even the first-order features of the time-averaged surface pressure distribution over the upper surface of the simplest fuselage geometry in hover or low-speed forward flight on the model scale. Early attempts to include airframe effects on the wake include various levels of empiricism beginning with the use of a prescribed wake structure some of these attempts to couple the wake and the airframe are described by Crouse and Leishman 1992 . Typical of these efforts is the work described by Lorber and Egolf 1990 who showed that including an unsteady potential in the rotor wake formulation improves agreement between generalized wake predictions and an experimental data set. They use the prescribed wake structure and specify the trajectory of the tip-vortex as it approaches the airframe a fixed offset distance defines the limiting distance allowed between the tip-vortex and the airframe. The results show fair agreement with experiment the time averaged mean pressure on the top of the airframe is in general underpredicted. Similar discrepancies appear in the instantaneous pressure on the top of the airframe. Time-averaged results for the pressure have also been obtained by Quackenbush et al .