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Aerodynamics of helicopter - part 8

Cuốn sách đã tìm thấy độc giả rộng rãi giữa các kỹ sư và những người khác, chẳng hạn như các phi công và trung cấp quản lý trong ngành công nghiệp và chính phủ, mà công việc đòi hỏi họ phải có nhiều hơn một sự hiểu biết chung về chất lượng, tại sao và từ đâu bay trực thăng. | 254 AERODYNAMICS OF THE HELICOPTER Note that the rate of change of angle of attack with azimuth at the blade tip is relatively smooth and sinusoidal in nature while farther inboard say at the 0.4 radius the rate of change of angle of attack is very rapid on the retreating side. Comparing the three figures it is seen that the angle of attack at the retreating tip increases from something over 12 degrees in Fig. 10-1 to over 16 degrees in Fig. 10-3. The area of the rotor operating in the stalled condition depends of course on the blade-section stall angle. If in Fig. 10-3 the blade stall angle were 12 degrees then the entire area to the left of the 12-degree contour would be stalled. Comparison between Calculated Stall Regions and Measured Stall Area The extent to which theory can predict stall may be seen in Fig. 10-4 which compares the theoretically stalled areas assuming a stall angle of 12 degrees of Figs. 10-1 10-2 and 10-3 with stalled areas as measured in flight reference II 9 Appendix IIA . Stall was measured in flight by analyzing pictures taken from a hubmounted camera of the action of wool tufts attached to a rotor blade at different spanwise stations. An angle of attack of 12 degrees was chosen in the theoretical comparison because wind-tunnel measurements on a section of the blade indicated this as the stall angle. It will be noted that the flight condition and point of occurrence of stall are properly predicted Fig. 10-4a and that the rate of growth of the stalled region is calculated with reasonable accuracy. Figures 10-1 to 10-3 show that the gradient of angle of attack near the retreating tip is small so that discrepancies in boundaries noted do not reflect large differences in angle of attack. By the same reasoning the contours make it clear that small reductions in the stalling angle of a blade can result in large increases in stalled area. Pilot reactions to the flight conditions represented in Fig. 10-4 are quite interesting. The condition shown