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Aerodynamics of helicopter - part 6

• Bốn chương mới trên autogiros, tua bin gió, phương pháp máy tính và tương tác-khung máy bay cánh quạt. Cuốn sách này là một giới thiệu tuyệt vời để các khí động học của máy bay trực thăng. Nó cung cấp một tài khoản của các nguyên tắc đầu tiên trong cơ học chất lỏng và động lực chuyến bay của máy bay trực thăng một cánh quạt. | 178 AERODYNAMICS OF THE HELICOPTER LAG MOTION IN FORWARD FLIGHT. In the preceding discussion it has been pointed out that for hovering flight the in-plane motion depends entirely upon the flapping motion of the blades with respect to the shaft. If no flapping exists no in-plane motion exists. In forward flight however an additional exciting torque is applied to the system as a result of the periodic variations in blade drag. This excitation and the resulting blade motion depend on the condition of flight but are independent of the particular combination of shaft flapping and feathering that is used to locate the control axis. The flapping relative to the shaft may be positive or negative depending on the fuselage center-of-gravity position and the tail setting. Because the resultant in-plane motion is the sum of the motion caused by periodic air forces and the motion caused by Coriolus torques the resultant motion may vary in amplitude and phase by changes of the Coriolus motion through shaft-position changes. Indeed it is possible to avoid first harmonic in-plane motion in forward flight by locating the constant-speed shaft in the proper position that position for which the Coriolus torques exactly compensate the air force torques. It should be kept in mind that in all practical cases the periodic in-plane blade motion is quite small of the order of I to 2 degrees. Variations of the mean lag angle from one flight condition to another however may be much larger. From the full-power condition to the autorotation condition the lag angle may vary from 10 degrees to 1 degree. HIGHER HARMONIC IN-PLANE MOTION. In-plane motion like flapping motion contains higher harmonics as well as first harmonics. While these are usually small compared to the first harmonic motion and accordingly were ignored in the discussions of the nature of the in-plane motion they are important as sources of vibration. The mechanism by which higher harmonic motions produce rotor vibrations will be