Các khái niệm vật lý của chuyển động Blade và kiểm soát Rotor Khí động lực học của chuyến bay chuyển tiếp Forward-Flight Hiệu suất Dự đoán và hiệu ứng của Stall Blade Rotor Giới thiệu về tính ổn định máy bay trực thăng Giới thiệu về các vấn đề rung máy bay trực thăng Phụ lục: Danh mục, Thư mục | 312 AERODYNAMICS OF THE HELICOPTER greater than would be expected if the forces were applied statically to the blade. This dynamic amplification is important in conventional rotor blades because the exciting forces at 2 rev. and 3 rev. are near enough to the natural bending frequency to cause large amplifications. The 2 rev. and 3 rev. flexings of the blade are important not only because of the input forces they may transmit to the rotor hub but also because of the high alternating blade stresses and accompanying possibilities of fatigue failure. Conventional blades are stiffer in the in-plane direction th n in the flapping direction and the natural bending frequencies are correspondingly higher. The first bending frequency is often about four times the rotational speed. Considerable amplification of 4 may therefore occur. Forces Transmitted to the Rotor Hub The preceding paragraphs have considered the rotor blades as the vibrating system. It is also of interest to regard the rotor as the source of alternating forces applied to the helicopter structure. Flapping or inplane motions or blade flexing in either plane cause periodic forces to be applied to the hub in the vertical and horizontal plane. Also variations in blade-pitching moment are transmitted through the control system. It is therefore important to determine the manner in which the periodic input forces from the rotor blades combine to produce a resultant force acting on the rotor hub. The fundamental rule governing the force inputs to the rotor hub is that for alternating forces which are identical on each rotor blade the only alternating forces and moments which the blades may transmit to the rotor hub are those which are integral multiples of the number of blades. Thus the hub of a three-bladed rotor receives only 3 rev. 6 rev. 9 rev. etc. force inputs. The following paragraphs while not a rigorous mathematical proof of the above theorem are intended to show physically why it is so. .
