Aerodynamics of helicopter - part 5

Các tính năng • Tác giả là một chuyên gia quốc tế công nhận về khí động học máy bay trực thăng • Chỉ có hiện đại văn bản đầy đủ bao gồm các cánh khí động học, khí động học không ổn định, gian hàng năng động, và cánh quạt đánh thức • tiêts sửa đổi | Cf a Fig. 6-11 Experimental vertical-autorotative-descent data and a comparison with theory. 136 AUTOROTATION IN VERTICAL DESCENT 137 curves shown in Fig. 6-11 in that the curves are relatively flat over the normal operating range. EFFECT OF TWIST. As noted in Fig. 6-11 the data include measurements for both twisted and untwisted plywood-covered blades of identical plan form. It should be noted that within the accuracy of the measurements the twisted blades low angles at the tip for good hovering and forward-flight performance gave the same rate of descent as untwisted blades. It appears therefore that helicopter twist is not detrimental to vertical-autorotative performance as has often been suspected from considerations of high inboard angles of attack. 7 PHYSICAL CONCEPTS OF BLADE MOTION AND ROTOR CONTROL Before developing quantitative expressions for the forces and moments acting on a lifting rotor moving edgewise through the air it is well to gain a physical understanding of the rotor in forward flight. As shown in Fig. 7-1 the advancing blades of the rotor encounter higher velocities than the retreating blades as the rotor moves forward. Considering first a rigid propeller it is seen that a sizable rolling moment would be present in forward flight as a result of the inequality in lift produced on the advancing and retreating blades. While it is possible to utilize two side-by-side rigid propellers rotating in opposite directions to cancel the rolling moments high alternating blade loads are involved. Two standard means are available to overcome the dissymmetry of lift in forward flight 1 The blades may be hinged at their roots so that no moments can be transmitted through the hub. Control is then achieved by tilting the hub axis until the resultant rotor vector points in the desired direction. 2 The blades may be rigidly attached to the shaft but cyclically feathered decreasing the pitch on the advancing high velocity side and increasing the pitch on the .

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