tailieunhanh - INTRODUCTION TO HELICOPTER AERODYNAMICS WORKBOOK phần 3

Xác định độ cao mật độ. Nhớ lại những tác động của nhiệt độ và độ ẩm độ cao mật độ. Tính độ cao mật độ sử dụng một biểu đồ độ cao mật độ. Tính độ cao mật độ sử dụng các quy định của ngón tay cái công thức. Nhớ lại những mối quan hệ giữa hoạt động của máy bay trực thăng và độ cao mật độ. | HELICOPTER AERODYNAMICS WORKBOOK CHAPTER 2 Figure 2-5 The Momentum Theory adequately provides an explanation for no-wind hovering flight but it does not cover all of the bases. INDUCED DRAG Figure 2-6 The Blade Element Theory picks up where the Momentum Theory leaves off. The conditions at the blade element are diagramed in figure 2-6. The blade sees a combination of rotational flow and downward induced flow figure 2-7 called relative wind a downward pointing velocity vector. The AOA is the angle formed between the relative wind and the chord line and the pitch angle is formed between the TPP and the chord line. Lift which is the total aerodynamic force perpendicular to the local vector velocity or relative wind is tilted aft. This rearward component generated by lift is induced drag formed from the acceleration of a mass of air downwash and the energy spent in the creation of trailing vortices. The remaining arrow labeled profile drag is the result of air friction acting on the blade element. Profile drag is made up of viscous drag skin friction and wake drag which is the drag produced from the low velocity low static pressure air formed in the wake of each blade. ROTOR BLADE AERODYNAMICS 2-5 CHAPTER 2 HELICOPTER AERODYNAMICS WORKBOOK Figure 2-7 AIRFOILS Airfoils fall into two categories symmetrical and nonsymmetrical. A symmetrical airfoil has identical size and shape on both sides of the chord line while a nonsymmetrical airfoil has a different shape and size on opposite sides of the chord line. Cambered airfoils are in the nonsymmetrical category figure 2-2 . PITCHING MOMENTS Now let us investigate the different aerodynamic characteristics of these airfoils regarding the aerodynamic center and center of pressure of each type. The aerodynamic center is the point along the chord where all changes in lift effectively take place and where the sum of the moments is constant. The sum of the moments is constant for any AOA. On a symmetrical blade the moment is zero. .