Không khí này tiếp tục xoắn ốc và giảm dần để tạo thành xoáy đầu dấu. Xoáy này tiếp tục quay, và tốc độ giảm ngày càng tăng với khoảng cách từ nguồn gốc. Trong một di chuột, các xoáy của một cuộc cách mạng ảnh hưởng đến các xoáy của cuộc cách mạng sau đây, gây ra một đường dẫn không đồng đều của các xoáy, cuối cùng | HELICOPTER AERODYNAMICS WORKBOOK CHAPTER 4 Figure 4-8 graphically describes the range variations with RPM changes. Since the amount of blade surface producing positive autorotative driving force varies according to RPM and this driving force is synonymous with thrust produced it is obvious the pilot has additional control over rate of descent by changing pitch through collective application. Excessively high Nr produces less driving force and a higher rate of descent and very low Nr leads to low driving force in proportion to high drag associated with a stalled profile. There is an optimum RPM range 94-95 for the TH-57 which produces the greatest net driving force and minimum descent rate. It is in the best interest of the pilot to strive for this RPM range until reaching flare altitude. Figure 4-8 HEIGHT-VELOCITY DIAGRAM No matter how well the pilot can execute an autorotation there remain some combinations of initial altitudes and airspeeds from which a safe autorotational landing will be extremely difficult to perform. The diagram illustrating this is the Height-Velocity H-V Diagram also known as the Deadman s Curve. The purpose of an H-V diagram is to identify the portions of the flight envelope from which a safe landing can be made in the event of a sudden engine failure. The H-V diagram figure 4-9 generally depicts two areas to be avoided The low-airspeed high-altitude region and the high-airspeed low-altitude region. AUTOROTATION 4-9 CHAPTER 4 HELICOPTER AERODYNAMICS WORKBOOK Figure 4-9 There are H-V diagrams for each type of helicopter. They are found in their respective NATOPS Manuals. Helicopter pilots should be familiar with these diagrams. Taking a closer look at the H-V diagram we see several definite points define the curve the first being the low hover height. Up to this height a pilot can handle a power failure by coming straight down using collective increase to cushion the landing. Above that altitude in combination with low speed the rotor blades
