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直升機(jī)飛行手冊 Helicopter Flying Handbook

時間：2014-11-09 12:30來源：FAA 作者：直升機(jī)翻譯 點(diǎn)擊：次

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When a helicopter is hovering, the tail rotor is operating in very disturbed airflow. As the helicopter achieves ETL, the tail rotor begins to generate much more thrust because of the less disturbed airflow. The helicopter reacts to the increased thrust by yawing. Therefore, as the helicopter achieves ETL, you must reduce tail rotor thrust by pedal input at about the same time that you need to make cyclic adjustments for lateral tracking, acceleration, and climb.
Induced Flow
As the rotor blades rotate, they generate what is called rotational relative wind. This airflow is characterized as flowing parallel and opposite the rotor’s plane of rotation and striking perpendicular to the rotor blade’s leading edge. This rotational relative wind is used to generate lift. As rotor blades produce lift, air is accelerated over the foil and projected downward. Anytime a helicopter is producing lift, it moves large masses of air vertically and down through the rotor system. This downwash or induced flow can significantly change the efficiency of the rotor system. Rotational relative wind combines with induced flow to form the resultant relative wind. As induced flow increases, resultant relative wind becomes less horizontal. Since AOA is determined by measuring the difference between the chord line and the resultant relative wind, as the resultant relative wind becomes less horizontal, AOA decreases. [Figure 2-40]
Transverse Flow Effect
As the helicopter accelerates in forward flight, induced flow drops to near zero at the forward disk area and increases at the aft disk area. These differences in lift between the fore and aft portions of the rotor disk are called transverse flow effect. [Figure 2-39] This increases the AOA at the front disk area causing the rotor blade to flap up, and reduces AOA at the aft disk area causing the rotor blade to flap down. Because the rotor acts like a gyro, maximum displacement occurs 90° in the direction of rotation. The result is a tendency for the helicopter to roll slightly to the right as it accelerates through approximately 20 knots or if the headwind is approximately 20 knots.
Transverse flow effect is recognized by increased vibrations of the helicopter at airspeeds just below ETL on takeoff and after passing through ETL during landing. To counteract transverse flow effect, a cyclic input to the left may be needed.
Sideward Flight
In sideward flight, the tip-path plane is tilted in the direction that flight is desired. This tilts the total lift-thrust vector sideward. In this case, the vertical or lift component is still straight up and weight straight down, but the horizontal or thrust component now acts sideward with drag acting to the opposite side. [Figure 2-41]
Sideward flight can be a very unstable condition due to the parasitic drag of the fuselage combined with the lack of horizontal stabilizer for that direction of flight. Increased altitudes help with control and the pilot must always scan in the direction of flight. Movement of the cyclic in the intended direction of flight causes the helicopter to move, controls the rate of speed, and ground track, but the collective and pedals are key to successful sideward flight. Just as in forward flight, the collective keeps the helicopter from contacting the ground and the pedals help maintain the correct heading; even in sideward flight, the tail of the helicopter should remain behind you. Inputs to the cyclic should be smooth and controlled, and the pilot should always be aware of the tip-path plane in relation to the ground. [Figure 2-42]
Contacting the ground with the skids during sideward flight will most likely result in a dynamic rollover event before the pilot has a chance to react. Extreme caution should be used
Resultant
Resultant
Lift
Thrust
Thrust
Drag
Drag
Helicopter.movement
Weight Resultant
Lift Weight
Downward.force.from.the.horizontal.stabilizer
Figure 2-41. Forces acting on the helicopter during sideward flight.

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