直升機教員手冊 Helicopter Instructor’s Handbook
時間：2014-11-10 08:35來源：FAA 作者：直升機翻譯 點擊：次
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To view this page ensure that Adobe Flash Player version 9.0.124 or greater is installed. 1. Larger angles of attack create more lift on an airfoil. 2. Smaller angles of attack result in a reduction of lift on the airfoil. 3. Exceeding the maximum (critical) angle can produce a stall. Maximum angle of attack is 15° to 20° on most airfoils. Hovering Flight It is essential for the student to understand the aerodynamics of hovering. Explain that for a helicopter to hover, lift produced by the rotor system must equal the total weight of the helicopter. An increase of blade pitch through application of collective increases the angle of incidence and generates the additional lift necessary to hover. As forces of lift and weight are in balance during stationary hover, those forces must be altered through application of collective either to climb or to descend. Describe to the student how, at a hover, the rotor-tip vortex reduces effectiveness of the outer blade portions. [Figure 3-13] Vortices of the preceding blade affect the lift of the other blades in the rotor system. When maintaining a stationary hover, this continuous creation of vortices combined with the ingestion of existing vortices is the primary cause of high power requirements for hovering. Rotor-tip vortices are part of the induced flow and increase induced drag. Ensure that the student understands that, during hover, rotor blades move large amounts of air through the rotor system in a downward direction. This movement of air also introduces induced flow into relative wind, which alters the AOA of the airfoil. If there is no induced flow, relative wind is opposite and parallel to the flightpath of the airfoil. With a downward airflow altering the relative wind, the AOA is decreased so that less aerodynamic force is produced. This change requires an increase in collective pitch to produce enough aerodynamic force to hover. Translating Tendency or Drift Explain to the student that the thrusting characteristics of a tail rotor during hovering flight create a tendency for the helicopter to drift laterally, which is called translating tendency. A single-rotor helicopter with a counterclockwise rotating main rotor tends to drift laterally to the right. Stress the cause: thrust exerted by the tail rotor compensates for main rotor torque. Translating tendency is to the left in a helicopter with a clockwise rotation of the main rotor. Explain to the student that the helicopter fuselage will remain relatively level to slightly left side low. The amount of fuselage tilt varies between types and design of helicopters. The tip path plane of the main rotor will not be level and will have to be adjusted accordingly with cyclic to counteract translating tendency and adverse wind conditions. The ability to tilt or adjust the wings of the helicopter allows the helicopter to maintain its position over the ground. Describe the methods used to correct for translating tendency: 1. Flight control rigging may be designed by the manufacturer so the rotor disk is tilted slighted when the cyclic control is centered to compensate for drift. 2. Transmission may be mounted so the mast is tilted slightly when the helicopter fuselage is laterally level. 3. Pilot applies cyclic in the opposite direction to arrest the drift. Pendular Action Pendular action is the result of the CG being below the supporting structure (rotor system). Tilting the rotor in one direction results in the fuselage swinging in the opposite direction. Stress to the student that this swinging is normal for helicopter operation since the helicopter fuselage is below the rotor system and overcontrolling can result in exaggerated pendular action and should be avoided. The cyclic should always be moved at a rate that allows the main rotor and fuselage to move as a unit. Emphasize that the student should use slow, smooth, cyclic inputs while hovering. The student must understand that it is the relationship of the tip path plane to the horizon, and not the position of the fuselage, that determines the helicopter’s direction of travel. Coning Coning is the upward flexing of the rotor blades. Point out to the student that coning is a normal phenomenon in all rotors producing lift. The amount a blade cones is a resultant between lift and centrifugal force. When lift is stronger than centrifugal force, the blade cones upward. When centrifugal force is stronger than lift, the blade moves downward, reducing the coning angle. [Figure 3-14] Explain the relationship between lift and excessive coning and describe the causes of excessive coning to the student: . Low revolutions per minute (rpm)—less centrifugal force . High gross weight—more lift needed . High G maneuvers—more lift needed . Turbulent air—point out to the student that any maneuvers requiring additional lift could lead to excessive coning. Give examples of excessive coning. Ensure the student understands: . Flight conditions that require large amounts of lift may lead to an excessive coning condition in the rotor. . As lift forces increase in the rotor, they overcome the rigidity produced by centrifugal force. The rotor blades begin flexing upward, which could lead to an excessive coning angle. Guide the student in identifying the adverse effects of excessive coning in the rotor system. [Figure 3-15] 1. Loss of disk area. 2. Loss of total lift available. 3. Stress on blades. 4. Excessive stress forces in the rotor could lead to blade cracking or blade separation from the rotor system. 5. Excessive coning combined with low rotor rpm may cause the blades to droop much lower than normal. This condition is likely to occur at the end of an autorotation and may allow the rotor blades to damage or remove the tail boom. 6. Excessive coning may become unrecoverable in flight. Coriolis Effect (Law of Conservation of Angular Momentum) |
