直升机教员手册 Helicopter Instructor’s Handbook
时间：2014-11-10 08:35来源：FAA 作者：直升机翻译 点击：次
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To view this page ensure that Adobe Flash Player version 9.0.124 or greater is installed. The law of conservation of angular momentum states that the value of angular momentum of a rotating body will not change unless external torques are applied. Explain to the student that, in other words, a rotating body continues to rotate with the same rotational velocity until some external force is applied to change the speed of rotation. Angular momentum can be expressed by the formula: Mass Angular × Velocity × Radius Squared Discuss how changes in angular velocity, known as angular acceleration or deceleration, take place if the mass of a rotating body is moved closer to or further from the axis of rotation. The speed of the rotating mass increases or decreases in proportion to the square of the radius. These forces cause acceleration and deceleration. Tell the student that the coriolis effect may be stated in the following terms. A mass moving radically— . Outward on a rotating disk exerts a force on its surroundings in the direction opposite to rotation. . Inward on a rotating disk exerts a force on its surroundings in the direction of rotation. The major rotating elements in the system are the rotor blades. As the rotor begins to cone due to G-loading maneuvers, the diameter of the disk shrinks. Due to conservation of angular momentum, the blades continue to travel the same speed even though the blade tips have a shorter distance to travel due to reduced disk diameter. This action results in an increase in rotor rpm. Most pilots arrest this increase with an increase in collective pitch. Conversely, as G-loading subsides and the rotor disk flattens out from the loss of G-load induced coning, the blade tips now have a longer distance to travel at the same tip speed. This action results in a reduction of rotor rpm, and is corrected by reducing collective pitch. Ground Effect Define ground effect for the student as the increased efficiency of the rotor system caused by interference of the airflow when near the ground. Discuss how ground effect permits relative wind to be more horizontal, the lift vector to be more vertical, and induced drag to be reduced, all allowing the rotor system to be more efficient. Maximum ground effect is achieved when hovering over smooth hard surfaces. When hovering over such terrain as tall grass, trees, bushes, rough terrain, and water, ground effect is reduced. Figure shows reactions to forces applied to a spinning Explain the two reasons for this phenomenon: induced flow rotor disk by control input or wind gusts. and vortex generation. [Figure 3-16] Gyroscopic Precession Explain to the student that precession occurs in rotating bodies that manifest an applied force 90° after application in the direction of rotation. Point out that although precession is not a dominant force in helicopter aerodynamics, pilots and designers must consider it since turning rotor systems exhibit some of the characteristics of a spinning gyro. Figure illustrates effects of precession on a typical rotor disk when force is applied at a given point. A downward force applied to the disk at point A results in a downward movement of the disk at point B. Aircraft designers take gyroscopic precession into consideration and rig the cyclic pitch control system to create an input 90° ahead of the desired action. Vertical Flight A student must understand that for climbing flight to occur, lift must be greater than weight. This is true whether at a hover or in steady state flight. Refer back to the forces acting on an aircraft in flight when explaining this concept. Forward Flight When explaining forward flight to the student, refer to the section on forces acting on an aircraft in flight. Remind the student that flight is the result of all forces, and that lift and thrust must be equal to the result of weight and drag for steady state flight. Point out that acceleration in forward flight is the result of thrust being greater than drag. Translational Lift Translational lift is the additional lift obtained from increased efficiency of the rotor system with airspeed obtained either by horizontal flight or by hovering into a wind. Describe the airflow patterns during directional flight and explain the causes of transitional lift. The relative wind entering the rotor system becomes more horizontal and results in the following: 1. A more vertical lift component 2. Less induced drag 3. An increased AOA 4. Less turbulent air entering the rotor system The airspeed range at which effective translational lift occurs is approximately 16–24 knots. As rotor efficiency increases and additional lift is produced due to more beneficial AOA, the rotor disk flaps upward causing the nose to pitch up; additional forward cyclic pressure is necessary at this point. As the airspeed increases and more lift is produced in the aft portion of the rotor disk, the nose tends to lower, requiring some aft cyclic to maintain an accelerative attitude and safe climb angle. Provide the student with a graph depicting drag at different airspeeds. Using a graph like Figure and guided discussion, ensure the student understands: 1. Each knot of forward airspeed increases the efficiency of the helicopter rotor system up to a point where retreating blade stall aerodynamics negate any further rotor system gains. 2. At effective translational lift (ETL), the rotor system completely outruns the recirculation of old vortices and begins to operate in smooth, undisturbed air. 3. Induced drag and total drag are reduced and overall rotor efficiency increases. 4. Increased efficiency continues with increased airspeed until best climb speed is reached. [Figure 3-1, Point E] 5. Airspeeds greater than best rate of climb speed result in lower efficiency of the helicopter due to increased parasite drag. Translational Thrust Translational thrust occurs as the helicopter transitions to forward flight and the tail rotor begins to operate in smooth undisturbed air. As the takeoff proceeds, the pilot notices the nose yaw (to the left in a counterclockwise turning system). This is the result of the increased translational thrust. To regain trimmed flight, a little right pedal is normally required. At about this same aerodynamic point, the airflow begins to smooth over the vertical stabilizer which carries some of the antitorque load in forward flight. This allows for slightly more reduction in tail rotor thrust, requiring further reduction in left pedal application. If there is no governor, a throttle change may be required to reduce the rpm slightly since the power demand was reduced. Depending on the helicopters position and airspeed, the rotor resultant rpm increase can be controlled by a slight increase in collective to maintain the rpm setting. |
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