直升機(jī)教員手冊(cè) Helicopter Instructor’s Handbook
時(shí)間：2014-11-10 08:35來源：FAA 作者：直升機(jī)翻譯 點(diǎn)擊：次
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To view this page ensure that Adobe Flash Player version 9.0.124 or greater is installed. Induced Flow Explain to the student that, at flat pitch, air leaves the trailing edge of the rotor blade in the same direction it moved across the leading edge; thus, no lift or induced flow is being produced. Demonstrate how, as blade pitch angle is increased, the rotor system induces a downward flow of air through the rotor blades, creating a downward component of air that is added to the rotational relative wind. Point out that because the blades are moving horizontally: . Some of the air is displaced downward. . The blades travel along the same path and pass a given point in rapid succession. . Rotor blade action changes still air to a column of descending air. This downward flow of air is called induced flow (downwash). Emphasize that it is most pronounced at a hover under no-wind conditions. Transverse Flow Effect Advise the student that in forward flight, air passing through the rear portion of the rotor disk has a greater downwash angle than air passing through the forward portion. Explain that this difference in downwash angle is due to the fact that the greater the distance air flows over the rotor disk, the longer the disk has to work on it and the greater the deflection is on the aft portion. Ensure the student understands: . Downward flow at the rear of the rotor disk causes a reduced AOA, resulting in less lift. . ThefrontportionofthediskproducesanincreasedAOA and more lift because airflow is more horizontal. . These differences in lift between the fore and aft portions of the rotor disk are called transverse flow effect. . Transverse flow effect causes unequal drag in the fore and aft portions of the rotor disk and results in vibration easily recognizable by the pilot. . Transverse flow occurs between 10 and 20 knots. Stress to the student that transverse flow effect is most noticeable during takeoff and, to a lesser degree, during deceleration for landing. Demonstrate how gyroscopic precession causes the effects to be manifested 90° in the direction of rotation, resulting in a right rolling motion requiring left cyclic input to maintain a more level fuselage attitude and proper ground track. Dissymmetry of Lift Dissymmetry of lift is the difference in lift that exists between the advancing half of the rotor disk and the retreating half. Explain to the student how to determine the total relative wind velocity on the advancing and retreating blades. Discuss the relative wind velocity of blades at a hover and during translational flight. Hover At a hover, relative wind velocity is: . Approximately 400 knots at the tips. . Approximately 300 knots one-fourth of the way in from the tips. . Approximately 200 knots one-half of the way in from the tips. . Approximately 100 knots three-fourths of the way in from the tips. . 0 knots at the center of the hub. Translational Flight In translational flight, relative wind velocity: . Is a combination of blade speed and airspeed. . Of the advancing blade is blade speed plus airspeed. . Of the retreating blade is blade speed minus airspeed. Develop the relative wind velocity for the advancing and retreating blades in the 090° to 270° position. [Figure 3-19] Show area of reverse flow. Emphasize that equal lift is created by advancing and retreating blades. 1. Advancing blade—greater lift. 2. Retreating blade—less lift. Discuss roll and explain that American-designed helicopters (counterclockwise rotation) would roll to the left and pitch up if transverse flow and dissymmetry of lift were not overcome. Explain main rotor method of overcoming dissymmetry of lift (flapping). 1. Advancing blade produces more lift; when flapping up, AOA decreases due to an increase in induced flow—loses lift. 2. Retreating blades produce less lift; when flapping down, AOA increases due to a decrease in induced flow—gains lift. 3. When blade flapping has compensated for dissymmetry of lift, the rotor disk is tilted to the rear. 4. Cyclic feathering also compensates for dissymmetry of lift (changes AOA) in the following ways: a. Cyclic feathering changes the angle of incidence differently around the rotor system. b. Forward cyclic decreases angle of incidence on advancing blade, resulting in reduced AOA, and increases angle of incidence on retreating blade resulting in increased AOA. 5. Tail rotor compensates for dissymmetry of lift by both flapping and feathering at the same time, accomplished by rotor design and mounting. A delta hinge allows for flapping, which automatically introduces feathering of the tail rotor. Exercise caution during a low-altitude, high-speed takeoff as pitch attitude is very low. If an engine failure or partial power condition were experienced, the pilot would not be able to safely place the aircraft in an autorotative profile. A quick review of the height velocity diagram would be very useful here. Sideward, Rearward, and Turning Flight Explain to the student that to accomplish these different modes of flight, the rotor disk is tilted in the desired direction. The forces acting on the helicopter remain the same, only the resultant vectors are different. Sideward hovering flight requires more pedal control to maintain heading. Depending on the lateral speed of travel, some fuselage tilting can be expected. Rearward flight must be accomplished slowly and cautiously due to wind effects on the horizontal stabilizer and the lowering of the tail rotor making surface contact easier to occur. Autorotation To help students better understand autorotation, divide it into four distinct phases: entry, steady-state descent, deceleration, and touchdown. Guide the student through each phase, stressing how it is aerodynamically different from the others. |
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