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. Instructional Points A hovering turn is initiated in either direction by applying antitorque pedal pressure toward the desired direction. Explain that during a turn to the left more power is needed because application of left pedal increases the pitch angle of the tail rotor which, in turn, requires additional power from the engine. A turn to the right requires less power. (On helicopters with a clockwise rotating main rotor, right pedal increases the pitch angle and, therefore, requires more power.) As the turn begins, use the cyclic as necessary (usually into the wind) to keep the helicopter over the desired spot. [Figure 10-7] To continue the turn, additional pedal pressure is required as the helicopter turns to the crosswind position. This is because the wind is striking the tail surface and the tail rotor area, making it more difficult for the tail to turn into the wind. As pedal pressures increase due to crosswind forces, additional cyclic pressure into the wind is required to maintain position. Use the collective with the throttle to maintain a constant altitude and rpm. After the 90° portion of turn, pedal pressure is decreased slightly to maintain the same rate of turn. Approaching the 180° (downwind) portion, opposite pedal pressure must be anticipated due to tail movement from an upwind position to a downwind position. At this point, the rate of turn has a tendency to increase at a rapid rate due to the weathervaning tendencies of the tail surfaces. Because of the tailwind condition, hold rearward cyclic pressure to keep the helicopter over the same spot. Note: The horizontal stabilizer can increase the difficulty of hovering with a strong tailwind. Depending on the specific design and mounting of the stabilizer, a tailwind may tend to lift the tail, requiring more aft cyclic, or lower the tail boom, decreasing the amount of cyclic needed into the wind to hold your position. The student needs to understand the stabilizer’s response to tailwind conditions so as to anticipate control movements required. Because of the helicopter’s tendency to weathervane, maintaining the same rate of turn from the 180° position actually requires some pedal pressure opposite the direction of turn. If opposite pedal pressure is not applied, the helicopter tends to turn at a faster rate. The amount of pedal pressure and cyclic deflection throughout the turn depends on the wind velocity. As the turn finishes on the upwind heading, apply opposite pedal pressure to stop the turn. Gradually apply forward cyclic pressure to keep the helicopter from drifting. Control pressures and direction of application change continuously throughout the turn. The most dramatic change is the pedal pressure (and corresponding power requirement) necessary to control the rate of turn as the helicopter moves through the downwind portion of the maneuver. The instructor can have the student make turns in either directions; however, in a high wind condition the tail rotor may not be able to produce enough thrust, which means the student will not be able to control a turn to the right in a counterclockwise rotor system. Therefore, if control is ever questionable, have the student first attempt to make a 90 degree turn to the left. If there is sufficient tail rotor thrust to turn the helicopter crosswind in a left turn, a right turn can be successfully controlled. The opposite applies to helicopters with clockwise rotor systems. Hovering turns should be avoided in winds strong enough to preclude sufficient aft cyclic control to maintain the helicopter on the selected surface reference point when headed downwind. Common Student Difficulties In addition to the difficulties already discussed in the Takeoff to a Hover section, there are some difficulties associated specifically with the hovering turn. Improper Rate of Turn Until the student has gained some experience in hovering turns, the amount of pedal required for the desired rate of turn is not known. The result is a turn that is either too slow or too fast, often varying rapidly between the two. The first hovering turns should be practiced in calm or light winds, so a certain pedal input results in a specific rate of turn. Compensating for Crosswind Students usually fail to anticipate the effect of the wind as the helicopter turns. The student must understand that, throughout the turn, the cyclic is displaced into the wind, and is independent of the direction of the turn. Also, pedal input must be increased as the turn approaches the crosswind position, then decreased as the downwind position is approached. Passing the downwind position, the student should anticipate an increase in the rate of turn as a result of the wind force. Coordination Before attempting hovering turns, the instructor should explain and demonstrate the effects of pedal input. For example, explain how a left pedal input causes a right drifting tendency, which must be compensated for by using left side cyclic. Even more noticeable is the effect on engine rpm. Left pedal input causes a decrease in rpm and right pedal input causes an increase. As the student gains an understanding of these effects, the tendency to overcontrol the antitorque pedals should diminish. Hovering Forward Forward hovering should be accomplished at hovering altitude and at a speed no faster than a brisk walk with heading remaining constant. The forward track should be defined by markings on the ground or by the alignment of two reference points. [Figure 10-8] Instructional Points Instruct students on the importance of maintaining a landing gear height high enough to allow adequate ground clearance before hovering in any direction. Stress to the student that the risk of dynamic rollover is greatest during any hovering maneuver. This also stresses the importance of keeping the landing gear aligned with the direction of travel. Apply forward cyclic to start the forward motion, then release some cyclic pressure to prevent the helicopter from accelerating. Hold enough forward cyclic pressure to keep forward motion no faster than a brisk walk. Any speed higher than this requires a higher landing gear height to allow adequate ground clearance for the tail landing gear when bringing a helicopter to a stop using rearward cyclic. As the helicopter begins to move forward and lift is diverted, add a little power to compensate for the loss of lift. |
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