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. Point out to the student that when the rotor rpm drops, the blades tend to maintain the same amount of lift by increasing pitch. As the pitch increases, drag increases, which requires more power to keep the blades turning at the proper rpm. When power is no longer available to maintain rpm, and therefore lift, the helicopter begins to descend. This changes the relative wind and further increases the angle of attack. At some point, the blades stall unless rpm is restored. If all blades stall, it is almost impossible to get smoother air flowing across the blades. To make matters worse, the main rotor rpm to tail rotor rpm ratio is one to six. Therefore, a one percent reduction in main rotor rpm can also result in a six percent reduction in tail rotor rpm, which corresponds directly to a loss of antitorque thrust available and loss of yaw control. Stress that any time the rotor rpm falls below the rpm limits while power is still available, the student should simultaneously add throttle and lower the collective. If in forward flight, gently applying aft cyclic loads up the system and helps increase rotor rpm. If there is no power available, immediately lower the collective and apply aft cyclic. Recovery From Low Rotor RPM Before the student is allowed to solo, the techniques for recovery from low rotor rpm in both a hover and in flight must be practiced. This low rotor training should not exceed any limitations. The instructor should not place undue stress on the dynamic parts of the helicopter, as letting the rpm drop too low can result in excessive blade bending, internal engine stress (including localized overheating not monitored by a gauge or sensor), and blade stop pounding. While in a hover, rotor rpm is reduced until the throttle alone will not increase the rpm. The student should then take the controls and attempt to recover rpm by lowering the collective just enough to allow the helicopter to settle gently toward the ground while increasing the throttle. The objective is to regain rpm without allowing the helicopter to touch down. To prevent touchdown, the collective should be raised slightly to stop the rate of descent. Practicing this maneuver graphically demonstrates to the student that if rotor speed is lost and the helicopter begins to settle, collective pitch alone should not be increased in an attempt to maintain altitude. While in flight, rpm may be regained by lowering the collective slightly and increasing the rpm. Aft cyclic while lowering the collective may also help increase rotor rpm, but is usually not required unless the rpm is critically low. Under certain conditions of high weight, high temperature, or high density altitude, a situation might exist in which the rpm is low even though maximum throttle is being used. This is usually the result of the main rotor blades having an angle of attack that creates so much drag that engine power is not sufficient to maintain or attain normal operating rpm. In a low rpm situation, the lifting power of the main rotor blades can be greatly diminished. Therefore, as soon as a low rpm condition is detected, immediately apply additional throttle, if available, while lowering the collective. This reduces main rotor pitch and drag. Under training conditions, make sure the skids or landing gear wheels do not contact the ground. In an actual situation in which the engine does not have sufficient power to accelerate the rotor, smoothly lower the helicopter to the ground, if conditions permit. Once on the ground, the collective can be lowered a little more to regain rpm. Do not try to maintain a hover by raising the collective when the rpm is too low and the throttle is wide open. As the helicopter begins to settle, smoothly raise the collective to stop the descent. At hovering altitude, this procedure might need to be repeated several times to regain normal operating rpm. This technique is called “milking the collective.” When operating at altitude, the collective may need to be lowered only once to regain rotor speed. The amount collective can be lowered depends on altitude. Since the tail rotor is geared to the main rotor, low main rotor rpm may prevent the tail rotor from producing enough thrust to maintain directional control. If pedal control is lost and the altitude is low enough that a landing can be accomplished before the turning rate increases dangerously, slowly decrease collective pitch, maintain a level altitude with cyclic control, and land. As instructors, we are aware that in most modern helicopters low or inadequate rotor rpm is an indicator of probable overloading or engine performance problems. Show the student that increased awareness to higher density altitudes is critical. Have the student determine the density altitude at the time this maneuver is performed and note to the student that recoveries at higher altitudes may not be possible. Common Student Difficulties As the helicopter begins to settle, students may have a tendency to increase collective pitch to stop the descent. Remember, students often fail to correlate the requirement to reduce collective pitch and increase throttle because previous training emphasized reducing throttle when lowering collective. Brownout/Whiteout Brownout and whiteout are two more helicopter hazards that cannot be demonstrated, but should be discussed with the student. Stress that the helicopter’s capability of landing in many conditions make it susceptible to visual obscuration when flying over ground material that can be blown up into the rotors during hover flight at low altitude. Differentiate between the brownout caused by dust and sand and the whiteout experienced in snow conditions. If during normal training sessions, seasonal conditions warrant the need to encounter such conditions, use the time to discuss the proper actions, taking care not to expose the student to an unfamiliar environment. Most often, an approach to the ground will suffice to compensate for these conditions. It should be noted that brownout conditions occur to varying degrees for a given landing zone depending on the aircraft in use (single versus tandem rotor), its configuration (weight), tactics being employed (rapidity of approach and landing, accompanying aircraft, time on deck, etc.), and environmental conditions (humidity and/or rain, day versus night, and temperature and density altitude). In general, explain that helicopters tend to begin to experience brownout during an angled, no-hover approach to landing at approximately 1–2 rotor diameters above ground level (50–150 feet), with the most serious conditions being experienced at approximately 50 feet and below. As the aircraft slows, the thrust vector of the main rotor disk becomes more vertical (as the aircraft pitches its nose up to decelerate), and the thrust becomes greater as power is added to sustain a hover, or near-hover condition prior to landing. Also, the rotor thrust tends to circulate down, out, then back up and down again through the rotor disk just prior to touchdown. All of these conditions combine just prior to landing, the most critical time for the pilot to eliminate lateral drift. |
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