旋翼机飞行手册 ROTORCRAFT FLYING HANDBOOK

时间：2011-04-05 11:32来源：蓝天飞行翻译 作者：航空 点击：次

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Figure 11-9. In a low G condition, improper corrective action could lead to the main rotor hub contacting the rotor mast. The contact with the mast becomes more violent with each successive flapping motion. This, in turn, creates a greater flapping displacement. The result could be a severely damaged rotor mast, or the main rotor system could separate from the helicopter.
Since a low G condition could have disastrous results, the best way to prevent it from happening is to avoid the conditions where it might occur. This means avoiding turbulence as much as possible. If you do encounter turbulence, slow your forward airspeed and make small control inputs. If turbulence becomes excessive, consider making a precautionary landing. To help prevent turbulence induced inputs, make sure your cyclic arm is properly supported. One way to accomplish this is to brace your arm against your leg. Even if you are not in turbulent conditions, you should avoid abrupt movement of the cyclic and collective.
If you do find yourself in a low G condition, which can be recognized by a feeling of weightlessness and an uncontrolled roll to the right, you should immediately and smoothly apply aft cyclic. Do not attempt to correct the rolling action with lateral cyclic. By applying aft cyclic, you will load the rotor system, which in turn produces thrust. Once thrust is restored, left cyclic control becomes effective, and you can roll the helicopter to a level attitude.
LOW ROTOR RPM AND BLADE STALL
As mentioned earlier, low rotor r.p.m. during an autorotation might result in a less than successful maneuver. However, if you let rotor r.p.m. decay to the point where all the rotor blades stall, the result is usually fatal, especially when it occurs at altitude. The danger of low rotor r.p.m. and blade stall is greatest in small helicopters with low blade inertia. It can occur in a number of ways, such as simply rolling the throttle the wrong way, pulling more collective pitch than power available, or when operating at a high density altitude.
When the rotor r.p.m. drops, the blades try 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 r.p.m. When power is no longer available to maintain r.p.m., 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 will stall unless r.p.m. is restored. If all blades stall, it is almost impossible to get smooth air flowing across the blades.
Even though there is a safety factor built into most helicopters, anytime your rotor r.p.m. falls below the green arc, and you have power, simultaneously add throttle and lower the collective. If you are in forward flight, gently applying aft cyclic loads up the rotor system and helps increase rotor r.p.m. If you are without power, immediately lower the collective and apply aft cyclic.
RECOVERY FROM LOW ROTOR RPM

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