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. When the helicopter is clear of the disturbed air, or downwash, confirm a forward speed indication and initiate a climb to regain the lost altitude. Retreating Blade Stall The student must understand the limits of high speed in the helicopter and the reasons for imposing them. While structural and airframe design limit helicopter airspeed, the most frequently referenced aerodynamic limitation to helicopter airspeed is retreating blade stall. The symptoms of retreating blade stall are main rotor vibrations, nose pitch up, and a rolling tendency, usually to the left in a helicopter with a counterclockwise main rotor blade rotation. High gross weight, maneuvering, and turbulence all tend to aggravate the retreating blade stall condition. Retreating blade stall normally cannot be demonstrated without exceeding the never-exceed speed (VNE) or maneuvering limits of the helicopter. However, the student must be able to explain that the cause of retreating blade stall is excessive forward speed for the existing circumstances. The manufacturer’s recommended VNE provides protection for normal situations. If the helicopter is heavily loaded and then flown into turbulence at or near VNE, or if it is maneuvered abruptly, retreating blade stall can occur. Ensure the student becomes familiar with the procedure (lower the collective, increase rpm, if possible, and reduce speed) for recovery from retreating blade. The instructor should advise the student that retreating blade stall is only one of many factors that produce the limiting VNE in coaxial and close tandem rotor systems, such as the Kaman K-Max, VNE may not be characterized as retreating blade stall due to the counterrotating blade system. This design may mask the huge stresses on the rotor system until the stresses overcome the structure. The instructor should ensure the student abides by all limitations. When corrective action is introduced at the first vibration indicating retreating blade stall, there should be very little effect. If the blade stall is allowed to progress to the point the helicopter pitches up and rolls, trying to stop the pitch and roll with cyclic inputs only aggravates the situation. Allowing a helicopter to pitch up reduces speed and alleviates the blade stall condition. Roll control is then effective. The best way to prevent students from encountering retreating blade stall is to instill in them the practice of flying at air speeds below VNE. The margin should be increased if turbulence is encountered. Emphasize to the student that retreating blade stall can be avoided by adhering to the VNE. The decrease in VNE speed with increasing density altitude must also be explained thoroughly. Ground Resonance Ground resonance is a hazard associated with an articulated rotor system. Have the student explain which types of rotor systems are susceptible to ground resonance, the factors that tend to cause it, and the means to avoid it or recover from it if it occurs. The student must understand that if the helicopter is allowed to touch down in a manner that creates a jolt or bounce to the airframe, ground resonance can occur. Improperly serviced landing gear struts in some helicopters can aggravate the tendency to oscillate and contribute to ground resonance. This contact with the ground sends vibrations through the aircraft to the rotor system, causing an imbalance and center of gravity shift in the rotor system. The imbalanced rotor system energy is transferred, in the form of a rhythmic back and forth vibration (normally a lateral, or side to side, motion), from the surface to the rotor system. Although the frequency of this vibration remains constant, the strength of the vibration is amplified until an uncontrollable oscillation develops. [Figure 12-4] Since the skids or landing gear wheels contacting the ground cause the rotor to become unbalanced, the obvious means of fixing the situation is to lift the helicopter free of the ground and allow the blades to assume a balanced condition. If rpm is too low for flight, the next best corrective action is to lower the collective to place the blades in low pitch and reduce powerplant to idle or cutoff. The vibration increases in severity only if there is power applied to the system. Simply lowering the collective is not sufficient to stop the destruction; power must be removed. Dynamic Rollover Dynamic rollover is another potentially hazardous situation peculiar to helicopter operation. The student should be able to explain that, for dynamic rollover to occur, some factor must first cause a helicopter to roll and pivot around the skid or landing gear wheel, until its critical rollover angle is reached. Beyond this point, main rotor thrust contributes to the roll and recovery becomes impossible, regardless of any cyclic corrections made. Tell the student that dynamic rollover can occur on a flat surface and most often does due to obstructions such as tiedowns, hot asphalt, and frozen or deep mud. Once the helicopter reaches an angle such that the cyclic cannot keep the rotor system level with the horizon, it is subject to uncontrollable rolling. It is this horizontal thrust component that makes this effect dynamic. Helicopters tend to be top heavy and easily disturbed in the vertical mode. All of the heavy components, such as engines, transmissions, drive shafts and bearing mounts, tail rotor gearbox, rotor head, swashplates, and blades are mounted high in the airframe in most helicopters. This contributes to a very delicate balance above what can be relatively narrow landing gear. The only possible recovery action is to lower the collective. If the static balance point has not been passed and the roll rate is low, the helicopter might land back roughly on its landing gear. An easy but dramatic demonstration of dynamic rollover can be illustrated by having the student stand straight with his or her feet shoulder width apart and then gradually lean to the side until his or her body extends past his or her center of gravity. At this point, the student will lose balance and will be unable to straighten up. This exercise demonstrates to the student that once the center of gravity of the human body extends too far in any direction, the person topples. The same principle holds true for the helicopter. |
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