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 the factors that influence dynamic rollover to include a skid or landing gear wheel in contact with the landing surface, pedal inputs, lateral loading (asymmetrical loading), crosswind condition, and a high roll rate. Explain that smooth and moderate control inputs are most effective in preventing dynamic rollover as it reduces the rate at which lift/thrust is introduced. Thorough preflights ensure the landing gear is fully free, prudent control movements slowly lift the helicopter off the surface or land without any lateral motion. The rising of one side of the helicopter for compensation for the translating effect should be expected and countered. More or less tilt than normal or usual is reason enough to stop the motion and check for control response. If in doubt, set the helicopter back down and double check for obstructions and skids stuck in hot asphalt, tie down chains or ropes over skids or gear points, etc. While landing the helicopter, the pilot should feel for any sliding or sinking motions, signaling an unsafe landing surface. If such a surface is encountered, descent must be stopped by increasing collective. If after coming to a level attitude, no further attempts should be made to touch down, the helicopter should be repositioned to a different area for another landing attempt. Low-G Conditions and Mast Bumping A student pilot must understand the potentially hazardous implications of intentionally or inadvertently performing low-G maneuvers. Too often, students study topics but fail to correlate their impact on the aircraft. Low-G maneuvers and the resulting consequences of mast bumping must be thoroughly addressed. Explain to the student that abruptly pushing the cyclic control forward from either straight-and-level flight or after a climb can put the helicopter into a low-G (weightless) flight condition, with catastrophic results. The student must be reminded that the semirigid rotor system suspends the airframe below the rotor. Therefore, in normal flight, the helicopter is using the pull of gravity for a normal airframe/rotor relationship or normal load on the rotor system. In a low-G maneuver, with the helicopter in an excessively nose-low or tilted attitude, gravity is no longer pulling the airframe down, and the feeling of weightlessness is experienced. With negligible gravitational pull between the rotor and airframe, any cyclic control input tends to fly the rotor disk over. The airframe does not respond, resulting in mast bumping. Ensure the student understands what is happening and that improper corrective action can lead to mast bumping or may allow a blade to contact the airframe. [Figure 12-5] Since a low-G condition could have disastrous results, it cannot be demonstrated, but the student must learn the correct response. The other part of the Low-G effect is the thrust of the tail rotor being above the center of gravity. This is often the initial cause of the fuselage to roll. Stress that abrupt movement of the cyclic and collective should be avoided, but if the student gets into a low-G condition, (be sure to describe the feeling of weightlessness and an uncontrolled roll to the right), he or she should immediately and smoothly apply aft cyclic. Warn the student against trying to correct the rolling action with lateral cyclic. Explain that by applying aft cyclic, lift is redirected to counteract weight and gravity restores the balance of forces. This in turn requires antitorque so the tail rotor forces then become balanced as well. Unless the collective is lowered, the thrust never changes. Only the direction of the rotor system thrust and lift is affected. Prevention is the only proper cure for the hazard. Since the best way to prevent low-G conditions is to avoid the conditions in which they might occur, suggest the student avoid turbulence as much as possible (since this type of weather induces low-G conditions). If the student encounters turbulence, advise him or her to slow forward airspeed and make small control inputs. If turbulence becomes excessive, explain that a precautionary landing should be considered. To help prevent turbulence induced inputs, demonstrate how to properly support the cyclic arm by bracing it against the leg. Low Rotor RPM Although rotor rpm and airspeed are repeatedly emphasized during training, it is important for the student to understand not only that there are limits to both, but also why those limits have been specifically defined. Ensure the student understands and can explain that the limits for powered operation are dictated by the operating limits of the engine. At low rpm, the engine cannot develop full power, and the high limit is imposed by engine structural limits. Also, point out that if the engine and main rotor rpm are allowed to get too low, tail rotor rpm is also greatly reduced. This situation could lead to an inability to keep the helicopter from turning. At the low limit on the rotor tachometer, the rotor may not produce enough lift to sustain level flight. The high limit of rotor rpm is imposed to protect the structural integrity of the rotor and drive components. Blade Stall Impress on the student that low rotor rpm can also lead to blade stall. If the rotor rpm decays to the point at which all the rotor blades stall, the result is usually fatal, especially when it occurs at altitude. In addition to the main rotor blades stalling, if centrifugal force is decreased too much, there is not sufficient force to keep the blades horizontal. Explain that, even at normal operating rpm, the blades cone upward when producing lift. If the required lift remains high coupled with high angles of attack, the blades bend upward further until there is no recovery. The danger of low rotor rpm and blade stall is greatest in small helicopters with low blade inertia. Explain to the student that a pilot can create the situation in a number of ways, such as simply rolling the throttle the wrong way, or pulling more collective pitch than power available, or when operating at high density altitude. |
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