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. Other Types of Antitorque System Explain to the student that there are several different types of anti-torque systems. One is the fenestron, or “fan-in-tail,” design. A fenestron is a fully enclosed tail rotor. It is essentially a ducted fan. The housing is integral with the tail skin, and, like the conventional tail rotor it replaces, is intended to counteract the torque of the main rotor. Fenestrons have between eight and eighteen blades. These may have variable angular spacing so that the noise is distributed over different frequencies and, thus, is quieter. The housing allows a higher rotational speed than a conventional rotor, allowing it to have smaller blades. [Figure 5-21] The smaller diameter allows use of higher fan speeds and sometimes requires higher fan rpm ranges to equal thrust from a much larger unducted system. The housing, although somewhat heavier, does offer some protection on the ground and is more streamlined in forward flight. Discuss with the student that propellers and rotors alike are designed to be less than transonic at the tips. The other type of tail rotor is the NOTAR. system (no tail rotor). The NOTAR system represents the first significant configuration change to conventional helicopters since 1939 when Igor Sikorsky flew the first conventional rotorcraft. "The new system uses the Coanda effect of air flowing over or around the surface of the tail boom to create lateral lift. This counteracts the torque of the main rotor. The NOTAR system shortens drive shafts, gearboxes, and the rotor unit itself. This reduction in the parts count is a distinct advantage over conventional tail rotor craft. [Figure 5-22] In operation, the NOTAR system draws low-pressure air in through an air intake located at the top of the airframe to the rear of the main rotor shaft. A variable-pitch fan pressurized the tail boom to a relatively constant 0,5 psi. The air is fed to two starboard side slots and a direct jet thruster. The slots provide the necessary antitorque force. The rotating jet thruster provides direction control. The two slots are located at 70 and 140 degrees and allow ejected air to mix with the main rotor downwash to establish the Coanda effect. The main rotor downwash is normally dissipated as essentially symmetric separation on both sides of the tail boom in a hover. The pressurized boom inject low-pressure air at 250 fps onto the Coanda surface (outer surface of the tail boom), which results in the deflection and produces about two-thirds of the required antitorque force. This force is predictable. It is controlled by the appropriate location of the slot and control of the air jet that exits from the slot. In other words, the tail boom reacts like an airplane wing, only sideways. The increased air speed over the starboard side of the tail boom causes lateral lift, pushing against the torque forces trying to spin the helicopter clockwise. This is the same result that a tail rotor achieves when it propels the tail in a counterclockwise motion. The main rotor downwash skews as velocity is increased, and the circulation control slot is uncovered, resulting in proportional loss of antitorque force. The vertical tail surface provides the directional stability with forward speed. In sideward flight, the effective angle of attack is changed as a function of the main rotor thrust and sideward velocity inflow effects. When the downwash is altered by motion other than hovering, the system reduces the Coanda effect, and the thruster picks up more of the load. This keeps the system forces balanced. The tail fin, which does not come into play when hovering, also becomes effective when flying forward. The direct thruster provides the remaining one-third of the force needed to counter the torque of the main rotor. The thruster rotates, moving the opening either to the right or left. In this way, directional control is achieved. Engines Discuss the different types of engine that may be found on most modern helicopters: reciprocating (or piston) and turbine. Discuss with the student the emergency procedures for engine related problems, such as loss of power (underspeed) or rapid increase in power (overspeed) while in flight. Authorized fuel types for a specific engine should also be topics of discussion at this time. Reciprocating Engine (Piston) Explain that the reciprocating engine is the most widely used powerplant in light helicopters and is designed to specific standards of reliability. It must be capable of sustained high power output for long periods. Explain to the student the cycle of the reciprocating engine, as depicted in Figure 5-23. Discuss the intake cycle (induction stroke, fuel/air mixture), compression cycle (fuel/air mixture ignited by spark plug), power cycle (burning mixture expands), and the exhaust cycle (burned gases escape). The manufacturer may be able to provide a diagram of the internal components of the reciprocating engine. This allows further discussion with the student of the internal workings of the engine. It is very important for instructors to teach the student to understand what the engine is supposed to do, how it works while flying and what happens when something breaks and how the pilot should react. The instructor should be able to discuss the octane requirements of a gasoline engine or jet fuel classifications if teaching in a turbine engine powered machine. Some engines require the settings to be changed for different fuels. The instructor must ensure the student can determine the difference between jet fuel and avgas when sumping the tanks. Explain that besides smell and the oiliness tests, there is the white paper test, where a drop is placed on a piece of white paper or paper towel. Avgas will leave a distinctive ring from the dye in the fuel whereas jet fuel tends to leave an oily yellow ring. Turbine Engine Explain to the student that the turbine engine is also widely used today on larger and most all of the military helicopters. Because turbine engines have a continuous combustion process which allows more horsepower to be developed form a smaller unit. Since the power is developed from circular rotation instead of reciprocating motion, power is smoother and engine stresses are reduced which contributes to reliability. The expense comes from the high temperature tolerant materials and close-tolerance manufacturing processes needed to produce the turbine engine. A turbine engine provides a high power-to-weight ratio, which a reciprocating engine cannot provide. Some have a power-to-weight ratio three times that of the piston engine. |
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