直升機(jī)教員手冊(cè) Helicopter Instructor’s Handbook
時(shí)間：2014-11-10 08:35來(lái)源：FAA 作者：直升機(jī)翻譯 點(diǎn)擊：次
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To view this page ensure that Adobe Flash Player version 9.0.124 or greater is installed. The purpose of the drag hinge and dampers is to absorb the acceleration and deceleration of the rotor blades caused by Coriolis Effect. [Figure 5-11] Older hinge designs relied on conventional metal bearings. By basic geometry, this precludes a coincidental flapping and lead/lag hinge and is cause for recurring maintenance. Newer rotor systems use elastomeric bearings, arrangements of rubber and steel that can permit motion in two axes. Other than solving some of the above-mentioned kinematic issues, these bearings are usually in compression, can be readily inspected, and eliminate the maintenance associated with metallic bearings. Elastomeric bearings are naturally fail-safe and their wear is gradual and visible. The metal-to-metal contact of older bearings and the need for lubrication is eliminated in this design. Coning or flapping hinges-allows the blades to flap up and as airspeed is increased, allows the main rotor blades to flap due to differences in the relative wind speeds. [Figure 5-11] Feathering hinges allow the main rotor system blades to change pitch individually as they cycle around the rotor disk to allow for direction thrust control application. This is a good time to reiterate to the student what was covered in the Helicopter Flying Handbook, Chapter 4, Helicopter Flight Controls, and how each input from the controls (cyclic and collective) independently or collectively affects the rotor system. Figures and depict how the blade acts in its rotation about the mast. Explain to the student that the blade is normally kept in a horizontal plane during its rotation by centrifugal force. However, high winds during runup or shutdown when the blades are turning at a low speed could affect this (and cause damage as well). The damage occurs when the blades flex up or down greater than normal. Another factor to consider is how the flapping force is affected by the severe rigor of the maneuver (rate of climb, forward speed, aircraft gross weight, hard landing, etc.). Coning or flapping hinges allow the blades to flap up and as airspeed is increased, allows the main rotor blades to flap due to differences in the relative wind speeds. The feathering hinges allow the main rotor system blades to change pitch individually as they cycle around the rotor disk to allow for direction thrust control application. Explain to the student that modern rotor systems may use the combined principles of the rotor systems mentioned above. Some rotor hubs incorporate a flexible hub, which allows the blade to bend (flex) without the need for bearings or hinges. These systems, called flextures, are usually constructed from composite material. Elastomeric bearings may also be used in place of conventional roller bearings. Elastomeric bearings are constructed from a rubber-type material and have limited movement that is perfectly suited for helicopter applications. Flextures and elastomeric bearings require no lubrication and, therefore, require less maintenance. They also absorb vibration, which means less fatigue and longer service life for the helicopter components. Bearingless Rotor System When discussing the bearingless rotor system, explain to the student how the structures of the blades and hub are manufactured differently to absorb stresses. Bearingless rotor systems, such as the Eurocopter systems, have contact surfaces or load points made of elastomeric composite components that deform and twist to allow blade movement. Most of these components are “on condition” life items versus metal components which must be changed at certain times due to metal fatigue. The composite components are designed so that even if a portion fails, the aircraft can make a safe landing. [Figure 5-12] The hingeless (bearingless) rotor system functions much as the articulated system does, but uses elastomeric bearings and composite flextures to allow flapping and lead lag movements of the blades in place of conventional hinges. Its advantages are improved control response with less lag and substantial improvements in vibration control. It does not have the risk of ground resonance associated with the articulated type unless the landing gear system needs servicing. The hingeless rotor system is also considerably a more expensive system. Tandem Rotor On a tandem rotor helicopter, two rotors turn in opposite directions at opposite ends of a long hull. The rotors are usually synchronized through a transmission system so that the main rotor shafts can be little more than a blade length apart. Tandem rotor helicopters operate a little differently from the single rotor variety. Tandem rotor helicopters have no tail rotor, so there is no translating tendency to combat, but there are pedals for directional control at a hover. The cyclic control, which is used as it always has been in single rotor helicopters, has not changed either. [Figure 5-13] One deviation to the tandem rotor system is the side-by-side twin rotor system. Figure shows an example of the Kamen K-Max intermeshing (side-by-side) rotor system, which dates back to the old H-4 Husky, and is a modified tandem rotor system. It is optimized for external load operations, and is able to lift a payload of over 6,000 pounds, which is more than the helicopter's empty weight. The K-MAX relies on the two primary advantages of synchropter over conventional helicopters: 1) it is the most efficient of any rotor-lift technology, and 2) it has a natural tendency to hover. This increases stability, especially for precision work in placing suspended loads. At the same time, the synchropter is more responsive to pilot control inputs, making it easily possible to sling a load thus to scatter seed, chemicals, or water over a larger area. Coaxial Rotor System Students should be shown the Coaxial rotor system, which consists of a pair of helicopter rotors mounted one above the other on concentric shafts, that is one shaft inside another with the same axis of rotation, but that turn in opposite directions. [Figure 5-15] Explain that this configuration is a feature of helicopters produced by the Russian Kamov helicopter design bureau. Coaxial rotors solve the problem of angular momentum by turning each set of rotors in opposite directions. The equal and opposite torques from the rotors upon the body cancel out. Rotational maneuvering, yaw control, is accomplished by increasing the collective pitch of one rotor and decreasing the collective pitch on the other. This causes a controlled differential of torque. |
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