直升機(jī)飛行員手冊(cè) 直升機(jī)操作手冊(cè) The Helicopter Pilot’s Handbook
時(shí)間：2011-04-05 11:37來(lái)源：藍(lán)天飛行翻譯 作者：航空 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 speed at which the retreating blade tip stalls depends on the total pitch of the blade, that is, whatever is set by the combination of collective and cyclic. The cyclic input will increase with speed, and the outer part of the blade will stall first, the maximum effect being felt just aft of the trailing edge. In the cabin, you will detect a rolling tendency (usually towards the advancing blade) and a rearward tilt, together with stick and aircraft vibration and reverse cyclic behaviour. Thus, the helicopter stalls as a function of going too fast, rather than too slowly, as with an aeroplane – the retreating blade flapping down to increase its lift gets a very high angle of attack, which announces itself with a lot of vibration. Try to avoid the problem if possible, by watching your airspeed and keeping away from VNE in gusty conditions. Recover by lowering the collective. approaching the speed of sound, which will limit forward speed. The rotor disc behaves like a gyroscope, and is subject to precession, meaning that an input doesn't have an effect until 90° later in the direction of rotation (see Instruments for more on this). Thus, if you pushed the cyclic forward, and the controls were not corrected, you would actually move left or right, according to which way round the blades were going. To cater for this, control inputs are applied in advance of the blades' movement. Their delayed response is phase lag. The effects of this can be seen when increasing the collective in forward flight (say when taking off)– there will be a roll towards the advancing blade because the front portion of the disc is always more efficient than the rear, due to Transverse Flow, which is a fore and aft disymmetry of lift. When you raise the collective, the front portion of the disc creates more lift, which actually takes effect over the retreating side, causing a roll towards the advancing blades (right, in a 206). The reason why the disc produces more lift at the front is because there is more induced velocity at the rear, and less angle of attack, and less lift. The point about flying controls is that they should always be moved smoothly. Good helicopter flying is essentially downwash management, which has some lag built in. If you jerk the controls, you will get all the drag without the lift when the blades get into position before the airflow has a chance to catch up. Tail Rotor Drift There is another way of looking at it, though. If you had contra-rotating main blades, the body would stay still, because the counteracting forces are in line with each other. The tail rotor, however is out on the end of the tailboom, and therefore has a moment arm, and enough leverage to cause movement. |
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