直升機飛行員手冊直升機操作手冊 The Helicopter Pilot’s Handbook

時間：2011-04-05 11:37來源：藍天飛行翻譯 作者：航空 點擊：次

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In autorotation, the function of downwash velocity is replaced by air going up through the rotors and creating a larger angle of attack.
The flight velocity is the reciprocal of the relative airflow, made up of downwash and tangential velocity and movement of the machine through the air.
Translation is the conversion from hover to forward movement, where the helicopter is supported by other means than its own power, that is, relative airflow. Translational lift is the extra thrust you get from forward movement, when the new airflow enters the disc. The helicopter flies better because you get more air through per unit of time, which has a lower induced velocity because it hasn't had a chance to speed up just before going through the rotor. As tip vortices are also being left behind, your lift vector becomes more vertical, for more thrust with less drag. The reason you have to lower the collective to maintain height at this point is because the angle of attack has increased against the new relative airflow. This also means less engine power is required. Of course, all the while rotor efficiency is increased with forward flight, at some point you need to increase power to overcome drag from the fuselage, which is increasing at a faster rate. This is why you should not reduce power at the end of a climb until you have both the speed and height you want (if you reduced power at, say, 1500 feet and 60 knots, but you really wanted 100 knots, you wouldn't be able to accelerate beyond a certain point without applying more power than you would have used before.

All changes in velocity from cyclic movements are known as transitions.

Ground Resonance
In flight, most parts of a helicopter vibrate at their own natural frequency. On the ground, they collect through the landing gear - if its natural vibration matches that of the main rotor, every time a blade rotates, the present vibrations receive another reflected pulse to increase their amplitude, and which can cause the aircraft to tip over and be destroyed. Peculiar to some helicopters, with fully articulated rotors, because they have dragging hinges (they are there to counteract vibration caused by movement of the blade's centre of mass), this is indicated by an uncontrollable lateral oscillation increasing rapidly in sympathy with rotor RPM. It could also be caused by blades not being in balance, unequal tyre pressures or finger trouble, but will only occur if the gear is in contact with the ground. It's best avoided by landing or taking off as cleanly as possible, but, if it does occur, you must either lift off or lower the collective and close the throttle.

Dynamic Rollover
This occurs when your helicopter has a tilted thrust vector with respect to the C of G, commonly encountered with some side drift when you have one skid or wheel on the ground acting as a pivot point, but you can also get a problem when your lateral C of G falls outside the width of the skids or wheels. Every object has a static rollover angle, to which it must be tilted for the C of G to be over the roll point, for most helicopters being 30-35°. As your lateral cyclic control at that point is a lot less effective than if you were hovering, because it is not rotating around the C of G, but the rollover point, you have less chance to get out of trouble, and the only effective control is through the collective (do not raise it). In other words, the lift from the rotor disc that should be vertical is inclined and converted into thrust, above the centre of gravity, so trying to use the cyclic to level, and the collective to get you off the ground is wrong!

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