Helicopter Flight Theory

3. Autorotation

Thus far, we have been concerned primarily with the helicopter's flight while the rotor is under power, being driven around by the engine. However there is another condition of flight, autorotation, when the blades will continue to turn and produce lift even though the power has been reduced or stopped entirely. In autorotation the helicopter is descending with the airflow coming up through the rotor from the front and below, rather than from above as happens in normal powered flight. This characteristic can hardly be overestimated since it provides an important safety factor. If the engine should fail the helicopter can still descend safely—provided the pilot handles the controls correctly and quickly lowers his collective stick to the low pitch angle required for the blades to autorotate. (The autorotational descent of a helicopter has been compared to the glide of an airplane with a dead engine.) In autorotation the airflow meets the rotor blades at angle of attack which induces a lift force inclined slightly forward, so that it serves to pull the blades around and thus keeps the rotor turning. To permit the autorotational forces to do their work in an emergency, it is necessary to design various mechanisms into the power drive system to the rotor. These are freewheeling systems which ensure that the rotor can keep turning freely if the engine stops, or even if the transmission or some other part were to jam. On most helicopters there is not just one system but three or more, each functioning independently, to guarantee that the blades will continue to rotate. Of the instruments installed in the cockpit, one in particular is worth describing because it is unique to the helicopter and is important for the pilot's control of rotor speed. This is the rotor tachometer, the instrument that indicates the number of revolutions per minute. In an airplane this instrument shows the revolutions of the engine alone. But in a helicopter, this one instrument provides the revolutions per minute of both the engine and the rotor on one clock face. This is accomplished by having two pointers, or needles, superimposed on the same shaft. One needle points to an inner scale on the face of the instrument with one set of values while the other needle points to an outer scale with a different set of numbers; one needle indicates the rotor revolutions, the other shows the revolutions of the engine.* In normal flight these two pointers will ordinarily remain together, since an increase in engine revolutions will of course result in a speeding up of the rotor. However, if engine power is suddenly reduced, as when the pilot chops back the throttle to go into an autorotational glide, the engine needle will fall away from the rotor needle since the rotor tends to maintain its speed — at first because of the inertia of the rotating mass, and then because of the aerodynamic force of autorotation. This is termed "splitting the needles". C.Gablehouse "Helicopters and Autogiros", 1969

1. Helicopter Flight | 2. Flight Controls | 3. Autorotation | 4. Rotor Mechanism