Transition From Flight With Engine Operating to Flight. in the Main Rotor Autorotation Regime
Gliding in the autorotative regime is not an emergency flight mode ; rather it is a normal, stable flight mode which is often used even with a normally functioning, sound engine. Gliding in the autorotative regime is
used for working out basic piloting techniques or for rapid loss of altitude. However, in order to assure safety, a definite order and sequence of actions must be followed in transitioning from flight with the engine operating into the autorotative regime. What happens with the helicopter in the case of sudden engine stoppage or in case of rapid decrease of the engine rpm?
The main rotor continues to rotate momentarily, and the rpm does not change, since the freewheeling clutch automatically disengages the engine from the transmission. The rotor continues to turn by inertia, since it has definite angular momentum. The greater the mass or weight of the blades, the larger the moment of inertia, the longer time the rotation will continue by inertia. Therefore, heavy blades have an advantage in the autorotative regime.
Under the action of the reactive moment, the main rotor rpm decreases, and therefore the thrust decreases. If the engine fails in the hovering regime, then as a result of main rotor thrust reduction, the helicopter will transition to vertical descent. However, if the engine fails in horizontal flight, reduction of the thrust and lift will cause the helicopter to descend along an inclined trajectory. In both cases, the air flow will approach the main rotor from below.
The presence of the vertical velocity causes increase of the blade element angles of attack by the magnitude Да and deflection of the force vector AR forward, i. e., a driving torque appears; therefore, there is an increase of the rpm or at least no further reduction of the rpm. Moreover, along with reduction of the main rotor rpm, there is reduction of the centrifugal force of each blade, which leads to increase of the main rotor coning angle, i. e., simultaneous upward flapping of the blades. When the flapping angle increases, there is a reduction of blade pitch under the influence of the flapping compensator, i. e., there is an increase of the main rotor rpm.
Thus, we conclude that in the case of engine failure, there are objective factors which facilitate transition of the main rotor into the autorotative regime. But the pilot must not rely on these conditions and expect the rotor
itself to transition into autorotation. Therefore, in case of engine failure, /135 the pilot must immediately reduce main rotor pitch to the minimal value. To this end, the collective-throttle lever is lowered fully. The main rotor rpm increases, and the circumferential velocity of the blade elements increases.
This leads to reduction of the blade element angles of attack and aft deflection of the force AR. Therefore, the main rotor rpm will increase up to some limit, and then the constant rpm regime is established, i. e., the autorotation becomes steady. However, if the rpm is too high, the pitch must be increased somewhat. During flight with the engine not operating, the rpm should correspond to the engine rated power rpm. In this case, the rotor will develop the maximal thrust force, and the vertical rate of descent will be minimal.
Transition of the main rotor into the autorotative regime is facilitated
by the stabilizer mounted on the tail boom. The stabilizer incidence angle
changes with change of the main rotor pitch: when the pitch is reduced to
the minimal value, the stabilizer incidence angle becomes negative (Figure 85).
If at the time of transition into the autorotative regime the helicopter is
moving with a horizontal velocity, the negative lift force Y develops on
the stabilizer. The moment of this force M = Y L causes helicopter
st st st
nose-up pitch. The main rotor angle of attack becomes positive, and the air flow approaches the rotor from below. The angle Да of each blade element increases, and the rpm increases, i. e., the main rotor transitions into the autorotative regime.
So far, we have discussed the factors which accelerate or decelerate autorotation of the main rotor. We have devoted considerable attention to this factor, since main rotor rpm in autorotation is the primary index of flight safety. If the rpm is less than the minimal permissible value during autorotation, the rotor can come to a stop — which is a problem which cannot be rectified.
However, during transition into the autorotative regime, the pilot must devote some attention to factors other than main rotor rpm. The helicopter behavior at this time differs markedly from the behavior in steady-state flight:
First of all, there is a marked reduction of the main rotor reactive moment. As a result of this, the helicopter tends to turn to the right about the vertical axis. Moreover, if there is a horizontal velocity, there will be flapping motions of the blades, and this means that the main rotor coning axis will tilt to the right.
As a result of the main rotor thrust force side component, the helicopter will bank and slip to the right.
At the moment of transition into the autorotative regime, the pilot must prevent rotation of the helicopter about the vertical and longitudinal axes by reversing the tail rotor thrust force and deflecting the main rotor cone of rotation to the left. The tilt of the helicopter fuselage relative to the horizon depends on the flight speed. At low speed, the tilt reaches 10-15°, i. e., the nose of the helicopter is quite high. This cannot be permitted, as the helicopter tail rotor may come in contact with the ground, and tail rotor failure may occur.
In steady-state autorotation the main rotor blades develop a driving torque. Under the influence of this torque, the blades are rotated forward relative to the vertical hinge to a negative lag angle. During glide, the driving torque depends on the azimuth angle. Therefore, the lag angle will vary, i. e., during glide, the blades will oscillate about the vertical hinges.