(3)
The mechanical link between the engine and the helicopter drivetrain can be explained in three different examples.
(a)
The first example is pushing a car. Your leg power (Np) is mechanically transferred through your arms to the car (Nr). The connection is one way, you may push the car, but the car cannot pull you. If your foot speed is constant at 3 mph, then the car speed is a constant 3 mph. This explains the statement that if the Np is constant, then the Nr is constant.
(b)
The second example of the mechanical link between the engine and helicopter is peddling a bicycle. The leg power (Np) is transferred to the wheel (Nr) via the chain (highspeed shaft). This connection is a one-way connection, your legs can drive the rear wheel, but when coasting down a hill, the rear wheel will not drive your legs. If your leg speed is constant at 100%, then the wheel speed will be a constant 100%. Again, this is an example of the mechanical link between Nr and Np.
(c)
The last example is called a mechanical loop. Due to the mechanical interaction of both power turbines, if one power turbine speeds up, driving the Nr up, the other engines power turbine becomes unloaded. An example of this would be Person No. 1 pushes harder on the car than Person No. 2. Person No. 2 senses the speed of the car increasing and instead of increasing speed with it he maintains the constant speed of 3 mph.
(4)
Np Arrangement-Each power turbine is driven at 100% rpm (by it's own Ng section), which yields 100% Revolutions Per Minute Rotor (RPM R). Use the % RPM 1-R-2 Vertical Instrument Display (VID) on the Pilots Display Unit (PDU) to visualize the mechanical connection between Np and Nr. Scan the % RPM R VID to identify which power turbine (normally both) is mechanically connected to the main rotor system. Next, use the % TRQ 1 and 2 VID to identify how much torque (work) each power turbine is delivering to the main rotor. In this situation, both power turbines are hooked to the main rotor, and delivering equal amounts of power as indicated on the PDU.
(a)
The Nr directly affects the Np, should the main rotor load demand increase to the point that the main rotor load exceeds the power capabilities of the engines; the Nr will push the Np down. An example of this is observed during a main rotor droop, the % RPM 1 and 2 are slowed down by the main rotor load as shown on the PDU.
(b)
During an autorotation, the main rotor speed (Nr) may build to a point where the power turbines decouple from the drivetrain. The % RPM R rises above the % RPM 1 and 2. The Nr won't pull the Np up due to the free-wheeling units in the drivetrain system.
(c)
When experiencing a high speed shaft failure, the RPM of the affected engine will increase to the Np over speed limit of 106±1%. Since the affected engine is no longer coupled to the Main Module, the affected engine torque will reduce to 0 %. The Rotor RPM will be dependant on the remaining engines ability to provide sufficient power to maintain speed. If the remaining engine has insufficient power capability to maintain 100 % due to reaching max torque available, the rotor will slow and maintain the same speed as the remaining engine and possibly result in a DECREASING % RPM R situation. If this occurs, pilot corrective action from memory is required.
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