(a)
Ng Section.  The Ng section is the part of the engine that generates power in the form of gas pressure.  The volume/pressure of this gas is varied to meet the power needs of the helicopter.  The Ng section can be thought of as "the engine".

(b)
Np Section.  The Np section is the part of the engine that converts power generated by Ng into shaft rotation, and delivers it to the main rotor (Nr) via the input and main transmission modules.  The Np section can be thought of as a "drivetrain component".

The relationship between Ng, Np, and Nr can be explained as follows: Ng makes power and drives Np, which is connected to Nr; Nr must rotate at a constant speed so the power turbines connected to it must be driven at a constant speed; the Nr load does not remain constant because the collective pitch is frequently adjusted; the gas pressure output of the Ng section acting upon the power turbine must be varied to match the main rotor load; Ng, and anything mechanically connected to it, is a variable; the Np/Nr are a constant.
Engine Operational sections (NG & NP) & NG/NP/NR relationship
Combustion pressure acting upon the Gas Generator Turbine Rotors is what powers the engine
(Compressor & Accessory components).

Combustion pressure acting upon the “FREE” Power Turbine Rotors is converted into rotational shaft
energy to power the drivetrain system.  NP’s largest power user is the Main Rotor, other loads include:
Tail Rotor, Hydraulic Pumps, AC Generators, and the Transmission Oil Cooler.
c. Mechanical Link.

(1)
To have a better understanding of the T-700 operational characteristics, you need to understand the mechanical link between the engines, main rotor system, and the gauges associated with this link.  Each power turbine is mechanically connected to the main rotor system through shafting and gearboxes.  This mechanical connection is a one-way connection. The power turbine drives the main rotor system, but when the main rotor speed exceeds power turbine speed (as in autorotation), the power turbine will decouple from the drivetrain via the freewheeling unit.

(2)
The disengagement of the engine from the main rotor system is accomplished by over running clutches (free-wheeling unit) in the input module.  The free-wheeling unit allows for engine disengagement during autorotation, or in the case of a non-operating engine, from the main rotor system.

(a)
Engaged:  The free-wheeling unit is engaged by the input drive gear of the engine and a compression spring.  As the engine drive gear turns the spring, it assists in forcing the rollers out of their cage, which forms a coupling between the engine input drive gear and transmission cam.

(b)
Disengaged:  When the engine input drive gear slows down below the speed of the transmission cam, the rollers are forced back into their cage.  This disengages the engine drive from the transmission drive in the input module and allows the main rotor to continue to drive the accessory and main modules during autorotation or single-engine operation.
　
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