Negative torque sensing is a condition wherein
propeller torque drives the engine and the propeller is
automatically driven to high pitch to reduce drag. The
function of the negative torque sensing system is to
limit the torque the engine can extract from the
propeller during windmilling and thereby prevent large
drag forces on the airplane. The NTS system causes a
movement of the propeller blades automatically
toward their feathered position should the engine
suddenly lose power while in flight. The NTS system
is an emergency backup system in the event of sudden
engine failure. It is not a substitution for the feathering
device controlled by the condition lever.
SPLIT SHAFT/ FREE TURBINE ENGINE
In a free power-turbine engine, such as the Pratt &
Whitney PT-6 engine, the propeller is driven by a
separate turbine through reduction gearing. The
propeller is not on the same shaft as the basic engine
turbine and compressor. [Figure 14-5] Unlike the fixed
shaft engine, in the split shaft engine the propeller can
be feathered in flight or on the ground with the basic
engine still running. The free power-turbine design
allows the pilot to select a desired propeller governing
r.p.m., regardless of basic engine r.p.m.
A typical free power-turbine engine has two
independent counter-rotating turbines. One turbine
drives the compressor, while the other drives
the propeller through a reduction gearbox. The
compressor in the basic engine consists of three axial
flow compressor stages combined with a single centrifugal
compressor stage. The axial and centrifugal
stages are assembled on the same shaft, and operate as
a single unit.
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Inlet air enters the engine via a circular plenum near
the rear of the engine, and flows forward through the
successive compressor stages. The flow is directed
outward by the centrifugal compressor stage through
radial diffusers before entering the combustion
chamber, where the flow direction is actually reversed.
The gases produced by combustion are once again
reversed to expand forward through each turbine stage.
After leaving the turbines, the gases are collected in a
peripheral exhaust scroll, and are discharged to the
atmosphere through two exhaust ports near the front of
the engine.
A pneumatic fuel control system schedules fuel flow to
maintain the power set by the gas generator power
lever. Except in the beta range, propeller speed within
the governing range remains constant at any selected
propeller control lever position through the action of a
propeller governor.
The accessory drive at the aft end of the engine
provides power to drive fuel pumps, fuel control, oil
pumps, a starter/generator, and a tachometer
transmitter. At this point, the speed of the drive (N1) is
the true speed of the compressor side of the engine,
approximately 37,500 r.p.m. |
