70A Constructional arrangement and operation
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TTP |
B1-L3 |
ATA 70A |
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Beech 90 Series |
B2-L1 |
Constructional
arrangement and operation |
The engine is divided into eight major sections as follows:
1. Compressor air inlet
2. Compressor
3. Combustion
chamber
4. Compressor turbine
5. Power turbines
6. Exhaust
7. Reduction gearbox
8. Accessory drive gearbox
Compressor Air Inlet
The compressor air inlet, at the rear of the engine,
forms the main air inlet
to the gas generator. The inlet includes a circular
light alloy casting forming a plenum chamber
to allow air to enter the compressor. An integral oil tank is part of this
assembly, immediately to the rear
of the inlet plenum chamber. The inlet assembly houses the
No. 1 (aft) compressor bearing. A circumferential screen is
around the air inlet to
prevent foreign
objects from entering
the compressor. The
function of the compressor air inlet is to direct
the ram airflow from the nacelle air inlet to the compressor assembly. It also
supports the #1 bearing, accessory gearbox,
and forms the oil tank.
The annular configuration
of the PT6 inlet case facilitates the protection against
FOD. Since the
compressor rotor intake is not in line with the
flight path, air coming in to the engine has to make a sharp turn
prior to entering the inlet. This, combined with the inertial anti-ice
system, and inlet screen, prevent FOD from entering the engine.
The inlet case struts are anti-iced through heat conduction from the oil contained in the oil tank.
The
compressor section is contained in the rear portion of the gas
generator case just forward of the compressor air inlet. The compressor rotor
assembly, rotating inside
a three-stage stator housing,
is made up of three axial stages and one centrifugal stage. From stage to stage the compressor progressively increases
air pressure to supply combustion
air for engine operation and to supply compressed air for engine cooling, bearing cavity sealing, compressor bleed valve operation,
and aircraft pneumatic services.
The compressor rotor assembly consists of three axial stages and one centrifugal impeller joined together by six tie rods and spacers. The impeller and first stage blades are made of titanium while the remaining parts are steel.
Each axial stage of compression accelerates the air and alternately slows the air mass as it passes through the first three stages of compression. The centrifugal compressor design causes the air mass to slow as the air is forced through the diffuser tubes decreasing the air mass speed and increasing compressor discharge pressure or P3 air available for engine operation, cooling air, and service air.
Only the first stage blades may be blended at field level. All blade repair must be documented and the repaired blade must be tracked using a blade drawing to show where the repairs are made. The engine compressor should be washed at regular intervals to maximize performance and prevent corrosion.
Combustion Section
The
combustion section consists of a reverse- flow, annular combustion chamber
in the front section of the gas
generator case and surrounds the compressor turbine. The combustion chamber
liner is held in place by the two igniter plugs and the 14 fuel nozzle adapters.
Air from the compressor enters the combustion chamber through machined holes to provide
airflow patterns conducive to combustion
chamber insulation and for producing the desired turbulence to support
proper fuel atomization and flame
propagation. The function of the combustion chamber
is to support and contain combustion
and hot gas flow.
During engine operation, the expanding gases flow
in a
rearward direction
through the large exit duct, attached to and aft of, the combustion chamber, and reverses the gas flow 180° to flow forward through the small exit duct to the compressor turbine guide vane assembly.
Turbine Section
A two-stage reaction
turbine arrangement
is made up of two separate and independent rotors.
Though aerodynamically connected,
they are mechanically separate units. The
first stage turbine is called the compressor
turbine and is rigidly connected to the four-stage compressor
through the rotor shaft. The
compressor and the compressor
turbine combination form the rotating assembly of the gas generator. The second turbine stage is splined
to the aft end of a shaft, which drives the propeller
reduction gearbox .
An inlet guide vane assembly precedes each stage of the turbine to direct the gas flow to its associated turbine disc at an angle that creates maximum energy extraction. The compressor turbine inlet guide vane has cored passages through which compressor air flows for cooling purposes. The compressor turbine rotates in a close-tolerance shrouded housing to ensure maximum operational efficiency. The power turbines rotate within the turbine stator housing which is bolted at its front end to the exhaust duct.
As the compressor and power turbines are separate units, their functions are also separate. The rotating compressor turbine, whose rpm is expressed as Ng or N1, extracts energy from the hot gas stream to drive the compressor and accessory gearbox while the power turbines extract the remaining energy from the hot gas stream to drive the propeller through the reduction gearbox. The rotating power turbine rpm is referred to as Nf. The compressor turbine and the power turbine operate at different speeds and rotate in different directions.
Reduction Gearbox
The reduction gearbox (RGB) is comprised of a two-stage planetary reduction gear system. A semi-flexible coupling interconnects the two planetary reduction gear assemblies and dampens any vibrations between the two assemblies. Input to the RGB is from the power turbine rotor shaft. The propeller shaft, supported by a thrust ball type bearing at the front of the RGB, drives the propeller. Accessory drive pads for the primary/(Nf) fuel topping governor, the overspeed governor, and a propeller tachometer generator are at the 12 o’clock, 9 o’clock and 2 o’clock positions at the front of the RGB, as viewed from the rear of the engine. The primary function of the reduction gear assembly is to reduce the high rotational output of the power turbine shaft (33,000 rpm) at takeoff to the maximum value (2,200/1900 rpm depending on engine type) for the propeller. The reduction ratio of the power turbine rpm to propeller rpm is approximately 15:1 on the PT6A-21 and 17.3:1 on the PT6A-135. The direction of the propeller rotation, as viewed from the rear of the engine, is clockwise.
Accessory Gearbox (Chapter 83)
The accessory gearbox (AGB) consists of a gear train assembly on the aft end of the engine. An oil tank compartment is between the compressor inlet case and the AGB and is isolated from the AGB by an oil-tight diaphragm plate. The gas generator rotor shaft extends through a sealed passage in the oil tank to the AGB to drive the accessories, which include:
•
Oil pressure pump
(80–100/85-105 psi), depending on
engine type
• Four
oil scavenge pumps
• Fuel control
unit (FCU)
• High-pressure
engine-driven fuel pump
•
DC starter-generator/centrifugal
engine breather impeller
• Ng (gas generator rpm) tachometer generator
Accessory Gearbox (Chapter 83)
The accessory gearbox (AGB) consists of a gear train assembly on the aft end of the engine. An oil tank compartment is between the compressor inlet case and the AGB and is isolated from the AGB by an oil-tight diaphragm plate. The gas generator rotor shaft extends through a sealed passage in the oil tank to the AGB to drive the accessories, which include:
•
Oil pressure pump
(80–100/85-105 psi), depending on
engine type
• Four
oil scavenge pumps
• Fuel control
unit (FCU)
• High-pressure
engine-driven fuel pump
•
DC starter-generator/centrifugal
engine breather impeller
• Ng (gas generator rpm) tachometer generator
Engine Bearings (Chapter 79)
There are six main bearings throughout the length of the engine used to support the rotating mass of the gas generator and power sections. Three are ball and three are roller. Bearings No. 1, 4, and 6 are ball bearings and support thrust loads generated by the compressor rotor (rearward) and power turbines (forward) and the propeller (forward). Bearings No. 2, 3, and 5 are roller bearings. They support radial loading and permit axial rotor movement required for thermal expansion.
ROTOR SUPPORTED BY
Compressor shaft No. 1 ball No. 2 roller
Power turbine shaft No. 3 roller No. 4 ball
Propeller shaft No. 5 roller No. 6 ball
Stations
Seven station numbers are assigned to the locations in the engine starting from the engine inlet. Either a T or a P designating them as a temperature or pressure station area prefixes each station number. They are labeled numerically starting from the rear of the engine.
Flanges
The flanges on the engine are the mating faces where the
main engine components are joined together. They are labeled alphabetically starting from the
front of the engine. Three are field accessible.
Flange letter identification
and functions are as follows:
E—Joins
the compressor turbine vane assembly together. Field accessible for hot section inspection maintenance.
Bearing Labyrinth Seals (Chapter 79)
Bearing compartments for bearings No. 1, 2, and 3 are sealed to prevent oil migration into the gas path using air, or labyrinth-type, seals. Each air seal consists of two separate parts, one stationary and one rotating (compressor rotor or power turbine shaft). One part incorporates machined knife-edged grooves. A small clearance is maintained between the stationary and rotating components to create a high velocity, turbulent compressor bleed-air flow that pushes the oil toward the center of the bearing cavity effectively containing the lubrication oil within the seal cavity. On the No. 3 bearing, there is one seal assembly on the rear side of the bearing cavity. On the No. 2 bearing, there is one seal assembly on the front side and two seals on the rear side of the bearing cavity. On the No. 1 bearing, there is only one seal and it is on the forward side of the cavity. The compressor P2.5 is used to seal the No. 1 bearing and P3 air is used for the No. 2 and 3 bearings. There is no mechanical contact between the stationary and rotating components.
Labyrinth Seal Malfunctions
Certain conditions can affect the performance of
the labyrinth seals (Figure 72-13). They include, but are not
limited to:
• Lack of airflow through the labyrinth seal.
• Wear on the stator knife edges due to rotor imbalance or bearing distress.
• Oil coking
in the grooves
between the knife edges, diminishing air turbulence across the seal which can lead to oil migration into the gas path.
• Flooding of
bearing cavity due to mal- function in
the oil scavenge system.
Identifying a suspect labyrinth seal can be accomplished by observing the following
symptoms:
SYMPTOM CAUSE
Compressor contamination,
oil smell in cabin,
oily bleed valve exhaust..................No. 1 bearing
Oil smell in cabin............................No. 2 bearing (rear)
Coking around No. 2 bearing and compressor turbine area, possible smoke on start and shutdown.........................................No. 2 bearing (front)
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PTP Beech 90 Series |
B1 |
LOC |
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B2 |
LOC |
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