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Bell 407 > Powerplant > Flashcards

Powerplant Flashcards

1
Q

What engine do we have?

A

RR 250 C47B

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2
Q

Describe the engine

A 
RR 250 C47B
Internal combustion turboshaft engine
Featuring a free turbine
Thermodynamically rated at 813 SHP
Derated by HMU and Fuel control to: 674 SHP takeoff, 630 SHP continuous - to provide reserve for high and hot operations

Single stage, single entry, centrifugal flow compressor directly coupled to a 2 stage gas generator turbine, coupled to a 2 stage free power turbine mated via drive splines to the reduction gear box. The output shaft is below the centerline of the engine rotor.

Engine has a single ignition, single combustion chamber, and single exhaust outlet directed upward.

Utilizes a Full Authority Digital Electronic Control (FADEC)

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3
Q

Describe RR engine rating

A

RR guarantees HP @ sea level @ specific FF & MGT so all 250 C47B engines given to Bell have data plate reading 650 SHP so they all meet or exceed HP specifications

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4
Q

Describe the engine mounting

A

Mounted horizontally by 3 bipod mounts and 1 horizontal mount
Mounted behind transmission and above fuselage to:
1. Simplify drive system
2. Improve inlet/exhaust arrangement
3. Reduce cabin noise

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5
Q

What are the 4 main engine components

A
  1. Compressor section
  2. Combustion section
  3. Turbine section
  4. Accessory Gearbox
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6
Q

Describe the throttle mechanism

A

Twist grip throttle controls mechanical movement of the HMU’s control lever (PLA) via a flexible cable which connects throttle arm to a bell crank assembly.

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7
Q

What are the components of the compressor assembly?

A

Front support, containing anti-ice provisions, and enclosing the front bearing
Shroud housing
Centrifugal impeller
Diffuser
Scroll
Rear support, enclosing the rear bearing (lubricated by the gear box)
Mounting

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8
Q

Describe the compressor impeller

A 
A single piece of forged titanium
With vanes transitioning from axial to centrifugal - eliminating the need for stators
51,000 RPM at 100% Ng
Compression ratio: 9.2:1
Temperature increase: 291C/555F
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9
Q

What is the purpose of the impeller vanes transitioning from axial to centrifugal?

A

Axial to centrifugal vanes aid in the acceleration of the air rearward and rotationally. As the vanes transition to centrifugal the air continues to accelerate and gain velocity as it is propelled outward.

Stators are not necessary as they are replaced by the diffuser.

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10
Q

What is the diffuser and its purpose?

A

The diffuser is a scroll with a series of divergent ducts

Its purpose is to trade the flow’s kinetic energy (high velocity) for increased potential energy (static pressure) by gradually slowing (diffusing) the gas velocity.

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11
Q

What is the inducer bleed port and its purpose

A

Inducer bleed port is a narrow slot in the compressor shroud housing that adjusts the volume of the air entering the compressor for optimal operations
Low Ng = airflow vented to atmosphere = unloading compressor to allow for rapid accelerations
High Ng = atmospheric air enter compressor = increases volume of air when max engine efficiency is required

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12
Q

What is the purpose of the bleed air valve

A

Aids in maintaining proper air flow pressure ratios, avoiding compressor stalls
Also used in the cabin heat systems

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13
Q

Where is the bleed air valve and how does it work?

A

Located on the upper left side of the engine it works off internal pressure sensor based on the relationship between ambient temp and Ng speed.

Works to unload the compressor at low Ng, modulate to closed at a predetermined pressure ratio and closes during flight operating speeds

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14
Q

Describe the Anti-ice system, its purpose, and how it works

A

Purpose: to precent ice formation on the compressor front support
Operation: after exiting the diffuser, air enters the anti-icing valve and flows to the compressor front support. Air is exhausted through either the small slots in the trailing edges of the 5 hollow radial struts or from the double wall bullet nose hub of the compressor front support.
These serve as the only 2 components with anti-ice provisions
Rise in MGT when in operation.
Fail safe - on
Must be on in visible moisture when ambient temp is at or below 5C/40F

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15
Q

ENG ANTI-ICE CAS

A
  1. With Anti-ice switch - ON = engine receiving anti-ice provisions
  2. With Anti-ice switch - OFF = avoid operations requiring max power
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16
Q

Describe the compressor section as air flows through.

A

Air enters at the single point entry, through the front support which encloses the front bearing.
Air passes the 5 hollow support inlet struts which provide both anti-ice provisions and the to/from flow of oil to lubricate the front bearing.
Air moves to the single piece of forged titanium compressor impeller which forces the air from an axial flow to centrifugal increasing its velocity. The impeller spins at 51,000 RPM @ 100% Ng with a compression ratio of 9.2:1 with a resulting temperature increase of 291C/555F. The need for stators is eliminated with the implementation of a diffuser which trades the high kinetic energy of the flow to for increased static pressure.
Airflow into the compressor can be manipulated through a narrow slot in the compressor shroud housing called the inducer bleed port. Its purpose is to optimize the flow for performance needs. At low Ng the air is expelled out so that rapid acceleration can take place. At high Ng air is drawn in to increase volume therefore maximizing engine efficiency.
A bleed air valve located on the left side of the bleed air manifold is open during start and idle and modulates as a predetermined pressure ratio is reached, and closed at flight speeds.
If the ambient outside air temperature is below 5C/40F with visible moisture, and the Anti-Ice is on, the air is drawn through the anti-ice valve and directed to the compressor front support where it is exhausted through the only 2 components with anti-ice provisions, either the small slots in the trailing edge of the 5 hollow radial struts or from the double wall bullet nose hub of the compressor front support.
Following the impeller and diffuser, air is directed through the scroll and ducting to the rear of the engine and the combustion section.

17
Q

Components of the combustion section

A 
Outer case
Inner liner
Single fuel nozzle
Single ignition plug
Burner drain valves
18
Q

Percentage divisions of air use in the combustion section

A

75-80% cooling of internal parts
20-25% required for fuel burn
2% used to seal oil passages

19
Q

What are burner drain valves?

A

Drains that prevent accumulation of water or fuel in the combustion section.
Start = valves close when air pressure in the chamber > air pressure outside the section by a predetermined value
Shutdown = open by means of spring action

20
Q

Describe the combustion section as air flows through

A

Air enters the single combustion liner at the aft end through holes in the liner dome and skin.
75-80% of the air is used to cool internal parts. Most of the cooling air enters the combustion liner in such a manner that the flame pattern is prevented from touching the wall of the combustion liner.
The air mixes with fuel sprayed from the single fuel nozzle and 20-25% is used for combustion to take place.
The remaining roughly 2% of the air is used to seal oil passageways.
Combustion gasses and cooling air mix resulting in temperatures within acceptable limits.
The air gasses move forward out of the combustion liner to the first stage Ng gas producer turbine.

21
Q

Describe the ignition system

A

During the start sequence, the excited box transforms 28 VDC input into a pulsed high voltage output for the igniter plug, which is mounted to the outer combustion case and threaded into the inner liner.
Once the engine starts, combustion is continuous.

22
Q

Describe the use of the Starter/Generator

A

During the start sequence the Starter/Generator is used as a DC motor to drive the engine.
Once started, with the GEN switch - ON, the Starter/Generator functions as a DC generator to supply the main source of electrical power and to charge the battery.

23
Q

What are the components of the turbine section

A

N1/Ng Gas Producer Turbine
N2/Np Power Turbine
MGT Thermocouples

24
Q

Describe Ng and its drive train

A

N1/Ng Gas Producer Turbine, a 2 stage turbine directly coupled to the compressor
51,000 RPM @ 100% Ng
Drives: HMU, Start/GEN, Oil pump (inside the gearbox)

25
Q

Describe Np and its drive train

A

N2/Np Power Turbine, free turbine coupled to the gas producer turbine
32,183 RPM @ 100% Np
To the Torquemeter Gear spinning at 10,528 RPM @ 100% Np
To the Power Output Shaft spinning at 6,317 RPM @ 100% Np
Drives: PMA, Torquemeter, Freewheeling Unit

26
Q

Where are the MGT Thermocouples located?

A

The turbine section

27
Q

Where are the MGT thermocouple signals sent?

A
  1. FADEC ECU

2. Engine Airframe Processor (GEA)

28
Q

Describe the Turbine section as air moves through.

A

High energy gas flows from the combustion section and first encounters N1/Ng, the gas producer turbine.
The gas turns the 2 stage turbine at 51,000 RPM @ 100% Ng. The turbine is directly coupled to and drives the centrifugal flow compressor as well as certain accessories through the Ng drive train (HMU, Start/GEN, Oil Pump-inside the gearbox).
The gas then flow to N2/Np, the power turbine.
The gas turns the 2 stage free turbine at 32,183 RPM @ 100% Np. The turbine is coupled to the gas producer turbine, and drives the power output shaft and certain engine accessories through the Np drive train (PMA, Torquemeter, Freewheeling unit)
The temperature of the gases is measure as it flows through the turbine section by MGT thermocouples. The readings are sent to both the FADEC ECU and the Engine Airframe Processor.
The gases are then directed to the single exhaust outlet, directed upward.

29
Q

What is the function and what are the components of the Accessory Gearbox

A

The Accessory Gearbox serves as the structural support of the engine as well as speed reduction drive lines to power rotors and other accessories.
All engine components are attached to the case
Enclosed are the: Ng and Np drive trains, oil pump for the engine oil system, torquemeter and chip detectors

30
Q

Describe the 2 Stage of Helical gearing reduction in the Np drive train.

A
  1. First stage reduction - helical powertrain drive gear @ 32, 183 RPM to the larger diameter gear on the helical torquemeter gearshaft @ 10, 528 RPM
  2. Second stage reduction - smaller diameter gear on the helical torquemeter gearshaft to the helical power takeoff gearshaft @ 6,317 RPM
31
Q

What is the function of the Helical gear set in the Np drive train?

A

Provides a 2 stage reduction of RPM from 32,183 to 10,528 to 6,317 at the output drive spline.
Helical gears have slanted tooth traces to provide a larger contact radio, reduced noise, less vibrations, and the ability to transmit larger forces.

The helix angles on the gears are engineered to ensure that both stages of reduction produce a high forward axial thrust on the torquemeter gearshaft in order to minimize friction effects and provide accurate torque measurements. So that if friction is neglected, axial thrust would be directly proportional to the torque transmitted through the gears.

32
Q

Describe the purpose of the Torquemeter and its relationship to engine oil pressure

A

Torquemeter gauges changes in torque to provide an indication in the cockpit.

It is a Hydraulic-type torquemeter located inside the gearbox uses engine lubrication system as its (hydraulic) pressure source, therefore, engine oil pressure must always be greater than torquemeter oil pressure which is regulated at 115-130 PSI. (Digitally displayed by a thin green marker on the engine oil pressure gauge, if the indication is below that line you may not have an accurate indication of torque.)

33
Q

Describe the torquemeter operations

A
  1. Torquemeter gear transmits thrust to a non-rotating piston via a ball bearing
  2. The piston slides freely on the non-rotating support shaft serving as a variable position valve
  3. The piston forms an internal chamber in which regulated engine oil is admitted
  4. As gear thrust increases so does the piston inlet opening
  5. Since the piston outlet openings remain constant, the piston chamber pressure is directly proportional to torque
  6. Chamber pressure is read by the torquemeter pressure sensing port
34
Q

Give a step by step example of the Torquemeter gear operation

A
  1. Increase in torque output of engine
  2. Increase in axial thrust acting on the piston
  3. Creates an imbalanced condition on piston
  4. Therefore, piston moves forward
  5. Which increases piston inlet opening
  6. Decreasing the restriction to the flow of oil into the chamber
  7. Therefore, oil pressure in the chamber increases
  8. The pressure stops increasing when the forces on the piston counter balance axial thrust on the piston
  9. When both forces (axial and chamber) equal, the piston stops
  10. The torquemeter indicating system registers and increase in torque, when it sensed a higher pressure in the chamber
  • At a stabilized power condition, axial thrust force acting on the piston counter balance by the torquemeter pressure in the oil chamber

Bell 407 (19 decks)

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