Powerplant Flashcards
(34 cards)
What engine do we have?
RR 250 C47B
Describe the engine
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)
Describe RR engine rating
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
Describe the engine mounting
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
What are the 4 main engine components
- Compressor section
- Combustion section
- Turbine section
- Accessory Gearbox
Describe the throttle mechanism
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.
What are the components of the compressor assembly?
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
Describe the compressor impeller
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
What is the purpose of the impeller vanes transitioning from axial to centrifugal?
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.
What is the diffuser and its purpose?
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.
What is the inducer bleed port and its purpose
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
What is the purpose of the bleed air valve
Aids in maintaining proper air flow pressure ratios, avoiding compressor stalls
Also used in the cabin heat systems
Where is the bleed air valve and how does it work?
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
Describe the Anti-ice system, its purpose, and how it works
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
ENG ANTI-ICE CAS
- With Anti-ice switch - ON = engine receiving anti-ice provisions
- With Anti-ice switch - OFF = avoid operations requiring max power
Describe the compressor section as air flows through.
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.
Components of the combustion section
Outer case Inner liner Single fuel nozzle Single ignition plug Burner drain valves
Percentage divisions of air use in the combustion section
75-80% cooling of internal parts
20-25% required for fuel burn
2% used to seal oil passages
What are burner drain valves?
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
Describe the combustion section as air flows through
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.
Describe the ignition system
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.
Describe the use of the Starter/Generator
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.
What are the components of the turbine section
N1/Ng Gas Producer Turbine
N2/Np Power Turbine
MGT Thermocouples
Describe Ng and its drive train
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)
