Ch. 2.1 Engine Flashcards
How much airflow is used for combustion? The remainder?
Approximately 30% of the total airflow through the engine is used for the combustion process. The remainder is utilized for the following:
- Compressor inlet temperature (T2)
- Compressor discharge pressure (P3)
- Combustor and turbine cooling
- Engine oil seal pressurization
What is the Engine Driven Boost Pump designed for?
- Provide reliable suction feed from the aircraft fuel tank to the engine, minimizing vulnerability and fire hazard in the event of damaged fuel lines.
- Provide discharge pressure to satisfy the minimum inlet pressure requirement of the HMU or high pressure fuel pump
Fuel is tapped off in the HMU to operate what?
- Positioning a metering valve to ensure proper fuel flow to the engine.
- Positioning a servo piston that actuated the variable geometry vane servo and start bleed valve.
- Amplifying various signals (T2, P3, Ng) that influence fuel flow and variable geometry vane servo position.
What are the two mechanical and one electrical linkage the HMU responds to?
The first mechanical input from the LDS directly coordinates Ng speeds to the approximate power required by the rotor system based on collective position.
The second mechanical input is through the PCL. The position of the PCL manipulates the PAS at the HMU setting the desired power setting.
A third input is in the form of an electrical signal from the EDECU, which actuates the torque motor servo in the HMU to precisely trim Ng speed for power turbine control and load sharing.
The HMU responds to the PCL for:
- Fuel shutoff
- Setting engine start fuel flow with automatic acceleration to ground idle.
- Setting permissible Ng up to maximum.
- Fuel priming.
- EDECU override capability (LOCKOUT).
The HMU also responds to T2, P3, and Ng. These inputs aid the HMU in controlling variable stator vanes and anti-ice/start bleed valve position during engine start and normal operation, reducing the chance of compressor stall.
Why is the Ng schedule intentionally placed at a higher than required power level?
- Fail safe to high power. The torque motor, when energized, is designed to reduce the schedule to the desired power level. Therefore, loss of the torque motor electrical current causes the schedule to return to the highest power level. A schedule that is biased high due to engine electrical failure does not cause power limiting and can be manually retarded to a more desirable level using the PCL. With all the engine protection functions in the HMU operational, neither engine damage not stall can occur during or following loss of the electrical signal to the torque motor.
- Power available with One Engine Inoperative. In the event of a failure of one engine, the remaining engines gas generator can increase power sufficiently up to it’s limit (contingency power) to carry the load at the given LDS setting. A load demand signal is introduced to the HMU through the LDS. When the LDS is reduced from it’s maximum setting by adjusting the collective, the Ng is reset from the PAS setting to provide intermediate and accurate gas generator response. This new Ng setting is trimmed by the EDECU to satisfy the Ng governing and load sharing functions.
Functions of the HMU
- Rapid engine transient response through collective compensation
- Automatic fuel scheduling for engine start
- Ng overspeed protection. The HMU mechanically limits Ng to 110+-2 percent. If the Ng servo, within the Ng governor, reaches a position corresponding to an overspeed, a centrifugal valve secures fuel flow to the engine. Once the overspeed condition has passed, the valve reopens, allowing normal operation to resume.
- Ng governing. The HMU receives T2, P3, and Ng inputs from their respective sensors, which are used to schedule fuel for minimum flow, maximum flow, and variable geometry vane control.
- Acceleration limiting. The Ng governor ensures any PCL motion will result in safe engine operation and will not cause engine damage. Except for intentional shutoff of the PCL, an inadvertent shutdown will not occur during PCL motion.
- Flameout and compressor stall protection. The HMU adjusts variable geometry vane position and opens the anti-ice)start bleed valve to prevent compressor instability.
Functions of the ODV
- Provides main fuel flow to the 12 fuel injectors during engine start and operation.
- Purges the mail fuel manifold overboard, after engine shutdown, through a shutoff and drain valve to prevent coking of the fuel injectors.
- Traps fuel upstream, which keeps the fuel/oil heat exchanger full, so that system priming is not required prior to next start.
- Returns fuel back to the HMU if the Np overspeed is energized or if the EDECU hot start preventer is activated
Difference between engine and parameter limiting?
Engine limiting is when the engine control system prevents further increases to power output (TGT, HMU fuel flow limiting)
Parameter limiting is when you reach a limit from Ch4 (torque or Ng limited). Pilot is responsible for keeping in continuous limits
Types of HMU fuel flow limiting
- Under certain conditions of pressure altitude and temperature and with a high power setting from the collective, the maximum fuel flow provided by the fuel pump is limited by the physical size of the fuel lines within the HMU and ODV. For EDECU engines this condition may occur with Contingency power either On or Off (prior to the IRP TGT limiter of the EDECU)
- The HMU is also designed to protect the compressor section by limiting fuel flow as a function of Ng and has also been associated with the term Ng limiting. T700 EPR limiting can occur under cold ambient temperatures at any altitude.
Control parameters of the EDECU
- Np sensing (governing)
- Np overspeed and torque sensing (load sharing, cockpit torque indication, and Np overspeed protection)
- TGT monitoring (temperature limiting circuit)
EDECU inputs from the cockpit
- ENG SPD TRIM switch
- CONTGCY PWR switch
- ENG OVERSPEEDTEST A and B buttons
EDECU receives the following inputs from the helicopter
- Torque from the other EDECU
- Np demand
- 400 Hz backup power
- HMU (LVDT)
The EDECU sends the following signals to the cockpit
- Torque
- Np
- TGT
- Contingency power
Functions of the EDECU
4NCHEFTASTEDAD
- Np governing
- Np overspeed protection
- Np overspeed test
- Ng decay relight feature
- Contingency power (manual)
- Hot start prevention
- Engine load sharing
- Fault diagnostic system
- TGT limiting
- Auto ignition system
- Signals (cockpit)
- TDI
- Engine speed trim
- [E]DECU lockout
- Auto Contingency power
- Dual engine auto Contingency power
