Time for that nice cup of tea and a biscuit or two!!!!!!!
How about a quick run through the start sequence and fuel scheduling of your R66 RR300 engine!
The idea is to give the R66 pilot a rough insight into how your engine gets from a static position to ground idle rpm.
We need to look at a few components:
Firstly, the igniter switch.
This has two positions: ‘off’ and ‘enable’.
With the battery switched on and the key in the off position I can press the starter button and the starter motor will engage and accelerate N1 rpm.
It will continue to do so until I stop pressing the starter button.
Once I release the starter button, the starter motor will disengage and the N1 rpm will decay.
With the battery switched on and the key selected to enable, I can press the starter button and release it, as this time the Generator Control Unit (GCU) will latch the starter motor into the starter mode until the N1 rpm reaches approximately 58% or the pilot places the key back into the off position.
It will also activate the igniter system for ignition purposes.
Therefore, the starting system is live in the conditions mentioned above; however, the starting system can be disabled by applying the rotor brake.
There are two fuel control valve operating knobs:
One below and adjacent to the pilot’s collective lever, which is the fuel shut off selector. This switches the fuel off/on at a valve located at the cabin bulkhead, so before the engine.
The other is on the instrument pedestal, which is the fuel cutoff selector.
The fuel cut off switches the fuel off/on at the fuel control unit (FCU) on the right hand side of the engine and is used during the starting and stopping of the engine.
There are several system indication gauges that we must consider:
N1 compressor rpm, (gas producer turbine wheels 1 & 2) is from a signal taken from a speed sensor, located at the top of the engine gearbox, which is looking at the rotation of the compressor spur adaptor gear-shaft.
N2 rpm (power turbine wheels 3 & 4) is a signal taken from a second speed sensor on the lower left hand side of the engine gearbox, which is looking at the rotation of an N2 idler gear.
Measured Gas Temperature (MGT). This temperature measurement is taken from a location in the turbine combustion gas path, at the exit of number 2 gas producer turbine wheel and the entry to no 3 power turbine wheel by a thermal couple harness.
Engine oil pressure indication. The pressure indication is taken from a transducer on the right hand side of the engine, which changes a wet oil pressure to an electrical signal.
The engine oil pump is driven by the N1 gear train, hence, you should see some engine oil pressure as soon as the N1 rpm starts to increase and that is why you will notice that the oil pressure increases with N1 rpm increase, until the engine oil pressure relief operates at the set engine oil pressure.
The torque meter. This is operated by the torque meter situated inside the engine gearbox. Very simply, there is a torque meter piston, which is subjected to an axial thrust produced by helical gears within the N2 gear train. In a non friction World, this axial thrust would be directly proportional to the power being demanded by the pilot.
Again, this is a wet oil pressure converted to an electrical signal by a transducer that is located under the engine oil pressure transducer.
The engine fuel scheduling components:
Engine driven fuel pump, driven by N1 gear train, receives gravity fed fuel pressure.
The FCU, is driven by N1 gear train and located as said above, on the right hand side of the engine.
It has has two controls attached to it, which are operated by the pilot:
Fuel cut off
The power turbine governor, which is situated on the left hand side of the engine and is driven by N2 gear train and is controlled by the pilot through the collective lever and an electrically operated beep actuator, whose operation is via a beep switch located at the end of the pilots collective lever.
The RR300 has a pneumatic fuel scheduling system, with the following pressures used:
Pc = pressure compressor, taken from the compressor scroll and delivered to the N2 governor and the N1 FCU via plumbing lines.
Pc is divided into Px bleed and Py bleed within the FCU
Px = Pressure acts on the Accelerator evacuated bellows.
Py = pressure acts on the governor evacuated bellows
Pa = pressure ambient
Pr = pressure regulated air
Pg = pressure governor reset (this pressure resets Py once the pilot opens the throttle to go from ground idle rpm to flight idle rpm).
From this point the power turbine governor will schedule the fuel based on N2 rpm.
therefore, Pr & Pg really only come into play once the pilot has opened the throttle towards flight idle, so are not relevant during the start and acceleration to ground idle.
Subject to the pilot having completed the POH pre start check list, the pilot will continue to follow through with the start check list (the pilot is responsible for making sure that their reference material used is to the latest amendment, which if in doubt can be checked on the RHC website).
I am not going through the engine start procedure in this post, I am only trying to simply explain how the FCU schedules the fuel during the start sequence.
When the pilot starts the engine the following happens:
With the throttle closed, fuel cut off in the cut off position and the igniter key in the enabled position the pilot will press the starter button and release.
At this point the starter generator will default to the starting mode and the GCU will latch the starter into the starting sequence until the N1 rpm has reached approximately 58% (at this N1 rpm the engine subject to ignition will become self sustained and the starter motor will disengage and revert to generator standby mode, the engine will continue to accelerate up to ground idle rpm).
The pilot is waiting for the N1 rpm to reach 15% (RHC have put a small white triangle mark on the N1 gauge face to assist the pilot in referencing the required rpm).
As the compressor rpm increases so does the pressure in the compressor increase, therefore the Pc pressure will increase.
This increase in pressure will be sensed in the following areas within the FCU:
The start derichment valve evacuated bellows.
A divider that will divide the Pc pressure into Px bleed and Py bleed pressures. However, at this point Py pressure is vented to atmosphere through the start derichment valve Pa orifice.
Therefore, the only pressure increase is Px bleed, which acts upon the acceleration evacuated bellows.
At 15% N1 the pilot will move the fuel cutoff control knob fully in, thus allowing fuel to be introduced to the fuel nozzle.
At this point the igniter will cause ignition and the engine will ‘light off’.
The pilot now has to monitor MGT, oil pressure and acceleration time.
The combustion plus the starter motor will cause the engine to accelerate, increasing Pc pressure.
The increase in Pc pressure will at this point only be felt on the acceleration bellows via Px bleed pressure as the Py bleed pressure is still being vented to atmosphere through the start derichment valve Pa orifice.
The increased Px bleed pressure will cause the evacuated acceleration bellows to contract.
The acceleration bellows are mechanically connected to the fuel metering valve, such that when it contracts it will move the fuel metering valve towards the open position, thus increasing the fuel delivered to the fuel nozzle and causing an acceleration of the engine.
As the engine accelerates, still with the aid of the starter motor, the Px bleed pressure also increases, (Pc = Pc at this point) thus further contracting the acceleration bellows, causing the fuel metering valve to supply more fuel to the engine and the acceleration continues.
At approximately 30%N1 rpm, Pc pressure will be sufficient to close the start derichment valve, preventing Py bleed pressure from being vented to atmosphere.
Py bleed pressure now starts to increase, which is acting on the governor evacuated bellows, causing it to contract. Just like the acceleration bellows, it is mechanically connected to the fuel metering valve and the contraction will cause the metering valve to move towards the open position increase fuel flow.
The engine is now accelerating due to Py, Px bleed pressure differential acting on the governor bellows.
At approximately 47% N1 rpm a set of fly weights driven by the FCU via the N1 gear train will produce a centrifugal force sufficient to overcome a spring tension which holds a speed enrichment lever on to the Px restrictor orifice, when the lever moves away from the orifice this will allow Px bleed pressure to vent to atmosphere through the Pa orifice.
This causes a further change in the Py, Px differential pressures, Py now being greater (Py = Pc) is acting on the governor bellows causing the fuel metering valve to move further towards the open position.
The engine continues accelerating (still with the aid of the starter motor up to approximately 58% N1, after which it will disengage and revert to generator stand by mode) therefore Py bleed pressure continues to increase, acting on the governor bellows causing the engine to continue to accelerate towards ground idle rpm.
At 65/67% N1 rpm the fly weight force is now sufficient too over come a spring tension holding the governor lever onto the Py restrictor orifice, thus allowing the Py pressure to vent to atmosphere via the Pa orifice.
The effect of this will be a slight reduction in Py bleed pressure, allowing the governor bellows to expand slightly and settle in a position that will meter the fuel for the given idle rpm required.
We are at idle rpm and you should understand that at this point the engine speed is governed by the FCU (N1 speed).
I hope that sort of makes sense!!!!
I think that is enough as you will have run out of biscuits by now.
The next tea and biscuit interlude in your busy lockdown schedule will cover fuel scheduling up to flight idle and in flight.
KNOWLEDGE IS FLIGHT SAFETY HELPING TO KEEP YOUR RPM IN THE GREEN.
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