Note: Descriptions are shown in the official language in which they were submitted.
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SYSTEM AND METHOD FOR OPERATING A MULTI-ENGINE ROTORCRAFT
TECHNICAL FIELD
The present disclosure relates generally to a multi-engine system, and more
particularly to
methods and systems of operating a multi-engine rotorcraft.
BACKGROUND OF THE ART
In dual-engine helicopters, an engine output shaft is mated to the helicopter
transmission clutch
system that drives the helicopter main and tail rotors. When the engine output
shaft speed is
lower than the helicopter rotor speed, the engine de-clutches from the
transmission clutch
system.
In certain circumstances, the engine may be required to accelerate rapidly
from a low speed to
a higher operating speed of the helicopter rotor. The amount of engine torque
needed for rapid
acceleration, as well as the engine inertia following rapid acceleration, can
potentially over-
stress or damage the clutch mechanism upon re-clutching of the engine.
Therefore, improvements are needed.
SUMMARY
In accordance with a broad aspect, there is provided a method for operating a
multi-engine
rotorcraft. The method comprises driving a rotor of the rotorcraft with a
first engine while a
second engine is de-clutched from a transmission clutch system that couples
the rotor and the
second engine, instructing the second engine to accelerate to a re-clutching
speed, and
controlling an output shaft speed of the second engine during acceleration of
the second engine
to the re-clutching speed by applying a damping function to a speed control
loop of the second
engine.
In accordance with another broad aspect, there is provided a system for
operating a multi-
engine rotorcraft. The system comprises a processing unit, and a non-
transitory computer-
readable medium having stored thereon program instructions executable by the
processing unit.
The program instructions are executable for: driving a rotor of the rotorcraft
with a first engine
while a second engine is de-clutched from a transmission clutch system that
couples the rotor
and the second engine, instructing the second engine to accelerate to a re-
clutching speed, and
controlling an output shaft speed of the second engine during acceleration of
the second engine
to the re-clutching speed by applying a damping function to a speed control
loop of the second
engine.
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Features of the systems, devices, and methods described herein may be used in
various
combinations, in accordance with the embodiments described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference is now made to the accompanying Figs. in which:
Fig. 1A is a schematic view of a multi-engine rotorcraft;
Fig. 1B is a schematic representation of an exemplary multi-engine system for
the rotorcraft of
Fig. 1A, showing axial cross-sectional views of two gas turbine engines;
Fig. 2 is a cross-sectional view of an example turboshaft engine of the
rotorcraft of Fig. 1A;
Fig. 3 is a block diagram of an example architecture for operating a
rotorcraft;
Figs. 4A-4B are graphical illustrations of example approaches for applying a
damping function to
a torque request;
Figs. 5A-5D are graphical illustrations of example approaches for applying a
damping function a
speed reference;
Figs. 6A-6D are graphical illustrations of example approaches for applying a
damping function
to an acceleration;
Figs. 7A-7B are graphical illustrations of example approaches for applying a
damping function to
a fuel flow;
Figs. 8A-8B are graphical illustrations of example approaches for applying a
damping function to
a rate of change of a fuel flow;
Fig. 9 is a block diagram of an example embodiment for a speed control loop;
Fig. 10 is a flowchart of an example method for operating a dual-engine
rotorcraft; and
Fig. 11 is a block diagram of an example computing device for implementing the
method of Fig.
10.
It will be noted that throughout the appended drawings, like features are
identified by like
reference numerals.
DETAILED DESCRIPTION
There are described herein methods and systems for controlling the rotational
speed of an
output shaft of an engine as it re-clutches into a transmission clutch system,
so as to minimize
transmission clutch system over-torque during re-clutching on a rapid engine
speed run-up. The
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methods and systems may be used when exiting an asymmetric operating regime,
as will be
explained in more detail below. The methods and systems may also be used in
normal engine
run-ups, for example during ground decoupled engine run-ups, or when
transitioning from One
Engine Operative (0E0) to All Engines Operative (AEO). Other applications are
also
considered.
Fig. 1A depicts an exemplary multi-engine rotorcraft 100, which in this case
is a helicopter. The
rotorcraft 100 includes at least two gas turbine engines 102, 104. These two
engines 102, 104
may be interconnected to a transmission clutch system (TCS) 105, as shown in
Fig. 1B, to drive
a main rotor 108.
Turning now to Fig. 1B, illustrated is an exemplary multi-engine system 105.
The multi-engine
system 105 may include two or more gas turbine engines 102, 104. In the case
of a helicopter
application, these gas turbine engines 102, 104 will be turboshaft engines.
Control of the multi-
engine system 105 is effected by one or more controller(s) 210, which may be
FADEC(s),
electronic engine controller(s) (EEC(s)), or the like, that are programmed to
manage the
operation of the engines 102, 104 to reduce an overall fuel burn, particularly
during sustained
cruise operating regimes, wherein the aircraft is operated at a sustained
(steady-state) cruising
speed and altitude. The cruise operating regime is typically associated with
the operation of
prior art engines at equivalent part-power, such that each engine contributes
approximately
equally to the output power of the system 105. Other phases of a typical
helicopter mission
include transient phases like take-off, climb, stationary flight (hovering),
approach and landing.
Cruise may occur at higher altitudes and higher speeds, or at lower altitudes
and speeds, such
as during a search phase of a search-and-rescue mission.
In the present description, while the aircraft conditions (cruise speed and
altitude) are
substantially stable, the engines 102, 104 of the system 105 may be operated
asymmetrically,
with one engine operated in a high-power "active" mode and the other engine
operated in a
lower-power (which could be no power, in some cases) "standby" mode. Doing so
may provide
fuel saving opportunities to the aircraft, however there may be other suitable
reasons why the
engines are desired to be operated asymmetrically. This operation management
may therefore
be referred to as an "asymmetric mode" or an "asymmetric operating regime",
wherein one of
the two engines is operated in a lower-power (which could be no power, in some
cases)
"standby mode" while the other engine is operated in a high-power "active"
mode. The
asymmetric operating regime may be engaged for a cruise phase of flight
(continuous, steady-
state flight which is typically at a given commanded constant aircraft
cruising speed and
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altitude). The multi-engine system 105 may be used in an aircraft, such as a
helicopter, but also
has applications in suitable marine and/or industrial applications or other
ground operations.
Referring still to Fig. 1B, according to the present description the multi-
engine system 105 is
driving in this example a helicopter (H) which may be operated in the
asymmetric operating
regime, in which a first of the engines (say, 102) may be operated at high
power in an active
mode and the second of the engines (104 in this example) may be operated in a
lower-power
(which could be no power, in some cases) standby mode. In one example, the
first engine 102
may be controlled by the controller(s) 210 to run at full (or near-full) power
conditions in the
active mode, to supply substantially all or all of a required power and/or
speed demand of the
-- common load 170. The second engine 104 may be controlled by the
controller(s) 210 to operate
at lower-power or no-output-power conditions to supply substantially none or
none of a required
power and/or speed demand of the common load 170. A clutch may be provided to
declutch the
low-power engine. Controller(s) 210 may control the engine's governing on
power according to
an appropriate schedule or control regime. The controller(s) 210 may comprise
a first controller
for controlling the first engine 102 and a second controller for controlling
the second engine 104.
The first controller and the second controller may be in communication with
each other in order
to implement the operations described herein. In some embodiments, a single
controller 210
may be used for controlling the first engine 102 and the second engine 104.
In another example, an asymmetric operating regime of the engines may be
achieved through
the one or more controller's 210 differential control of fuel flow to the
engines, as described in
pending application 16/535,256, the entire contents of which are incorporated
herein by
reference. Low fuel flow may also include zero fuel flow in some examples.
Although various differential control between the engines of the multi-engine
system 105 are
possible, in one particular embodiment the controller(s) 210 may
correspondingly control fuel
flow rate to each engine 102, 104 accordingly. In the case of the standby
engine, a fuel flow
(and/or a fuel flow rate) provided to the standby engine may be controlled to
be between 70%
and 99.5% less than the fuel flow (and/or the fuel flow rate) provided to the
active engine. In the
asymmetric operating regime, the standby engine may be maintained between 70%
and 99.5%
less than the fuel flow to the active engine. In some embodiments, the fuel
flow rate difference
between the active and standby engines may be controlled to be in a range of
70% and 90% of
each other, with fuel flow to the standby engine being 70% to 90% less than
the active engine.
In some embodiments, the fuel flow rate difference may be controlled to be in
a range of 80%
and 90%, with fuel flow to the standby engine being 80% to 90% less than the
active engine.
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In another embodiment, the controller 210 may operate one engine (say 104) of
the multi-
engine system 105 in a standby mode at a power substantially lower than a
rated cruise power
level of the engine, and in some embodiments at substantially zero output
power and in other
embodiments less than 10% output power relative to a reference power (provided
at a reference
fuel flow). Alternately still, in some embodiments, the controller(s) 210 may
control the standby
engine to operate at a power in a range of 0% to 1% of a rated full-power of
the standby engine
(i.e. the power output of the second engine to the common gearbox remains
between 0% to 1%
of a rated full-power of the second engine when the second engine is operating
in the standby
mode).
In another example, the multi-engine system 105 of Fig. 1 B may be operated in
an asymmetric
operating regime by control of the relative speed of the engines using
controller(s) 210, that is,
the standby engine is controlled to a target low speed and the active engine
is controlled to a
target high speed. Such a low speed operation of the standby engine may
include, for example,
a rotational speed that is less than a typical ground idle speed of the engine
(i.e. a "sub-idle"
engine speed). Still other control regimes may be available for operating the
engines in the
asymmetric operating regime, such as control based on a target pressure ratio,
or other suitable
control parameters.
Although the examples described herein illustrate two engines, the asymmetric
operating
regime is applicable to more than two engines, whereby at least one of the
multiple engines is
.. operated in a low-power standby mode while the remaining engines are
operated in the active
mode to supply all or substantially all of a required power and/or speed
demand of a common
load.
In use, the first engine (say 102) may operate in the active mode while the
other engine (say
104) may operate in the standby mode, as described above. During the
asymmetric operating
regime, if the helicopter (H) needs a power increase (expected or otherwise),
the second engine
104 may be required to provide more power relative to the low power conditions
of the standby
mode, and possibly return immediately to a high- or full-power condition. This
may occur, for
example, in an emergency condition of the multi-engine system 105 powering the
helicopter,
wherein the "active" engine loses power, and the power recovery from the lower
power to the
high power may take some time. Even absent an emergency, it will be desirable
to repower the
standby engine to exit the asymmetric operating regime.
In some embodiments, the standby engine may be de-clutched from the TCS 105 of
the
rotorcraft. As illustrated in Fig. 1B, first and second engines 102, 104 each
having a respective
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transmission 152 are interconnected by a common output gearbox 150 to drive a
common load
170. In one embodiment, the common load 170 may comprise a rotary wing of a
rotary-wing
aircraft. For example, the common load 170 may be a main rotor 108 of the
aircraft 100.
Depending on the type of the common load 170 and on the operating speed
thereof, each of
engines 102, 104 may be drivingly coupled to the common load 170 via the
output gearbox 150,
which may be of the speed-reduction type.
For example, the gearbox 150 may have a plurality of transmission shafts 156
to receive
mechanical energy from respective output shafts 154 of respective engines 102,
104. The
gearbox 150 may be configured to direct at least some of the combined
mechanical energy from
the plurality of gas turbine engines 102, 104 toward a common output shaft 158
for driving the
common load 170 at a suitable operating (e.g., rotational) speed. It is
understood that the
transmission clutch system 105 may also be configured, for example, to drive
accessories
and/or other elements of an associated aircraft. The gearbox 150 may be
configured to permit
the common load 170 to be driven by either of the gas turbine engines 102, 104
or by a
combination of both engines 102, 104 together.
With reference to Fig. 2, the gas turbine engines 102, 104 can be embodied as
turboshaft
engines. Although the foregoing discussion relates to engine 102, it should be
understood that
engine 104 can be substantively similar to engine 102. In this example, the
engine 102 is a
turboshaft engine generally comprising in serial flow communication a low
pressure (LP)
compressor section 12 and a high pressure (HP) compressor section 14 for
pressurizing air, a
combustor 16 in which the compressed air is mixed with fuel and ignited for
generating an
annular stream of hot combustion gases, a high pressure turbine section 18 for
extracting
energy from the combustion gases and driving the high pressure compressor
section 14, and a
lower pressure turbine section 20 for further extracting energy from the
combustion gases and
driving at least the low pressure compressor section 12.
The low pressure compressor section 12 may independently rotate from the high
pressure
compressor section 14. The low pressure compressor section 12 may include one
or more
compression stages and the high pressure compressor section 14 may include one
or more
compressor stages. The low pressure compressor section 12 may include one or
more variable
guide vanes at its inlet or inter stage. The high pressure compressor section
14 may include one
or more variable guide vanes at its inlet or inter stage. A compressor stage
may include a
compressor rotor, or a combination of the compressor rotor and a compressor
stator assembly.
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In a multistage compressor configuration, the compressor stator assemblies may
direct the air
from one compressor rotor to the next.
The engine 102 has multiple, i.e. two or more, spools which may perform the
compression to
pressurize the air received through an air inlet 22, and which extract energy
from the
combustion gases before they exit via an exhaust outlet 24. In the illustrated
embodiment, the
engine 102 includes a low pressure spool 26 and a high pressure spool 28
mounted for rotation
about an engine axis 30. The low pressure and high pressure spools 26, 28 are
independently
rotatable relative to each other about the axis 30. The term "spool" is herein
intended to broadly
refer to drivingly connected turbine and compressor rotors.
The low pressure spool 26 includes a low pressure shaft 32 interconnecting the
low pressure
turbine section 20 with the low pressure compressor section 12 to drive rotors
of the low
pressure compressor section 12. In other words, the low pressure compressor
section 12 may
include at least one low pressure compressor rotor directly drivingly engaged
to the low
pressure shaft 32 and the low pressure turbine section 20 may include at least
one low pressure
turbine rotor directly drivingly engaged to the low pressure shaft 32 so as to
rotate the low
pressure compressor section 12 at a same speed as the low pressure turbine
section 20. The
high pressure spool 28 includes a high pressure shaft 34 interconnecting the
high pressure
turbine section 18 with the high pressure compressor section 14 to drive
rotors of the high
pressure compressor section 14. In other words, the high pressure compressor
section 14 may
include at least one high pressure compressor rotor directly drivingly engaged
to the high
pressure shaft 34 and the high pressure turbine section 18 may include at
least one high
pressure turbine rotor directly drivingly engaged to the high pressure shaft
34 so as to rotate the
high pressure compressor section 14 at a same speed as the high pressure
turbine section 18.
In some embodiments, the high pressure shaft 34 may be hollow and the low
pressure shaft 32
extends therethrough. The two shafts 32, 34 are free to rotate independently
from one another.
The engine 102 may include a transmission 38 driven by the low pressure shaft
32 and driving a
rotatable output shaft 40. The transmission 38 may vary a ratio between
rotational speeds of the
low pressure shaft 32 and the output shaft 40.
The engine controller 210 can modulate a fuel flow rate provided to the engine
102, the position
and/or orientation of variable geometry mechanisms within the engine 102, a
bleed level of the
engine 102, and the like. In some embodiments, the engine controller 210 is
configured for
controlling operation of multiple engines, for instance the engines 102 and
104.
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With reference to Fig. 3, the rotorcraft 100, comprising the engines 102, 104
and the rotor 108,
is illustrated using a block diagram. More than two engines 102, 104 may be
present on a same
rotorcraft 100. The engines 102, 104 are mechanically coupled to the main
rotor 108 via the
transmission clutch system (TCS) 105, for instance as illustrated in Fig. 1B,
for causing the rotor
108 to rotate and produce thrust for the rotorcraft 100. Collectively, the
engines 102, 104, and
the rotor 108 form part of a multi-engine system which is controlled by the
engine controller 210.
The engine controller 210 is configured for receiving various instructions
from an operator of the
aircraft 100, for example via operator input 230, which can include one or
more flight control
inputs.
The engine controller 210 can be composed of various devices, including one or
more FADECs,
one or more rotor controllers, or any other suitable devices for controlling
operation of the
engines 102, 104, and/or the rotor 108. In some embodiments, the operation of
the engines
102, 104, and of the rotor 108 is controlled by way of one or more actuators,
mechanical
linkages, hydraulic systems, and the like. The engine controller 210 can be
coupled to the
actuators, mechanical linkages, hydraulic systems, and the like, in any
suitable fashion for
effecting control of the engines 102, 104 and/or of the rotor 108. For
example, if a change in the
operating conditions of the rotorcraft 100 is detected without any
corresponding change in
inputs from an operator of the rotorcraft 100, the engine controller 210 can
adjust the inputs to
compensate for the uncommanded change.
One or more sensors 202, 204 are coupled to the engines 102, 104, for
acquiring data about the
operating parameters of the engines 102, 104. Additionally, sensors 208 are
coupled to the rotor
108 for acquiring data about the operating parameters of the rotor 108. The
sensors 202, 204,
208 may be any suitable type of sensor used to measure operating parameters
including, but
not limited to, speed sensors, acceleration sensors, pressure sensors,
temperature sensors,
altitude sensors, and the like. The sensors 202, 204, 208, can be coupled to
the engine
controller 210 in any suitable fashion, including any suitable wired and/or
wireless coupling
techniques.
In certain conditions, the rotor 108 of the rotorcraft 100 is driven with a
first engine, for example
engine 102, while a second engine, for example engine 104, is unclutched from
the
transmission clutch system 105. The engine controller 210 can be configured to
control a speed
of the output shaft 154 of the second engine 104 during a re-clutching
procedure of the second
engine 104. In some embodiments, while the second engine 104 is accelerating
to reach a re-
clutching speed, i.e. a speed at which the output shaft 154 can engage with a
clutch in the
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gearbox 150, a damping module 206 applies a damping function to a speed
control loop 207 of
the second engine 104. This will be referred to as "dampened re-clutching"
herein.
The damping function applied by the damping module 206 to the speed control
loop 207 may
vary according to various implementations. The damping function has the effect
of slowing down
the rotational speed of the output shaft of the re-clutching engine prior to
the re-clutching engine
reaching a re-clutching speed. Controlling the rotational speed of the output
shaft in the manner
described herein limits the torque/power at the moment of re-clutching. During
the un-clutched
engine's run-up to the moment before re-clutching, torque cannot be measured
as there is no
opposing load on the output shaft to create a twist. If the engine output
shaft is still accelerating
at the moment of re-clutching, i.e. the moment torque starts to be measured,
the torque may
already be too high and thus damage the clutch. Therefore, the rotational
speed of the output
shaft is controlled to limit torque/power. The engine may be accelerated
before re-clutching and
then acceleration is limited at the moment of re-clutching using the damping
function.
In some embodiments, the damping function corresponds to a limit on a torque
request for the
re-clutching engine. An example is illustrated in Figs. 4A-4B. Fig. 4A
illustrates an example of a
rotational speed 402 of the output shaft of the re-clutching engine, when the
engine has been
instructed to accelerate to a re-clutching speed 404. Fig. 4B illustrates an
example of a torque
request 412 for the re-clutching engine. The torque request 412 is used by the
speed control
loop 207 to determine a fuel flow needed to obtain a desired power turbine
speed. When the
rotational speed 402 reaches a target speed 406 that is within a tolerance 408
of the re-
clutching speed 404, a limit 414 is applied to the torque request 412.
The torque request limit 414 is momentarily applied to the torque request 412,
to prevent over-
torque during re-clutching. The torque request limit 414 dampens the
acceleration of the re-
clutching engine, thereby slowing down the output shaft speed 402 in its final
approach to the
re-clutching speed 404 and reducing a re-clutching torque.
The value (or schedule) of the torque request limit 414 may be found through
testing and/or
simulations, to determine an optimal value. In some embodiments, a plurality
of torque request
limits 414 are available and a given torque request limit is selected as a
function of one or more
parameters, such as acceleration, or directly per mechanical design limit of
the clutch. The
selection may be made by the engine controller 210, such as by the damping
module 206.
The torque request limit 414 may be removed from the torque request 412 once
the re-clutching
speed 404 has been reached by the output shaft of the re-clutching engine.
Removal may be
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determined as a function of a speed error (i.e. a difference between the
output shaft speed 402
and a speed reference 410), a sensed torque, a collective lever pitch (CLP),
time, or any other
suitable parameter.
The tolerance 408 used to trigger the application of the torque request limit
414 may be
predetermined or may be determined dynamically, for example as a function of
acceleration
where tolerance 408 is proportional to acceleration. In other words, the
higher the acceleration,
the sooner the speed needs to be slowed down. Other embodiments are also
considered.
In some embodiments, the damping function is applied to a speed reference to
which the output
shaft speed tracks. A portion of the speed reference is shaped in accordance
with a profile that
will cause the rotational speed of the output shaft of the re-clutching engine
to slow down upon
its final approach to the re-clutching speed. Examples are illustrated in
Figs. 5A-5D. Fig. 5A
illustrates a first example, whereby a portion of a speed reference 410 is
shaped according to a
ramp profile 502. The ramp profile 502 acts as a tapering off of the speed
reference 410 starting
from the target speed 406.
Fig. 5B illustrates another example, whereby the speed reference 410 is
tapered off in a hold
profile 504 followed by a ramp profile 502 to the re-clutching speed. In other
words, when the
rotational speed 402 of the output shaft reaches the target speed 406, the
speed reference 410
is held to the target speed 406 for a given time period (the hold profile 504)
and then ramped up
to the re-clutching speed (the ramp profile 502). The duration of the hold
profile 504 may be
predetermined or set dynamically. For example, the duration of the hold
profile 504 may be
based on acceleration. Other embodiments are also considered.
A slope of the ramp profile may be predetermined or set dynamically. For
example, the slope of
the ramp profile 502 may be proportional to a difference between the re-
clutching speed 404
and the rotational speed 402 (i.e. a speed error) when the speed 402 is above
the hold profile
504 and acceleration is below a threshold. If the speed error is small, the
ramp profile 502 can
be steeper as the re-clutching torque will be minor. If the speed error is
large, the ramp profile
502 will need to be more gradual. Other embodiments are also considered. In
some
embodiments, the ramp profile 502 has an infinite slop and the hold profile
504 and ramp profile
502 together form a step profile.
In some embodiments, the ramp profile 502 begins at the target speed 406
(following a hold
profile 504 or not) and continues to a reference speed that is greater than
the re-clutching speed
404, before decreasing to the re-clutching speed 404. An example is shown in
Fig. 5C. When a
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reference speed 506 is reached, the speed reference 410 may be shaped in
various manners to
return to the re-clutching speed 404, such as but not limited to a step, a
decreasing ramp, a
combination of a hold and a ramp, and the like. The reference speed 506 may be
predetermined
or set dynamically. For example, when the hold profile 504 transitions to the
reference speed
506 through the ramp profile 502, the reference speed 506 is set such that the
difference
between the reference speed 506 and the re-clutching speed 404 is proportional
to the
difference between the re-clutching speed 404 and the ramp profile 502. Other
embodiments
are also considered.
Fig. 5D illustrates yet another example of applying the damping function to
the speed reference
410. In this example, the speed reference 410 is tapered off using a reference
model 508, for
example, exhibiting a first-order transfer function response in the final
approach to the re-
clutching speed 404. Other reference models may also be used, such as but not
limited to first
order or second-order transfer functions.
When applied to the speed reference 410, the damping function may correspond
to any profile
that causes the output shaft speed 402 to slowdown in its final approach to
the re-clutching
speed 404. The rate of change of the profile, the duration of the profile, and
the level of the
profile can be dependent on any one or more of time, CLP, sensed speed, and
acceleration.
In some embodiments, the damping function is applied to an acceleration of the
re-clutching
engine. An example is shown in Figs. 6A-6B. Fig. 6A illustrates the output
shaft speed 402 of
the re-clutching engine. Fig. 6B illustrates the acceleration 602 of the power
turbine (NPdot). An
acceleration limit 604 is applied to the acceleration 602 in order to slowdown
the output shaft
speed 402 in its final approach to the re-clutching speed 404. The
acceleration limit 604 is
scheduled as a function of the output shaft speed 402, and can remain active,
be modified, or
phased out after the output shaft speed 402 has reached the re-clutching speed
404.
In some embodiments, the damping function is applied to a fuel flow for the re-
clutching engine.
An example is shown in Figs. 7A-7B. Fig. 7A illustrates the output shaft speed
402 of the re-
clutching engine. Fig. 7B illustrates the fuel flow 702 for the re-clutching
engine. A fuel flow limit
704, or maximum fuel flow command, is momentarily lowered as the output shaft
speed 402
approaches the re-clutching speed 404. A sensed speed of the output shaft may
trigger the
lowering of the fuel flow limit 704, for example when the output shaft speed
402 reaches the
target speed 406. The tolerance 408 may be predetermined or dynamically
selected. For
example, the tolerance 408 may be proportional to acceleration, where the
higher the
acceleration, the sooner the rotational speed 402 is slowed down. This
suppression in fuel flow
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reduces the engine output torque at the moment of re-clutching. The level to
which the fuel flow
limit 704 is lowered may be dependent on the speed error and/or a rate of
change of the
rotational speed 402 (i.e. acceleration). The fuel flow limit 704 may be
raised back to its original
value, either gradually or in a step profile, after the output shaft speed 402
has reached the re-
clutching speed 404, as a function of one or more of time, sensed speed error,
acceleration,
sensed torque, and CLP.
In some embodiments, the damping function is applied to a rate of change of
the fuel flow to the
re-clutching engine. An example is shown in Figs. 8A-8B. Fig. 8A illustrates
the output shaft
speed 402 of the re-clutching engine. Fig. 8B illustrates the rate of change
of the fuel flow 802
for the re-clutching engine. A rate of change of fuel flow limit 804 is used
to vary the increasing
and decreasing rate of change of fuel flow command. For example, an increasing
rate of change
of the fuel flow is boosted to control engine rapid acceleration and then a
decreasing rate of
change of fuel flow is boosted to lower the output torque of the re-clutching
engine upon re-
clutching. The conditions and moments to boost the rate of change of fuel flow
may be a
function of one or more of time, sensed speed error, acceleration, sensed
torque, and CLP. The
engine is accelerated by increasing fuel flow but limited by a maximum rate of
change of the fuel
flow. Boosting an increasing rate of change refers to increasing the allowable
maximum rate of
change of fuel flow (e.g. from 1000 pph/s to 1500 pph/s), hence allowing fuel
flow to increase
more rapidly which in turn increases the acceleration of the engine. Similarly
for decelerating the
engine or slowing the acceleration of the engine, boosting a decreasing rate
of change of fuel
flow refers to decreasing the allowable minimum rate of change of fuel flow
(e.g. from -1000
pph/s to -1500 pph/s), hence allowing fuel flow to decrease more rapidly which
in turn increases
the deceleration of the engine.
An example embodiment of the speed control loop 207 is illustrated in Fig. 9.
In this example,
the engine speed is governed by an outer control loop 902 that receives a
desired power turbine
speed signal (NP_REF) and an estimated power turbine speed signal (NP_EST).
The outer
control loop 902 may also receive a signal indicative of a collective lever
angle command (CLA).
Based at least in part on the received signals, the outer control loop
determines a torque
request of the power turbine (QPT_REQ) that will bring NP_EST in line with
NP_REF. A signal
indicative of QPT_REQ is sent from the outer control loop 902 to an inner
control loop 904. The
inner control loop 904 may also receive a signal indicative of an estimated
power turbine torque
(QPT_EST) as well as other inputs, such as signals indicative of a gas
generator speed
maximum limit (NG_MAX), inlet guide vane and stability bleed schedules (IGV &
BLD
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schedules), and other limits. The inner control loop 904 determines a fuel
flow (WF) that will
deliver the power turbine torque request as determined by the outer control
loop 902.
When using the embodiment of Fig. 9 for the speed control loop 207, the
damping module 206
may be configured to apply a torque request limit to the outer control loop
902 or the inner
control loop 904, as the torque request is determined in the outer control
loop 902 and provided
to the inner control loop 904. The damping module 206 may be configured to
shape the speed
reference prior to having it received at the outer control loop 902, or to
instruct the outer control
loop 902 to shape the speed reference accordingly. The damping module 206 may
be
configured to apply the acceleration limit to the inner control loop 904 as
one of the limits input
thereto. The damping module 206 may be configured to apply the fuel flow limit
and/or the rate
of change of fuel flow limit to the inner control loop 904 as one of the
limits input thereto. Other
implementations are also considered.
With reference to Fig. 10, there is shown a flowchart illustrating a method
1000 for operating a
multi-engine aircraft, for instance the rotorcraft 100. In some embodiments,
the rotorcraft 100 is
a helicopter, which can comprise a plurality of engines which are configured
to provide motive
power to the rotorcraft, and at least one rotor coupled to the plurality of
engines, for example the
rotor 108.
At step 1002, the rotor 108 of the rotorcraft 100 is driven with at least a
first engine, such as
engine 102, while at least a second engine, such as engine 104, is de-clutched
from a
transmission clutch system that couples the rotor 108 and the second engine
104. This may
happen while the rotorcraft is inflight or when the rotorcraft is on the
ground, for example during
engine run-ups. Additional engines may also be driving the rotor 108 in
addition to the first
engine 102.
At step 1004, the second engine is instructed to accelerate to a re-clutching
speed. This
instruction may come, for example, from the engine controller 210, in response
to a change in
engine and/or aircraft parameters, such as a failure to the first engine 102
or to any other engine
driving the rotor 108. The instruction may also be triggered in response to a
pilot input for
increased power, for example via an increase to a power lever angle (PLA) in a
cockpit of the
rotorcraft. Other circumstances may also trigger the instructions to
accelerate to the re-clutching
speed.
At step 1006, the output shaft speed of the second engine is controlled during
its acceleration to
the re-clutching speed by applying a damping function to a speed control loop
of the second
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engine. As indicated above, the damping function may take various forms, such
as a limit
applied to an operating parameter of the re-clutching engine that causes the
output shaft speed
to slowdown on its approach to the re-clutching speed. The slowdown prevents a
speed
overshoot upon re-clutching, thus minimizing the torque applied to the
transmission clutch
system as the re-clutching engine engages the gearbox. The rapid engine run-up
followed by
dampened re-clutching can be achieved by the engine controller 210 using one
or a
combination of the different embodiments described herein. While the re-
clutching engine is
decoupled from the transmission clutch system, the rotor operating speed is
maintained by one
or more other engines, by the pilot manipulating the CLP, or via automatic
control of the CLP
through a flight control system.
In some embodiments, the method 1000 is performed by the FADEC of the
rotorcraft 100, or
any other suitable engine electronic controller, which can implement part or
all of the engine
controller 210. In some embodiments, a portion of the method 1000 is performed
by the FADEC
or other suitable engine electronic controller.
With reference to Fig. 11, the method 1000 may be implemented by a computing
device 1110,
which can embody part or all of the engine controller 210, the speed control
loop 207, and/or the
damping module 206. The computing device 1110 comprises a processing unit 1112
and a
memory 1114 which has stored therein computer-executable instructions 1116.
The processing
unit 1112 may comprise any suitable devices configured to implement the
functionality of the
engine controller 210 and/or the functionality described in the method 1000,
such that
instructions 1116, when executed by the computing device 1110 or other
programmable
apparatus, may cause the functions/acts/steps performed by the engine
controller 210 and/or
described in the method 1000 as provided herein to be executed. The processing
unit 1112 may
comprise, for example, any type of general-purpose microprocessor or
microcontroller, a digital
signal processing (DSP) processor, a central processing unit (CPU), an
integrated circuit, a field
programmable gate array (FPGA), a reconfigurable processor, other suitably
programmed or
programmable logic circuits, custom-designed analog and/or digital circuits,
or any combination
thereof.
The memory 1114 may comprise any suitable known or other machine-readable
storage
medium. The memory 1114 may comprise non-transitory computer readable storage
medium,
for example, but not limited to, an electronic, magnetic, optical,
electromagnetic, infrared, or
semiconductor system, apparatus, or device, or any suitable combination of the
foregoing. The
memory 1114 may include a suitable combination of any type of computer memory
that is
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located either internally or externally to device, for example random-access
memory (RAM),
read-only memory (ROM), compact disc read-only memory (CDROM), electro-optical
memory,
magneto-optical memory, erasable programmable read-only memory (EPROM), and
electrically-erasable programmable read-only memory (EEPROM), Ferroelectric
RAM (FRAM)
or the like. Memory 614 may comprise any storage means (e.g., devices)
suitable for retrievably
storing machine-readable instructions 616 executable by processing unit 1112.
The methods and systems for operating a rotorcraft as described herein may be
implemented in
a high level procedural or object oriented programming or scripting language,
or a combination
thereof, to communicate with or assist in the operation of a computer system,
for example the
computing device 1110. Alternatively, the methods and systems described herein
may be
implemented in assembly or machine language. The language may be a compiled or
interpreted
language.
Embodiments of the methods and systems described herein may also be considered
to be
implemented by way of a non-transitory computer-readable storage medium having
a computer
program stored thereon. The computer program may comprise computer-readable
instructions
which cause a computer, or more specifically the processing unit 1112 of the
computing device
1110, to operate in a specific and predefined manner to perform the functions
described herein,
for example those described in the method 1000.
Computer-executable instructions may be in many forms, including program
modules, executed
by one or more computers or other devices. Generally, program modules include
routines,
programs, objects, components, data structures, etc., that perform particular
tasks or implement
particular abstract data types. Typically the functionality of the program
modules may be
combined or distributed as desired in various embodiments.
The above description is meant to be exemplary only, and one skilled in the
art will recognize
that changes may be made to the embodiments described without departing from
the scope of
the present disclosure. Still other modifications which fall within the scope
of the present
disclosure will be apparent to those skilled in the art, in light of a review
of this disclosure.
Various aspects of the systems and methods described herein may be used alone,
in
combination, or in a variety of arrangements not specifically discussed in the
embodiments
described in the foregoing and is therefore not limited in its application to
the details and
arrangement of components set forth in the foregoing description or
illustrated in the drawings.
For example, aspects described in one embodiment may be combined in any manner
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aspects described in other embodiments. Although particular embodiments have
been shown
and described, it will be apparent to those skilled in the art that changes
and modifications may
be made without departing from this invention in its broader aspects. The
scope of the following
claims should not be limited by the embodiments set forth in the examples, but
should be given
the broadest reasonable interpretation consistent with the description as a
whole.
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