ARCHITECTURE OF A MUL Tl I:'NGINE HELICOPTER PROPULSION SYSTEM
AND CORRESPONDING HELICOPTER
1. Technical field of the invention
The invention relates to an architecture of a propulsion system of a multi-engine
helicopter, in particular a twin-engine or three-engine helicopter, and to a
helicopter comprising a propulsion system having an architecture of this kind.
2. Technological background
As is known, a twin-engine or three-engine helicopter has a propulsion system
comprising two or three turbos haft engines, each turboshaft engine comprising a
gas generator and a free turbine which is rotated by the gas generator and is
rigidly connected to an output shaft. The output shaft of each free turbine is
suitable for inducing the movement of a power transmission gearbox (referred to
in the following by the abbreviation PTG), which itself drives the rotor of the
helicopter which is equipped with blades having a variable pitch.
Each turboshaft engine is generally equipped with a starter-generator for the
initial start-up of the turbos haft engine and also for supplying power to the low DC
voltage onboard network (referred to in the following by the abbreviation OBN)
during flight. The OBN is generally connected to a device for storing low-voltage
electricity, for example a 28-volt storage battery.
There are also architectures in which the OBN is also supplied with power via an
auxiliary power unit (APU) and via an AC/DC converter.
There are also architectures in which the starter and generator functions of each
turboshaft engine are separate. In this case, the generator function is achieved
by taking power from the PTG (generally of 115 volts AC), followed by conversion
by an ACIDC converter.
Furthermore, it is known that, when the helicopter is in a cruise flight situation
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(i.e. when it is progressing in normal conditions, in AEO (all engines operative)
mode, during all the flight phases apart from transitional phases of take-off,
landing or hovering flight), the turboshaft engines operate at low power levels,
below their maximum continuous output (hereinafter MCO). In some
arrangements, the power provided by the turboshaft engines during a cruise flight
can be less than 50 % of the maximum take-off output (hereinafter MTO). These
low power levels result in a specific consumption (hereinafter SC), which is
defined as the relationship between the hourly fuel consumption by the
combustion chamber of the turboshaft engine and the power provided by said
turboshaft engine, which is approximately 30 % greater than the SC of the MTO,
and a reduction in the efficiency of the gas turbines.
In order to reduce this consumption during cruise flight (or during holding on the
ground for example), it is possible to stop one of the turbos haft engines and to
put it into a mode known as standby. The active engine or engines then operate
at higher power levels in order to provide all the necessary power, and therefore
at more favourable SC levels.
In the following, "economical flight phase" will denote a flight phase during which
at least one turboshaft engine is in standby mode, and "conventional flight phase"
will denote a flight phase during which none of the turboshaft engines are in
standby mode.
In FR1151717 and FR1359766, the applicants proposed methods for optimising
the specific consumption of the turboshaft engines of a helicopter by the
possibility of putting at least one turboshaft engine into a stable flight mode,
referred to as continuous flight mode, and at least one turboshaft engine into a
particular standby mode that it can leave in emergencies or in a normal manner,
according to need. A transition out of the standby mode is referred to as 'normal'
when a change in the flight situation requires the turboshaft engine in standby to
be activated, for example when the helicopter is going to transition from a cruise
flight situation to a landing phase. A normal transition out of standby mode of this
kind occurs over a period of between 10 seconds and 1 minute. A transition out
of the standby mode is referred to as an 'emergency' when a there is a power
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failure or a power deficit in the active engine, or when the flight conditions
suddenly become difficult. An emergency transition out of standby mode of this
kind occurs over a period of less than 10 seconds.
A turboshaft engine leaves a standby mode and transitions from an economical
flight phase to a conventional flight phase for example by means of an
emergency assistance device that comprises incandescent "glow-up" spark plugs
as a near-instantaneous ignition device, supplementing the conventional spark
plugs, and a propellant cartridge that feeds an auxiliary micro-turbine as a
mechanical means for accelerating the gas generator of the turbos haft engine.
Such a device for restarting the turboshaft engine in standby has the
disadvantage of substantially increasing the total weight of the turboshaft engine.
The benefit in terms of fuel consumption obtained by placing the turboshaft
engine in standby is thus partly lost by the excess weight brought about by the
restart device, in particular when each turboshaft engine is equipped with an
emergency restart device of this type.
The inventors have thus sought to solve problems which are incompatible a
priori, namely the possibility of placing the helicopter in the economical flight
phase, i.e. of placing at least one turboshaft engine in standby, without increasing
the weight of the overall propulsion system too much but whilst also allowing the
OBN to be supplied with electrical power.
In other words, the inventors have sought to propose a new architecture of the
propulsion system of a twin-engine or three-engine helicopter.
3. Aims of the invention
The invention aims to provide a new architecture of the propulsion system of a
multi-engine helicopter.
The invention also aims to provide an architecture of a propulsion system of a
multi-engine helicopter which allows a turboshaft engine to be placed in standby
4
and allows the quick restart thereof.
The invention also aims to provide, in at least one embodiment of the invention,
an architecture of a propulsion system which has a mass and a volume which are
acceptable for being installed in a helicopter.
The invention also aims to provide, in at least one embodiment of the invention,
an architecture of a propulsion system which has a lower cost than the
architectures from the prior art that have the same performance.
4. Disclosure of the invention
In order to achieve this, the invention relates to an architecture of a propulsion
system of a multi-engine helicopter, comprising turboshaft engines that are
connected to a power transmission gearbox (referred to in the following by the
abbreviation PTG), and comprising a low DC voltage onboard network (referred
to in the following by the abbreviation OBN) for supplying power to the helicopter
equipment during flight.
The architecture according to the invention is characterised in that it comprises:
one turboshaft engine among said turboshaft engines, referred to as a
hybrid turboshaft engine, which is capable of operating in at least one
standby mode during a stable flight of the helicopter, the other turboshaft
engines operating alone during this stable flight,
an electrotechnical pack for quickly restarting said hybrid turboshaft
engine in order to bring said engine out of said standby mode and to
reach a mode referred to as the nominal mode, in which it provides
mechanical power to said power transmission gearbox, said restart pack
being connected to said OBN,
at least two sources of electrical power for said OBN.
Therefore, the architecture of a propulsion system of a multi-engine helicopter
according to the invention has just one hybrid turbos haft engine that is capable of
operating in a standby mode. The architecture according to the invention is
5
therefore asymmetrical and has just one hybrid turboshaft engine. An
architecture according to the invention thus minimises the number of components
by providing that just one turboshaft engine can be put in standby mode. Only the
hybrid turboshaft engine is equipped with an electrotechnical restart pack, thus
limiting the total weight of the propulsion system.
Furthermore, the architecture provides two sources of electrical power for the
OBN. The architecture therefore has redundancy in the electrical power
generation for supplying power to the OBN, meaning that a possible failure of the
first power source for the OBN is compensated for by the second power source.
Advantageously and according to the invention, said electrotechnical pack for
quick restart is a high-voltage pack, and a low voltage-high voltage converter is
arranged between the OBN and the restart pack.
A high-voltage pack makes it possible to achieve a quick restart of the turboshaft
engine. In addition, a low voltage-high voltage converter is arranged between the
low-voltage OBN and the restart pack in order for it to be possible for the OBN to
supply power to the restart pack.
Advantageously and according to the invention, said sources of electrical power
for said OBN are selected from the group comprising:
at least one generator that is arranged between the PTG and the OBN
and is associated with an AC-DC converter,
an auxiliary power unit that is connected to the OBN and associated with
an AC-DC converter,
a starter-generator that is arranged between a non-hybrid turboshaft
engine and the OBN.
Advantageously and according to the invention, each generator and said auxiliary
power unit are capable of providing an AC voltage of 115 volts, and said
associated converter is capable of converting said 115-volt AC voltage into a DC
voltage of 28 volts.
L ..
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Any combination is possible in order to ensure redundancy in the supply of
electrical power for the OBN.
According to an advantageous variant, a first electrical power source is a
generator that is arranged between the PTG and the OBN and associated with
an AC-DC converter, and a second electrical power source is a starter-generator
that is arranged between a non-hybrid turboshaft engine and the OBN.
According to this advantageous variant, during what is known as a conventional
flight phase of a twin-engine helicopter (i.e. when all the turboshaft engines are
operating in a mode for providing mechanical power to the power transmission
gearbox}, all the turboshaft engines provide the PTG with the power required for
driving the rotor of the helicopter. The electrical generation of the OBN is made
reliable by the redundancy of the starter-generator on the non-hybrid turboshaft
engine and of the generator that is arranged on the PTG and associated with the
AC-DC converter.
During an economical flight phase, the hybrid turboshaft engine is in standby and
the other turboshaft engine provides the necessary power to the PTG. The
electrical generation of the OBN is made reliable by the redundancy of the
starter-generator on the non-hybrid turboshaft engine and of the generator that is
arranged on the PTG and associated with the AC-DC converter.
In the event of the loss of the non-hybrid turboshaft engine, the hybrid turboshaft
engine is emergency restarted by means of the electrotechnical restart pack.
Even though the non-hybrid turboshaft engine has stopped, the supply of power
to the OBN is ensured by the generator that is arranged on the PTG and
associated with the AC-DC converter.
Advantageously, the restart pack further comprises a device for storing highvoltage
electrical energy, which is capable of accumulating electrical energy from
the OBN during said nominal mode of said hybrid turboshaft engine, making it
possible to provide mechanical power to the power transmission gearbox, and
which is capable of providing, on demand, the accumulated electrical energy that
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is required for said hybrid turboshaft engine to leave standby mode.
The turboshaft engine in standby is thus restarted by means of the high-voltage
energy storage device which is arranged between the turboshaft engine and the
low voltage-high voltage converter.
Advantageously and according to the invention, said restart pack is capable of
providing, when said hybrid turboshaft engine is in the standby mode, electrical
energy for keeping said hybrid turboshaft engine in a predetermined standby
mode.
In particular, a turboshaft engine comprises, as is known, a gas generator and a
free turbine that is supplied with the gases from the gas generator. The gas
generator comprises a shaft and a combustion chamber that is supplied with fuel.
Advantageously and according to the invention, the standby mode can be one of
the following modes:
a standby mode referred to as normal idling, in which said combustion
chamber is ignited and said shaft of the gas generator rotates at a speed
of between 60 and 80 %of the nominal speed,
a standby mode referred to as normal super-idling, in which said
combustion chamber is ignited and said shaft of the gas generator rotates
at a speed of between 20 and 60% of the nominal speed,
a standby mode referred to as assisted super-idling, in which said
combustion chamber is ignited and said shaft of the gas generator
rotates, with mechanical assistance, at a speed of between 20 and 60 %
of the nominal speed,
a standby mode referred to as turning, in which said combustion chamber
is extinguished and said shaft of the gas generator rotates, with
mechanical assistance, at a speed of between 5 and 20 % of the nominal
speed,
a standby mode referred to as shutdown, in which said combustion
chamber is extinguished and said shaft of the gas generator is at a
complete stop.
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Each of the standby modes mentioned above can thus be assisted by the
electrotechnical pack in order to keep the hybrid turboshaft engine in a
predetermined standby mode. This electrical assistance can be taken directly
from the onboard network of the helicopter or taken from the energy storage
device. Preferably, the assistance is taken from the onboard network so that the
storage device keeps the maximum amount of energy in order for the hybrid
turboshaft engine to be brought out of standby, in particular in emergencies. For
example, in the case of the turning mode, the on board network can supply power
to a device for mechanically assisting the gas generator of the hybrid turboshaft
engine.
Advantageously and according to the invention, the quick restart pack comprises
an electrical machine that is capable of restarting said turboshaft engine when
leaving standby in normal conditions, and a device for leaving standby that is
capable of restarting said turboshaft engine when leaving standby in emergency
conditions.
According to this variant, the electrotechnical restart pack comprises an energy
storage device, an electrical machine and a device for leaving standby in an
emergency. This device for leaving standby in an emergency can be an
electrotechnical, pyrotechnic, pneumatic or hydraulic device.
A mode for leaving standby in an emergency is a mode in which the combustion
chamber is ignited and the shaft of the gas generator is brought to a speed of
between 80 and 105 % within a period of less than 10 seconds following an order
to leave standby.
A mode for leaving standby normally is a mode in which the combustion chamber
is ignited and the shaft of the gas generator is brought to a speed of between 80
and 105 % within a period of between 10 seconds and 1 minute following an
order to leave standby mode.
The invention also relates to a helicopter comprising a propulsion system,
characterised in that said propulsion system has an architecture according to the
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invention.
The invention also relates to an architecture of a propulsion system of a multi~
engine helicopter, and to a helicopter equipped with a propulsion system having
an architecture of this kind, characterised in combination by all or some of the
features mentioned above or in the following.
5. List of drawings
Other aims, features and advantages of the invention will emerge from reading
the following description, which is given purely by way of non~limiting example
and relates to the accompanying drawings, in which:
Fig. 1 is a schematic view of an architecture of a propulsion system of a
twin-engine helicopter according to an embodiment of the invention,
Fig. 2 is a schematic view of an architecture of a propulsion system of a
twin-engine helicopter according to a further embodiment of the invention,
Fig. 3a is a schematic view of the architecture from Fig. 1 during a
conventional flight phase, in which all the turboshaft engines are
operating in a mode for providing mechanical power,
Fig. 3b is a schematic view of the architecture from Fig. 1 during an
economical flight phase, in which one turboshaft engine is in standby
mode,
Fig. 3c is a schematic view of the architecture from Fig. 1 as the
turboshaft engine in standby is leaving standby in a normal manner,
Fig. 3d is a schematic view of the architecture from Fig. 1 during a phase
of leaving standby in an emergency following failure of the other
turbos haft engine.
6. Detailed description of an embodiment of the invention
Fig. 1 is a schematic view of an architecture of a propulsion system of a twin~
engine helicopter according to an embodiment of the invention. This architecture
comprises two turboshaft engines 1, 2 that are connected to a power
JO
transmission gearbox 3. Each turboshaft engine 1, 2 is controlled by its own
inspection-control device, which is not shown in the drawings for reasons of
clarity. The architecture further comprises a low DC voltage 28-volt onboard
network 7 intended for supplying current to various items of equipment of the
helicopter, which are not shown in the drawings for reasons of clarity.
Each turboshaft engine comprises a gas generator and a free turbine that is
rigidly connected to an output shaft rotated by the gas generator. The output
shaft of each free turbine is suitable for inducing the movement of the power
transmission gearbox 3 (referred to in the following by the abbreviation PTG),
which itself drives the rotor of the helicopter which is equipped with blades having
a variable pitch.
According to the invention, the turboshaft engine 1 is a hybrid turboshaft engine
that is capable of operating in at least one standby mode during a stable flight of
the helicopter.
This standby mode is preferably selected from the following operating modes:
a standby mode referred to as normal idling, in which the combustion
chamber is ignited and the shaft of the gas generator rotates at a speed
of between 60 and 80 % of the nominal speed,
a standby mode referred to as normal super-idling mode, in which the
combustion chamber is ignited and the shaft of the gas generator rotates
at a speed of between 20 and 60 % of the nominal speed,
a standby mode referred to as assisted super idling, in which the
combustion chamber is ignited and the shaft of the gas generator rotates,
with mechanical assistance, at a speed of between 20 and 60 % of the
nominal speed,
a standby mode referred to as turning, in which the combustion chamber
is extinguished and the shaft of the gas generator rotates, with
mechanical assistance, at a speed of between 5 and 20 % of the nominal
speed,
a standby mode referred to as shutdown, in which the combustion
chamber is extinguished and the shaft of the gas generator is at a
II
complete stop.
The architecture further comprises an electrotechnical pack 20 for quickly
restarting the hybrid turboshaft engine 1 in order to bring it out of the standby
mode and to reach a mode for providing mechanical power to the power
transmission gearbox. This pack 20 is a high-voltage pack that is arranged
between the turboshaft engine 1 and the OBN 7 by means of a high voltage-low
voltage converter 14.
According to the embodiment in the drawings, the electrotechnical quick restart
pack 20 comprises an electrical machine 5 that is capable of restarting the hybrid
turboshaft engine 1 when leaving standby in normal conditions (i.e. within a
period of between 10 seconds and 1 minute following the order for the turbos haft
engine 1 to leave standby). Said pack also comprises a device 6 for leaving
standby in an emergency that is capable of restarting the turboshaft engine 1
when leaving standby in emergency conditions (i.e. within a period of less than
10 seconds. following the order for the turboshaft engine 1 to leave stanaby). Said
pack also comprises an energy storage device 15 which is capable of
accumulating electrical energy provided by the OBN 7 when the hybrid turboshaft
engine 1 is in standby, and which is capable of providing the electrical machine 5
and the device 6 for leaving standby with the electrical energy that is necessary
for restarting the hybrid turboshaft engine 1.
An architecture according to the invention further comprises two sources of
electrical power for the OBN 7.
According to the embodiment in Fig. 1, the first source of power for the OBN 7 is
a generator 16 that provides an AC voltage of 115 volts. The generator 16 is
arranged between the PTG 3 and the OBN 7, and is associated with an AC-DC
converter 17. According to the embodiment in Fig. 1, the second source of power
for the OBN 7 is a starter-generator 4 that provides a DC voltage of 28 volts and
is arranged between the turboshaft engine 2 and the OBN 7. This startergenerator
4 is capable of ensuring the first start-up of the turbos haft engine 2 and
of ensuring the supply of power to the OBN 7 during flight.
12
According to the embodiment in Fig. 2, the first source of power for the OBN 7 is
always the generator 16 that is arranged between the PTG 3 and the OBN 7 and
associated with the A C-DC converter 17. In contrast, the second source of power
for the OBN 7 is an APU 18 that provides an AC voltage of 115 volts and is
connected to said AC-DC converter 17. In this embodiment, the starter-generator
4 of the architecture of Fig. 1 is replaced by a starter 40 of which the only function
is to start up the turboshaft engine 2.
According to other embodiments that are not shown in the drawings, other
combinations of sources of power for the OBN 7 can be used, such as two
generators arranged between the PTG 3 and the OBN 7.
Each architecture further comprises items of equipment 9 that are supplied with
high-voltage AC current directly by the generator 16 or by the APU 18. Said
architecture also comprises a low-voltage storage battery 8.
In the following, the operating principle of the architecture of Fig. 1 is explained in
detail with reference to Fig. 3a to 3d. In Fig. 3a to 3d, the bold lines show the
main power circuits (mechanical or electrical) that are active between the various
members shown.
Fig. 3a is a schematic view of the architecture from Fig. 1 during a conventional
flight phase, i.e. a flight phase during which both turboshaft engines 1, 2 provide
power to the PTG 3. This is, for example, a take-off or landing phase, during
which the helicopter needs to have available the total power of the engines. The
two power paths 21, 22 that are active between the turboshafl engines 1, 2 and
the PTG 3 are shown schematically by bold lines in Fig. 3a. The supply of
electrical power to the OBN 7 is made reliable by having two separate power
supply circuits. The first circuit 23 supplies power to the OBN 7 by means of the
starter-generator 4 of the turboshaft engine 2. The second circuit 24 supplies
power to the OBN 7 by means of the generator 16 that is associated with the A CDC
converter 17.
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__ , _;__._--- ---·---~-~c..-:~-
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Fig. 3b is a schematic view of the architecture from Fig. 1 during an economical
flight phase, Le. a flight phase during which the turboshaft engine 1 is put in a
standby mode such that only the turboshaft engine 2 provides power to the PTG
3. Thus, only the power path 22 is active. The supply of electrical power to the
OBN 7 is made reliable by the two power supply circuits described in connection
with Fig. 3a. The first circuit 23 supplies the OBN 7 by means of the startergenerator
4 of the turboshaft engine 2. The second circuit 24 supplies the OBN 7
by means of the generator 16 that is associated with the AC-DC converter 17.
The standby mode of the turboshaft engine 1 is assisted by the electrotechnical
pack 20, which keeps the turboshaft engine 1 in a predetermined standby mode.
This assistance is shown schematically in Fig. 3b by the circuit 25. This circuit
comprises a low voltage-high voltage converter 14 that is arranged between the
OBN 7 and the electrotechnical pack 20.
Fig. 3c is a schematic view of the architecture from Fig. 1 during a phase of
restarting the turboshaft engine 1 when leaving standby normally. The turboshaft
engine 2 provides power to the PTG 3. The power path 22 is therefore active.
The power path 21 is being activated. In order to do this, the electrical machine 5
ensures the start-up of the turbos haft engine 1 by using the energy stored in the
energy storage device 15. The supply of power to the electrical machine 5 is
shown schematically by the circuit 26 in bold in Fig. 3c. The supply of electrical
power to the OBN 7 is made reliable by the two power supply circuits described
in connection with Fig. 3a and 3b. The first circuit 23 supplies power to the OBN
7 by means of the starter-generator 4 of the turboshaft engine 2. The second
circuit 24 supplies power to the OBN 7 by means of the generator 16 that is
associated with the AC-DC converter 17.
Fig. 3d is a schematic view of the architecture from Fig. 1 during a phase in
which the turboshaft engine 2 has been lost and the turboshaft engine 1 is being
emergency restarted. During this phase, the turboshaft engine 2 has therefore
failed and no longer provides any power to the PTG 3. The power path 22 is
therefore inactive. The power path 21 is being activated. In order to do this, the
device 6 for leaving standby in an emergency ensures the emergency start-up of
the turboshaft engine 1. The activation of the device 6 for leaving standby in an
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14
emergency depends on the type of device used. A device 6 of this kind for
leaving standby in an emergency is, for example, the propellant device described
in FR 1358996 in the name of the applicant. The supply of electrical power to the
OBN 7 is ensured by the second circuit 24, which supplies the OBN 7 by means
of the generator 16. In contrast, the first circuit 23 no longer allows power to be
supplied to the OBN 7 by means of the starter-generator 4 in the event of failure
of the turbos haft engine 2.
The invention is not limited to the described embodiments only. In particular, the
architecture can comprise three turbos haft engines for the equipment of a threeengine
helicopter, while operating in a manner that is mutatis mutandis identical
to that described in connection with a twin-engine application.

CLAIMS
1. Architecture of a propulsion system of a multi-engine helicopter,
comprising turboshaft engines (1, 2) that are connected to a power transmission
gearbox (3), and comprising a low DC voltage onboard network (7) for supplying
power to helicopter equipment during flight,
characterised in that it comprises:
one turboshaft engine among said turboshaft engines, referred to as a
hybrid turboshaft engine (1 ), which is capable of operating in at least one
standby mode during a stable flight of the helicopter, the other turboshaft
engines (2) operating alone during this stable flight,
an electrotechnical pack (20) for quickly restarting said hybrid turboshaft
engine in order bring said engine out of said standby mode and to reach a
mode referred to as the nominal mode, in which it provides mechanical
power to said power transmission gearbox, said restart pack (20) being
connected to said onboard network (7),
at least two sources ( 4, 16, 18) of electrical power for said on board
network (7).
2. Architecture according to claim 1, characterised in that said
electrotechnical pack (20) for quick restart is a high-voltage pack, and in that a
low voltage-high voltage converter (14) is arranged between said onboard
network (7) and said rapid restart pack (20).
3. Architecture according to either claim 1 or claim 2, characterised in that
said sources (4, 16, 18) of electrical power for said onboard network (7) are
selected from the group comprising:
at least one current generator ( 16) that is arranged between said power
transmission gearbox (3) and said onboard network (7) and is associated
with an A C-DC converter (17),
an auxiliary power unit (18) that is connected to said onboard network (7)
and associated with an AC-DC converter (17),
a starter-generator (4) that is arranged between a non-hybrid turboshaft
engine (2) and said onboard network (7).
16
4. Architecture according to claim 3, characterised in that each current
generator (16) and said auxiliary power unit (18) are capable of providing an AC
voltage of 1 15 volts, and in that said associated converter ( 1 7) is capable of
converting said 1 15-volt AC voltage into a DC voltage of 28 volts.
5. Architecture according to any of claims 1 to 4, characterised in that said
electrotechnical pack (20) comprises an electrical energy storage device (15)
which is capable of accumulating electrical energy from said on board network (7)
during said nominal mode of said hybrid turboshaft engine (1 ), and which,
following a command to leave standby, Is capable of providing the hybrid
turboshaft engine (1) with the accumulated electrical energy that is necessary to
ensure restart thereof.
6. Architecture according to any of claims 1 to 5, characterised In that said
restart pack (20) is capable of providing, when said hybrid turboshaft engine (1)
is in standby mode, electrical energy for keeping said hybrid turboshaft engine {1)
in a predetermined standby mode.
7. Architecture according to any of claims 1 to 6, characterised in that said
quick restart pack {20) comprises an electrkal machine (5) that is capable of
restarting said turboshaft engine (1) when leaving standby in normal conditions,
and a device (6) for leaving standby in an emergency that is capable of restarting
said turboshaft engine (1) when leaving standby in emergency conditions.
8. Architecture according to claim 7, characterised in that said device (6) for
leaving standby in an emergency is selected from the group comprising an
electrotechnical device, a pyrotechnic device, a pneumatic device and a hydraulic
device.
9. Helicopter comprising a propulsion system, characterised in that said
propulsion system has an architecture according to any of claims 1 to 8.