Technical context
The invention lies in the field of assemblies
comprising a plurality of free-turbine engines, as are
5 commonly to be found on helicopters.
It should be recalled that a free-turbine engine
includes a power turbine or ''free turbine'' that, in a
helicopter, drives the rotors of the helicopter via an
overrunning clutch or "freewheel" and a main gearbox
10 (MGB), and also includes a gas generator constituted
mainly by a compressor, a combustion chamber, and a high
pressure (HP) turbine.
The shaft of the gas generator is connected by
stepdown gearing or an accessory gearbox to an electrical
15 machine constituted by a stator and a rotor, and capable
of operating equally well as a motor (starter) or as a
generator. In motor mode, the electrical machine is
powered by a source of electricity and it develops
driving torque so as to drive the gas generator of the
20 engine in rotation, particularly for the purpose of
starting it, thus providing assistance in starting. In
generator mode, the electrical machine is driven in
rotation by the gas generator in order to take mechanical
power off therefrom, which mechanical power is converted
25 into electrical power.
While an aircraft having two free-turbine engines is
in cruising flight, proposals have been made in Documents
FR 2 967 132 and FR 2 967 133 to put one of the two
engines in a standby mode so as to desynchronize its free
30 turbine from the MGB while increasing the power drawn
from the other engine, thereby enabling the overall fuel
consumption of the system to be reduced. Several
variants of that standby mode have been proposed.
In a first variant, referred to as ''super idle'' the
35 gas generator of the desynchronized gas turbine can be
regulated on a slow idling speed.
2
In a second variant, referred to as ''assisted super
idle", the gas generator of the gas turbine that is
desynchronized from the MGB can also be regulated on a
slow idling speed, while assistance driving torque is
5 simultaneously being applied to the gas generator via the
electrical machine and the accessory gearbox.
In a third variant, the combustion chamber of the
engine may be completely extinguished, and it is then
proposed to maintain the gas generator in rotation at a
10 speed that facilitates re-ignition at the end of the
stage of cruising flight. The appropriate range of
speeds may be called a "preferred ignition window". This
mode of operation, referred to as "turning" mode,
involves providing the gas generator with assistance that
15 is prolonged.
In these three modes of operation, which may be
maintained throughout the duration of cruising flight,
the power transmitted to the MGB by the engine on standby
is generally zero, and it is generally not possible to
20 draw power from its gas generator.
In the three variants mentioned above, it is
necessary to be able to reactivate the desynchronized
engine quickly, in particular in an emergency, e.g. in
the event of another engine failing, if there are three
25 or more engines in all, or indeed of the other engine
failing if there are two engines. That is why the gas
generator is kept rotating at a speed that facilitates
re-ignition in the system where the combustion chamber is
extinguished.
30 Maintaining the gas generator in rotation in the
preferred ignition window ("turning" mode) and providing
prolonged assistance to the gas generator while it is
regulated on an idling speed ("assisted super idle" mode)
both require relatively little power, but end up
35 requiring considerable energy, since the purpose of the
system lies in using it throughout a flight of long
duration. Among other solutions, FR 2 967 132 and
3
FR 2 967 133 propose using an electric starter powered by
a starter/generator connected to the gas generator of the
other engine, or a generator that is driven directly or
indirectly by the free turbine of the other engine.
5 As for emergency restarting from a low speed
situation or with the combustion chamber extinguished, it
is necessary to apply high power to the shaft of the gas
generator because of the considerable inertia of the
rotary assemblies and because of the opposing torque from
10 the compressor of the engine. That power needs to be
delivered during a short duration, of the order of a few
seconds, in order to enable the engine to restart
quickly. In FR 2 967 133 it is suggested, among other
possibilities, that electrical energy can be used, in
15 particular taken from a supercapacitor, to provide the
gas generator with a burst of assistance.
In Document EP 2 602 458, proposals are made to use
power taken from the power turbine of the first engine in
order to assist rotating the gas generator of the second
20 engine. Power is transferred by using two electrical
machines. It enables fuel consumption to be reduced.
The second engine is maintained in idle mode.
In Document EP 2 581 586, proposals are also made to
use two supercapacitors (which are members for storing
25 electrical energy), each of which is charged by a
respective electricity generator driven by the gas
generator of one of the two engines, and each of which
serves to provide a burst of energy for starting the
other engine from that engine being in an extinguished
30 state.
In Document FR 2 914 697, a burst of acceleration
assistance is given to the gas generator of an engine, in
particular by delivering mechanical power to the gas
generator via an electrical machine that is driven in
35 rotation by the free turbine. The system also operates
to provide assistance in deceleration.
--- --- --------- -------
4
In this context, the present invention seeks to
propose a structure making it possible both to deliver
power continuously to the gas generator of a first
turbine engine from the other turbine engine particularly
5 but not necessarily in the context of providing prolonged
assistance in rotating the gas generator of said first
engine in its preferred ignition window, and also to
propose the use of an electrical storage member that is
charged by one engine and that serves to provide a burst
10 of energy to the gas generator of the second engine when
restarting or assisting the acceleration of said second
engine. The structure may preferably, but not
necessarily, be independent of the on-board electricity
network (in particular it may be independent in terms of
15 electrical power supply, and it may also be electrically
isolated therefrom) and it may be easy to implement in
practice in an aircraft.
Description of the invention and the associated
20 advantages
For this purpose, there is provided an assistance
device for a free-turbine engine of an aircraft having at
least two free-turbine engines, the device comprising an
electrical starter machine and an electric generator
25 machine, the electrical starter machine providing
prolonged assistance to the gas generator of a first
engine using energy produced by the electric generator
machine driven by the second engine, the assistance
device further comprising at least one electricity
30 storage member electrically connected to said electrical
starter machine for providing a burst of assistance to
said gas generator, the assistance device further
comprising a first power converter and a second power
converter, the electrical starter machine being powered
35 by the first power converter that enables it to exchange
energy with the storage member for providing the burst of
assistance, and that transmits thereto the energy
5
supplied by the second power converter for the prolonged
assistance, the assistance device being characterized in
that it further comprises a computer for cutting off the
flow of fuel to the gas generator during a determined
5 period during the prolonged assistance and for
maintaining said gas generator at a reduced speed for
facilitating re-ignition of said gas generator.
In certain implementations, the above-mentioned
electrical machines may function equally well in motor
10 mode and in generator mode, in which case the
architecture may be symmetrical, each of the two engines
being capable of being assisted in turn. Nevertheless,
an asymmetrical architecture is also possible, with
assistance being provided for only one of the two
15 engines.
Because of this structure, it is possible with
limited weight and with a limited number of components to
install the various functions for burst assistance in
flight, for burst assistance on starting, and for
20 prolonged assistance for rotating the gas generator, such
as for example maintaining it in rotation in prolonged
manner in the absence of combustion in the combustion
chamber. It is also possible to start the engine in
conventional manner, or to provide it with dry motoring.
25 It should be recalled that dry motoring consists in
driving the gas generator in rotation at a low speed for
about ten seconds, while the fuel supply is cut off, so
as to use the stream of air generated by the compressor
to cool certain internal subassemblies of the engine, and
30 so as to clear from the combustion chamber any
accumulation of un-burnt fuel resulting from a failure to
ignite during starting.
The system with two power converters makes it
possible to manage exchanges of energy between the
35 electrical machine driven by the gas generator of the
first engine, which generally supplies alternating
current (AC), the electrical machine for providing
6
assistance to the gas generator of the second englne,
which is generally also an AC machine, and the storage
member, which may in particular deliver direct current
(DC). Thus, these two converters enable to energy
5 sources to be used that are not of the same nature (DC or
AC) or that do not have the same characteristics (low or
high voltage, different frequencies).
The device may include a bus, e.g. a high-voltage DC
bus between the electricity storage member and the first
10 converter, the bus being independent (electrically
isolated) from the electricity network of the aircraft.
Thus, the requirements of regulations concerning the onboard
network do not apply to this bus, and its voltage
may be different from the voltage of the on-board
15 network, and adapted for storing energy in the storage
member and also for optimizing the weight of the
electrical machines and the power converters.
Advantageously, the first converter is servocontrolled.
Thus, the first converter serves to control
20 the speed (frequency) and the torque applied to the
starter rotary machine (AC machine) .
Advantageously, a disconnector member (static,
electromechanical, or other) enables the two converters
to be electrically isolated from each other, the storage
25 member remaining connected to the first power converter.
Thus, the storage member may transmit energy solely to
the gas generator of the first engine, without applying
any to the second power converter.
Advantageously, the first electrical machine is also
30 a generator. This makes it possible to recharge the
electricity storage device with energy coming from the
first electrical machine via the first power converter.
Advantageously, a switch member enables the second
power converter to be connected to the electrical storage
35 member. This makes it possible to recharge the
electricity storage device with energy coming from the
second engine, via the second power converter.
7
Advantageously, with the help of the electricity
storage member, the device may be controlled, via the
first power converter, to provide a burst of assistance
in optionally accelerating or decelerating said gas
5 generator of the first engine ln controlled manner during
twin-engined flight. As set out in document
FR 2 914 697, this method makes it possible to improve
the transient performance of the engine, and thus to
reduce the amplitude of the drop or the increase in the
10 speed of the rotors of the aircraft resulting from a
rapid variation in the power demanded from said engine.
It is specified that in the event of a deceleration of
the gas generator, the burst of assistance involves
taking energy away, whereas in the event of the power·
15 generator being accelerated, the burst of assistance
involves delivering energy without taking any energy from
the on-board network of the aircraft.
20
Advantageously, the second power converter is
powered by an electrical machine operating as a generator
and driven by the gas generator of a second engine of the
aircraft. As a result of this technical option, an
assistance device is obtained that can be independent of
the on-board network, thereby significantly reducing
problems of electromagnetic disturbances, in particular
25 those conveyed by electric cables, simplifying
installation, and avoiding any need to overdimension the
on-board network, the electricity generation system, or
the battery of the aircraft. Furthermore, the assistance
device may be provided by the engine manufacturer in the
30 context of a design and certification process that is
distinct from the design and certification process for
the aircraft.
For example, the generator second electrical machine
also has a function of starting the second engine. Thus,
35 weight is saved and the number of devices needed is
decreased, and it is possible to implement conventional
starting of the first engine, or indeed dry motoring of
8
the engine, using the assistance device.
Advantageously, the device includes one storage
element per engine in order to participate in burst
accelerations of the gas generators of either of the
5 engines.
In addition to each of the two engines thus being
provided with a burst assistance function that is
specific thereto, the fact of having one storage element
per engine makes it possible to segregate electrically
10 the assistance devices of the two engines when they are
contributing simultaneously to the propulsion of the
aircraft (twin-engined flight condition) .
In certain embodiments, the prolonged assistance may
be performed during periods in which said gas generator
15 is in operation, the mechanical energy being adapted to
maintain said gas generator at a low speed that minimizes
fuel consumption. Under such circumstances, the computer
maintains the flow of fuel to the gas generator for a
determined period during prolonged assistance and it
20 reduces the speed of the gas generator so as to minimize
fuel consumption.
Finally, the electricity storage member may comprise
at least a supercapacitor, .a hybrid competitor, a
lithium-ion battery, or a flywheel optionally having an
25 integrated DC/AC power converter.
It is advantageously proposed that the electricity
storage member should be recharged by taking energy from
the gas generator of the first or second engine during
periods in which said gas generator is supplied with
30 fuel.
It is advantageously specified that the storage
member is more particularly designed to accept, without
damage, discharge sequences that are characterized by
high power and short duration, of the order of a few
35 seconds, and that in this respect it is dedicated solely
to the function of providing a burst of assistance to the
gas generator of the engine. In this context, it is used
9
in particular for normal starting functions, emergency
starting, and dry motoring, and also for assistance in
flight.
The invention also provides an aircraft having at
5 least two free-turbine engines, and including an
assistance device as mentioned.
10
15
The aircraft may be a helicopter.
The invention is described below with reference to
the accompanying figures.
List of figures
Figure 1 is an overall diagram of a system of the
invention, incorporated in the propulsion and electrical
systems of the aircraft.
Figure 2 is a more particular diagram of an
embodiment of the invention.
Figures 3 to 9 each show a stage or a mode of use of
the embodiment of Figure 2.
Figure 10 shows an alternative embodiment of the
20 invention.
25
Figure 11 shows another alternative embodiment of
the invention.
Figures 12 and 13 show two other alternative
embodiments of the invention.
Detailed description
With reference to Figure 1, the general electrical
architecture of an embodiment of the proposed system is
as follows.
30 The generation of electricity on the aircraft is
35
provided by at least two alternators ALT1 and ALT2 driven
by the main gearbox MGB, typically three-stage type
115 VAC/400 Hz machines, although other rotary machines
could be envisaged.
This architecture is advantageous in the context of
single-engined low-cost cruising flight since it
guarantees functional and organic independence between
10
electricity generation and the operation of the turbine
engines, thus making it possible to conserve a sufficient
level of availability and redundancy for generating
electricity during low-cost cruising flight, while one of
5 the two engines is being maintained in a standby mode
that is incompatible with taking any power from its gas
generator.
Furthermore, this architecture is less penalizing
for the operation of the engines than taking power from
10 the gas generators of the engines, in particular in terms
of impact on acceleration and specific consumption
performance, insofar as the mechanical power
corresponding to the electrical power consumed by the onboard
network of the aircraft is taken from the free
15 turbine and not from the gas generator.
ALTl and ALT2 power the electricity network of the
aircraft, other available energy sources for powering
this network possibly being constituted by an on-board
auxiliary power unit APU, one or more storage batteries,
20 or indeed, when on the ground, a ground power unit.
The main gearbox MGB is driven by the engines GTl
and GT2. In this example they are free-turbine
turboshaft engines. Each of them has a gas generator and
a power turbine (free turbine) driving the MGB via a
25 freewheel.
Each engine GTl and GT2 has a respective rotary
machine (respectively G/31 and G/32) suitable for
operating as a starter and as a generator, and in the
embodiment described mechanically connected to the gas
30 generator of the engine via an accessory gearbox. In
order to optimize the compactness and the weight of the
device, it is preferred for G/31 and G/32 to present a
machine architecture that is compatible with being driven
at high speed by the gas generator, and thus without a
35 rotor winding, such as for example and in non-exhaustive
manner a brushless synchronous machine with permanent
magnets, a variable reluctance machine, or an
11
asynchronous machine. The two machines G/S1 and G/S2 are
included in an independent electrical assembly 100 that
operates independently of the electricity network of the
aircraft.
5 With reference to Figure 2, the independent
electrical assembly 100 comprises the following
subassemblies.
Firstly there are two buses, Bus No. 1 and Bus No. 2
operating with direct current (DC) and at high voltage
10 (of the order of several hundreds of volts), which buses
are independent of each other and of the on-board
network.
The independent electrical assembly further
comprises two reversible DC/AC static power converters
15 SPCl and SPC2 (e.g. of the two level inverter type or of
some other type) that serve in particular to power and
control the rotary machines G/S1 and G/S2 in torque and
in speed. While the electrical machine is being driven
by the gas generator, each of these converters is capable
20 of operating as a controlled rectifier and of regulating
the voltage of the corresponding bus.
The independent electrical assembly 100 further
comprises electricity storage members S1 and S2 that are
optimized for delivering short and intense discharges of
25 power. By way of example, they may be supercapacitors,
or hybrid capacitors (possibly fitted with their own
control system), lithium ion (Li-ion) batteries fitted
with their own battery management systems (BMSs) or a
flywheel (with its loading/unloading AC/DC converter).
30 The independent electricity assembly 100 also
includes an electrical disconnector member 120 of the
electromechanical contactor type or of the solid sate
power controller (SSPC) type serving to connect together
the two DC buses (in parallel), and conversely to isolate
35 electrically on one side the assembly S1, Bus 1, SPC1,
G/S1, GT1 and on the other side the assembly S2, Bus 2,
SPC2, G/S2, GT2.
12
With reference to Figure 3, there follows a
description of a stage of twin-engined flight.
Once both engines GTl and GT2 have started, the two
electrical machines G/Sl and G/S2 are driven by the gas
5 generators of the engines GTl and GT2 and they operate in
generator mode, with the DC/AC converters being
controlled as rectifiers in application of an appropriate
current/voltage relationship for the purpose of
recharging and/or maintaining the charge in the storage
10 members Sl and S2. The disconnector member 120 is open
circuit.
Since the storage members might possibly have been
discharged while starting the engines on the ground, it
may be necessary to wait for the storage members Sl and
15 S2 to be charged once more to their nominal level prior
to authorizing takeoff.
Recharging or maintaining charge of the storage
members Sl and S2 (in order to compensate for internal
losses e.g. due to balancing cells in a pack of
20 supercapacitors or a battery, or indeed due to friction
in a flywheel) is performed in a manner that is
independent from the on-board network by taking energy
from the gas generators of the respective engines GTl and
GT2. Depending on their technology, compensation for
25 losses in each storage member Sl and S2 may represent no
more than a few tens of watts under steady conditions.
In this mode of operation, the DC buses are
electrically isolated from each other and they operate
independently of each other.
30 Furthermore, in a variant, sequential management of
the recharging of the storage members is implemented
using a hysteresis type relationship: the member is
charged up to an energy threshold El, and then recharging
is inhibited (i.e. no more power is taken from the gas
35 generator) until the energy stored drops - as a result of
internal losses - to below a threshold E2 that is less
than El. With battery or hybrid capacitor type
13
technologies involving active balancing circuits, quite
long pauses are achieved between two recharging stages.
With reference to Figure 4, in twin-engined flight,
the energy stored in the members Sl and S2 is used to
5 optimize the operation of the engines. The general idea
is to provide a transient input of mechanical energy to
the gas generator of one of the engines. Two modes of
operation can then be envisaged, and they are described
in detail below. They are described for operation while
10 the disconnector member 120 is open circuit.
Firstly, it is possible to deliver a burst of
assistance for accelerating the gas generator. In the
event of pitch being increased quickly from a low speed,
delivering mechanical power to the gas generator serves
15 to improve its acceleration, and therefore increase the
speed with which the engine delivers power to the free
turbine, and consequently significantly decrease the
transient drop in the speed of rotation of the rotor of
the helicopter as occurs at the end of such a maneuver,
20 thus increasing the safety margin of the crew.
This function provides a significant improvement to
performance when the initial speed of the gas generator
is slow. This mode of operation may be activated
automatically at the request of the turbine computer when
25 various criteria are satisfied, e.g. and in non-limiting
manner: the system is available (no failure detected),
the level of energy stored in the members Sl and S2 is
sufficient, the engine is operating, the initial speed of
the gas generator lies in a given range, and a rapid
30 increase in power demand is detected as a result of the
operating line of the engine approaching its limit for
protection against surging.
The surge of assistance is deactivated when the
operating line of the engine moves away from its limit
35 for protection against surging, when the level of energy
stored drops below a certain threshold, or when the speed
exceeds a certain threshold.
14
Thereafter, it is possible to deliver a burst of
assistance for decelerating the gas generator. In the
event of a rapid reduction in pitch during which
deceleration of the gas generator is limited by the anti-
5 flameout relationship, it is proposed to increase the
generation setpoint of the converters for a few instants
so as to take off a large amount of power from the gas
generators. As a result, it is possible to decelerate
the gas generator more quickly, and thus increase the
10 speed with which regulation of the engine decreases the
flow rate of fuel injected into the combustion chamber
and consequently decrease the amplitude of the transient
increase in the speed of the rotor.
This mode of operation is activated automatically at
15 the request of the turbine computer when various criteria
are satisfied, for example and in non-limiting manner:
the operating line comes close to the anti-flameout flow
rate limit, and providing there is the capacity available
for storing the energy that is taken from the gas
20 generator while it is being decelerated.
In a variant, the capacity of the storage member is
thus slightly overdimensioned in order to ensure under
all circumstances that there is a margin for storing
energy.
25 In alternative manner, a device may be added to the
DC bus for dissipating the energy taken to decelerate the
gas generator, such as for example an assembly made up of
a resistance element and a braking chopper arm.
It is also possible to provide a burst of assistance
30 of the power injection type. Mechanical power Pmec is
injected to the gas generator so as to obtain an
increased effect on the free turbine. In certain zones
of the flight envelope, it is possible to recover power·
K.Pmec from the free turbine, and thus from the MGB of the
35 helicopter with gain K greater than l. It should be
observed that when appropriate conditions are present,
delivering assistance to the gas generator can thus be
15
more efficient than delivering assistance that is
equivalent except that it is injected directly to the
free turbine or to the MGB. This mode of operation may
be activated when the energy stored in the members S1 and
5 S2 is sufficient, either in preventative manner on
request of the crew, e.g. taking off with high load
and/or at high altitude and/or at high temperature, or
else automatically at the request of the engine computer
in order to provide additional power for a short duration
10 at the one engine inoperative (OEI} contingency rating,
e.g. in the event of the speed of the free turbine
dropping below a certain threshold or on detecting a loss
of power from the other engine.
With reference to Figure 5, there can be seen a
15 single-engined low-cost cruising stage of flight.
When the conditions for allowing this mode are
present, which implies amongst other things that there is
sufficient energy stored in the members S1 and/or S2, the
avionics sends a go-to-standby instruction to the
20 computer of the gas turbine (e.g. GT2 in the figure}.
In a first variant as shown in Figure 5, referred to
as the "super idle" variant, the computer of GT2 reduces
the flow rate of fuel and regulates the speed of the gas
generator to a low setpoint value, enabling the power
25 turbine to be desynchronized from the MGB (so the power
delivered to the helicopter is then zero} and enabling
fuel consumption to be low. Simultaneously, the
electrical machine G/S2 and the associated converter SPC2
are inhibited so as to avoid taking power from the gas
30 generator of GT2.
The electrical machine G/S1 and its converter SPC1
pass to ''generator'' mode (if they were not already
there}; the two DC buses are then electrically connected
together by reconfiguring the disconnector member 120.
35 Energy taken from the gas generator of GT1 is used for
maintaining charge in the storage members S1 and S2: this
charge-maintaining function may be performed continuously
16
or else discontinuously and sequentially on each of the
two members.
In a second variant, as shown in Figure 6, the
computer of the engine GT2 cuts off the flow of fuel and
5 regulates the speed of the gas generator on a setpoint
value. Since the combustion chamber of GT2 is
extinguished, fuel consumption is zero and the free
turbine becomes desynchronized with the MGB.
Simultaneously, the electrical machine G/S2 and the
10 associated converter SPC2 pass into motor mode with the
speed setpoint as defined by the regulator and
corresponding to the ideal ignition window for the
combustion chamber. The gas generator passes into
autorotation, and after a few seconds, its speed
15 stabilizes on this setpoint, the combustion chamber being
extinguished. The electrical machine G/Sl and its
converter SPC1 pass into generator mode, if they were not
already there.
The two DC buses are electrically connected together
20 by reconfiguring the disconnector member 120. Energy
taken from the gas generator of the engine GT1 is used to
maintain charge in the storage members S1 and S2 and to
power the electrical machine G/S2 via the power converter
SPC2. This aspect constitutes prolonged assistance to
25 the gas generator of the engine GT2, and is referred·to
as "turning" mode.
In a variant shown in Figure 7, the computer
maintains the supply of fuel to the engine GT2, and it is
provided with prolonged assistance in rotating its gas
30 generator, on the same principle as that described with
reference to Figure 6. For this purpose, the computer
regulates the speed of the gas generator to a setpoint
value so as to optimize the operation of the turbine and
so as to minimize fuel consumption. In such a mode of
35 operation, referred to as ''assisted super idle'', SPC2 and
G/S2 operate in motor mode.
5
17
During these stages of operation, the electrical
assembly 100 remains independent from the on-board
network.
Single-engined low-cost cruising flight mode can be
exited in two different ways. Firstly, with reference to
Figure 8, when restarting GT2 is not urgent, it is
started at the request of the avionics using the normal
procedure: initially, the two DC buses are electrically
isolated from each other by reconfiguring the electrical
10 disconnector member 120.
If the engine GT2 was initially on standby with its
combustion chamber ignited ("super idle" or "assisted
super idle" mode), the electrical machine G/S2 is
controlled so as to deliver driving torque to provide a
15 burst of assistance using energy stored in the storage
member S2 to accelerate the gas generator.
Simultaneously, the computer of the engine GT2 increases
the fuel flow rate in application of a predefined
relationship. If the engine GT2 was initially on standby
20 with the combustion chamber extinguished ("turning"
mode), the computer initiates a starting sequence
analogous to that described above except that the gas
generator of the engine GT2 is already being driven in
the ideal ignition window. When ignition of the
25 combustion chamber is detected, the torque delivered by
the electrical machine G/S2 is increased and the computer
of the engine GT2 increases the flow rate of fuel in
application of a predefined relationship. Either way,
when the speed NG exceeds a sustainable threshold, the
30 electrical assistance is disconnected and the engine GT2
accelerates by its own means up to flight speed.
It should be observed that an analogous sequence can
be used for starting the engine while the helicopter is
on the ground, before takeoff, except that the gas
35 generator of each engine is initially fully stopped. The
engines are usually started sequentially, one after t.he
other. Once both engines have started, and before
18
takeoff, the storage members Sl and S2 are recharged
using the procedure describe above (see Figure 3).
With reference to Figure 9, under certain conditions
of single-engined low-cost flight, the crew might require
5 power from the engine GT2 quickly: this can happen for
example in the event of power being lost from the engine
GTl, or in the event of an unexpected power need
requiring power from both engines and thus justifying a
quick exit from the single-engined mode (avoiding an
10 obstacle, etc.). Under such circumstances, restarting is
performed using the emergency procedure, the purpose
being to cause the engine GT2 to reach its flight speed
or even its OBI rating in a short period of time.
Initially, the two DC buses are electrically
15 isolated by reconfiguring the electrical disconnect
member 120. If the engine GT2 was initially on standby
with its combustion chamber ignited ("super idle" or
"assisted super idle" mode), then the electrical machine
G/S2 is operated so as to deliver driving torque in order
20 to provide a burst of assistance in accelerating the gas
generator, this assistance being at a level that is
substantially higher than for the normal restarting
procedure. Simultaneously, the computer of the engine
GT2 increases the fuel flow rate in application of a
25 predefined relationship, likewise optimized for fast
restarting of the turbine.
If the engine GT2 was initially on standby with the
combustion chamber extinguished ("turning" mode), the
computer triggers ignition of the combustion chamber,
30 with this operation being made easier by the fact that
the gas generator is already being driven in rotation in
the ideal ignition window. Thereafter, as above, the
computer proceeds to request a burst of electrical
assistance for accelerating the gas generator and it
35 increases the flow rate of fuel in application of a
predefined relationship, likewise optimized for fast
restarting of the turbine.
19
In both situations, the burst of electrical
assistance to the gas generator may be extended beyond
the starter cut-off speed threshold used in the normal
starting procedure in order to minimize the time taken by
5 the engine to accelerate to its flight or OEI rating.
Once the helicopter is on the ground, before
switching off the turbines, it may be advisable to
recharge the storage members of the electrical
hybridizing device so that they are ready for subsequent
10 starting. This procedure may be performed during the
required passage to the ''ground idle" speed used for
balancing temperatures in the engines before stopping
them.
15
A variant is described with reference to Figure 10.
The independent electrical assembly 101 is similar
to the independent electrical assembly 100 as described
above, but the storage members S1 and S2 are replaced by
a single storage member S. By way of example, it is
dimensioned so as to be capable of emergency starting a
20 single engine. The advantage is then a saving of almost
two in terms of weight and compactness. In recharging
mode (twin-engined operation), one of the two power
converters SPC1 or SPC2 is specified as the "master" by
the supervisor computer and is in charge of recharging
25 the storage member S. The reconfiguration member 121
enables the storage member S to be connected to the
converter SPC2 and enables the assembly S-SPC2-G/S2 to be
electrically isolated from the assembly SPC1-G/S1, or on
the contrary, it enables the storage member S to be
30 connected to the converter SPC1 and the assembly S-SPC1-
G/S1 to be electrically isolated from the assembly SPC2-
G/S2 for the stages of charging the member S with one or
the other of the engines or of providing a burst of
assistance to an engine.
35 The reconfiguration member 121 is also capable of
keeping the assembly SPC2(G/S2)-SPC1(G/S1) electrically
connected together for single-engined stages of flight
5
20
involving powering one of the electrical machines
operating as a motor by the other electrical machine
operating as a generator (''turning'' mode and ''assisted
super idle" mode).
Another variant is described with reference to
Figure 11.
In this example, the electrical assembly 102 is not
independent from the on-board network. Electrical
connection between electrical machine G/Sl and the
10 converter SPCl takes place via the on-board network.
There is only one electrical storage member S and it is
dedicated to providing a burst of assistance to the
engine GT2 via the converter SPC2 and the electrical
machine G/S2. It may be charged by the converter and the
15 electrical machine G/Sl or by the converter SPC2 and the
electrical machine G/S2, in particular as a function of
the position of the reconfiguration member 122. The
engine GTl is not put into a standby mode during low-cost
cruising flight. In contrast, the engine GT2 may be put
20 into a standby mode with its combustion chamber ignited
("assisted super idle" mode) or with its combustion
chamber extinguished ("turning" mode), with the
electrical energy needed for prolonged assistance to the
gas generator then coming from the engine GTl via G/Sl,
25 SPCl, SPC2, ond G/S2, or via ALTl, SPCl, SPC2, and G/S2
(see ALTl in association with Figure 1). In this second
example, G/Sl may be replaced by a simple, non-controlled
starter.
The reconfiguration member 122 enables the storage
30 member s to be connected to the converter SPC2 and
enables the assembly S-SPC2-G/S2 to be electrically
isolated from the assembly SPCl and on-board network, or
on the contrary enables the storage member S to be
connected to the converter SPCl and the assembly S-SPCl-
35 G/Sl to be electrically isolated from the assembly SPC2-
G/S2 for the stages of charging the member S or of
providing a burst of assistance to an engine.
21
Another variant is described with reference to
Figure 12. The engine GT1 has an accessory board
including motion takeoffs for two electrical machines,
specifically a starter electrical machine 01 and a
5 generator electrical machine G1. The machine 01, which
is used for normal starting of the engine GT1, is powered
by the on-board network, while the machine G1 is
connected to the converter SPC1. The remainder of the
electrical circuit is similar to that of Figure 9. The
10 single storage member S is dedicated to providing a burst
of assistance to the engine GT2.
The reconfiguration member 123 enables the storage
member S to be connected to the converter SPC2 and
enables the assembly S-SPC2-G/S2 to be electrically
15 isolated from the assembly SPC1-G1, or on the contrary it
enables the storage member S to be connected to the
converter SPC1 and the assembly S-SPC1-G/S1 to be
electrically isolated from the assembly SPC2-G/S2-GT2 for
the stages of charging the member S by one or other of
20 the engines or of providing a burst of assistance to the
engine GT2.
The reconfiguration member 123 is also capable of
keeping the assembly SPC2(G/S2)-SPC1(G1) connected
together for the stages of flight that involve powering
25 the electrical machine G/S2 by the electrical machine G1.
30
The assembly constituted by the elements G1, SPC1,
123, S, SPC2, and G/02 is an independent electrical
assembly referenced 103.
board network.
It is independent from the on-
Another variant is described with reference to
Figure 13.
The electrical assembly 104 comprises a converter
SPC1 connected to the on-board network.
It also comprises a converter SPC2 connected to
35 switch members 130 for connecting it either to the
electrical machine G/Sl connected to the engine GT1, or
to the electrical machine G/S2 connected to the engine
22
GT2. The two electrical machines G/Sl and G/S2 must not
both be connected at the same time to the converter SPC2.
A configuration member 124 also serves to connect
the sole storage member S to the converter SPCl in order
5 to be charged by the on-board network, or to the
converter SPC2 to provide a burst of assistance to one of
the engines GTl and GT2, as a function of the position of
the switch member 130. The storage member S can also be
connected to both converters SPCl and SPC2
10 simultaneously.
The reconfiguration member 104 enables the converter
SPCl to be connected via the switch member 130 to the
converter SPC2 in order to provide prolonged assistance
to the gas generator of the engine connected to the
15 converter SPC2 (in particular in standby mode with the
combustion chamber extinguished while the gas generator
is maintained in the preferred ignition window, i.e. in
"super idle" mode, and in standby mode with the chamber
ignited, i.e. in "assisted super idle" mode).
20 It should be recalled that the on-board network is
powered by one or more alternators that are driven
indirectly or directly by at least one of the engines GTl
or GT2, and that when one 0 r the other of them is
extinguished, it is necessarily the other one that
25 provides power in prolonged manner to the on-board
network.
30
The invention is not limited to the embodiments
described, but extends to all variants within the ambit
of the scope of the claims.

CLAIMS
1. An assistance device (100; 101; 102; 103; 104) for a
free-turbine engine (GTl) of an aircraft having at least
two free-turbine engines (GTl, GT2), the device
5 comprising an electrical starter machine (Dl) and an
electric generator machine (G2), the electrical starter
machine (Dl) providing prolonged assistance to the gas
generator of a first engine (GTl) using energy produced
by the electric generator machine (G2) driven by the
10 second engine (GT2), the assistance device further
comprising at least one electricity storage member (S1;
S) electrically connected to said electrical starter
machine (D1) for providing a burst of assistance to said
gas generator, a first power converter (SPC1) and a
15 second power converter (SPC2), the electrical starter
machine (01) being powered by the first power converter
(SPC1) that enables it to exchange energy with the
storage member (Sl; S) for providing the burst of
assistance, and that transmits thereto the energy
20 supplied by the second power converter (SPC2) for the
prolonged assistance, the assistance device being
characterized in that it further comprises a computer for
cutting off the flow of fuel to the gas generator during
a determined period during the prolonged assistance and
25 for maintaining said gas generator at a reduced speed for
facilitating re-ignition of said gas generator.
2. An assistance device according to claim 1, wherein a
disconnector member enables the two converters (SPCl,
30 SPC2) to be electrically isolated from each other, the
storage member (S1; S) remaining connected to the first
converter (SPCl).
3. An assistance device according to claim 1 or claim 2,
35 wherein the first electrical machine (Gl/Sl) is also a
generator.
24
4. An assistance device according to any one of claims 1
to 3, wherein the second electrical machine (G2) is
driven by the gas generator of the second engine (GT2)
5 5. An assistance device according to any one of claims 1
to 4, wherein a switch member (120; 121; 122; 123; 124)
enables the second converter (SPC2) to be connected to
the electrical storage member (Sl; S).
10 6. An assistance device according to any one of claims 1
to 5, wherein the second converter (SPC2) is powered by a
generator electrical machine (G2) driven by the gas
generator of a second engine (GT2) of the aircraft.
15 7. An assistance device according to any one of claims 1
to 6, wherein the electricity storage member (Sl; S) can
be used, where appropriate, for assisting in controlled
acceleration or deceleration of said gas generator under
twin-engined flight conditions.
20
8. An assistance device according to any one of claims 1
to 7, including one storage element (Sl, S2) per engine
(GTl, GT2) in order to participate in burst accelerations
of the gas generators of either of the engines (GTl,
25 GT2).
9. An assistance device according to any one of claims 1
to 8, wherein the computer maintains the flow of fuel to
the gas generator for a determined period during
30 prolonged assistance and it reduces the speed of the gas
generator so as to minimize fuel consumption.
10. An assistance device according to any one of claims 1
to 9, wherein the electricity storage member (Sl; S)
35 comprises a supercapacitor, a hybrid competitor, a
lithium-ion battery, or a flywheel having an integrated
DC/AC converter.
25
11. An assistance device according to any one of claims 1
to 10, wherein the electricity storage member (S1; S) is
recharged by taking energy from the gas generator of the
5 first or second engine (GT1; GT2) during periods in which
said gas generator is supplied with fuel.
12. An aircraft having at least two free-turbine engines,
and including an assistance device according to any one
10 of claims 1 td 11.