Aircraft propulsion assembly and thermal management method
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- SAFRAN HELICOPTER ENGINES
- Filing Date
- 2024-07-02
- Publication Date
- 2026-05-13
AI Technical Summary
Aircraft propulsion systems with multiple engines face challenges in maintaining engine temperature during standby modes, particularly at low altitudes or in cold weather, which can lead to icing and lubrication issues, making it difficult to restart the engines efficiently.
A propulsion assembly with a heat exchanger in one engine connected via an air interconnection duct to a second engine, allowing heat from the first engine to be transferred to the second engine, even when it's in standby mode, to maintain temperature and facilitate defrosting and re-ignition.
This solution effectively maintains engine temperatures during standby modes, preventing icing and ensuring smooth operation by providing sufficient heat for lubrication and re-ignition, thus enhancing the reliability and efficiency of the propulsion system.
Smart Images
Figure FR2024050880_16012025_PF_FP_ABST
Abstract
Description
Description Title of the invention: Aircraft propulsion system and thermal management method Technical Field
[0001] The present invention relates to the field of aeronautical propulsion and in particular to engine assemblies for aircraft comprising at least two engines such as turboshafts or turboprops. In particular, the invention relates to the thermal management of the components of a propulsion assembly for aircraft comprising at least two engines, and a method for thermal management of such a propulsion assembly. Prior art
[0001] Climate change is a major concern for many legislative and regulatory bodies around the world. Indeed, various restrictions on carbon emissions have been, are being, or will be adopted by various states. In particular, an ambitious standard applies both to new types of aircraft and those in circulation requiring the implementation of technological solutions in order to make them compliant with current regulations. Civil aviation has been mobilizing for several years now to make a contribution to the fight against climate change.
[0002] Technological research efforts have already led to very significant improvements in the environmental performance of aircraft. The Applicant takes into consideration the impact factors in all phases of design and development to obtain less energy-intensive, more environmentally friendly aeronautical components and products whose integration and use in civil aviation have moderate environmental consequences with the aim of improving the energy efficiency of aircraft.
[0003] Consequently, the Applicant is constantly working to reduce its negative climate impact through the use of methods and the use of virtuous development and manufacturing processes and minimizing greenhouse gas emissions to the minimum possible to reduce the environmental footprint of its activity.
[0004] The search for minimizing polluting emissions linked to air transport involves, in particular, improving all the efficiencies of propulsion systems, and more particularly the propulsive efficiency which characterizes the efficiency with which the energy used is converted into useful thrust.
[0005] For reasons of redundancy, among other things, it is common in the aeronautical field to equip aircraft with at least two engines. In particular, it is common to equip rotary-wing aircraft such as helicopters or convertiplanes with propulsion systems of at least two engines, typically gas turbine engines, which can be mechanically coupled together. Such multi-engine propulsion systems allow operation in economical mode, known as "ECO" mode. ECO mode is an operating mode, generally corresponding to a cruising flight phase, of a twin-engine architecture, in which one or more of the engines of the propulsion system are put into standby mode to provide very reduced power, or even zero power, with one or more other engines of the propulsion system then providing a large part, or even all of the power supply.
[0006] As proposed, in particular in French patent applications FR 2 967 132 A1 and FR 2 967 133 A1, the at least one engine in standby mode can be switched off. In order to accelerate restarting, in particular to allow a possible emergency start to replace or support another faulty power source, it has also been proposed, in these disclosures, to keep one shaft of each switched-off thermal engine rotating, for example with an electric machine. It is nevertheless alternatively conceivable that the standby mode is a super-idle regime at very low speed, for example at a rotation speed of less than 40% of the nominal rotation speed, without switching off.
[0007] However, in such a standby mode, the heat engine generates little or no heat and, depending on atmospheric conditions, particularly at altitude or in cold weather, its temperature is likely to drop rapidly to very low values, with risks of icing, difficulty in re-igniting and / or loss of lubrication. Indeed, these engines typically need adequate lubrication before being able to deliver power, which imposes a minimum temperature of the lubricant, usually of the order of 273 K to 278 K depending on the lubricant. Thus, in French patent application FR 3 01 1 277 A1, it was proposed to provide at least one heat source to warm the lubricant of the heat engine maintained in standby mode.More specifically, this document has proposed using as a heat source the electric machine driving a shaft of the heat engine in standby mode and / or the power supply of this electric machine. However, it may be appropriate to find other heat sources that are more efficient and / or simpler to integrate into the heat engine intended to be put into standby mode and / or into its lubrication circuit. Statement of the invention
[0008] This disclosure is the result of technological research aimed at significantly improving the performance of aircraft and, in this sense, contributes to reducing their environmental impact, particularly in terms of energy consumption and greenhouse gas emissions. For this purpose, a first aspect of this disclosure relates to an aircraft propulsion unit, comprising a first engine with a heat exchanger traversed by an air duct, and a second engine, which is a heat engine, as well as an air interconnection duct connecting the air duct, downstream of the heat exchanger of the first engine, to the second engine.
[0009] By means of the arrangement of the interconnecting duct, heat released by the first engine can be supplied, through the heat exchanger of the first engine and the air flowing in the air duct and the air interconnecting duct, to the second engine in order to maintain its temperatures even at very low speeds or in the turning-off mode.
[0010] The second engine may in particular be a gas turbine engine, in order in particular to offer a high power / mass ratio. In this case, the second engine may in particular comprise a gas generator with a combustion chamber and a shaft configured to be driven in rotation in a turning mode with the combustion chamber off. Thus, the heat supplied through the air interconnection duct can maintain temperatures of the second engine even in turning mode with the combustion chamber off, so as to facilitate its defrosting, lubrication and / or subsequent relighting. For this, the air interconnection duct may in particular open into the gas generator of the second engine, and more particularly into an air intake of this gas generator.Thus, the air supplied by the air interconnection duct can in particular ensure defrosting of the gas generator and / or a sufficient temperature in its combustion chamber to facilitate its subsequent re-ignition.
[0011] Alternatively or additionally, the second engine may also include a heat exchanger and the air interconnection duct may open upstream of the heat exchanger of the second engine. Thus, the heat transmitted by the air supplied by the air interconnection circuit may be received by the second engine through the heat exchanger of the second engine. In particular, the heat exchanger of the second engine may be an air-oil heat exchanger, crossed by the lubricant circuit of the second engine, so as to maintain the temperature of the lubricant of the second engine.
[0012] The first engine may also have a lubricant circuit and the heat exchanger of the first engine may then be an air-oil heat exchanger crossed by the lubricant circuit of the first engine. Thus, the heat supplied to the second engine may come from the cooling of the lubricant of the first engine.
[0013] The assembly may include at least one valve for redirecting airflow from the air duct to the air interconnecting duct, and optionally a piloted actuator for actuating the at least one valve. However, the at least one valve could optionally include a check valve directly actuated by pressure differentials in the ducts, rather than actively piloted.
[0014] The first engine may in particular be, like the second engine, a heat engine and in particular a gas turbine engine. It is nevertheless alternatively possible for the first engine to be an engine of another type and more particularly an electric motor.
[0015] The first engine and the second engine may be mechanically coupled to the same gearbox, which may in particular be a main gearbox of an aircraft, and in particular of a rotary-wing aircraft such as for example a helicopter.
[0016] A second aspect of the present disclosure relates to such an aircraft comprising the propulsion unit of the first aspect, and a third aspect of the present disclosure relates to a method for thermal management of a propulsion unit comprising the steps of heating, in a heat exchanger of a first engine, an air flow circulating in an air duct passing through the air exchanger of the first engine, and diverting the heated air flow downstream of the heat exchanger of the first engine, through an air interconnecting duct, to a second engine, which is a heat engine, and which can operate at reduced speed or be switched off. Thus, heat released by the first engine is supplied, through the heat exchanger of the first engine and the air circulating in the air duct and the air interconnecting duct, to the second engine in order to maintain its temperatures even in standby mode.
[0017] In particular, the second engine may be a gas turbine engine comprising a gas generator with an extinguished combustion chamber and a shaft driven to rotate in turning mode. In this case, the heated air flow may be supplied to the gas generator of the second engine. However, alternatively or additionally, the heated air flow may be supplied upstream of a heat exchanger of the second engine, which may in particular be an air-oil heat exchanger crossed by a lubrication circuit of the second engine. Brief description of the drawings
[0018] The invention will be well understood and its advantages will appear better, on reading the detailed description which follows, of embodiments shown in as non-limiting examples. The description refers to the attached drawings which are schematic and are intended above all to illustrate the principles of the disclosure.
[0019] In these drawings, from one figure to another, identical or equivalent elements (or parts of elements) are identified by the same reference signs. In these attached drawings:
[0020] [Fig. 1] Figure 1 schematically illustrates an aircraft with a propulsion system comprising two engines;
[0021] [Fig. 2] Figure 2 illustrates more specifically a first embodiment of this propulsion unit;
[0022] [Fig. 3] Figure 3 illustrates an air circuit of a variant of the first embodiment of the propulsion unit;
[0023] [Fig. 4] Figure 4 illustrates a second embodiment of the propulsion assembly;
[0024] [Fig. 5] Figure 5 illustrates a third embodiment of the propulsion assembly; and
[0025] [Fig. 6] Figure 6 illustrates a fourth embodiment of the propulsion unit. Description of the embodiments
[0026] In order to make the disclosure more concrete, embodiments are described in detail below, with reference to the accompanying drawings. It is recalled, however, that the invention is not limited to these embodiments.
[0027] The first figure illustrates a rotary-wing aircraft 1, more specifically a helicopter with a main rotor 2 and an anti-torque tail rotor 3 coupled to a propulsion unit 4 for their actuation. The propulsion unit 4 illustrated comprises a first engine 5a and a second engine 5b. Power take-off shafts 6a, 6b of these engines 5a, 5b are connected to a main gearbox 7 to actuate the main rotor 2 and the tail rotor 3.
[0028] The propulsion assembly 4 according to a first embodiment is illustrated in greater detail in Fig. 2. As illustrated in this figure, each engine 5a, 5b may be a heat engine, and more specifically a gas turbine engine comprising a compressor 8a, 8b, a combustion chamber 9a, 9b, a first turbine 10a, 10b operatively connected by a rotary shaft 11a, 11b to the compressor 8a, 8b and a second turbine 12a, 12b, or free turbine, coupled to the power take-off shaft 6a, 6b. The assembly of the compressor 8a, 8b, combustion chamber 9a, 9b, first turbine 10a, 10b and rotary shaft 11a, 11b of each engine 5a, 5b may form a gas generator 20a, 20b. The rotating shaft 11a, 11b of each gas generator 20a, 20b can be operatively coupled to an actuator 13a, 13b and to a fan 14a, 14b.
[0029] Although in Figs. 2 and 4 to 6 the actuator 13a, 13b associated with each gas generator 20a, 20b is shown disposed on the same rotating shaft as the respective fan 14a, 14b, it is also conceivable that the actuator 13a, 13b and the fan 14a, 14b associated with each gas generator 20a, 20b are disposed on separate shafts, and operatively coupled to the gas generator 20a, 20b directly or through an intermediate transmission, which may in particular comprise one or more pinions.
[0030] The actuation device 13a, 13b may in particular be an electrical machine, more specifically an electric motor-generator connected to an electrical network of the aircraft 1. Thus, the actuation device 13a, 13b may be used both for starting the corresponding engine 5a, 5b and for generating electricity after this start. In the first case, the electrical machine of the actuation device 13a, 13b may be electrically powered by the electrical network of the aircraft to operate in motor mode. In the second case, the electrical machine of the actuation device 13a, 13b may operate in generator mode to power the electrical network of the aircraft.
[0031] Furthermore, however, the actuating device 13b of the second motor 5b can serve to maintain this second motor 5b in turning mode, by rotating the rotary shaft 11b, with the combustion chamber 9b switched off, at a reduced speed N Vireur, which can be, for example, between 5 and 20% of a nominal regime Ni of the rotating shaft 1 1 b. Keeping a gas turbine engine in turning mode helps to accelerate its possible re-ignition.
[0032] Each fan 14a, 14b may be arranged in an air duct 15a, 15b passing through an air-oil heat exchanger 16a, 16b through which a lubricant circuit 17a, 17b of the corresponding engine 5a, 5b passes. Each lubricant circuit 17a, 17b may in particular also comprise a pump 18a, 18b and in particular be connected to a transmission casing 19a, 19b of the corresponding engine 5a, 5b. As illustrated, each fan 14a, 14b may be operatively coupled to the rotary shaft 11a, 11b of the corresponding gas generator 20a, 20b, so as to be driven by the latter to ensure an air flow, via the air duct 15a, 15b, through the corresponding heat exchanger 16a, 16b.
[0033] The power supplied by the propulsion unit 4 may vary significantly depending on the flight stage of the aircraft 1. Thus, the power required for the cruising speed is normally significantly lower than the maximum continuous power of the propulsion unit 4, and even less compared to its maximum take-off power. However, since the propulsion unit 4 is normally sized according to the latter, it may be significantly oversized compared to the power required for the cruising speed. Consequently, in cruising, with the two engines 5a, 5b in operation, they could find themselves far from their optimal operating speed, which would result in a relatively high specific consumption.
[0034] In principle, with a propulsion unit comprising a plurality of engines, it is possible to maintain the cruising speed with at least one of these engines in standby mode. With the at least one remaining engine operating at a speed then closer to its optimal speed, the overall specific consumption can be reduced. In order to allow such an economical mode of operation of the propulsion unit 4, while allowing an immediate increase in power if necessary, one of the engines 5a, 5b, for example the second engine 5b, can be put into standby mode. This standby mode can be the aforementioned turning mode, or a super idle speed at very low speed, for example at a rotation speed of less than 40% of the nominal rotation speed, without switching off.
[0035] In the propulsion unit 4 illustrated in FIG. 2, the second engine 5b can therefore be in standby mode during the cruising speed of the aircraft 1, while the first engine 5a provides a majority, or even all, of the power used to drive the main rotor 2 and the tail rotor 3 through the main gearbox 7. In order to remain capable of ensuring an emergency power increase, in particular in the event of failure of the first engine 5a, the second engine 5b can be maintained in turning mode by actuation of its rotary shaft 11b by the actuation device 13b, or in super-idle mode without extinction.
[0036] However, in flight, with the combustion chamber 9b switched off and ambient temperatures that can be very low, especially at altitude, the temperatures of the second engine 5b can drop very significantly. Even in super-idle mode without switching off the combustion chamber 9b, the heat released by the latter may be insufficient to maintain the temperatures of the second engine 5b and avoid icing of the compressor 8b or excessive viscosity of the lubricant. To continue to provide sufficient heat to the second engine 5b, even in standby mode, the propulsion unit 4 may therefore also comprise an air interconnection duct 21, connecting the air duct 15a of the first engine 5a, downstream of its heat exchanger 16a, to the second engine 5b. More specifically, in this first embodiment, the air interconnection duct 21 may open into the air duct 15b of the second engine 5b, upstream of its heat exchanger 16b.The propulsion assembly 4 may further comprise a valve 22, which may be arranged on the air interconnection duct 21 and connected to an actuator 23. The actuator 23 may be controlled so as to open the valve 22 when the second engine 5b is in standby mode. The actuator 23 and / or the valve 22 may further comprise a return device (not shown), for example elastic or magnetic, to close the valve 22 when the second engine 5b is no longer in standby mode.
[0037] Thus, during operation of the propulsion unit 4, a transfer of heat from the lubricant circulating in the lubricant circuit 17a of the first engine 5a to a flow of air driven by the fan 14a in the air duct 15a of the first engine 5a takes place in the air-oil heat exchanger 16a of the first engine 5a, so as to heat this air flow and cool the lubricant of the first engine 5a. When the second engine 5b enters standby mode, the piloted actuator 23 can open the valve 22 so as to divert this air flow, heated by the air-oil heat exchanger 16a of the first engine 5a, from downstream of the heat exchanger 16a in the air duct 15a of the first engine 5a to upstream of the heat exchanger 16b in the air duct 15b of the second engine 5b. In this heat exchanger 16b of the second engine 5b, the heat transfer can then take place in the reverse direction, thus heating the lubricant circulating in the lubricant circuit of the second engine 5b with the heat carried by the air flow diverted from the first engine 5a.
[0038] Although in the example illustrated in Figure 2 the valve 22 is actively controlled, it is also conceivable to use non-return valves in its place, calibrated so as to respond in a similar manner to the transition to standby mode of the second motor 5b. Thus, in a variant of this first embodiment, as illustrated in Figure 3, a non-return valve 22b can be located in the air duct 15b of the second motor 5b, upstream of the arrival of the air interconnection duct 21, and calibrated so as to pass from an open position (dotted line) to a closed position following the reduction in speed of the fan 14b when the second motor 5b transitions to standby mode.Another check valve 22c may be located in the air interconnecting duct 21 and calibrated to move from a closed position (dotted line) to an open position in response to the pressure in the air duct 15b of the second motor 5b decreasing due to the check valve 22b in that air duct 15b closing. Yet another check valve 22a may be located in the air duct 15a of the first motor 5a, downstream of the bypass to the air interconnecting duct 21, and calibrated to move from an open position (dotted line) to a closed position in response to the pressure in the air duct 15a of the first motor 5a decreasing due to the check valve 22c opening in the air interconnecting duct 21.
[0039] The air interconnection duct 21 may, however, open elsewhere than into the air duct 15b of the second engine 5b. In particular, in a second embodiment illustrated in FIG. 4, the air interconnection duct 21 may open into the gas generator 20b of the second engine 5b, for example example in an air intake duct of the compressor 8b of the second engine 5b. The remaining elements of the propulsion assembly according to this second embodiment are similar to those of the first embodiment and consequently receive the same references in Figure 4 as in Figure 2.
[0040] In this way, during the operation of the propulsion unit 4 according to this second embodiment, when the second engine 5b goes into standby mode, the controlled actuator 23 can open the valve 22 so as to divert this air flow, heated by the air-oil heat exchanger 16a of the first engine 5a, from downstream of the heat exchanger 16a in the air duct 15a of the first engine 5a to the gas generator 20b of the second engine 5b, and in particular to the intake of the compressor 8b, so as to directly supply heat to this gas generator 20b, in particular to ensure its defrosting and / or maintain the temperature of the combustion chamber 9b above a minimum threshold. As in the variant of Figure 3, the hot air bypass can alternatively be carried out by a set of duly calibrated non-return valves, rather than actively controlled through a piloted actuator.
[0041] It is also conceivable to supply the hot air derived from the air duct 15a of the first engine 5a to several locations of the second engine 5b. Thus, in a third embodiment illustrated in FIG. 5, the air interconnection duct 21 can branch off so as to open on the one hand into the air duct 15b of the second engine 5b, upstream of its heat exchanger 16b, as in the first embodiment, and on the other hand into the gas generator 20b of the second engine 5b, for example in an air intake duct of the compressor 8b of the second engine 5b, as in the second embodiment. The remaining elements of the propulsion assembly according to this third embodiment are similar to those of the previous embodiments and consequently receive the same references in FIG. 5 as in FIGS. 2 and 4.
[0042] In this way, during the operation of the propulsion unit 4 according to this third embodiment, when the second engine 5b goes into standby mode, the piloted actuator 23 can open the valve 22 in such a way that to divert this air flow, heated by the air-oil heat exchanger 16a of the first engine 5a, from downstream of the heat exchanger 16a in the air duct 15a of the first engine 5a simultaneously to upstream of the heat exchanger 16b in the air duct 15b of the second engine 5b and to the gas generator 20b of the second engine 5b, and in particular to the intake of the compressor 8b. As in the variant of Figure 3, the diversion of the hot air can alternatively be carried out by a set of duly calibrated non-return valves, rather than actively controlled through a piloted actuator.
[0043] Although in the preceding embodiments the first engine 5a is, like the second engine 5b, a heat engine and more particularly a gas turbine engine, it is also possible to use other types of engine as the first engine, and in particular an electric motor so as to form a hybrid propulsion unit.Thus, in a fourth embodiment as illustrated in Figure 6, the first motor 5a may be an electric motor, with a single rotary shaft 6a operatively connectable to a main gearbox 7 for driving the main rotor 2 and the tail rotor 3, as well as to a fan 14a disposed in an air duct 15a passing through an air-oil heat exchanger 16a through which passes a lubricant circuit 17a of the corresponding first motor 5a, 5b, which may in particular be connected not only to a transmission casing 19a of the corresponding motor 5a but, alternatively or in addition to this, to a casing 60 surrounding the motor 5a itself. The remaining elements of the propulsion unit according to this fourth embodiment are similar to those of the third embodiment and consequently receive the same references in Figure 6 as in Figure 5.It is however conceivable that the air interconnection duct 21 only opens into the air duct 15b of the second engine 5b, upstream of its heat exchanger 16b, as in the first embodiment, or into the gas generator 20b of the second engine 5b, for example in an air intake duct of the compressor 8b of the second engine 5b, as in the second embodiment, rather than in both.
[0044] Thus, in a manner analogous to the previous embodiments, during the operation of the propulsion unit 4 according to this fourth mode of embodiment, when the second engine 5b enters standby mode, the piloted actuator 23 can open the valve 22 so as to divert the air flow, heated by the air-oil heat exchanger 16a of the first engine 5a, from downstream of the heat exchanger 16a in the air duct 15a of the first engine 5a to upstream of the heat exchanger 16b in the air duct 15b of the second engine 5b and / or to the gas generator 20b of the second engine 5b, and in particular to the intake of the compressor 8b. As in the variant of FIG. 3, the diversion of the hot air can alternatively be carried out by a set of duly calibrated non-return valves, rather than actively controlled through a piloted actuator.
[0045] Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that various modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. Furthermore, individual features of the various embodiments discussed may be combined in additional embodiments. Therefore, the description and drawings are to be considered in an illustrative rather than restrictive sense.
Claims
Claims
1. Propulsion assembly (4) for aircraft (1), comprising: a first engine (5a) with a heat exchanger (16a) crossed by an air duct (15a), a second engine (5b), which is a heat engine, and an air interconnection duct (21) connecting said air duct (15a), downstream of the heat exchanger (16a) of the first engine (5a), to the second engine (5b), the propulsion assembly (4) being characterized in that the first engine (5a) comprises a lubricant circuit (17b) and the heat exchanger (16a) of the first engine (5a) is an air-oil heat exchanger crossed by the lubricant circuit (17b) of the first engine (5a).
2. A propulsion unit (4) according to claim 1, wherein the second engine (5b) is a gas turbine engine.
3. A propulsion assembly (4) according to claim 2, wherein the second engine (5b) comprises a gas generator (20b) with a combustion chamber (9b) and a shaft (11b) configured to be rotated in a turning mode with the combustion chamber (9b) off.
4. Propulsion assembly (4) according to claim 3 in which the air interconnection duct (21) opens into the gas generator (20b) of the second engine (5b).
5. Propulsion assembly (4) according to any one of claims 1 to 4, in which the second engine (5b) also comprises a heat exchanger (16b) and the air interconnection duct (21) opens upstream of the heat exchanger (16b) of the second engine (5b).
6. Propulsion assembly (4) according to claim 5, in which the second engine (5b) comprises a lubricant circuit (17b) and the heat exchanger (16b) of the second engine (5b) is a heat exchanger air-oil heat, crossed by the lubricant circuit (17b) of the second engine (5b).
7. A propulsion assembly (4) according to any one of claims 1 to 6, comprising at least one valve (22; 22a, 22b, 22c) for redirecting an air flow from said air duct (15a) to the air interconnecting duct (21).
8. Propulsion assembly (4) according to claim 7, comprising a piloted actuator (23) for actuating the at least one valve (22; 22a, 22b, 22c).
9. Aircraft (1) comprising the propulsion unit (4) according to any one of claims 1 to 8.
10. A method for thermally managing a propulsion unit (4) comprising the following steps: heating, in a heat exchanger (16a) of a first engine (5a), an air flow circulating in an air duct (15a) passing through a heat exchanger (16a) of the first engine (5a), and diverting the heated air flow downstream of the heat exchanger (16a) of the first engine (5a), through an air interconnection duct (21), to a second engine (5b), which is a heat engine, the method being characterized in that the first engine (5a) comprises a lubricant circuit (17b) and the heat exchanger (16a) of the first engine (5a) is an air-oil heat exchanger crossed by the lubricant circuit (17b) of the first engine (5a).
11. A method according to claim 10, wherein the second motor (5b) is running at reduced speed or is switched off.
12. A method according to claim 11, wherein the second engine (5b) is a gas turbine engine comprising a gas generator (20b) with an extinguished combustion chamber (9b) and a shaft (11b) driven in rotation in turning mode.
13. A method according to claim 12, wherein the heated air flow is supplied to the gas generator (20b) of the second engine (5b).
14. A method according to any one of claims 11 to 13, wherein the heated air flow is provided upstream of a heat exchanger (16b) of the second engine (5b).