Patent Description:
Reverse-flow gas turbine engines draw air into a central core of the engine near a rear portion of the engine, and exhaust combustion gases from a front portion of the engine. Gases therefore flow through the core from the rear to the front of the engine.

In some conventional reverse-flow engines, air is drawn into the core and compressed with a compressor stage driven by a first turbine stage. A second turbine stage, separate from the first turbine stage and rotating a separate shaft, provides the rotational output of the engine.

<CIT> discloses a reverse flow gas turbine engine with offset reduction gearbox.

<CIT> discloses an aircraft turboprop engine provided with an electric machine.

<CIT>, <CIT> and <CIT> disclose further aircraft reverse flow gas turbine engines.

According to an aspect of the present invention, there is provided a reverse-flow gas turbine engine in accordance with claim <NUM>.

In an embodiment of the above, the reverse-flow gas turbine engine comprises a gear train, the electric motor drivingly engaged to the propeller via the gear train.

In an embodiment of any of the above, the gear train is disposed axially between the electric motor and the propeller.

In an embodiment of any of the above, the gear train is operable to selectively drivingly engage the electric motor to the propeller.

In an embodiment of any of the above, the core further comprises an output shaft drivingly engaged to the propeller via the RGB, the electric motor and the output shaft operable to concurrently drive the propeller.

In an embodiment of any of the above the electric motor is drivingly engaged only to the propeller.

In an embodiment of any of the above, the reverse-flow gas turbine engine comprises an electric generator configured to provide electrical power to the electric motor, the electric generator disposed axially between the RGB and the propeller.

In an embodiment of any of the above, the electric generator and the electric engine are positioned in series axially between the RGB and the propeller.

In an embodiment of any of the above, the RGB has an RGB output shaft drivingly engaged to the electric generator.

According to another aspect of the present invention, there is provided a method of modifying a reverse-flow gas turbine engine in accordance with claim <NUM>.

<FIG> illustrates a gas turbine engine <NUM> of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication an air inlet <NUM>, a compressor section <NUM> for pressurizing the air from the air inlet <NUM>, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, a turbine section <NUM> for extracting energy from the combustion gases, an exhaust outlet <NUM> through which the combustion gases exit the gas turbine engine <NUM>. The gas turbine engine <NUM> has a longitudinal center axis <NUM>. The engine <NUM> in <FIG> is a turboprop engine <NUM> and includes a propeller <NUM> which provides thrust for flight and taxiing. The propeller <NUM> includes a nose cone 16A and propeller blades 16B which rotate about the center axis <NUM> to provide thrust.

The gas turbine engine <NUM> (sometimes referred to herein simply as "engine <NUM>") has a central core <NUM> through which gases flow and which includes most of the turbomachinery of the engine <NUM>. The engine <NUM> is a "reverse-flow" engine <NUM> because gases flow through the core <NUM> from the air inlet <NUM> at a rear or aft portion of the engine <NUM>, to the exhaust outlet <NUM> at a front portion of the engine <NUM>. This is in contrast to "through-flow" gas turbine engines in which gases flow through the core of the engine from a front portion to a rear portion. The direction of the flow of gases through the core <NUM> of the engine <NUM> is shown in <FIG> with arrows F. The direction of the flow of gases through the core <NUM> of the engine <NUM> can be better appreciated by considering that the gases flow through the core <NUM> in the same direction D as the one along which the engine <NUM> travels during flight. Stated differently, gases flow through the engine <NUM> from a rear end of the core <NUM> towards a front end adjacent the propeller <NUM>.

It will thus be appreciated that the expressions "forward" and "aft" used herein may refer to the relative disposition of components of the engine <NUM>, in correspondence to the "forward" and "aft" directions of the engine <NUM> and aircraft including the engine <NUM> as defined with respect to the direction of travel D. In the embodiment shown, a component of the engine <NUM> that is "forward" of another component is arranged within the engine <NUM> such that it is located closer to the propeller <NUM>. Similarly, a component of the engine <NUM> that is "aft" of another component is arranged within the engine <NUM> such that it is further away from the propeller <NUM>.

Still referring to <FIG>, the core <NUM> of the engine <NUM> has multiple spools <NUM>. One or more of the spools <NUM> rotate about the center axis <NUM> to perform compression to pressurize the air received through the air inlet <NUM>, and to extract energy from the combustion gases before they exit the core <NUM> via the exhaust outlet <NUM> a forward end of the core <NUM>. The core <NUM> may include other components as well, including, but not limited to, gearboxes, tower shafts, and bleed air outlets.

A first spool 20A includes at least one component to compress the air that is part of the compressor section <NUM>, and at least one component to extract energy from the combustion gases that is part of the turbine section <NUM>. More particularly, the first spool 20A has a low pressure turbine <NUM> which extracts energy from the combustion gases, and a low pressure compressor <NUM> for pressurizing the air. The low pressure turbine <NUM> (sometimes referred to herein simply as "LPT <NUM>") in <FIG> is separated mechanically from the low pressure compressor <NUM> (sometimes referred to herein simply as "LPC <NUM>"). Both the LPT <NUM> and the LPC <NUM> are disposed along the center axis <NUM>. In the depicted embodiment, both the LPT <NUM> and the LPC <NUM> are axial rotatable components having an axis of rotation that is coaxial with the center axis <NUM>. They can each include one or more stages of rotors and stators, depending upon the desired engine thermodynamic cycle, for example.

The LPT <NUM> is forward of the LPC <NUM>. The LPT <NUM> is aft of the exhaust outlet <NUM>. The LPC <NUM> is forward of the air inlet <NUM> and in fluid communication therewith. The LPC <NUM> is closer to, or at, an aft end of the core <NUM>. The LPC <NUM> is disposed between the air inlet <NUM> and the LPT <NUM> along a direction parallel to the center axis <NUM>. This arrangement of the LPT <NUM> and the LPC <NUM> provides for a reverse-flow engine <NUM> that has one or more low pressure compressors located at the rear of the engine <NUM> which are driven by one or more forwardly-positioned turbines. Still referring to <FIG>, the core <NUM> includes an output drive shaft <NUM>. The drive shaft <NUM> extends forwardly from the LPT <NUM> and is drivingly engaged thereto. In <FIG>, the drive shaft <NUM> is coaxial with the center axis <NUM>.

A rotatable load, which in the embodiment shown includes the propeller <NUM>, is mountable to the engine <NUM>, and when mounted, is drivingly engaged (e.g. directly or indirectly connected) to the LPT <NUM>, and is located forward of the LPT <NUM>. In such a configuration, during operation of the engine <NUM>, the LPT <NUM> drives the rotatable load such that a rotational drive produced by the LPT <NUM> is transferred to the rotatable load. The rotatable load can therefore be any suitable component, or any combination of suitable components, that is capable of receiving the rotational drive from the LPT <NUM>, as now described.

In the embodiment shown, a reduction gearbox <NUM> (sometimes referred to herein simply as "RGB <NUM>") is drivingly engaged to the core <NUM> to be driven by one or more components thereof. In <FIG>, the RGB <NUM> is disposed axially between the core <NUM> and the propeller <NUM>. In <FIG>, the RGB <NUM> is disposed axially between the LPT <NUM> and the propeller <NUM>. In an alternate embodiment, the RGB <NUM> may be part of the core <NUM>. In <FIG>, the RGB <NUM> is mechanically coupled to a front end of the drive shaft <NUM>, which extends between the RGB <NUM> and the LPT <NUM>. The output shaft <NUM> of the core <NUM> is thus drivingly connected to the propeller <NUM> via the RGB <NUM>. The RGB <NUM> processes and outputs the rotational drive transferred thereto from the LPT <NUM> via the drive shaft <NUM> through known gear reduction techniques. The RGB <NUM> allows for the propeller <NUM> to be driven at its optimal rotational speed, which is different from the rotational speed of the LPT <NUM>.

The propeller <NUM> is mechanically coupled to the output of the RGB <NUM> via a propeller shaft <NUM>. The propeller shaft <NUM> allows the rotational drive outputted by the RGB <NUM> (and the core <NUM>) during operation of the engine <NUM> to be transferred to the propeller <NUM> to provide propulsion during flight.

Still referring to <FIG>, the engine <NUM> includes a second spool <NUM> with at least one component to compress the air that is part of the compressor section <NUM>, and at least one component to extract energy from the combustion gases that is part of the turbine section <NUM>. The second spool <NUM> is also disposed along the center axis <NUM> and includes a high pressure turbine <NUM> drivingly engaged (e.g. directly connected) to a high pressure compressor <NUM> by a high pressure shaft <NUM>. Similarly to the LPT <NUM> and the LPC <NUM>, the high pressure turbine <NUM> (sometimes referred to herein simply as "HPT <NUM>") and the high pressure compressor <NUM> (sometimes referred to herein simply as "HPC <NUM>") can include axial rotary components. They can also each include one or more stages of rotors and stators, depending upon the desired engine thermodynamic cycle, for example. In the depicted embodiment, the HPC <NUM> includes a centrifugal compressor 42A or impeller which is driven by the HPT <NUM>. During operation of the engine <NUM>, the HPT <NUM> drives the HPC <NUM>.

The HPT <NUM> is aft of the LPT <NUM>, and forward of the combustor <NUM>. The HPC <NUM> is aft of the combustor <NUM>, and forward of the LPC <NUM>. The HPT <NUM> is forward of the HPC <NUM>. The HPC <NUM> is disposed axially between the LPC <NUM> and the HPT <NUM>, and the HPT <NUM> is disposed axially between the HPC <NUM> and the LPT <NUM>. The HPT <NUM> and LPT <NUM> are in fluid communication, such that the combustion gases from the combustor <NUM> flow through the HPT <NUM> and then through the LPT <NUM>. From this arrangement of the HPT <NUM> and the HPC <NUM>, it can be appreciated that during operation of the engine <NUM>, the LPC <NUM> feeds pressurized air to the HPC <NUM>. Therefore, the pressurized air flow produced by the LPC <NUM> is provided to the HPC <NUM>. In <FIG>, the HPC <NUM> is mechanically coupled to the LPC <NUM> such that the HPC <NUM> (and thus the HPT <NUM>) performs all of the compression work.

In light of the preceding, it can be appreciated that the LPT <NUM> is the "low pressure" turbine section when compared to the HPT <NUM>, which is sometimes referred to as the "gas generator". The LPT <NUM> is sometimes referred to as a "power turbine". The turbine rotors of the HPT <NUM> spin at a higher rotational speed than the turbine rotors of the LPT <NUM> given the closer proximity of the HPT <NUM> to the outlet of the combustor <NUM>. The engine <NUM> shown in <FIG> is thus a "two-spool" engine <NUM>.

The HPT <NUM> and the HPC <NUM> can have any suitable mechanical arrangement to achieve the above-described functionality. For example, and as shown in <FIG>, the second spool <NUM> includes the high pressure shaft <NUM> extending between the HPC <NUM> and the HPT <NUM>. The high pressure shaft <NUM> is coaxial with, and drivingly engaged to, the LPC <NUM> such that the high pressure shaft <NUM> drives the LPC <NUM>.

Still referring to the embodiment shown in <FIG>, the engine <NUM> also includes an accessory gearbox <NUM>. The accessory gearbox <NUM> (sometimes referred to herein simply as "AGB <NUM>") receives a rotational output and in turn drives accessories (e.g. fuel pump, starter-generator, oil pump, scavenge pump, etc.) that contribute to the functionality of the engine <NUM>. The AGB <NUM> can be designed with side-facing accessories, top-facing accessories, or rear-facing accessories depending on the installation needs. The AGB <NUM> is aft of the air inlet <NUM>. During operation of the engine <NUM>, the high pressure shaft <NUM> transmits a rotational drive to the AGB <NUM> which in turn drives the accessories of the AGB <NUM>. In an alternate possible embodiment the engine <NUM>, the engine <NUM> is free of an AGB <NUM>. The AGB <NUM> can be arranged relative to the core <NUM> of the engine <NUM> differently than as shown in <FIG>. For example, the AG <NUM> may be mounted on the side of the engine <NUM>, and forward of the air inlet <NUM>. The circumferential angular position of the AGB <NUM> may be selected to suit specific installation needs. Other positions and arrangements for the AGB <NUM> are thus possible.

Still referring to <FIG>, the engine <NUM> has an electric motor <NUM>. The electric motor <NUM> is drivingly engaged to the propeller <NUM> or to some component thereof to providing a rotational output to the propeller <NUM> to rotate the propeller blades 16B and generate thrust during any suitable aircraft flight condition. The electric motor <NUM> is provided with an electrical input such as electrical power and generates a mechanical, rotational output to drive the propeller <NUM>. In <FIG>, the electric motor <NUM> is provided only with an electrical input and is not also provided with a mechanical input. The output of the electric motor <NUM> is coupled, directly or indirectly, only to the propeller <NUM> and is free of mechanical connection to another component. For example, in <FIG>, the output of the electric motor <NUM> is coupled to the propeller shaft <NUM> of the propeller <NUM>, and the propeller shaft <NUM> is itself driven by an output of the RGB <NUM> and coupled thereto.

The electric motor <NUM> is disposed between the RGB <NUM> and the propeller <NUM>, when considered in a direction parallel to the center axis <NUM>. The electric motor <NUM> is thus positioned axially between the RGB <NUM> and the propeller <NUM> or its components, to drive the propeller <NUM>. This arrangement of the electric motor <NUM> may allow for matching the rotational output speed of the electric motor <NUM> to the rotational speed of the propeller <NUM> at a given flight condition. Therefore, the electric motor <NUM> may be designed or selected so that its output speed is the same or similar to the rotating speed of the propeller <NUM>. This is in contrast to the output speed of the drive shaft <NUM> of the LPT <NUM> or "power turbine" which is typically used to drive the propeller <NUM>, but which often rotates at a much higher speed than the propeller <NUM> and thus requires speed reduction via the RGB <NUM>. Positioning the electric motor <NUM> between the RGB <NUM> and the propeller <NUM> also places the electric motor <NUM> in a colder part of the engine <NUM>, which may contribute to improving the working life of the electric motor <NUM>. Positioning the electric motor <NUM> between the RGB <NUM> and the propeller <NUM> may facilitate servicing or repair of the electric motor <NUM> because the only component that may need to be removed to access the electric motor <NUM> is the propeller <NUM>. Positioning the electric motor <NUM> between the RGB <NUM> and the propeller <NUM> may allow the electric motor <NUM> to be provided as a stand-alone or self-sufficient module which is free of any structural attachment to the casing of the engine <NUM>. The engine <NUM> disclosed herein is thus a reverse-flow, multi-spool engine with an electric motor <NUM> built into the engine <NUM> and disposed in between the RGB <NUM> and the propeller <NUM>.

The electric motor <NUM> may have any suitable structure or component to achieve the functionality ascribed to it herein. The electric motor <NUM> may be selected to be sufficiently powerful to drive the propeller <NUM> either without using fuel in the engine <NUM>, or with using a reduced amount of fuel by the engine <NUM> during at least one mode of operation of the engine <NUM>. Electricity for driving electric motor <NUM> may be supplied by an electric power source <NUM> under the control of a suitable controller <NUM> such as an EEC (Electronic Engine Controller) or FADEC (Full Authority Digital Engine Control). The electric power source <NUM> may, for example, include one or more batteries 62A, an auxiliary power unit (APU) and/or an electric generator from another engine of the same aircraft onto which the engine <NUM> is mounted. The controller <NUM> may be configured to control the operation of the electric motor <NUM> by providing suitable control signals to the electric motor <NUM> and/or providing suitable conditioning of the electric power supplied to the electric motor <NUM> by the electric power source <NUM>. The controller <NUM> may actuate the amount of electric power supplied to the electric motor <NUM> in response to control signals it receives, such as for example, commands sent via a control interface (e.g., panel) from a pilot of an aircraft to which engine <NUM> is mounted. The controller <NUM> and the electric power source <NUM> may be configured to supply enough electric power to the electric motor <NUM> in order to produce some or all of the torque required to rotate the propeller <NUM> during at least one mode of operation of the aircraft.

The electric motor <NUM> may comprise one or more rotors and one or more respective stators. In some embodiments, the plurality of rotor/stator pairs may be angularly or circumferentially distributed about a shaft axis of rotation. One or more of rotors may have a respective rotor axis of rotation that is radially offset from a center axis of the electric motor <NUM>. In some embodiments, each rotor axis may be radially offset from the center axis at a substantially uniform offset distance. Each rotor may be drivingly engaged (e.g., coupled via a shaft) to a respective drive gear for transferring motive power from the rotors to the propeller <NUM>. The electric motor <NUM> may be drivingly engaged to transmit and/or receive motive power to/from the propeller <NUM> in any suitable manner. In some embodiments, the electric motor <NUM> may be drivingly engaged to the propeller <NUM> via the drive gears drivingly engaged to a common gear, which is in turn drivingly engaged with the propeller shaft <NUM> via suitable meshed gearing. The structure and principle of operation of possible configurations for the electric motor <NUM> are described in <CIT> and in <CIT> The electric motor <NUM> may be "built-in" into the engine <NUM>, such that the electric motor <NUM> has all of its components assembled together to provide a single output to the propeller <NUM>. For example, and as shown in <FIG>, the electric motor <NUM> and its components may be housed in an annular electric motor housing <NUM> which is attached to the bearings supporting the propeller shaft <NUM> at a forward end, and which is attached to an RGB housing 31A at an aft end. The electric motor <NUM> may therefore be relatively easily inserted and mounted within the engine <NUM>. Accordingly, the electric motor <NUM> and its physical integration within the engine <NUM> may, in some embodiments, allow for modifying an existing reverse-flow, multiple-spool engine <NUM> to be provided with the electric motor <NUM>.

In <FIG>, the electric motor <NUM> is coaxial with the spools <NUM> and with the center axis <NUM>.

In an alternate arrangement that is not covered by the claims, the electric motor <NUM> may have components, such as its rotor or internal gears, which rotate about an axis that is transverse to the center axis <NUM>, such that the electric motor <NUM> is not coaxial with the spools <NUM> or the center axis <NUM>. The electric motor <NUM> is mounted at a location within the engine <NUM> that is spaced a distance measured in a radial direction from the center axis <NUM>, from the drive shaft <NUM> of the LPT <NUM>, and from the propeller shaft <NUM>. A component of the electric motor <NUM>, such as its rotor and the axis about which the rotor rotates, is spaced a distance measured in a radial direction from the center axis <NUM>, from the drive shaft <NUM> of the LPT <NUM>, and from the propeller shaft <NUM>. The electric motor <NUM> is therefore radially offset from the propeller <NUM> or the drive shaft <NUM>.

Referring to <FIG>, the electric motor <NUM> is indirectly mounted to the propeller shaft <NUM>. The engine <NUM> includes a gear train <NUM> drivingly engaged to both the output of the electric motor <NUM> and the propeller shaft <NUM>, so as to drivingly engage the electric motor <NUM> to the propeller <NUM>. The electric motor <NUM> is thus indirectly coupled to the propeller attachment via the gear train <NUM>. The gear train <NUM> has any suitable arrangement of gearing and ratios to allow an output from the electric motor <NUM> to be supplied to the propeller <NUM>. In <FIG>, the electric motor <NUM> has a motor output shaft <NUM> which meshes with, and drives, an input gear 66A of the gear train <NUM>. An output gear 66B of the gear train <NUM> engages and meshes with a radial gear 35A of the propeller shaft <NUM>, to transfer the rotational drive from the motor output shaft <NUM> to the propeller <NUM>. In <FIG>, the gear train <NUM> modifies the speed and torque of the output of the electric motor <NUM> as desired, to supply the modified output directly to the propeller <NUM>. The gear train <NUM> is disposed axially between the electric motor <NUM> and the propeller <NUM>. The gear train <NUM> is enclosed or housed within the electric motor housing <NUM>. In <FIG>, the gear train <NUM> is a separate component from the electric motor <NUM>, and is separate from the internal gearing of the electric motor <NUM>. In embodiments, one of which is described in greater detail below, the electric motor <NUM> is coupled directly to the desired component of the propeller <NUM>, and there is no gear train <NUM> provided between the electric motor <NUM> and the propeller <NUM>.

In <FIG>, the gear train <NUM> is operable to selectively drivingly engage the electric motor <NUM> to the propeller <NUM>. The gear train <NUM> allows the electric motor <NUM> to engage the propeller <NUM> to transfer a rotational drive thereto, and also allows the electric motor <NUM> to disengage from the propeller <NUM> such that the output of the electric motor <NUM> is not supplied to the propeller <NUM>. This selective engagement may be achieved using any suitable mechanism, such as a clutch. This selective engagement of the electric motor <NUM> via the gear train <NUM> may allow for the electric motor <NUM> to provide the sole rotational drive to the propeller <NUM>, to provide rotational drive concurrently with the drive shaft <NUM> of the core <NUM>, or to provide no rotational drive to the propeller <NUM> at all such that the propeller <NUM> is driven entirely by the output of the core <NUM>. This selective engagement of the electric motor <NUM> may be used, for example, to allow only the electric motor <NUM> to provide rotational drive to the propeller <NUM> during a cruise, taxi, or descent flight condition. This selective engagement of the electric motor <NUM> may be used, for example, to allow both the electric motor <NUM> and the core <NUM> to provide rotational drive to the propeller <NUM> during a take-off flight condition, such that the electric motor <NUM> and the output shaft of the core <NUM> (i.e. the drive shaft <NUM> of the LPT <NUM>) are operable to concurrently drive the propeller <NUM>. The engine <NUM> may therefore have a dual connection to propeller <NUM> - one output connection from the electric motor <NUM> and the second output connection from the core <NUM> and its LPT <NUM>.

Referring to <FIG>, the engine <NUM> has an electric generator <NUM>. During operation, the electric generator <NUM> converts the mechanical output of the core <NUM> into electrical power that is supplied to the electric motor <NUM>. The electric generator <NUM> is configured to provide electrical power to the electric motor <NUM>. In <FIG>, the electric generator <NUM> is a separate component from the electric motor <NUM>. One possible configuration of this separateness may include the electric generator <NUM> and the electric motor <NUM> being enclosed in separate containers with wiring extending between them to supply electrical power to the electric motor <NUM>. In <FIG>, the electric generator <NUM> during operation supplies electrical power only to the electric motor <NUM>. In <FIG>, the wiring extends only between the electric generator <NUM> and the electric motor <NUM> to supply electrical power to the electric motor <NUM>. Additional wiring from the controller <NUM> to the electric motor <NUM> and to the electric generator <NUM> may be routed outside the structure of the engine <NUM>. Referring to <FIG>, and like the electric motor <NUM>, the electric generator <NUM> is also disposed axially between the RGB <NUM> and the propeller <NUM>. In <FIG>, the electric generator <NUM> is disposed axially between the RGB <NUM> and the electric motor <NUM>. In <FIG>, the electric motor <NUM> and the electric generator <NUM> are disposed in series or sequentially between the RGB <NUM> and the propeller <NUM>. In <FIG>, the electric generator <NUM> and the electric motor <NUM> are axially adjacent, or next to, one another. The engine <NUM> disclosed herein may therefore be a reverse-flow, multi-spool engine <NUM> with a cooperating electric motor <NUM> and electric generator <NUM> disposed in between the RGB <NUM> and the propeller <NUM>. In an alternate embodiment of the engine <NUM>, there is no electric generator <NUM>, and the electric motor <NUM> is supplied with electrical power from another electrical power source <NUM>, such as the batteries 62A. The electric generator <NUM> may be located elsewhere in the engine <NUM> in alternate configurations. The electric generator <NUM> may be connected to the batteries 62A. Referring to <FIG>, the RGB <NUM> has an RGB output shaft 31B that transmits the rotational output of the RGB <NUM>. The RGB output shaft 31B, which is itself driven by the drive shaft <NUM> of the LPT <NUM>, is drivingly engaged with the electric generator <NUM> to provide the motive power thereto. The "power turbine" shaft <NUM> in <FIG> thus provides some or all of the mechanical input to the electric generator <NUM>, via the RGB <NUM>. In an alternate embodiment, the electric generator <NUM> is driven by another component, such as the HPT <NUM>, to be used as an electrical power source for the electric motor <NUM>. The controller <NUM> may provide full digital envelope protection, to optimize "hybrid" operation of the engine <NUM> through all phases of flight. The controller <NUM> may be configured to control the operation of the electric motor <NUM> by optimizing the hybrid engine functionality either via the batteries 62A or directly from the electric generator <NUM>.

Referring to <FIG>, the starter-generator of, or in, the AGB <NUM> is a separate component from the electric motor <NUM> and from the electric generator <NUM> described above. The starter-generator of the AGB <NUM> is spaced apart from the electric motor <NUM> and from the electric generator <NUM>, and is housed in a separate enclosure. The starter-generator of the AGB <NUM> may be configured as, or include, an electric starter/generator drivingly engaged to a drive shaft of the core <NUM>, to start rotation of the rotatable components of the core <NUM>, such as the compressor section <NUM>. In certain engine operating conditions, the high pressure shaft <NUM> of the core <NUM> may provide rotational drive to the starter-generator of the AGB <NUM> to generate electrical power for various functions unrelated to the operation of the engine <NUM>. This functionality of the starter-generator of the AGB <NUM> is thus separate from that of the electric motor <NUM> which is used to provide rotational drive only to the propeller <NUM>. Furthermore, although the electric generator <NUM> may also be driven by the core <NUM>, the electrical power thus generated by the electric generator <NUM> is supplied only to the electric motor <NUM>.

<FIG> shows a configuration of the engine <NUM> where the electric motor <NUM> is mounted directly to the propeller <NUM>, whereby this example is not covered by the claims. Features shown in <FIG> which are not provided
with reference numbers and which are similar to the features shown in other figures bear the same reference numbers as the features shown in other figures. In <FIG>, the electric motor <NUM> is coupled directly to the propeller shaft <NUM> of the propeller <NUM>, and there is no gear train <NUM> provided between the electric motor <NUM> and the propeller <NUM>. The electric motor <NUM> is thus drivingly engaged only to the propeller <NUM>. This direct configuration may take different forms. In <FIG>, the motor output shaft <NUM> of the electric motor <NUM> is drivingly engaged to the propeller shaft <NUM> to provide the rotational output of the electric motor <NUM> to the propeller <NUM>. The motor output shaft <NUM> is supported by bearings 68A at an aft end of the motor output shaft <NUM>, and is supported via the propeller shaft <NUM> via its bearings 35AB. In <FIG>, the propeller shaft <NUM> and the motor output shaft <NUM> are integral with one another. In <FIG>, the propeller shaft <NUM> and the motor output shaft <NUM> are one integral shaft. This direct configuration of the electric motor <NUM> and the propeller <NUM> may allow for the output speed of the electric motor <NUM> to be selected to exactly match the desired rotational speed of the propeller <NUM> at a specific flight condition, such as cruise or take-off. In <FIG>, the engine <NUM> is a reverse-flow, multi-spool engine with an electric motor <NUM> having a direct shaft connection with the propeller <NUM>. In <FIG>, the output of the LPT <NUM> may not be used to drive the propeller <NUM>, and the engine <NUM> may not have an RGB <NUM>.

Referring to <FIG>, there is disclosed a method of modifying or upgrading an existing reverse-flow gas turbine engine <NUM> having multiple spools <NUM>, an RGB <NUM>, and a propeller <NUM>. The method includes mounting the electric motor <NUM> within the gas turbine engine <NUM> and positioning it axially between the RGB <NUM> and the propeller <NUM>. The method includes drivingly engaging the electric motor <NUM> to the propeller <NUM>. This may include drivingly engaging the electric motor <NUM> only to the propeller <NUM>. The method may also include mounting an electric generator <NUM> axially between the RGB <NUM> and the propeller <NUM> to provide electrical power to the electric motor <NUM>. This method may allow for modifying or upgrading an existing engine in the after-market, for example to improve its performance by adding the electric motor <NUM> in the desired location. This may transform the existing engine into a hybrid electric-fuel gas turbine engine <NUM>.

Referring to <FIG>, there is disclosed a method of operating the gas turbine engine <NUM> and the propeller <NUM>. The method includes operating the engine <NUM> to drive the core <NUM> and the RGB <NUM> drivingly coupled thereto. The method includes operating the electric motor <NUM> positioned axially between the RGB <NUM> and the propeller <NUM> to drive, at least partially, the propeller <NUM>.

Claim 1:
A reverse-flow gas turbine engine (<NUM>), comprising:
a propeller (<NUM>) having a propeller shaft (<NUM>) mounted for rotation about a center axis (<NUM>) of the engine (<NUM>);
a first spool (20A) having a low pressure compressor (LPC) (<NUM>) in fluid communication with an air inlet (<NUM>) and a low pressure turbine (LPT) (<NUM>), the LPC (<NUM>) disposed axially between the air inlet (<NUM>) and the LPT (<NUM>);
a second spool (<NUM>) having a high pressure compressor (HPC) (<NUM>) in fluid communication with the LPC (<NUM>) to receive pressurized air therefrom, and a high pressure turbine (HPT) (<NUM>) drivingly engaged to the HPC (<NUM>) and in fluid communication with the LPT (<NUM>), the HPC (<NUM>) disposed axially between the LPC (<NUM>) and the HPT (<NUM>) and the HPT (<NUM>) disposed axially between the HPC (<NUM>) and the LPT (<NUM>); and
a reduction gearbox (RGB) (<NUM>) drivingly engaged to, and disposed axially between, the LPT (<NUM>) and the propeller (<NUM>), the RGB (<NUM>) having an RGB housing (31A); characterised in that it further comprises:
an electric motor (<NUM>) drivingly engaged to the propeller (<NUM>) and disposed axially between the RGB (<NUM>) and the propeller (<NUM>), the electric motor (<NUM>) coaxial with the first and second spools and with the center axis (<NUM>), the electric motor (<NUM>) having an annular electric motor housing (<NUM>), the annular electric motor housing (<NUM>) having an aft end attached to the RGB housing (31A).