Patent Description:
A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-energy exhaust gas flow. The high-energy exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section typically includes low and high pressure compressors, and the turbine section includes low and high pressure turbines.

A high pressure turbine drives the high pressure compressor through an outer shaft to form a high spool, and the low pressure turbine drives the low pressure compressor through an inner shaft to form a low spool. The fan section may also be driven by the low pressure turbine through the inner shaft.

The engine is typically started by driving the high spool through a tower shaft through an accessory gearbox. Once the high spool is up to speed, the low spool follows and the engine is brought to an idle condition. When the engine is operating, the accessory gearbox is driven through the same tower shaft to drive accessory components such as hydraulic pumps and electric generators. The loads from the accessory gearbox on the high spool reduce efficiency.

Turbine engine manufacturers continue to seek further improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies.

<CIT> discloses a twin turboshaft engine.

<CIT> discloses an aircraft starter generator.

<CIT> discloses a gas turbine engine comprising a rotatable shaft used to drive an auxiliary unit.

Viewed from a first aspect, a gas turbine engine as claimed in claim <NUM> is provided.

The gas turbine engine may include a first clutch assembly for selectively coupling the first tower shaft to the ring gear, a second clutch assembly for selectively coupling the sun gear to the carrier, and an accessory gearbox driven by an output of the superposition gearbox.

The output of the superposition gearbox may include a lay shaft coupled to the carrier.

The first clutch assembly and the second clutch assembly may include one-way mechanical clutches.

The superposition gearbox may not be fixed to a static structure of the engine.

A high speed spool drive train gear ratio may be between <NUM> to <NUM>, the high speed spool drive train gear ratio being measured from the high speed output gear to the sun gear shaft drive gear.

An epicycle gear ratio may be between <NUM> to <NUM>, the epicycle gear ratio being measured from the ring gear to the sun gear
Viewed from a second aspect, a gas turbine engine as claimed in claim <NUM> is provided.

An epicycle gear ratio may be between <NUM> to <NUM>, the epicycle gear ratio being measured from the ring gear to the sun gear.

Viewed from a third aspect, a gas turbine engine as claimed in claim <NUM> is provided.

The detailed description explains embodiments of the present disclosure, together with advantages and features, by way of example with reference to the drawings.

The inner shaft <NUM> is connected to a fan section <NUM> through a speed change mechanism, which in exemplary gas turbine engine <NUM> is illustrated as a geared architecture <NUM> to drive fan blades <NUM> at a lower speed than the low speed spool <NUM>.

For example, gear system <NUM> may be located aft of the low pressure compressor <NUM> and the fan blades <NUM> may be positioned forward or aft of the location of the geared architecture <NUM> or even aft of turbine section <NUM>.

In a further example, the engine <NUM> bypass ratio is greater than about six (<NUM>), with an example embodiment being greater than about ten (<NUM>), the geared architecture <NUM> is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about <NUM> and the low pressure turbine <NUM> has a pressure ratio that is greater than about five (<NUM>). In one disclosed embodiment, the engine <NUM> bypass ratio is greater than about ten (<NUM>:<NUM>), the fan diameter is significantly larger than that of the low pressure compressor <NUM>, and the low pressure turbine <NUM> has a pressure ratio that is greater than about five (<NUM>:<NUM>).

The example gas turbine engine includes the fan section <NUM> that comprises in one non-limiting embodiment less than about <NUM> fan blades <NUM>. In another non-limiting embodiment, the fan section <NUM> includes less than about <NUM> fan blades <NUM>. Moreover, in one disclosed embodiment the low pressure turbine <NUM> includes no more than about <NUM> turbine rotors schematically indicated at <NUM>. In another non-limiting example embodiment, the low pressure turbine <NUM> includes about <NUM> turbine rotors. A ratio between the number of fan blades <NUM> and the number of low pressure turbine rotors is between about <NUM> and about <NUM>. The example low pressure turbine <NUM> provides the driving power to rotate the fan section <NUM> and, therefore, the relationship between the number of turbine rotors <NUM> in the low pressure turbine <NUM> and the number of blades <NUM> in the fan section <NUM> disclose an example gas turbine engine <NUM> with increased power transfer efficiency.

The example engine <NUM> includes an accessory drive system <NUM> that receives power from both the high speed spool <NUM> and the low speed spool <NUM>. The accessory drive system <NUM> drives an accessory gearbox <NUM> that includes accessory component <NUM> and lubricant pump <NUM>. The accessory component <NUM> may include pumps, generators and other devices driven to enable operation of different engine and aircraft systems. The accessory gearbox <NUM> is also coupled to a starter <NUM>. The starter <NUM> is capable of driving the accessory drive system <NUM> to start the engine <NUM>. In this example, a tower shaft assembly <NUM> is coupled to both the low speed spool <NUM> and the high speed spool <NUM> to distribute power extraction between the two spools <NUM>, <NUM>.

Excessive power extraction from a single spool, such as the high speed spool <NUM>, can limit operation and degrade overall performance and engine efficiency. Accordingly, the example accessory drive system <NUM> extracts power from both the low speed spool <NUM> and the high speed spool <NUM> to meet the overall power demands of the engine <NUM> and the aircraft associated with the engine.

Referring to <FIG>, with continued reference to <FIG>, the example accessory drive system <NUM> includes a superposition gearbox <NUM> that is coupled between accessory gearbox <NUM> and the tower shaft assembly <NUM>. The superposition gearbox <NUM> is an epicyclic gearbox that includes a sun gear <NUM> that rotates about an axis <NUM>. A plurality of intermediate gears <NUM> are engaged with the sun gear <NUM> and supported by a carrier <NUM>. A ring gear <NUM> circumscribes and is engaged with the plurality of intermediate gears <NUM>. The example superposition gearbox <NUM> is not coupled to a static structure of the engine <NUM> and, therefore, is operated by various input combinations to provide the desired distribution of power.

In the disclosed example, the tower shaft assembly <NUM> includes a first tower shaft <NUM> that is driven by a low speed output gear <NUM> disposed on the low speed spool <NUM>. A first gear <NUM> on the first tower shaft <NUM> is coupled to the low speed output gear <NUM>. A second gear <NUM> is disposed on a second end of the first tower shaft <NUM> and engages a ring gear shaft drive gear <NUM> disposed on a ring gear shaft <NUM>.

A second tower shaft <NUM> is coupled to a high speed output gear <NUM> that is driven by the high speed spool <NUM>. The second tower shaft <NUM> includes a first gear <NUM> driven by the high speed output gear <NUM> on the high speed spool <NUM>. A second gear <NUM> of the second tower shaft <NUM> is engaged to sun gear shaft drive gear <NUM> disposed on a sun gear shaft <NUM>. The first tower shaft <NUM> and the second tower shaft <NUM> are disposed concentrically about a common axis <NUM>. Moreover, the axis <NUM> is disposed at an angle relative to the engine longitudinal axis A and an axis <NUM> of the superposition gearbox <NUM>.

First tower shaft <NUM> is coupled to the ring gear shaft <NUM> that is selectively coupled to the ring gear <NUM>. The second tower shaft <NUM> is coupled to the sun gear shaft <NUM> that is coupled to drive the sun gear <NUM>. The sun gear shaft <NUM> is directly coupled to the sun gear <NUM> and is not selectively engaged to the sun gear <NUM>.

The superposition gearbox <NUM>, therefore, has a first input provided by the first tower shaft <NUM> through the ring gear shaft <NUM> coupled to drive the ring gear <NUM> and a second input provided by the second tower shaft <NUM> to drive the sun gear shaft <NUM> and, thereby, the sun gear <NUM>. An output from the superposition gearbox <NUM> is provided by a lay shaft <NUM> that is coupled to the carrier <NUM>. The lay shaft <NUM> drives the accessory gearbox <NUM> in the disclosed example embodiment. The accessory gearbox <NUM> includes another gear system or plurality of gears as is required to the drive accessory components schematically illustrated at <NUM> and <NUM>. Moreover, the accessory gearbox <NUM> is coupled to the starter <NUM> to provide a driving input through the lay shaft <NUM> to the superposition gearbox <NUM> to drive the high speed spool <NUM> during starting operation.

Referring to <FIG>, with continued reference to <FIG>, the example superposition gearbox <NUM> includes a first clutch assembly <NUM> that selectively couples the ring gear shaft <NUM> to drive the ring gear <NUM>. The first clutch <NUM> selectively couples driving input from the low speed spool <NUM> through the first tower shaft <NUM> to the ring gear shaft <NUM> and ring gear <NUM>.

A second clutch assembly <NUM> selectively couples the sun gear <NUM> to the carrier <NUM>. The second clutch assembly <NUM> is shown uncoupled such that the sun gear <NUM> and the carrier <NUM> may rotate at different relative speeds. Coupling of the sun gear <NUM> to the carrier <NUM> enables the second tower shaft <NUM> to directly drive the carrier <NUM>. Accordingly, through a selective coupling of the first clutch assembly <NUM> and the second clutch assembly <NUM> different inputs from the high speed spool <NUM> and the low speed spool <NUM> can be input through the superposition gearbox <NUM> to drive the lay shaft <NUM> and, thereby, the accessory gearbox <NUM>.

The first clutch assembly <NUM> and the second clutch assembly <NUM> are one-way mechanical clutches that do not require control or separate independent actuation. Each of the clutch assemblies <NUM>, <NUM> are automatically engaged and disengaged depending on the speed and direction of torque input. The selective actuation of the mechanical clutches <NUM>, <NUM> enables both the low speed spool <NUM> and the high speed spool <NUM> to provide torque input to drive the lay shaft <NUM>.

In the configuration for operation shown in <FIG>, the first clutch <NUM> is engaged to couple the ring gear shaft <NUM> to the ring gear <NUM>. Accordingly, the first tower shaft <NUM> driven by the low speed spool <NUM> is coupled to drive the ring gear <NUM>. The second clutch assembly <NUM> is not engaged and is free running. The second tower shaft <NUM> drives the sun gear shaft <NUM> to drive the sun gear <NUM>. Accordingly, the low speed spool <NUM> is driving the ring gear <NUM> and the high speed spool <NUM> is driving the sun gear <NUM>. The driving inputs are both in the same rotational direction and result in an overall output through the carrier <NUM> to drive the lay shaft <NUM>.

Accordingly, a rotational input, schematically indicated at <NUM>, of the low speed spool <NUM> combined with a rotational input <NUM>, schematically shown by the rotational arrows, combined within the superposition gearbox <NUM> generate an output, schematically shown at <NUM>, to drive the accessory gearbox <NUM>.

Differing rotational inputs provided by <NUM> and <NUM> may be of differing speeds and torques. The superposition gearbox <NUM> receives and automatically distributes the torques to provide the driving input <NUM> through the lay shaft <NUM> to drive the accessory gearbox <NUM>.

Referring to <FIG>, another schematic illustration of the disclosed drive system <NUM> is schematically shown configured for a starting operation configuration. In the starting configuration, starter <NUM> provides input torque <NUM> to drive the accessory gearbox <NUM> and the lay shaft <NUM> to rotate high speed spool <NUM>. In this position, the first clutch assembly <NUM> is not coupled because the ring gear <NUM> due to the direction of torque input and rotation. Accordingly, the first tower shaft <NUM> is not back driven by the superposition gearbox <NUM>. The second clutch assembly <NUM> is engaged to couple the carrier <NUM> to the sun gear shaft <NUM>. Accordingly, rotation of the carrier <NUM> by the lay shaft <NUM> drives the sun gear shaft <NUM> and, thereby, the second tower shaft <NUM>. Driving the second tower shaft <NUM> rotates the high speed spool <NUM> to provide a rotational input, schematically shown at <NUM>. None of the torque or rotational input provided by the lay shaft <NUM> is transmitted to the low spool <NUM>. The high speed spool <NUM> is driven until the engine starts and begins spinning under its own power as is known and understood by those skilled in gas turbine engine structure and operation.

Referring to <FIG>, with continued reference to <FIG>, a forward wind milling operating condition is schematically shown and includes an input of torque on the low speed spool <NUM> from the fan section <NUM>. Airflow schematically shown at <NUM> passing through the fan <NUM> when the engine is not operating will cause rotation of the low spool <NUM>. Rotation of the low spool <NUM> in turn causes rotation of various structures in the engine. Forward rotation of the engine requires lubricant to be supplied. In this example embodiment, rotation of the low spool <NUM> causes rotation of the geared architecture <NUM> (<FIG> and <FIG>) and therefore creates a need to provide lubricant to rotating components. Moreover, other components of the engine such as the support bearings as well of the superposition gearbox <NUM> require lubricant when rotated. Accordingly, the disclosed accessory drive system <NUM> drives the lay shaft <NUM> to drive the accessory gearbox <NUM> that in turn drives a lubricant pump <NUM>. The lubricant pump <NUM> drives lubricant through a system of conduits schematically shown at <NUM> of the lubrication system schematically indicated at <NUM> that provides lubricant to engine components as indicated at <NUM> and the superposition gearbox <NUM>. The example lubrication system <NUM> is shown schematically and is contemplated to include features that distribute lubricant throughout the engine <NUM> as would be understood by one skilled in turbine engines.

Operation of the accessory drive system <NUM> in the illustrated forward wind milling operating condition includes rotation of the low speed spool <NUM> caused by airflow <NUM> through the fan section <NUM>. Rotation of the low speed spool <NUM> drives the first tower shaft <NUM>. The first clutch <NUM> is coupled such that rotation of the first tower shaft <NUM> rotates the ring gear shaft <NUM> and the ring gear <NUM>. The high speed spool <NUM> is stationary and does not provide an input to the second tower shaft <NUM> and the sun gear <NUM>. The sun gear <NUM> is therefore held stationary. The sun gear <NUM> rotates with the carrier <NUM> because the second clutch assembly <NUM> does not allow the carrier <NUM> to rotate faster than the sun gear <NUM>. Accordingly, because the high speed spool <NUM> is attached to the sun gear <NUM>, the high speed spool <NUM> becomes a parasitic drag on the system to slow or prevent rotation of the sun gear <NUM>. Rotation of the ring gear <NUM> in combination with the sun gear <NUM> drives the intermediate gears <NUM> about the axis <NUM>. Rotation of the intermediate gears <NUM> drives the carrier <NUM> and thereby the lay shaft <NUM>. Accordingly, the input schematically indicated at <NUM> from the low speed spool <NUM> into the superposition gearbox <NUM> generates an output <NUM> to drive the accessory gearbox <NUM>. Driving of the gearbox <NUM> in turn drives the pump <NUM> to operate the lubrication system <NUM>.

Referring to <FIG>, an aft wind milling operating condition is schematically shown where airflow <NUM> from aft of the engine is driving rotation of the fan <NUM>. Airflow <NUM> from the aft direction causes the fan section <NUM> and thereby the low speed spool <NUM> to rotate in about the axis A in a direction that does not cause coupling between the ring gear shaft <NUM> and the ring gear <NUM>. Accordingly, neither the torque from the low speed spool <NUM> or the high speed spool <NUM> is transmitted through the superposition gearbox <NUM>. In the reverse wind milling operating condition, rotation of the high speed spool <NUM> is not possible and therefore lubricant flow is not necessary. Accordingly, the first clutch <NUM> does not couple the ring gear shaft <NUM> to the ring gear <NUM> and no torque is transferred to the accessory gearbox <NUM>.

The example accessory drive system <NUM> includes a superposition gearbox <NUM> that automatically distributes input driving torque between the low speed spool <NUM>, the high speed spool <NUM> and the accessory gearbox <NUM> as required during engine operation. The selective operation of the superposition gearbox <NUM> is enabled by first and second one-way clutches that provide different combinations of inputs and outputs that automatically couple based engine operating conditions.

Referring now to <FIG> and <FIG>, with continued reference to <FIG> and <FIG>, various performance charts of the accessory drive system <NUM> are illustrated in <FIG>, in accordance with the present disclosure. The speed into the superposition gearbox <NUM> is a function of both the speed of the high pressure compressor <NUM> and the speed of the low pressure compressor <NUM> because the superposition gearbox <NUM> has two input shafts including the sun gear shaft <NUM> that is driven by the high pressure compressor <NUM> and the ring gear shaft <NUM> that is driven by the low pressure compressor <NUM>. Since the low speed spool <NUM> has a large speed excursion (i.e., max speed minus min speed) and the high speed spool <NUM> has a relatively small speed excursion, the design of the superposition gearbox <NUM> is a tradeoff of many factors, including, but not limited to, output speed excursion, power split, starting torque, and idle thrust. Therefore, the selection of gear ratios in the low speed spool drive train, high speed spool drive train, and the superposition gearbox <NUM> are difficult to determine.

A low speed spool drive train runs from the low speed output gear <NUM> to the ring gear shaft drive gear <NUM> and includes the low speed output gear <NUM>, the first gear <NUM>, the second gear <NUM>, and the ring gear shaft drive gear <NUM>. The low speed spool drive train gear ratio is measured from the low speed output gear <NUM> to the ring gear shaft drive gear <NUM>. The low speed spool drive train gear ratio may be defined as a ratio of the number of gear teeth of the first gear <NUM> to the number of gear teeth of the low speed output gear <NUM> (i.e., the number of gear teeth of the first gear <NUM> divided by the number of gear teeth of the low speed output gear <NUM>) multiplied by a ratio of the number of gear teeth of the ring gear shaft drive gear <NUM> to the number of gear teeth of the second gear <NUM> (i.e., the number of gear teeth of the ring gear shaft drive gear <NUM> divided by the number of gear teeth of the second gear <NUM>), as illustrated by equation (i) below.

A high speed spool drive train runs from the high speed output gear <NUM> to the sun gear shaft drive gear <NUM> and includes the high speed output gear <NUM>, the first gear <NUM>, the second gear <NUM>, and the sun gear shaft drive gear <NUM>. The high drive spool drive train gear ratio is measured from the high speed output gear <NUM> to the sun gear shaft drive gear <NUM>. The high speed spool drive train gear ratio may be defined as a ratio of the number of gear teeth of the first gear <NUM> to the number of gear teeth of the high speed output gear <NUM> (i.e., the number of gear teeth of the first gear <NUM> divided by the number of gear teeth of the high speed output gear <NUM>) multiplied by a ratio of the number of gear teeth of the sun gear shaft drive gear <NUM> to the number of gear teeth of the second gear <NUM> (i.e., the number of gear teeth of the sun gear shaft drive gear <NUM> divided by the number of gear teeth of the second gear <NUM>), as illustrated by equation (ii) below.

The epicycle gear ratio of the superposition gearbox <NUM> is measured from the ring gear <NUM> to the sun gear <NUM>. The epicycle gear ratio may be defined as a ratio of the number of gear teeth of the ring gear <NUM> to the number of gear teeth of the sun gear <NUM> (i.e., the number of gear teeth of the ring gear <NUM> divided by the number of gear teeth of the sun gear <NUM>), as illustrated by equation (iii) below.

The embodiments disclosed herein seek to provide a superposition gearbox <NUM> in which the low speed spool drive train gear ratio and the high speed spool drive train gear ratio are in a range that optimizes four factors including: (<NUM>) minimize speed excursion of the accessory drivetrain (i.e., minimize speed excursion of lay shaft <NUM> drives the accessory gearbox <NUM>); (<NUM>) maximize power extraction from the low pressure compressor <NUM> at idle; and (<NUM>) minimize starting torque on the second tower shaft <NUM>; and (<NUM>) reduce idle thrust to maximum acceptable level.

The low speed spool drive train gear ratio may be between <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. The low speed spool drive train gear ratio may be about equal to <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>.

The high speed spool drive train gear ratio may be between <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. The low speed spool drive train gear ratio may be about equal to <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>.

The epicycle gear ratio may be between <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. The epicycle gear ratio may be about equal to <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>.

As illustrated in <FIG>, data point (a) represents a conventional engine designed have <NUM>% power extracted from the low speed spool <NUM>, data point (b) is the low speed spool extraction engine that utilizes <NUM>% power extracted from the low speed spool <NUM>, and data point (c) is the low speed spool extraction engine that utilizes a combination of power extracted from the low speed spool <NUM> and the high speed spool <NUM>. Data point (c) represents an optimal location of the charts illustrated in <FIG>.

As illustrated in <FIG>, a low speed spool drive train gear ratio between <NUM> to <NUM>, a high speed spool drive train gear ratio between <NUM> to <NUM>, and an epicycle gear ratio may be between <NUM> to <NUM> produce an output speed <NUM> between <NUM> and <NUM> at a low spool power extract percentage <NUM> of between <NUM>% and <NUM>%. Therefore accomplishing the goals to (<NUM>) minimize speed excursion of the accessory drivetrain (i.e., minimize speed excursion of lay shaft <NUM> drives the accessory gearbox <NUM>) and (<NUM>) maximize power extraction from the low pressure compressor <NUM> at idle.

As illustrated in <FIG>, illustrates that a large HR gear ratio (e.g., a gear ratio ><NUM>) may be necessary to slow down the high spool speed <NUM> to the sun gear <NUM> and as a result the torque on the second tower shaft <NUM> increases. Advantageously, since the second tower shaft <NUM> is on the outside of the first tower shaft <NUM>, the second tower shaft <NUM> can better carry this increase in torque. Therefore, the optimum can be at greater than <NUM>% torque since the second tower shaft <NUM> is larger. Data point (a) represents a conventional engine and represents a baseline where data point (c) is assumed to be about the same as the baseline because even with <NUM>% power extracted from the low speed spool <NUM>, the gas turbine engine <NUM> by turning the high speed spool <NUM>.

As illustrated in <FIG>, a low speed spool drive train gear ratio between <NUM> to <NUM>, a high speed spool drive train gear ratio between <NUM> to <NUM>, and an epicycle gear ratio may be between <NUM> to <NUM> produce an idle thrust <NUM> between <NUM>% and <NUM>% at a low spool power extract percentage <NUM> of between <NUM>% and <NUM>%. Therefore accomplishing the goal to (<NUM>) reduce idle thrust to maximum acceptable level. The "maximum acceptable level" of idle thrust depends on the bypass ratio and fan geometry of the gas turbine engine <NUM>. It is advantageous to reduce idle thrust <NUM> just enough so that the aircraft can taxi without riding the brakes (e.g., too much idle thrust) or by having to throttle up excessively and burning fuel (e.g., too little idle thrust).

Advantageously, a low speed spool drive train gear ratio between <NUM> to <NUM>, a high speed spool drive train gear ratio between <NUM> to <NUM>, and an epicycle gear ratio may be between <NUM> to <NUM> result in a parameterized optimization of an epicyclic superposition drive for multi-rotor power extraction. Some benefits of a low speed spool drive train gear ratio between <NUM> to <NUM>, a high speed spool drive train gear ratio between <NUM> to <NUM>, and an epicycle gear ratio may be between <NUM> to <NUM> include an increased high pressure compressor <NUM> stall margin at idle, reduced idle thrust, a reduced generator speed excursion (e.g., therefore smaller lighter generator as the accessory component <NUM>), minimized packaging of the second tower shaft <NUM> (e.g., less obstruction to the gas path), and optimized idle thrust level.

Claim 1:
A gas turbine engine (<NUM>) comprising:
a low speed spool (<NUM>) including a low pressure compressor (<NUM>);
a low speed output gear (<NUM>) disposed on the low speed spool (<NUM>);
a high speed spool (<NUM>) including a high pressure compressor (<NUM>);
a high speed output gear (<NUM>) disposed on the high speed spool (<NUM>);
a first tower shaft (<NUM>) engaged to the low speed spool (<NUM>) at the low speed output gear (<NUM>);
a second tower shaft (<NUM>) engaged to the high speed spool (<NUM>) at the high speed output gear (<NUM>);
a superposition gearbox (<NUM>) including a sun gear (<NUM>), a plurality of intermediate gears (<NUM>) engaged to the sun gear (<NUM>) and supported in a carrier (<NUM>), and a ring gear (<NUM>) circumscribing the intermediate gears (<NUM>);
a ring gear shaft (<NUM>) coupled to drive the ring gear (<NUM>);
a ring gear shaft drive gear (<NUM>) disposed on the ring gear shaft (<NUM>) and engaged with the first tower shaft (<NUM>);
a sun gear shaft (<NUM>) coupled to drive the sun gear (<NUM>); and
a sun gear shaft drive gear (<NUM>) disposed on a sun gear shaft (<NUM>) and engaged with the second tower shaft (<NUM>);
characterized in that:
a low speed spool drive train gear ratio is between <NUM> to <NUM>, the low speed spool drive train gear ratio being measured from the low speed output gear (<NUM>) to the ring gear shaft drive gear (<NUM>);
the sun gear shaft (<NUM>) is directly coupled to the sun gear (<NUM>); and
the first tower shaft (<NUM>) and the second tower shaft (<NUM>) are concentric about a common axis (<NUM>).