# Patent

## Title
Counter-rotating gearbox for tip turbine engine

## Abstract

A tip turbine engine provides an axial compressor rotor that is counter-rotated relative to a fan. A planetary gearset couples rotation of a fan to an axial compressor rotor, such that the axial compressor rotor is driven by rotation of the fan in a rotational direction opposite that of the fan. By counter-rotating the axial compressor rotor, a final stage of compressor vanes between the final stage of compressor blades and inlets to the hollow fan blades of the fan are eliminated. As a result, the length of the axial compressor and the overall length of the tip turbine engine are decreased.

## Background
This application is a divisional of U.S. application Ser. No. 11/719,224, now U.S. Pat. No. 7,937,927, filed on May 14, 2007, which is a National Phase of to PCT Application No. PCT/US2004/039961 filed on Dec. 1, 2004.


This invention was conceived in performance of U.S. Air Force contract F33657-03-C-2044. The government may have rights in this invention.


BACKGROUND OF THE INVENTION
The present invention relates to a tip turbine engine, and more particularly to a gearbox for counter-rotatably driving an axial compressor of a tip turbine engine.
An aircraft gas turbine engine of the conventional turbofan type generally includes a forward bypass fan, a low pressure compressor, a middle core engine, and an aft low pressure turbine, all located along a common longitudinal axis. A high pressure compressor and a high pressure turbine of the core engine are interconnected by a high spool shaft. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in a combustor, where it is ignited to form a high energy gas stream. The gas stream flows axially aft to rotatably drive the high pressure turbine, which rotatably drives the high pressure compressor via the high spool shaft. The gas stream leaving the high pressure turbine is expanded through the low pressure turbine, which rotatably drives the bypass fan and low pressure compressor via a low spool shaft.
Although highly efficient, conventional turbofan engines operate in an axial flow relationship. The axial flow relationship results in a relatively complicated elongated engine structure of considerable length relative to the engine diameter. This elongated shape may complicate or prevent packaging of the engine into particular applications.
A recent development in gas turbine engines is the tip turbine engine. Tip turbine engines include hollow fan blades that receive core airflow therethrough such that the hollow fan blades operate as a centrifugal compressor. Compressed core airflow from the hollow fan blades is mixed with fuel in an annular combustor, where it is ignited to form a high energy gas stream which drives the turbine that is integrated onto the tips of the hollow bypass fan blades for rotation therewith as generally disclosed in U.S. Patent Application Publication Nos.: 20030192303; 20030192304; and 20040025490. The tip turbine engine provides a thrust-to-weight ratio equivalent to or greater than conventional turbofan engines of the same class, but within a package of significantly shorter length.
In the known tip turbine engine designs, an axial compressor rotor is directly driven by rotation of the hollow fan blades. The axial compressor rotor includes one or more stages of radially-extending compressor blades each of which followed by a stage of static compressor vanes extending radially inwardly from a compressor case. Increasing the number of stages of compressor blades and compressor vanes increases the compression of the core airflow and the efficiency of the engine, but increases the overall length and weight of the engine and the number of parts.
SUMMARY OF THE INVENTION
A tip turbine engine according to the present invention provides at least one gear coupling the rotation of the fan to an axial compressor rotor, such that the axial compressor rotor is driven in a rotational direction opposite that of the fan. In the disclosed embodiment, the at least one gear is an epicyclic gearset, such as planetary gearset. The axial compressor rotor is coupled to a sun gear and the fan is coupled to a planet carrier. The planet carrier carries a plurality of first planet gears engaging the sun gear and a plurality of second planet gears engaging the first planet gears and a ring gear.
In this manner, the direction of rotation of the axial compressor rotor is reversed relative to the direction of rotation of the fan by the planetary gearset. Because the axial compressor blades are rotating in a direction opposite that of the inducer sections of the fan blades, it is unnecessary to redirect the flow of the compressed core airflow prior to the inducer sections. Therefore, a final stage of compressor vanes between the final stage of axial compressor blades and the inducer sections is eliminated, thereby reducing the length of the axial compressor and the overall length of the tip turbine engine. Additionally, the angle of the inducer relative to the engine axis is increased, which thereby shortens the overall axial dimension of the inducer, thereby further reducing the overall length of the engine.



BRIEF DESCRIPTION OF THE DRAWINGS
Other advantages of the present invention can be understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
FIG. 1 is a partial sectional perspective view of a tip turbine engine according to the present invention.
FIG. 2 is a longitudinal sectional view of the tip turbine engine of FIG. 1 taken along an engine centerline.
FIG. 3 is a perspective view of the gearbox assembly of FIG. 2.
FIG. 4 illustrates the relationship of the angles of the last stage of compressor blades and the inducer sections in the counter-rotating axial compressor rotor of FIG. 2.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a general perspective partial sectional view of a tip turbine engine (TTE) type gas turbine engine 10. The engine 10 includes an outer nacelle 12, a rotationally fixed static outer support structure 14 and a rotationally fixed static inner support structure 16. A plurality of fan inlet guide vanes 18 are mounted between the static outer support structure 14 and the static inner support structure 16. Each inlet guide vane preferably includes a variable trailing edge 18A.
A nosecone 20 is preferably located along the engine centerline A to improve airflow into an axial compressor 22, which is mounted about the engine centerline A behind the nosecone 20.
A fan-turbine rotor assembly 24 is mounted for rotation about the engine centerline A aft of the axial compressor 22. The fan-turbine rotor assembly 24 includes a plurality of hollow fan blades 28 to provide internal, centrifugal compression of the compressed airflow from the axial compressor 22 for distribution to an annular combustor 30 located within the rotationally fixed static outer support structure 14.
A turbine 32 includes a plurality of tip turbine blades 34 (two stages shown) which rotatably drive the hollow fan blades 28 relative a plurality of tip turbine stators 36 which extend radially inwardly from the rotationally fixed static outer support structure 14. The annular combustor 30 is disposed axially forward of the turbine 32 and communicates with the turbine 32.
Referring to FIG. 2, the rotationally fixed static inner support structure 16 includes a splitter 40, a static inner support housing 42 and a static outer support housing 44 located coaxial to said engine centerline A.
The axial compressor 22 includes the axial compressor rotor 46, from which a plurality of compressor blades 52 extend radially outwardly, and a fixed compressor case 50. A plurality of compressor vanes 54 extend radially inwardly from the compressor case 50 between stages of the compressor blades 52. The compressor blades 52 and compressor vanes 54 are arranged circumferentially about the axial compressor rotor 46 in stages (three stages of compressor blades 52 and two stages of compressor vanes 54 are shown in this example). The axial compressor rotor 46 is mounted for rotation upon the static inner support housing 42 through a forward bearing assembly 68 and an aft bearing assembly 62.
The fan-turbine rotor assembly 24 includes a fan hub 64 that supports a plurality of the hollow fan blades 28. Each fan blade 28 includes an inducer section 66, a hollow fan blade section 72 and a diffuser section 74. The inducer section 66 receives airflow from the axial compressor 22 generally parallel to the engine centerline A and turns the airflow from an axial airflow direction toward a radial airflow direction. The airflow is radially communicated through a core airflow passage 80 within the fan blade section 72 where the airflow is centrifugally compressed. From the core airflow passage 80, the airflow is diffused and turned once again toward an axial airflow direction toward the annular combustor 30. Preferably, the airflow is diffused axially forward in the engine 10, however, the airflow may alternatively be communicated in another direction.
A gearbox assembly 90 aft of the fan-turbine rotor assembly 24 reverses the direction of rotation and provides a speed increase between the fan-turbine rotor assembly 24 and the axial compressor 22. In the embodiment shown, the speed increase is at a 3.34-to-one ratio. The gearbox assembly 90 may be an epicyclic gearbox, such as a planetary gearbox that provides counter-rotating engagement between the fan-turbine rotor assembly 24 and an axial compressor rotor 46. The gearbox assembly 90 is mounted for rotation between the static inner support housing 42 and the static outer support housing 44. Referring to FIGS. 2 and 3, the gearbox assembly 90 includes a sun gear 92, which rotates the axial compressor 22, and a planet carrier 94, which rotates with the fan-turbine rotor assembly 24. A plurality of first planet gears 93 each engage the sun gear 92 and one of a plurality of second planet gears 98 (visible only in FIG. 3). The second planet gears 98 each engage one of the first planet gears and a rotationally fixed ring gear 95. The first planet gears 93 and second planet gears 98 are mounted to the planet carrier 94. The gearbox assembly 90 is mounted for rotation between the sun gear 92 and the static outer support housing 44 through a gearbox forward bearing 96 and a gearbox rear bearing 99. The sun gear 92 is rotationally engaged with the axial compressor rotor 46 at a splined interconnection 100 or the like.
It should be noted that the gearbox assembly 90 could utilize other types of gear arrangements or other gear ratios and that the gearbox assembly 90 could be located at locations other than aft of the axial compressor 22. For example, the gearbox assembly 90 could be located at the front end of the axial compressor 22. Alternatively, the gearbox assembly 90 could provide a speed decrease between the fan-turbine rotor assembly 24 and the axial compressor rotor 46.
FIG. 4 illustrates the relationship of the angle of the compressor blades 52 (one shown) in the last stage of counter-rotating compressor blades 52 to the inducer sections 66 in the engine 10 of FIG. 2. As indicated above, the counter-rotation of the compressor blades 52 permits the final stage of compressor blades 52 to be positioned immediately adjacent the inducer sections 66, thereby eliminating an otherwise-necessary intervening stage of compressor vanes 54 (FIG. 2). Referring to the compressor blade velocity triangle 112 in FIG. 4, the compressor blade 52 is angled relative to the engine centerline A, which gives an angle of a relative velocity vector, vr1. The velocity of the counter-rotating compressor blade 52 gives a blade velocity vector, vb1. The resultant vector, indicating the resultant core airflow from the compressor blade 52, is the absolute velocity vector, va1. The leading edge 114 of the inducer section 66 is angled to efficiently receive the core airflow from the compressor blade 52, which flows toward the inducer section 66 at the absolute velocity vector, va1. The absolute velocity vector, va1, is reproduced as absolute velocity vector, va2 in the inducer velocity triangle 116. Since the inducer section 66 is moving as shown by blade velocity vector va2, the leading edge 114 is angled parallel to a relative velocity vector vr2, which together with blade velocity vector vb2 would result in absolute velocity vector, va2, to match the angle of the core airflow incoming from the compressor blade 52.
Although the specific angles will depend on a variety of factors, including anticipated blade velocities and the design choices made in the earlier stages of the compressor blades 52, two general benefits are provided by counter-rotation. First, there is no need for a non-rotating stage of compressor vanes between the last stage of compressor blades 52 and the inducer sections 66, since compression is provided by the counter-rotation of the compressor blades 52 and inducer sections 66 which would not be provided by adjacent, co-rotating components. This reduces the overall length and weight of the engine 10 and the number of parts in the engine 10. Second, the absolute velocity vector va2 of the core airflow into the inducer sections 66 has a component that is opposite the blade velocity vector vb2, which would not exist in a co-rotating arrangement with a compressor vane between the compressor blades 52 and the inducer section 66. As a result, the angle α of the leading edge 114 of the inducer section 66 relative the engine centerline A is significantly increased, thereby shortening the axial length of the inducer (since it requires a certain absolute length, in order to turn the core airflow from axial flow to radial flow), and decreasing the overall length of the engine 10.
In operation, core airflow enters the axial compressor 22, where it is compressed by the compressor blades 52. The compressed air from the axial compressor 22 enters the inducer section 66 in a direction generally parallel to the engine centerline A, and is then turned by the inducer section 66 radially outwardly through the core airflow passage 80 of the hollow fan blades 28. The airflow is further compressed centrifugally in the hollow fan blades 28 by rotation of the hollow fan blades 28. From the core airflow passage 80, the airflow is turned and diffused axially forward in the engine 10 into the annular combustor 30. The compressed core airflow from the hollow fan blades 28 is mixed with fuel in the annular combustor 30 and ignited to form a high-energy gas stream.
The high-energy gas stream is expanded over the plurality of tip turbine blades 34 mounted about the outer periphery of the fan-turbine rotor assembly 24 to drive the fan-turbine rotor assembly 24, which in turn counter-rotatably drives the axial compressor 22 via the gearbox assembly 90. As explained above, counter-rotation of the axial compressor 22 permits elimination of a stage of compressor vanes. As a result, the length of the axial compressor 22 and the overall length of the tip turbine engine 10 are decreased.
The fan-turbine rotor assembly 24 discharges fan bypass air axially aft to merge with the core airflow from the turbine 32 in an exhaust case 106. A plurality of exit guide vanes 108 are located between the static outer support housing 44 and the rotationally fixed static outer support structure 14 to guide the combined airflow out of the engine 10 and provide forward thrust. An exhaust mixer 110 mixes the airflow from the turbine blades 34 with the bypass airflow through the fan blades 28.
In accordance with the provisions of the patent statutes and jurisprudence, exemplary configurations described above are considered to represent a preferred embodiment of the invention. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.

## Claims

1. A method of operating a turbine engine including the steps of:
rotatably driving a fan having a plurality of hollow fan blades with a turbine mounted to an outer periphery of the fan, wherein the turbine includes turbine blades disposed radially outward of the plurality of hollow fan blades; and
counter-rotatably driving an axial compressor rotor via rotation of the fan, wherein the axial compressor rotor is counter-rotatably driven by a planetary gearset that couples the fan to the axial compressor rotor.

2. The method of claim 1 further including the step of compressing air with the axial compressor rotor and guiding the compressed air radially outwardly through interiors of the plurality of fan blades toward the turbine.

3. The method of claim 2 further including the step of centrifugally compressing the compressed air in the interiors of the plurality of fan blades.

4. The method of claim 1 wherein a rate at which the axial compressor rotor is driven is higher than a rate of rotation of the fan.

5. The method of claim 4, wherein the ratio of the rate at which the axial compressor rotor is driven to the rate of rotation of the fan is 3.34-to-1.

6. The method of claim 1 further including the step of coupling the axial compressor rotor to a sun gear of the planetary gearset.

7. The method of claim 6 further including the step of coupling a planet carrier of the planetary gearset with the fan, the planetary gearset including a first planet gear engaging the sun gear and a second planet gear engaging the first planet gear, at least one of the first planet gear and the second planet gear mounted to the planet carrier.

8. The method of claim 1, wherein an axial compressor corresponding to the axial compressor rotor comprises at least one stationary compressor vane.

9. The method of claim 1, wherein the planetary gearset is aft of the fan and the turbine.

10. The method of claim 1, wherein an axial compressor corresponding to the axial compressor rotor only includes a plurality of compressor blades that are counter-rotatably driven relative to the plurality of fan blades.

11. A method of operating a turbine engine including the steps of:
rotatably driving a fan having a plurality of fan blades with a turbine mounted at an outer periphery of the fan; and
counter-rotatably driving an axial compressor rotor via rotation of the fan, wherein the axial compressor rotor is counter-rotatably driven by a planetary gearset that couples the fan to the axial compressor rotor, wherein an axial compressor corresponding to the axial compressor rotor comprises at least one stationary compressor vane between two of a plurality of compressor blades.

12. The method of claim 11 further including the step of compressing air with the axial compressor rotor and guiding the compressed air radially outwardly through interiors of the plurality of fan blades toward the turbine.

13. The method of claim 11 further including the step of coupling the axial compressor rotor to a sun gear of the planetary gearset.

14. The method of claim 13 further including the step of coupling a planet carrier of the planetary gearset with the fan, the planetary gearset including a first planet gear engaging the sun gear and a second planet gear engaging the first planet gear, at least one of the first planet gear and the second planet gear mounted to the planet carrier.

15. The method of claim 11, wherein the planetary gearset is aft of the fan and the turbine.

16. The method of claim 11, wherein the ratio of a rate at which the axial compressor rotor is driven to the rate of rotation of the fan is 3.34-to-1.

17. The method of claim 11, wherein an axial compressor corresponding to the axial compressor rotor only includes a plurality of compressor blades that are counter-rotatably driven relative to the plurality of fan blades.