Patent Application
20110120079
This disclosure relates to a variable area fan nozzle for a turbofan engine that includes features for controlling mode shape of the airfoil.
Gas turbine engines that have an engine cycle modulated with a variable area fan nozzle (VAFN) provide a smaller fan exit nozzle diameter during cruise conditions and a larger fan exit nozzle diameter during take-off and landing conditions. The VAFN typically includes an airfoil that moves between desired positions.
A design requirement for the VAFN is to maintain structural integrity throughout the flight envelope of the aircraft. Flow turbulence and mechanical vibrations subject the VAFN to both tonal and broadband aerodynamic loads that cause the nozzle to elastically deflect from a desired position.
A nacelle assembly for a high-bypass gas turbine engine is disclosed and includes a variable area fan nozzle (VAFN) including at least one second fan nacelle section movable relative to the first fan nacelle section to modify an area of the bypass flow path. At least one stiffener is mounted between the second fan nacelle section and the first fan nacelle section.
An example gas turbofan engine includes a core engine that drives a fan through a gear drive mechanism. The core engine is housed within a core nacelle and a fan nacelle at least partially surrounds the core nacelle and defines a bypass flow path around the core nacelle. Airflow into the fan nacelle is communicated into the core engine. The VAFN is operated to effectively vary the fan nozzle exit area to adjust fan bypass air flow to provide desired engine operation over a range of flight conditions with respect to performance and other operational parameters.
The stiffeners are mounted to between the second fan nacelle section and the first fan nacelle section and modify mode shapes and natural frequencies produced by the unsteady aerodynamic loads on the VAFN to substantially reduce and/or eliminate flutter within a desired flight envelope.
These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description.
Referring to
The example engine 10 is a high-bypass geared aircraft engine with a high bypass ratio. The turbofan 14 is much larger than the core engine 12 and provides most of the thrust produced by the engine 10. The gear mechanism 16 provides a desired reduction in speed of the fan 14 to provide the desired thrust output. As appreciated, other engine configurations will benefit from this disclosure and are within the contemplation of this invention.
A fan nacelle 40 at least partially surrounds the core nacelle 38 and defines a bypass flow path 44 around the core nacelle 38. Airflow into the fan nacelle 40 is communicated into the core engine 12. The airflow is compressed by the low pressure compressor 18 and the high pressure compressor 20 before being directed into the combustor 22. Fuel is mixed with air in the combustor 22 and ignited to produce a gas flow that drives the turbines 24, 26. The turbines 24, 26 in turn drive the compressors 18 and 20 and the fan 14 through the gear drive mechanism 16. The gas flow expands through the turbines 24, 26 and is exhausted through the core flow path 52 defined as an annular opening between a nose cone 48 and the core nacelle 38.
The core engine 12 and core nacelle 38 are supported within the fan nacelle 40 along the axis 36 by mount supports 32. The bypass flow path 44 is defined between the core nacelle 38 and the fan nacelle 40 as an annular channel for bypass air flow 50. The example engine 10 is a high bypass configuration where the majority of airflow is directed through the bypass flow path 44. Bypass air flow 50 is discharged through a variable area fan nozzle (VAFN) 45 that defines a nozzle exit area 46 between the fan nacelle 40 and the core nacelle 38.
Thrust produced by the engine 10 is a function of density, velocity and area. Manipulating these parameters varies the direction and magnitude of thrust generated by the bypass flow 50. The VAFN 45 varies the area of the fan nozzle exit area 46 to adjust the pressure ratio of the bypass flow 50. Low pressure ratio turbofans are desirable for their high propulsive efficiency. However, low pressure ratio fans are susceptible to fan stability and flutter problems at low power and low flight speeds. The VAFN 45 provides for the engine to operate at a more favorable fan operating line at low power, avoiding the instability region and still provide the relatively smaller nozzle area necessary to obtain a high-efficiency fan operating line at cruise.
A significant amount of thrust is provided by the bypass flow 50 due to the high bypass ratio. The fan 14 of the engine 10 is preferably designed for a particular flight condition—typically cruise at 0.8M and 35,000 feet. As the fan 14 is efficiently designed at a particular fixed stagger angle for an efficient cruise condition, the VAFN 45 is operated to effectively vary the fan nozzle exit area 46 to adjust fan bypass air flow 50 such that the angle of attack or incidence on the fan blades is maintained close to the design incidence for efficient engine operation at other flight conditions, such as landing and takeoff to thus provide optimized engine operation over a range of flight conditions with respect to performance and other operational parameters.
The example VAFN 45 includes a second fan nacelle section 42 that is movable axially relative to the first fan nacelle section 40 to adjust the exit area 46. The second nacelle section 42 is driven axially by actuators 54 commanded by a controller 56.
Referring to
The second fan nacelle section 42 slides axially to change the physical area and geometry of the bypass flow path 44 during particular flight conditions. The bypass flow 50 is effectively altered by sliding of the second fan nacelle section 42 relative the fan nacelle section 40 between a closed position (
In operation, the VAFN 45 communicates with the controller 56 to move the second fan nacelle section 42 relative the first fan nacelle section 40 to effectively vary the area defined by the fan nozzle exit area 46. Various control systems including an engine controller or an aircraft flight control system may also be usable with the present invention. By adjusting the axial position of the entire periphery of the second fan nacelle section 42, engine thrust and fuel economy are maximized during each flight regime by varying the fan nozzle exit area 46.
The VAFN 45 encounters unsteady loads caused by mechanical vibrations and high pressure differences on opposing sides of the nacelle structures 40, 42. The structural responses from unsteady loads upon the VAFN 45 are greatest at the resonant frequencies so that the VAFN 45 is designed to withstand many cycles of this forced vibration. Moreover, under certain flow conditions the phasing between the unsteady aerodynamic loads and the displacement of the resonant modes of the structure are such that the net work of the fluid on the structure over an oscillation period is greater than zero. When this occurs, the amplitude of the deflections grows substantially over time resulting in high stress in the structure. This phenomenon is referred to as aeroelastic instability or flutter.
Such flutter or aeroelastic instability of the example second fan nacelle section 42 can cause undesired deflections and variation of the exit flow area 46. Moreover, flow turbulence and mechanical vibration subject the VAFN 45 to both tonal and wide ranging aerodynamic loads. Further under certain flow conditions the phasing between the unsteady aerodynamic loads and the displacement of the resonant modes of the VAFN 45 can cause and magnify deflections of the second fan nacelle section 42. These loads can cause undesired deflections or flutter of the second fan nacelle section 42 in the radial direction indicated by arrows 62. Such fluttering and deflection of the second fan nacelle sections 42 can result in undesired variation of the nozzle exit area 46.
The example VAFN 45 includes stiffeners 64 mounted between the first fan nacelle section 40 and the second fan nacelle section 42 to modify mode shapes and natural frequencies produced to substantially reduce and/or eliminate flutter caused by unsteady aerodynamic loads within desired flight operating ranges.
The example stiffeners 64 include a first base 66 mounted to the fan nacelle 40 and a second base 68 mounted at the leading edge 78 of the second fan nacelle section 42. Although the example stiffener base 68 is mounted near the leading edge 78, other locations that provide the desired flutter control are within the contemplation of this disclosure. A connector 70 is received in both the first and second bases 66, 68 and controls radial movement to the second fan nacelle section 42 relative to the fan nacelle 40. The connector 70 slides axially with movement of the second fan nacelle section 42 between the open and closed positions. The connector 70 is disposed and received in each of the first base 66 and the second base 68 so as to control and prevent undesired outward radial movement of the second fan nacelle section 42. The example stiffeners 64 are not powered to cause movement of the second nacelle section 42 axially. The example stiffeners 64 are driven by movement, and do not restrict axial movement of the second nacelle section 42. Because the stiffeners 64 control and substantially reduce radial movement, the frequency and mode shape of the second fan nacelle section 42 are maintained within desired operating parameters.
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Each of the first and second bases 66, 68 include openings 74 for fasteners that extend through the base and into the core portion 72. The openings 74 may then be filled with a sealant to provide a smooth outer surface and to maintain fastener integrity.
The second base 68 is mounted to the second nacelle section 42 at the leading edge 78. The leading edge 78 is that edge that is most upstream on the second nacelle section 42. As appreciated, the second nacelle section 42 includes a trailing edge 80 (
The example connector 70 is attached with fasteners that extend through openings 76 in the second base. The opposing end of the example connector 70 extends into the first base 66 but is not fastened to the first base. Instead, the connector 70 moves freely within the first base 66 to provide unrestricted axial movement of the second nacelle section 42 relative to the first nacelle section 40.
Axial placement of the second base section 66 at and adjacent to the leading edge 78 of the second nacelle section 42 provides the desired mode shape and frequency changes required to maintain the desired position of the nacelle section 42 when extended axially away from the first fan nacelle section 40.
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The sectors 82, 84 are movable in unison or independently by actuators 54 to provide either a uniform exit area modifications or asymmetric exit area modification that can be utilized to provide a directional thrust. The example illustrated in
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Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
