The present invention relates to auxiliary power units (APUs). In particular, the present invention relates to a mounting system for restraining the APU should one or more struts retaining the APU in position fail.
Many aircraft typically include on-board APUs, located in the tail sections of the aircraft, to provide electrical power and compressed air for systems throughout the aircraft. When an aircraft is on the ground, the primary propulsion engines of the aircraft are shut down, and the APU provides the main power source for a variety of systems and serves as a main engine starter. The APU may also provide power during in-flight operations for various systems.
The APU is supported within a compartment in the tail section of the aircraft by a plurality of mount struts (typically six or more). These mount struts operate to constrain the APU from movement relative to other components within the tail section (e.g., a bell mouth, tailcone structure, an exhaust silencer, and an exhaust pipe). APUs must be certified as failsafe. This certification requires that the APU cannot become disattached from the tail section under any operating condition. In the past, to meet this requirement redundant mount struts were added to the tail section to further restrain the APU. Unfortunately, redundant mount struts add to the overall weight of the aircraft, add preload into the system that must be accounted for, and make installation of the APU more complicated.
An auxiliary power unit assembly includes an auxiliary power unit with an auxiliary power unit exhaust duct, a bell mouth, and one or more bell mouth struts. The auxiliary power unit exhaust duct extends from the auxiliary power unit and has an aft end portion that is received within the bell mouth. The bell mouth struts extend from an inner surface of the bell mouth and extend generally radially toward the auxiliary power unit exhaust duct.
APU nacelle 14 houses APU 16 and mount struts 18a, 18b, 18c, and 18d therein. Mount struts 18a, 18b, 18c, and 18d extend in various directions from APU 16 to contact structure 12. In
Bell mouth 24 is arranged downstream of and is in fluid communication with APU 16. Bell mouth 24 has an axial length with an upstream end that is flared and a downstream end that has a smaller diameter than the upstream end. Bell mouth 24 is an airflow system that extends annularly around aft end portions of APU exhaust duct 22. Bell mouth 24 extends axially aft to connect to and communicate with exhaust pipe 26. Exhaust silencer 28 is disposed annularly around exhaust pipe 26 and is configured to attenuate the noise of APU 16. More particularly, exhaust silencer 28 has an array of baffles spaced apart axially from one another and extending annularly around exhaust pipe 26 to form tuned chambers to attenuate noise that results from the combustion gases of APU 16. Bell mouth struts 30a, 30b, and 30c (
APU 16 compresses combustion air, adds fuel, and combusts the resulting fuel/air mixture. The resulting hot, high-pressure combustion gas then expands through a turbine stage (not shown) within turbine section 20. The resulting rotation of the turbines is used to generate electrical power for associated devices of the aircraft (not shown). The spent combustion gases exit through APU exhaust duct 22, and flow into bell mouth 24.
Bell mouth 24 acts to draw cooling air from APU compartment 14 and mix the cooling air with the spent combustion gases from APU 16 passing through APU exhaust duct 22. Exhaust pipe 26 communicates with bell mouth 24 and provides a channel for expelling the combustion and cooling gases from aircraft tail section 10. Bell mouth 24 and exhaust pipe 26 generally comprise an annular duct constructed of a sheet metal. Bell mouth struts 30a and 30b do not inhibit the compartment cooling air drawn out exhaust pipe 26.
In the embodiment shown in
Bell mouth struts 30a-30c connect to and extend generally radially from the inner annular surface of bell mouth 24 inward toward the outer surface of APU exhaust duct 22 but terminate adjacent APU exhaust duct 22 without making contact. This leaves a gap 32 between bell mouth struts 30a-30c and APU exhaust duct 22. The size of gap 32 will be application specific but should account for expected movement of APU exhaust duct 22 and bell mouth 24 relative one another due to aircraft maneuvers (e.g., hard landings), thermal growth, and spatial tolerances.
As discussed previously, fail safe struts 30a-30c are configured to support APU 16 via APU exhaust duct 22 should a failure of one of mount struts 18a, 18b, 18c, and 18d or additional mount struts (not shown) occur. Such a failure would cause APU 16 to become unconstrained and deflect slightly, which would bring APU exhaust duct 22 into contact with one or more of bell mouth struts 30a-30c. Bell mouth struts 30a-30c are constructed and sized to prevent deflection of APU 16 out of communication with bell mouth 24 and will prevent unconstrained movement (deflection) of APU 16 that would cause failure of additional components (e.g., additional mount struts 18a, 18b, 18c, and 18d, exterior structure 12, bell mouth 24, exhaust pipe 26, and exhaust silencer 28).
By using bell mouth struts 30a-30c, the use of redundant mount struts (in addition to mount struts 18a, 18b, 18c, and 18d) can be avoided. By avoiding the use of redundant mount struts, the overall weight of the aircraft can be maintained rather than increased, additional preload in the system can be avoided, and installation of APU 16 can be simplified.
While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.
This Application claims the benefit of Provisional Application No. 61/486,544 filed on May 16, 2011, and entitled “AUXILIARY POWER UNIT MOUNTING FEATURE”, the entire content of which is expressly incorporated by reference.
Number | Date | Country | |
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61486544 | May 2011 | US |