Abstract:
A method for exhausting gas from an aircraft engine assembly is provided. The method includes coupling a first exhaust duct in fluid communication only to a first engine. The first exhaust duct includes a primary outlet and a secondary outlet. The method further includes coupling a second exhaust duct in fluid communication only to a second engine. The second exhaust duct includes a primary outlet and a secondary outlet. The method also includes aligning a portion of the first engine secondary outlet concentrically within a portion of the second engine secondary outlet.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT 
     The U.S. Government has certain rights in this invention as provided for by the terms of Contract No. MDA 972-98-9-0009 awarded by the Defense Advanced Research Projects Agency (DARPA). 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to aircraft turbine engines, and more specifically, to propulsion systems used with a multiple turbine engine assembly. 
     In some known aircraft, propulsion systems are used to control a flow of exhaust gases for a variety of aircraft functions. For example, such systems can be used to provide thrust for Vertical Take-Off and Landing (VTOL), Short Take-Off Vertical Landing (STOVL) and/or Extreme Short Take-Off and Landing (ESTOL) aircraft. At least some known STOVLs and ESTOLs use vertical thrust posts to facilitate short and extremely short take-offs and landings. In aircraft using vertical thrust posts or nozzles, exhaust from a common plenum is channeled to thrust posts during take-off and landing operations, and, at a predetermined altitude, through a series of valves, the exhaust is channeled from the common plenum to a cruise nozzle. 
     Other known STOVLs and ESTOLs use rotor tip gas reaction driven rotors and rotor tip driven rotors to facilitate vertical take-offs and landings. During flight, some such rotor-driven aircraft can transform into fixed wing aircraft. In some of such aircraft, the engine exhaust gases may also be used to control yaw through aft mounted variable area exhaust nozzles. Other known aircraft use the propulsion system to reduce drag on the aircraft, and/or cool aircraft wings. In such aircraft, gases are channeled through the wings of the aircraft and discharge through a plurality of openings defined in a trailing portion of the wing. Such aircraft are sometimes referred to as having “blown wings”. 
     Because single engine assemblies may limit the survival capabilities of the aircraft when engine power loss occurs, each of the above described propulsion systems may be preferred to be used with a multiple, especially a dual, engine assembly. In a single engine assembly, the engine exhausts into a plenum where it is then channeled via the propulsion system for use by the aircraft. In a dual engine propulsion control system, each engine exhausts into a common plenum wherein the exhaust is channeled for use by the aircraft. However, because the exhausts are mixed in a common plenum, if one engine stalls, fails, or is in any way rendered inoperable, the exhaust flow from that engine may not enter the plenum. The associated pressure drop in the plenum may adversely affect the operation of the propulsion system and valves must be provided to prevent back flow of exhaust gases into the inoperable engine. 
     A control system must also be provided to actuate these valves at a high slew rate to prevent the operable engine from stalling. Such a control system capability has been envisioned but is not known to have been demonstrated. Such a control system would be required to make major airflow adjustments in the propulsion system that may include closing valves to prevent loss of plenum pressure through the inoperable engine and to attempt to redirect nozzle areas to facilitate retaining flight stability. Such significant readjustments of nozzle area configuration may lead to the stall of the operational engine and may cause a complete loss of lift. Furthermore, it may be difficult for the control system to increase the power setting of the operable engine to an emergency level. 
     Because of the time and cost of developing a control system and implementing the mechanical complexity created by such propulsion systems having multiple engines exhaust into a common plenum, such propulsion systems may be much more costly than those used with single engine assemblies. For example, to provide exhaust control, such propulsion systems include a plurality of sensors and valves that can be selectively actuated to isolate an inoperable engine. More specifically, known propulsion systems used with dual engine assemblies include a plurality of butterfly valves and actuation systems that direct the exhaust gases in the event of an engine power loss. The valves enable an inoperable engine to be isolated to prevent back-flow through the inoperative engine. In addition, the isolation valves enable emergency flight capability and limited aircraft control with an inoperable engine. However, because such valves must operate at high temperatures, such valves and associated actuation systems may require large actuation forces. Furthermore, in known propulsion systems, once the valves have been closed to isolate an inoperable engine, it is not possible to attempt to restart an inoperable engine. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect a method for exhausting gas from an aircraft engine assembly is provided. The method includes coupling a first exhaust duct in fluid communication only to a first engine. The first exhaust duct includes a primary outlet and a secondary outlet. The method further includes coupling a second exhaust duct in fluid communication only to a second engine. The second exhaust duct includes a primary outlet and a secondary outlet. The method also includes aligning a portion of the first engine secondary outlet concentrically within a portion of the second engine secondary outlet. 
     In another aspect an engine assembly is provided. The engine assembly includes a first engine including a first exhaust duct coupled in fluid communication only to the first engine. The first exhaust duct includes a primary outlet and a secondary outlet. The engine assembly further includes a second engine including a second exhaust duct coupled in fluid communication only to the second engine. The second exhaust duct includes a primary outlet and a secondary outlet. A portion of the first engine secondary outlet is aligned substantially concentrically within a portion of the second engine secondary outlet. 
     In a still further aspect a propulsion system for an aircraft is provided. The propulsion system includes a first engine including a first exhaust duct coupled in flow communication only to the first engine. The propulsion system further includes a second engine including a second exhaust duct coupled in flow communication only to the second engine. The first exhaust duct includes a primary exhaust and a secondary exhaust. The second exhaust duct includes a primary exhaust and a secondary exhaust. A portion of the first engine secondary exhaust is aligned substantially concentrically within a portion of the second engine secondary exhaust. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top schematic view of an exemplary propulsion system that may be used with a multiple engine assembly; 
         FIG. 2  is a top schematic view of an alternative embodiment of a propulsion system that may be used with a multiple engine assembly; 
         FIG. 3  is a perspective view of a portion of the propulsion system shown in  FIG. 2 ; 
         FIG. 4  is a perspective view of an exemplary plenum that may be used with the propulsion system shown in  FIG. 2 ; and 
         FIG. 5  is a top schematic view of another alternative embodiment of a propulsion system that may be used with a multiple engine assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a top schematic view of an exemplary propulsion system  100  that may be used with a multiple engine assembly  10  used with a VTOL, ESTOL and/or STOVL fixed wing aircraft (not shown). In the exemplary embodiment, engine assembly  10  includes a first engine  102  and a second engine  104 . Alternately, propulsion system  100  can be used with engine assemblies  10  having more than two engines. In an exemplary embodiment, engine  102  includes a first inlet  106  and engine  104  includes a second inlet  108 . Inlets  106  and  108  enable air to be channeled into engines  102  and  104 , respectively. Exhaust gases  110  from first engine  102  are discharged through a first exhaust duct  114  and exhaust gases  112  from second engine  104  are discharged through a second exhaust duct  116 . 
     In the exemplary embodiment, exhaust duct  114  is coupled in flow communication to a first lift duct  118  and to a first cruise duct  120 . A first passageway  122  is coupled to duct  114  to enable exhaust gases  110  to be channeled from first duct  114  to first lift duct  118 . In the exemplary embodiment, at least a portion of first lift duct  118  is oriented substantially perpendicularly to a center line  124  that extends between engines  102  and  104 . Engine assembly  10  and propulsion system  100  are symmetric on each side of center line  124 . In the exemplary embodiment, first lift duct  118  is coupled in flow communication to a starboard lift nozzle  126  and to a port lift nozzle  128 . Alternately, lift nozzles  126  and  128  may be positioned other than starboard and port of center line  124 . Furthermore, alternately, propulsion system  100  can have more or less than two lift nozzles  126  and  128 . In the exemplary embodiment, lift nozzles  126  and  128 , lift nozzles  136  and  138 , and cruise nozzles  130  and  142 , described in more detail below, are each variable area nozzles. The design, construction, and control of variable area nozzles are known in the art. The nozzles  126 ,  128 ,  130 ,  136 ,  138 , and  142  may utilize clamshell or shutter valve designs as is known in the art. Actuation of nozzles  126 ,  128 ,  130 ,  136 ,  138 , and  142  may be hydraulic, electric, or any other method known in the art. Alternately, propulsion system  100  can have more or less than two lift nozzles  126  and  128 . Lift nozzles  126  and  128  extend from lift duct  118  generally toward the ground (not shown). More specifically, in the exemplary embodiment, lift nozzles  126  and  128  are each oriented such that during pre-defined engine operations, exhaust gases  110  discharged therefrom facilitate enabling generally vertical take-off and landing (VTOL) of the aircraft, short take-off and vertical landing (STOVL), and/or extremely short take-off and landing (ESTOL). 
     First cruise duct  120  is coupled in flow communication to duct  114  and extends generally parallel to center line  124 . In the exemplary embodiment, during pre-defined engine operations, exhaust gases  110  are channeled from duct  114  into cruise duct  120 , wherein the gases  110  may be discharged through a first cruise nozzle  130  to generate thrust from the aircraft. More specifically, first cruise nozzle  130  facilitates propelling the aircraft in a direction that is generally parallel to center line  124 . 
     In the exemplary embodiment, second duct  116  is coupled in flow communication to a second lift duct  132  via a second passageway  134 . During pre-defined engine operations, exhaust gases  112  discharged from engine  104  may be channeled from duct  116  through passageway  134  to lift duct  132 . In the exemplary embodiment, at least a portion of second lift duct  132  is oriented substantially perpendicularly to center line  124 . Moreover, in the exemplary embodiment, second lift duct  132  is coupled in flow communication to a second starboard lift nozzle  136  and to a second port lift nozzle  138 . Alternately, lift nozzles  136  and  138  may be positioned other than starboard and port of center line  124 . Furthermore, alternately, propulsion system  100  can have more or less than two lift nozzles  136  and  138 . Lift nozzles  136  and  138  extend from lift duct  132  and are oriented such that exhaust gases  112  discharged therefrom are discharged generally towards the ground. More specifically, in an exemplary embodiment, second lift nozzles  136  and  138  are each oriented such that during pre-defined engine operations, exhaust gases  112  discharged from lift nozzles  136  and  138  facilitate enabling generally vertical take-off and landing of the aircraft, STOVL, and/or ESTOL. Furthermore, in the exemplary embodiment, a portion of each second lift nozzle  136  and  138  extends substantially concentrically within a portion of each of lift nozzles  126  and  128 , respectively. Alternately, a portion of each of first lift nozzles  126  and  128  extends substantially concentrically within a portion of each of lift nozzles  136  and  138 , respectively. 
     In the exemplary embodiment, second duct  116  is coupled to a second cruise duct  140 . Cruise duct  140  extends generally parallel to center line  124  and is positioned adjacent to cruise duct  120 . In the exemplary embodiment, during pre-defined engine operations, exhaust gases  112  are channeled from second duct  116  into cruise duct  140  wherein the gases  112  may be discharged through a second cruise nozzle  142 . More specifically, second cruise nozzle  142  facilitates propelling the aircraft in a direction that is generally parallel to center line  124 . 
     For generally vertical take-off and landing, STOVL, and/or ESTOL, cruise nozzles  130  and  142  are closed from a first position to a closed second position, and lift nozzles  126 ,  128 ,  136 , and  138  are opened from a first position to an open second position. Control design and control laws required for progressive actuation of nozzles  126 ,  128 ,  130 ,  136 ,  138 , and  142  during pre-defined engine operations are known in the art. When first cruise nozzle  130  is closed, exhaust gases  110  are forced from first duct  114  into first passageway  122  wherein the gases  110  are discharged into first lift duct  118 . Exhaust gases  110  are then discharged from first lift duct  118  through first lift nozzles  126  and  128  to facilitate vertical thrust. Similarly, in the exemplary embodiment, when second cruise nozzle  142  is closed, exhaust gases  112  are forced from second duct  116  into second lift duct  132  via second passageway  134  wherein exhaust gases  112  are the discharged through second lift nozzles  136  and  138 . Thrust from exhaust gases  112  discharged through second lift nozzles  136  and  138  combine with exhaust gases  110  discharged through first lift nozzles  126  and  128  to facilitate generally vertical take-off and landing, and/or STOLV, and/or ESTOL of the aircraft. 
     Once the aircraft has reached a pre-determined altitude, in the exemplary embodiment, cruise nozzles  130  and  142  are opened from the closed second position to the first position while lift nozzles  126 ,  128 ,  136 , and  138  are closed from the open second position to the first position. With lift nozzles  126 ,  128 ,  136 , and  138  closed, exhaust gases  110  and  112  are channeled from first duct  114  and second duct  116 , respectively, into cruise ducts  120  and  140 , respectively, wherein exhaust gases  110  and  112  are discharged through cruise nozzles  130  and  142 , respectively. As exhaust gases  110  and  112  are discharged through cruise nozzles  130  and  142 , respectively, the thrust from the exhaust gases  110  and  112  facilitates propelling the aircraft in a direction that is generally parallel to center line  124 . During pre-defined engine operations in which propulsion in a direction that is generally non-parallel to center line  124  is changed to propulsion in a direction that is generally parallel to center line  124 , propulsion system  100  enables the inoperability of engine  102  or  104  without unbalanced lift or thrust forces. Furthermore, in the exemplary embodiment, propulsion system  100  enables controlled propulsion in a direction that is generally parallel to center line  124  or controlled decent in a direction that is generally non-parallel to center line  124 . 
     In the exemplary embodiment, each engine includes a starboard lift nozzle and a port lift nozzle that facilitates providing balanced lift. For example, if one engine is rendered inoperable during aircraft operation, the exhaust gases from the other engine enables a continued, balanced lift to be provided. The maintenance of balanced lift does not require any control intervention and/or the actuation of any exhaust valves to redirect exhaust gases, either internally or at the lift nozzles. The total lift may be reduced, but in a balanced and controlled manner due to the symmetric position of the lift nozzles and the symmetric geometry of the exhaust gas supply to the nozzles. A simple control command can increase the power of the functioning engine to potentially facilitate allowing continued flight. In the above-described exemplary embodiment, either engine may be rendered inoperable, yet the propulsion system may facilitate maintaining control of the aircraft, and/or increase the probability of aircraft survival, without enhanced control technology and the placement of air control valves in the internal flow circuits. 
     When the aircraft is in forward flight and wing borne, the loss of engine power will not require any valves present in the system to be actuated to prevent back flow from the functioning engine to the inoperable engine. The forward thrust will be maintained via the cruise nozzles without any control system action. Furthermore, an effort to restart the inoperable engine could be made without endangering the continued operation of the aircraft. Moreover, during the transition from channeling gases through the lift nozzles to channeling gases through the cruise nozzles, the propulsion system allows the inoperability of either engine without unbalanced lift or thrust forces such that operation of the aircraft may be continued. 
       FIG. 2  is a top schematic view of an alternate embodiment of a propulsion system  200  that may be used with a rotary wing aircraft that transitions to a fixed wing aircraft having a multiple engine assembly  20 .  FIG. 3  is a portion of propulsion system  200  shown in  FIG. 2 .  FIG. 4  is a perspective view of an exemplary plenum configuration that may be used to supply exhaust gases to the circuit associated with rotor blade  202  of propulsion system  200  shown in  FIG. 2 . 
     In the exemplary embodiment, engine assembly  20  includes a rotor blade  202 , a first engine  204 , and a second engine  206 . Alternately, propulsion system  200  can be used with engine assemblies  20  having more than two engines. In an exemplary embodiment, engine  204  includes a first inlet  208 , and second engine  206  includes a second inlet  210 . Inlets  208  and  210  enable air to be channeled into engines  204  and  206 , respectively. Exhaust gases  212  from first engine  204  are discharged through a first exhaust duct  216  coupled in flow communication to first engine  204 . Similarly, exhaust gases  214  from second engine  206  are discharged through a second duct  218  coupled in flow communication to second engine  206 . 
     In the exemplary embodiment, first exhaust duct  216  is coupled in flow communication to a first rotor duct  220  and to a first cruise duct  222 . First rotor duct  220  is coupled in flow communication to a first rotor plenum  224 . In the exemplary embodiment, at least a portion of rotor plenum  224  is oriented generally perpendicularly and upward from a center line  226  that extends between engines  204  and  206 . Engine assembly  20  and propulsion system  200  are symmetric on each side of center line  226 . Rotor plenum  224  is coupled in flow communication to a plurality of first rotor blade ducts  228  that extend through rotor blade  202  in a direction that is generally oriented from a rotor plenum center  235  to rotor blade ends  236 . In the exemplary embodiment, rotor plenum  224  is coupled in flow communication to four first rotor blade ducts  228 , two of which extend through one portion of rotor blade  202 , and the remaining first rotor blade ducts  228  extend through the remaining portion of blade  202 . Alternately, propulsion system  200  may include more or less than four first rotor blade ducts  228 . 
     Each of the plurality of rotor blade ducts  228  is coupled in flow communication to a rotor nozzle  230 . In the exemplary embodiment, each rotor nozzle  230  has a variable area. More specifically, in the exemplary embodiment, the area of each rotor nozzle  230  is varied via an upper flap  232  and a lower flap  234 . Upper flap  232  and lower flap  234  are rotatably coupled to an end portion  236  of rotor blade  202 . Upper flap  232  and lower flap  234  can be rotated with respect to rotor blade  202  to facilitate increasing or decreasing the area of rotor nozzles  230 . In the exemplary embodiment, a plurality of rotor nozzles  230  share a single upper flap  232  and lower flap  234  to facilitate uniformly controlling the area of a plurality of rotor nozzles  230 . Actuation of rotor tip nozzles  230 , upper flap  232 , and/or lower  234  may be hydraulic, electric, or other known actuation devices. The control of rotor tip tip nozzles  230 , upper flap  232 , and/or lower  234  is known in the art. 
     In the exemplary embodiment, during pre-defined engine operations, exhaust gases  212  may be used to facilitate directional control of the aircraft. More specifically, in the exemplary embodiment, first cruise duct  222  is coupled in fluid communication to a first yaw duct  238  and to a first cruise nozzle  240 . In the exemplary embodiment, cruise nozzles  240  and  256 , described in more detail below, are each variable area nozzles. The design, construction, and control of variable area nozzles are known in the art. Nozzles  240  and  256  may utilize clamshell or shutter valve designs as is known in the art. Actuation of nozzles  240  and  256  may be hydraulic, electric, or any other method known in the art. Moreover, first cruise duct  222  includes a starboard yaw control  242  defined therein. In the exemplary embodiment, yaw duct  238  is oriented generally perpendicularly to cruise duct  222 , and is coupled to a port yaw control  244 . During pre-defined engine operations, exhaust gases  212  are discharged from first cruise duct  222 , through at least one of starboard yaw control  242 , port yaw control  244 , and cruise nozzle  240 . 
     In the exemplary embodiment, second exhaust duct  218  is coupled in flow communication to a second rotor duct  246  and to a second cruise duct  248  that is positioned adjacent to cruise duct  222 . Second rotor duct  246  is coupled in flow communication to a second rotor plenum  250 . In the exemplary embodiment, at least a portion of second rotor plenum  250  is oriented generally perpendicularly and upward from center line  226 . In the exemplary embodiment, a portion of first rotor plenum  224  extends substantially concentrically within a portion of second rotor plenum  250 . Alternately, a portion of second rotor plenum  250  extends substantially concentrically within a portion of first rotor plenum  224 . Second rotor plenum  250  is coupled in flow communication to a plurality of second rotor blade ducts  252  that extend through rotor blade  202  in a direction that is generally oriented from rotor plenum center  235  to rotor blade ends  236 . In the exemplary embodiment, second rotor plenum  250  is coupled in flow communication to four second rotor blade ducts  252 , two of which extend through one portion of rotor blade  202 , and the remaining second rotor blade ducts  252  extend through the remaining portion of blade  202 . Alternately, propulsion system  200  may include more or less than four second rotor blade ducts  252 . Each of the plurality of second rotor blade ducts  252  is coupled to one rotor nozzle  230 , which is described in more detail above. 
     In the exemplary embodiment, during pre-defined engine operations, exhaust gases  214  may be used to facilitate directional control of the aircraft. More specifically, in the exemplary embodiment, second cruise duct  248  is coupled in flow communication to a second yaw duct  254  and to a second cruise nozzle  256 . Moreover, cruise duct  248  has port yaw control  244  defined therein. In the exemplary embodiment, second yaw duct  254  is oriented generally perpendicularly to cruise duct  248 , and is coupled to starboard yaw control  242 . During pre-defined engine operations, at a starboard yaw control outlet  258 , exhaust gases  212  discharged through first cruise duct  222  combine with exhaust gases  214  discharged through second yaw duct  254  to control a yaw of the aircraft. Similarly, at a port yaw control outlet  260 , exhaust gases  212  discharged through first yaw duct  238  combine with exhaust gases  214  discharged through second cruise duct  248  to control yaw of the aircraft. Alternately, propulsion system  200  may be used to control directions other that the yaw of the aircraft. Moreover, in the exemplary embodiment, exhaust gases  214  may be discharged through duct  218  into cruise duct  248 , and then through at least one of starboard yaw control  242 , port yaw control  244 , and second cruise nozzle  256 . 
     For generally vertical take-off and landing, STOVL and/or ESTOL, cruise nozzles  240  and  256  are closed from a first position to a second position, and rotor nozzles  230  are open via moving upper flap  232  and lower flap  234  from a closed position to an open position. Movement of cruise nozzles  240  and  256  is coordinated with the movement of rotor nozzles  230 . Control design and control laws required for progressive actuation of nozzles  230 ,  240 , and  256  during pre-defined engine operations are known in the art. During pre-defined engine operations, exhaust gases  212  are channeled from first duct  216 , through first rotor duct  220 , and then channeled into first rotor plenum  224 . Exhaust gases  212  are channeled from first rotor plenum  224  through first rotor blade ducts  228 , and are then discharged from propulsion system  200  through rotor nozzles  230  to facilitate rotating rotor blade  202 . Rotation of rotor blade  202  facilitates propelling the aircraft in a direction that is generally non-parallel to center line  226 . 
     Similarly, in the exemplary embodiment, when second cruise nozzle  256  is closed from a first position to a second position, and rotor nozzles  230  are open via moving upper flap  232  and lower flap  234  from a closed position to an open position, exhaust gases  214  are channeled from second duct  218 , through second rotor duct  246 , and then channeled into second rotor plenum  250 . During pre-defined engine operations, exhaust gases  214  are channeled from second rotor plenum  250  through second rotor blade ducts  252 , and then are discharged from propulsion system  200  through rotor nozzles  230 . More specifically, in the exemplary embodiment, exhaust gases  212  discharged through rotor nozzles  230  facilitate rotating rotor blade  202  wherein rotation of rotor blade  202  facilitates providing propelling the aircraft in a direction that is generally non-parallel to center line  226 . 
     Once the aircraft has reached a pre-determined altitude, in the exemplary embodiment, cruise nozzles  240  and  256  are opened from the second position to the first position while rotor nozzles  230  are closed via upper flap  232  and lower flap  234 . Movement of cruise nozzles  240  and  256  is coordinated with the movement of rotor nozzles  230 . When rotor nozzles  230  are closed, rotor blade  202  may cease rotation such that blade  202  is oriented substantially perpendicularly to center line  226 , and the aircraft transforms into a fixed wing aircraft. When the aircraft is operating as a fixed wing aircraft, upper flap  232  and lower flap  234  close to facilitate preventing exhaust gases  212  and  214  from discharging through rotor nozzles  230 , and to facilitate reducing drag on rotor nozzles  230  facing fore. With flaps  232  and  234  closed, propulsion system  200  may have a greater capability for propelling the aircraft at a velocity in a direction that is generally parallel to center line  226 . In an exemplary embodiment, a control system (not shown) may modify the pitch of rotor blade  202  to facilitate allowing the blade  202  to function more effectively as a wing. 
     During pre-defined engine operations, with rotor nozzles  230  closed, exhaust gases  212  and  214  are channeled from rotor ducts  220  and  246  through cruise ducts  222  and  248 , respectively, and then through cruise nozzles  240  and  256 , respectively. As exhaust gases  212  and  214  are discharged through cruise nozzles  240  and  256 , respectively, the thrust from the exhaust gases  212  and  214  facilitates propelling the aircraft generally parallel to center line  226 . In the exemplary embodiment, by opening and closing starboard yaw control  242  and/or port yaw control  244 , exhaust  212  and  214  can be used to change a yaw of the aircraft. Control design and control laws required for the actuation of yaw control  242  and  244  during pre-defined engine operations are known in the art. Alternately, exhaust gases  212  and  214  may be channeled through propulsion system  200  for other directional control uses. 
     The propulsion system described above facilitates providing a balanced loss of rotor tip thrust if either engine is rendered inoperable without requiring an aircraft/engine control system beyond what is currently known in the art. The engine exhaust gases are channeled separately from each engine to each of the rotor blade tip nozzles to facilitate providing the balanced loss of rotor tip thrust. The propulsion system also enables aircraft stability to remain uncompromised. Furthermore, the yaw flow circuit incorporates a crossover of the engine exhaust circuits to supply exhaust gases to the yaw nozzles. The crossover of the yaw circuit to each of the engine exhaust circuits facilitates maintaining the required yaw forces without any significant aircraft/engine control system action when one engine is inoperable. 
       FIG. 5  is a top schematic view of another alternative embodiment of a propulsion system  600  that may be used with a multiple engine assembly  60 . In the exemplary embodiment, engine assembly  60  includes a wing  602 , a first engine  604 , and a second engine  606 . Alternately, propulsion system  600  can be used with engine assemblies  60  having more than two engines. In an exemplary embodiment, engine  604  includes a first inlet  608 , and second engine  606  includes a second inlet  610 . Inlets  608  and  610  enable air to be channeled into engines  604  and  606 , respectively. Exhaust gases  612  from first engine  604  are discharged through a first exhaust duct  616  coupled in flow communication to first engine  604 , and exhaust gases  614  from second engine  606  are discharged through a second exhaust duct  618  coupled in flow communication to second engine  606 . 
     In the exemplary embodiment, first duct  616  is coupled in flow communication to a first plenum duct  620  and to a first cruise duct  622 . First plenum duct  620  is coupled in flow communication to a first plenum  624 . In the exemplary embodiment, at least a portion of plenum  624  is oriented generally perpendicularly and upward from a center line  626  extending between engines  604  and  606 . Engine assembly  60  and propulsion system  600  are symmetric on each side of center line  626 . In the exemplary embodiment, first plenum  624  is configured similarly to first rotor plenum  224  shown in  FIG. 4 . 
     Furthermore, in the exemplary embodiment, first plenum  624  is coupled in flow communication to a plurality of first wing ducts  628  and  630  that extend axially through wing  602  and are oriented in a substantially non-parallel direction to center line  626 . In the exemplary embodiment, plenum  624  is coupled to a starboard first wing duct  628  and a port first wing duct  630 . Starboard wing duct  628  extends from center line  626  towards a wing starboard end  636  through one portion of wing  602 . Port wing duct  630  extends from center line  626  towards a wing port end  640  through the remaining portion of wing  602 . Alternately, propulsion system  600  may include more or less than two first wing ducts  628  and  630 . Each of first wing ducts  628  and  630  is coupled to a first wing outlet  632  and  638 , respectively. In the exemplary embodiment, starboard wing outlet  632  is defined on an aft side  634  of wing  602  on starboard end  636 . Similarly, port first wing outlet  638  is defined on aft side  634  of wing  602  on port end  640 . 
     Alternately, exhaust gases  612  may be channeled into first exhaust duct  616 , and air (not shown) from an engine fan (not shown) may be channeled into a first fan exhaust duct (not shown). The air from the first fan exhaust duct is discharged into first plenum  624  to facilitate providing fluid to wing ducts  628  and  630  at a lower temperature than exhaust gases  612 . In an exemplary embodiment, the air is channeled through wing ducts  628  and  630  to facilitate cooling wing  202 . The air is then discharged from wing  202  through outlets (not shown) at the ends of wing ducts  628  and  630 . Similarly, exhaust gases  614  may be channeled into second exhaust duct  618 , and air (not shown) from an engine fan (not shown) may be channeled into a second fan exhaust duct (not shown). The air from the second fan exhaust duct is discharged into second plenum  648  to facilitate providing fluid to wing ducts  650  and  652  at a lower temperature than exhaust gases  614 . In an exemplary embodiment, the air is channeled through wing ducts  650  and  652  to facilitate cooling wing  202 . The air is then discharged from wing  202  through outlets (not shown) at the end of wing ducts  650  and  652 . 
     In the exemplary embodiment, first duct  616  is coupled in flow communication to a first cruise duct  622 . In the exemplary embodiment, at least a portion of cruise duct  622  is oriented generally parallel to center line  626 . In the exemplary embodiment, during pre-defined engine operations, exhaust gases  612  are discharged from duct  616  through first cruise duct  622 . First cruise duct  622  is coupled in flow communication to a first cruise nozzle  642 . In the exemplary embodiment, cruise nozzles  642  and  658 , described in more detail below, are each variable area nozzles. The design, construction, and control of variable area nozzles are known in the art. Nozzles  642  and  658  may utilize clamshell or shutter valve designs as is known in the art. Actuation of nozzles  642  and  658  may be hydraulic, electric, or any other method known in the art. Cruise nozzle  642  is oriented to facilitate enabling exhaust gases  612  to be discharged through cruise nozzle  642  to facilitate propelling the aircraft in a direction that is generally parallel to center line  626 . 
     In the exemplary embodiment, second exhaust duct  618  is coupled in flow communication to a second plenum duct  644  and to a second cruise duct  646 . Second plenum duct  644  is coupled in flow communication to a second plenum  648 . In the exemplary embodiment, at least a portion of plenum  648  is oriented generally perpendicularly and upward from center line  626 . In the exemplary embodiment, a portion of first plenum  624  extends substantially concentrically within a portion of second plenum  648 . Alternately, a portion of second plenum  648  extends substantially concentrically within a portion of first plenum  624 . Moreover, in the exemplary embodiment, second plenum  648  is configured similarly to second rotor plenum  250  shown in  FIG. 4 . 
     Furthermore, in the exemplary embodiment, second plenum  648  is coupled in flow communication to a plurality of second wing ducts  650  and  652  that extend axially through wing  602  and are oriented in a substantially non-parallel direction to center line  626 . In the exemplary embodiment, second plenum  648  is coupled in flow communication to a starboard second wing duct  650  and a port second wing duct  652 . Starboard wing duct  650  extends from center line  626  towards wing starboard end  636  through one portion of wing  602 . Port wing duct  652  extends from center line  626  towards wing port end  640  through the remaining portion of wing  602 . Alternately, propulsion system  600  may include more or less than two second wing ducts  650  and  652 . Each of second wing ducts  650  and  652  is coupled in flow communication to a second wing outlet  654  and  656 , respectively. In the exemplary embodiment, starboard wing outlet  654  is defined on aft side  634  of wing  602  on starboard end  636  between starboard first wing outlet  632  and center line  626 . Similarly, port wing outlet  656  is defined on aft side  634  of wing  602  on port end  640  a between port first wing outlet  638  and center line  626 . Starboard second wing outlet  654  and port second wing outlet  656  are oriented to facilities enabling exhaust gases  612  and  614  to be discharged through outlets  632 ,  638 ,  654  and  656 , respectively, to facilitate reducing drag on the aircraft during pre-defined engine operations. Propulsion system  600  with wing ducts  628 ,  630 ,  650 , and  652  may also facilitate providing other functions, such as, but not limited to, yaw control, wing trailing edge thrust nozzles, or wing cooling. 
     In the exemplary embodiment, second duct  618  is coupled in flow communication to a second cruise duct  646 . In the exemplary embodiment, at least a portion of cruise duct  646  is oriented generally parallel to center line  626  and is positioned adjacent to cruise duct  622 . In the exemplary embodiment, exhaust gases  614  are channeled from duct  618  into second cruise duct  646 . In the exemplary embodiment, second cruise duct  646  is coupled in flow communication to a second cruise nozzle  658 . Cruise nozzle  658  is oriented to facilitate enabling exhaust gases  614  to be discharged through cruise nozzle  658  to facilitate propelling the aircraft in a direction generally parallel to center line  626  during pre-defined engine operations. 
     When the aircraft is in operation, in the exemplary embodiment, air is channeled through inlets  608  and  610  into engines  604  and  606 , respectively. From engines  604  and  606 , exhaust gases  612  and  614  are discharged through exhaust ducts  616  and  618 , respectively. A portion of exhaust gases  612  and  614  is then channeled through plenum ducts  620  and  644 , respectively. The remaining portion of exhaust gases  612  and  614  is channeled through cruise ducts  622  and  646 , respectively. The portion of exhaust gases  612  and  614  channeled into plenum ducts  620  and  644 , respectively, is then channeled through plenums  624  and  648 , respectively. From plenums  624  and  648 , exhaust gases  612  and  614  are channeled into wing ducts  628 ,  630 ,  650 , and  652 , respectively, to be discharged through wing outlets  632 ,  638 ,  654 , and  656 , respectively. When exhaust  612  and  614  is discharged through outlets  632 ,  638 ,  654 , and  656  drag on the aircraft is facilitated to be reduced. The portion of exhaust gases  612  and  614  that is channeled into cruise ducts  622  and  646 , respectively, is discharged through cruise nozzles  642  and  658 , respectively. Control design and control laws required for actuation of nozzles  642  and  658  during pre-defined engine operations are known in the art. Exhaust gases  612  and  614  are discharged through cruise nozzles  642  and  658 , respectively, to facilitate propelling the aircraft in a direction that is generally parallel to center line  626 . 
     The above-described methods and apparatus facilitates simplifying a propulsion system that can be used with a multiple engine assembly. The propulsion system facilitates maintaining independent exhaust circuits for a multiple engine assembly in an aircraft by providing each engine with an exhaust circuit independent from any other engine exhaust circuit. Each engine has an exhaust circuit from the engine to at least two outlets. The propulsion system facilitates isolating one engine from other engines within the assembly. With the engines isolated by the design of the exhaust circuit, isolation valves, actuators, and hydraulic lines are eliminated, facilitating reducing the complexity of the multiple engine installation. If an engine is rendered inoperable, the isolation of each engine facilitates allowing the ability to restart an inoperable engine, and facilitates maintaining control of an aircraft during the inoperability of an engine. The propulsion system facilitates maximizing control at a reduced thrust level since the amount of exhaust is reduced but the distribution fractions are facilitated to be held substantially constant. Control of the aircraft is also facilitated to be increased because, by isolating the exhaust circuits, the exhaust forces stay balanced. The propulsion system also facilitates attempting to restart an inoperable engine without affecting the operation of the other engines in the installation. Pilot operation of an aircraft is facilitated to be simplified because, by using the propulsion system of the current invention, the pilot only needs to attempt restart of an inoperable engine without having to actuate valves. 
     Exemplary embodiments of a method and apparatus for a multiple engine propulsion system are described above in detail. The method and apparatus is not limited to the specific embodiments described herein, but rather, components of the method and apparatus may be utilized independently and separately from other components described herein. For example, the propulsion system may also be used in combination with other aircraft exhaust gas functions, and is not limited to practice with only the generally horizontal propulsion function, the generally vertical take-off and landing, STOVL, and/or ESTOL functions, and/or the drag reduction function as described herein. Rather, the present invention can be implemented and utilized in connection with many other aircraft propulsion applications. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.