6 The partition between the two ducts includes an aperture covered by a selectively movable flap.

7 8 BRIEF DESCRIPTION OF DRAWINGS 9 10 The invention, in accordance with preferred and exemplary embodiments, together with further 11 objects and advantages thereof, is more particularly described in the following detailed description taken in 12 conjunction with the accompanying drawings in which: 13 Figure 1 is an axial sectional view through an exemplary turbofan engine mounted by a pylon to the 14 wing of an aircraft, and including a variable area fan nozzle in accordance with an exemplary embodiment of 15 the present invention.

16 Figure 2 is an isometric view of the turbofan engine illustrated in Figure 1 with an exemplary 17 embodiment of the fan nozzle thereof.

18 Figure 3 is an isometric view of a portion of the fan nozzle illustrated in Figure 2 including a 19 secondary flow duct.

20 Figure 4 is an enlarged sectional view of a portion of the variable fan nozzle illustrated in Figure 1.

21 Figure 5 is a top, partly sectional view of a portion of the fan nozzle illustrated in Figure 4 and taken 22 along line 5-5.

23 Figure 6 is a partly sectional axial view inside the secondary fan duct illustrated in Figure 5 and 24 taken along line 6-6.

25 Figure 7 is a top, partly sectional view of the secondary flow duct illustrated in Figure 6 and taken 26 along line 7-7.

27 Figure 8 is an isometric view of a lower portion of the fan duct illustrated in Figure 1 in accordance 28 with another embodiment.

29 Figure 9 is a partly sectional top view, like Figure 7, of a portion of the fan nozzle in accordance 30 with another embodiment.

31 Figure 10 is a partly sectional, elevational view of the secondary flow duct illustrated in Figure 9 32 and taken along line 10-10.

33 34 MODE (S) FOR CARRYING OUT THE INVENTION 35 36 Illustrated in Figure 1 is an exemplary turbofan aircraft gas turbine engine 10 mounted by a pylon to 37 the wing of an aircraft 12, shown in part. The engine includes in serial flow communication a fan 14, 1 multistage axial compressor 16, annular combustor 18, high pressure turbine 20, and low pressure turbine 22.

2'During operation, ambient air 24 is pressurized in the compressor and mixed with fuel in the 3 combustor for generating hot combustion gases 26 which flow through the high and low pressure turbines 4 that extract energy therefrom. The high pressure turbine powers the compressor through a shaft 5 therebetween, and the low pressure turbine powers the fan through another shaft therebetween.

6 The exemplary turbofan engine illustrated in Figure 1 is in the form of a high bypass ratio engine in 7 which most of the air pressurized by the fan bypasses the core engine itself for generating propulsion thrust.

8 The fan air 24 is discharged from the engine through a substantially annular fan exhaust nozzle 28 defined 9 radially between an outer shell or cowling 30 of the core engine and a fan nacelle 32 surrounding the fan and 10 the forward portion of the core engine.

11 The fan nozzle 28 is illustrated in more detail in Figure 2 in which the fan nacelle 32 coaxially or 12 concentrically surrounds the core engine cowling 30 to define a circumferentially extending fan duct 40 13 radially therebetween for discharging axially the fan air 24 pressurized by the upstream fan 14. As initially 14 shown in Figure 1, the fan duct 40 has a tubular inlet at the leading edge of the fan nacelle and an arcuate 15 outlet 42 disposed radially between the cowling and a trailing edge 44 of the nacelle from which the fan air is 16 discharged during operation for providing propulsion thrust to power the aircraft in flight.

17 The core exhaust gases 26 are discharged from the core engine through an annular core exhaust 18 nozzle 34 defined between the core cowling 30 and an optional center plug 36 disposed coaxially or 19 concentrically therein around an axial centerline axis 38 of the engine and plug.

20 The turbofan engine illustrated in Figures I and 2 is attached to the wing by a vertical pylon 46 21 which interrupts the circumferential continuity of the fan duct. Arcuate portions of the fan nacelle and core 22 cowling are joined together at circumferentially opposite endwalls 48 which result in the fan ducts being 23 C-shaped in two complementary halves which are typically hinge mounted to the pylon for permitting access 24 inside the engine. The C-shaped fan ducts and their longitudinal or axial endwalls 48 may have any 25 conventional configuration, and are typically mounted to the pylon for axial translation movement therealong 26 for uncovering conventional thrust reversers (not shown) in a typical manner.

27 However, the exemplary fan nozzle illustrated in Figures 1-3 may be modified in accordance with 28 the present invention for introducing therein another longitudinal wall or partition 50 spaced from a 29 corresponding endwall 48 to define a secondary fan flow duct 52 behind the partition.

30 Since the pylon illustrated in Figure 2 interrupts the circumferential continuity of the annular fan 31 duct, the two C-shaped fan duct halves have corresponding endwalls 48 on opposite sides of the pylon. And, 32 the two C-ducts 40 have another pair of the endwalls 48 at their lower ends opposite to the pylon.

33 Correspondingly, a pair of the partitions 50 may be introduced at the top of the fan ducts, or at the bottom 34 thereof, or at both locations to correspondingly define the two or four secondary flow ducts 52 at opposite 35 circumferential ends of the two primary fan ducts 40.

36 Whereas the primary ducts 40 have corresponding arcuate primary outlets 42 which extend over the 37 majority of the circumferential extent of the engine, the secondary ducts 52 have correspondingly small 1 secondary outlets 54 which occupy a relatively minor portion of the circumferential extent of the engine. The 2 secondary outlets 54 are generally rectangular in area, and circumferentially adjoin the arcuate primary 3 outlets 42 in a common annulus therewith interrupted by the pylon at the top of the engine, and the endwalls 4 48 at the bottom of the engine.

5 The primary and secondary ducts 40,52 are fixed-area structures, with the common trailing edge 44 6 of the fan nacelle having a constant radial spacing from the common outer diameter of the core cowling 30.

7 However, variable area capability may be introduced into the fan nozzle with the circumferential 8 bifurcation of the C-ducts 40 by the introduction of the partitions 50 and secondary flow ducts 52. As shown 9 in Figures 3 and 4, each partition includes a portal or aperture 56 initially covered by a suitably movable door 10 or flap 58. In this way, the flaps 58 may be suitably opened when desired for providing flow communication 1 I between the primary ducts and the secondary ducts for selectively increasing the collective outlet flow area 12 for the fan nozzle.

13 This new configuration of the fan nozzle may be used to advantage in a method for reducing noise 14 in the engine which otherwise has fixed-area outlets in the primary and secondary ducts. The flaps 58 may be 15 simply opened during takeoff operation for temporarily increasing the circumferential flow area of the fan 16 ducts for correspondingly reducing velocity of the fan air being discharged from the engine.

17 In this exemplary turbofan engine, a major component of noise generation is due to the high velocity 18 of the fan discharge air as it mixes with the relatively low velocity of the surrounding ambient airstream 19 during aircraft flight. By opening the flaps 58, the secondary ducts and their corresponding outlets provide a 20 temporary increase in available flow area for discharge of the fan air, which therefore reduces the velocity 21 thereof and correspondingly reduces noise during takeoff.

22 And, when the aircraft being powered by the engine reaches cruise operation at a predetermined 23 altitude above sea level, the flaps 58 may be suitably closed for blocking flow discharge through the 24 secondary ducts and terminating the area increase. Then at cruise operation of the engine, the reduced flow 25 area attributable solely to the primary ducts 40 then ensures efficient operation of the engine at cruise altitude 26 which does not require the additional discharge flow area useful during takeoff operation.

27 In this way, the fan nozzle is provided with variable area capability with otherwise fixed-area outlets 28 42,54 by the simple introduction of the secondary ducts and the selective communication therebetween 29 provided by the movable flaps. The primary ducts 40 may therefore be optimally sized for maximizing cruise 30 performance of the engine at altitude with a suitably small collective flow area at the primary outlets 42.

31 And, the secondary outlets 54 may be optimized for takeoff operation to provide a temporary increase in total 32 fan air discharge area.

33 Figures 5-7 illustrate an exemplary embodiment in which suitable means are provided for 34 selectively moving the flaps 58 between stowed and deployed positions for correspondingly blocking and 35 unblocking the partition apertures 56 for varying the area available for discharging the fan exhaust. The flaps 36 are preferably normally closed in their stowed position sealingly blocking the corresponding apertures 56 to 37 prevent flow discharge of the fan exhaust air through the secondary flow ducts 52. The flaps are preferably 1 stowed for cruise operation which requires minimum discharge area for the fan exhaust.

2'The flaps may then be moved to their deployed and open position in which the partition apertures 3 are unblocked for discharging a minor portion of the fan air from the primary ducts 40 through the secondary 4 ducts for discharge through the secondary outlets. In this way, the fan air from the common fan is 5 collectively discharged through both primary and secondary fan ducts and their corresponding primary and 6 secondary outlets 42,54 for providing maximum area for discharging the fan exhaust during takeoff.

7 The introduction of the partitions and movable flaps in the otherwise conventional fan nozzle 8 requires relatively few additional components and is relatively simple in configuration. The trailing edge of 9 the fan nacelle remains fixed in space for maintaining a simple design.

10 It is noted, however, that the aft end of the fan nacelle in which the secondary flow ducts 52 are 11 incorporated may be in the conventional form of a translating sleeve which covers thrust reversers (not 12 shown) during normal operation. When thrust reverse is required during landing of the aircraft, the nacelle 13 sleeve translates aft as shown by the doubled headed arrow in Figure 2, for uncovering the hidden thrust 14 reverser doors, and then is translated forward when thrust reverse operation is completed. The partitions 50 15 and corresponding secondary flow ducts 52 may be readily incorporated in the translating sleeve of the fan 16 nacelle in cooperation with the primary C-ducts typically incorporated therein.

17 Since the C-ducts 40 are provided in pairs in the engine, the secondary ducts 52 are also preferably 18 provided in either one pair at the top or bottom of the engine, or in two pairs thereat. The corresponding 19 secondary outlets 54 in two or four multiples may be suitably sized in flow area to significantly increase the 20 total discharge flow area of the fan nozzle during takeoff operation for correspondingly significantly reducing 21 flow velocity of the discharged fan air from the nozzle. By decreasing the velocity of the fan exhaust, a 22 corresponding reduction in fan-generated noise may be achieved.

23 Furthermore, and independent of noise attenuation, the additional discharge flow area provided by 24 the secondary outlets 54 during takeoff operation may be used for increasing takeoff thrust from the 25 discharged fan exhaust, which may be accompanied by a slight increase in fan rotor speed.

26 Significant noise attenuation or thrust increase may be obtained by temporarily increasing the fan 27 discharge flow area in the exemplary range of about 5-20 percent which area increase would be collectively 28 provided by the introduction of two or four of the secondary fan ducts and outlets 54.

29 In the exemplary embodiment illustrated in Figures 5-7, the moving means include a suitable hinge 30 mount 60 configured for pivoting an upstream or forward end of each flap 58 to the partition 50. The 31 downstream or aft distal end of the flap is thusly movable inside the secondary duct for unblocking the 32 partition aperture.

33 The flap 58 may have any suitable construction and composition and is preferably relatively thin.

34 The hinge mount 60 preferably includes reinforcing arms integrally joined to the back side of the flap and 35 mounted in suitable clevises attached inside the secondary duct 52.

36 In the exemplary embodiment illustrated in Figures 5-7 the moving means are active for powering 37 open and closed the flaps when desired. In this embodiment, articulated linkage 62 joins the flap to the 1 endwall 48 for kinematically controlling opening and closing pivotal movement of the flap between the 2 stowed and deployed positions. Correspondingly, a suitable actuator 64 is operatively joined to the linkage 3 62 for selectively deploying and stowing the flap atop the partition aperture.

4 As shown in Figure 6, the linkage 62 includes an elongate control rod 62a pivotally joined by 5 suitable clevises to the endwall 48 over the width or height of the flap 58. An input lever 62b is fixedly 6 joined to one end of the control rod, and a pair of output levers 62c are fixedly joined to the control rod 7 spaced apart from each other and from the input lever.

8 A pair of control arms 62d are pivotally joined by suitable clevises to the back side of the flap 58 9 near the downstream, distal end thereof, with opposite ends of the control arms being pivotally joined to 10 respective ones of the output levers 62c.

11 The actuator 64 as illustrated in Figures 5 and 7, includes an extendable output or piston rod 64a 12 which is pivotally joined to the input lever 62b for selectively rotating the control rod to drive the control 13 arms to open and close the flap 58 atop the partition aperture. Figure 5 illustrates the actuator rod 64a being 14 fully retracted, which in turn drives the linkage for closing and seating the flap 58 over the partition aperture 15 56. Figure 7 illustrates full extension of the actuator rod 64a, which correspondingly drives the linkage to 16 pivot open the flap 58 for permitting flow discharge from the primary duct 40 into the secondary duct 52.

17 Note in Figures 5 and 7 that the partition 50 may be simply introduced with a suitable inclination 18 from the corresponding endwall 48 for gradually increasing the flow area of the secondary duct 52 between 19 its upstream and downstream ends. The actual configuration and location of the partition 50 are selected for 20 providing preferred area distribution of the primary fan duct 40 as required for normal operation.

21 The introduction of the secondary flow ducts 52 may be separately optimized for providing 22 additional discharge flow area for the fan exhaust only when required during takeoff operation. Accordingly, 23 the primary fan ducts 40 may themselves be fixed in area and configuration and optimized specifically for 24 cruise performance, independently of the secondary flow ducts which are specifically configured and 25 optimized for takeoff operation.

26 The linkage illustrated in Figures 5-7 is merely exemplary of suitable linkage which may be 27 introduced for opening and closing the flap as desired. A particular advantage of the linkage illustrated in 28 this embodiment is the ability in a relatively small volume provided by the secondary flow duct 52 to 29 introduce robust and balanced moving means for the flap. The control arms are balanced on opposite edges 30 of the flap, and are powered by the common control rod 62a, and by the common actuator 64 which may be 31 located in the available space inside the fan nacelle and outside the small secondary duct 52.

32 As shown in Figure 5, the control arms 62d are inclined forward toward the control rod 62a to join 33 the output lever 62c in a joint located between the control rod and the flap in the illustrated stowed position.

34 In this way, the flap 58 is self-locked in its closed and stowed position when the actuator output rod 64a is 35 fully retracted. Note that the pivot joint between the control arm 62d and the control arm 62a is over-center, 36 or between the centerline of the control rod 62a and the flap. In this position, the control arm 62d is trapped 37 between the control rod and flap and cannot move unless permitted by the actuator.

1 By powering the actuator 64 for extending its output rod 64a as illustrated in Figure 7, the control 2 rod 62a rotates'the corresponding input and output levers 62b, c which overcomes the initial over-center 3 position of the control arms and permits opening of the individual flap.

4 Note that the stowed position illustrated in Figure 5 includes the clockwise over-center of the 5 forward end of the control arm 62d. As shown in Figures 6 and 7, the input lever 62b is angularly spaced 6 from the output levers 62c to pivot the adjoining ends of the control arms 62d between the control rod 62a 7 and the endwall 48 to permit static pressure P of the fan air atop the inner surface of the flap to drive open the 8 flap during deployment. In Figure 7, the output lever 62c has been rotated counterclockwise overcoming the 9 over-center position thereof initially between the control rod and the partition 50.

10 When the various flaps illustrated in Figure 2 are closed during cruise operation, fan discharge area 11 is provided solely by the two C-shaped primary fan ducts 40. The adjoining secondary outlets 54 12 nevertheless provide circumferential continuity of the common annulus of the two opposite fan ducts.

13 Accordingly, in order to reduce undesirable base drag from the inactive secondary flow ducts during cruise 14 operation, means in the exemplary form of a flow conduit or shunt 66 are provided for channeling air through 15 the secondary ducts when the flaps are stowed closed atop the partition apertures. In this way, some air may 16 flow through the secondary ducts when the flaps are closed for reducing or eliminating the base drag.

17 The conduit 66 is illustrated in Figures 2 and 3 in an exemplary embodiment, and preferably 18 extends through the fan nacelle for channeling ambient air external of the fan nacelle through the secondary 19 ducts when the flaps are stowed closed. Other forms of the conduit may be used, and other routing thereof 20 may be used for providing purge air through the secondary ducts when the flaps are closed. For example, the 21 partition may include one or more unblocked purge holes for channeling a portion of the fan exhaust 22 therethrough at all times including both takeoff and cruise operation.

23 The exemplary shunt conduit 66 illustrated in Figure 3 includes an inlet in the form of an aperture 24 mounted flush in the outer skin of the nacelle, and an outlet in the form of another aperture suitably located in 25 the secondary duct 52, such as in the top wall thereof.

26 Figure 8 illustrates an alternate embodiment of the shunt conduit 66 mounted through the fan nacelle 27 at the bottom of the engine below the pylon. The inlet end of the conduit 66 is mounted flush in the outer 28 skin of the fan nacelle, and the outlet end of the conduit is mounted through the corresponding endwall 48 in 29 the side of the bottom secondary duct 52.

30 Figures 9 and 10 illustrate an alternate embodiment of the flap moving means in the form of one or 31 more springs 68 which are passive devices that do not require an external energy source like the actuator.

32 The springs are disposed between the back side of the flap 58 and the endwall 48 for spring biasing closed 33 the flap atop the partition aperture in the normally closed stowed position.

34 The springs are sized to permit or utilize static pressure P of the fan air in the primary fan duct 40 to 35 develop a differential pressure across the flap relative to the secondary fan duct 52 as the pressure of fan air 36 increases during takeoff operation of the engine. The springs are sized so that developed static pressure 37 during takeoff is sufficient to overcome the spring returning forces for opening the flap away from the 1 partition aperture.

2'In this'way, static pressure alone of the fan air being discharged through the fan nozzle is sufficient 3 to selectively deploy open the flaps during takeoff. As the power of the engine is decreased for cruise 4 operation, the static pressure of the fan air correspondingly decreases, and the restoring force of the spring 68 5 is sufficient to self-stow closed the flap atop the partition aperture.

6 The springs 68 illustrated in Figures 9 and 10 may have any suitable form such as the three leaf 7 springs illustrated therein. The proximal ends of the three springs may be suitably clamped to the endwall 48, 8 with the opposite distal ends of the springs slidingly abutting the back side of the flap. In other embodiments, 9 compression, tension, or torsion springs might also be used.

10 The bifurcated turbofan fan exhaust nozzle disclosed above maintains a fixed-area, substantially 11 annular fan nozzle outlet while introducing variable area capability by circumferential bifurcation of the 12 nozzle. The secondary fan ducts 52 may be sized as small or as large as desired and the secondary outlets 13 thereof typically represent a relatively small percentage of the total fan discharge flow area represented by the 14 primary and secondary outlets.

15 With the flaps being stowed closed during most engine operation, the fixed-area primary fan ducts 16 and outlets thereof may be optimized for cruise performance. And, when the flaps are opened during takeoff 17 operation, additional fan discharge area is provided by the secondary fan ducts and outlets thereof for 18 temporarily increasing available discharge area for correspondingly reducing the velocity of the discharged 19 fan air through the primary fan duct for reducing engine noise during takeoff.

20 Additional features of the variable area fan exhaust nozzle have been introduced in the provisional 21 patent application and are summarized hereinbelow. More specifically, the variable area fan exhaust nozzle 22 disclosed above permits automatic adjustment to two extreme positions: a large fan exhaust area and a small 23 fan exhaust area. The large exhaust area is for takeoff from sea level to a preset altitude, while the small 24 exhaust area is for cruise.

25 Since the noise emitted by high bypass ratio turbofan engines during takeoff is a consequence of the 26 shearing between the fan airflow and the ambient air, it is of particular interest to reduce during takeoff the 27 velocity of the fan airflow so that the shearing between the fan airflow and the ambient air is reduced. To 28 reduce the velocity of the fan airflow during takeoff, the fan exhaust area may be increased.

29 The bifurcated fan nozzle is of particular interest for a C-duct nacelle with translating sleeve type 30 thrust reversers. With reference to Fig. 2, as known in the art, the fan nacelle is attached to the aircraft 31 structure via the pylon 46. The nacelle is of the C-duct type, and is composed of an air intake, two C-duct 32 halves, an outboard one, an inboard one, and a primary exhaust nozzle 28.

33 With reference to Figs. 2,3 each half is radially composed of an inner skin usually referred as inner 34 fan duct, an outer skin usually referred as outer fan duct, the outer skin of the C-duct and two longitudinal 35 walls or endwalls 48. The inner fan duct and its associated longitudinal walls form a structure that is 36 stationary and usually hinged on the pylon for maintenance access to the engine in a conventional manner.

37 The assembly C-duct outer skin and outer fan duct with associated closing ribs form what is know in 1 the art as a translating sleeve. The translating sleeve is the structure that supports and houses the thrust 2 reverser of the blocker-type doors and cascades (not shown). The translating sleeve is mounted on associated 3 guiding tracks attached to the fixed walls 48 in a conventional manner.

4 The inner fan duct, the side walls, and the translating sleeve form a half duct in the shape of a"C" 5 that ducts one half of the total fan airflow mass. The fan airflow 24 exists at the trailing edge 44 of each half 6 C-duct. As the two C-ducts are basically identical, the two C-duct halves may be configured the same way 7 and equipped with the same means for introducing variable area capability.

8 Conventional C-ducts have a fixed exhaust area. For long range aircraft, the exhaust area is usually 9 optimized for cruise performance conditions. The bifurcated fan nozzle provides a means to adjust the 10 exhaust area of each half of the C-ducts. More particularly it provides a large exhaust area for takeoff and a 11 small exhaust area for cruise.

12 When the amount of the exhaust area increase required is rather limited, for example in the order of 13 10% to 15% of the total fan mass flow, the bifurcated nozzle is particularly attractive as it allows a fixed 14 contour C-duct trailing edge, a fixed contour C-duct outer skin, and a fixed contour outer fan duct skin. The 15 nacelle trailing edge is formed by the trailing edge of outer skin and the trailing edge of the outer duct. A 16 significant characteristic of the bifurcated nozzle is that the contours and trailing edge of the C-duct are not 17 altered by the adjustment of the exhaust area of the C-duct. This is an important aspect of the fan nozzle, as 18 all of the prior art known by me use movable parts that move and alter the trailing edge and contours.

19 Since the fan air flows and exits through two half C-ducts, each half C-duct needs to have the 20 capability to increase its exhaust area, for takeoff, by only half of the value required. In other words and for 21 example, if for takeoff there is a requirement to increase by 12% the exit area for the total fan mass flow, then 22 each C-duct needs to have the capability to increase its exhaust area by only 6%. This aspect is of 23 importance as this small area increase allows designing the contour of the outer fan duct and of the outer skin 24 such that the exit area of the half C-duct is at its maximum value.

25 With reference to Fig. 2, each half C-duct exhaust area is at maximum when the flaps 58 are open, 26 and half of the total fan mass airflow produced by the engine exhausts through the exhaust area of each half 27 C-duct, formed by the addition of areas from the two primary outlets and the four secondary outlets. The 28 accumulation of areas of the secondary outlets in each C-duct equates to the half of the fan exhaust area 29 increase required.

30 Consequently, each of the four secondary outlets 54 provides only a quarter of the fan exhaust area 31 increase required. This represents the takeoff configuration of each of the two half C-ducts. When the flaps 32 are closed, each half C-duct exhaust area is minimum, and half of the total fan mass airflow produced by the 33 engine exhausts through each of the two primary outlets 52 only. No fan airflow flows through the secondary 34 outlets. This is the cruise configuration of each of the two half C-ducts.

35 As was explained previously, the nacelle trailing edge, the outer skin, and the outer fan duct have 36 not moved for either value of the exhaust area of the half C-duct. With reference to Fig. 2 there are two 37 longitudinal partition walls 50 installed in the half C-ducts. Each of these walls is attached to the inner fan

duct and installed at an inclination angle with their associated longitudinal endwalls of the half C-duct such that their upstream portion is attached to their respective endwalls, and their downstream portion is separated from the respective endwalls.

Each of the partition walls may be made of two parts, a fixed one attached to the inner fan duct and a movable one attached to the outer fan duct of the translating sleeve. In direct thrust mode, takeoff or cruise, the two parts of each partition wall form one entity. In reverse mode, the translating sleeve moves rearward, and the two parts of the partition 50 will be separated from each other.

If the C-duct is equipped with a thrust reverser of the pivoting doors type, or if there is no thrust reverser, then there are no translating sleeves and consequently each of the previous partition walls is made of one part only. For cruise performance purposes, since no fan airflow is flowing through the secondary outlets, it is desirable to limit the magnitude of the value of these outlets in order to avoid generating base drag. For each half C-duct it is possible to only use one secondary outlet instead of two.

With reference to Fig. 3 in the upstream portion of each of the partition walls 50, there is an opening 56. A flap 58 is hinged on its associated partition and is either closing the opening or opening it. With the flap closed, the system is in the cruise configuration, and the primary outlet 42 provides the exhaust area for half of the total fan mass airflow. With the flap opened, the system is in the takeoff configuration, and the exhaust area is the combination of the primary and secondary outlets.

With reference to Figs. 3,7, the flap 58 is hinged along a hinge axis on the fixed partition wall 50.

With reference to Figs. 5,6, 7, arms 62d control the position of the flap. One end of the arms connects to its associated clevis fitting, while the other end connects to rod 62a hinged on longitudinal wall 48 via support fittings.

Actuator 64 is pivotally mounted on a fitting attached to longitudinal wall 48 inside the space formed by the fixed partition wall and associated longitudinal wall. Piston rod 64a of actuator 64 connects to extension lever 62b of the control rod.

As is shown on Fig. 5, when the flap 58 is closed, arm 62d and the centerline axis of rod 62a are in an over-center position that forces the piston rod to stay in its retracted position. This is the locking mechanism of the flap in its closed position. Consequently the actuator does not need to be energized to maintain the flap in its closed position, and there is no need for an independent locking mechanism.

For opening the flap, the actuator is energized and the piston rod extends to break the over-center position of arm 62d and rod 62a. Once this is achieved, the actuator can be de-energized, as the static pressure differential that is acting on the flap will drive the flap to its opened position. Consequently there is no energy required to maintain the flap in its fully opened position. The actuator 64 can be electro- mechanical, electro-hydraulic or pneumatic. With reference to Fig. 7, when flap 58 is opened, it opens the secondary conduit 52 formed by the endwall, partition, inner fan duct, and outer fan duct of the translating sleeve.

In a first and simplest mode of operation the flaps are either closed or opened, whether there is one or two flaps on the inboard and outboard C-ducts. In an alternate configuration, and in the case two flaps are

installed on the inboard and two flaps on the outboard C-ducts, then for takeoff up to a predetermined altitude, all flaps are opened and the total fan exhaust area is equal to the accumulation of areas from the primary and secondary outlets.

From that preset altitude to part of climb, one flap on the outboard C-duct and its equivalent one on the inboard C-duct are closed. In this condition the total fan exhaust area is equal to the accumulation of areas of the two primary outlets and two secondary outlets. For the remainder of the climb and cruise, all four flaps are closed and the total fan exhaust area is equal to the accumulation of areas of the two primary outlets only.

In another mode of controlling the position of the flaps, open or close, it is also possible to use a completely passive system, i. e. , it is possible to totally eliminate all components of the actuation means previously described. In this mode of realization, each of the flaps is maintained closed by at least a calibrated spring 68 illustrated in Figs. 9 & 10. The springs are calibrated to deliver a force that generates a closing moment around the pivoting axis of the flaps, and that is greater than the opening moment generated by the static pressure differential acting on each flap in cruise. This ensures that each flap is closed in cruise.

Since for takeoff the static differential pressure acting on each flap is greater than the one in cruise, then the moment that is generated by the static pressure is greater than the closing moment that is generated by the spring. This causes each flap to open and stay open until the static differential pressure drops to the value it has in cruise. While the spring shown in Figs. 9 & 10 is of the leaf spring type, it will be understood that any other spring could be used instead.

It may be desirable when all flaps are closed (each half C-duct is in the cruise configuration) to blow ambient air through each of the secondary outlets 54. This ensures, in cruise, the elimination of the base area formed by the areas thereof with the flaps closed. As shown on Fig. 3, and as an example, this can be realized by providing an ambient air inlet in the outer skin and an outlet in the outer fan duct of the translating sleeve. The inlet and outlet are connected to the flow shunt or conduit 66. The air inlet can be located in any other area of its associated half C-duct, for example between the two half C-ducts.

With reference to Fig. 8 the outlet of conduit 66 for supplying ambient air to the secondary duct 52 is now located in the endwall 48. In this embodiment, it is necessary to only connect one extremity of the conduit 66, to either the inlet or the outlet thereof for allowing the opening of each half C-duct for maintenance access to the engine.

Still in reference to Fig. 8, the outlet of conduit 66 is attached to wall 48 that has been provided with a corresponding opening, and its inlet seals with the nacelle that holds the ambient air inlet and that is located between the two half C-ducts. Conduit 66 supplies outside ambient air that is blown through the secondary duct 52 when the flap is closed. A similar air ambient supply means is provided for each of the secondary outlets so that the external air is blown through these outlets when the associated flaps are closed.

The adjustment of the fan exhaust area in the bifurcated fan nozzle is simple, and with all flaps opened, i. e. , during takeoff, it will reduce the difference in velocity between the fan airflow and the ambient air, and consequently will reduce the noise. Another characteristic of the fan nozzle is that it does not reduce

the amount of acoustic treatment that is usually installed on the C-duct. This means that the fan nozzle provides a means to further reduce the noise emitted by the turbofan engine during takeoff.

While the above description relates to a short nacelle C-duct type with separate flow, i. e. , the fan air and the core gases having separate exhaust nozzles, the above flap arrangement can be installed on a long <BR> <BR> nacelle C-duct type, i. e. , the fan air and the core gases have a common exhaust nozzle. While Fig. 2 shows that the trailing edges of the C-ducts are planar, they can be any shape.

While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.