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A turbofan exhaust nozzle includes a fan duct defined between a fan nacelle and a core engine cowling. The fan duct includes a longitudinal endwall and a partition spaced therefrom to define a secondary flow duct. The fan duct also includes a primary outlet, and the secondary duct includes a seconda

A turbofan exhaust nozzle includes a fan duct defined between a fan nacelle and a core engine cowling. The fan duct includes a longitudinal endwall and a partition spaced therefrom to define a secondary flow duct. The fan duct also includes a primary outlet, and the secondary duct includes a secondary outlet. The partition between the two ducts includes an aperture covered by a selectively movable flap.

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1. A turbofan exhaust nozzle comprising:a fan nacelle surrounding a core cowling to define an arcuate fan duct radially therebetween for discharging fan air;said fan duct including a longitudinal endwall interrupting circumferential continuity thereof, and a partition spaced from said endwall to def

1. A turbofan exhaust nozzle comprising:a fan nacelle surrounding a core cowling to define an arcuate fan duct radially therebetween for discharging fan air;said fan duct including a longitudinal endwall interrupting circumferential continuity thereof, and a partition spaced from said endwall to define a secondary flow duct;said fan duct having an arcuate primary outlet disposed radially between said cowling and a trailing edge of said nacelle, and said secondary duct having a secondary outlet disposed between said cowling and nacelle trailing edge; andsaid partition including an aperture covered by a movable flap. 2. A nozzle according to claim 1 further comprising means for selectively moving said flap from a stowed position blocking said aperture to a deployed position unblocking said aperture for discharging fan air through both said primary and secondary outlets. 3. A nozzle according to claim 2 wherein said moving means are configured to deploy said flap during takeoff operation of a turbofan engine configured for discharging said fan air through said fan duct. 4. A nozzle according to claim 3 wherein said moving means are configured to stow said flap during cruise operation of said turbofan engine at a selected altitude. 5. A nozzle according to claim 3 wherein said secondary outlet is sized to increase discharge flow area during takeoff operation for reducing flow velocity of said discharged fan air. 6. A nozzle according to claim 3 wherein said secondary outlet is sized to increase discharge flow area during takeoff operation for increasing takeoff thrust from said discharged fan air. 7. A nozzle according to claim 3 wherein said moving means comprise a hinge mount pivoting an upstream end of said flap from said partition, with a downstream end of said flap being movable inside said secondary duct for unblocking said aperture. 8. A nozzle according to claim 7 wherein said moving means further comprise:articulated linkage joined to said flap for kinematically controlling pivotal movement thereof between said stowed and deployed positions; andan actuator operatively joined to said linkage for selectively deploying and stowing said flap atop said partition aperture. 9. A nozzle according to claim 8 wherein said linkage comprises:a control rod pivotally joined to said endwall, and including an input lever at one end, and a pair of output levers spaced apart therefrom;a pair of control arms pivotally joined to said flap and to respective ones of said output levers; andsaid actuator includes an output rod pivotally joined to said input lever for selectively rotating said control rod to drive said control arms to open and close said flap atop said partition aperture. 10. A nozzle according to claim 9 wherein said control arms are inclined forward toward said control rod to join said output levers between said control rod and flap in said stowed position to self-lock said flap closed with said actuator output rod being retracted. 11. A nozzle according to claim 10 wherein said input lever is angularly spaced from said output levers to pivot adjoining ends of said control arms between said control rod and said endwall to permit static pressure of said fan air atop said flap to drive open said flap. 12. A nozzle according to claim 7 wherein said moving means further comprise a spring disposed between said flap and endwall for spring biasing closed said flap atop said partition aperture in said stowed position, with said spring being sized to permit static pressure on said flap during takeoff operation to overcome spring force in said spring for opening said flap away from said partition aperture. 13. A nozzle according to claim 12 wherein said spring comprises a plurality of leaf springs mounted at opposite ends between said flap downstream end and said endwall. 14. A nozzle according to claim 3 further comprising a flow conduit extending through said fan nacelle in flow communication with said secondary duct for channeling am bient air external of said nacelle into said secondary duct for discharge through said secondary outlet. 15. A nozzle according to claim 14 wherein said flow conduit includes an inlet in an outer skin of said nacelle, and an outlet in said secondary duct. 16. A turbofan exhaust nozzle comprising:a fan nacelle surrounding a core engine cowling with arcuate portions joined together at circumferentially opposite endwalls to define a pair of C-shaped fan ducts for discharging fan air therefrom;each of said fan ducts including a partition spaced from one of said endwalls to define a corresponding pair of secondary flow ducts;said pair of fan ducts having corresponding primary outlets disposed radially between said cowling and a trailing edge of said nacelle;said pair of secondary ducts having corresponding secondary outlets adjoining said primary outlets;said partitions including respective apertures covered by corresponding flaps; andmeans for selectively moving said flaps between stowed and deployed positions for correspondingly blocking and unblocking said partition apertures. 17. A nozzle according to claim 16 wherein said moving means are configured to deploy said flaps during takeoff operation of a turbofan engine configured for discharging said fan air through said fan ducts. 18. A nozzle according to claim 17 wherein said moving means are further configured to stow said flaps during cruise operation of said turbofan engine at a selected altitude. 19. A nozzle according to claim 18 wherein said moving means comprise a hinge mount pivoting an upstream end of each of said flaps from a corresponding partition, with a downstream end of said flaps being movable inside corresponding ones of said secondary ducts for unblocking said partition apertures. 20. A nozzle according to claim 19 wherein said moving means are active; and include an actuator for deploying said flaps. 21. A nozzle according to claim 19 wherein said moving means are passive, and utilize static pressure of said fan air to selectively deploy open said flaps. 22. A nozzle according to claim 19 further comprising means for channeling air through said secondary ducts when said flaps are stowed closed atop said partition apertures. 23. A nozzle according to claim 22 wherein said channeling means are configured for channeling ambient air external of said nacelle through said secondary ducts when said flaps are stowed closed. 24. A nozzle according to claim 19 further comprising a pair of said secondary ducts, partitions, and flaps disposed at said endwalls on one side of said core engine cowling, and another pair of said secondary ducts, partitions, and flaps disposed at said endwalls on an opposite side of said core engine cowling. 25. A method of reducing noise in a turbofan engine having a fan discharging fan air through a fixed flow area outlet in a fan nozzle defined between a fan nacelle and core engine cowling comprising increasing circumferential extent of outlet flow area in said fan nozzle during takeoff operation of said engine for correspondingly reducing velocity of said fan air being discharged therefrom, and terminating said area increase at cruise operation of said engine at altitude.