Variable area plug nozzle

1. An exhaust nozzle comprising:

an outer shell having a trailing edge;

a plug disposed coaxially inside said shell for defining an annular exhaust duct therebetween, and extending forward from said shell trailing edge to define an annular duct inlet for receiving exhaust gases, and extending aft from said trailing edge to define an annular duct outlet therebetween for discharging said exhaust gases;

said plug including an inlet adjacent said duct inlet, a flap closing said plug inlet, and an outlet adjacent said duct outlet; and

means for selectively moving said flap to bypass a portion of said exhaust gases from said exhaust duct and through said plug to said plug outlet for discharge near said duct outlet.

2. A nozzle according to claim 1 wherein said plug includes an annular bypass band disposed in said exhaust duct axially between said plug inlet and plug outlet, and said plug outlet provides a local increase in flow area downstream of said band when said inlet flap is open.

3. A nozzle according to claim 2 wherein said plug further comprises:

a radially outer skin including a portion forward of said band having a row of apertures defining said plug inlet and selectively closed by respective ones of said flaps, and including a portion converging aft from said band and having said plug outlet therein; and

an inner skin spaced radially inwardly from said outer skin to define a bypass duct therebetween disposed in flow communication with said inlet apertures and plug outlet.

4. A nozzle according to claim 3 wherein said plug further comprises an annular manifold sealingly enclosing said flaps at said inlet apertures.

5. A nozzle according to claim 4 wherein:

said flaps are pivotally mounted inside said plug; and

said flap moving means are configured for permitting pressure of said exhaust gases flowable in said exhaust duct to pivot said flaps radially inwardly into said manifold for flow through said bypass duct and out said plug outlet.

6. A nozzle according to claim 5 wherein said moving means are configured for pressurizing said manifold to pressurize closed said flaps against said exhaust gas pressure inside said exhaust duct, and venting said manifold to permit said exhaust gas to open said flaps.

7. A nozzle according to claim 6 wherein said flap moving means comprise a respective return spring disposed in said manifold for biasing closed each of said flaps.

8. A nozzle according to claim 7 wherein said return spring is sized to produce a closing force on said flap sufficient to retain closed said flap against exhaust gas pressure in said exhaust duct occurring at aircraft cruise operation notwithstanding venting of said pressure in said manifold.

9. A nozzle according to claim 6 wherein said flap moving means further comprise a bleed conduit extending from said manifold to join in flow communication a compressor of a gas turbine engine, and a valve operatively disposed in said bleed conduit for selectively pressurizing said manifold with compressor bleed air, and venting said bleed air therefrom.

10. A nozzle according to claim 6 wherein:

said inner skin includes a row of manifold apertures aligned with respective ones of said inlet apertures; and

said flaps extend through said manifold apertures to close said inlet apertures.

11. A nozzle according to claim 10 wherein each of said flaps includes a radially outer perimeter flange sized to overlap said inlet aperture for provide a seal therewith, and a radially inner perimeter flange sized to overlap said manifold aperture for providing a seal therewith.

12. A nozzle according to claim 6 wherein said plug outlet is disposed forward of said duct outlet.

13. A nozzle according to claim 6 wherein said plug outlet is open.

14. A nozzle according to claim 6 wherein said plug outlet comprises annular rows of outlet apertures.

15. A nozzle according to claim 14 wherein said outlet apertures extend parallel to the centerline axis of said plug, and have elliptical profiles exposed on said outer skin.

16. A nozzle according to claim 6 wherein said plug outlet comprises an arcuate cut-out in said outer skin being larger in flow area than one of said inlet apertures.

17. An exhaust plug comprising:

an annular outer skin including an annular hump, a portion extending forward therefrom, and a portion converging aft from said hump;

an annular inner skin disposed concentrically inside said outer skin to define an annular bypass duct radially therebetween;

a row of plug inlet apertures disposed in said forward portion of said outer skin in flow communication with said bypass duct;

a plug outlet disposed in said aft portion of said outer skin in flow communication with said bypass duct;

a plurality of flaps, each disposed in a respective one of said inlet apertures; and

means for selectively moving said flaps between closed and open positions for selectively bypassing exhaust gases through said bypass duct around said hump.

18. An exhaust plug according to claim 17 further comprising:

an annular manifold sealingly enclosing said flaps at said inlet apertures;

said flaps being pivotally mounted inside said plug manifold by pairs of hinges; and

said flap moving means are configured to selectively pressurize said manifold to pressure-close said flaps, and selectively vent said manifold to permit said flaps to open into said manifold.

19. An exhaust plug according to claim 18 wherein:

said inner skin includes a row of manifold apertures aligned with respective ones of said inlet apertures;

said flaps extend through said manifold apertures to close said inlet apertures; and

each of said flaps includes a radially outer perimeter flange sized to overlap said inlet aperture for providing a seal therewith, and a radially inner perimeter flange sized to overlap said manifold aperture for providing a seal therewith.

20. A method of attenuating noise in an aircraft gas turbine engine exhaust nozzle having a center plug, comprising:

discharging hot exhaust gas flow from a turbine into said exhaust nozzle;

bypassing a portion of said exhaust gas flow from said exhaust nozzle through said center plug during takeoff operation of said nozzle in an aircraft for temporarily increasing outlet flow area of said nozzle; and

terminating exhaust gas bypassing through said center plug during cruise operation of said nozzle in said aircraft.