variable-geometry nozzle


variable-geometry nozzle
A nozzle whose diameter can be changed. It is normally in the shape of a convergent nozzle at subsonic speeds and of a convergent-divergent nozzle at supersonic speeds, or when a reheat is on. A variable nozzle permits the use of after-burner noise reduction and easier starting. The variable geometry may be achieved automatically, depending on the throttle position and/or speed or by controlling it manually from the cockpit. On most aircraft, only the primary (convergent) nozzle position is controlled.
The illustration shows a typical operation of a variable geometry exhaust nozzle. The nozzle is fully open at start up and remains in the same position till throttle is advanced to about 65% when it is fully closed. The nozzle remains fully closed until beyond 100% RPM and until minimum afterburner is selected, when it takes up an intermediate position between the maximum and the minimum. It opens up fully when throttle is just short of fully open position and remains so until the maximum afterburner position. The same sequence is followed when throttle is brought back except that the nozzle opens up to fully open position at about 60% RPM instead of 65% as was the case when throttle was being advanced.

Aviation dictionary. 2014.

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