Originally Posted by Will Fraser
Generally the Iris is fully open with any AB setting??
Hi Will
The VEN variable area nozzle/Iris has an area that varies depending on throttle settings, flight conditions, atmospheric conditions and engine condition. Generally speaking the nozzles are fully open (or close to fully open) at ground idle. At military power they are near fully closed. As the afterburner is selected they begin to gradually open from mil setting and continue to do so as maximum afterburner is selected. At maximum AB they are near fully open under optimal conditions.
The reason the nozzles at max AB are not fully open under normal circumstances is that additional 'margin' is needed to ensure the engine can operate safely with require performance throughtout the flight envelope. Also on a hot day an engine will make less thrust so the nozzles are set with a smaller area than on a cold day where more thrust is avaliable.
Obi