Editing Propelling nozzle (section)

==Principles of operation==
* A nozzle operates according to the [[Venturi effect]] to bring the exhaust gasses to ambient pressure, while forming them into a propulsive jet; if the pressure upstream of the nozzle is high enough, the flow will reach sonic speed ([[choked flow|choke]]). The role of the nozzle in back-pressuring the engine is explained [[#The other purpose of the propelling nozzle|below]].
* The energy to accelerate the stream comes from the temperature and pressure of the gas. The gas [[adiabatic expansion|expands adiabatically]] with low losses and hence [[Carnot cycle|high efficiency]]. The gas accelerates to a final exit velocity which depends on the pressure and temperature at entry to the nozzle, the ambient pressure it exhausts to (unless the flow is [[choked flow|choked]]), and the efficiency of the expansion.<ref>"Jet Propulsion" Nicholas Cumpsty, {{ISBN|0 521 59674 2}}, p243</ref> The efficiency is a measure of the losses due to friction, non-axial divergence as well as leakage in C-D nozzles.<ref>"Exhaust nozzles for Propulsion Systems with Emphasis on Supersonic Aircraft" Leonard E. Stitt, NASA Reference Publication 1235, May 1990, para 2.2.9</ref>
* Airbreathing engines create forward thrust on the airframe by imparting a net rearward momentum to the exhaust gas. If thrust exceeds the resistance incurred by the aircraft moving through the air, it will accelerate. The jet may or may not be [[#Reasons for C-D nozzle under-expansion and examples| fully expanded]].
* On some engines that are equipped with an afterburner the nozzle area is varied during non-afterburning or dry thrust conditions. Typically the nozzle is fully open for starting and at idle. It may then close down as the thrust lever is advanced reaching its minimum area before or at the Military or maximum dry thrust setting. Two examples of this control are the General Electric [[J-79]]<ref>J79-15/-17 Turbojet Accident Investigation Procedures, Technical Report ASD-TR-75-19, Aeronautical Systems Division, Wright-Patterson Air Force Base Ohio, Fig60 "Nozzle area v Throttle angle</ref> and the Klimov [[RD-33]] in the [[MIG-29]].<ref>"Flight Manual MIG-29" Luftwaffenmaterialkommando GAF T.O.1F-MIG-29-1, Figure1-6 "Primary nozzle area v throttle angle"</ref> Reasons for varying the nozzle area are explained in section: [[#Nozzle area control during dry operation|Nozzle area control during dry operation]].