Editing Turbojet (section)

== Net thrust ==
The net [[thrust]] <math>F_N\;</math> of a turbojet is given by:<ref name="JP3.1" >{{Cite book | last=Cumpsty  |first=Nicholas | year=2003 | title=Jet Propulsion | edition=2nd | publisher=Cambridge University Press | isbn=0-521-54144-1 | ref=Cumpsty, Jet Propulsion | chapter=3.1 }}</ref><ref>{{cite web | title = Turbojet Thrust | publisher = NASA Glenn Research Center | url = http://www.grc.nasa.gov/WWW/K-12/airplane/turbth.html | access-date =6 May 2009 }}</ref>

<math>F_N =( \dot{m}_{air}  + \dot{m}_f) V_{j} - \dot{m}_{air} V</math>

where:

{| style="border: none"
|-
|<math>\dot{m}_{air}</math>
| is the rate of flow of air through the engine
|-
|<math>\dot{m}_f</math>
| is the rate of flow of fuel entering the engine
|-
|<math>V_j\;</math>
| is the speed of the jet (the exhaust plume) and is assumed to be less than [[sonic velocity]]
|-
|<math>V\;</math>
| is the [[true airspeed]] of the aircraft
|-
|<math>(\dot{m}_{air} + \dot{m}_f) V_j</math>
| represents the nozzle gross thrust
|-
|<math>\dot{m}_{air} V</math>
| represents the ram drag of the intake
|}

If the speed of the jet is equal to [[sonic velocity]] the nozzle is said to be "[[Choked flow|choked]]".  If the nozzle is choked, the pressure at the nozzle exit plane is greater than atmospheric pressure, and extra terms must be added to the above equation to account for the pressure thrust.<ref name="Cumpsty, 6.3" >[[#Cumpsty, Jet Propulsion|Cumpsty, Jet Propulsion]], Section 6.3</ref>

The rate of flow of fuel entering the engine is very small compared with the rate of flow of air.<ref name=JP3.1/> If the contribution of fuel to the nozzle gross thrust is ignored, the net thrust is:

<math>F_N = \dot{m}_{air} (V_{j} - V)</math>

The speed of the jet <math>V_j\;</math> must exceed the true airspeed of the aircraft <math>V\;</math>if there is to be a net forward thrust on the airframe. The speed <math>V_j\;</math> can be calculated thermodynamically based on [[adiabatic expansion]].<ref>{{Cite web|url=http://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node85.html#SECTION06364000000000000000|title=11.6 Performance of Jet Engines|website=web.mit.edu}}</ref>