Net Thrust
The net thrust of a turbojet is given by:
where:
| is the rate of flow of air through the engine | |
| is the rate of flow of fuel entering the engine | |
| is the speed of the jet (the exhaust plume) and is assumed to be less than sonic velocity | |
| is the true airspeed of the aircraft | |
| represents the nozzle gross thrust | |
| 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. 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.
The rate of flow of fuel entering the engine is very small compared with the rate of flow of air. If the contribution of fuel to the nozzle gross thrust is ignored, the net thrust is:
The speed of the jet must exceed the true airspeed of the aircraft if there is to be a net forward thrust on the airframe. The speed can be calculated thermodynamically based on adiabatic expansion.
A simple turbojet engine will produce thrust of approximately: 2.5 pounds force per horsepower (15 mN/W).
Read more about this topic: Turbojet
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