Introduction of jet aircraft

Introduction of jet aircraft


In this set of posts I will write about the performances of an jet engine JAR/FAR-25. The most important of the performance subject, is the relation between a manoeuvre such as, cruise, climb …. And the engine of the plane. For this reason I am going to start explaining the basics of an jet engine.



How does a jet engine works?





Jet engines are based on the second and third Newton´s law. Basically it increases the air backwards for moving forwards

We define thrust as follows: the amount of mass of air per second by the variation of gas velocity.





How to know the jet engine power?


In the cockpit there are some instruments related to the measurement of the engine power.
This instruments give us the revolutions oh the hight pressure turbine (N2) and low pressure turbine (N1).





Engine pressure ratio (EPR), is the ratio between of the total output pressure and engine input pressure, therefore the EPR is proportional to the thrust and is always greater than one.






How do we know that the engine is working properly?



The manufacturer of any jet engine establishes operating margins based on the temperature of the exhaust gas temperature  (EGT)

The motor settings are time-limited:

– Take off thrust (TO): It is the maximum power that a jet engine can reached, its limitation depends on the engine, and it varies between 5 min-10 min

– Go-around thrust (GA): matches with the time and temperature with the take off thrust.

– Maximum continuous thrust (MCT): Maximum thrust adjustment for maximum power with no time limitation. It is mainly used in an engine failure


Jet Engine power considerations


The thrust is function of the quantity of the air that enter to the engine, as well as the speed increment between the inlet air and outlet air.


T= G(Vs-Ve)


T: Thrust, force in the sense of the movement.


G : the waste of air. It is the quantity of air per unit of time (air mass/ time).


Vs: velocity of the outflow air.


Ve : velocity of the inlet flow.


All that affects the waste of air, velocity of the outflow air, and velocity of the inlet flow, will result in a change in the thrust.

For example:

Density altitude (DA): If the density altitude is high, the density drops down, the mass of air drops down, the Waste drops down so, the thrust drops down.  DA ↑: ρ ↓ G ↓ T↓.




There are other factors, which are controllable from cockpit:

Water Injection: It makes that the exhaust gas temperature drops down, so the temperature limitation is higher, leading in an increased Thrust.

Engine Bleed: The plane uses some air from the engines to feed the air conditioning among other systems, that makes that the air used to create the thrust is reduced, So the Thrust is lower.







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