Air Intake

[Bubblehead]

What physical phenomenon occurs within the jet engine as the aircraft speeds up during take off and when it slows down during landing. What about as it cruises at high altitudes where the air is thinner but the aircraft is moving at a much greater speed?

Bubblehead

[Mobius]

If you mean what mechanically moves, there's very little actually, other than in supersonic aircraft.  Conventional jet engines (other than ramjets and scramjets) don't work with supersonic airflows into the engine, so there are various methods of slowing down the airflow, usually by a static shockwave generator type thing, like the little bar in the front of the F-16 intake, becuase airflow behind supersonic shockwaves is subsonic.  There are a few aircraft specific things, like the drooping intakes on the F-15 to direct air into the intakes at high AOAs, or the alternate intakes on the top of the MiG-29 so they don't ingest debris.  But the only thing that will change in a jet engine throughout a flight is the engine RPM, nozzle deflection, and maybe the size of the opening of the fuel valves, but jet engines usually don't have too many parts, and in the case of ramjets and scramjets, they have no moving parts, which is really amazing.

If you mean what is happening in the engine to produce thrust, just remember "suck, squeeze, bang, blow", which usually works for most power production cycles.  The engine pulls air in from the outside with a fan or series of fans on the front.  The air is then passed through a series of compressors, then a portion goes through the burners where fuel is added and combusted, which heats it and the rest of the air, which adds a large amount of energy to the airflow.  It is then forced out the back where it expands, and forces itself out the nozzle where all that extra energy pushes the jet engine through the air, and if the engine is attached to an airplane, the airplane goes with it.  This physical processes of this cycle don't change throughout a flight unless the airflow into the engine is to little to sustain engine RPM, which causes a flameout.  Other than that, the only thing that changes throughout the flight is the energies imparted on the air and the engine, and the flowrates and velocities of the air flow throughout the enigine.

If that's not what you meant, than what the heck do you mean?

[expat]

Not Quite sure what you mean by physical phenomenon. There are many different factors that have to be taken into account. The main ones are outside air temperature, altitude, fuel flow, ram air effect to name just a few. I could write a pages on this, but try this website out. It may help. It takes a few moments to load the Sim part.

Jet engine simulator

Matt

[Bubblehead]

Patience gentlemen.

The reason I brought this up was because a good friend of mine who retired as a Navy Aviation Maintenance Officer was explaining me the difference between when a jet engine going full blast while stationary and going full blast while in flight. Certainly there has to be some effect when air is simply being sucked in (as in stationary) and when air is being forced in in great amounts, as in during flight. I remember him saying that there needs to be more fuel during landing and that explains why the black smoke trailed by the aircraft. Of course the advancement in engine design was much difference when compared to the Boeing 707 days. Nonetheless, thaks for the technical explanation.

Bubblehead

[Mobius]

This is pretty muh a total guess, but are you sure he didn't say there is more fuel during landing instead of there needs to be more fuel?  That would make sense to me (hypothetically, I don't know technically).  During high speed flight, there is more ram air pressure, which means there's a higher mass flow rate of air, so more fuel would be needed to keep the proper air-fuel mixture at high speeds.  At low speeds, if the same A/F ratio were used, the same amount of fuel would be used, with less air, meaning it would run rich.  If the mixture was leaned for take-off and landing, then that same ratio was used at high speeds, the mixture would be lean.  I would imagine there's computer controlled fuel injectors to keep the A/F mixture optimum, but I don't know how it actually works, so maybe someone who actually knows will have a better answer.