QUOTE(UALdave @ Oct 22 2009, 09:54 PM)
That's interesting that it's automatic on the A320 series; I have the W.A.R. United Airlines 777-200 and the captain in that DVD said that it's automatic on that aircraft. He also said that they can start both engines at the same time on it. So, he said that, to paraphrase, "you won't hear us calling out N1 or N2 like you do on other aircraft". Does that have anything to do with the fact that both the 777 series and the A320 series are fly-by-wire? And how do you tell when the engine on such a plane has fully started?http://en.wikipedia.org/wiki/File:Turbofan_operation.svg
My main question is: how does a pilot start an engine on a commercial aircraft where it's not automatic? And what do those numbers (in an EFIS-equiped) cockpit refer to in the EPR, N1, and N2 gauges? RPMs, or temperature in fahrenheit?
HERE IS A COOL LINK ON THE JET
We don`t like posting this in case some bozo nicks the aircraft - y`know whadda mean. . ?
But very basically, you have to shove air into the turbine at very high pressure to turn it, then you put fuel into it and ignite it, actually, ignite it first - this is very important, or you have lots of fuel to ignite which is not healthy, then, when it catches light, you monitor the ITT which gets hotter, peaks and then stabilises, you have to check if it is going into meltdown, if not and when everything is stable within a specific time limit (stopwatch), then you can start the rest of the engines, one by one. Thats basically it. Today`s Airbuses do start automatically, well, its actually semi-automatic, but once on their way through the start procedure FADEC takes over.
Answering your other bit. . The captain mentioned that because of FADEC, which is fully autonomous digital engine control or something like that, which monitors the start process for you (but you still have to do it yourself too!) if all the parameters are ok then it will just go through the process, there is also a re-start procedure which is also automatic and even an annuciation and shutdown . . bit. It is quite a clever little bit of kit. Ranger was well up on all this stuff, you might wanna ask him, as he has got it in his MD-11.
Your core of the engine, if you like, is generally referred to as N2, as you know, N2 will be the first turbine that turns, and unless it is a fixed axial thingy then the subsequest draft of air being drawn from the front of the engine will also start to move the N1 - as airflow through the front of the engine increases so does N1, as N1 is the front fan that you can see from in front of the aircraft when you look. It takes a lot of airflow to push this fan around at its optimum N1 speed thats why it seems to rotate slowly, at first, also, don`t confuse the noise of the APU starting up first, which is in itself a jet engine too - as you know.
If you just put Jet Turbine Engines into a search engine, it will come up with loads of links for you to click on and have a look at and you will eventually find the bits you want. Also, if you post in TECHNICAL on here then you will get more of a technical response.
On the old RB211 in the Lockheed Tri-Star, the Front fan is the N1 which is the LP fan this is also attached to an LP rotor at the back of the engine. Tthe N2, was an IP turbine with a similair IP turbine further at the back attached to it. So when one rotor rotates it turns its other rotor which it is attached to. Same with the HP rotor which is all in the core yet has its attached mate at the back of the engine. The airflow being drafted through by the bleed pressure being direct onto the rotor blades and the air get compressed trying to squish its way around the rotors and stators. In its compressed state it enters an area where there is an injector spraying atomised / vapour fuel into the airflow. Lower down the engine are the ignitors which are like the ones on your Mom`s Gas Cooker. These ignite the fuel and air mix which burns at a fearsome rate and immediately expands and tries to get out of the engine quickly by shoving its way through the remaining rotors and stators and expanding into the atmosphere - by doing this it also speeds up the engine so the air coming in is also turning the turbine as well as the air going out. When it is all balanced out we say it is at self-sustaing RPM which means it is idling nicely, so you can turn off the ignitors, for that particular engine and concentrate on the remaining engines. you have to make sure that there is no ice or heavy rain or snow or severe crosswind to the engine intake especially during takeoff, if there is then you might want to leave the ignitors on because the chances are that you are ging to need them - often a flame out has been prevented by the fact that continuous ignition has been selected by the pilot.
The stabilized self sustaining idle on an F-16 is typically 70 percent RPM of N
and the A320 is 20% N1, 398 EGT, 60% N2 and 600 LBS per hour fuel flow.
As you noted it is necessary to monitor the thrust on the jet engine and we do this by the N1 reading or the EPR or both!
The EPR, as you know, measures the difference in pressure between the thrust going out the back end and the airflow pressure in the compressor - the EPR is actually the exhaust pressure divided by the compressor pressure - so if you take one to be "x" and then the other to be "y" then you can play around with the fraction to get your EPR Exhaust Pressure Ratio - which decreases as forward acceleration increases as air flows or rather gets forced into the intake due to forward movement of the aircraft. It is easy to see how effecient these engines are as the faster you go, the greater thrust you get and it gets cheaper too as you get higher. The most power you get is in the middle altitudes and power per say tends to slack off a bit as you get higher - but not noticebly - when the pilots do their calcs they work out the optimum alt for their cruise based on the best performance and/or economy for that flight - like altitude, air density, temperature and their weight -as, if it costs you mega tons of fuel to get up there - it may be more cost effective to stay down here, that sort of thing and they have a load of charts and graphs to work it all out with. Nowadays, they probably have it all stuffed into a laptop.
but don`t quote me on that as I am too poor to ever be insured for being sued. LOL.