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Roger Mazengarb

Engine Idle

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Do some of you know how to read or calculate the N1 when the engines are on idle?On my twin-engine airliner, the idle N1 is around 27% on ground, and around 30-31% at high altitudes. I don't know if it also depends from the temperature.I need to know the exact value of the idle N1 value, so any info on this topic is greatly welcome.Thanks !!Eric

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N1 is (basically) the percentage of gaseous flow through the turbine. Ok, that's really over-simplifying it but it will do for this discussion. (N1 is actually more specific than that, but that description will do for now.)Remember, air is a fluid... in fact, any sort of gaseous vapor is actually a fluid. N1 at engine idle will have a lower reading than N1 when moving forward through the air at some speed simply because the act of moving rams air into the engines which is in addition to the normal idle gases produced by the engine.At ground idle, the only thing that can do that is a really stiff wind... which might get you 1/2% increase in N1, if that.27% N1 seems odd though. When starting a helicopter turbine, specifically the 206b3, you have to keep the starter turning continuously until the engine reaches 58% N1 or the flow isn't fast enough to be self-sustaining. To let go of the starter too early will case what's known as a "hot start", meaning that the flame trails up into the turbine and it goes boom.... and you have a really bad day.All my exp is in helos... maybe the N1 percentage is very different when using the turbine exhaust to create thrust because you're probably running at a much higher rate of flow through the turbine.But remember, N1 is read in percentage of maximum for that aircraft. 40% on an F-15 is probably way over 100% for a 206b3 Bell helo.Scott / Vorlin

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The starter on a helicopter is released at 58% N1 to ensure there is enough airflow to continue combustion. If it released to early you get a HUNG START where the engine can't continue to speed up to idle RPM. ( ALot of things are happening inside the fuel control unit at the same time, Air pressures on bellows and counterweights spinning to open little valves and it all depends on N1 Speed. a HOT START happens when there isn't enough airflow to cool the internal parts of the engine. and on the start, airflow is N1 speed. When the fuel in a helicopter is interduced makes a big difference. At 10% N1 with fuel and it will go HOT, 15% N1 and fuel is added good start. 75% of the air going into say a Allison 250 C20B engine is used for cooling the internal parts of the engine....... Ramble on or what...LOL

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Eric,I know of two possible methods:1) Modify the a/c .air file, table 1502, CN1 vs CN2; increasing CN1 on the second row (Mach 0.9) for the lowest CN2 values and up to the next CN2 column greater than "ground" idle.2) If you don't want to play with .air files, just include in a gauge a polinomial that "converts" the original CN1 idle values to those needed at high altitude/speed. With this method you will need to code the engine N1 gauge using LVars of your own.Tom

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Hello Tom,This is interesting. I would like to have a look at the table 1502, but I don't have the appropriate air file editor. I only have "AirEd", which only displays hexadecimal numbers for the table.Is there another editor that gives more understandable reading?Thanks,Eric

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I've got it, thanks for the link. Indeed, I had an old version and this one is much better !!Can you tell me more about these 3D tables? I didn't really understand what they mean.For example, table 1502 is "Turbine CN1 vs CN2 and Mach No." When I open this table, the first row (grey) shows CN1, and the first comlumn (also grey) shows the mach. The other cells show CN2. Is this right?Thanks for your help !!Eric

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"" When I open this table, the first row (grey) shows CN1, and the first comlumn (also grey) shows the mach. The other cells show CN2. Is this right? ""Nope. First row are CN1 values at 0 Mach for each CN2 column value.Second row are CN1 at 0.9 Mach for the same. FS makes a direct interpolation of the two values for a given (Mach) speed, which in real world is rather incorrect, but FS doesn't use polinomials...Tom

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The Aircraft Airfile Manager (AAM) is a very good tool for graphically depicting and manipulating 3D tables. It's still out there...and still free.click on the downloads link at:http://www.aircraftmanager.com/CheersBob ScottATP IMEL Gulfstream II-III-IV-V L-300Santiago de Chile

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Well, I know that South America has special charms in it, but so far I didn't realise, that You are still able to download from a long gone site. At least for us in Europe the aircraftmanager.com is offline for last six months if not more and that is why Herve Sors kindly made the AAM available for download from his site.

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