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Joined: Thu Jul 15, 2004 3:07 am

Engine Derate...I Don't Understand

Tue Aug 31, 2004 4:45 am

I understand the fundamentals behind derate but here is what I don't understand. When you do a flex temp derate and input the air temp at say 40C and the actual temp is 30C how does this result in less power? Wouldn't a higher outside air temp actually require MORE power (less dense air, therefore less power)? I might be way off but I use this stuff all the time in Flight Simulator (Phoenix Simulations A320' has a all of this simulated and you have to do all the FMIC input stuff as well.

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Joined: Sun Nov 23, 2003 7:09 am

RE: Engine Derate...I Don't Understand

Tue Aug 31, 2004 4:55 am

The power output of jet engines is limited by the RPM limits of the rotors.
The power output of jet engines is limited by internal pressures allowed.
The power output of jet engines is limited by internal temperatures allowed.

On any give day one or another of the above limits is going to be reached before the others are. On a hot day it might be temp limit first. At sea level it might be pressure first. Et cetera.

Now this one and particular engine has a thust rating of, say, 32,000 lbs. Well it does not produce that amount on every takeoff, just on the ones were conditions are ideal. That is the maximum rated thrust.

When we "assume" a higher temperature it sets the fuel flow limits lower so that it does not over temp itself.

Now there is actually more to it than this, but I think you can see where I'm headed with that. Sometimes when I insert an assumed temperature I am surprised by the takeoff N1 it sets.
Happiness is not seeing another trite Ste. Maarten photo all week long.
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Joined: Thu Jul 15, 2004 3:07 am

RE: Engine Derate...I Don't Understand

Tue Aug 31, 2004 5:09 am

Got it! Makes total sense. Thanks for that.

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Joined: Thu Sep 11, 2003 10:27 pm

RE: Engine Derate...I Don't Understand

Tue Aug 31, 2004 11:37 pm

The engine FADEC "power manages" the engine using N1 for GE engines and EPR for PW/RR. I'll explain N1 power management since that's what I'm most familiar with.

Power management is designed to produce constant thrust for all temperatures up to some limit. The FADEC includes tables of N1K (corrected N1). Corrected N1 is a N1 value adjusted for the current ambient temperature. This allows the FADEC to determine a demand N1 speed for all temperatures. The tables of N1K are functions of Mach number, ambient pressure and ambient temperature. The goal of the designer is to define the N1K tables to produce constant thrust at all temperatures. This is called maximum flat rated thrust.

However, there is a limit where the engine hot section components must be protected from extreme temperatures. Above this limit, the N1K tables are pulled back. The point where the flat-rated thrust breaks off is often called cornerpoint temperature. If you were to plot thrust versus ambient temperature for a given power setting, you would see a clear "corner point". Below this cornerpoint you have flat-rate thrust, above the cornerpoint you have decreasing thrust as temperature increases.

Now, back to your question. Derate using the assumed temperature method is accomplished by setting the temperature input above the cornerpoint. If set below the cornerpoint, not much would change, but if set above the cornerpoint N1K will be pulled back. The higher you push the temperature, the more pull back you get.


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