N1 setting with regards to atmospheric conditions.

[Dariuszw]

My understanding is that N1 is set lower when temp, density alt. is high due to ITT increase in order to protect engine which makes sense. But I still dont understand why we set N1 lower when temp, density alt. is lower then standard. Here ITT seems to play no role so why N1 decreases ?
Is it because of takeoff, landing computations which are based on cetrain constant power output so increased power in cold and dense conditions is unecessary and for that N1 is reduced or is it because more power would exceed some engine stractural limit ?

You know, I fly my jets for years now and what a shame that such a inteligent, highly experianced and knowledgeable pilot as ME does not have an answer to this a silly little question.:hmm:

[de facto]

What jet do you fly?

[Stuck_in_an_ATR]

Most probably this one, judging by some of his posts: Fisher-Price Airplane | Flickr - Photo Sharing! (http://www.flickr.com/photos/monikalel42/3676332497/)
:E

[Checkboard]

why we set N1 lower when temp, density alt. is lower then standard. Here ITT seems to play no role so why N1 decreases ?

Because the engine is "flat rated" to produce a set amount of thrust. If you set the same N1 in denser conditions is would produce MORE than its rated thrust, pressures inside the engine would be MORE than rated, engine mounts would have to take MORE than their designed load.

Of course, sometimes the engine is flat rated to a lower thrust than it's design load, for economy savings (in engine wear and tear) - those savings would be negated by having the engine run MORE than it's flat rating.

[barit1]

Checkboard is correct. And the REASON for flat rating is economics; within the flat-rating thrust region, the airplane performance improves at colder temps (denser air over the wings), so there is no good reason to use excess thrust, when engine life can be extended by flat rating.

Also consider that if we didn't cut back N1, Compressor discharge pressure would keep rising as OAT drops, and since the engine case is a pressure vessel, it would have to be made heavier to contain the pressure safely. Bearing loads, etc. would also increase.

[FE Hoppy]

@barit1
Not sure I agree with your reasoning. It's like saying the reason for an ITT limit is economics. It's not the reason for the limit. But the consequence of lower ITT is savings. Unless that gives too little thrust for your intended operation. In which case you need a bigger engine and that will cost more.

The REASON for flat rating is to put a design limit on case construction and therefore weight.

[Dariuszw]

Thanks guys , now Im not only very pretty but more inteligent by the hour.:ok:

[barit1]

The REASON for flat rating is to put a design limit on case construction and therefore weight.

Absolutely. Not only the case, but the "long tom" fan shaft and the torque load it sees.

All of which comes back to weight, and thus economics!