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enter image description here

It's easy to understand why the takeoff N1 limit decreases with an increase in the OAT in the chart above; the engines get less dense air at higher temperatures, and less dense air means less engine power capability. But why does the takeoff N1 limit start to decrease with a decrease in the OAT below 30 degrees Celsius? For your information, the chart is an airport analysis chart for Boeing 737.

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    $\begingroup$ Not sure, so not giving an answer. But I expect another parameter becomes the limiting factor at cold temperatures, and the N1 maximum allowed must be reduced to avoid exceeding it. $\endgroup$ – MikeY Dec 2 at 3:16
  • $\begingroup$ I definitely don't find it easy to understand why the N1 limit decreases with an increase in the OAT. Because less dense air means a turn of the fan moves less air, but that also gives it less resistance, so it gives less power, but should be able to spin as fast. $\endgroup$ – Jan Hudec Dec 2 at 19:15
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    $\begingroup$ Since this Q is in hot network questions: What are N1 and N2? for background for the idly curious wondering what this question is actually about. $\endgroup$ – Peter Cordes Dec 3 at 6:25
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What you see is called a flat rated engine. It means the maximum thrust from the engine is constant below the flat rated temperature (usually 30°C). Above that temperature, thrust will decrease due to the EGT (exhaust gas temperature) limit. In order to achieve a constant thrust at lower temperatures, the N1 needs to be decreased accordingly.

Flat Rated Engine
(CFM56-5A Lufthansa Training Manual, page 4)

See also:

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There are basically 3 limits that the engine faces, temperature (maximum turbine entry temperature or maximum compressor exit temperature), pressure (maximum compressor exit pressure) and stress (maximum stress in the blades as a result of spool speed).

Varying the OAT for a specific engine design will hit one of these limits. When the OAT increases, the amount of fuel you can add to the system reduces as you get over-temperature, when the OAT decreases you could add more fuel, but that will increase spool speed which is also limited. From this figure you see the effects on SOT (turbine Stator Outlet Temperature, equivalent to Turbine Inlet Temperature, TIT):

enter image description here

The limit on the left is the decrease in spool speed, note that the corrected spool speed is constant (N / sqrt(T)) so for a lower temperature, the N also decreases. The constraining of the power is called flat rating.


This effect can be simulated with an engine model. If we would vary OAT for a certain power setting, e.g. constant (corrected) spool speed for a large bypass turbofan engine with imposed compressor exit pressure and turbine inlet temperature limit a model would look like:

GSP model of a large bypass turbofan engine

After the variation of OAT you will yield the blue performance curves for a non-limited engine, and the black dashed curves for a limited engine:

enter image description here

Note that the compressor outlet pressure is the limiting factor in the graph on the left, but spool speed and compressor pressure are linked. The question is which of the 2 is reached first, this depends on the input and the design constraints.

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    $\begingroup$ Good Lord! An answer with a simulation! Unreal. $\endgroup$ – Fattie Dec 2 at 18:43
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    $\begingroup$ What software is that? Is it free? $\endgroup$ – Thomas Weller Dec 3 at 10:18
  • $\begingroup$ @ThomasWeller Yes it is free. $\endgroup$ – 0scar Dec 3 at 14:56

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