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Old 30th Oct 2014, 10:22
  #5 (permalink)  
FE Hoppy
Join Date: Sep 1998
Location: wherever
Age: 52
Posts: 1,618
Normally the term Margin refers to engine temperature. The difference between the ITT/EGT achieved when setting thrust and the limit for that thrust.

The greater the difference between actual and assumed temperature, the greater the margin.

This also applies to any actual temperatures below the flat rated temp. A take off with full thrust at 4C will have a lower ITT than a take off at 30C.

So the flat rated temperature of an engine does not define the Margin when using assumed thrust. It's the difference between actual and assumed which defines the margin.

Now, if it's density you are interested in then we can say that we gain uncorrected benefits in terms of Lift and TAS when we use an assumed temperature compared to taking off at the equivalent actual temperature. This performance gain is also proportional to the difference between actual and assumed temps.

The only real significance the flat rated temperature has is to define the minimum acceptable assumed temperature.

Setting an assumed temperature of 25C on an engine which is flat rated to 30C is simply asking for max take off thrust. i.e. no reduction. If you could do this you would find that the N1 at assumed temperature would be higher than the N1 at actual temperature. You would then correct this N1 for the difference in density between actual and assumed and if you did the sums correctly you would get the same N1 as for your actual temperature.

Because the thrust scheduled at actual and assumed is the same and therefore the N1 required to achieve that thrust is the same.

No difference in thrust, N1 or ITT.

Bit of a ramble but hope it helps.
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