All engines are designed to be most efficient at a particular RPM. For a modern jet that is somewhere around 90% n3 or n2 as the case may be. This means that on the thrust/fuel flow graph 90% RPM would be the most efficient..
Now lets level off at twenty thousand feet and set 90% and see what happens. The airspeed will soon increase above limits which in turn causes more drag and decreases efficient operation. So what do we do, we climb up to an alttitude where that same 90% RPM sets us up at the airframe's optimum speed somewhere around .80 mach. We also get the benifit of increased TAS as compared to levels below somewhere around 26000 ft.
Then we say to our selves if climbing is good lets keep going, but then more thrust is needed to maintain .8 mach which increases the N3 above its 90% optimum. So this explains why optimum is lower than max altitude (still air of course). As we proceed we get lighter and less than optimum 90% RPM is the result so we climb to where we get that nice 90% again. This explains the fact that optimum altitude rises as the aircraft gets lighter.
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Last edited by airbus757; 10th September 2007 at 18:03.