If you look in the books you will find a TAS versus power graph for a jet engine. I was also a bit stumped at the fact that jet power increases with TAS instead of IAS.
The way I made sense out of it: A climbing jet:
1. At a higher TAS there is a greater column of air flowing through the engine per time.
2. This may allow the fan/ compressor to rotate faster, improving it's compression ratio. This should increase the N1 in theory.
3. I'm not sure about this; but I think that as long as the thrust lever is not moved while climbing, the engine wants to maintain a constant N1 RPM, so it will simply decrease fuel flow to compensate for the improved compression ratio.
Also, I believe the highest level flight TAS for a jet can be achieved at the changeover altitude where, in a climb, one would go from a constant IAS to a constant Mach climb speed. this is at about 25.000ft.
But, jets climb higher. One of the reasons for this must be for compressibility effects, which will also improve fuel flow.
Also, at the tropopause you have a reasonably good TAS at a cold temperature, which also means a denser column of air through the engine per time.
It may make more sense if you think of IAS as a certain pressure working on the airplane per time, and TAS as a column of air flowing over the airplane per time.
Please correct me if i'm wrong, this certainly is an interesting topic, but not always easely explained in the books!
