Originally Posted by
pattern_is_full
I think the key point here is holding a constant N1 (fan rpm).
As you accelerate in forward speed, your increasing airspeed is ramming air into the fan/compressor, and that "helps" them to spin (think of a pinwheel in a breeze), requiring less effort from the power section, so N2 probably rolls back a bit to hold a constant
Not quite correct. As A/C fwd speed increases, air pressure to the Fan increases due to ram effect. As this ram air hits the Fan, the increasing pressure means that blades are now rotating in denser air and therefore increase the load on the rotor. While the pressure ratio increases the turbine now has to work harder to maintain N1 (Fan) speed.
It really depends on what is the controlling parameter, either spool speed or turbine temperature and how that controlling parameter is scheduled during the T/O run. Some engines may either maintain or increase fuelling to overcome the increased A/C drag.
Hope this helps.