Oz, reference your quote:
"Drag then decreases as the air density reduces, thrust is now greater than drag so the aircraft accelerates.
It is exactley the same theory as a normal, constant IAS climb in an aircraft with a flat rated engine. For the same thrust and IAS, TAS will increase with altitude"
If it is a constant ias climb (can we talk cas?) as you say, then how can drag decrease (compressibilty aside)?
Secondly, I know engines can be flat rated with respect to temperature. Can you explain this flat rating with altitude and speed? New to me.
Hawk