I should have given more details. When analysing best range we use the equation:
(V/Tr) X (1/SFC) where V = TAS, Tr = thrust required and SFC= specific fuel cunsumption.
The issue in class was the following. Do we get any advantage gaining altitude (for a jet) on the airframe point of vue (V/Tr)?
We know the thrust requried curve shifts right when increasing the altitude. We want to have the max for V/Tr. Since Tr does not move and V increases with alt we have a tremendous advantage flying high from the airframe point of vue (from the engine also but this is another issue).
Now my question was: Above the trop, is the TAS varying? If not, there is no advantage nor disavantage flying within this layer.
Your thoughts?