Assumes a normally aspirated engine (non-turbocharged). THP decreases with altitude due to decreased air density.
The difference between THP available and the THP required is excess THP, which can be used to climb. The point of maximum excess THP is the maximum rate of climb airspeed (Vy).
The maximum rate of climb depends on the excess THP available.
Power required to maintain altitude at a given airspeed.
The true airspeed for a maximum rate of climb increases with altitude.
The indicated airspeed for a maximum rate of climb decreases with altitude.