Vx/Vy
 

Just reading up on some POF - looked through some old posts and came across this: "Vy is the speed of maximum excess power = power available - power required. If power available were not a function of speed, that would be the same as minimum power required speed (minimum sink rate for a glider). But since power available increases with speed, Vy occurs at a higher speed than minimum power required."

*"But since power available increases with speed" - can someone explain this to me?

The power available for jet aircraft does increase with increasing speed, assuming constant thrust with increasing speed:

P = F * v

If thrust force (F) is constant and the speed (v) is increasing, power will be increasing as well.

The power available for a propeller-driven (whether it be piston or turboprop) aircraft also increases with speed up to a point. Essentially 'power available' is 'thrust available multiplied by speed'. Propeller thrust decreases with increasing speed.