Vx is simply the speed were you have maximum excess of thrust from the engine compared to the drag (thrust/drag)
Vy is the speed were there is maximum excess of power availible (mathematically: excess thrust*speed)
As you see an excess of thrust is an important factor in both as this is the force that makes you climb in the first place.
In Vy the speed also becomes important since you want to get up along your climbing path as quick as possible to reach your altitude in minimum time.
The fundamental difference is that power includes some kind of movement and the speed of the movement where thrust is just a force. In the previous explanation the 90 degree climb could be made at 1 feet / 100000 years and it would still be considered 90 degrees of climb = good
Last edited by kme; 14th October 2007 at 13:28.