Well start with some definitions.
Thrust is a force, and power is a rate at which work is done. Work is the product of force times the distance moved in the direction of the force.
Vx is the speed at which you get the best angle of climb. Climbing is a matter of overcoming the force of gravity. Therefore the more force (excess of thrust over drag) you have, the better you can do it (ingoring all other considerations). So thrust is a key issue
Vy is the speed at which you get the best rate of climb. Rate of climb is connected to rate of delivery of work, and therefore power is a key issue.
Vy is the lowest point of the drag curve. Any other speed (including Vx if not the same) will have higher drag.
Hope that helps.
Last edited by puntosaurus; 11th May 2010 at 21:40.