Thanks Mike, Mark's description is along the lines that I was trying to describe except I would suggest that the flight path of the airplane in inertia space is the "keel" of the iceboat analogy. The propelling force is just a vector of the wind relative to the flight path at the top of the hill.

Energy is added at the roughly 12 oclock position each time the airplane comes around. Low drag means it is retained at 3, 6 and 9. The gravity energies cancel each other out (up and down).