"David E. Powell" David_Powell3006 msn.com wrote:

One thing is for sure it can't be too high or low relative to
center of gravity.

That may be less critical than you think. Propeller propulsion
would be the freest form since it would not depend on the ground,
the full benefit of skating depends on freedom of movement. Push
stick based propulsion (a bulky prototype is currently being used
here) is stable, even when being pushed from knee level. I get a
third wheel effect, and stroking with my legs is not required.
Pushing/stroking is a HUGE source of instability while inline
street skating.