Hi Ski Professor--
Thanks for digging this one up again. While it's pretty technical, this discussion involves some very important concepts, including some common misconceptions that cause many skiers unnecessary problems!
While we probably agree on "the forest," I think we're seeing "the trees" a little differently. Here's my response to some of the points and questons you raise:
|If you put a ski on edge and turn it across your line-of-motion, pressure or what's perceived as weight is created. Do you agree with that?
Well, yes--if you pivot your skis crosswise to your direction of travel, and engage their edges, pressure you feel underfoot will increase. But regardless of how we may perceive it, this pressure is NOT the same thing as "weight." This movement does NOT cause us to gain weight!
|... you should also agree that you can turn a ski and not create a change of direction. With the shapes of skis these days, a self steering effect is built into the ski so it is very difficult, if not impossible, to put a ski on edge and not get a change of direction. Do you agree with that?
I agree with the first part--pivoting your skis without changing your direction of travel is the definition of a "hockey stop" or a "pivot slip." And I somewhat agree with the second part--doing it with NO change of direction is a tough test of skill, on ANY ski--as many instructor certification exam candidates have learned, to their chagrin. But it is certainly not impossible. Challenging, yes, impossible, no!
|If you do [agree with the previous statement] then you should agree with the statement that I made about forces that act upon your C.O.M. are felt through your B.O.S.
Not sure why this follows, but since I don't 100% agree with your previous statement anyway, it's a moot point. Yes, we feel many forces in skiing as pressure on the soles of our feet. No--as I've noted--not all those forces are "weight."
|To flatten your skis the B.O.S. needs to be under the C.O.M., as all of your photos and mine show so you must agree with that
No! Generally, flat skis coincide with the moment the CM (center of mass) crosses over the skis, and our photos do show this. But it is not necessary! Are you familiar with what Georges Joubert called "surf technique"? Just as angulation movements of the feet/ankles, knees, hips, and spine can increase edge angle at any point, so can they DECREASE edge angle. Whether I SHOULD or not may be debatable, but I CAN flatten my skis on the snow without my CM being over my feet.
|...a release of pressure or "unweighting".
As my reply to Learner above describes, while unweighting may involve a release of presure, not all releases of pressure constitute unweighting!
Anyway, I'm not sure any of these picky points is really critical to this discussion, beyond just defining terms, but since you've raised them, I've replied.
The real crux of our possible disagreement seems to be the following:
|What I was saying is flattening the skis on the snow raises the mass ... the C.O.M. does and HAS to rise to change edges.
As I've elaborated, the CM rising as the edges change is a common coincidence, but it is NOT a necessity. It often represents a common intermediate mistake! I can most certainly flatten my skis while LOWERING my CM, if I choose, by actively retracting (flexing, shortening) my legs beneath me as I tip my skis flat. I can flatten my skis while maintaining the heght of my CM too, and I can flatten my skis while raising my CM. As I noted in my reply to Learner, separating the movement pools of flexing/extending and tipping/flattening is a critical skill for expert skiing.
Your experiment of holding a ski pole at the bottom and tipping its grip left and right is flawed as an analogy of a skier--because that pole obviously CAN'T flex and extend as it tips, but a skier CAN. Unlike that pole, a skier's length need NOT "remain the same."
I contend that the sequence of Sarah Schleper shows the edge change without rising/unweighting as she demonstrates a "retraction turn." But if others don't see this, clearly all she would have to do is flex a little more deeply during the edge change in frame 5, and it would be obvious.
Accurate and independent movements of the CM relative to the feet, forward and back, left and right, and up and down, appropriate to the skier's desire and the task at hand, are a sign of expert skiing. Developing the skills to do this is hardly easy, and most skiers are not experts! But we must not confuse what most skiers DO with what MUST BE DONE. We must not confuse common practice with logical or physical necessity.
For sure, expert skiers routinely do things that most skiers cannot!
To summarize the relevance of these points to the original topic of this thread--unweighting IS often useful and often desirable, but it is NOT always necessary. Simply releasing the edges (flattening the skis) finishes a turn, releasing the pressure of centripetal force, but without a coinciding raising or lowering of the CM relative to the feet, it does NOT alter the pressure caused by the skier's body weight. Releasing the edges is not the same as unweighting!
PS--for those who REALLY want to get technical, consider that it actually makes no difference whether the CM is rising or dropping anyway, as far as "unweighting" goes. We can momentarily unweight--OR momentarily INCREASE the pressure on our skis--while rising OR sinking. What matters is not the direction of movement of the CM, but the direction of its ACCELERATION. Skis are unweighted when the CM accelerates downward--which means either slowing down an UP movement or speeding up a DOWN movement. And pressure increases when the CM accelerates upward. For what it's worth....