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Can one be out of balance in the air? - Page 2

post #31 of 45
Quote:
4) Overhead branches ()

...wires, lift cables, ....

I was at the Denver Ski Expo last November, where they had some incredible freestylers and trampoline gymnasts giving a great show on the trampoline. When one of the stars repeatedly looked up at the (very high) ceiling of the Denver Convention Center before beginning his routine, I knew we were in for a great show. With the help of the others "pumping" the trampoline, he did, indeed, launch himself to the rafters!

Best regards,
Bob
post #32 of 45

I like where you guys have been going with this. Here's my 2 cents:

 

Quote:
Well, I guess this boils down to semantics, but I don't know about using the term balance to refer to the orientation of ones body during free fall. I would say it's more accurate to state that ones body is correctly positioned in the air rather than correctly balanced.

I agree that one typically thinks of balance as balance against something. Yet in the time when I've been in the air and in trouble, I've most certainly felt out of balance cues from my inner ear. If it's not really balance in the air, then what is it/should be called? You'd think one could come up with a better name than positioning.

 

At speeds where air friction is significant, I'd argue that it is balance in the air because the force is significant enough to allow body movements to significantly effect the flight path of the center of mass. Yes, this is beyond recreational speeds, but it could be as slow as 40mph for kids and 60mph for adults. Think of the ski jumpers.

 

With respect to positioning there are a number of movements that can be done to effect position rotationally either flipping or spinning and effecting either the body as a whole or via separation of the upper and lower body. For example, Albrecht Kitzbuhel shows an example of attempt to stop flipping rotation by bending forward at the waist and windmilling. It was not enough to stop the crash, but it looks like he had enough air to do a back flip if he had attempted to help the flip instead of fight it. Could he have saved himself by opening his ankles to drop his ski tips and use air drag on the ski tops to retard the flip? We can initiate and retard rotational movements by tensing and relaxing muscles or by moving parts closer or farther from the axis of rotation. I once saw a guy do a beautiful 360 with almost no spin on takeoff. After takeoff he dropped his ski tips so the skis were vertical to his body and brought his hands and poles in to hug his body. Like a skater spin, this caused his slight rotation to increase enough to complete the spin before landing. All he had to do was unscrunch to stop the spin and land.

 

What I'm interested in learning is whether windmilling causes a gyroscopic effect to resist movement or a paddling effect.

 

post #33 of 45
Quote:
What I'm interested in learning is whether windmilling causes a gyroscopic effect to resist movement or a paddling effect.

Not really either, Rusty--although with enough wind resistance, I suppose you could "paddle" a bit against the air.

But as you begin to "windmill" your arms, it involves angular acceleration in the direction of the windmill. That acceleration involves an equal and opposite acceleration of your body in the opposite direction, giving you at least some ability to control your rotation about that transverse axis (fore-aft tumbling, or "pitch"). Of course, to sustain the force, you must continue to accelerate (speed up) the rotation, so there is a brief limit to how long you can do it. And if it is successful, you must continue the windmilling at a constant speed until you land, because slowing or stopping it would introduce an angular (rotational) force in the opposite direction--bringing your original problem right back!
Quote:
I've most certainly felt out of balance cues from my inner ear. If it's not really balance in the air, then what is it/should be called?

Good question. I don't think anyone here would assert that you can't screw up, and know it, in the air--only that at least some common definitions of "balance" (particularly, relation between CM and base of support) do not apply. I suggest that for most of us, our inner ear mechanisms will go berserk with most aerial maneuvers other than straight, upright air. No matter how "balanced" and "perfect" or (more likely) completely out of whack I might end up if I tried to throw an off-axis, multi-rotation spin, I guarantee that my inner ear would freak out, because I am not accustomed to these things. I would feel very disoriented even if I could somehow throw it just right.

And that furthers the point, I think! I'm sure that the inner ear gives us all sorts of important information when we're airborne, but I don't think it tells us about our "balance."

Best regards,
Bob
post #34 of 45

post #35 of 45
Quote:
What I'm interested in learning is whether windmilling causes a gyroscopic effect to resist movement or a paddling effect.
I'll take bite at that apple - Yes, windmilling (done properly) does affect our forward/aft rotation! It's just that other factors affect us a lot more.

Angular momentum must be conserved so when a body-part is rotated, something else must give - but the result is not necessarily 'equal' and not necessarily 'opposite' in our typical intuitive way of seeing things. Rotational results are tricky to pin down because the reaction is not always what you'd expect. Just hold the axle of a bicycle wheel, spin the wheel and try to "tip it" to the side. Dang thing will try and rotate 90-degrees from the direction you tried to tip it!


With this in mind, consider that once in the air, a skier can still change their orientation (position) by implementing independent body-part rotations. If anyone needs proof, carefully examine this video from the days of Skylab.

Watching this video, note that a few of his gymnastics begin as directly-forward whole-body rolls then turn into whole-body spins and eventually end up as forward rolls again. He does several routines like this where new whole-body rotations in a different direction seem to come out of nowhere.

How? Look carefully at his overall orientation and especially at the axis of each rotation with respect to the orientation of that large 'wheel' he's performing in. Notice that when spinning his whole body has changed its axis from parallel to the wheel to being off by quite a few degrees? And when he comes back to a forward roll, his axis is back where it originally was?

We see the same idea when a cat falls from an upside-down state and it quickly rights itself before landing (given enough time). If we drop the cat from a North/South orientation we'll find it lands with something of an East/West orientation. This is a result of rotational transformations the cat makes to achieve flipping over - and staying flipped over until landing, even though it has stopped rotating anything.

Oh, and if you don't think this applies to skiing ...check out the first segment of this video. wink.gif

Once again this isn't a matter of 'balance' while in flight but rather a matter of orientation in relation to something else. For jumping skiers it's a matter of orientation to the surface at the moment of landing. Most of the input for a successful landing comes at the moment of lift-off but controlling one's orientation to airflow is essential for longer jumps. Implementing rotational contortions can help correct launch-errors but may cause more harm than good if airflow is mucked up by such arm and leg motions.

.ma
post #36 of 45

I was wondering if windmill paddling could be caused by off center rotation of the arms relative to the core (as the hands move around the circle, in one side of the circle they would paddle while they pumped on the other side - i.e. the hands not traveling in a perfect circle and thus providing no net change in momentum).  In theory, the muscles of the arms could impart force to the shoulders the same way that oars row a boat.  Even if the hands traveled in a perfect circle if the center was off axis relative to the core, that would impart gyroscopic force like the way a spinning top stands up.

 

I know that when I did my big crash in a super pipe, my inner ear told me I was upside down and moving down even though I had no idea where I was relative to the wall or the bottom of the pipe. I knew I was in trouble but I did not know where to move to get out of trouble. With experience, I would have known to bend at the waist to reorient myself perpendicular to the wall (and rotate my upper body to face "down"), regain snowboard to wall contact and balance against the wall instead of just dropping hip first into the transition from 12 feet up. D'oh! I know that when I was in a little slab slide, the little fog bank I was in made it impossible to see that I was moving. But my inner ear commanded my stomach to empty contents until I came out of the fog bank and my eyes regained agreement with my inner ear. However, when I've done flips and barrel rolls in the wind tunnel or under water I've had no stomach problems. People say the inner ear plays a key role in balance. Am I reading too much into this?

post #37 of 45
Oops! Good first post, James Elston. Welcome to EpicSki!

Best regards,
Bob Barnes
post #38 of 45
TheRusty,

Since the inner-ear elements cannot 'see' outside and have no contact with anything external, there's no way they can tell up from down during free flight. All that those components can sense is directional accelerations.

Gravity provides continual acceleration so we can sense a lack of free-fall when standing on the ground. When accelerating downhill on skis we sense the reduced effect of Gravity (because we're giving in to part of it) but we can still sense the remaining portion still pulling us downward. The overall friction of airflow past us when skydiving still leaves us with a 'down' direction because there's still a component of Gravity remaining unsatisfied by our incomplete degree of plummet (an old technical term I just invented). In a vacuum we'd sense no 'down' because we'd be entirely in free-fall ... that and because we'd probably be unconscious as well.

At the speeds we skiers typically experience flailing along a ballistic path I don't think air resistance is enough for our inner-ear to notice the remaining component of Gravity left unsatisfied, thus we lose any sense of 'down'. At that point I suspect a variety of proprioceptors are probably all screaming too loudly for us to hear those inner-ear thingies anyway.

.ma
post #39 of 45

Balance = a state of equilibrium

 

Equilibrium = a stable situation in which forces cancel one another

 

So, in my mind, yes, one can be out of balance while in the air.  Flailing arms trying to regain one's center of mass to be reasonably related to one's legs would IMHO imply an unstable situation.  In physics a stable situation is one in which when acted upon by an outside force, the situation returns to it's original state.  (Tip a glass a quarter of an inch and it returns to an upright position.  Tip it two inches and it topples over.  The first is stable, the second unstable)

 

As a (former) meteorologist, we are concerned with the state of the atmosphere and almost always refer to it as stable (think smoggy days) or unstable (think thunderstorms).

 

Bob

post #40 of 45
Interesting analogy, WVSkier (and good to see your name around here again!).

But consider that, except for a little bit of air resistance (which, of course, can actually be quite substantial in some cases), there ARE no external forces acting on us when we're airborne beyond the constant downward pull of gravity.

And we must be careful about adhering too rigidly or exclusively to any strict physics definition of things like "balance." Consider the consequences of your "equilibrium" definition: according to Newton's First Law of Motion, an object's motion will not change--an object in motion will remain in constant, straight-line motion--unless acted upon by an external net force. Since turning is a form of acceleration, and acceleration describes a change of motion, that means that a skier in balance (equilibrium--no net external force) cannot turn.

That's right--if balance = equilibrium, then a balanced skier cannot turn, and a turning skier cannot be in balance, according to the most basic laws of physics.

I suggest we keep digging for a more applicable, practical definition of balance! (Or at least a different frame of reference....)

wink.gif

Best regards,
Bob
post #41 of 45
Thread Starter 

I bet Newton never caught much air!  biggrin.gif

post #42 of 45
Fig Newton: the force required to accelerate a fig one meter per second per second.
post #43 of 45
While I generally talk about "being in balance" with regular students I frequently point out to instructors that in the real world we deliberately ski in a constant state of planned imbalance.

In each moment while making linked turns we're deliberately creating the exact degree of imbalance necessary to 'fall toward' our next desired position. By the time we get to that position, we're already deliberately creating the next necessary state of imbalance - to redirect our current 'fall' toward the next necessary position. In reality, we're never actually "in balance" at any given moment during continuously linked turns.

Of course, this isn't something I'd try to explain to regular students although the subject comes up with engineers and mathematically inclined people who quickly realize the actual situation.

In my own view, stability doesn't really have much to do with balance. We can say an object is 'in balance' while also being unstable (a tall lamp with a tiny base on a breezy patio). We can also say an object is 'out of balance' while also being stable (Leaning Tower of Pisa, a leaning telephone pole). A skier is considered 'out of balance' when leaning heavily on the tails - yet they can also be quite stable in that position. Oh, and when the formula is just right (perfectly balanced) Nitroglycerin can be (is) highly unstable too. biggrin.gif

.ma
post #44 of 45
Thread Starter 

As promised, here is the video conclusion to the montage that was the basis for this thread.

 

Kueng,SG,Web.jpg

 

 

 

 

 

 

Now, the question of how Patrick go so rotationally out of balance should be answerable.

post #45 of 45
Quote:
Originally Posted by michaelA View Post

While I generally talk about "being in balance" with regular students I frequently point out to instructors that in the real world we deliberately ski in a constant state of planned imbalance.

In each moment while making linked turns we're deliberately creating the exact degree of imbalance necessary to 'fall toward' our next desired position. By the time we get to that position, we're already deliberately creating the next necessary state of imbalance - to redirect our current 'fall' toward the next necessary position. In reality, we're never actually "in balance" at any given moment during continuously linked turns.

Of course, this isn't something I'd try to explain to regular students although the subject comes up with engineers and mathematically inclined people who quickly realize the actual situation.

In my own view, stability doesn't really have much to do with balance. We can say an object is 'in balance' while also being unstable (a tall lamp with a tiny base on a breezy patio). We can also say an object is 'out of balance' while also being stable (Leaning Tower of Pisa, a leaning telephone pole). A skier is considered 'out of balance' when leaning heavily on the tails - yet they can also be quite stable in that position. Oh, and when the formula is just right (perfectly balanced) Nitroglycerin can be (is) highly unstable too. biggrin.gif

.ma

Good post Michael,

 

Balance is a state where the force vectors (both static and dynamic) align. Funny enough leaning back in a snowplow is a state of balance whereas in a dynamic carved turn the only point where you are in balance is a brief moment close to the apex of the turn (this is also why I prefer to look at turns from apex to apex because this is where you can plan the next trajectory)

 

In mathematics, structural stability is a fundamental property of a dynamical system which means that the qualitative behavior of the trajectories is unaffected by small perturbations.

So, a beginner beginner hanging back in the boots has balance but a poor structural stability (a small bump or ice will throw him off)

An expert skier is seldomly in a state of balance but has a very good structural stability.
 

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