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28 off from a heli at 100ft


gator1
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We filmed one of team Spokane stroking 28 off. Tried pretty hard to keep the respective position of Heli to skier constant to allow the tech heads to get a good consistent look at the path. Anybody want to bet what differences there will be between this path and 38?

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Man you're doing some cool stuff right now @Gator1! Trying to figure out how you're making time!!

 

Anyhow my prediction: The -38 path will be slightly sharper out of the buoy, cross the centerline slightly earlier, and have a slightly straighter looking path into the ball. Another way of saying it is that will bend a hair closer to a square wave.

 

If that doesn't happen, that's when I'm going to get really interested. :)

 

Well actually, I won't have to change my thinking much if centerline-earlier thing is untrue, because of course a skier who is capable of running -38 is going to run -28 far ahead -- he/she is not taking the easiest path, but rather the lowest-risk path. These are potentially quite different. Indeed, most of us focus on skiing our early passes "just as hard" even though that is not necessary just to complete.

 

If you compare someone who can just barely run -28 with someone who can just barely run -38, THEN I'd really expect the earlier crossing for the -38 guy.

 

My theories are moderately well-informed, but that doesn't mean they're right! Some data is always interesting.

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@Than_Bogan I'm in a travel heavy mode right now, following 6 weeks down time waiting for scoped knee to heal, got a grand total of 11 passes so far this spring. Going nuts and killing time while sitting in airports. Fly two days for a 1 hour meeting. Travel is so glamorous.
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That is really cool! Seems like he's pretty far past the ball when he hooks up and heads across course, but is still there in plenty of time at the next ball.

 

It's official, I need to come to Washington if for nothing else to get a ride in your helicopter. Maybe you could fly down to CA and pick me up when I'm there in two weeks.

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@Dusty‌ , two sets in a row, we're only two balls away from the stateline. Damn, I can almost smell the desperation and stale beer in the carpet. Can't wait for our boy to get it done and get me back to the home of my youth.
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@Colebrah‌ Thanks! very cool. My Heli got conscripted to go dry cherries so we were in a hold mode.

 

Shape is basically the same between 28 and 38: Two straight lines with a hook at the end. Easier to see at 38.

 

From hookup to a little past the whitewater at about 40 degrees (acceleration), then sharp 15 degree turn, then straight line while coasting to turn. Basic difference between 28 and 38 is angle of the acceleration line and angle of the coasting line, and sharpness of turn at ball.

 

amazing what perspectives show. Bears no resemblance to any pendulum. And, therefore, I submit to the BOS intelligensia, pendulum physics as a mathematical model to discuss speeds and accel is a waste of time.

 

I'm mesmerized by straightness of those two lines. Just like that recent video of AM gates from the side perspective. Makes it very easy to model speeds from a visual.

 

BT dubs, nice pass brah.

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This is awesome visual data! SO COOL!

 

The statement "bears no resemblance to any pendulum" goes a little too far. At some point I hope to have time to explain why (with charts even!). I may start by emailing you directly. Possibly not today; we'll see.

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@gator1 I've noticed the same thing from forward mounted GoPro footage. The ski goes straight, edge change, straight, turn, repeat . . . . But the handle scribes a perfect arc around the pylon like a pendulum. Herein lies a lot of the complication in skiing. The energy source is scribing arcs while the ski is tracking relatively straight lines, and the skier in between these two things has to keep them efficiently connected. It has to be one of the most complex moves in sports.
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@SkiJay‌ There are a couple of things I've learned from these videos: When people speak of the ski "grabbing angle" and "building angle to the wakes" I'm convinced they are just kidding themselves. It may be the ski holds the straight line we see with less effort. And, in the category of "something actually useful to getting better" I'm convinced these vids prove you will add no angle once the arms go straight and the load comes on. Which kind of validates the whole patience, finish the turn, complete your rotation, blah blah.

 

Maybe the razor 14 or whatever Horton tested awhile back is different, but I think all other skis with the three part rocker prescribe this straight line due to their shape.

 

And, at the risk of getting overly into the realm of rocket science, the handle swings an arc back and forth at the end of a string of constant length. However, that arc is not an natural, pendulum profile of gravity accelerating a mass. Instead, the skier is providing acceleration to the mass.

 

The rocket science boys would say draw a control volume "an imaginary movng box" around the boat, skier and rope. If we assumed drag from the water was the stand in for gravity, put a guy on a saucer out at buoy width, and let him swing back and forth he'd present a decaying pendular motion in the control volume. Lots of drag, so the motion would decay pretty quickly. But, within the control volume pendulum physics would describe the motion exactly.

 

But, the box is moving. So the decaying pendulum would have to be mapped onto the bottom of the lake as the control volume passes over the lake at 34 mph. It would not look like a pendulum, neither handle or saucer rider.

 

But, there is more. The skier adds acceleration on the "downswing", and deceleration on the "upswing". So, within the control volume, we would not see a pendular path. It would pick up speed too quickly on the "downswing", and lose speed too quickly on the upswing.

 

Then, we have to map the artificially boosted and retarded pendular motion onto the bottom of the lake. A real mess that requires a computer and sophisticated measuring devices to analyze.

 

And so, after all this physics and human force adding and subracting, which really IS rocket science, the skier path mapped onto the lake bottom is TWO STRAIGHT F--CKING LINES. HOW CAN THIS BE?

 

My mind is blown by this. Why does A=pi r squared? Why is the derivative of speed acceleration? Why does f=ma? Not kindof. exactly. WTF! We ski two straight lines. There must be a god.

 

Or maybe the fact that I just finished four beer lawnmowing is having an affect.

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a guy whose knowledge on the subject i trust has told me ' no matter whether someone thinks slalom is related to pendulum physics or not they never seem to consider that slalom is actually a closer model to a * double pendulum *. the rope to the handle being the first pendulum with the pylon as the pivot point and the handle to the skier being the second pendulum with the handle being the pivot point. the action of the secondary pendulum is generally under the direct influence of skiers input '. i copied that out of his email and dont pretend to understand completely what it means.

 

but there is one thing he said that makes pretty good sense -apparently those who study pendulum physics agree that a double pendulums behavior is very much tied to its initial conditions. which sounds a lot like the advice of top coaches who say getting wide and far up on the boat for your gate shot sets up your run for the rest of the pass. its seems those coaches often make reference to similarity with a pendulum when their dispensing that advice.

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Nope. As I have written before, the skier's reach can serve to lengthen or shorten the effective length of the rope.

 

But, the handle and rope have comparatively no mass. And, they can only impart a resultant force vector pointed at the boat, with no moment. Consequently, the handle does not serve as a pivot point.

 

A golf swing has two main pivot points: Shoulder and wrist. The arm can provide a force vector other than one pointed at the shoulder. And the wrist muscles can provide a moment. Thus, the club does pivot around the wrist.

 

Entirely different. The rope has no muscles, and the handle can't provide a moment.

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@gator1 -must be wonderful to know everything but some very highly regarded skiers and coaches have been known to use the phrase ' pivot around the handle ' when describing technique and even a dummy like me who doesn't know everything knows that if you pivot around a point then that point can probably be described as a ' pivot point '. of course if you are a better skier and know more about skiing then chris rossi you might be right.....

 

a friend of mine who is an avid golfer has a training club with a hinge part way up the shaft and that hinge point has no effective mass either but somehow the club behaves exactly like a double pendulum if you swing it wrong. i'm just glad he's not a physics genius either or that club would have been a big waste of money.

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@mwetskier‌ Its like the two guys getting chased by the bear. The guy who is gonna live doesn't have to be the fastest human on the planet. Same with you and me and physics.

 

VERY clear I'm not a better skier than rossi, or many other people, maybe including you. However, I may be a better engineer.

 

You've missed the point. Your club example illustrates this perfectly. Take your buddy's club. Tie a rope to the handle. Swing the club with the rope.

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Ok, time to get esoteric here. This should be fun. For me. :)

 

I contend that the straight line phenomenom is about 50% due to human interpretation. The human visual system is loaded with what are called "oriented filters." Their entire purpose (and indeed the only thing they can do) is to detect straight lines. However, such a filter also generates some response when what it sees is pretty close to a straight line. This actually has to be true, since an oriented filter is -- as it's name suggests -- oriented. We have lots of them at various angles, but each one is oriented at a specific angle. If it didn't detect anything but perfect matches, then we could only see edges that just happened to be at exactly the same angle as our oriented filters!

 

A consequence of this "design" (for lack of a better term) is that we tend to see straight lines. Furthermore, our brains organize things into smaller components (although the details of this are not well understood, and especially not by me!) So a section of low curvature (i.e. fairly straight) followed by a section of high curvature (i.e. bent) followed by a section of low curvature, is going to be mapped by our brains to two line segments. It's not because it's literally there, it's because that's how our brain can associate it with other similar things it has seen, instead of only being able to perform exact matches (which is totally effin' useless since we almost never experience exact matches).

 

That's not the entire explanation, of course. It's still interesting why there are two sections of such low curvature. But I don't think this phenomenon quite falls into the shocking elegance of e^(i*pi) = -1.

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@gator1 -yeah i get your bear analogy. you're saying you're not necessarily smart just smarter than me. the thing about guys with an ego as big as yours is that sometimes the bear surprises them when they least expect it. but until i see you skiing at the level of rossi, winter, raley, stisher and other ski coaches who regularly draw analogies to a pendulum i feel pretty confident its better to listen to them.
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@mwetskier‌ Hang on, let me check...............Yep, hasn't changed. In regards to my thoughts on ski technique, I don't care who you listen to. In fact, please do the opposite of whatever you may perceive I have suggested.

 

To be clear, my reason for spending time on this site is to advance my knowledge of how to get better.

 

Towards that end, from here on out you're on "ignore" to me. I put you there after the smear string, as you seem incapable of discussing technique without getting personal. My mistake for taking you off that status. It won't happen again.

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