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Author Topic: Centre of gravity  (Read 2712 times)
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zippy8
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« on: December 07, 2010, 05:08 PM »

Or if you prefer "Center of Gravity".
Nigh on a hundred views at The Other Place™ but no replies so I'll try you lot on for size. Of course it's not going to make a lot of sense without reading the first thread:-

Not being one to easily let something go  Roll Eyes I thought that I might finally follow through on something I raised here, a mere 5 years ago.

I set up an improvised rig to see where the CoG moved to with and without weights.


First up, the tail weighted Cosmic TC XS; on the left weighted and on the right without:-



Secondly, a full sized Fury that uses both nose and tail weights; same deal:-



As I hope this demonstrates 14g. of tail weighting moves the static CoG a little bit (it's about 2cm along the spine on the CTC XS). The two 15g. weights make somewhere in the region of bugger all difference to this point on the Fury.

However, the difference in how these kites fly and especially how they Yoyo is enormous. I'm not sure I'd even bother trying to Yoyo an unweighted Fury other than to try to prove whether or not it was possible.

So... even a very minor change to static CoG makes a huge difference to this one aspect of a kite's performance but those weights really are important. Thus their effect isn't due to CoG changes - right ?

Mike,
original article now here. It's a good article in may respects and worth a read.
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Texanpilot
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« Reply #1 on: December 07, 2010, 06:14 PM »

I'm not a physicist or dynamic mechanic, but it would seem that the only static CG effect would be seen when the kite is fading.

I would think the weight effect on yo-yo's is more about inertia/momentum.  With weights at the end of the spine (nose or tail) the rotational kinetic energy is higher than for an unweighted kite.  That extra kinetic energy carries the kite around the yo-yo when the kite otherwise wouldn't go. 

I've noticed on high-end road bikes, the spoke nipples are at the hub instead of at the rim.  I think the same principle is working in reverse.  On the bike, you want less inertia on the rim to get less resistance in adding or subtracting wheel rotational speed (i.e., quicker acceleration and braking). 

To quote from the Wikipedia "moment of inertia" article:
Quote
The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. Therefore, it encompasses not just how much mass the object has overall, but how far each bit of mass is from the axis. The farther out the object's mass is, the more rotational inertia the object has, and the more force is required to change its rotation rate. For example, consider two hoops, A and B, made of the same material and of equal mass. Hoop A is larger in diameter but thinner than B. It requires more effort to accelerate hoop A (change its angular velocity) because its mass is distributed farther from its axis of rotation: mass that is farther out from that axis must, for a given angular velocity, move more quickly than mass closer in. So in this case, hoop A has a larger moment of inertia than hoop B.


I don't think a lower CG caused by tail weights will actually help the kite rotate (assuming that the moment of inertia doesn't change).  The essence of a yo-yo is about getting the tail up and over the nose.  It seems as if a lower CG (in and of itself) would actually serve to hinder the kite going inverted.   I don't think a kite with 8 oz of lead on the tail (assuming it would fly at all) would even backflip, much less yo-yo (unless started in a dive).  The weight would keep the tail down, wouldn't it?  Who wants to try in the next hurricane?  Huh

This seems to sum up the yo-yo aspect of kite weights for me. 
Of course, I'm open to correction on this, since I have no official training on physics or mechanics.  I just try to pay attention to what's going on around me.   Undecided

« Last Edit: December 07, 2010, 06:27 PM by Texanpilot » Logged

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Stuart99
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« Reply #2 on: December 07, 2010, 06:27 PM »

But maybe you should think about incorporating the other axis' center of gravity in there and see what results you get. Static center of gravity is a 3D concept, not just a 2D one Wink
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zippy8
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« Reply #3 on: December 07, 2010, 07:13 PM »

But maybe you should think about incorporating the other axis' center of gravity in there and see what results you get.
Considering that it's effect on axis is minimal I would reasonably estimate that any off axis change will be less than bugger all. Also it'd be a swine to dream up a rig to demonstrate this  Wink

And, as I hope the example of the Fury shows, CoG simply isn't relevant to Yoyo performance. You don't improve performance in that respect because you've moved the CoG (be it in 2D or 3D) but because of increased inertia. The minor change in CoG is incidental.

Mike.
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mikenchico
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« Reply #4 on: December 07, 2010, 07:47 PM »

Surprised to see such a small change actually. Texan pretty much covered my first thoughts and as I was reading his post the same thought intimig8r99 brought up was churning through my mind.

The change in CoG in relation to the center of lift is possibly your answer, I'm sure the added inertia comes into play also. How would you figure all that out? A quick picture, imagine the kite laying on it's belly flat on the ground, the center of lift may be say 6" off the ground and on a kite usually just in front of your static CoG. By weighting the spine which is on the ground you have added inertia offset from the center of lift. Weight displacement can dramatically change the center of lift on an airplane I think, I'm not a pilot but I work with them and keeping a proper balance in an airplane seems to be a concern. So even though your static measurement doesn't show a lot of difference it may change the center of lift far more.

So you going to be snowed in long enough to do an Orville & Wilbur Wright and construct a wind tunnel?

« Last Edit: December 07, 2010, 07:49 PM by mikenchico » Logged

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zippy8
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« Reply #5 on: December 07, 2010, 08:10 PM »

The change in CoG in relation to the center of lift is possibly your answer

The nose-and-tail weighted Fury shows almost no change in CoG when they are removed, yet it is straightforward to Yoyo with them but without them it is, for me, out of reach. I continue to contend that CoG is a red herring.

We also discussed how kites move in a Yoyo here. I may resurrect the video. I may not. I may also try to get footage of the Element which moves more strangely.  Huh

Quote
So you going to be snowed in long enough to do an Orville & Wilbur Wright and construct a wind tunnel?

I know a local university that has one.  Wink Not sure what I'd do with it though.

Mike.
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Allen Carter
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« Reply #6 on: December 08, 2010, 12:55 AM »

It's a pendulum sort of thing. Think about spinning a lenth of string with a weight on the end. The less weight the more difficult it is keep spinning.
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« Reply #7 on: December 08, 2010, 01:26 AM »

I always sucked at Dynamics compared to Statics.

1 Assuming a symmetric kite, you should be able to construct a simple rubber tipped fulcrum which slightly cradles the exposed spine spar to create a single point static CG locator. Although you will probably end up on your back unless you locate the fulcrum about 7 feet off the floor. Your setup looks fine to me.

2 Inertia is inherent in any body having mass. Momentum is basically inertia in motion. A body at rest has inertia but no momentum.

3 Just as tires are equalized for both static (bubble balance in the old days) and dynamic (spin), looking at the static balance alone, while easier, is not a total picture.

4 I do know that adding weights to the keel speeds up the pitch rotation and adding weight to the nose slows the pitch rotation down. Adding both keel and nose weight increases the inertia of the kite, but in pitch, the 2 weights fight each other.

5 There is a term called "radius of gyration": http://en.wikipedia.org/wiki/Radius_of_gyration
The rails on billiard/snooker/pool tables are not located half way up the height of the balls where one would normally think they should be looking at it statically. The rails are 40% higher than the center of the ball. It's a radius of gyration thing and the balls would go off the tables if the rails were down at the ball radius height.

6 An ice skater's CG nor inertia does not change when they move their arms from a capital T position into one where their arms hug their chest, but their spin speed increases dramatically.

7 I have long thought that adding weight to the keel allows the kite to pitch while minimizing the amount of sail exposed to air resistance. When properly weighted or balanced, the lower part of the kite is following in the path of the nose as best it can. If my theory is correct, then there should be a point that if you kept adding weight or moving the weight further away from the dynamic center, the pitch speed should get worse.

8 I've stopped caring why kites do what they do. They are pretty and they make me smile. Satellites, in many ways were much easier to understand than a sport kite.

9 Mike, that is a beautiful kite.  Huh
« Last Edit: December 08, 2010, 01:30 AM by chilese » Logged

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« Reply #8 on: December 08, 2010, 03:24 PM »

It's a pendulum sort of thing. Think about spinning a lenth of string with a weight on the end. The less weight the more difficult it is keep spinning.

Also think about holding a length of wood in your hand.  Hold it at one end then toss it so it flips around and you catch the other end - note the speed of rotation and the distance it travels away from your hand.

Now take a hammer and do the same thing.  For the same throwing effort, the hammer will spin much faster and will move away from your hand much less.

It isn't about centre of gravity, it's about the distribution of the mass
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Kevin Sanders

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« Reply #9 on: December 08, 2010, 05:32 PM »

My money is on momentum for the yoyos, but CoG seems important in order to get a good, even, fast, predictable rotation.  If its not naturally in a good spot, I don't think that the kite will yoyo well or easily.

It would be interesting to try your experiment on an older, higher aspect-ratio kite that is generally not weighted and does not pitch well to see where it lies.

One thing that I frequently notice on different kites is how they behave while doing a jacobs ladder and I think it's related to their CoG.  My favorites can just sit there and JL in-place with little to no movement upward or downward.  Some others JL well, but you can cleary see them rotating about their CoG in a much more awkward way - in the same way, they unroll at a non-constant speed since their CoG is not near the "middle" of the kite.

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zippy8
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« Reply #10 on: December 08, 2010, 05:50 PM »

CoG seems important in order to get a good, even, fast, predictable rotation.  If its not naturally in a good spot, I don't think that the kite will yoyo well or easily.

Define "a good spot" ? The few kites I looked at had them a bit above the centre T but that was about all I could generalise about them.

Going back to the original article, consider this:-
your kite’s Center of Gravity. This is the spot that marks the axis around which the kite will pitch when doing all those flippy-floppy moves.

Firstly, a point in space such as the CoG doesn't define an axis but it also certainly doesn't the point about which a kite travels in a Yoyo. The video that I removed from the other thread showed this quite well but so do dozens of other videos. Kites rarely rotate about a point in space, but execute a fairly complex glide 'n' rotate combo. Only the more "out there" kites (step forward the Element) come close to rotating about a point and I'm not sure if it's really the CoG.

Quote
It would be interesting to try your experiment on an older, higher aspect-ratio kite that is generally not weighted and does not pitch well to see where it lies.

I'll see what I can rustle up but my stock of older kites have been thinned over the years.

Mike.
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« Reply #11 on: December 08, 2010, 07:32 PM »

Define "a good spot" ? The few kites I looked at had them a bit above the centre T but that was about all I could generalise about them.

Wherever Lam puts it!  Smiley

Like John, I don't care as much exactly how they work, just that they do.

-Tom
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JimB
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« Reply #12 on: December 08, 2010, 08:43 PM »

bingo.

It's a pendulum sort of thing. Think about spinning a lenth of string with a weight on the end. The less weight the more difficult it is keep spinning.

Also think about holding a length of wood in your hand.  Hold it at one end then toss it so it flips around and you catch the other end - note the speed of rotation and the distance it travels away from your hand.

Now take a hammer and do the same thing.  For the same throwing effort, the hammer will spin much faster and will move away from your hand much less.

It isn't about centre of gravity, it's about the distribution of the mass
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zippy8
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« Reply #13 on: December 11, 2010, 08:46 PM »

I don't care as much exactly how they work, just that they do.

A slightly Juggalo position but moving on....  Wink

Quote from: KaoS
It isn't about centre of gravity, it's about the distribution of the mass

Verily.  Undecided

However.... what about those amongst us who employ a tail weight but not at the tail ? I've had good results with weight placed at the centre T and midway twixt this and the tail itself. What about those souls with Prism's "Micro-adjustable machined counterweight system" on their QPros that I seem to recall doesn't even reach the base of the tail ? With these setups it definitely isn't about CoG 'cos the effect is even more diminished so what is it doing ?

Mike.
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« Reply #14 on: December 11, 2010, 08:58 PM »

The most efficient pitch driven circle PDC™, will be one in which the sail exhibits minimum air resistance as the kite rotates. In other words, the weight of the kite should be placed so that the major surface area of the kite follows the rotational vector of the nose.

This doesn't always mean that the weight will be placed at the tip of the keel. On some kites, the ideal spot may not even be on the spine, but below the keel tip and therefore, never be realized.

I have no proof of this other than some amounts of common sense, engineering mumbo-jumbo and eye-witness good old-fashioned watching a kite pitch backwards from the side.
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