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Author Topic: Centre of gravity  (Read 2542 times)
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tpatter
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« Reply #15 on: December 11, 2010, 11:35 PM »

Perhaps the title of this thread is simply mis-placing our search for an explanation!  Smiley

I've enjoyed watching many skaters, dancers, swimmers, and gymnasts over the years during the Olympics.  For sure they all likely have varying CoGs, but it didn't seem to adversely affect their performance.  This is one variable among hundreds, maybe more.  These individuals learn to work the system they have been given (their bodies) in order to get where they want to go.

I've been astounded by how much the addition of 5g in the right place can make on one kite, but have added far more to other designs with little to no detectable difference and I sure didn't like them with the extra weight.  I've also seen flyers do incredible things with a kite that I dismissed as "too odd" to spend any time trying to fly.   Some kites are a dream in little to no wind, but lose all their charm in anything over 8 or so.  You can go on and on......

"Junk in the trunk" is just ONE variable.


« Last Edit: December 12, 2010, 12:43 AM by tpatter » Logged

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JimB
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« Reply #16 on: December 12, 2010, 12:56 AM »

Well that is a point.

There are other variables including CoG at work. It may not be the key variable but it must be having some effect.

AoA for one. Drop the nose back and it gets easier to roll up. Bring it forward and some tricks are easier.

Wind pressure for another. This may be the primary variable. How many tricks are wind dependent? One could argue all of them, but I wouldn't go that far. It'd be close though.

The pilot is another candidate for primary variable and a very good one.

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werner
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« Reply #17 on: December 12, 2010, 08:54 AM »

What to think about this? A kite doing a yoyo has to move at first nose away and down, tail going up. Depending on the speed of this movement you have some possibilities. If "nose away" goes really fast you risk to create an invertion in the billow of the nose and thus a pressure that will stop the rotation or redirect it in a horizontal way and let it kind of glide away from you. These are the kites wich are doing the first 180 degrees of a yoyo rather well and than stop in a turtle. If "nose away" goes to slowly there is not enough momentum to push the kite through the trick. The speed of this "nose away" movement depends on the difference between the centre of lift and the centre of gravity. To make this difference bigger you can lower the center of gravity by adding weight.
So you have at least two things to consider, that is: the static behaviour of a kite and the dynamic one.
I think the most important reason why a kite does a yoyo well is the fact that it keeps performing aerodynamically while flying a yoyo. I try to explain: when a kite flies it has air moving along the sail that induces it into a specific aerodynamic form. It creates lift in the nose section and along the leading edges. It also creates drag in the tail section and at the wingtips. When giving slack these forces automatically result in a turtle position but when this movement happens in the natural radius of the kite ( can be influenced by adding weight ) it means that the kite will fly through and will hold its nose billow to pull it from horizontal position to vertical nose down and finish the rotation in a rather small radius.
I don't know if I have a point here but I think this vision can explain a lot of the different yoyo-behaviour and sometimes unexpected results when adding weight or changing sailsurface etc..
Another thought that came up is that having a spine that bends as an arrow and lets the kite rock while on its belly should give easier yoyo's too. Aerodynamically this should create some lift in the tailarea going from normal flight into turtle position and thus work together with the forces at the nose of the kite forming an even bigger rotational momentum.     
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zippy8
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« Reply #18 on: December 12, 2010, 11:31 AM »

Perhaps the title of this thread is simply mis-placing our search for an explanation!  Smiley
But it comes from the original KiteLife article that kicked off this kerfuffle (at least in my mind).

Quote from: JimB
There are other variables including CoG at work. It may not be the key variable but it must be having some effect.
Why must ? In the case of a purely tail weighted kite I could replicate the same CoG by using a heavier (possibly far heavier) weight away from the base of the spine. For instance, rather than 14g. at the tail of that CTC XS would it take 40g. just below the centre T ? CoG would be the same but, extrapolating from empirical evidence (or guessing as it's sometimes called) I don't expect the Yoyo performance or style to be the same.

Quote from: werner
The speed of this "nose away" movement depends on the difference between the centre of lift and the centre of gravity. To make this difference bigger you can lower the center of gravity by adding weight.

I think that I've made my position clear on this - CoG is a red herring. I refer you to the Fury; weighted and unweighted. Same CoG, different Yoyo performance.

Quote
I think the most important reason why a kite does a yoyo well is the fact that it keeps performing aerodynamically while flying a yoyo.
Mostly true. I will repost the video I made of the Next and Next Team UL to show what I mean. It's a sort of glide 'n' stall movement. Bendy spines, I would suggest, aren't really a factor.

Mike.
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JimB
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« Reply #19 on: December 13, 2010, 12:05 AM »

zippy yadayada:


"Quote from: JimB
There are other variables including CoG at work. It may not be the key variable but it must be having some effect.
Why must ? In the case of a purely tail weighted kite I could replicate the same CoG by using a heavier (possibly far heavier) weight away from the base of the spine. For instance, rather than 14g. at the tail of that CTC XS would it take 40g. just below the centre T ? CoG would be the same but, extrapolating from empirical evidence (or guessing as it's sometimes called) I don't expect the Yoyo performance or style to be the same."

Why must? No reason other than the sneaking suspicion that it is all important to a greater or lesser degree.

How about spreader placement, weight, stiffness, etc.? There are a lot of little things that go into setting up a kite. Varying the placement of spreaders can have a dramatic effect on kite performance just as a for instance.


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stapp59
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« Reply #20 on: December 13, 2010, 04:01 AM »

/ramble on

One thing I really like about this hobby is while the principle idea of kites is simple; a piece of fabric and sticks tethered and controlled by a length(s) of string; the details of flight are quite complex.

As JimB points out, there are lots of static factors involved: center of gravity, aspect ratio, wing span, height, sail shape, sail loading, tunnel depth, standoff number and placement, trailing edge profile, leading edge shape, frame weight and stiffness, frame geometry, weight distribution, bridling and all its nuances. Don't forget broad seaming, venting, etc, etc....

Then there are all the dynamic factors: sail pressure, center of pressure, lift, drag, boundary layer, Reynolds numbers, angle of attack, turning radius, inertia, fabric vibration, frame flex, wind speed and smoothness, 2 dimensional flight (carving turns, corners, figures), 3 dimensional flight (tricks, flailing, falling with style), etc, etc.

What about pilot skill all thing being equal?

The craft and art of building kites is a world unto itself.

After applying the best aerodynamic engineering principles towards a particular design there is still the need for prototyping, test flying, tweaking, lather, rinse, repeat...

I do not understand all the factors involved in flight but the observable simplicity of flying a kite is a thing of beauty.

/ramble off  Huh Roll Eyes Smiley
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zippy8
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« Reply #21 on: December 13, 2010, 05:24 AM »

Why must? No reason other than the sneaking suspicion that it is all important to a greater or lesser degree.
Fair enough.

Quote
How about spreader placement, weight, stiffness, etc.? There are a lot of little things that go into setting up a kite.
Ah right. I think we all need to go back to the KiteLife article once again. The suggestion therein was that simply slipping in some weight to move the CoG of your pre-existing kite would have you "flipping your kite all over the place in no time".

I agree with the method but not the reasoning. That is all.

Mike.
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JimB
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« Reply #22 on: December 13, 2010, 09:46 AM »

Okay.

That was more about every little bit having some effect. Cue Kumbaya

But, yeah, not CoG.

We can all check that of our bucket lists.

What a relief.



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