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Zippy8
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Centre of gravity

Sun Dec 05, 2010 1:00 am

Not being one to easily let something go :roll: 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.

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

Image Image

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

Image Image

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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misterbleepy
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Re: Centre of gravity

Wed Dec 08, 2010 9:57 am

OK - shamed into replying after seeing your GWTW post

My feeling is in agreement with yours - it's all about inertia & momentum.
For a given yoyo input, without weights your Fury will be slowed down by air resistance, whereas with weights it will have enough momentum to overcome the air resistance and complete the yoyo.

Disclaimer - O-level physics is my highest qualification related to this, and it is a qualification with over 30 years of rust surrounding it.
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mobius
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Re: Centre of gravity

Wed Dec 08, 2010 3:55 pm

Someone once said that the CoG is where the bridle collection point should be directly above. Kind of makes sense I think?

I would have thought you should add weight such that the CoG remains the same, however even a small amount of weight at both poles will create a fly-wheel effect.

Am I the only person to add weight at the end of the bottom spreaders to assist in yaw rotations like backspins? Makes a big difference if you're in to such things.
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misterbleepy
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Re: Centre of gravity

Wed Dec 08, 2010 5:31 pm

mobius wrote:
Am I the only person to add weight at the end of the bottom spreaders to assist in yaw rotations like backspins? Makes a big difference if you're in to such things.


I tried that once with my Outer Space UL, to try and get it to axel better (using a Blue Moon weight kit - great for experiments like this).
I did improve the axels, but took nearly all the "UL" out of it in the process - #-o
Keith B

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Zippy8
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Re: Centre of gravity

Wed Dec 08, 2010 6:25 pm

mobius wrote:
Someone once said that the CoG is where the bridle collection point should be directly above. Kind of makes sense I think?

Possibly... but you'd have to define exactly where the kite at this time. At the top of the window the CoG would be above the entire bridle.

Am I the only person to add weight at the end of the bottom spreaders to assist in yaw rotations like backspins?

Not remotely and this again reinforces the notion that CoG shift isn't what's important (mass distribution perhaps ?). You should also only do things in moderation - I saw your teammate Sasha stick Fury weights on the LSs of a Dot Matrix once :shock:

Mike.
 
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Re: Centre of gravity

Wed Dec 08, 2010 11:09 pm

It's not the CoG that's directly in line with the strings, but the centre of lift. The CoG IMHO, only has an effect when the kite is in a stall, or in other words, when the effect of the wind can be considered irrelevant. The relation between the CoG and CoL has an effect when you start changing the relation between lift/wind pressure and gravity.
CoG above CoL and a bit of slack will cause the kite to fall forward and gently glide towards you. While this can be fun with a no-wind SLK, it's certainly not what we are looking for here.
CoG below CoL and a bit of slack will cause the kite to tip backwards = here we go!

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Positivo
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Re: Centre of gravity

Fri Dec 17, 2010 1:37 pm

So the centre of lift (COL) is the point where you pull the kite, the point of pull (PUP), right? Bridle Connection Point, maybe that's a better way of saying it (BCP).
Recap: if the COG is below the BCP then yoyo's would be easier?
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seales
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Re: Centre of gravity

Mon Dec 20, 2010 4:56 pm

Unless it's been used elsewhere, 'bridle connection point' seems a source of confusion. I assume you mean where the bridle legs join the flying line, at the 'tow point'?
Centre of lift is the aerodynamic centre of lift, the centre of pressure for the sail. I'm assuming it's the point about which no pitching moment is generated by the lift.
It is the separation between these two points (mostly by the bridle) that gives the kite stability in the pitch axis during normal flight as, when it is rotated from this position, the resulting force couple returns the kite to the balanced position.

The recap could then be: yo-yo's are only possible if the CoL is above (nosewards) of the CoG.

I can't get my head around what the distance between the two points would change though. The stable yo-yo rotation rate for a given airspeed? (assuming the kite 'flies' round the rotation, rather than just batters the air out the way as it rotates. would do a bit of both.)

The nose+tail weighting without much change to the CofG could be for both rotational momentum and inertia:
Rotational Momentum-minimise the loss in rotation rate due to air resisting the rotation
Rotational Inertia-allow more forward momentum to be generated (which will ensure the lift vector remains positive and at the front of the kite during a yo-yo) instead of being transferred into rotational motion (which will be damped more effectively by the air during the rotation) before the upper bridle legs become taut again.

The second is probably more of a side effect rather than a design purpose, pushing the centre of rotation further from the back of the kite if the rotational losses are the same.

It should/could be noted that the CoL switches position with reversal of the airflow over the kite and that the lift vector will change direction at negative incidences. Changing of position enables 2 pop roll-ups and, IMO in addition to the firm input, the change of direction initiates yo-fades.
 
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Lex B
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Re: Centre of gravity

Tue Dec 21, 2010 9:27 pm

Could anyone make me a little drawing, to give me a visual clue of what we are talking about here.
Somehow can't picture it in my mind, guess there's more of us, who can not.
remember: amateurs built the ark ..
professionals built the Titanic.

PLEASE......NO TAILS ....
 
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jr
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Re: Centre of gravity

Tue Dec 21, 2010 11:28 pm

 
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Positivo
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Re: Centre of gravity

Wed Dec 22, 2010 8:51 am

Thank you for clearing that out :-)

Now explain how one can calculate the Center Of Pressure of a kite?
Flying now: B'zar 2010 STD and UL, B'zar 2011, BatQuartz, E2, Elixir, Modded Gemini, Flashlight, Sweety, Deepspace, Superfly.
 
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seales
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Re: Centre of gravity

Wed Dec 22, 2010 11:43 am

Drawing attempt, mainly for bridle effect in normal flight:
Image
At all points (forward flight), removal of the green bridle forces would pitch the kite back. It does throw up the question of wrapped flight stability - separation of the mean sail depth from the yo-yo stops?
Personal interpretation of course, may well be incorrect. Ah well, even NASA didn't seem to state their uniform gravitational field assumption ;).

And from the second NASA source: "When computing the trim of an aircraft, model rocket, or kite, we usually apply the aerodynamic forces at the aerodynamic center of airfoils and compute the center of pressure of the vehicle as an area-weighted average of the centers of the components." Maybe not today, but sometime.
 
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Re: Centre of gravity

Thu Dec 23, 2010 9:47 pm

seales wrote:
Drawing attempt, mainly for bridle effect in normal flight:
Image

I Thank you veeeeeeeeeeery much!
remember: amateurs built the ark ..
professionals built the Titanic.

PLEASE......NO TAILS ....
 
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Positivo
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Re: Centre of gravity

Fri Dec 24, 2010 6:20 pm

Thank you for your time and effort with the drawings!
Flying now: B'zar 2010 STD and UL, B'zar 2011, BatQuartz, E2, Elixir, Modded Gemini, Flashlight, Sweety, Deepspace, Superfly.