View unanswered posts | View active topics It is currently Mon Jul 28, 2014 10:18 am



Post new topic Reply to topic  [ 14 posts ] 
Centre of gravity 
Author Message
User avatar

Joined: Mon Sep 29, 2003 7:43 pm
Posts: 5953
Location: Vihtavuori, Finland
Post Centre of gravity
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.


Sun Dec 05, 2010 1:00 am
Profile WWW
FA Supporter
FA Supporter
User avatar

Joined: Fri Aug 20, 2004 9:29 pm
Posts: 2760
Location: near Newquay, Cornwall
Post Re: Centre of gravity
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.

_________________
Keith B
--
Kite Tricks mind map

bleep bleep bleep


Wed Dec 08, 2010 9:57 am
Profile YIM WWW
FA Supporter
FA Supporter
User avatar

Joined: Fri Sep 05, 2003 3:36 pm
Posts: 1729
Location: Romsey, Hampshire, UK
Post Re: Centre of gravity
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.

_________________
Dave Morley


Wed Dec 08, 2010 3:55 pm
Profile WWW
FA Supporter
FA Supporter
User avatar

Joined: Fri Aug 20, 2004 9:29 pm
Posts: 2760
Location: near Newquay, Cornwall
Post Re: Centre of gravity
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
--
Kite Tricks mind map

bleep bleep bleep


Wed Dec 08, 2010 5:31 pm
Profile YIM WWW
User avatar

Joined: Mon Sep 29, 2003 7:43 pm
Posts: 5953
Location: Vihtavuori, Finland
Post Re: Centre of gravity
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.

Quote:
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.


Wed Dec 08, 2010 6:25 pm
Profile WWW
User avatar

Joined: Tue Mar 11, 2008 8:44 pm
Posts: 276
Location: Munich
Post Re: Centre of gravity
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!

Stuart

_________________
the lunatic is on the grass


Wed Dec 08, 2010 11:09 pm
Profile WWW
User avatar

Joined: Fri Apr 22, 2005 11:58 am
Posts: 102
Location: Heule - Belgium
Post Re: Centre of gravity
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?

_________________
Flying now: B'zar 2009, 2010 and 2011, BatQuartz, E2, Elixir, Modded Gemini, Flashlight, Sweety, Nirvana UL, Deepspace.


Fri Dec 17, 2010 1:37 pm
Profile WWW
User avatar

Joined: Tue Sep 09, 2008 9:52 pm
Posts: 67
Location: Bristol / Fleet, Hants
Post Re: Centre of gravity
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.


Mon Dec 20, 2010 4:56 pm
Profile
User avatar

Joined: Sat Aug 09, 2008 10:22 am
Posts: 151
Location: Baarn, The Netherlands
Post Re: Centre of gravity
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 ....


Tue Dec 21, 2010 9:27 pm
Profile
User avatar

Joined: Wed Feb 25, 2004 5:47 pm
Posts: 674
Location: Berkshire
Post Re: Centre of gravity
http://www.grc.nasa.gov/WWW/K-12/airplane/cp.html

http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html

Enjoy!


Tue Dec 21, 2010 11:28 pm
Profile
User avatar

Joined: Fri Apr 22, 2005 11:58 am
Posts: 102
Location: Heule - Belgium
Post Re: Centre of gravity
Thank you for clearing that out :-)

Now explain how one can calculate the Center Of Pressure of a kite?

_________________
Flying now: B'zar 2009, 2010 and 2011, BatQuartz, E2, Elixir, Modded Gemini, Flashlight, Sweety, Nirvana UL, Deepspace.


Wed Dec 22, 2010 8:51 am
Profile WWW
User avatar

Joined: Tue Sep 09, 2008 9:52 pm
Posts: 67
Location: Bristol / Fleet, Hants
Post Re: Centre of gravity
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.


Wed Dec 22, 2010 11:43 am
Profile
User avatar

Joined: Sat Aug 09, 2008 10:22 am
Posts: 151
Location: Baarn, The Netherlands
Post Re: Centre of gravity
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 ....


Thu Dec 23, 2010 9:47 pm
Profile
User avatar

Joined: Fri Apr 22, 2005 11:58 am
Posts: 102
Location: Heule - Belgium
Post Re: Centre of gravity
Thank you for your time and effort with the drawings!

_________________
Flying now: B'zar 2009, 2010 and 2011, BatQuartz, E2, Elixir, Modded Gemini, Flashlight, Sweety, Nirvana UL, Deepspace.


Fri Dec 24, 2010 6:20 pm
Profile WWW
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 14 posts ] 


Who is online

Users browsing this forum: Majestic-12 [Bot] and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group.
Forum style by STSoftware for PTF.