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Author Topic: Center of Gravity of a Kite  (Read 2499 times)
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Ara Ararauna
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« on: February 22, 2014, 02:45 AM »

How can one calculate or find the center of gravity of a kite?
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« Reply #1 on: February 22, 2014, 05:01 AM »

Now there is an innocent question that could take a couple of textbooks to explain all the related aerodynamic principles!

Directly though, the CG is the central balance point of an object where gravity appears to act.  Read this.

http://www.wikihow.com/Calculate-Center-of-Gravity

For humans, the CG somewhere around the waist/hips (as are this morning's doughnuts) which makes us unstable when walking on ice like outside my front door right now since we just had a thaw and refreeze the last couple of days.

For light aircraft, the CG is normally forward of the center of lift for stability. Move the CG towards the nose for more stability (Cessna 150).  Move the CG back for less stability but more maneuverability (Pitts Special, jet fighters).

With a sportkite laying on its back, the CG is going to be along the spine (symmetrical wing panels) between the center T and the nose.  Lower than the spine vertically towards the table as a kite is a 3D object. You can get an idea by balancing the kite on your finger tip or holding the kite by the pig tails up off the table and observing how the nose tips. Picking the kite up at the center T is a good indicator.

Mass density is not distributed evenly on a sport kite (frame, sail, bridle, weights) so there is not a simple equation to find the exact CG in all three dimensions.

We change the CG of the kite and moment arms by adding weight to the tail to enhance pitch tricks.  Recent designs for hot tricks pay close attention to the location of the CG, frame locations, bridle attachment points etc.

You not only have to think about static CG, but center of lift, moment arms, and how they all are affected by flight forces.

Others with more college Aero classes chime in...

Why do you ask?
« Last Edit: February 22, 2014, 05:19 AM by stapp59 » Logged

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« Reply #2 on: February 22, 2014, 06:17 AM »

Zippy did some photographic studies and shared his results on the forum..... not certain of which kites though.....
Doug S seems to be the higher thinking engineer on the forum I am certain he can elaborate more.
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« Reply #3 on: February 22, 2014, 06:50 AM »

Ara,

I am pleased to answer your question.  To find the center of gravity (CG) for a kite with a stiff frame is quite easy.  At the bottom of this reply is a copy of my write-up on the CG from my stunt kite article, where the link is one contained with my thoughts regarding stunt kite designs.  I included the links from Post Reply 12 below.

 - Technical Article - Stunt Kite Aerodynamics by Doug Stout.pdf - https://docs.google.com/open?id=0B_TPx1W-lkmSRnZuQWJVbzFYRVk

 - Geokite 8 with Bird of Prey Stunt Kite Design - Program and Stunt Kite Design by Doug Stout - May 2 2012.xlsx - https://docs.google.com/open?id=0B_TPx1W-lkmSQ3dRQVdwaFBiZ0U

 - Geometric Stunt Kite Comparison by Doug Stout - May 2 2012.xls - https://docs.google.com/open?id=0B_TPx1W-lkmSOVFHZUM5dUdQTms

The question is what do you want to do with your CG knowledge?  If you wish to compare kites, then you will need more information about each kite as indicated in my article.  If you are interested in the effects of extra weight at the tail of a stunt kite, then you will need to perform a moment of inertia calculation.  This is quite easy, but not in the posted version of my stunt kite program at this time.  I have a separate simple spreadsheet for this.

Enjoy,

Doug

Center of Gravity -
 
The center of gravity of a stunt kite is the location where the mass of your stunt kite can be represented by one point.  The center of gravity is measured from the nose of the stunt kite along the center spine.  With your stunt kite fully assembled, position the stunt kite so the center spine is parallel to the ground.  The nose and tail of the stunt kite must be the same distance to the ground.  The tips of the stunt kite also must be the same distance to the ground.  Balance your stunt kite on your thumbnail, along the center spine, so your stunt kite is parallel to the ground.  If the tail is lower, move your thumb towards the tail of the stunt kite.  If the nose is lower, move your thumb towards the nose of the stunt kite.

The distance from the nose to your stunt kite to your thumbnail is the distance we measure to determine the center of gravity location for your stunt kite.  Another way to locate the center of gravity is to suspend your stunt kite by the center spine by using a small section of flying line.  The flying line would be attached to your ceiling and tied to the center spine.  This will allow you to adjust the position of the flying line on the center spine to assure the nose and tail are the same distance from the floor.
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Ara Ararauna
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« Reply #4 on: February 22, 2014, 02:29 PM »

Thanks!

Concerning "what" the CG is, I had a clear idea of what it is. I also imagined it could be found by balancing the kite on some point of the spine (in a similar way as you balance an airplane by placing your index fingers on either tip of the wings).

What I wasn't sure of was whether this was the same point around which rotations and spins occurred during flight and tricks. From what you explain it looks like it is indeed the same point.

Now, concerning the "why" I ask, you are all going to kill me...  Roll Eyes   Roll Eyes    Embarrassed

I had this crazy idea... I almost don't dare say it...   Embarrassed

Imagine we placed a small electric motor with a small propeller at the CG to avoid altering the properties of the kite too much (I stress this last part because I know the properties would be altered anyway). The propeller would be oriented blowing toward the tail of the kite.
The idea would be to be able to use an STD kite when no wind is available...

I know there are some kites that have been converted to RC flying gadgets.
But here the idea would be to still fly the kite with the lines and not use a RC.

I was just toying around with this crazy idea and thought I would ask.
 Sad

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« Reply #5 on: February 22, 2014, 03:40 PM »

Now that sounds suspiciously similar to the models some of us used to fly  Grin

http://www.control-line.org

It is feasible to combine the two given today's model aviation electric motor and battery systems.  Not sure how much fun it would be ultimately.

I've always wanted to borrow a few of those big portable ventilation fans I've seen at the stadium.  Cool

You could also get a good SUL/indoor kite, light lines, and walk backwards a bit  Cheesy 

Don't let me or anyone else discourage you from trying something new.  Just be prepared to try a whole bunch of things that don't work that well at first...
« Last Edit: February 22, 2014, 03:46 PM by stapp59 » Logged

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« Reply #6 on: February 22, 2014, 05:05 PM »

One yoyo tells the tale on this in my experience.  If the kite rolls around almost 'in place' then it's about perfect.     Backspin tells you the other axis and the comete combines several.

Does this seem rational thinking?
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« Reply #7 on: February 22, 2014, 06:47 PM »

Oh Ara you would enjoy a Shanti cube turned into a Rc plane...google it...a small square single cell box kite with a propeller.... the way you turned was by changing the CoG... very cool piece!!!!!!
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« Reply #8 on: February 22, 2014, 08:24 PM »

CG is exactly the point a kite rotates around in yoyo's and flat spins  IF the kite was flat and had no wind resistance. Dynamic balance affects it mostly. If you were to mount a motor on a dualie you would need to find the static balance point first.  Then you will need TWO props because you will need to balance the thrust to keep the kite from nosing up or down.  Its actualy easily done and fun for a bit until you get your lines cut by a prop Tongue  That was the hardest thing to do when I did it was finding a way to keep the lines from getting sucked into the props during slack line attempts.  BTW I used micro rc motors and props in the .09 size class on a-frame mounts similar to the premier mounts but smaller.
« Last Edit: February 22, 2014, 11:14 PM by Ca Ike » Logged
Jim Foster
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« Reply #9 on: February 22, 2014, 09:47 PM »

The center of gravity of any object may be found by hanging that object from any point on that object, then finding a line directly downward from the hanging point. Then, hang the object from a different point on the object then finding the line directly down from that hanging point. Where the two lines cross within the object is the center of gravity of the object.
Thank you Mr. Munger, my high school physics teacher.
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Ara Ararauna
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« Reply #10 on: February 23, 2014, 07:22 AM »

Now that sounds suspiciously similar to the models some of us used to fly  Grin

http://www.control-line.org


Yes, I flew those when I was a teen back in the late 70's until my inner ear balance system lost its robustness due to growing older...  Cheesy
However, this is not exactly the referent I had in mind.

Don't let me or anyone else discourage you from trying something new.


True! Thanks   Smiley
« Last Edit: February 23, 2014, 07:34 AM by Ara Ararauna » Logged

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« Reply #11 on: February 23, 2014, 07:26 AM »

Oh Ara you would enjoy a Shanti cube turned into a Rc plane...google it...a small square single cell box kite with a propeller.... the way you turned was by changing the CoG... very cool piece!!!!!!

Yes I had seen that video already quite a few times.
It does look like a blast of fun.

However, I wasn't thinking on an RC kite at all this time.
I really thought of a self-propelled kite that takes away frustration from wind variability and despite the contraption still allows a certain degree of trickability.

Thanks
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Ara Ararauna
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« Reply #12 on: February 23, 2014, 07:33 AM »

Its actualy easily done and fun for a bit until you get your lines cut by a prop Tongue  That was the hardest thing to do when I did it was finding a way to keep the lines from getting sucked into the props during slack line attempts.

Yes, I had thought of that problem. But I imagined something like a tube to house the motor & propeller inside and make it much more difficult for the lines to get caught or even sucked.

Now, you are not 100% clear about it but you seem to have tried that before...
Do you have any videos?
Or is it only mere speculation  Wink
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« Reply #13 on: February 23, 2014, 11:25 AM »

Its actualy easily done and fun for a bit until you get your lines cut by a prop Tongue  That was the hardest thing to do when I did it was finding a way to keep the lines from getting sucked into the props during slack line attempts.

Yes, I had thought of that problem. But I imagined something like a tube to house the motor & propeller inside and make it much more difficult for the lines to get caught or even sucked.

Now, you are not 100% clear about it but you seem to have tried that before...
Do you have any videos?
Or is it only mere speculation  Wink
I gave up on it.  There were several issues I couldn't find good fixes for.  First was the line cutting.  Shrouds helped but quickly became too cumbersome.  Second was mount stability.  I fixed that by running guy wires to the LE's at the spreader points but that added control issues and dynamic balance issues.  Third was speed control.  Fourth was prop pitch in relation to the kite.  All among a myriad of sail pressure/flow issues.

My last design was using an inline set up with the batt on the spine at CG and micro motor/prop on the tail using a trick line to keep the lines out of the prop.  Micro servos and reciever (mounted near the batt) for pitch and yaw control on a 2 axis premier motor and mount.  IT worked the best but still had issues.  There was never enough sail pressure to get good enough air flow to have good steering control and stability from the lines themselves.  You have to be able to change speed and prop angles dynamically and even though I had a modified transmitter with the "sticks" on extensions so I could hold them and the straps together it all became too much to work with.  The only set up that worked was a hang glider design but then that took away the flight line control.

All in all it quickly became an expensive experiment with no real workable solution.  The only workable control set up was a quad line deal using Jon's q-flaps and a single speed , manual adjust variable power circuit but still couldn't dynamically adjust pitch to keep good air pressure/flow on the face of the kite.  Without the q-flaps you couldn't turn well at all.
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« Reply #14 on: February 23, 2014, 01:22 PM »

Kite design and aerodynamics are dependent on higher pressure on the face of the kite with lower pressure on the back side. Just moving the kite forward through the air isn't going to provide the airflow and pressures a current design needs, as Anthony discovered and found no easy solution to overcome it.

I bet there's a design waiting to be discovered that would work though, something between a kite and a control line airplane, something you don't have to spin yourself dizzy with and could fly to one side.

Keep dreaming, maybe you're the one   Smiley   Smiley
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