Balance

Someone didn't understand why I said this...

"Changes to static balance often occur when changing from heavy pultruded
spars to a lightweight wrapped frame. Given that the lower spreaders are much
longer than the upper, there's quite a bit more carbon low in the kite. So, when
changing to a lighter frame the greatest saving in weight will be in the bottom
half of the kite".

~ I can't understand this point. Making the frame lighter will change
~ proportionally
~ all the weight distribution. Therefore the center of mass of the frame will
~ remain the same. Probably the change in static balance is due to the extra
~ weight of sail, connectors, standoffs, etc. which becomes relatively heavier
~ than the frame. What's your opinion ?

This was my reply

I based that comment on experience gained from various re-framing experiments, however to prove this to myself, and to illustrate the point I've just got a kite and done some measuring.
The kite was one of my Acaras: Conventional full sized sport kite, the frame is composed of std length rods and a 1m spine, nothing unusual about the frame. I marked a point at 50 percent of the kites height. This fell a few inches in front of the T (where the CoG usually is on a sportkite) and horizontally in line with the ferrules (as you would expect).

Measuring above this point the kite has:
57cm of spine, 57cm top spreader, 2 x 82.5cm upper leading edges = 279cm of carbon

Measuring below this point the kite has:
44cm of spine, 2 x 82.5cm lower LE's and 2 x 82.5cm lower spreaders = 374cm of carbon

Lets suppose I'm going to change a from an Avia .2300 frame to a Skyshark 5P frame. A fairly realistic type of re-framing exercise.
The weight of a 5P = 13.67g,
The weight of a .2300 = 20.6g

Therefore:
-the weight of .2300 frame above 50% of the kites height is
279/82.5 * 20.6 = 69.665455g
-the weight of 5P frame above 50% of the kites height is
279/82.5 * 13.67 = 46.229455g
-the weight of .2300 frame below 50% of the kites height is
374/82.5 * 20.6 = 93.386667g
-the weight of 5P frame below 50% of the kites height is
374/82.5 * 20.6 = 61.970667g

Therefore in changing from a .2300 frame to a 5P frame:
- the upper half of the kite has got lighter by 23.436g
- the lower half of the kite has got lighter by 31.416g

i.e.: in changing to a lighter frame the greatest saving was in the lower half of the frame, 7.98g greater. 7.98g is not a huge amount, but it's roughly equivalent to say half a spar. I'm sure that taping an additional half a spar to the top spreader of a kite would affect it's balance.

kitedesign/balance_eg.html last modified 19:57:09 30-Jul-2017


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