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Frame Stress From High Winds


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Poll: Rev Arcana Poll Version 1.0 alpha (42 member(s) have cast votes)

Which parts of the Rev 1.5's frame are most apt to be riven asunder by the raw elemental fury of high winds?

  1. The vertical spars. (24 votes [55.81%])

    Percentage of vote: 55.81%

  2. The leading edge spars. (11 votes [25.58%])

    Percentage of vote: 25.58%

  3. liek who carez LOL (8 votes [18.60%])

    Percentage of vote: 18.60%

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#41 quaa714

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Posted 26 January 2009 - 01:39 AM

Nope, full on wind pressure, center of the window.


I'm gonna have to agree with John here on this one.
As I recall, the break took place coming up thru the window is a strong gust of wind. I was downwind adjusting my kite on the ground almost underneath the flying kites.
I definitely looked up right away and it was clear to me and the others that something popped.

"Cya in the Sand!....."

"Slack lines are fine lines!"


"You don't need a weatherman to know which way the wind blows" BD
"One good thing about music, when it hits you feel no pain" BM
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#42 bobw

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Posted 26 January 2009 - 07:16 AM

Yup. Heard a loud pop, looked up, and saw Laura's kite spiraling and then a rod flying away. It was cool! Pretty surprising to me. It was a weird, variable day, and I was flying a 1.5 w. 2 wraps in it, and except for the occasional gust, it never felt like the wrong choice.

#43 Love2fly

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Posted 26 January 2009 - 11:14 AM

Nope, full on wind pressure, center of the window.



That's exactly what happened! It was definitely a gust from a sudden shift more than 45 degrees to the right of where the center of the window was, and the right downspar.
Pretty cool.... sort of... you know what I mean?
Laura
 
*** Any day flying is a good day; have a great one! 
**** REVS: Fly it, you'll like it!
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#44 Dominic_G

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Posted 23 March 2009 - 06:40 PM

interesting post! I has sure that the middle LE spar will blow in mid air under too much stress. I cant wait to see and hear the verticals explode in mid air... but i wont search for it for sure!

i find it very satisfying to break a spar in mid air, when its not a stupid crash or accident. Like i often blow lower spreaders on my dual line kite, in mid air while pulling a hard half-axel or something like that in the center of the window. It feels great to push the kite to its max.

Never broke my rev yet.... hope it will not be my race rods!

#45 Dano

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Posted 26 March 2009 - 06:54 PM

Good stuff here, Thanks!

Made me think the "Advantage Stiff" verticals i got with my new vented 1.5 SLE might just be a good thing. :)

-dano
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#46 FortFlyer

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Posted 26 March 2009 - 07:54 PM

In all my time flying I've only broken 2 rods, 1 was a down spar in my power blast 2-4 in 25 mph winds probably due to the fact that my 300 lb lump of a body was holding the kite back.

It exploded into 4 pieces so that i know was from over stress, 2nd was a LE right at the ferrule and that was due to slippage of the ferrule.

All my kites have 2 wrap down spars reg and vented and I've never had a problem my hands seem to automatically adjust for heavy gusts without even thinking about it.

Only thing lately that has me concerned is with the race rods they seem to wear right where the downspar crosses the LE on both the down spar and the LE i've noticed blemishes forming, I try to rotate the spars every time i use them just to keep the wear down but I can see this becoming a problem eventually.

Like John Mitchell's picture I've had tips touching many times without breakage its scary and I don't recommend doing it often but it goes to show you the flexibility of these rods.

Most times I have seen LE breaks were from ground slams at an angle, Straight on they seem to take a beating well as proof on my EXP which has taken more beatings then UFC fighters from my 2 sons and people test flying.
Jim,
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Rev's are like a carbon framed out-of-body experience

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#47 John F

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Posted 26 March 2009 - 08:17 PM

I have broken two outer LE spars. One on a rev 2 with supersonic crash into a rock. The other on a 1.5 SLE where the outer was not seated in the the center spar. My bad.

On verticals I have had them pop out of the connectors under high wind conditions. Solution was tightening the bungees on the bottom. I wonder if the fracture doesn't occur is the bungees are to tight and do not have enough give. The give in the bungees is a wonderful feature in the REV. It takes most of the force unless it is too tight.

#48 MrDenny

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Posted 27 March 2009 - 04:31 AM

I have "popped" :angry: a few rods in my time but never realized just how much pressure is on the rods. I had a vertical rod on my Blast shoot straight up in the air. It was quite a sight. I don't know what the kite looked like coming down because I was totally watching the rod. I watched the rod go up and almost out of sight! :o It kind of fluttered down into the sea oats about a hundred feet down wind. Nothing was broken so it was a great experience. I am waiting on my Blast race leading edge to arrive soon.

Denny #12

.. .


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#49 quaa714

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Posted 27 March 2009 - 05:32 AM

I have "popped" :angry: a few rods in my time but never realized just how much pressure is on the rods. I had a vertical rod on my Blast shoot straight up in the air. It was quite a sight. I don't know what the kite looked like coming down because I was totally watching the rod. I watched the rod go up and almost out of sight! :o It kind of fluttered down into the sea oats about a hundred feet down wind. Nothing was broken so it was a great experience. I am waiting on my Blast race leading edge to arrive soon.


HAHAHAHAHAHA..............There's some funny lines that come to mind here......think I'll just leave it alone though!!!!!! :lol: :lol: :lol:

"Cya in the Sand!....."

"Slack lines are fine lines!"


"You don't need a weatherman to know which way the wind blows" BD
"One good thing about music, when it hits you feel no pain" BM
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#50 Kitelife

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Posted 27 March 2009 - 07:00 AM

omg - behave - LOL

John Barresi

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#51 sammy

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Posted 27 March 2009 - 08:42 AM

AHAHAHA rich! after rich said that it went from a rev post to something on the other side of the Island lol. What are we going to do with this guys rich, crazy man he is lol
RABBI שמואל aka SAMMY


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#52 stroke survivor

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Posted 09 July 2010 - 07:31 AM

Only broken one rod so far - knock on wood, assembling LE and didn't have outer rod seated fully, gust came up and cracked end of rod! ::kid_cussing::My Bad :censored: Still have 4 out of 5 original rods from my Rev 1!! Tough little buggers!!!:)

Edited by stroke victim, 12 July 2010 - 10:22 PM.

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#53 Felix Mottram

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Posted 05 March 2011 - 06:56 AM

Only broken one rod so far - knock on wood, assembling LE and didn't have outer rod seated fully, gust came up and cracked end of rod! ::kid_cussing::My Bad :censored: Still have 4 out of 5 original rods from my Rev 1!! Tough little buggers!!!:)



I wonder if anyone can translate the following into something that relates to tubular rods <grins>

Felix

"Imagine we have a solid circular rod, and consider the forces in the
middle as we try to bend it. I'm going to draw some cross sectional
diagrams.

Seen from the side:

y coordinate
| axes plane of interest
|___x v
<=== ===> F
---------------------------------------------
C _ ^ stretched : : _
/ \ |2r : deviation, / \ C
\> | : exaggerated </
v squashed *
---------------------------------------------


Seen end-on:
+----> expansion
y e
| ,-------. | /
|___z / \ | /
| | |/ graph of
| | / expansion
| | /| vs y
\ / / |
`-------' / |


So we have a rod of radius r. We apply a bending couple C. This will
cause it to bend slightly, into (by symmetry) a section of a very
large radius hoop. We will neglect the 2nd-order effects of squeezing
of the rod in the y direction and expansion in the z direction.

>From the geometry, we can see that the expansion or contraction
distance is proportional to y. At the middle (y=0) the rod neither
stretches or compresses; at the top edge (y=r) it is maximally
extended. Ie e(y) = d * y where e is the proportion by which the
material is extended and d is a value indicates how tight a bend we
get.

While the tube is still in its elastic region (ie, it hasn't started
to bend permanently ie fail), the force is proportional to the
extension. Since the force varies across the cross-section of the
rod, we want the force at each point - ie, the pressure. The
geometrical symmetry means the force varies only according to y and
not x or z.

Ie, P(y) = E * e(y) Hooke's law, where E is a material
property (Young's modulus).

Consider the component of the couple at the plane of interest,
provided by the horizontal strip of the plane of interest at the
vertical position y:

dC(y) = dF(y) * y

where dC is the sliver of the couple and dF is the sliver of the
normal force. But force is pressure times area:

dF(y) = P(y) * w(y) * dy

where w is the horizontal width across (ie, in the z direction) the bar
at the height y. Since the bar is circular in cross-section,

w(y) = sqrt( r^2 - y^2 )

So substituting all that in,

dC(y) = ( E * ( d * y ) ) * y * sqrt(r^2 - y^2) * dy

= E * d * y^2 * sqrt(r^2 - y^2) * dy

(NB I have neglected to think too hard about the overall sign of
this couple.)

Ie to calculate C we have to integrate the contribution of the P at
the various y:

r
/\
C = | E * d * y^2 * sqrt(r^2 - y^2) * dy
\/
y = -r

I didn't fancy remembering how to integrate that so I cheated and used
Wolfram Mathematica online integrator[1] which tells me:

C =

- E * d * 1/8 *

[ -1 y ] +r
[ y * sqrt(r^2-y^2) * (2*y^2 - r^2) + r^4 tan ------------- ]
[ sqrt(r^2-y^2) ] y=-r

The LH term is zero at both limits because sqrt(r^2-y^2) is zero at
both limits. So we are left with the RH term. This is slightly
awkward the way Wolfram has integrated it (using the 1-argument atan
rather than the computer person's 2-argument atan) because the
denominator is zero, but atan is actually well-defined if we take the
appropriate limit. So:

[ -1 y ] +r
C = -1/8 * E * d * [ lim l^4 * tan ------------- ]
[ y=0..->l sqrt(r^2-y^2) ] l=-r

The sqrt() denominator approaches 0 from the positive side in each
case. l^4 is always positive. The numerator is positive at the upper
limit and negative on the lower, so:

-1 -1
C = -1/8 * E * d * r^4 * ( lim tan f - lim tan f )
f->inf f->-inf

= -1/8 * E * d * r^4 * ( pi/2 - -pi/2 )

= -pi/8 * E * d * r^4

The minus sign is simply due to carelessness about the overall sign
and can be disregarded. Recollect that E is a material parameter and
d is a geometrical meausre of how much we've bent the rod. Let K =
pi/8 * E, and dropping the minus sign, we have:

C = K * d * r^4

So for any given material, and amount of bending, the couple
(informally, force) required to bend the rod by that amount is
proportional to the 4th power of the rod's radius. The rod's
stiffness against bending goes as the 4th power of the radius.

This is the Luggage's 4th power.


But if we are interested in the _failure strength_ we want to know
when the rod will fail. Failure occurs when the expansion of the rod
at any point exceeds the material's elastic limit, ie where:

r
max e(y) >= e
y=-r lim

As soon as that is reached anywhere that bit of the rod will start to
yield, and the other parts will need to take up the strain and won't
be able to do so, and it will buckle (this is not true for all
structures but it is for this one). e_lim is a material property too.

Since e(y) = d * y obviously the maximum e is at r. (It will
probably fail at the top, in tension, but if it fails in compression
that's -r, which will make no difference to the analysis.)

So we have, at failure:

d * r = e
lim

What we want to know is how C_lim (the couple at which the rod fails)
depends on r.

Above we had:

C = K * d * r^4

Substituing in to eliminate d, we get:

e
lim
C = K * ---- * r^4
lim r

or

C = K * e * r^3
lim lim


So the rod's _strength_ against bending is only proportional to the
third power of its thickness, not the fourth.

The discrepancy between third power for strength and the fourth power
for stiffness is because a thicker rod bends less before it fails.


[1] http://integrals.wol...expr=x^2 * Sqrt[r^2+-+x^2]"

#54 --Pete

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Posted 05 March 2011 - 07:36 AM

This thread might be what you are looking for:

http://www.engineers...posts/3844.html
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#55 Stone in Shoe Bob

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Posted 05 March 2011 - 11:10 AM

Sorry Felix, had to check the date on that last post, your almost a month early with that one. ;)
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#56 stroke survivor

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Posted 05 March 2011 - 12:48 PM

Can anyone translate that into something a common "joe" can get his mind around???Posted Image I think I gave up after the first paragraph!!!Posted Image

wayne from portland
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#57 Reef Runner

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Posted 05 March 2011 - 04:05 PM

Can anyone translate that into something a common "joe" can get his mind around???Posted Image I think I gave up after the first paragraph!!!Posted Image


I think that I recollect, sometime ago, that I read where John said "the verticals will always go first". John, please forgive me if I have misquoted, and please set us straight.....................Posted Image Posted Image
nick

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#58 Kitelife

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Posted 05 March 2011 - 06:07 PM

In my experience, barring unusual circumstance (like impact), this is true.

John Barresi

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#59 --Pete

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Posted 05 March 2011 - 10:05 PM

No no; Felix is serious. A solid rod is just a tube with zero inside diameter.

Of course, Felix, you have to consider the fact that wrapped carbon fiber tubes have different strengths in the lateral and longitudinal directions.
--Pete
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#60 Felix Mottram

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Posted 06 March 2011 - 04:49 AM

No no; Felix is serious. A solid rod is just a tube with zero inside diameter.

Of course, Felix, you have to consider the fact that wrapped carbon fiber tubes have different strengths in the lateral and longitudinal directions.


Uh! Quite...

Rods break, very occasionally. Loose ferrules seem to be the main danger in the leading edge. It is worth checking them as a matter of habit, especially with team kites.

Felix




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