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Mar 16, 2011, 09:43 AM
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Last edited by DLGjunkyard; Aug 22, 2018 at 11:40 PM.
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Mar 16, 2011, 10:29 AM
G_T
G_T
Registered User
Material creep generally has a period of fast creep where things are settling in and strain is becoming uniformly distributed in the material. Then there follows a period of work hardening which is likely the period referred to in your post. Lastly there is a period of progressive fiber failure. This is where individual fibers fail - the weakest ones - thereby increasing the strain on the rest of the fibers. This process obviously accelerates...

Creep rates are also (generally speaking) a function of temperature. Approach closer to the melting point of the material and the creep rate increases quite a bit.

The graph above where your quoted value comes from pertains to the initial elongation behavior to applied load. There we see 0.2%, or for our application about 2mm, in 100 hours. This is not negligable.

You can test creep easily enough for yourself. Take a length of your chosen line, hang a length with a reasonable weight, and take an immediate measurement. Let it hang for quite a while - say a couple of weeks at least, and see what it does. I'm expecting you will see some initial elongation occurring on a short time scale, and then for practical purposes it stops elongating. That is my experience with Kevlar fiber in the past when I used it quite a bit. Just don't use something like a wood ruler to test. That can change more in length due to humidity changes than the Kevlar will due to creep.

For rudder applications, the spring is only applying a small portion of the maximum load the line may experience. The first launch yaw oscillation provides much higher load. I experienced about 3mm of creep in the first day of flying my Twister-II. I had to re-tie the line to compensate. It has gone an additional 2mm since then and I should re-tie the line again. Not Kevlar though. Kevlar would have done better. Had I pre-stretched the line, I would not likely have encountered this issue.

The early creep of a material is somewhat logarithmic in time for its behavior. This is when a load is applied, AND when it is removed. So it is possible that when one stores a DLG that the rudder will drift in the direction of the line contracting as it is under lower average load than is the case when it is being thrown and the load spikes in launch and is generally a little higher due to flying. So one could at least theoretically get the case where one assembles the model and notes the rudder has drifted. Then one trims out the drift. After flying a bit, one has to trim back the other direction.

One also has the situation where one pre-stretches a string, stores it, and then installs it. Some of the benefit of pre-stretching is lost.

Kevlar has an advantage that its coefficient of thermal expansion is slightly negative, as is carbon fiber. So as temperature changes at least the fiber is expanding or contracting in the same direction as the tailboom! * I'm assuming here that the carbon being stiffer, oriented in the same direction, and of greater content will dominate any typically biased glass in a tailboom. Kevlar in a tailboom would behave more like the carbon if uni in the direction of the tailboom which nobody does (that I know of). If the Kevlar was oriented perpendicular to the carbon, then its expansion coefficient is opposite that of the carbon but the stiffness is so low it won't matter. As a Kevlar fiber is warmed, it contracts lengthwise but grows fatter.

Kevlar also has the disadvantage that it absorbs moisture so keep it dry. I do know that this reduces the mechanical properties and damages the fibers, but I do not know the effect on length in the fiber direction. One could test. Anyway for this reason Kevlar is often coated or impregnated with something. This may be a wax or some polymer. Or even some sort of oil. I have seen all of these. Or you may find uncoated.

Kevlar degrades during UV exposure but our lines are protected within the fuselage. This degradation is visually evident as the color progresses from a light yellow to a golden brown over time and exposure.

Kevlar is self-abrasive so if the fibers are allowed to slide against each other, they will cause damage. Not likely an issue for us as long as the lines do not cross. HOWEVER as has been pointed out, Kevlar will also saw through many other things... So it should not be allowed to slide against unprotected surfaces, particularly if there is pressure between the Kevlar and the surface.

Gerald
Mar 17, 2011, 02:14 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Quote:
Originally Posted by DLGjunkyard
Use spectra fishing line instead. Its equally as strong and found most every where
What breaking strain of Spectra do you use?.. it comes in a wide range.
Mar 17, 2011, 04:15 AM
Registered User
MGeo's Avatar
Quote:
Originally Posted by discostu956
Are you only holding it with CA?

Uni knot is a good knot, use about 8 turns for best results with spectra
I've been using 20 pound Berkely FireLine (http://www.berkley-fishing.com/produ...fused-original), using a uni knot with 4-5 turns, fastened with CA. Tensile pull tests on this have been disappointing. CA does not bond well to the line. I can try more turns on the uni.

Perhaps I am using the wrong line. Can someone provide a link to line they are using with good pull test results.

I have also had the problem of not being able to get the line tight. I am trying off at the servo arm followed by the surface horn.

I like the idea of locking off at the servo arm under a screw head, but my servos lay on their sides in the fuse with no access to the scew heads once installed. Perhaps I need to rethink that end. Thanks for your input.

George
Last edited by MGeo; Mar 17, 2011 at 04:23 AM.
Mar 17, 2011, 04:26 AM
Is my CG correct?
discostu956's Avatar
I'm suprised that a 5 turn uni knot isn't holding well enough. I will try and see if there are any knots that will work better for this
Mar 17, 2011, 06:01 AM
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Last edited by DLGjunkyard; Aug 22, 2018 at 11:39 PM.
Mar 17, 2011, 11:05 AM
Throw it like you hate it
RCPC's Avatar
Quote:
Originally Posted by MGeo

I have also had the problem of not being able to get the line tight. I am trying off at the servo arm followed by the surface horn.

I like the idea of locking off at the servo arm under a screw head, but my servos lay on their sides in the fuse with no access to the scew heads once installed. Perhaps I need to rethink that end. Thanks for your input.

George
I run two lengths of the line (spiderwire 50 or 65lb test) in my setups and tie a half-hitch at the control horn side where the line turns around and then fix the servo horn side with a crimp and thin CA followed by black CA. Does this make sense?

To be clearer, I pull a length of line that is over twice the length needed. I fold it in half so at one end there are two free ends and the other has a U when the line turns around. I pull the line through the fuse with a small wire so the U is at the control horn side. I tie a half hitch here before the control horn is fixed. Then i fix the control horn. It's easier to make the knot before the horn is fixed. This is not mandatory, however.

Once this is done, slide a crimp over the free ends, thread the free ends through the servo horn, and then thread the free ends through the crimp again. I power up the servo at center and then pull line through the crimp until the control surface is at it's neutal setting. I usually pull just a little more through so the line is about 1-2mm too short. This accounts for the initial creep that happens in the first day or so when the line is under tension. Once you are happy with the length of the line, collapse the crimp around the line and wet with CA. The crimp is just a small piece of brass tubing of about 1-2mm diameter and about 1cm long. I am out of town but i can get pictures this weekend when I get home if you want. Hope this helps.
Paul
Mar 18, 2011, 05:29 AM
Registered User
MGeo's Avatar
Quote:
Originally Posted by RCPC
I run two lengths of the line (spiderwire 50 or 65lb test) in my setups and tie a half-hitch at the control horn side where the line turns around and then fix the servo horn side with a crimp and thin CA followed by black CA. Does this make sense?
Paul
Yes Paul this makes senses and I like the concept. My problem may be the type of line I am using (FireWire), which does not seem to take CA well at all. I'm going to try alternates.

Disco, problem with Uni may by my technique. I'll keep working at it.

Thanks to all for the input.

George
Mar 18, 2011, 07:12 AM
Is my CG correct?
discostu956's Avatar
Braided stuff might take up the CA better (ie you can see that its woven)
Mar 18, 2011, 03:50 PM
Throw it like you hate it
RCPC's Avatar
Quote:
Originally Posted by MGeo
My problem may be the type of line I am using (FireWire), which does not seem to take CA well at all. I'm going to try alternates.
try just a crimp and see if it holds. here are some pics of how i did the install on my salonit. the pictures are pretty self-explanatory coupled with my previous post.
paul
Mar 19, 2011, 03:53 AM
Registered User
MGeo's Avatar
Thanks Paul, the photos make it clear. I like your bushing idea on the horn, as well as the crimp. I will look for some 50 lb Spiderwire EZ Braid and give it a try.

George


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