Wires

Please tell what happened and how it might have been avoided. Names should be ommitted. This forum should help others learn from mistakes that caused or nearly caused a mishap.
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Steve D
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Wires

Post by Steve D » Fri Sep 18, 2015 9:20 pm


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lswendt
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Post by lswendt » Thu Sep 24, 2015 3:06 pm

Hmmm...load testing my side wires has always been a part of my preflight. Learned it from Rob Kells.

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Ken Andrews
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Stepping on side wires isn't enough

Post by Ken Andrews » Sat Oct 17, 2015 12:13 pm

Hah! I finally found my misplaced copy of Dennis Pagen’s “Performance Flying�. On page 318, he says that the side wires are under 125 pounds of tension, and this number sounds more plausible to me than the 50-75 pounds that the Vimeo author quotes. Moreover, the loads can double in a steeply banked turn or when pulling out of a dive, and they increase even more in wire-slapping turbulence. Hence, I wouldn’t go thinking that his “step on it� test is anywhere close to the normal flight loads that the side wires must handle.

While I’m attacking this Vimeo guy, kinks in side wires make me really nervous. Such a kink gets pulled straight every time there’s tension on the wire, and returns to its bent state when the tension is released. Each such cycle work-hardens the metal, weakening the wire. While a glider is set up on launch and moving around a little in the breeze, it could go through several cycles a minute, just as it does on every step when ground-handling.

I’m certainly not against testing the side-wires by stepping on them, but it’s not sufficient. Visual inspection is necessary, and I believe that side wires should be replaced immediately if there are any kinks, and periodically even if there aren’t.

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JD
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Re: Stepping on side wires isn't enough

Post by JD » Sat Oct 17, 2015 6:36 pm

Ken Andrews wrote:....I’m certainly not against testing the side-wires by stepping on them, but it’s not sufficient. Visual inspection is necessary, and I believe that side wires should be replaced immediately if there are any kinks, and periodically even if there aren’t.
Ken,
If you do the trigonometry and work out the force vectors, doing a 50# step-on test can produce several times that amount of pressure into tension on the side wires. It is best to pull full VG before performing the test on a topless glider. The less, the side wire sags during the test for any given amount of pressure placed between the side wore and the leading edge, the greater the amount of tension load on the wires. The primary purpose of this test is to insure that the glider does not fold up immediately after launch and result in a serious or fatal accident such as the one up a Fort Funston recently.

Perhaps you recall the incident about 5 years ago when a certain Sylmar pilot made a high speed pass over our LZ on high Litespeed that was equipped with overage 2mm side-wires? His right-hand side-wire failed when he pulled up resulting in loss of roll authority followed by a long spiral into the wash, broken bones and aluminum. Had this pilot performed the simple sire-wire stomp test before launching there is a good chance his compromised side-wire would have failed before he took to the skies.

Yes, you are correct in assuming that visual inspections for kinks, corrosion, abrasion and other damage is very important. But not all damage is visible and can occur inside the Nico-press sleeve and elsewhere. And so stressing the side-wire to 100 to 150# of tension during pre-flight by stepping on it is not a bad idea and only takes a few seconds.

This pre-flight check is in every Wills Wing owner's manual I have ever read and that is how it found its way into Jan's video even if he did not explain all the force vectors adequately.

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Jim
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Post by Jim » Sat Oct 17, 2015 7:44 pm

That certain pilot's wires probably would not have failed, if he had followed the prescribed maintenance interval. The wires were the same age as the glider: 6 years old. Even if the wires passed the 'stomp' test, I doubt any of us would put as much pressure on the side wires as we may encounter in flight.

What do the manufacturers have to say about testing wires before flight? How do manufacturers test wires before installation or release for replacement?

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JD
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Post by JD » Sat Oct 17, 2015 9:18 pm

Jim wrote:That certain pilot's wires probably would not have failed, if he had followed the prescribed maintenance interval. The wires were the same age as the glider: 6 years old. Even if the wires passed the 'stomp' test, I doubt any of us would put as much pressure on the side wires as we may encounter in flight.

What do the manufacturers have to say about testing wires before flight? How do manufacturers test wires before installation or release for replacement?
Those 2mm side wires were rated for 50 hours before replacement according to the Litespeed manual. Even standard 3/32" side wires are recommended to be replaced each year. The stomp test is good enough to allow the pilot to launch and get far enough away from the hill to use his reserve in the event of a failure.

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Jim
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Post by Jim » Sun Oct 18, 2015 8:15 am

Tangent Man wrote:The stomp test is good enough to allow the pilot to launch and get far enough away from the hill to use his reserve in the event of a failure.
That's very comforting, Jonathan.

I'm going to check with another source about manufacturer's testing. As I think you know, there have been failures of new wire sets, too.

Anyway, both you and Ken are correct: check everything, be nice to your wires when packing up and adhere to mfgs. maintenance intervals.

brianscharp
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Post by brianscharp » Mon Oct 19, 2015 7:53 am

Funston Accident
http://www.hanggliding.org/viewtopic.ph ... 066#375066
spork wrote:
flyzguy wrote:Someone could do the trigonometry to figure out the resulting line tensions and how they compare to flight loads...
It depends entirely on how bent the wire is at the load. For sake of argument let's say the 50 lb load is applied in the middle of the wire, and the wire is bent by an angle of theta. We can compute the resulting tension on the wire this way...

sin(theta/2) = 50 / tension

Or:

tension = 50 / sin(theta/2)

So if the wire is bent 10 degrees the tension would be 573 lbs
If it was bent 30 degrees the tension would be: 193 lbs
http://www.hanggliding.org/viewtopic.ph ... 072#375072
spork wrote:Doing some rough calculations and using the attached HG pic, I get that you want the cable to withstand 360 lbs for a 200 lb pilot to do a coordinated 60 degree banked turn with a 50% safety margin (with some assumptions as indicated).

This seams reasonably in line with the 50 lb test.

Calcs:

Length of 1 wing: 568 pixels
wire attach point: 298 pixels
200 lb pilot (we don't count the weight of the wing since it's being supported by the air)
1/2 pilot's weight goes to each side wire.
Let's put the center of lift at 1/3 of the wing span (based on an elliptical loading and some guess work).
Assume the load is taken entirely by the cables - not the cross-bar.

Moment reguired to hold wing: (568/3) x 200/2
Same moment must be applied by wire attached at 298: 298 * vert_force = (568/3) x 200/2 = 18933
So vert_force on the wire is: 63 lbs (when flying at 1G).
Based on the image the side wires are at an angle of 31 degrees.
Using the equation from the previous post we see that a 63 lb vertical force
applied by a wire at 31 degrees results in a wire tension of: 122 lbs
In a coordinated 60 degree bank we'd be doing 2G's - so each wire would be at about 240 lbs.
And you'd want at least a 50% safety margin so that suggests we should test it to: 360 lbs.

Image

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