Weak links

[Zack C]

Also, if the function of the link is to save the glider then why not go all the way up to 2 G's?

I'll suggest one more reason...you want the glider's weak link to break before the tug's. While USHPA's SOPs state that the tug's weak link should always be stronger than the glider's, good luck finding one that actually is when you go up to 2 Gs.

Zack

[Zack C]

Although, having said that, at most flight parks you'll be over the tug's at 1.5 Gs anyway...

Not an issue at our local club though.

Zack

[Tad Eareckson]

Must be nice. I NEVER ONCE went up behind a Dragonfly when I wasn't scared of either losing the tow:
- 'cause of the weak link they conned ME into using
- and getting the rope because of the weak link THEY were using

[Steve Davy]

How much time does it take to make a shear link?
What is the normal life span of a shear link (unprotected vs. protected)?
You have them at 35 lbs. increments, why that number?
How could shear links be mass produced?
What other questions should I be asking?
As always thanks for your time.
Steve

[Tad Eareckson]

How much time does it take to make a shear link?

There are three flavors of Shear Link: Towline, Ribbon Bridle, and Bridle Link.

The Towline Shear Link I would recommend ONLY for the front end of an aero towline - where it doesn't get dragged. If you're gonna drag a weak link use a Tost.

The Ribbon (two point) Bridle is a nightmare. Takes a VERY LONG day to do ONE and you're half blind when you're finished. It's not the critical ends that are the problem but hand stitching three or four ten foot elements together without distortion. They're not practical and I'll never do another one.

I like the way I set Zack up with hollow braid spliced its entire length for uniform stiffness and - I'm predicting - virtual elimination of wrap potential. Appropriate string loop weak link at the top and sailmaker's thimble engaging appropriate Bridle Link at the bottom.

The Bridle Link is relatively easy. It's essentially a Towline Shear Link but folded in half so it can take twice what it could loaded end to end (like the Towline Shear Link) - which means you only have to do half as many of the critical and tedious Strength Stitches. And the Strength Stitching is split into port and starboard runs so they're not compromised by contact with the thimble or tow ring.

How much time...

One or two of them is a pain. But when you set up a production line - measure, cut, fuse the leechline Base elements; have needle, threader, floss, color coding thread and heatshrink ready to go - I dunno... Maybe three or four an hour?

What is the normal life span of a shear link (unprotected vs. protected)?

Forever. Don't leave them baking in the sun 'cause the Strength Stitching is nylon but other than that...

I've bench tested these things by cranking them up to maybe five percent shy of predicted breaking strength a couple of times and backing off and had them blow right on target.

And, just as it's a good idea for everybody to fly with a parachute even though the chances of a non aerobatic dude ever needing one are microscopic... Tow pilots NEVER get NEAR the point at which they NEED a weak link - assuming they're using half decent release (which they don't). I've never heard of an aerotow pilot with a finger on the trigger who couldn't beat even a loop of 130 pound Greenspot when the shit hit the fan.

And, also just like a parachute, if you NEED one under a couple hundred feet you're gonna die anyway.

But idiots like Marc Fink...

http://ozreport.com/forum/viewtopic.php?t=24846
Is this a joke ?

Marc Fink - 2011/08/31 08:11:05 UTC

I was actually in the process of reaching for the release and just about to pull it...

The point of all this being that you're NEVER gonna get anywhere NEAR the point of stressing a one and a half G weak link in any case.

You have them at 35 lbs. increments, why that number?

Dental floss is WONDERFUL for hand stitching hang gliding stuff - including the barrel releases I've been making for a long time. About five years ago I figured that it would be a REAL GOOD idea for me to determine what the stitching would hold so I wouldn't hafta go too nuts with massive overkill to keep things from blowing out of sequence.

First I tried the determine the breaking strength of the floss itself and think I got something around eight pounds. So I was figuring eight pounds per stitch - but was suspicious 'cause there's always the issue of how the ends are being held to screw things up.

Then I realized that that was stupid anyway - I wanted to know the strength of the actual stitching so I should be testing the ACTUAL STITCHING.

So I did a few runs of various numbers and noticed that I was getting REAL LINEAR results. LIGHTBULB!!!

Before I went into production I realized I needed to set some standards that anyone could duplicate. I went to a sailmaker's supply shop and got some standard Dacron thread sizes but the predictability sucked - I'm guessing because the elastic and parallel fiber nylon floss equalizes and seats better. So that was my choice.

I get about 17-18 pounds pounds per stitch and on a bridle end or as a bridle that's roughly doubled. And that's plenty fine enough an increment - it's not a narrow target you're shooting for. 1.3 to 2.0 Gs - as long as you don't overload the release.

How could shear links be mass produced?

It's somewhat tedious hand stitching. I don't think there's a great way around that. But one per pilot is all you need - although I encourage people to have a spare (like I do). One person could flood the market in a few weeks of the winter.

What other questions should I be asking?

If you're curious... How we got into this bullshit mess to begin with. The history is fascinating - reads like everything the US did to give the Japanese a free pass on the morning of 1941/12/07.

As always thanks for your time.

As always thanks for your questions.

[Tad Eareckson]

Thought experiment...

Good thermal day, Dragonfly, 250 pound glider, dolly launch, ribbons along the runway, gun to your head, pick one:
A. 130 pound Greenspot and good two point release.
B. Two G weak link and your release is welded shut until you clear two hundred feet and the Dragonfly's mirror has been shot out.

Based upon what I've experienced, seen, read about...

I'd be HIGHLY tempted to go with B.

Both options can kill you but low level dolly launched aerotow lockouts with competent pilots, ribbons, and solid equipment are virtually nonexistent and, in any case, even if you're flying a good finger-on-the-trigger release, your chances of surviving a low level lockout are probably a LOT worse than surviving a tumble - with a hand deployed parachute - at altitude.

And with B you don't have to do multiple relaunches and landings which themselves are not by any means risk free and you actually get to have a shot at doing what you came to do instead of this...

http://ozreport.com/3.066
Weaklinks

Davis Straub - 1999/06/06

He was especially upset that I had written that I had doubled my weaklink after three weaklinks in a row had broken on me.

http://ozreport.com/9.011
2005 Worlds

Davis Straub - 2005/01/13

Tom Lanning had four launches, and two broken weaklinks and a broken base tube. He made it just outside the start circle.

http://ozreport.com/13.238
Adam Parer on his tuck and tumble

Adam Parer - 2009/11/25

Airborne Gulgong Classic
New South Wales

Due to the rough conditions weak links were breaking just about every other tow and the two tugs worked hard to eventually get everyone off the ground successfully.

http://ozreport.com/forum/viewtopic.php?t=24846
Is this a joke ?

Davis Straub - 2011/08/26 14:04:52 UTC

We had six weaklink breaks in a row at Zapata this year.

...kinda moronic bullshit.

I'm going with B. I suffered WAY too much with A throughout my entire previous aerotowing career.

[Steve Davy]

Is this what Zack has on the bottom?

http://www.flickr.com/photos/aerotowrelease/8313955230/


http://www.flickr.com/photos/aerotowrelease/8312907083/

[Tad Eareckson]

No, that's what I flew with on the bottom until 2006 when I both came up with the Shear Link concept and debunked the 130 pound Greenspot bullshit.

Then I thought I could execute the Shear Link concept on a two point bridle just by stitching Dacron leechline onto 3/16 inch Dacron line and the bottom end looked like:

http://www.flickr.com/photos/aerotowrelease/8306174203/


http://www.flickr.com/photos/aerotowrelease/8307233210/

But the bulge at the bottom was also present at the top:

http://www.flickr.com/photos/aerotowrelease/8305996819/


I thought I could get away with it but it wasn't long before I got my first ever wrap. Back to the drawing board but it was a pretty good demonstration of how effective the secondary weak link was at handling it - even at normal wave-off tension.

Then the Ribbon Bridle...

Bottom:

http://www.flickr.com/photos/aerotowrelease/8313782786/


http://www.flickr.com/photos/aerotowrelease/8312737353/

and top:

http://www.flickr.com/photos/aerotowrelease/8313607716/
http://www.flickr.com/photos/aerotowrelease/8313614876/



http://www.flickr.com/photos/aerotowrelease/8313617012/

At first I was too generous with the heat shrink and that irregularity in the stiffness soon earned me another demonstration of how effective the secondary weak link was at blowing off a wrapped bridle but after I minimized that length I had nothing but smooth feeds.

But when I was setting Zack up it dawned on me that the job could be done very well and very easily with a traditional but heavy string loop at the top of a spliced hollow braid bridle, no weak link on the bottom end - just a thimble in the eye, and a light Bridle Link geared only as a secondary weak link and not for one point towing. I'm pretty happy with it and that's how I'd set myself up if I were gonna do it again.

You can see Zack's setup at:

http://www.kitestrings.org/post248.html#p248

[Steve Davy]

First, thanks for the detailed explanations. I'm getting a pretty good understanding of what I'm looking at when I view your photos.

Second, if his primary fails and he has to use the secondary then that thimble has to go though the ring, correct?

Also, your setups seem to be based on a primary release that never fails and his is known to be problematic. In other words, if I was towing with release that had known problems I might start to question just how strong a link I would use. So I wonder if some other folks feel the same.

Could it be that getting releases that never ever fail and triggers on the base tube is the only (or at least the first step) way to get rid of "whip-stallers"?

[Tad Eareckson]

First, thanks for the detailed explanations.

Never happier than when I'm doing them. Beats the crap out of being totally ignored decade after decade.

I'm getting a pretty good understanding of what I'm looking at when I view your photos.

http://www.kitestrings.org/post38.html#p38

Zack C - 2011/01/10 14:28:40 UTC

When I first saw your release years ago on the Oz Report forum my impression was the same as most people's. I didn't know what the pictures were showing but it looked way more complicated than it needed to be. After seeing the problems that even the best releases on the market have and learning more about your release, however, I understand why you made it the way you did and the advantages it provides.

Second, if his primary fails and he has to use the secondary then that thimble has to go though the ring, correct?

Yeah, but then he wouldn't be using his secondary as a secondary - he'd be MISusing it as a BACKUP.

And while the thimble may not help any when you're misusing the secondary as a backup...

http://ozreport.com/forum/viewtopic.php?t=11591
Where to put the weaklink - the HGFA rules

Rohan Holtkamp - 2008/04/21

Once again history has shown us that this thread-through system can hook up and the hang glider remains being towed by the keel only, with the bridle well out of reach of even a hook knife. I know of just one pilot to survive this type of hook-up, took him some twelve months to walk again though.

...there's no guarantee that a bottom bridle end minus a thimble is gonna clear the ring EITHER. So that's not really a situation you can afford to get yourself into - even though Flight Park Mafia instructors will tell students, "The primary on the trainer doesn't work, so just use the backup." (And I've heard them do it.) I'd roll to blow the weak link high or have one hand on the secondary and the other on the parachute low. (Of course if I were low and NEEDED to release I wouldn't have that luxury.)

Also, your setups seem to be based on a primary release that never fails and his is known to be problematic.

Yes and yes. (How ya comin' with that hole, Zack?)

In other words, if I was towing with release that had known problems I might start to question just how strong a link I would use.

You well might.

You might EVEN have a light weak link blow at about the right time in a lockout to allow you to pull out after using up almost all of the 250 feet with which you started out.

And you might also:

- forget about all the people who ain't around no more when their releases jammed or they were unable to get to them in time and start thinking that you're gonna be able to get away with a roll of the dice next time too

- fail to consider what might happen when you're standing on your tail going up like a rocket in a thermal or dust devil and that same weak link figures you need the same kind of help it gave you last time

So I wonder if some other folks feel the same.

That's EXACTLY how about 99.5 percent of the brainwashed towing public feels - and have for the thirty years since Donnell tore up his copy of Manned Kiting and eliminated the whipstall from towing by a combination of royal decree and creative crash analysis.

The Flight Park Mafia sells them whatever shitrigged junk they're able to weld and velcro together, tells them it WILL fail on a regular basis but that's the best anyone can do after twenty years of nonstop engineering evolution, and assures them that they'll be just fine with a combination of 130 pound Greenspot, bent pin backups, and readily accessible hook knives.

And 99.5 percent of the towing public will believe this because they WANT TO; because no one in his right mind would ever connect himself to a towline if he really considered the reality of what he was doing.

Could it be that getting releases that never ever fail...

Tommy Crump - 1986/10

The release that I am using works every time and is mechanically sound. You need not have an additional release in case this one fails. A good mechanical release, under most all circumstances, is going to release every time. There are some things that you must rely on hundreds of thousands of times without failure. A release mechanism that is properly designed can do that.

...and triggers on the base tube...

Gregg B. McNamee - 1996/12

To actuate the primary release the pilot does not have to give up any control of the glider. (Common sense tells us that the last thing we want to do in an emergency situation is give up control of the glider in order to terminate the tow.)

If your system requires you to take your hand off the control bar to actuate the release it is not suitable.

...is the only (or at least the first step) way to get rid of "whip-stallers"?

- If you do those two things your chances of survival go up astronomically.
- But there are other reasonably good ways to get killed in towing with which the equipment on your end can't and won't help you.

You need a heavy weak link and somebody on the other end - also, if he's on a tug, with a heavy weak link - who isn't an idiot.

Manned Kiting
The Basic Handbook of Tow Launched Hang Gliding
1974

"A bad flyer won't hurt a pin man but a bad pin man can kill a flyer." - Bill Bennett

That's still true and Cragin can still go fuck himself. You don't want Rooney or anyone else from Ridgely or Quest on the front end. You want the ten year old kid who knows that when he sees the kite going down like a brick he should NOT let go of the string - he should run like hell into what's left of the wind until the lull's over.

And there will still always be stuff you can't afford to launch into if you wanna live - just like in slope launching.