Mike,
And here's how the discussion went in the American theocracy - nothing dramatized, actual names.
From:
Manned Kiting
The Basic Handbook of Tow Launched Hang Gliding
Daniel F. Poynter
1974
The tow line should be about 125 feet long and made of 2000+ lb. material such as 3/8" polypropylene.
Often a flyer who appears to be in a hopeless position at 6', recovers and continues to fly.
The boat should not fly him wide open because a burst of speed may be needed to pick him up at one point.
A knot was not removed from the tow rope and on the second flight, it failed. The kite looped and folded around the pilot.
"The greatest dangers are a rope break or a premature release." - Richard Johnson
Broken ropes have put many kite flyers in the hospital and a few in the ground.
Donnell Hewett - 1980/12
Skyting requires the use of an infallible weak link to place an absolute upper limit to the towline tension in the unlikely event that everything else fails.
Now I've heard the argument that "Weak links always break at the worst possible time, when the glider is climbing hard in a near stall situation," and that "More people have been injured because of a weak link than saved by one." Well, I for one have been saved by a weak link and would not even consider towing without one. I want to know without a doubt (1) when I am pushing too hard, and (2) what will break when I push too hard, and (3) that no other damage need result because I push too hard.
Furthermore, I will not use a mechanical weak link no matter how elaborate or expensive because there is always the possibility that it may fail to operate properly. In skyting we use a simple and inexpensive strand of nylon fishing line which breaks at the desired tension limit. There is no possible way for it to jam and fail to release when the maximum tension is exceeded. Sure, it may get weaker through aging or wear and break too soon, but it cannot get stronger and fail to break. If it does break too soon, so what? We simply replace it with a fresh one.
A properly designed weak link must be strong enough to permit a good rate of climb without breaking, and it must be weak enough to break before the glider gets out of control, stalls, or collapses. Since our glider flies level with a 50 pound pull, climbs at about 500 fpm with a 130 pound pull, and retains sufficient control to prevent stalling if a weak link breaks at 200 pounds pull, we selected that value. Of course, a pilot could deliberately produce a stalled break at 200 lbs, just as he can stall a glider in free flight. But if he is trying to limit his climb rate and the forces exceed the break limit, the glider simply drops its nose to the free flight attitude and continues flying. If the weak link breaks (or should the towline break) at less than the 200 pound value, the effect is even less dramatic and controlled flight is still present.
Most people are amazed at how small a string is needed for the weak link of a tow system. In fact, many people upon seeing it in operation for the first time make a comment something like "Don't you need something a little stronger than that? It's going to break!" But, of course, that's the whole point, it's supposed to break. And in order to break at about 200 lbs, it needs to be a single strand (loop) of No. 21 or 24 size nylon cord or a double strand (loop) of No. 12 or 15 size. For our glider we have found through trial and error that a loop of No. 18 braided nylon twine is ideal. A single strand of this twine is rated at 140 lb breaking strength, so a double strength loop should break at 280 pounds. In practice, we have found that because of the knots, it actually breaks at about 200 lbs when tied in a loop and attached to the towline.
Although we suspect that the same weak link would work well with other gliders, we have not had the opportunity to run tests on other gliders to verify this suspicion. Until such tests can be run, we strongly recommend that considerable caution be exercised in determining the correct weak link for any other glider. One should start with a line that definitely breaks too soon, gradually increasing the strength until a point is reached where the glider is able to climb at a good rate without breaking the weak link, but where no stall occurs when the weak link does break.
Obviously, more work needs to be done in this area, but even so, we have found our current system to be quite satisfactory and able to provide the necessary tension limiting and regulation needed for safe and enjoyable flight under tow.
WORK? Not just WORK but MORE WORK? Get real dude - we're pretty wiped already just from skimming those paragraphs. And if it seems to have worked for a few flights for a 150 pound Hang One physics professor on a 50 pound Sandpiper second generation Rogallo it's obviously good enough for people who can't handle grade school arithmetic. Any objections? Two hundred pounds it is then.
"Well, only tested with two flights. On the first flight it operated perfectly, and on the second something jammed up on a ring and the guy was killed."
How did it do on the third?
http://www.hanggliding.org/viewtopic.php?t=16384
Tow Release Malfunction
Mike Lake - 2010/03/29 20:05:42 UTC
The cool guy in the video had plenty of time but Murphy says that this might not always be the case. Make no mistake this was 50% of a fatality.
http://www.sahga.com/forum3/viewtopic.php?f=1&t=2125
Shane Smith has passed away
John Lowery - 2011/01/16 03:19 UTC
Tucson
This is just heartbreaking. Shane Smith passed away today in a hang gliding accident while scooter towing at Phoenix Regional Airport.
http://www.hanggliding.org/viewtopic.php?t=21033
barrels release without any tension except weight of rope..
Bart Weghorst - 2011/02/25 19:06:26 UTC
Cowboy Up Hang Gliding, Jackson Hole
Most of the time. But I've had it once where the pin had bent inside the barrel from excessive tow force. My weaklink was still intact. The tug pilot's weaklink broke so I had the rope. I had to use two hands to get the pin out of the barrel.
No stress because I was high.
And here we go again. Anybody wanna start a pool on the date?
"Score that release as 'average' then"
http://www.hanggliding.org/viewtopic.php?t=14230
pro tow set-up
Jim Rooney - 2009/11/02 18:58:13 UTC
Oh yeah... an other fun fact for ya... ya know when it's far more likely to happen? During a lockout. When we're doing lockout training, the odds go from 1 in 1,000 to over 50/50.
http://www.hanggliding.org/viewtopic.php?t=16384
Tow Release Malfunction
bisleybob - 2010/03/26 19:08:27 UTC
now imagine a release that can never in no way jam up
Jim Rooney - 2010/03/26 20:54:43 UTC
Dude, quit bogarting that stuff ;)
How's it go? Never say never.
Bent pin releases are indeed very very reliable. But 100%? Nope. It's exceptionally rare, but they jam. All mechanical things do.
If they were perfection, everyone would be using them. They're not. As with all things, they are a tradeoff. Having that big ole chunk of metal on my sternum as I depart a launch dolly, just a couple feet off the ground, is not my idea of a good situation to be in.
What do they call them again? "Chest Crushers"?
So, good? Yes.
Perfect? No.
You say "average", we say "very very reliable". Tomatoes, tomahtoes.
And somehow that nice round, best guessed number stuck.
How odd. Precisely what that nice round pin that twenty years ago Bobby grabbed off the table a few feet away does when you put it inside a nice straight scrap of tubing. A round peg in a square hole - literally.
How could ANYONE in the early '80s do anything other than make a best guess?
By understanding what the German big glider boys had been doing since the early '20s?
How is any value likely to be relevant 30 years on?
The Brooks Bridle seems to have held up pretty well over the same period. Would've let Shane go back home happy 46 days ago.
Why 'never to be exceeded'.
It's a RULE. In a civilized society you hafta have RULES. Without rules we revert to the jungle. If it says "never to be exceeded" you don't exceed it. You exceed it you're one step down the slippery slope leading to chest crushers, necrophilia, cannibalism, and medical marijuana.
And you Tad might now be flying with your buddies giving them a never ending stream of abuse because they insist on using sh** rig releases.
I might have been flying a little while longer. But eventually I would've ended up the same way over very very reliable bent pin releases.
Zack,
Glad to have you with us, Mike.
Major honor and privilege actually. We owe Mike and a few of his crowd BIG TIME. And we need to lean on him to get down as much of that sort of history as possible.
Given your above post, I would now think you wouldn't have any problems with our surface tow weak links.
As you noted, I didn't have a significant problem with them then. And if they hadn't banned me I'd have already gone back and told them they were actually doing things better than I was thinking at the time.
But I really doubt that anybody really understood WHY what they were doing was pretty optimal and I notice that nobody engaged me to justify it. If you can't explain something all it is is another dime a dozen hang gliding OPINION. Yeah, that's the way the Houston guys do it. In Austin everybody goes three hundred. Tomatoes, tomahtoes.
I still believe the acceptable breaking range of weak links is quite large...
As do I and have so said many times.
But that said, I still think 600 lbs is pushing it. I've had 600 lb weak links break and it makes me very uncomfortable being anywhere near that tension.
Just 'cause you HAVE six hundred pounds doesn't mean you have to USE six hundred pounds. Just 'cause you HAVE a Porsche 911 Turbo doesn't mean you have to stomp it to the floor every time you go to the grocery store. The afterburners on your F-18 are there for if or when you NEED them.
If you don't wanna do six hundred pounds everybody has at least one piece of equipment he can use to stop doing six hundred pounds. And you also have a radio so it needn't be all or nothing.
But if you ever get into a Dennis Pagen or Arlan Birkett / Jeremiah Thompson situation I one hundred percent guarantee you your gonna be EXTREMELY uncomfortable NOT having all of those six hundred pounds at your disposal - even if the majority of them are still being held in reserve.
Even though we've done a bazillion tows without ever having a structural failure, I guess I'll always be paranoid. But I see no problem using a 400 lb weak link to address that fear.
If you're towing in fear of the difference between four and six hundred pound weak links you should be scared shitless of what happens between the time you release and land in thermal conditions. Do you do moderate aerobatics? If not start doing some wangs on the way down after the lift has switched off. Get a feel for what the glider can do no sweat. And remember that there's no regulation that requires that you have to just hang around doing nothing while the tension builds up to that point.
If I ever meet up with Steve Pearson again I'll try to find out if he has any idea just how much line tension gliders can take.
You don't need to. The glider doesn't know the difference between flavors of pounds delivered to its hang point. They can come as a consequence of pilot weight, pilot weight multiplied by a centrifugal factor, and/or tow tension. You have at least eighteen hundred pounds in your pocket and you can spend it however you want. And if you have a six hundred pound weak link, hook in at two hundred, and pick up an extra G in a hard turn away from tow you still have half of your spending money in reserve.
Another thing I've always wondered: we always talk about G loading as a scalar, but how does the direction of the tow force affect the glider's load capacity? Surely 600 lbs pulling straight down isn't going to affect the glider the same way as 600 lbs pulling forward...
The glider doesn't know anything about up, down, forward, or backwards. All it knows is the resultant of the vectors it's feeling at its hang point. If you're pulling straight down it's gonna think it has an eight hundred pound pilot. If you're pulling straight ahead it's gonna think it has a 632 pound pilot and the earth has shifted 71.6 degrees forward from where you're looking and will adjust it's trim (pitch up) correspondingly. If you're pulling straight aft... Never mind, you really don't wanna let yourself get into a situation to demonstrate what happens.
...(or backward in the case of the tug).
You can't worry about the tug. Even that ratty little loop of 130 that wouldn't even let you follow him through a normal easy turn on 2010/11/27 can kill him (or you) in other circumstances. That's why HE has a release he doesn't have to reach for. He's ALWAYS gonna be better off without us but we're the reason he even exists and we're paying him to get us going. If he's counting on a weak link of any rating to keep him safe he doesn't know what he's doing or talking about and shouldn't be up there.
I noticed last weekend our tug had six strands of 130 lb Greenspot instead of four of 205 leech as Gregg reported.
That's a stupid way to do things. I'm not crazy about double loops and TRIPLES? You're begging to trash your predictability. I could test it but I'm not sure what that would prove 'cause the strands all have to be loaded evenly. I'd go with my Shear Link. That's close to the top of the range of what I can do but still inside enough to make me comfortable. Tost doesn't hit close to six hundred but Ken Hammond could undoubtedly provide an insert that hits real close to whatever number you give him.
