Use of weak links

On 2010/06/07 07:00 PM, Martin Gregorie wrote:
On Mon, 07 Jun 2010 08:42:16 -0700, Cats wrote:

On Jun 7, 3:41 pm, Martin
wrote:
On Mon, 07 Jun 2010 13:54:50 +0200, Andreas Maurer wrote:
On Mon, 07 Jun 2010 05:20:48 -0500, brian whatcott
wrote:

Brian wrote:
On Jun 6, 5:54 pm, John wrote:
Brian wrote:
True the 90deg AOA was justa theoritical way to look at the
loads on a stalled wing.
As soon as the wing stalls, the load decreases.

Yes it decreases, but there is some load on it still. Even a
stalled wings produces some lift just not nearly as much as a
flying wing.

Brian C.

How about drag?

Forget the drag - you cannot go fast enough with 90 degrees AoA to
produce any significant amount of drag.

There's another point that hasn't been mentioned - many winches can't
pull in cable at anything like Vwinch for most gliders, e.g. most
single seaters have a Vwinch of 65kts or higher but I'm told our
Supacat's maximum cable speed is 55 kts.

This doesn't mean that it can't break gliders: try this thought
experiment. Launch without a weak link, wait until the glider is at
60-70 degrees from the winch and ramp the winch up to full power to
cause a gross overspeed. This is the situation that will put maximum
load on the wings and it is the situation where the weak link is
designed to fail.

At least one of these things is very likely to happen: (1) the pilot
pulls the bung
(2) a back release, if the glider gets far enough overhead of the winch
(3) the hook gets pulled out of the glider
(4) the wings break
(5) the cable snaps

arranged in increasing severity and (my guess) decreasing probability
since I think that one of the less severe events will happen before the
more severe ones.
snip

Think I'd place cable snaps at #3, hook gets pulled out at #4 and wings
break at #5 on the grounds that we practise cable breaks, the hook
pulling out shouldn't affect *this* flight and wings breaking certainly
will.

I was being pessimistic and assuming a new 3/16" steel cable is being
used and made a guestimate that a new cable is stronger than a set of
used glider wings. I'd certainly expect the hook to pull out before the
spars snapped. I've taken my hook out for a thorough clean-up and have a
pretty good idea of what its bolted to!

I'm also assuming that a less severe event occurrence will reduce the
load and prevent the more severe events from happening. e.g. pulling the
hook out will prevent the spar or cable from breaking.


Interestingly we actually had some engineer members intrigued by this go
and measure the ultimate strength of the winch cable in use.

So - dyneema is another case entirely, and I have no data for
multistrand cable joined with ferrules - but 2 metric ton breaking
strain single strand wire broke reliably at ~900kg at the knot joining
it to the ring on the weak link.

Ergo it is pretty redundant putting a black Tost weak link on.

Not suggesting that no weak link is a good idea, just that the chances
of simple mechanical failure on the cable is higher than you think.
Especially after the wire gets abraded and work hardened.

If you work out the weight of the cable hanging from the hook - it will
seldom exceed 150Kg. Most of the force vector on the hook is from the
winch pulling. In the limiting case this is pulling straight down. At
this point the winch effectively becomes a sling load. I have witnessed
a ham fisted combination of winch driver and Blanik L13 pilot do this.
The vertical component was sufficient to momentarily lift one side of
the winch off the ground. Clear air under the wheel and both jacks.

The Blanik was no doubt way above Vwinch and pulling back heroically to
slow things down.

The resultant massive birdsnest came from the cable failing at the
attachment to the yoke - not from the blue weak link.

--- news://freenews.netfront.net/ - complaints: ---

On Jun 7, 6:53*pm, brian whatcott wrote:
Martin Gregorie wrote:

/snip/



I was being pessimistic and assuming a new 3/16" steel cable is being
used and made a guestimate that a new cable is stronger than a set of
used glider wings. I'd certainly expect the hook to pull out before the
spars snapped. I've taken my hook out for a thorough clean-up and have a
pretty good idea of what its bolted to!

I'm also assuming that a less severe event occurrence will reduce the
load and prevent the more severe events from happening. e.g. pulling the
hook out will prevent the spar or cable from breaking.

Strength, according to this supplier:
Long URL:http://www.ematerialhandling.com/mat...less_steel/cab...

TinyURL format:http://tinyurl.com/6ztmkh

3/16 diam. stainless cable

Construction: 1X19
Material: * * * * * 304 * * * * 316
Weight/1000ft * * * *72 lb
wkg load limit * * *940 lb * * *854
breaking load * * *4700 lb * * 4270

Construction: 7X7
Weight/1000ft * * * * 62 lb
wkg * * * * * * * * *740 * * * *670
brk * * * * * * * * 3700 * * * 3350

Construction: 7X19
Wt/1000ft * * * * * * 65 lb
wkg * * * * * * * * *740 * * * *642
brk * * * * * * * * 3700 * * * 3210

(I believe galvanized is usually a little stronger?)

Supposing that sailplanes are designed to meet 6g limit loads,
the most flexible 316 stainless 7X19 cable would break
* when transmitting a 6g load to an airframe capable
of reacting over 535 lb at 1g * (6 X 535 = 3215 lb)

Brian W

Brian,

1x19 is seldom used, if ever. 7/7 5/32" (4.8lbs/100ft) or
3/16" (6.1lbs/100ft), or metric equivalent, is most commonly used
steel wire rope. Typically the product known as Galvanized Aircraft
Steel Wire Rope. A few places use solid wire. Never heard of
stainless in use, likely due to cost. Tends to be more expensive and
more brittle than the other. Rule of thumb for fatigue reduction is
roller/guide radius 60x the wire diameter. However, the SSA plan
underwriter (last I heard) is not providing liability insurance upon
renewal for use of steel wire rope. Synthetic Dyneema or Spectra
based rope products (or heavier poly ropes in some cases) are the
used. They are as strong or stronger than steel for the same
diameter, easily spiced, have no recoil and less mass so are more
safely handled, and have high UV tolerance. I believe Amsteel Blue
to be the superior product considering strength and longevity. It's
at least 4x the price of steel wire rope.

Rule of thumb, apart from the manufacturers limits, for winch weak
links is 1.3 x AUW. This is close to the charts, see the datasheets
link below.

See
http://www.gliding.co.uk/bgainfo/saf...hlaunching.htm

and the data sheets for glider by make/model

http://www.gliding.co.uk/bgainfo/tec...datasheets.htm

This is UK info, but quite useful.

Frank W

On Jun 7, 10:38*pm, Bruce wrote:
On 2010/06/07 07:00 PM, Martin Gregorie wrote:

On Mon, 07 Jun 2010 08:42:16 -0700, Cats wrote:

On Jun 7, 3:41 pm, Martin
wrote:
On Mon, 07 Jun 2010 13:54:50 +0200, Andreas Maurer wrote:
On Mon, 07 Jun 2010 05:20:48 -0500, brian whatcott
*wrote:

Brian wrote:
On Jun 6, 5:54 pm, John *wrote:
Brian wrote:
True the 90deg AOA *was justa theoritical way to look at the
loads on a stalled wing.
As soon as the wing stalls, the load decreases.

Yes it decreases, but there is some load on it still. Even a
stalled wings produces some lift just not nearly as much as a
flying wing.

Brian C.

How about drag?

Forget the drag - you cannot go fast enough with 90 degrees AoA to
produce any significant amount of drag.

There's another point that hasn't been mentioned - many winches can't
pull in cable at anything like Vwinch for most gliders, e.g. most
single seaters have a Vwinch of 65kts or higher but I'm told our
Supacat's maximum cable speed is 55 kts.

This doesn't mean that it can't break gliders: try this thought
experiment. Launch without a weak link, wait until the glider is at
60-70 degrees from the winch and ramp the winch up to full power to
cause a gross overspeed. This is the situation that will put maximum
load on the wings and it is the situation where the weak link is
designed to fail.

At least one of these things is very likely to happen: (1) the pilot
pulls the bung
(2) a back release, if the glider gets far enough overhead of the winch
(3) the hook gets pulled out of the glider
(4) the wings break
(5) the cable snaps

arranged in increasing severity and (my guess) decreasing probability
since I think that one of the less severe events will happen before the
more severe ones.
snip

Think I'd place cable snaps at #3, hook gets pulled out at #4 and wings
break at #5 on the grounds that we practise cable breaks, the hook
pulling out shouldn't affect *this* flight and wings breaking certainly
will.

I was being pessimistic and assuming a new 3/16" steel cable is being
used and made a guestimate that a new cable is stronger than a set of
used glider wings. I'd certainly expect the hook to pull out before the
spars snapped. I've taken my hook out for a thorough clean-up and have a
pretty good idea of what its bolted to!

I'm also assuming that a less severe event occurrence will reduce the
load and prevent the more severe events from happening. e.g. pulling the
hook out will prevent the spar or cable from breaking.

Interestingly we actually had some engineer members intrigued by this go
and measure the ultimate strength of the winch cable in use.

So - dyneema is another case entirely, and I have no data for
multistrand cable joined with ferrules - but 2 metric ton breaking
strain single strand wire broke reliably at ~900kg at the knot joining
it to the ring on the weak link.

Ergo it is pretty redundant putting a black Tost weak link on.

Not suggesting that no weak link is a good idea, just that the chances
of simple mechanical failure on the cable is higher than you think.
Especially after the wire gets abraded and work hardened.

If you work out the weight of the cable hanging from the hook - it will
seldom exceed 150Kg. Most of the force vector on the hook is from the
winch pulling. In the limiting case this is pulling straight down. At
this point the winch effectively becomes a sling load. I have witnessed
a ham fisted combination of winch driver and Blanik L13 pilot do this.
The vertical component was sufficient to momentarily lift one side of
the winch off the ground. Clear air under the wheel and both jacks.

The Blanik was no doubt way above Vwinch and pulling back heroically to
slow things down.

The resultant massive birdsnest came from the cable failing at the
attachment to the yoke - not from the blue weak link.

--- news://freenews.netfront.net/ - complaints: ---

Stainless, copper, and aluminum sleeves can produce 100% strength if
machine swaged and 95% when properly hand swaged. They are swaged in
pairs about 12 inches apart so slippage can be seen visually. I've
had the back end of an XJ-6 Jag lifted off the ground by an L-13 on
auto tow and one side of a winch lifted off the ground by an L-23.
IIRC, red links in use on both.

Frank Whiteley

On Jun 8, 5:49*am, Frank Whiteley wrote:
On Jun 7, 6:53*pm, brian whatcott wrote:





Martin Gregorie wrote:

/snip/

I was being pessimistic and assuming a new 3/16" steel cable is being
used and made a guestimate that a new cable is stronger than a set of
used glider wings. I'd certainly expect the hook to pull out before the
spars snapped. I've taken my hook out for a thorough clean-up and have a
pretty good idea of what its bolted to!

I'm also assuming that a less severe event occurrence will reduce the
load and prevent the more severe events from happening. e.g. pulling the
hook out will prevent the spar or cable from breaking.

Strength, according to this supplier:
Long URL:http://www.ematerialhandling.com/mat...less_steel/cab...

TinyURL format:http://tinyurl.com/6ztmkh

3/16 diam. stainless cable

Construction: 1X19
Material: * * * * * 304 * * * * 316
Weight/1000ft * * * *72 lb
wkg load limit * * *940 lb * * *854
breaking load * * *4700 lb * * 4270

Construction: 7X7
Weight/1000ft * * * * 62 lb
wkg * * * * * * * * *740 * * * *670
brk * * * * * * * * 3700 * * * 3350

Construction: 7X19
Wt/1000ft * * * * * * 65 lb
wkg * * * * * * * * *740 * * * *642
brk * * * * * * * * 3700 * * * 3210

(I believe galvanized is usually a little stronger?)

Supposing that sailplanes are designed to meet 6g limit loads,
the most flexible 316 stainless 7X19 cable would break
* when transmitting a 6g load to an airframe capable
of reacting over 535 lb at 1g * (6 X 535 = 3215 lb)

Brian W

Brian,

1x19 is seldom used, if ever. *7/7 5/32" (4.8lbs/100ft) or
3/16" (6.1lbs/100ft), or metric equivalent, is most commonly used
steel wire rope. Typically the product known as Galvanized Aircraft
Steel Wire Rope. *A few places use solid wire. *Never heard of
stainless in use, likely due to cost. *Tends to be more expensive and
more brittle than the other. *Rule of thumb for fatigue reduction is
roller/guide radius 60x the wire diameter. *However, the SSA plan
underwriter (last I heard) is not providing liability insurance upon
renewal for use of steel wire rope. *Synthetic Dyneema or Spectra
based rope products (or heavier poly ropes in some cases) are the
used. *They are as strong or stronger than steel for the same
diameter, easily spiced, have no recoil and less mass so are more
safely handled, and have high UV tolerance. * I believe Amsteel Blue
to be the superior product considering strength and longevity. *It's
at least 4x the price of steel wire rope.

Rule of thumb, apart from the manufacturers limits, for winch weak
links is 1.3 x AUW. *This is close to the charts, see the datasheets
link below.

Seehttp://www.gliding.co.uk/bgainfo/safety/safewinchlaunching.htm

and the data sheets for glider by make/model

http://www.gliding.co.uk/bgainfo/tec...datasheets.htm

This is UK info, but quite useful.

Frank W- Hide quoted text -

- Show quoted text -

What's wrong with UK data? We do more than 2/3rd of our glider
launches by winch, so hopefully we know what we are doing by now!

Derek Copeland

It is better not to use a reserve weak link. This requires a doubled
link which have to be of different designs, The main link has two
round holes and the reserve has one slightly slotted hole. We had a
fatality in the UK where somebody inadvertantly fitted two main links
to the carrier, which effectively doubled the strength of the weak
link, and then a glider had a mainspar failure during the launch on a
rather windy rough day. See:

http://www.aaib.gov.uk/sites/aaib/pu...eon_502118.cfm

There was some evidence that the wood/metal composite mainspar was
slightly corroded in this glider, but it is still unlikely that it
would have failed if launched on the correct weak link.

If you want to avoid weak link breaks using a single link, perhaps you
should change the link after 100 launches, or monitor its condition.
The Tost links have a hole drilled in them, like a tensile test piece,
and this will show signs of distorting or cracking before it fails.

Derek Copeland


On Jun 2, 2:53*pm, "Tim Mara" wrote:
unbelievable! especially since a weak link is such an easy thing to add,
especially for ground launching!
you might be quite amazed at the numbers of pilots who call me asking what
the "proper weak link" should be when it is such a fundamental question and
one that every prospective student pilot needs to know even before taking a
practical test for a private pilot certificate!

using the TOST weak links is easy and the reserve weak link makes a lot of
sense since ALL weak links fatigue in time and become weaker before actually
breaking.the idea of the reserve weak link is so that when this happened and
the main weak link fails from fatigue the reserve link will hold under
normal loads and the launch is still completed without an actual failure but
the broke link is then visible for the next launch (we check these before
each launch .right? ((you are supposed to) and you know then to replace the
broken link before continuing with another launch. simple to use, and even
simpler to change from one weak link strength to another as needed by using
the notch connectors or similar attachment.you can see more on my website
pagehttp://wingsandwheels.com/page30.htm

regards
Tim Mara

Please visit the Wings & Wheels website atwww.wingsandwheels.com

"T8" wrote in message

...
On Jun 2, 4:43 am, Paul wrote:





Currently the club that I am part of don't seem to be too concerned about
the safety aspects of using the correct weak links for winch launching
and I'd like to know how safe it is.

Currently I see two issues with the winch launching:

1. Sometimes a weak link is not used at all. The gliders are attached
directly to the winch cable (via drogue chute and various connectors).

2. When a weak link is included in the cable (only a primary link - no
reserve link) it is used for all glider types (G102 Astir, G103 Twin II,
ASW20, LS4) and I'm not even sure what the breaking strain is. My guess
is it's a Tost #1 black (1000 daN) weak link but that's hard to tell
because its weathered and the protective sleeve obscures the Tost number.