Winch Launch Stresses on Vintage Gliders

Double weak links???? Hmmm. This has the sound of "Let's put a 20 amp
fuse in there, instead of the 10 amp one that the manual calls for, just
in case the 10 amp one might blow."



Does this strike anyone else as dubious? Why use classified weak links
at all if you are going to ignore the breaking point of a particular
unit?



Larry









"Derek Copeland" wrote in
message :

Re the wing bending thing, almost the same happens
with ASW20's, which only have a 15 m wingspan. Neatly
demonstrates the forces involved.

I know of only one case (unfortunately fatal) where
a vintage glider has suffered a structural failure
during a winch launch. An investigation of the wreckage
revealed a somewhat corroded aluminium/wood laminated
mainspar. An additional factor was that it was launched
on a rather windy and turbulent day using identical
doubled up white weak links with round holes, which
effectively doubled the strength of the weak link.
Some clubs use double weak links to reduce the chances
of a cable break, but one of the links should have
a slotted hole so that it only takes over when the
first link breaks.

Derek Copeland

At 13:18 25 January 2006, Martin Gregorie wrote:
Bruce wrote:
The weak link is designed to limit the wing root bending
moment to a
safe number. (Equivalent to 2-2.5G without gust loads)
As previously
posted, there is no G relief on a winch launch, so
the strength of a
high aspect ratio glider weak link will generally
be a lower fraction of
MTOW than for a low AR wing. RTFM, do not assume.

As a slightly OTT sidelight on that, have you ever
seen an ASH-25 being
winched? Its an amazing sight. Its lucky there is no
Velcro on the tips
or it would be in big trouble after release.

The strength of the weak link is not 2xMTOW, it is
the limiting force
that will not exceed the safe structural strength
of the airframe when
the aircraft is at it's maximum winch speed, maximum
coeficient of lift,
and the cable is acting at it's designed release angle
(87 degrees if I
recall correctly)These days this takes into account
the 'standard'
vertical gusts.

In pole-bending pilot terms, for a modern glider Vwinch
and the weak
link strength are set so it can withstand a 'standard'
vertical gust if
the glider is at the top of the cable, carrying the
full cable weight,
at Vwinch and the pilot has the stick hard back. I
don't do that to a
JAR-22 glider (besides, the Discus I tail stalls first)
and would
certainly not to it to a vintage glider.

It is standard practice to use two weak links in parallel. One has slotted
holes, so there is no strain on it unless the primary weak link fails. If
there is a true overload, both weak links fail. However, if the primary
weak link fails due to fatigue, but the actual forces are below the weak
link limits, the secondary weak link does not fail, preventing an
unnecessary launch failure.

However, if you screw up and use two identical weak links (either both with
regular holes, or both with slotted holes) you have the problem you alluded
to, which is that you effectively have a single weak link with double the
intended strength.

Mike Schumann

Agreed, however we have found that gust loads that break one, generally
break two, and we've taken to a policy of using only one TOST dog bone
at a time. Recently at the beginning of the day we found someone had
assembled a TOST assembly with two dog bones. The chief instructor
took it apart and found two round holed links. We think a single dog
bone of the proper rating is sufficient.

Frank Whiteley

Is it possible to see that one is slotted and one round holed when the
cable is ready to be hooked up to the glider? Surely the pilot must
be given the opportunity to inspect the links before accepting the
cable. It's a while since I winched and we used rope weak links then.

Andy

Andy, that's a good question. Actually, as far as I know, there isn't a way
to visually inspect then once they are assembled into the protective case.
One wonders why.

It would be easy to dip one end of the slotted links in, say, tan paint.
Seeing a link assembly with two white dogbones but with one showing a tan
end would indicate that the link had been assembled correctly.

Bill Daniels


"Andy" wrote in message
oups.com...
Is it possible to see that one is slotted and one round holed when the
cable is ready to be hooked up to the glider? Surely the pilot must
be given the opportunity to inspect the links before accepting the
cable. It's a while since I winched and we used rope weak links then.

Andy

Andy wrote:
Is it possible to see that one is slotted and one round holed when the
cable is ready to be hooked up to the glider?

Easy: Pull at both ends. If everything is correct, then one link is now
fixed while the other can be moved freely.

And that's the idea of the arrangement: Over time, the weak link will
stretch, even when used within its load limit, and therefore weaken. In
other words, after some time, the link will be weaker than advertised.
The second link however will keep his strengh, as it never takes load.
So when the first link breaks prematurely, the second will still hold
the advertized load.

When you pull at both ends and one link can't be moved freely under the
load, then the first link has stretched and must be changed.

Stefan

Derek Copeland wrote:
Re the wing bending thing, almost the same happens
with ASW20's, which only have a 15 m wingspan. Neatly
demonstrates the forces involved.

I know - I had one - and winching is my preferred launch method.

--
martin@ | Martin Gregorie
gregorie. |
org | Zappa fan & glider pilot

We use a very similar system except the fitting is
L shaped, enough to protect the link but allowing easy
inspection. It is important to remember that the weak
link only works at it's rated value when new and the
force required to break it reduces over the number
of launches. Having the L shape protector allows easy
inspection of the link for necking or elongation of
the holes, both indicate a 'worn' link.
In practice links are used until they break, which
they inevitably will do if not changed.




At 16:30 29 January 2006, Robin Birch wrote:
We use a system where the weak link is held in an H
channel with one end
hole drilled and the other slotted so that the end
bolt is supported but
can pull out when the link breaks.

This gives a good protection to the link. We found
that the metal
shrouds bent and also you couldn't see the link colour.
With the H
channel you can see the whole link.

Robin

In message , Bill Daniels
writes
Andy, that's a good question. Actually, as far as
I know, there isn't a way
to visually inspect then once they are assembled into
the protective case.
One wonders why.

It would be easy to dip one end of the slotted links
in, say, tan paint.
Seeing a link assembly with two white dogbones but
with one showing a tan
end would indicate that the link had been assembled
correctly.

Bill Daniels


'Andy' wrote in message
groups.com...
Is it possible to see that one is slotted and one
round holed when the
cable is ready to be hooked up to the glider? Surely
the pilot must
be given the opportunity to inspect the links before
accepting the
cable. It's a while since I winched and we used rope
weak links then.

Andy




--
Robin Birch

I don't disagree that they'll eventually break, but we've made many
hundreds of launches before a fatigue. When we have broken two at a
time due to a gust load, we replaced them with two new ones. Next
launch we broke both again. We were launching into strong winds with a
shear about 500ft. Over time we have broken the wire much more often
than a weak link failure. Since we train for the proper response to a
launch failure, it no longer makes much sense to use up two at a time.
It does eliminate the potential for mis-assembly.

Frank