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.