Winch Launch Stresses on Vintage Gliders

In theory the weak link is set to break at about twice
the maximum gross weight of the glider, but remember
that the glider also has to lift the weight of the
cable, the cable pull is a point load through the centre
of the fuselage, and if you hit a gust there may be
a momentary overload before the link breaks. As I said
before, use the correct weak link and don't exceed
the placarded maximum winch speed (lift, and therefore
stress, increases with the square of the airspeed).
Then you have two safety factors on your side.

Derek Copeland

At 01:06 24 January 2006, Brian wrote:
Someone please correct me if I am wrong,

But as I recall the weak link is supposed break before
it carries twice
the gross weight of the glider.
So during the launch the wings must lift at a maximum
the weight of the
glider plus up to 2 times weight of the glider through
the weak link.
So the maximum load you should be able to put on the
glider is 3G's.

I don't think I would want to fly a glider that could
not withstand
3G's.

Brian

Not really the same as pulling 3 'G'. If you take
an example of a glider where the total weight is say
700 lbs, of which the non-lifting surfaces (ie everything
but the wings) weighs 400 lbs, the 1 'G' load on the
wing roots (wing root bending load) is 400 lb. When
the weak link is close to breaking, at 1400 lb (twice
the weight of the glider), the total load on the wing
roots is 1800 lb, a wing root bending load equivalent
to pulling 4 1/2 'G' in free flight, somewhat narrower
margins.

I have chosen a fairly benign case, if someone takes
the time to go through the actual figures for a few
different types, I'm sure they will find some worse
examples than my (hypothetical) one.

At 01:06 24 January 2006, Brian wrote:
Someone please correct me if I am wrong,

But as I recall the weak link is supposed break before
it carries twice
the gross weight of the glider.
So during the launch the wings must lift at a maximum
the weight of the
glider plus up to 2 times weight of the glider through
the weak link.
So the maximum load you should be able to put on the
glider is 3G's.

I don't think I would want to fly a glider that could
not withstand
3G's.

Brian

Yes,
Remember that on the winch there is no "G" relief of the increased lift
from the wings as you pull the G. This is worth between 1 and 1.5 G
which is why speed control in terms of slow is important as the stall
speeds are increased appropriately. It is also one of the reasons why
over speed can become dangerous far more quickly at the top of the wire
than in a dive.

Robin

In message , Chris Rollings
writes
Not really the same as pulling 3 'G'. If you take
an example of a glider where the total weight is say
700 lbs, of which the non-lifting surfaces (ie everything
but the wings) weighs 400 lbs, the 1 'G' load on the
wing roots (wing root bending load) is 400 lb. When
the weak link is close to breaking, at 1400 lb (twice
the weight of the glider), the total load on the wing
roots is 1800 lb, a wing root bending load equivalent
to pulling 4 1/2 'G' in free flight, somewhat narrower
margins.

I have chosen a fairly benign case, if someone takes
the time to go through the actual figures for a few
different types, I'm sure they will find some worse
examples than my (hypothetical) one.

At 01:06 24 January 2006, Brian wrote:
Someone please correct me if I am wrong,

But as I recall the weak link is supposed break before
it carries twice
the gross weight of the glider.
So during the launch the wings must lift at a maximum
the weight of the
glider plus up to 2 times weight of the glider through
the weak link.
So the maximum load you should be able to put on the
glider is 3G's.

I don't think I would want to fly a glider that could
not withstand
3G's.

Brian






--
Robin Birch

"Brian" wrote in message
ups.com...
Someone please correct me if I am wrong,

But as I recall the weak link is supposed break before it carries twice
the gross weight of the glider.
So during the launch the wings must lift at a maximum the weight of the
glider plus up to 2 times weight of the glider through the weak link.
So the maximum load you should be able to put on the glider is 3G's.

I don't think I would want to fly a glider that could not withstand
3G's.

Brian


The 2X GW weak link figure is from the US FAR's (Max 200% and Min 80% of GW)
and references only air tow. The FAR's don't seem to know much about winch
launch.

If you examine glider handbooks, the strength of the recommended weak link
for winch launch is usually about 1.2 to 1.4 times the gliders gross weight
but sometimes it's less. In the case of my Nimbus, the GW is 650 kilos and
the recommended weak link for both aero and winch launch is only 600 kg
force. I suspect it has more to do with the hook mounting structure than
wing strength.

As a previous poster noted, RTFM.

Bill Daniels

Bill Daniels wrote:
"Brian" wrote in message
ups.com...

Someone please correct me if I am wrong,

But as I recall the weak link is supposed break before it carries twice
the gross weight of the glider.
So during the launch the wings must lift at a maximum the weight of the
glider plus up to 2 times weight of the glider through the weak link.
So the maximum load you should be able to put on the glider is 3G's.

I don't think I would want to fly a glider that could not withstand
3G's.

Brian



The 2X GW weak link figure is from the US FAR's (Max 200% and Min 80% of GW)
and references only air tow. The FAR's don't seem to know much about winch
launch.

If you examine glider handbooks, the strength of the recommended weak link
for winch launch is usually about 1.2 to 1.4 times the gliders gross weight
but sometimes it's less. In the case of my Nimbus, the GW is 650 kilos and
the recommended weak link for both aero and winch launch is only 600 kg
force. I suspect it has more to do with the hook mounting structure than
wing strength.

As a previous poster noted, RTFM.

Bill Daniels


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.

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.

On some of the vintage gliders out there the limiting strength might be the
structure the hook is fitted to, I know of at least one case (Sagitta) where the
hook pulled out. However, I believe this is not generally the case.

You can be sure about compliance by the JAR-22 and newer gliders, but Remember
we are talking about vintage gliders here. Caveat aviator...
--
Bruce Greeff
Std Cirrus #57
I'm no-T at the address above.

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.

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

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.

Have on one notable occasion had a cowboy lift one wheel of the winch off the
ground with a Blanik L13 (35 years old - even if it is metal and enormously
strong) and a strategic gust... Funny thing is it had an appropriate weak link
in there and the thing did not fail.

Some gliders have more elevator authority than is idiot proof on a winch launch.
--
Bruce Greeff
Std Cirrus #57
I'm no-T at the address above.

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.

If I am not 100% certain of the structural integrety of a vintage glider, I
will "de-rate" it by lowering speeds and maybe using a weaker link that
originally specified. An example is not flying it at speeds above the green
arc and using a white link instead of a blue one.

Bill Daniels


"Derek Copeland" wrote in
message ...
Re the wing bending thing, almost the same happens
with ASW20's, which only have a 15 m wingspan. Neatlyf I am not 100%
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.

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.