Winch Launch Stresses on Vintage Gliders

[Mike Schumann]

Our club is exploring switching from aero tow to winch launching. Some of
our members own older wooden gliders and have expressed concerns about the
increased stress on the glider's structure resulting from winch launching
vs. aero tow.

Is this a valid concern?
--
Mike Schumann

[Charles Yeates]

Bluenose has winched K7 and K8 gliders for twenty plus years. The
gliders are now 35 years old -- no winch related maintenance required.

Mike Schumann wrote:
> Our club is exploring switching from aero tow to winch launching. Some of
> our members own older wooden gliders and have expressed concerns about the
> increased stress on the glider's structure resulting from winch launching
> vs. aero tow.
>
> Is this a valid concern?

[Robin Birch]

In message >, Mike
Schumann > writes
>Our club is exploring switching from aero tow to winch launching. Some of
>our members own older wooden gliders and have expressed concerns about the
>increased stress on the glider's structure resulting from winch launching
>vs. aero tow.
>
>Is this a valid concern?
Just make sure that your winch drivers are trained properly and don't
just launch as though they are coming off an aircraft carrier. Proper
speed control and suitable weak links and it works fine.

We launch T31s T21s K6s Skylark 2s and Oly 2bs with now issues at all.
Remember that a lot of older gliders had compromise hooks which won't
give as high a launch as the more recent designs with real CofG hooks.

Cheers

Robin
--
Robin Birch

[Dave Martin]

>In message , Mike Schumann writes
>>Our club is exploring switching from aero tow to winch
>>>>launching. Some of our members own older wooden gliders
>>>>and have expressed concerns about the increased stress
>>on >>the glider's structure resulting from winch launching
>>vs. aero >>tow.

>>Is this a valid concern?

In addition to Robins well made point the pilots need
a certain amount of training and discipline. Bill Daniel
web site on winch launching contains many useful documents
that may be worth reading. See

http://groups.yahoo.com/group/winchdesign/?yguid=180793033

You will have to register to down load the infomation
but it is freely available. If you are in the USA make
contact will Bill he is interested in developing winch
launching.

Dave




Just make sure that your winch drivers are trained
properly and don't just launch as though they are coming
off an aircraft carrier. Proper speed control and
suitable weak links and it works fine.
>
>We launch T31s T21s K6s Skylark 2s and Oly 2bs with
>now issues at all.
>Remember that a lot of older gliders had compromise
>hooks which won't
>give as high a launch as the more recent designs with
>real CofG hooks.
>
>Cheers
>
>Robin
>--
>Robin Birch
>

[GM]

Dave Martin wrote:
> >In message , Mike Schumann writes
> >>Our club is exploring switching from aero tow to winch
> >>>>launching. Some of our members own older wooden gliders
> >>>>and have expressed concerns about the increased stress
> >>on >>the glider's structure resulting from winch launching
> >>vs. aero >>tow.
>
> >>Is this a valid concern?
>
> In addition to Robins well made point the pilots need
> a certain amount of training and discipline. Bill Daniel
> web site on winch launching contains many useful documents
> that may be worth reading. See
>
> http://groups.yahoo.com/group/winchdesign/?yguid=180793033
>
> You will have to register to down load the infomation
> but it is freely available. If you are in the USA make
> contact will Bill he is interested in developing winch
> launching.
>
> Dave
>
>
>
>
> Just make sure that your winch drivers are trained
> properly and don't just launch as though they are coming
> off an aircraft carrier. Proper speed control and
> suitable weak links and it works fine.
> >
> >We launch T31s T21s K6s Skylark 2s and Oly 2bs with
> >now issues at all.
> >Remember that a lot of older gliders had compromise
> >hooks which won't
> >give as high a launch as the more recent designs with
> >real CofG hooks.
> >
> >Cheers
> >
> >Robin
> >--
> >Robin Birch
> >



Mike,

heed all of the above and check the operating manuals for each glider
you intend to launch. You should find a section regarding the required
weak-link strength. Order the proper weak-links from TOST. Contrary to
'home-made' weak links fashioned from hardware store polypropylene
rope, the TOST links are precision engineered devices with a breaking
strength within a few % of the rating. Also, check what type of
releases your gliders have and buy the appropriate rings.
Regarding the extra stress on the glider: a good rule of thumb is that
a normal winch launch imposes a load about equivalent of 2g onto the
glider.
As Dave Martin already stated - join our yahoo winchdesign group for
all your questions regarding safe winch launching.

Uli Neumann

[dumass]

Look at vintage glider winch launch video at Nijmegen Aero Club website.
http://www.nijac.nl/galerij/zweefvlieg.wmv
Vintage glider winch launch is no problem as long as you know what you (or your winch driver) are doing. (of course the oximoron for everything)
This is the originating club of the Van Gelder / MEL winch.
You can winch launch anything from hangglider/paraglider to fully ballasted DG505 (again, if you know what you are doing)

Our club is exploring switching from aero tow to winch launching. Some of
our members own older wooden gliders and have expressed concerns about the
increased stress on the glider's structure resulting from winch launching
vs. aero tow.

Is this a valid concern?
--
Mike Schumann

[Derek Copeland]

If the vintage wooden glider is truly airworthy and
you use the specified weak link (usually white or blue),
then no. We launch vintage gliders by winch most of
the time in the UK (America's fixed aircraft carrier,
just off Europe - if you don't know where that is).


However, winch launches can put a strain equivalent
to a 3.5g turn on the mainspar, and sometimes the tail,
without any obvious g loading on the pilot. If you
have a very powerful modern winch, then you need to
brief the winch driver to apply the power fairly gently
at the start. Placarded speed limits need to be strictly
observed.

Cheers,
Derek Copeland (part owner of several vintage gliders)


At 18:18 22 January 2006, Mike Schumann wrote:
>Our club is exploring switching from aero tow to winch
>launching. Some of
>our members own older wooden gliders and have expressed
>concerns about the
>increased stress on the glider's structure resulting
>from winch launching
>vs. aero tow.
>
>Is this a valid concern?
>--
>Mike Schumann
>
>
>

[Bruce]

Mike Schumann wrote:
> Our club is exploring switching from aero tow to winch launching. Some of
> our members own older wooden gliders and have expressed concerns about the
> increased stress on the glider's structure resulting from winch launching
> vs. aero tow.
>
> Is this a valid concern?
We fly almost exclusively vintage aircraft at our club. All launches are winch,
and the safety and maintenance record is excellent. (>30,000 vintage trainer
launches at last count) The two principal trainers are a 1965 Bergfalke III and
a 1955 Bergfalke II/55. with >13,000 and > 19,000 launches respectively.

Like with any mechanical system it is possible to damage an aircraft with winch
launches if you do not follow correct procedures. Conversely, simple procedures
can ensure safe operation.

My Advise is -
1] some engineer put a lot of work into designing and specifying the weak link
system. Use the right weak links and your chances of overstressing are minimal.
2] make sure your winch drivers are pilots and visa versa. That way there is a
lower risk of over exuberance...
3] Teach a progressive rotation into the steep climb, slamming the tail of a
wooden glider into the ground with full up elevator can result in a lot of work.
(if you are lucky) One pilot who did this at our club flew for 2:30 with a
cracked rudder and broken frame under the elevator hinge. He didn't think it was
a "loud bang..."
4] The point on the launch that exposes the structure to greatest stress is the
top of the launch where you have maximum downward force from the winch and
cable, often together with higher speed if the winch driver is insensitive. Add
a pilot who is bending the stick to wring the last bit of height out of a launch
(last year someone - different club - managed to spin off the cable) and you
have the limiting force being the down force on the elevator.

Respect the speed limits and use appropriate equipment and winching is no
problem for vintage types. Conversely, the sustained flexing on the wings of
some of the newer glass ships can result in premature gell coat crazing, and
some owners refuse to winch launch for this reason.

--
Bruce Greeff
Std Cirrus #57
I'm no-T at the address above.

[Brian]

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

[Mark Newton]

In article >,
"Mike Schumann" > wrote:

> Our club is exploring switching from aero tow to winch launching. Some of
> our members own older wooden gliders and have expressed concerns about the
> increased stress on the glider's structure resulting from winch launching
> vs. aero tow.
>
> Is this a valid concern?

Not really. If your glider is likely to be damaged by a winch
launch, then it's also likely to be damaged by a 3G pullup.

If a 3G pullup is going to harm it, perhaps you should check your
maneuvering envelope and consider grounding the aircraft for
good. :-)

- mark

[Derek Copeland]

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
>
>

[Chris Rollings]

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]

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

[Bill Daniels]

"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

[Bruce]

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.

[Martin Gregorie]

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

[Bruce]

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.

[Derek Copeland]

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.
>

[Bill Daniels]

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.
>>
>
>
>

[01-- Zero One]

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.
> >

[Mike Schumann]

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

[Frank Whiteley]

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

[Martin Gregorie]

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

[Andy]

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

[Bill Daniels]

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
>

[Robin Birch]

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
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
>>
>
>

--
Robin Birch

[Stefan]

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

[Don Johnstone]

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
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
>>>
>>
>>
>
>--
>Robin Birch
>

[Frank Whiteley]

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

[Ian Johnston]

On Wed, 25 Jan 2006 04:26:23 UTC, "Bill Daniels" >
wrote:

: 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.

As I recall, the Tost weak link system specifies 1.4 times the
all-up-weight for gliders without a recommended colour - normally
rarities.

Ian


--

[Ian Johnston]

On Tue, 24 Jan 2006 01:04:44 UTC, "Brian" > wrote:

: 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.

It would actually be much higher than that. In a normal 3G turn, only
3 times the weight of the fuselage and contents a gets transmitted
through the wing attachments - the extra load of the wings is absorbed
internally. If you add the extra 2G with a winch cable, the wing
fittings are taking 1 x the weight of the fuelage plus 2 x the weight
of the entire glider, which is twice the weight of the wings more than
in the 3G turn. Since the wings are typically half the mass of the
glider, that means that the wing root stresses in a 3G (weight plus
2G) launch are roughly the same as those in a 4G turn.

Hence the typical 1.4 x AUW weak link specification - that gives a
maximum load on the wing roots equivalent to just under 3.5G, which is
generally the maximum permitted loading in normal flight.

Ian
:


--

[Ian Johnston]

On Sun, 29 Jan 2006 15:35:45 UTC, "Bill Daniels"
<bildan@comcast-dot-net> wrote:

: 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.

I'm pretty sure that I have seen genuine Tost weak links in which the
slotted ones had a paint band round the middle. But that may have been
added by the club in question. It's a sensible thing to do.

Ian