Use of weak links

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 Smith 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.

Some time back during a rainstorm our then CFI gave a talk about how the
various airframe limits are set. I remember him saying that Vwinch is 95%
of the speed at which the wing's maximum design loading is reached when
the glider is overhead the winch with the stick fully back.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Jun 7, 3:41*pm, Martin Gregorie
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 Smith 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.

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

On Jun 7, 3:41Â*pm, Martin Gregorie
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 Smith wrote:
Brian wrote:
True the 90deg AOA Â*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.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Jun 4, 2:27*pm, Andreas Maurer wrote:
On Thu, 03 Jun 2010 19:43:46 -0500, brian whatcott

wrote:
Specifically what I am saying is that unless you are exceeding the
Maximum Winch Launch speed of the glider it should be impossible to
overstress the wings as the wing will stall before exceeding maximum
structural load of the aircraft.

Brian

This is a perfectly sound argument in many phases of flight.
*But a *tow capable of exerting enough force on a stalled structure
will certainly break it. A winch need not relax when a wing stalls....
Visualize an airframe being hauled along at an AoA of 90 degrees,
to exaggerate this point...

This won't break the wing spars either.

As long as the IAS is below VA (in other words: in the green range of
the airspeed indicator), no control input at all can damage the glider
- the definition of VA is that this is the speed under which a high
speed stall will prevent exceeding the maximum g-load. So, as long as
you are slower than VA, you are safe.

(Of course - combined control inputs can indeed break the structure
even below VA, usually by torsional load on the tail boom by a
deflected rudder at speeds close to VA combined with significant yaw
angle.)

The problem of exceeding the Maximum Winch Launch speed is not the
wing, but the attachment of the tow hook to the fuselage structure.
Pretty hard to explain why you landed with the belly hook missing...

Cheers
Andreas
Bye
Andreas

Hi Andreas,

I thought you would know better that that! Gliders have a lower
maximum winch launching speed than Va called Vw. This is because the
pull is applied as a point load in the centre of the fuselage and,
unlike in the manoeuvring case, there is no g unloading. Essentially
the cable is pulling the fuselage down while the wings are providing
large amounts of upward lift, which puts a large bending moment on the
mainspar. This is fairly obvious if you watch a floppy winged glider
being winch launched, when the wings bend quite alarmingly, especially
near the top of the launch when the forces are most opposed. The
recommended weak link is set by by the designer so that it fails well
before the mainspar does. It is possible that other bits of structure
such as the hook mounting will fail first. The only cases where I have
known this happen were to old wooden gliders where the wood has become
slightly rotten in the hook area.

Essentially the weak link acts as a fuse that breaks before the glider
does. Generally to damage a glider you would have to hit a sharp-edged
gust while pulling back hard against a powerful winch, near the top of
the launch, with an overstrength weak link, and exceeding VW, all at
the same time.

Derek Copeland

Some time back during a rainstorm our then CFI gave a talk about how the
various airframe limits are set. I remember him saying that Vwinch is 95%
of the speed at which the wing's maximum design loading is reached when
the glider is overhead the winch with the stick fully back.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

Your CFI may have said that but there's no engineering data or
certification specifications to back it up.

There's no chance of overstressing an airworthy glider on a winch
launch when using the correct weak link.

On Mon, 07 Jun 2010 12:21:31 -0700, bildan wrote:

Some time back during a rainstorm our then CFI gave a talk about how
the various airframe limits are set. I remember him saying that Vwinch
is 95% of the speed at which the wing's maximum design loading is
reached when the glider is overhead the winch with the stick fully
back.

--
martin@ Â* | Martin Gregorie
gregorie. | Essex, UK
org Â* Â* Â* |

Your CFI may have said that but there's no engineering data or
certification specifications to back it up.

He was talking us through JAR 22 at the time and JAR 22.583 looks pretty
prescriptive to me.

There's no chance of overstressing an airworthy glider on a winch launch
when using the correct weak link.

Maybe so but note that JAR 22.583 defines the max weak link strength as
the *lesser* of 83% of structural damage limit or max achievable lift
with full up elevator and JAR 22.585 says the hook must withstand the
larger of 125% of the weak link breaking load and the glider's MTOW.

I agree that 22.583 and 22.585 taken together guarantee no damage if the
right weak link is used, but I'd remind you that this discussion was
triggered by a question about a club that was winching with NO weak link.
JAR 22.583 has no guarantee against overstressing the glider under those
conditions because it specifically says that its OK to design a glider
for a weak link that breaks at a lower load than the wing can generate
with full up elevator. Unless there's a clause somewhere else in JAR 22
that would prevent it, this implies that a compliant glider can be broken
by pulling the stick back during an overspeed if it is winched on an
incorrect weak link.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

John Smith wrote:
Andreas Maurer wrote:
Forget the drag - you cannot go fast enough with 90 degrees AoA to
produce any significant amount of drag.

I'd rather say that with a 90° AoA you have *only* drag.

Quite. Cd reaches its peak (around 2) at 90 deg AoA, a notably bell
shaped curve 0 - 180 deg AoA

Cl peaks around 15, drops then rises again to peak around 45 deg AoA
falling to zero round 90 AoA according to some NASA trials of several
representative airfoils.

It was written of several WWI incidents, that a falling leaf full stall
could sometimes be walked away from.
In those machines, going fast was not a prerequisite.

Brian W

On Mon, 07 Jun 2010 20:55:46 +0000, Martin Gregorie wrote:

On Mon, 07 Jun 2010 12:21:31 -0700, bildan wrote:

Some time back during a rainstorm our then CFI gave a talk about how
the various airframe limits are set. I remember him saying that Vwinch
is 95% of the speed at which the wing's maximum design loading is
reached when the glider is overhead the winch with the stick fully
back.

--
martin@ Â* | Martin Gregorie
gregorie. | Essex, UK
org Â* Â* Â* |

Your CFI may have said that but there's no engineering data or
certification specifications to back it up.

He was talking us through JAR 22 at the time and JAR 22.583 looks pretty
prescriptive to me.

There's no chance of overstressing an airworthy glider on a winch
launch when using the correct weak link.

Maybe so but note that JAR 22.583 defines the max weak link strength as
the *lesser* of 83% of structural damage limit or max achievable lift
with full up elevator and JAR 22.585 says the hook must withstand the
larger of 125% of the weak link breaking load and the glider's MTOW.

I agree that 22.583 and 22.585 taken together guarantee no damage if the
right weak link is used, but I'd remind you that this discussion was
triggered by a question about a club that was winching with NO weak
link. JAR 22.583 has no guarantee against overstressing the glider under
those conditions because it specifically says that its OK to design a
glider for a weak link that breaks at a lower load than the wing can
generate with full up elevator. Unless there's a clause somewhere else
in JAR 22 that would prevent it, this implies that a compliant glider
^^^^^^^^^^^^^^^^^^
can be broken by pulling the stick back during an overspeed if it is
^^^^^^
winched on an incorrect weak link.

This should read "it is possible for some compliant gliders to be".


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

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...teel_cable.htm

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

On Jun 7, 5:32*pm, Martin Gregorie
wrote:
On Mon, 07 Jun 2010 20:55:46 +0000, Martin Gregorie wrote:
On Mon, 07 Jun 2010 12:21:31 -0700, bildan wrote:

Some time back during a rainstorm our then CFI gave a talk about how
the various airframe limits are set. I remember him saying that Vwinch
is 95% of the speed at which the wing's maximum design loading is
reached when the glider is overhead the winch with the stick fully
back.

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |

Your CFI may have said that but there's no engineering data or
certification specifications to back it up.

He was talking us through JAR 22 at the time and JAR 22.583 looks pretty
prescriptive to me.

There's no chance of overstressing an airworthy glider on a winch
launch when using the correct weak link.

Maybe so but note that JAR 22.583 defines the max weak link strength as
the *lesser* of 83% of structural damage limit or max achievable lift
with full up elevator and JAR 22.585 says the hook must withstand the
larger of 125% of the weak link breaking load and the glider's MTOW.

I agree that 22.583 and 22.585 taken together guarantee no damage if the
right weak link is used, but I'd remind you that this discussion was
triggered by a question about a club that was winching with NO weak
link. JAR 22.583 has no guarantee against overstressing the glider under
those conditions because it specifically says that its OK to design a
glider for a weak link that breaks at a lower load than the wing can
generate with full up elevator. Unless there's a clause somewhere else
in JAR 22 that would prevent it, this implies that a compliant glider

* * * * * * * * * * * * * * * * * * * * * * * * * * *^^^^^^^^^^^^^^^^^^

can be broken by pulling the stick back during an overspeed if it is
* ^^^^^^
winched on an incorrect weak link.

This should read "it is possible for some compliant gliders to be".

--
martin@ * | Martin Gregorie
gregorie. | Essex, UK
org * * * |- Hide quoted text -

- Show quoted text -

Right, the point, if I'm not mistaken, is to USE A WEAK LINK (of the
correct breaking strength).