Use of weak links

On Jun 4, 10:15*am, Chris Nicholas wrote:
For clarity, did the original poster mean Chief Flying Instructor by
"CFI" (UK and some other countries' usage), *or "some CFIG” as another
(USA?) poster thought?

Either way, what is the safety officer doing, not caring and deferring
to the the CFI whoever it is?

Chris N.

The original message was from South Africa. Presumably the OP meant
Chief Flying Instructor.

Darryl

On Jun 4, 6:27*am, Andreas Maurer wrote:

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

Are you sure about that? It was my understanding the Va is never
marked in the ASI and that the top of the green arc is the maximum
operating speed in rough air. Va may be lower than that.

Andy

On Jun 4, 1:53*pm, Andy wrote:
On Jun 4, 6:27*am, Andreas Maurer wrote:

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

Are you sure about that? *It was my understanding the Va is never
marked in the ASI and that the top of the green arc is the maximum
operating speed in rough air. Va may be lower than that.

Andy

andy - that is correct

On Jun 3, 6:43*pm, brian whatcott wrote:
Brian wrote:

/snip/

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...

Brian W

While technically correct, in practice it probably isn't much of a
factor.
For my 750lb glider with a 120sq.ft. of wing rated for 6g's and as 1g
stall speed of 40 mph the 6g stall speed is 97mph.
For the flat plate drag of the wing area to exceed 6g's on the wing a
120mph is required by my calculation, which means the winch operator
is going to have be pulling me at 120mph since I am stalled and no
longer have any angular acceleration.

A 200% weak link should break at 69mph if I understand it correctly
(3G load) Interestingly IIRC my max ground launch speed is 69mph.

Brian C.

On 6/2/2010 7:23 AM, Paul wrote:
On Wed, 02 Jun 2010 04:28:31 -0700, T8 wrote:
Here's hoping that the signal to noise ratio of a one on one
conversation with your CFI is better than that on r.a.s. (see concurrent
thread on altimeter settings, for example :-)).

Yours is a reasonable question that deserves a reasonable response. My
$0.02: you are not being 'paranoid' about anything.

-Evan Ludeman / T8


Thanks for the input everyone.

I'll take the matter up with my CFI and if not successful will stick to
aerotows while the rest of the club can risk their lives using the winch.

Weak links are there to protect the structural integrity of the glider.
If your club is not using weak links, they may be damaging the glider,
which might not be obvious. You are still putting yourself at risk if
you are flying a club glider that may have been damaged, regardless of
whether you are winch launching or using aero tows.

--
Mike Schumann

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

Better to think twice and write once.

For instance, how does your ASI indicate at 90 deg AoA ?
If an airframe is designed for 1.5 (the design factor)
times limit load (often 4 g)
then if it is pulled with a force exceeding this, at high AoA,
wing breakage is guaranteed.

Sincerely

Brian W

Andreas Maurer wrote:
On Thu, 3 Jun 2010 14:38:51 -0400, "Tim Mara"
wrote:

80-200% for aero-towing *
100-200% for ground launching *
of max certificated gross weight *

Most amazing.
This violates the POHs of all the gliders I know.

Cheers
Andreas

Bye
Andreas

There you go again, Andreas :-)
The thread indicated that the regs specify:
poh, or if not specified, the other.

Better to think twice and write once

Brian W

Brian wrote:
On Jun 3, 6:43 pm, brian whatcott wrote:
Brian wrote:

/snip/

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...

Brian W

While technically correct, in practice it probably isn't much of a
factor.
For my 750lb glider with a 120sq.ft. of wing rated for 6g's and as 1g
stall speed of 40 mph the 6g stall speed is 97mph.
For the flat plate drag of the wing area to exceed 6g's on the wing a
120mph is required by my calculation, which means the winch operator
is going to have be pulling me at 120mph since I am stalled and no
longer have any angular acceleration.

A 200% weak link should break at 69mph if I understand it correctly
(3G load) Interestingly IIRC my max ground launch speed is 69mph.

Brian C.


There is plenty I don't know about gliders. That you believe a glider is
designed for 6g limit loads for example. I thought the load limit was
lower, on condition parachutes are worn?
But then, there's plenty I don't know about sailplane design...

Brian W

On Jun 4, 6:54*pm, brian whatcott wrote:
I thought the load limit was
lower, on condition parachutes are worn?

Do any sailplanes require a parachute to be worn as a condition of
operation. I don't know of any. Schleicher required equipment for
my glider is a parachute or a cushion of a specified thickness.

Andy

On Jun 4, 7:41*pm, brian whatcott wrote:
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

Better to think twice and write once.

For instance, how does your ASI indicate at 90 deg AoA ?
If an airframe is designed for 1.5 (the design factor)
* times limit load (often 4 g)
then if it is pulled with a force exceeding this, at high AoA,
* wing breakage is guaranteed.

Sincerely

Brian W

Back to the OP question.

It's absolute insanity to winch launch a glider with anything but the
exact weak link demanded by the POH. There's simply no wiggle room -
you have to do what the manual says. However, with that link, there's
zero risk of overloading the wing structure. (If the POH doesn't
specify a winch weak link, I wouldn't winch launch the glider.)

While you're at it, buy a "preamble" from W&W (Tost) or Klaus Fey's
(www.eqip.de) and don't try to cobble up one with rope, hardware store
carabiners and a surplus 'chute. There's some real safety issues with
how these things are put together you don't want to learn the hard way.