Use of weak links

On Fri, 04 Jun 2010 20:41:48 -0500, brian whatcott
wrote:


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.


Sorry Brian,

I have to admit I don't understand what you mean.
What do you want to say with that 90 deg AoA?


My point was that a force exceeding the design load is simply not
possible below VA, no matter how high AoA is.

Andreas Maurer wrote:

I have to admit I don't understand what you mean.
What do you want to say with that 90 deg AoA?

That AoA makes your IAS observation rather difficult?
:-)

Brian W

On Jun 5, 6:22*pm, Andreas Maurer wrote:
On Fri, 04 Jun 2010 20:41:48 -0500, brian whatcott

Sorry Brian,

I have to admit I don't understand what you mean.
What do you want to say with that 90 deg AoA?

My point was that a force exceeding the design load is simply not
possible below VA, no matter how high AoA is.

Andreas,

What Brian W,was trying to say is that it is possible to overload the
wing by simply by the flat plate drag on the wing when it is attached
to the winch. Think of the plane just falling with no horizontal speed
(90deg Angle of Attack). With it not attached to the winch it would
reach a terminal velocity and limit the amount of load that is applied
to the wings. With it attached to the winch it could exceed it's
terminal velocity and apply additional load to the wings, perhaps
enough to fail the structure.

My gut feeling is that it would take quite a bit of vertical velocity
to fail the wing, but I tried applying my admittedly limited math
skills to it and came up with about 120mph to apply 6g's load to
fairly typical single place glider. This was a lower velocity than I
expected but still pretty unreasonable to think it is possible in that
it would mean that the winch operator is pulling the cable in at
120mph.

Brian C.

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

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.

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

Andy wrote:
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

I was way off base, by all accounts. I hear they can be required for
competition or aerobatics...

Brian W