Use of weak links

brian whatcott wrote:
/snip/
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

I received a private note that mentioned that gliders may well
be built to 6+g limit loads and that parachutes are not
mandatory - with some notable exceptions.
Where DO I get my preconceptions? :-)

Brian W

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

I understand completely the need for a weak link when ground launching
- it's there to protect the glider's structural integrity. The need
for a weak link for aerotow was less obvious to me. Searching the web
it became clear that an aerotow weak link is there to try to protect
the tow pilot. In case a glider "kites", the tow pilot has a much
better chance of survival if the tow rope breaks, as there's a history
of the tow plane's release not working in this case.

Despite this laudable purpose, I've never seen an aerotow weak link
used in my (admittedly limited) experience at some 10 airports in the
US. Maybe I've missed the weak links - are they attached at the tow
plane's tow hook? I think, though, most US glider operators use tow
rope with a breaking strength of 80 to 200% of max certified operating
weight of the glider, instead of a weak leak.

Just out of curiousity (really not trying to start something), are
there US gliderports that use aerotow weak links?

-John

On Jun 5, 3:09*pm, jcarlyle wrote:
I understand completely the need for a weak link when ground launching
- it's there to protect the glider's structural integrity. The need
for a weak link for aerotow was less obvious to me. Searching the web
it became clear that an aerotow weak link is there to try to protect
the tow pilot. In case a glider "kites", the tow pilot has a much
better chance of survival if the tow rope breaks, as there's a history
of the tow plane's release not working in this case.

Despite this laudable purpose, I've never seen an aerotow weak link
used in my (admittedly limited) experience at some 10 airports in the
US. Maybe I've missed the weak links - are they attached at the tow
plane's tow hook? I think, though, most US glider operators use tow
rope with a breaking strength of 80 to 200% of max certified operating
weight of the glider, instead of a weak leak.

Just out of curiousity (really not trying to start something), are
there US gliderports that use aerotow weak links?

-John

Yes.

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.

On Fri, 4 Jun 2010 11:53:04 -0700 (PDT), 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.

You are absolutely correct - the maximum operating speed in rough air
might be different from Va.


But:
Usually (at least in German gliders) only Va is marked on the ASI (top
of the green arc).

Example ASK-21:
Va 180 kp/h (top of green arc)
Maximum operating speed in rough air 200 kp/h (not marked in ASI).

It might well be possible that it's different for other gliders, but
so far I cannot remember haviong flown one.

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 Sun, 6 Jun 2010 07:21:57 -0700 (PDT), Brian
wrote:

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.

Ahhh.... I start to see the light.
A 90 degree AoA is, of course, a situation that can happen easily if
the pilot makes the slightest mistake.... lol.



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.

If you are attached to a winch cable at an AoA of 90 degrees, being
pulled towards the winch with 120 mph, I am pretty sure that a broken
wing spar is one of your minor problems.

Isn't the CG hook going to back release long before 90 degree AoA
anyway? It will release based on the angle between the wire and the
glider not the AoA...maybe 60 degrees AoA?

Scott