Yaw String in a Spin

Some very interesting thoughts there - especially about
whether or not the glider spins about an axis in a
vertically downwards line. The airflows - and especially
the behaviour of the yaw string - are very much easier
to understand if the glider c of g is thought of descending
in a spiral and the centre of the whole spin rotation
is inside the rotation circle of the yaw string.

From the pilot/glider frame of reference in a steep
spin the nearest point on the central axis of rotation
of the spin may be a short distance above the pilot's
head - or looking down the nose the point of interception
of the central axis may be a long way in front of the
nose (the first view being like looking down a radius
to the centre of an inverted verically descending cone
cone and the second being like looking down a side
of the cone to the vertically descending point)


There is a natural tendency to imagine the spinning
glider as rotating a point somewhere near its C of
G with the yaw string on the opposite side of the axis
of rotation from the tail. But, if the tail and the
yaw string were both to turn out to be on the same
side of the central axis of rotation of the spin, as
described above, then the loose end of the string would,
very obviously to everyone, be pointing to the inside
spin/downgoing wing side of the canopy - irrespective
of the other factors causing the same deviation of
the string to the inside that have been described previously
in this thread.

John Galloway

At 17:00 19 January 2005, wrote:
Andy,

Remember my spin entry tests in the V2b a year and
a half ago. One of
the departures was so violent, I lost reference, and
even though I knew
which way I entered, I can see how a surprise stall
with the same sort
of departure could cause disorientation and application
of pro spin
rudder. At low altitudes (during a save, for instance),
the delay in
recovery could be very bad news.

As for spin dynamics, it might be interesting to look
at airfoil tufts
throughout to see what's going on. Anyone have a link?
I think most of
us envision a spin as a straight line down, the aircraft
rotating about
this axis. That seems too simple. At any rate, the
yaw string should
always be displaced into the direction of spin (or
average there if
oscillating). Staying in the spin requires that the
inside wing be
producing higher drag (as a result of AOA), and thus
the pro-rotation
displacement.

I won't be able to fly until April, but maybe one of
you southwestern
types could provide some video of the yaw string through
2 or three
full rotations. We can discuss this ad nauseum, but
a few pictures
would make the discussion much more interesting and
possibly fruitfull.
Andy, you up for it?

I won't be able to fly until April, but maybe one of
you southwestern
types could provide some video of the yaw string through
2 or three
full rotations. We can discuss this ad nauseum, but
a few pictures
would make the discussion much more interesting and
possibly fruitfull.
Andy, you up for it?


Good idea.

I've just looked at some video I took in 2003 which
includes a 1 turn spin in each direction.

Initially, as the nose drops, the yaw string points
inside the spin, i.e. a spin to the right, the string
says use more right rudder. Once the glider starts
rotating, the string points outside the spin, i.e.
spinning right, string says use left rudder.

Can't post it, nowhere to put it.

Ed.
(Pilatus)

It means exactly the same rudder direction you apply
in any other, non spinning, yawed condition (how could
it be otherwise as the string doesn't change its mode
of action in a spin?). The front end of a yaw string
is stuck to the canopy and if the loose end is pointing
left in a spin you apply apply right rudder.

I have never heard of anyone referring to the front
end of the yaw string being the way that it is pointing.
If they do that in the States the wording could
be changed. However, since pilots have ingrained into
thenm the wording 'full opposite rudder' for spin recovery,
I would strongly suggest retaining the word 'opposite'
and referring to the loose end of the yaw string as
the way it points.

There is nothing new to learn in what I suggest - I
just picked it up from Reichmann's book. The change
in emphasisis just that the attention is directed to
the yaw string in any case of doubt rather than to
watching the houses whirling about.

John Galloway


At 19:00 16 January 2005, Greg Arnold wrote:
'Apply full rudder opposite to the direction of the
yaw string' -- what
does that mean? What is the direction of the yaw string?
If the loose
end of the yaw string is on the right side of the canopy,
is the
direction of the yaw string to the right, or is it
to the left?

I think you mean that if the loose end of the yaw string
is on the right
side of canopy, you apply right rudder? Or the opposite
of the normal rule?




The yaw string *always* points to the inside of a
spin
(according the Reichmann and others) and modifying
the teaching to 'apply full rudder opposite to the
direction of the yaw string' would be a more certain
way of choosing the life rudder pedal rather than
the
death one at low altitude

John Galloway wrote:
It means exactly the same rudder direction you apply
in any other, non spinning, yawed condition (how could
it be otherwise as the string doesn't change its mode
of action in a spin?). The front end of a yaw string
is stuck to the canopy and if the loose end is pointing
left in a spin you apply apply right rudder.

I have never heard of anyone referring to the front
end of the yaw string being the way that it is pointing.

Really? I picked it up somewhere in my instruction. Something to the
effect of "Think of the yaw string as an arrow with the head attached to
the canopy. It points to the the rudder pedal you need to push to
coordinate."
Seemed very natural and simple to me.

Shawn

Really? I picked it up somewhere in my instruction. Something to the
effect of "Think of the yaw string as an arrow with the head attached to
the canopy. It points to the the rudder pedal you need to push to
coordinate." Seemed very natural and simple to me.

Yes. I use this most often with transitioning power pilots who are
taught to "step on the ball" of the inclinometer ("step on the head of
the arrow"). With ab initio glider pilots I start with telling them to
add rudder on the opposite side of where the tail of the string is
pointing. If that doesn't work, I switch to the above.

Tony V.

John Galloway wrote:
It means exactly the same rudder direction you apply
in any other, non spinning, yawed condition (how could
it be otherwise as the string doesn't change its mode
of action in a spin?). The front end of a yaw string
is stuck to the canopy and if the loose end is pointing
left in a spin you apply apply right rudder.


Are you sure? Imagine a flat spin. If the loose end is pointing to the
left, doesn't that mean yoiu are spinning to the right? So don't you
want left rudder?

At 23:00 16 January 2005, Shawn wrote:
John Galloway wrote:
It means exactly the same rudder direction you apply
in any other, non spinning, yawed condition (how could
it be otherwise as the string doesn't change its mode
of action in a spin?). The front end of a yaw string
is stuck to the canopy and if the loose end is pointing
left in a spin you apply apply right rudder.

I have never heard of anyone referring to the front
end of the yaw string being the way that it is pointing.

Really? I picked it up somewhere in my instruction.
Something to the
effect of 'Think of the yaw string as an arrow with
the head attached to
the canopy. It points to the the rudder pedal you
need to push to
coordinate.'
Seemed very natural and simple to me.

Shawn

So that's the pedal you'll want to press in a spin.
Use your way of thinking about it. No dispute between
us. See Mike the Strike's posting.

John Galloway

At 23:00 16 January 2005, Shawn wrote:
John Galloway wrote:
It means exactly the same rudder direction you apply
in any other, non spinning, yawed condition (how could
it be otherwise as the string doesn't change its mode
of action in a spin?). The front end of a yaw string
is stuck to the canopy and if the loose end is pointing
left in a spin you apply apply right rudder.

I have never heard of anyone referring to the front
end of the yaw string being the way that it is pointing.

Really? I picked it up somewhere in my instruction.
Something to the
effect of 'Think of the yaw string as an arrow with
the head attached to
the canopy. It points to the the rudder pedal you
need to push to
coordinate.'
Seemed very natural and simple to me.

Yes, it is pointing into the relative wind; point the
glider toward that.

Shawn

Yikes...
This thread is well on its way to the top for adding
confusion to a topic!!

I don't even think of yaw string in terms of 'pointing'...

for me it solely reflects how air is passing over it,
the string always stays aligned to this airflow...and
most all of the time I want to keep my ship aligned
pointy end first into that same airflow. My contribution
to the confusion.


At 04:00 17 January 2005, Greg Arnold wrote:
John Galloway wrote:
It means exactly the same rudder direction you apply
in any other, non spinning, yawed condition (how could
it be otherwise as the string doesn't change its mode
of action in a spin?). The front end of a yaw string
is stuck to the canopy and if the loose end is pointing
left in a spin you apply apply right rudder.


Are you sure? Imagine a flat spin. If the loose end
is pointing to the
left, doesn't that mean yoiu are spinning to the right?
So don't you
want left rudder?

At 04:00 17 January 2005, Greg Arnold wrote:
Are you sure? Imagine a flat spin. If the loose end
is pointing to the
left, doesn't that mean yoiu are spinning to the right?
So don't you
want left rudder?

You better sort that out in your head quick!

Think. Start straight level and slow. Feed in full
left rudder. The glider rotates (yaws) left but continues
initially on the track it was going. The airflow is
now coming more from the right and blows the yaw string
out to the left (the slip ball, which is free to move
in its tube, goes out to the right sharply because
the airflow is decelerating the whole aircraft apart
from it).

The left wing reaches the stall, the wing drops and
the angle of attack increases even further. The increase
in drag on the wing causes the glider to continue rotating
to the left.
The glider is now sinking rapidly with the left wing
more badly stalled than the right due to the rotation.
This means that the glider continues to yaw and roll
left.

Looking from above the glider is now following a circular
anti-clockwise path with the nose pointing into the
circle and the tail out. The airflow is still coming
more from the right (over the whole aircraft and not
just forward of the centre of gravity) and the yaw
string is being blown out to the left whether the nose
pitches down or up into a flat attitude or not. The
slip ball (and you) are trying to continue in a straight
line and feel a force throwing you to the right. This
is a left hand spin!

The anti-spin action at this point is to reduce the
yaw to the left with full right rudder; pull the string,
push the ball or step on the head of the snake (sounds
like a position in the Kama Sutra) as your personal
mantra dictates and then move the stick forward from
its central position (where I hope you placed it as
the spin developed) until the wing unstalls. Now centralise
the rudder before loading the wing up on the pull out
or you'll be off the other way.