Yaw String in a Spin

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?

Seems like there is some potentially confusing terminology
being used here that I've never heard before. I've
always been taught that the the 'direction' of the
yaw string is the side of the glider it leans to, and
so you correct by pressing the rudder on the opposite
side.

Here I think people are saying that if the yaw string
is displaced to the right side of the glider it is
'pointing' left. While I can understand how you might
naturally want the front end to be the tip of the 'pointer',
I think it's confusing to refer to 'pointing' at all
because of this left/right confusion. I prefer to
say the yaw string is 'to the left' or 'to the right'
as position is less ambiguous that the 'pointing' direction.

To be honest, I've never looked at the yaw string in
a spin as it has never been ambiguous to me which way
the world was turning - if the world is going round
and round counter-clockwise how can this not be spinning
to the right? I guess I presumed that for a spin to
persist the glider would have to stay skidding, but
in a fully established spin you might wonder, if the
yaw string was far enough forward (say in a two-seater),
whether the rotation overcomes the skid in terms of
the local flow across the canopy - apparently not given
the comments here. Boy I'd hate for that to be wrong
though.

Years ago when I was flying a Ventus A 16.6 (easy to
spin unintentionally in my experience) I taught myself
that if the inside wing in a turn ever dropped, to
push the stick forward and into the turn and to hit
top rudder. You try to make it as instinctive as possibe,
but it takes practice. The top rudder is the easiest
part - the stick movement is against most people's
instincts.

9B

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?

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.

This vision was the basis for my remarks about not
recognizing a spin on turn to final. We need to re-think
the teaching of spin recognition to include a better
vision of what the aircraft movement really is -- a
circular skid path, to point that out and impress it
on students and ourselves. Successive spins at lower
and lower altitudes will demonstrate that the lower
one is the more obvious the sidewise motion will be.
But no demonstrations in the pattern, please!




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?

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)