Yaw String in a Spin

At 06:00 17 January 2005, Z Goudie wrote:
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!

Yep! We had better do some spins and observe the yawstring
while it develops; I'll confess that I have never paid
attention to the string while spinning. (Further comments
in body of post)


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.


I think everyone agrees to this point. Error in thought
takes place once the spin starts.


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!
(Here is another indicator; if you feel a force throwing
you to the right you need to add right rudder)

This is where the confusion begins; it is tempting
to think that the glider is simply rotating about its
CG without sideways movement; (it almost looks that
way above 3000ft. agl). If that were the case the
yaw string would switch sides because the airflow would
now be from the left ahead of the CG and from the right
aft of the CG.

In truth, the whole glider is still sliding through
the air to the right side in this left-hand spin, so
the yaw string should still be to the left side.


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.

This mental error in not recognizing the sideways component
of the spin is what accounts for stall/spin accidents
on turn to final; down low the sideways movement is
dramatically noticeable in a way that it is not at
altitude. This is the reason pilots do not recognize
a spin down low. We should emphasize this continuing
sideways component in all spins and call attention
to it by the behavior of the yaw string. I would like
to believe that focussing on this sideways movement
might prevent some future accidents.