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?