Yaw String in a Spin

At 15:00 18 January 2005, Don Johnstone wrote:
I think that the test pilots concerned knew exactly
how they got there but it may not have helped them
much. I re-call seeing a very good video of a Jaguar
that had departed and was tumbling end over end with
fuel being forced out of the jet intakes. I think the
pilot took about 20000ft to find a point where he could
break into the sequence and return to controlled flight.
As a result the action to be taken if a Jaguar departs
is seize the handle between your legs and pull hard.

Well, Don, its a good enough way of passing one's last
few moments but I would try ejecting rather than....
and the next part of the line is just so obvious I
can't even bring myself to finish.....damn - done it
again.

Sorry.

John

John Galloway wrote:
At 15:00 18 January 2005, Don Johnstone wrote:

I think that the test pilots concerned knew exactly
how they got there but it may not have helped them
much. I re-call seeing a very good video of a Jaguar
that had departed and was tumbling end over end with
fuel being forced out of the jet intakes. I think the
pilot took about 20000ft to find a point where he could
break into the sequence and return to controlled flight.
As a result the action to be taken if a Jaguar departs
is seize the handle between your legs and pull hard.


Well, Don, its a good enough way of passing one's last
few moments but I would try ejecting rather than....
and the next part of the line is just so obvious I
can't even bring myself to finish.....damn - done it
again.

Now how would the NTSB address *that* in the accident report.
Delicately, I'd hope.

Shawn

At 14:31 18 January 2005, Colin Lamb wrote:
'What is most important is 'situational awarness'.
A pilot must learn to
know where he is and how he got there.'

The last part of that statemenht may not always be
true. I have read
reports from test pilots who intentionally place the
aircraft in
out-of-control positions in order to determine a recovery
procedure. A
common response is that they ride the aircraft until
they are in a position
they recognize and as soon as that happens they can
recover. How they got
there was irrelevant.

Although few airplanes use yaw strings (the pilots
are still behind the
curve a bit), they are normally used on helicopters.
Although the pedals
are used to counteract the torque of the tail rotor,
we operate them the
same way - keep the yarn centered.

Colin N12HS

Wha-a-t?

How can you use a yaw string in a helicopter? Doesn't
the rotor wash it around? Educate me.

(I had one helicopter ride in an H-13 in 1954; didn't
see no stinkin' yaw string.)

"How can you use a yaw string in a helicopter? Doesn't
the rotor wash it around? Educate me.

(I had one helicopter ride in an H-13 in 1954; didn't
see no stinkin' yaw string.)"

Helicopter pilots have learned a lot since 1954. I fly helicopters (just
Schweizers). Perhaps the production crew kept putting them on from
sailplane days, but they work just like any other yaw string. And, they are
brightly colored yarn, too. Only one of the helicopters I have flown has a
turn and bank indicator - and all have the yarn.

When you are hovering, the yarn is meaningless - but you are not going
anywhere, either. As soon as you move to forward flight, the relative wind
goes from the front of the helicopter to the back, just like any other
aircraft.

At best glide ratio, I get just over 3:1, but with a headwind the glide
angle approaches that of an elevator.

Colin N12HS



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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)

The yaw string just indicates the direction of the flow in the boundary
layer at that point. I have seen in a stabilized spin in a 2-32 the yaw
string point in the direction of the turn, straight forward, straight
aft and in a direction opposite the direction of the rotation. This was
an induced spin with the CG almost at the max allowable aft position. I
have heard reports of an ASK-21 doing similar things but have not
experienced it.

Possibly looking through the string and just observing the direction
the world is moving and inputting rudder in the direction counter to
the direction of motion might have merit?