Spinning the SZD 50-3

On Wed, 28 Jan 2004 01:44:10 GMT, "Arnold Pieper"
wrote:

You shouldn't spin more than 3 turns unless you're practicing for aerobatic
flight.

If your purpose is International Aerobatic Club competition, you don't
need to do it for that either. According to the FAI catalog of
aerobatic figures, there is no spin allowed to be done with greater
than 2 turns. For unknown flights at the unlimited level, the max is
1.5 turns.

So I would modify what you say above to say, "you shouldn't spin more
than 2 turns unless you are practicing to be a test pilot."

Be safe,
Klein

The Puchacz WAS certified to JAR 22. When it was first
imported into the UK I repeated the tests, prior to
the BGA granting it certification. The testing included
five turns spins with the C of G at the manufacturers
aft limit; recovery was normal in well under one turn.

Having said that:

1. I once had a similar experience to the one described,
when flight testing the Grob Twin II for BGA certification.
Earlier spins at mid-range C of G had been typically
Grob - self recovery after less than a turn. I expected
that it would be just possible to hold it in at aft
C of G. In fact the spin started to flatten after
about two turns. I initiated recovery immediately,
it took three turns to recover (I was considering abandoning
the aircraft at that point).

After lots of phone calls to the manufacturer (who
said it had never happened in their testing), we resumed
testing (Cautiously) and approached the aft C of G
in small increments. Eventually we got to the C of
G position at which I had had the problem - the spin
was perfectly normal! So far I was aware, I had used
exactly the same spin entry technique in both cases.
Clearly there must have been some small difference
(ailerons not quite central, or some such) that made
an important difference. Whatever the cause, I couldn't
get it to repeat, all subsequent spins were unremarkable.

Rogue spins can happen, IN ANY TYPE.

2. The Puchacz described may have had some repair,
or equipment change, that moved the C of G further
aft, and this failed to get into the aircraft's records
(appalling, I know, but it has happened).

3. The elevator deflections may have become mis-set,
allowing greater upward deflection, and less downward.
This can happen unintentionally, I don't know how,
but it would very likely produce the result described.


At 01:36 28 January 2004, Geir Raudsandmoen wrote:
If you were within the permitted CoG range, and used
the standard recovery method, the spin behaviour you
described is definitely non-compliant with JAR 22 certification
rules. JAR22.221 states that a sailplane certificated
for intentional spinning must be able to recover from
a fully developed spin (5 turns) within 1 turn after
recovery action is done. This has to be demonstrated
in several loading and control conditions.

Additionally, this paragraph states that it must be
impossible to obtain uncontrollable spins with any
use of the controls.

The Puchacz may not have been certificated to JAR 22,
but possibly to the older OSTIV rules. However, I very
much doubt that this type of behaviour would have been
acceptable under older certification rules, although
the verification/testing requirements might have been
less strict in earlier days.

Geir

At 01:00 28 January 2004, Tim Shea wrote:
I love to spin. It's exciting. I took aerobatic training
with Wayne
Handley and was taught spin recoveries by him.

I have direct experience spinning the Puchacz at Minden.
This is what
I remember from my experience. Your mileage may vary.
With friends (usually lighter than me) in the front,
I spun it while
sitting in the back seat more than a dozen times. The
CG was within
the published range and I didn't have any trouble with
simple
recovery- stick centered and forward and rudder away
from the
direction of rotation. Worked great.
I should mention that I used to be 50 lbs heavier than
I am now, but
still in the published range for the plane.
During the training towards my instructors rating,
I spun the Puch
twice with my instructor. The first 2 or so rotation
spin I was able
to recover normally, no sweat. The second manuver was
quite different.
I was asked to let the spin develop a little deeper
for the second.
After 4 or so rotations, the nose seemed to float up
and the rotation
*seemed* to slow considerably. I remember thinking
that this is cool!
Kind of like floating. When it was time for the recovery
I applied the
control inputs I'd been taught (as specified above)
and much to my
surprise, nothing different happened.....for a long
time. I estimate
that we completed another 5+ rotations nose high before
it broke,
rolled over and recovered. I had the stick centered
and against the
front stop with the rudder also pegged away from the
rotation. We
recovered with several (4 or 5) thousand feet under
us (we'd been
playing at cloudbase at about 15K).
Once on the ground, we discussed this incident in the
grumpy bar for
at least an hour. I (and he) decided to never spin
the Puch again. I
didn't. I doubt he did either.
I had heard of this happening before. I assumed that
it was from
operation outside of the design envelope. Apparently
I was wrong.
John Shelton probably said it best: 'On my own as a
test pilot, I will
certainly get killed'. I felt like a dumb-ass for quite
a while (more
than usual) after that.

Clearly there must have been some small difference
(ailerons not quite central, or some such) that made
an important difference. Whatever the cause, I couldn't
get it to repeat, all subsequent spins were unremarkable.

Rogue spins can happen, IN ANY TYPE.


Could it be that a spinning glider is governed by laws that are
not well described by more traditional linear equations of
aerodynamics (or JAR-22)? I am reminded of some articles related to the
loss of ships at sea to "Rogue Waves" or "Freak Waves".
http://members.shaw.ca/diesel-duck/l...ogue_waves.htm

Ships designed to the standard engineering models of expected maximum
wave heights for the worst predicted conditions were being lost and
those describing witnessing these waves at sea were dismissed as
crackpots until only very recently.

Now, application of a non-linear model i.e. chaos
theory, seems to be describing these freak ocean waves more accurately,
where there very existance was doubted until jut a few years ago. Your
description of a very small variation in the initial conditions,
resulting in a very much different situation describes an outcome that
might be predicted by chaos theory better than some of these other
explanations. The non-linear SchrÖdinger equation was originally
developed in the field of quantum mechanics but is now being applied to
modeling freack ocean waves. Could we be dealing with a "quantum"
phenomenon when dealing with a spinning glider where it behaves nicely
according to our traditional model most of the time, but every once in a
while it produces a "Freak Spin" do to the complex interaction of all
the forces involved?

Bob wrote:

Clearly there must have been some small difference
(ailerons not quite central, or some such) that made
an important difference. Whatever the cause, I couldn't
get it to repeat, all subsequent spins were unremarkable.

Rogue spins can happen, IN ANY TYPE.


Could it be that a spinning glider is governed by laws that are
not well described by more traditional linear equations of
aerodynamics (or JAR-22)? I am reminded of some articles related to the
loss of ships at sea to "Rogue Waves" or "Freak Waves".
http://members.shaw.ca/diesel-duck/l...ogue_waves.htm

Ships designed to the standard engineering models of expected maximum
wave heights for the worst predicted conditions were being lost and
those describing witnessing these waves at sea were dismissed as
crackpots until only very recently.

Now, application of a non-linear model i.e. chaos
theory, seems to be describing these freak ocean waves more accurately,
where there very existance was doubted until jut a few years ago. Your
description of a very small variation in the initial conditions,
resulting in a very much different situation describes an outcome that
might be predicted by chaos theory better than some of these other
explanations. The non-linear SchrÖdinger equation was originally
developed in the field of quantum mechanics but is now being applied to
modeling freack ocean waves. Could we be dealing with a "quantum"
phenomenon when dealing with a spinning glider where it behaves nicely
according to our traditional model most of the time, but every once in a
while it produces a "Freak Spin" do to the complex interaction of all
the forces involved?

Maybe you don't need to invoke such complex things like chaos. Things
which are not taken in account by JAR-22 and usual procedure for weight and
balance, as somebody pointed it, are the moments of inertia around the
3 axis of the glider. Of special importance is the moment of inertia around
the pitch axis, a higher inertia around this axis favors flattening the
spin. And 2 gliders may have exactly the same weight and same CG postion
with different such moments.

Robert Ehrlich wrote:
...
Maybe you don't need to invoke such complex things like chaos. Things
which are not taken in account by JAR-22 and usual procedure for weight and
balance, as somebody pointed it, are the moments of inertia around the
3 axis of the glider. Of special importance is the moment of inertia around
the pitch axis, a higher inertia around this axis favors flattening the
spin. And 2 gliders may have exactly the same weight and same CG postion
with different such moments.

Correcting myself: what favors flattening the spin is rather the difference
of the moments of inertia around the 2 other axis (roll and yaw). Practically
what has influence on both (moment around pitch axis or difference between
moments around the 2 other axis) is the same thing, i.e. weights in the
fuselage far from CG, e.g. tail repair balanced by nose ballast.

Reading of these brave souls who have been testing the Puchaz spinning
characteristics with multi-turn spins at high altitudes reminds me of an
anecdotal rumour which reached me about at least one other glider type,
and I think also some power aircraft, which similarly misbehaved until
many turns/it got lower. It made me wonder a few things:

At the heights people here have been writing about - 10 to 17 thousand
feet - what is the true airspeed at which it enters the spin on command
and how does that differ from the lower altitude airspeed used for
certification tests? Bear in mind also one poster's comments that a
glider does not instantly cease forward motion and go instead into
vertical motion with a rotational component - in the absence of infinite
forces, the first is subject to some deceleration taking time and space,
and the second some vertical acceleration taking time and height.

Similarly, what is the true vertical velocity at onset and when stable
in the spin?

What is the ratio of those two velocities compared with the ratio at
test air densities?

Does the rotation rate remain identical, whether at height (lower air
densities) or at lower altitude (higher density)?

Does all that have an effect on true angle of attack?

Could such things account for high altitude spins when fully developed
requiring more turns to recover?

I wonder if the people who conduct these high altitude tests were in a
regime not tested by the maker or the certification test pilots such as
Chris Rollings?

Chris N.

I have been told two things:

1. That the Reynolds number applicable to a glider changes with altitude,

2. That the Reynolds number affects the stall/spin behaviour and recovery.

I can remember an anecdote (I am vague as to who or when) about a K21 at
Aboyne. The glider was at about 20,000 ft., and the crew wanted to get
down for the next pupil. They put it into a spin (which it entered without
difficulty), and then held the spin without moving the controls. It span
down to about 7,000 ft. and then self-recovered without any
change in control position. Heights are QNH.

I don't understand Reynolds numbers, but I know it matters; and it would
seem not only to designers. It might well be that some gliders more easily
spin and are harder to recover at height. I would expect this to apply at
altitudes above about 7,000 ft. QNH, above which height they are almost
certainly not test flown.

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.


"Chris Nicholas" wrote in message
...

Reading of these brave souls who have been testing the Puchacz spinning
characteristics with multi-turn spins at high altitudes reminds me of an
anecdotal rumour which reached me about at least one other glider type,
and I think also some power aircraft, which similarly misbehaved until
many turns/it got lower. It made me wonder a few things:

At the heights people here have been writing about - 10 to 17 thousand
feet - what is the true airspeed at which it enters the spin on command
and how does that differ from the lower altitude airspeed used for
certification tests? Bear in mind also one poster's comments that a
glider does not instantly cease forward motion and go instead into
vertical motion with a rotational component - in the absence of infinite
forces, the first is subject to some deceleration taking time and space,
and the second some vertical acceleration taking time and height.

Similarly, what is the true vertical velocity at onset and when stable
in the spin?

What is the ratio of those two velocities compared with the ratio at
test air densities?

Does the rotation rate remain identical, whether at height (lower air
densities) or at lower altitude (higher density)?

Does all that have an effect on true angle of attack?

Could such things account for high altitude spins when fully developed
requiring more turns to recover?

I wonder if the people who conduct these high altitude tests were in a
regime not tested by the maker or the certification test pilots such as
Chris Rollings?

Chris N.

W.J. (Bill) Dean (U.K.). wrote:

I have been told two things:

1. That the Reynolds number applicable to a glider changes with altitude,

2. That the Reynolds number affects the stall/spin behaviour and recovery.

I can remember an anecdote (I am vague as to who or when) about a K21 at
Aboyne. The glider was at about 20,000 ft., and the crew wanted to get
down for the next pupil. They put it into a spin (which it entered without
difficulty), and then held the spin without moving the controls. It span
down to about 7,000 ft. and then self-recovered without any
change in control position. Heights are QNH.

I don't understand Reynolds numbers, but I know it matters; and it would
seem not only to designers. It might well be that some gliders more easily
spin and are harder to recover at height. I would expect this to apply at
altitudes above about 7,000 ft. QNH, above which height they are almost
certainly not test flown.

W.J. (Bill) Dean (U.K.).
Remove "ic" to reply.

So I'm a test pilot every time I fly from my home field at 7,500 ft MSL?

On Wed, 28 Jan 2004 14:12:44 UTC, "W.J. \(Bill\) Dean \(U.K.\)."
wrote:

: I don't understand Reynolds numbers

They are simply a way of comparing flow patterns round gliders in
different situations. If two gliders have the same Reynolds number
then the flow round them will look the same - the stream lines are the
same shape - regardless of the velocity. It's given by density *
velocity * characteristic length / viscosity. So if you go to a high
altitude (- low density) you need a higher speed for the same flow
pattern [1]. Or if you make a 50% scale model (- smaller length) you
also need a higher speed.

But beware - Reynolds numbers can be used to compare smaller parts of
aerodyamics as well. For example, separation of a boundary layer
typically occurs at a particular Reynolds number based on distance
from the leading edge of the wing, frontal drag from the fuselage will
depend on the Reynolds number based on the mean diameter of the
fuselage and so on. This is one of the main reasons people find
Reynolds numbers confusing ...

Ian, with his Fluid Dynamics Lecturer's hat on.

[1] Yes, I know viscosity must change with pressure and density, but I
can't offhand remember how!
--

Heh, in Poland, there is a short story of two bored pilots who wanted to =
have some fun in Puchacz when returnig from a wave flight. They spun the =
Puchacz from the 7000m, and kept it spinning till 2000m. When they =
started the recovery, there was.... nothing, the Puchacz still spinned!
Finally, they stopped after the 6'th or 7'th turn since the recovery =
procedure has begun.

Remember about inertia guys!!!


--=20
Janusz Kesik

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