Spinning the SZD 50-3

On Wed, 28 Jan 2004 01:43:36 UTC, "Bill Daniels"
wrote:

: Remember the old spin-the-hammer trick from freshman physics? It seems
: solid objects don't like to spin around their long axis - they prefer to
: spin about their shortest.

Actually, the physics says that objects are stable when rotating
around the axes corresponding to the greatest and least moments of
inertia. For a glider I'd expect yaw to be the highest moment of
inertia and pitch to be the lowest. Howver, that ignores aerodynamic
effects, which I'd expect to be much more significant.

Ian
--

On 28 Jan 2004 11:17:41 GMT, "Ian Johnston"
wrote:

On Wed, 28 Jan 2004 00:56:12 UTC, (Tim Shea)
wrote:

: 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 am told that a Junior oscillates nose up and down while spinning,
and that recovery is much much snappier nose down.


That's all in the POH. A Junior has three different spin behaviours
depending on cockpit load. IIRC the oscillation occurs with a light
pilot.

I'm in the middle group (180 with a chute) and it recovers
automatically after just over 2 rotations, even with the controls
still fully crossed. The last half rotation gets really slow. I was a
bit annoyed. Having just done Silver height and wanting down in a
hurry, I was after 3 turns and was intending to come most of the way
down in a 3-turn - recover - spin the other way sequence. Still, I
repeated the experiment in the other direction and with recovery by
merely centreing the controls and got consistent recovery after just
over 2 rotations.

Be sure to read the POH before attempting more than one rotation.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

I'm in the middle group (180 with a chute) and it recovers
automatically after just over 2 rotations, even with
the controls
still fully crossed. The last half rotation gets really
slow. I was a
bit annoyed. Having just done Silver height and wanting
down in a
hurry, I was after 3 turns and was intending to come
most of the way
down in a 3-turn - recover - spin the other way sequence.
Still, I
repeated the experiment in the other direction and
with recovery by
merely centreing the controls and got consistent recovery
after just
over 2 rotations.


I quite frankly find this a quite scary post! I do
hope you allowed suficient height above the start of
your 1000m gain. I take it you are also one of the
'glider pilot hero' types? Still, i'm sure you impressed
everyone at the bar afterwards..........

So, what was wrong with a more conventional 'rapid
decent', you know, the one that uses full airbrake
circling in sink, or sideslipping with full airbrake.

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.

On 28 Jan 2004 13:42:00 GMT, Pete Zeugma
wrote:

I'm in the middle group (180 with a chute) and it recovers
automatically after just over 2 rotations, even with
the controls
still fully crossed. The last half rotation gets really
slow. I was a
bit annoyed. Having just done Silver height and wanting
down in a
hurry, I was after 3 turns and was intending to come
most of the way
down in a 3-turn - recover - spin the other way sequence.
Still, I
repeated the experiment in the other direction and
with recovery by
merely centreing the controls and got consistent recovery
after just
over 2 rotations.


I quite frankly find this a quite scary post! I do
hope you allowed suficient height above the start of
your 1000m gain.

I was at 5300 ft when I decided to come down and to practise spinning
on the way, ending the last spin at about 2500 ft. That's quite low
enough for me: I won't deliberately initiate a spin under 3000 ft.

So, what was wrong with a more conventional 'rapid
decent', you know, the one that uses full airbrake
circling in sink, or sideslipping with full airbrake.

Not as much fun. Besides I hadn't spun the Junior for a while and
thought I needed the practise.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Shawn Curry writes:

Anyone else spin the Puch for more than three turns? What happened
(obviously you survived)? I've heard that some other aircraft also
have a flatter spin mode that after several turns that is hard to
recover from. Any knowledge of why this happens? (Now where's my
copy of Stick and Rudder?)

The Pitts S-1S is the one for this. Get it spinnong, then let the
controls train and open the throttle. It will go into a FAST flat
spin, picking the nose up to above the horizon. If you are not carfull
and precise in the recovery you can inadvertantly flick it into a spin
in the other direction, or into an outside spin. Not ever heard that
it is hard to recover from though.

--
Paul Repacholi 1 Crescent Rd.,
+61 (08) 9257-1001 Kalamunda.
West Australia 6076
comp.os.vms,- The Older, Grumpier Slashdot
Raw, Cooked or Well-done, it's all half baked.
EPIC, The Architecture of the future, always has been, always will be.

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!
--

On Wed, 28 Jan 2004 15:51:28 UTC, Martin Gregorie
wrote:

: On 28 Jan 2004 13:42:00 GMT, Pete Zeugma
: wrote:

: So, what was wrong with a more conventional 'rapid
: decent', you know, the one that uses full airbrake
: circling in sink, or sideslipping with full airbrake.
:
: Not as much fun.

Coo. You've been and gone and said it now. "Fun", eh?

Ian

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

I suspect you may be, if you go in for multi-turns spins at much above
that sort of altitude. That's the point I was asking about. What do
you think?

More conventional high flying is probably within the range of Reynolds
numbers that correspond with tests, provided you don't push the envelope
at the edges. Also the difference in density and RN is not great from
7,000 to 7500 feet.

If you know all this, of course, you can educate me by telling me the
answers.

If you don't - . . . back to your own question, I think, or perhaps an
aerodynamicist could tell us both (and any others who may be
interested).

The higher you go, of course, the more difference it makes. As pointed
out in other threads, if you go high enough, you stall at the same speed
as flutter onset, which leaves no usable envelope at all.

In my earlier post about true velocities/IAS/density/AoA/rotational
speed etc., as I don't know if everyone realises their tie up with
Reynolds numbers, I deliberately didn't refer to RN. Few (certainly not
me) would know off by heart the formulae, even if they have heard of the
things, or how the other factors and RN change with height. I did,
however, presume that all post bronze or equivalent people will have
done some reading on true vs IAS, flight envelopes, etc.. and might
therefore appreciate that the geometry of a spin, effectiveness of
control surfaces, and rotational aspects, high up could be different
from lower down.

Chris N.