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Grob Twin Astir Tailshaking



 
 
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  #11  
Old January 14th 07, 05:27 PM posted to rec.aviation.soaring
snoop
external usenet poster
 
Posts: 40
Default Grob Twin Astir Tailshaking

"shudder"? I think the glider just liked the high speed run and 90
degree turn! Snoop


Peter wrote:
On a recent thermalling flight with a friend in the front seat I commenced a
high speed run from 3000 feet to lose height and demonstrate glider flight at
higher speed. I trimmed out for 100kts and flew a straight course until down
to about 2000ft where I then turned left 90 degrees. This took the
glider beneath some late afternoon cumulus that was still providing lift.
Immediately the glider began fishtailing rapidly giving the impression the
horizontal tail surface was moving side to side by a few inches. It sure got
my attention and I immediately opened the airbrakes and raised the nose to
reduce speed to about 60 kts. I honestly felt there was some chance of
structural failure.It gave another shudder and then settled down. I asked the
previous owners of the glider, another gliding club, if they had experienced
this phenomenon but nobody had. However another of our instructors had a
similar experience back in February and again late last year. The engineer is
not too concerned as he has not found anything loose or obviously amiss that
might cause this tail oscillation. It does seem to be rough air related as I
have carried out loops in this glider where I have dived to 105 knots with
none of the tail shaking evident. Our glider is the fixed gear G103 Twin
Astir.

I wonder if anyone else has experienced this. I cant find ant relevant service
bulletins that address the matter.


  #12  
Old January 15th 07, 12:54 AM posted to rec.aviation.soaring
John Sinclair
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Posts: 49
Default Grob Twin Astir Tailshaking


Sure sounds like your doing all the right things, Peter.
What does it do when you grasp the top of the vertical
fin and give it a wicked shaking? Do this to a Libelle,
it keeps shaking for a while. They have been known
to shake like this in the air, also. Not sure I would
be doing acro in that bird or flying it over 80 knots,
for that matter. Grob's got an AD out that strengtherns
the aft fuselage after the whole tail section fell
off of a G-103.
Take Care,
JJ

At 18:30 13 January 2007, Peter wrote:

The glider recently had its 7000 hr inspection carried
out and the engineer
has assured me the rudder balance was checked and corrected
at the time.

Here is what he has come up with to date.

Quote:


Banjo joint?? – Is he talking about the vertical spar
that runs down the
inside of the tail and bells out to the shape of the
fuselage right at the
back of the fin, that the rudder is attached to…. This
is ‘the’ main load
bearing thing that carries the horizontal and vertical
tail loads. The
forward structural pick-up for the tailplane is a shorter
spar that goes
part-way down the fin (from memory).

The glue joints of the fin spar were visually inspected
when we had the rudder
off for the 7000 hr inspn and subsequently tap testing
was done in this area.
If there was a problem here then I am quite confident
we would have found it
during the inspn, or at least during subsequent inspections
of the area where
I have put fairly high side-loadings on the tail to
try to find the problem.
I cannot say categorically that the structure is perfect,
only that there is
no internal noises apparent when the fin is side-loaded
in an oscillatory
manner, and the fuselage/fin torsional damping appears
to be O.K.

Also done at the 7000 hr inspn was a weight and total
moment check of the
rudder while off, which was found to be out of limits
and corrected.

I am open to suggestions as to what else can be the
cause. It would ease our
minds to find and fix the problem.



An interesting piece of information I have found relates
to a known phenomenon
called ‘tail-snaking’ which was defined as a snaking
/ twisting of the tail
unit, but not flutter of the control surface as such.
The original article
related to large RC model gliders in competition at
high speed and another
article made mention of (full size) aircraft manufacturers
using an add-on
ridge on both sides of a control surface at the T.E
to prevent boundary layer
separation shift fore and aft on alternate sides of
the surface.

It is also known that the trailing edges of control
surfaces should not be
rounded but kept abrupt as this is an area that can
cause problems. (the T.E
of the rudder on MW is not rounded.. J). Below is
a bit on the subject and is
bound to help you sleep.



Snaking Oscillations



Another stability problem that was quite common in
airplanes of the period
around WW II was a tendency for a continuous small-amplitude
lateral
oscillation in straight and level flight. This problem
was called 'snaking'
and its cause was quite mysterious. Among the explanations
offered were
response of the normal lateral oscillation of the airplane
to continuous
small-amplitude turbulence, periodic flow separation
from the wing root that
affected the vertical tail, or nonlinear aerodynamic
characteristics of the
wing or tail surfaces for small changes in angle of
attack. One explanation,
which will be discussed subsequently, was the unsteady
lift characteristics of
the vertical tail at low frequencies.

While some of these explanations may have had some
influence in rare
instances, the true explanation was first given by
George Schairer of the
Boeing Company in an analysis of this problem on the
Boeing 314 flying boat,
one of the China Clippers. He pointed out that at small
angles of sideslip,
the rudder had a tendency to float against the relative
wind, which caused the
airplane to swing around and yaw in the opposite direction.
Friction in the
rudder system, however, held the rudder in this position
as the airplane swung
through zero sideslip. On reaching a sideslip in the
opposite direction, the
rudder hinge moments would eventually break through
the friction force and the
cycle would be repeated in the opposite direction.
Thus, energy was fed into
the oscillation by the rudder, which caused the oscillation
to build up to an
amplitude where this energy equaled that removed by
the inherent damping of
the airplane.

On learning of this explanation, efforts were made
to verify it. A convenient
test airplane was the Fairchild 22 on which an experimental
all-moveable
vertical tail had been installed. This type of tail
surface was an invention
of Robert T. Jones and had the advantage that hinge
moments due to angle of
attack and due to deflection could be adjusted separately
with changes in the
hinge location and tab gearing. The tests were made
covering a range of
conditions and friction values, and the validity of
the theory was established
(ref. 4.8).

The question arises as to how such an apparently obvious
control motion could
have escaped detection. The explanation is that because
of the relatively low
damping of the Dutch roll oscillation, the rudder motion
required to sustain a
constant-amplitude oscillation is only a small fraction
of the amplitude of
the yaw or sideslip. For example, in a typical snaking
oscillation of plus or
minus two degrees of sideslip, the rudder motion required
might have been only
plus or minus two-tenths of a degree. This small motion
was less than the
sensitivity of control position recorders used at that
time, and [31] this
motion could be absorbed by stretch in the control
cables without being felt
at the pilot's rudder pedals.

Another little-known aspect was the tendency of the
rudder to float against
the relative wind at small sideslip angles. Most control
surfaces float with
the relative wind at larger sideslip angles. In typical
wind-tunnel tests,
measurements had been made at increments of angles
of attack or sideslip of
five degrees, and as a result, the small changes in
characteristics at very
small values of angle of sideslip were not detected.

In addition to flight tests, theoretical studies were
made to explain and
quantitatively predict the oscillation. These studies
are discussed in a
subsequent chapter.

I and our CFI had a quick check yesterday of the rudder
and all we found was a
small amount of play in the top hinge bearing. The
components themselves seem
well connected to the structure.





 




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