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Grob Twin Astir Tailshaking



 
 
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  #1  
Old January 11th 07, 06:59 AM posted to rec.aviation.soaring
Peter[_5_]
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Posts: 4
Default Grob Twin Astir Tailshaking

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.

Ads
  #2  
Old January 11th 07, 06:35 PM posted to rec.aviation.soaring
P Ilatus
external usenet poster
 
Posts: 10
Default Grob Twin Astir Tailshaking

my old club in the UK had an Astir which exhibited
the same tail shaking at high speed. the problem was
the 'banjo' joint where the tail met the fuselage.
unfortunately I don't have any other details except
that it was at the Lakes gliding club back in '94 or
there abouts. I'm sure they'd remember.








At 06:06 11 January 2007, 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.




  #3  
Old January 11th 07, 06:35 PM posted to rec.aviation.soaring
P Ilatus
external usenet poster
 
Posts: 10
Default Grob Twin Astir Tailshaking

my old club in the UK had an Astir which exhibited
the same tail shaking at high speed. the problem was
the 'banjo' joint where the tail met the fuselage.
unfortunately I don't have any other details except
that it was at the Lakes gliding club back in '94 or
there abouts. I'm sure they'd remember.








At 06:06 11 January 2007, 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.




  #4  
Old January 12th 07, 09:30 AM posted to rec.aviation.soaring
Michael Huber
external usenet poster
 
Posts: 38
Default Grob Twin Astir Tailshaking

Peter,

check the rudder hinges and the ribs that holds the hinges.
Michael


  #5  
Old January 12th 07, 09:59 AM posted to rec.aviation.soaring
[email protected]
external usenet poster
 
Posts: 209
Default Grob Twin Astir Tailshaking

Forget that....

Check your parachute has had a fresh repack!!!
Is it too late to ask for your money back?

Al

Michael Huber wrote:
Peter,

check the rudder hinges and the ribs that holds the hinges.
Michael


  #6  
Old January 12th 07, 06:29 PM posted to rec.aviation.soaring
KP
external usenet poster
 
Posts: 8
Default Grob Twin Astir Tailshaking

Peter
I second Michael's rudder statement. I have a Grob 103 C Twen Acro III
and the rib that is glued to the vertical skin by the upper hinge came
unglued. You can check this by putting pressure on the right side of
the rudder at the upper hinge point and see if the vertical fin moves.
The glider had five hours on it at the time.

Ken (KP)

On Jan 12, 12:30*am, "Michael Huber" wrote:
Peter,

check the rudder hinges and the ribs that holds the hinges.
Michael


  #7  
Old January 13th 07, 03:30 PM posted to rec.aviation.soaring
John Sinclair
external usenet poster
 
Posts: 49
Default Grob Twin Astir Tailshaking

Peter,
You may have experienced low frequency rudder flutter.
I have seen a 2 cycle per second rudder flutter at
100 knots in a Duster. Recommend you check the rudder
balance to see if it is within factory specs. If it
did flutter, the hinges and drive rib may be damaged
as well. have a good look at the fin spar with the
rudder off.
JJ


At 06:06 11 January 2007, 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.





  #8  
Old January 13th 07, 03:56 PM posted to rec.aviation.soaring
Andy[_1_]
external usenet poster
 
Posts: 1,565
Default Grob Twin Astir Tailshaking


John Sinclair wrote:
Peter,
You may have experienced low frequency rudder flutter.


Can the rudder have low frequency flutter without the rudder pedals
following. I assume not, so were the pedals moving?

Andy

  #9  
Old January 13th 07, 06:23 PM posted to rec.aviation.soaring
Dave Rolley
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Posts: 4
Default Grob Twin Astir Tailshaking

JJ is spot on! I've been through this myself in a Concept 70.

Early certification guidelines indicated that balancing the rudder was
not necessary on aircraft with a Vne below a certain speed. Of course
the guidelines were written for aircraft with steel tube or monocote
aluminum construction. GRP structures behave differently.

In the case of my Concept 70, the rudder was grossly out of balance. It
took almost 12 ounces of lead ahead of the hinge line to balance the
rudder. That was the short term fix. About two years later, while
refinishing the glider, I had a new rudder built out of glass, carbon,
and kevlar. Plus I had the fin spar inspected and, as it turned out, it
and the hinges needed repair. We stripped the old rudder and found at
least 4 layers of finish on it.

BTW, how serious is rudder balance? On a V-tail Bonanza the rudder
being out of balance by the weight of a silver dollar (1 ounce) on the
rudder trailing edge is enough to cause flutter and catastrophic
structural failure at speeds below the yellow arc. It is not something
to play with.

As to the rudder pedals moving, you would be surprised how much the
rudder cable can stretch. Since most glider rudder systems are cable
driven at the rudder regardless if push rods are used up front, unless
you are on the rudder pedal real hard, you might not notice the pedal
moving until the rudder deflections get rather large.

Since the control surface extracts energy from the airflow, the only way
to deal with flutter in flight is to slow down. Right Now! Don't wait
and don't think you can control it with control inputs from the cockpit.
You have to get rid of the active energy in the situation.

Dave

Andy wrote:
John Sinclair wrote:
Peter,
You may have experienced low frequency rudder flutter.


Can the rudder have low frequency flutter without the rudder pedals
following. I assume not, so were the pedals moving?

Andy

  #10  
Old January 13th 07, 07:23 PM posted to rec.aviation.soaring
Peter[_5_]
external usenet poster
 
Posts: 4
Default Grob Twin Astir Tailshaking


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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