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.