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.