Recent Discussions from the Comanche Owner’s Forum –
Stabilator Flutter
A
s many ICS members can
attest, the best source of
information is often other
Comanche owners who have faced
similar maintenance and operational
issues. As a service to all ICS
members, the following is a series of
online postings from the Comanche’s
Owner’s Forum. This discussion
between Comanche owners con-
cerns stabilator flutter. It is printed
as it appeared in the Forum.
I just installed an S-TEC 55X
autopilot in my 1965 twin and
when I go to a vertical speed decent
and the airspeed approaches 195
or just about the yellow arc, the
stabilator starts to flutter. As soon
as I disengage the A/P, it stops. I
have checked the cable tension on
the elevator and the bridle cables
on the A/P. I have had the A/C for
nine years with the old Altimatic
A/P and this is a new problem.
Any insight from anyone would be
appreciated thanks.
I would be very interested in hear-
ing about any such flutter problems
and their resolution in this forum.
I get stabilator flutter on my TC at
about 195 IAS if the bushing or bolt
between the control rod and the trim
actuator. Just fixed it again last week.
I wonder if the pitch trim bridle
is reaching its nose down cable limit
and slipping the clutch?
I have seen a low frequency 5 HZ
(+-) oscillation with the autopilot on,
out-of-trim and some play in the trim
drum. I did it by leveling off, cranking
in cruise trim (before speeding up)
and hitting altitude hold. My guess
is that what you are seeing is a
similar effect, as the autopilot doesn’t
have the speed to produce a fast
oscillation on its own.
I fixed the tab play and don’t hit
the hold so early and have not
had a repeat. Check the stabilator
trim tab play.
I’ve got a 55X in my 1966 PA 30
and have never experienced anything
like you describe. The highest I’ve
ever let my airspeed get was about
220 indicated on a very smooth day
and still no flutter whatsoever.
Normally, just to ensure no issues
in a descent, I’ll reduce power to
keep the airspeed 5 to 10 mph below
the yellow. The 55X has always
been rock steady.
Not directed at you Todd but to
everyone, are we really talking about
flutter guys? Not a term I would
throw around loosely! I wasn’t aware
that you could have a true flutter
incident that didn’t result in loss
or near loss of the aircraft.
Hans, help me out on this will you?
According to Maurice, and other
postings on this forum and my own
experience as well, stabilator flutter
will occur in different degrees of
intensity.
Severe flutter will result in serious
damage and possible loss of control
of the aircraft. The plane in the
famous NASA flutter movie managed
to land safely although it did need
some (a lot of?) repairs.
At the other end, I have experi-
enced what I would call mild flutter,
where the airframe vibrated and a
slight sensation was also felt in the
yoke for a few seconds. This was also
an alarm bell to get things fixed
immediately.
I would consider all incidents of
flutter as quite serious since they
all are at least a first step (or last)
before loss of control and damage
and reflect an airframe in need of
immediate maintenance.
Comments are welcome.
To everyone interested in this
discussion: True flutter occurs
when torsional instability couples
to the bending frequency of the
structure. Normally, up to flutter
speed, structural damping and air
damping, prevent these vibration
modes from coupling. The torsional
mode is when the stabilator tips
rotate about the spar, and the
bending mode is when the stabilator
tips flex up and down.
Looking at the NASA flutter film,
one observes that the stabilator tips
are both rotating and flexing. The
motion is somewhat similar to a
manta ray swimming.
What Jack01267, T6, Charlie,
James and others have experienced
(including myself at one time) is a
buffeting caused by looseness in the
stabilator trim system, which can
be the start of torsional instability.
The two main causes of stabilator
trim tab buffeting are 1) Washing the
airplane, and getting water into the
trim tab (increases tab mass), and 2)
Looseness of the trim system (exci-
tation force and reduced damping).
There are three locations where
this looseness can occur. The two
ends of the trim tab control rod
contain bushings, a thru pin, and an
AN bolt. The trim drum contains two
bushings, which are preloaded by
the two cable guard bolts. The entire
assembly must allow movement,
but with minimum excess play.
So, what is the maximum allow-
able up/down movement, measured
at the aft center of the trim tab? I
don’t recall if the service manual
has a requirement, but I will offer
up that ± .050 is maximum.
If there are airplanes out there
with more play than that, then the
word needs to get out for problem
correction. We don’t want to lose an
JANUARY 2004 Comanche Flyer • 17
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