From: The Lancair Mailing List
Flutter is not simply
caused by something being loose; that is a gross oversimplification.
Flutter is a complicated science and is sometimes confused with
buffeting and
vibration. Typically flutter is experienced at or near the Vne speed of
the
aircraft. It is generally known to happen very quickly and involves a
catastrophic failure. But wait, there is more - a lot more; read on.
http://www.angelfire.com/music/thugboy/thesis/part1.htm
Flutter
Flutter is a
dangerous phenomenon encountered in flexible structures subjected to
aerodynamic forces.
Flutter
occurs as a result of interactions between aerodynamic, stiffness, and
inertial
forces on a structure. For an aircraft, flutter may occur when the
aircraft is
accelerated to a speed where, when disturbed, the wings flex, and the
resultant
vibrations do not have sufficient damping. The damping of an aircraft’s
vibrations is a function of the speed at which it is flying.
Also at:
http://www.geocities.com/mgd3/flying/flutter
Flutter is
the resonance of a structure that occurs when the elastic properties of
the
structure are in harmony with a load being applied. Although pilots
normally
think in terms of "aileron flutter", flutter can be experienced in
the fuselage, stabilizer, rudder, wings, or even propellers.
There are
several type of flutter modes that can occur. Most pilots think of
aileron
flutter as looking out and seeing their ailerons buzzing up and down.
Although an
easy answer is "unbalanced control surfaces", you need to understand
why. In an unbalanced control surface, there is a positive rotational
moment of
inertia. In other words, the CG of the surface is not at the hinge
point. So if
load the aircraft in G (or shake the wing up and down), the aileron
will want
to rotate up and down accordingly. In my case, the aileron vibration
translated
to wing vibration, which overloaded the wing attach fitting. Now
typically the
opposing ailerons will counterbalance this tendency. But importantly,
all of
the intervening pushrods, slave-struts, hinges, idlers, bellcranks,
bolts and
bearings add a degree of flexibility. And that's what causes problems.
The
independent variables include control surface moment and balance
characteristics, but they also include vibration sources (such as the
rotating
heavy thing up front), turbulence, air density, velocity, G-loading,
CG,
weight, shock wave formation, etc.
At:
http://www.findarticles.com/p/articles/mi_m0JZX/is_2_6/ai_78360106
Flutter is
the dynamic instability of an elastic body in an airstream. Flutter
speed (Uf)
and the corresponding frequency (vf) are defined as the lowest airspeed
and
frequency at which a flying structure will exhibit sustained, simple
harmonic
oscillations. Flutter is a dynamic instability (self-sustaining and
increasing)
that may result in failure of the structure. In aircraft, the failure
of a main
structure generally results in the loss of the aircraft. Aircraft are
designed
such that their airframe flutter will occur at airspeeds and conditions
outside
the aircraft envelope by a safety margin of at least 15 percent.
Modifications
that change the vibrational modes of an aircraft cause the flutter
speed to
change.
The
frequency and airspeed at which flutter occurs generally increases with
increased structural stiffness. However, many times increased stiffness
in a
structural component changes the vibrational frequencies of that
component and
result in changes of frequencies in the overall aircraft structures.
These
changes can cause unforeseen consequences such as vibration or flutter,
and
their effect must be evaluated by analyses or testing. Usually, a
ground
vibration test is made to determine changes in the vibrational modes of
a
modified airframe. These modes are used to validate or update the
structural
dynamic analysis model that determines the flutter speeds and
frequencies.
Flutter, buffeting,
and vibration can affect handling qualities. This is caused by the
uncompensated motion of the flight control surfaces relative to the
airflow.
For instance, an elevator rotated upward is expected to cause an
aircraft to
climb. Deflection of the horizontal stabilizer caused by buffet,
flutter, or
vibration can result in the elevator providing a nose-down rotation.
Asymmetric
bending of the horizontal stabilizer from flutter, buffet, or vibration
can
cause a roll or yaw. In general, remedies for flutter, buffet, and
vibration
are also remedies for these types of handling problems. These are
usually
high-speed problems.
See:
http://www.auf.asn.au/groundschool/flutter.html
When aerodynamic
forces applied to the wing or a control surface alter the aoa, the
dynamic
pressure distribution changes. These changes plus the structure's
elastic
reactions may combine as an oscillation or vibration (probably
initially
noticed as a buzz in the airframe) which will either damp itself or, as
the
airspeed is increased, may begin to resonate at the natural frequency
of the
structure and thus rapidly increase in amplitude if the phase
relationships are
right. (Pushing a child on a swing is an example of phase relationships
and
amplification). This latter condition is flutter and, unless airspeed
is very
quickly reduced, will cause control surface separation within a very
few
seconds.
Inertia has
a role in flutter development requiring that control surfaces –
ailerons,
elevators, rudder – be mass balanced (i.e. the centre of gravity of the
control surface coincides with the hinge line) to limit the mass moment
of
inertia; and also to prevent them becoming heavier as airspeed
increases. It
may be acceptable for the control surface to be over-balanced, i.e. the
cg is
slightly forward of the hinge line.
The critical
flutter airspeed [or something akin to it] may eventuate well below Vd
or Vdf
(See Note at bottom) if wear in control surface hinges, slop in
actuating
rods/cables/cranks/torque tubes, water or ice inside control surfaces
or
absorbed within a foam core, mud outside, faulty trim tabs or other
system
weaknesses exist which alter the structure's reaction.
The
following paragraph is an extract from an article by William P. Rodden
appearing in the McGraw-Hill Dictionary of Science and Technology; it
provides
a succinct description of flutter:
"Flutter
(aeronautics) – An aeroelastic self-excited vibration with a sustained
or
divergent amplitude, which occurs when a structure is placed in a flow
of
sufficiently high velocity. Flutter is an instability that can be
extremely
violent. At low speeds, in the presence of an airstream, the vibration
modes of
an aircraft are stable; that is, if the aircraft is disturbed, the
ensuing
motion will be damped. At higher speeds, the effect of the airstream is
to
couple two or more vibration modes such that the vibrating structure
will
extract energy from the airstream. The coupled vibration modes will
remain
stable as long as the extracted energy is dissipated by the internal
damping or
friction of the structure. However a critical speed is reached when the
extracted energy equals the amount of energy that the structure is
capable of
dissipating, and a neutrally stable vibration will persist. This is
called the
flutter speed. At a higher speed, the vibration amplitude will diverge,
and a
structural failure will result."
NOTE: Flight
at airspeeds outside the envelope (or at inappropriate speeds in
turbulent
conditions or when applying inappropriate control loads in a high-speed
descent
or, indeed, at any time) is risky and can lead to airframe failure. Vne
is the
IAS which should never be intentionally exceeded in a descent or other
manoeuvre and is normally set at 90% of Vd, the 'design diving speed'.
For a
normal category aircraft, Vd is required to be 1.4 times Vno and, to
receive
certification, it must be demonstrated, possibly by analytical methods,
that
the propeller, engine, engine mount, and airframe will be free from
overspeeding, severe vibration, buffeting, flutter, control reversal
and
divergence. To provide some safety margin, Vne is then set at 90% of
the lower
of Vd or Vdf. Vdf is a diving speed which has been demonstrated without
problem
in test flights and which must be lower than, or equal to, Vd.
I hope this
has been a help.
Warm
regards,
Gary
FXE (Fort Lauderdale Executive)
http://www.uslan.com/hinge-kit.html
Proper Aileron Alignment
More than one case of ailerons hanging low has been documented on the
RV-10. Ideally, you want the flaps full-up, and align the
ailerons to them and have the wingtips aligned with them too, all at
the same time.
The tip that I found was that you need to perform the alignment of the
ailerons with the elevator completely neutral. I originally did
my alignment with the stick secured, however if you have up-elevator
applied your ailerons will both rise slightly and cause them to be low
while in-flight. Aligning them with a perfectly neutral stick
will fix this issue.
Watch
for this Elevator Trim Issue!
Here is a link to the original post:
http://forums.matronics.com/viewtopic.php?t=20576&highlight=elevator+trim+tabs
Read through the information below, but let me tell you my opinion on
it first...
Basically the gist of the issue is that you need to ensure that both
sides of the trim tabs go to the neutral, trailing position, at the
same time, and that no matter what you do, will you end up with a
situation where one trim tab goes up while the other goes down.
It's that simple. So have someone actuate the trim while you
watch...if you have a position where one tab is up and the other is
down, fix it. In normal operation, the trim tabs do NOT move
exactly the same. If memory serves correctly, they both go down
together, but as they pass through neutral towards up, only one
continues upwards. There is a small period of time though where
the cam action of the actuator may briefly make one side move a small
amount down as it's on it's way upward. At any rate, just inspect
your trim and ensure that never is there a time when the trim tabs are
opposite eachother. If you achieve that, you should be set, as
long as you're getting somewhere near the listed travel
requiremements. I haven't personally verified Bill's measurements
below but I'm sure they're at least close.
Bill's Post
I went through this issue with Vans couple of months ago hoping for a
plans revision or service bulletin but those folks seem to be busy in
other areas - but I believe it to be important and want to share my
findings.
I setup my elevator tabs per the instructions with both left and right
35 degrees below trail position when the servo is at the full up
position. It is possible to meet this spec by making adjustments at the
servo end of the cable.
During my first cross country I happened to look back at the tail and
saw it was twisted. The elevator counter weight was above the
horizontal stablizer on one side and below on the other. This bothered
me more than a little as the forces would need to be high to cause this
twist. I had not noticed this during my test flights because I now had
luggage in the back and needed more down trim. After a lot of
measurements I determined that the problem was I followed the
instructions. If you begin with both elevator trim surfaces down at
exactly 35 degrees the port side will never rise to trail position do
to the cam action of the two actuators. Yet, the starboard will rise
.75 inch above trail causing the twist.
At a minimum effort, all flying RV-10s should check for this condition.
The fix is easy.
Highly recommend that you set the trim as follows:
1. Run the trim servo to full nose up.
2. Set the starboard trim tab trailing edge to 3 inches below elevator
trailing edge.
3. Run the starboard trim tab to trail.
4. Set the port trim tab to trail.
Using these settings you will be able to trim out all pitch forces
during final with the CG forward.
Bill DeRouchey
N939SB, flying straight