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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 reactions.
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
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