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lighten), until either the rudder or the ailerons reach
full deflection or the maximum sideslip angle is
reached.
(2) At no time should there be a tendency
toward a force reversal, which could lead to an overbalance
condition or a rudder lock.
(3) Release the ailerons while still holding
full rudder. When the ailerons are released, the low
wing should return to the level position. Do not assist
the ailerons during this evaluation.
(4) To check static directional stability,
trim the aircraft at a low cruise setting above 5,000
feet AFL. Slowly yaw the aircraft left and right using
the rudder. Simultaneously the wings should be kept
level by using the ailerons. When the rudder is
released, the aircraft should tend to return to straight
flight.
g. Spiral Stability. This is determined by the
aircraft’s tendency to raise the low wing when the
controls are released in a bank. To test for spiral
stability, apply 15 to 20 degrees of bank either to
the left or right, and release the controls. If the bank
angle decreases, the spiral stability is positive. If the
bank angle stays the same, the spiral stability is neutral.
If the bank angle increases, the spiral stability
is negative. Negative spiral stability is not necessarily
dangerous, but the rate of divergence should
not be too great or the aircraft will require frequent
pilot attention and will be difficult to fly, especially
on instruments.
NOTE: Friction in the aileron control system
can completely mask the inherent spiral
characteristics of the airframe.
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AC 90-89A 5/24/95
SECTION 4. A WORD OR TWO ABOUT FLUTTER
‘‘Stay up on the edge of your seat.’’ Scott Crossfield, Test Pilot
1. OBJECTIVE. To understand the causes and
cures of the condition known as flutter.
2. DESCRIPTION. Flutter in an aircraft structure
is the result of an interaction between aerodynamic
inputs, the elastic properties of the structure,
the mass or weight distribution of the various elements,
and airspeed.
a. To most people, the word ‘‘flutter’’ suggests
a flag’s movement as the wind blows across it. In
a light breeze, the flag waves gently but as the wind
speed increases, the flags motion becomes more and
more excited. It takes little imagination to realize
if something similar happened to an aircraft structure,
the effects would be catastrophic. The parallel
to a flag is appropriate.
b. Think of a primary surface with a control
hinged to it (e.g., an aileron). Imagine that the airplane
hits a thermal. The initial response of the wing
is to bend upwards relative to the fuselage.
c. If the center of mass of the aileron is not
exactly on the hinge line, it will tend to lag behind
the wing as it bends upwards.
d. In a simple, unbalanced, flap-type hinged
control, the center of mass will be behind the hinge
line and the inertial lag will result in the aileron being
deflected downwards. This will result in the wing
momentarily generating more lift, increasing its
upward bending moment and its velocity relative to
the fuselage. The inertia of the wing will carry it
upwards beyond its equilibrium position to a point
where more energy is stored in the deformed structure
than can be opposed by the aerodynamic forces
acting on it.
e. The wing ‘‘bounces back’’ and starts to
move downward but, as before, the aileron lags
behind and is deflected upwards this time. This adds
to the aerodynamic down force on the wing, once
more driving it beyond its equilibrium position and
the cycle repeats.
f. Flutter can happen at any speed, including
take-off speed. At low airspeeds, however, structural
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5/24/95 AC 90-89A
and aerodynamic damping quickly suppress the flutter
motion. But as the airspeed increases, so do the
aerodynamic driving forces generated by the aileron.
When they are large enough to cancel the damping,
the motion becomes continuous.
g. Further SMALL INCREASES will
produce a divergent, or increasing oscillation, which
can quickly exceed the structural limits of the airframe.
Even when flutter is on the verge of becoming
catastrophic it can still be very hard to detect. What
causes this is the high frequency of the oscillation,
typically between 5 and 20 Hz (cycles per second).
It will take but a small increase in speed (1⁄4 knot
or less) to remove what little damping remains and
the motion will become divergent rapidly.
h. Flutter also can occur on a smaller scale
　
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