Page 589 - Aircraft Stuctures for Engineering Student

570 Elementary aeroelasticity

The type of flutter described above, in which two distinctly different types of
oscillating motion interact such that the resultant motion is divergent, is known as
classical flutter. Other types of flutter, non-classical flutter, may involve only one
type of motion. For example, stallingflutter of a wing occurs at a high incidence
where, for particular positions of the spanwise axis of twist, self-excited twisting
oscillations occur which, above a critical speed, diverge.
Another non-classical form of flutter, aileron buzz, occurs at high subsonic speeds
and is associated with the shock wave on the wing forward of the aileron. If the
aileron oscillates downwards the flow over the upper surface of the wing accelerates,
intensifying the shock and resulting in a reduction in pressure in the boundary layer
behind the shock. The aileron, therefore, tends to be sucked back to its neutral
position. When the aileron rises the shock intensity reduces and the pressure in the
boundary layer increases, tending to push the aileron back to its neutral position.
At low frequencies these pressure changes are approximately 180" out of phase
with the aileron deflection and therefore become aerodynamic damping forces. At
higher frequencies a component of pressure appears in phase with the aileron velocity
which excites the oscillation. If this is greater than all other damping actions on the
aileron a high frequency oscillation results in which only one type of motion, rotation
of the aileron about its hinge, is present, i.e. aileron buzz. Aileron buzz may be
prevented by employing control jacks of sufficient stiffness to ensure that the natural
frequency of aileron rotation is high.
Bufeting is produced most commonly in a tailplane by eddies caused by poor
airflow in the wing wake striking the tailplane at a frequency equal to its natural
frequency; a resonant oscillation having one degree of freedom could then occur.
The problem may be alleviated by proper positioning of the tailplane and clean
aerodynamic design.

13.4.1 Coupling -

We have seen that the classical flutter of an aircraft wing involves the interaction of
flexural and torsional motions. Separately neither motion will cause flutter but
together, at critical values of amplitude and phase angle, the forces produced by
one motion excite the other; the two types of motion are then said to be coupled.
Various forms of coupling occur: inertial, aerodynamic and elastic.
The cross-section of a small length of wing is shown in Fig. 13.21. Its centre of gravity
is a distance gc ahead of its flexural axis, c is the wing section chord and the mass of the
small length of wing is m. If the length of wing is subjected to an upward acceleration j;
an accompanying inertia force my acts at its centre of gravity in a downward direction,
thereby producing a nose down torque about the flexural axis of mygc, causing the wing
to twist. The vertical motion therefore induces a twisting motion by virtue of the inertia
forces present, i.e. inertial coupling. Conversely, an angular acceleration ti about the
flexural axis causes a linear acceleration of gc& at the centre of gravity with a corre-
sponding inertia force of mgcii. Thus, angular acceleration generates a force producing
translation, again inertial coupling. Note that the inertia torque due to unit linear accel-
eration (mgc) is equal to the inertia force due to unit angular acceleration (mgc); the
inertial coupling therefore possesses symmetry.