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Dynamic cases
Structural flutter
Flutter is the name given to a form of structural vibration that normally involves a combination of motions; typically bending and twisting. It is most likely to occur on wings, but tailplane flutter is not uncommon. The Handley Page 0/400 bomber produced an early, well documented, case of tailplane flutter, during the First World War.
Figure 14.4 illustrates one flutter mode. In this example, the wing is oscillating in torsion (twisting) as well as bending. As it bends upwards as in position-1 in Fig. 14.4, it is twisting in a nose-down sense. When it passes the limit of upward travel and starts to spring back down, it presents a negative (nose-down) angle of attack as in position-4. As the lift force is now downwards, it aids the motion.
Near the bottom limit of travel, the wing starts to twist nose-up, so that when it springs upwards (position-9), the angle of attack has become positive,
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Fig. 14.4 Flutter The wing twists as it flaps up and down. The changes in angle of attack mean that the aerodynamic force is always tending to help the motion, which does not, therefore, damp out |
and the lift force again aids the motion. Because the lift force is assisting the movement, the motion does not damp out like a normal vibration, and it can continue, sometimes with the amplitude increasing until failure occurs.
It will be seen, that in the case described above, the torsional and bending oscillations are 90° out of phase; that is, the bending deflection reaches its maximum as the twist approaches its mid-position.
The normal remedy is to increase the torsional stiffness of the wing, but flutter can also occur by a coupling of the wing flexure with a pitching oscillation of the whole aircraft. This problem was encountered on some tailless aircraft, for which the pitching inertia was low. In such cases, increasing the torsional stiffness would not help, and it was the flexural stiffness that had to be increased.
The mass distribution also critically affects the flutter behaviour of the wing, and the location of wing-mounted engines is an important factor.
As with divergence, the onset of flutter depends on the aircraft speed. The lowest speed at which flutter occurs, is known as the critical flutter speed, and again, it is necessary to ensure that the aircraft speed never reaches the critical value.
