In addition to the fluttering of a wing or tail, the control surfaces may flutter. This is a problem on mechanically operated systems, due to the flexibility of the cables or rods. A common case occurs with ailerons when their centre of gravity is behind the pivot line. In this case, as the wing accelerates upwards, the inertia of the control surface causes it to swing downward relative to the wing. This increases the lift and assists the motion which is, therefore, not damped out.
The traditional solution was mass balancing; the introduction of weights, which alter the natural frequencies, and change the position of the centre of gravity of the surface, as described in Chapter 11. Other forms of control surface oscillation are associated with vibrational resonances, and their cure requires a proper analysis of the system.
Aircraft with power-operated controls are less prone to control flutter, as the hydraulic actuating rams provide a virtually inelastic linkage.