The problems involving vibration which affect helicopters can be divided into two major categories:

• (a)Forced vibration effects

• (b)Aeroelastic and aeromechanical instabilities.

Forced vibration effects are subdivided into two categories.

• (i)Effects due primarily to aerodynamic forcing; and

• (ii)Effects due primarily to mechanical forcing.

Forced vibration effects have important repercussions in the area of:

• (i)Crew comfort and efficiency

• (ii)Fatigue strength of dynamic components and the airframe

• (iii)Reliability of electronic equipment.

The most important forcing effect is aerodynamic in origin, and this will be considered first.