Aerodynamic sources of noise and vibrations

Rotational noise comprises two mechanisms related to linear aerodynamic theory thickness noise caused by the displacement and reconnection of the air by the rotor blades loading noise due to the forces that the rotor blades exert on the air. For a fixed location of the rotor plane, these processes occur 

The impulsive noise of a helicopter leads to several effects with pulsing characteristic and high amphtudes at discrete frequencies. Again, we are concerned with the higher harmonics of the blade passage frequency. These types of noise occur in different flight situations and are recognized by the human ear 

The examination of the helicopter main rotor has yielded a multitude of excitation mechanisms for noise and vibration. Dealing with such a complicated system, it is unlikely that a single solution exists to produce relief in all aspects. Thus, a variety of partially very different approaches has been discussed and developed. In this section, the major ideas involving non-active elements will be presented for the classical helicopter configuration. Details on these elements are given by Bielawa [21], Brarnwell et al. [23]. 

As vibrations have historically been the dominant problem and are easier to solve without a detailed understanding of their generation processes, a number of devices to improve the situation locally at specific mount points have been developed. 

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Fig. 2.2. Aerodynamic sources of noise and vibration at the helicopter main rotor original photograph by Eurocopter.

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