Noise- and vibration-related problems of the helicopter

The lateral thrust of the tail rotor of a helicopter has to compensate for the main rotor torque. For maximum effectiveness, it is operated with a rotational speed as high as permitted by the blade tip velocity, but well below the speed of sound. In general, the noise- and vibration-generating mechanisms are similar to those of the main rotor. While there is no cyclic blade pitch, the interaction with the main rotor outflow has to be taken into account. Due to the comparatively small diameter of the tail rotor, its rotational speed is significantly higher than that of the main rotor and thus the emitted noise and vibrations have higher frequencies, see Staufenbiel et al. [168]. 

The noise and vibration sources discussed above have very different characteristics. Consequently, the various implications and their respective perception depend strongly on the location of the observer. The situation will be discussed briefly in the following from the three basic points of view. 

The soimd radiated by the rotorcraft into its environment is what the critical observer perceives as noise pollution and what delivers a characteristic acoustic signature for aircraft detection and classification. The tjq)ical rotor-craft sound is composed of several components with destinctive directivity and intensity, depending on the flight conditions. In general, at a distance, the main rotor noise is dominant, the high frequency emissions of the tail rotor have some relevance, and the engine noise is secondary. 

Oscillations travel through the structure to the cabin and reach pilots and passengers, as well as vibration sensitive navigation equipment some of this energy is radiated to the air inside the cabin. In addition to this structural sound path, there is the direct air sound path, see Gembler [79] for example 

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Fig. 2.1. Complexity of noise- and vibration-related problems of the helicopter.

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