Methods of Rotor Noise Reduction

It will be clear that in the quest to reduce helicopter rotor noise not only does the rotor noise intensity and directivity need to be predicted accurately, but strategies need be devised to either reduce or defocus rotor noise. One approach is to first try to relate the far-field rotor noise levels to their source points on the rotor. This has been recognized by several authors, including Sim et al. (1995), Lowson (1996), and Strawn (1997). Therefore, at least in principle, it may be possible to modify the aerodynamics at specific source points on the rotor and change the propagated noise in a profitable way. This, however, is not an easy goal, and even if possible theoretically by means of calculations and analysis, much work will be needed to actually implement any such proposed system on a helicopter.

The use of higher harmonic cyclic blade pitch [see Brooks et al. (1991) and Yu et al.

(1994) ] and active trailing edge flaps [see Charles et al. (1994) and Dawson (1995)] have been suggested to modify the unsteady blade airloads and alter the intensity of propagated BVI noise. While some benefits have been realized on actual rotor tests, these approaches have been only “open-loop” processes and so require extensive mapping out of the combina­tions of conditions where significant noise reductions are actually obtained. These reduced noise flight conditions, unfortunately, often coincide with increases in vibration levels on the rotor, which will never be acceptable. Passive designs such as blade tip sweep and unequal blade spacing have also been proposed to dephase rotor noise sources [see Baeder (1997) and Sullivan et al. (2002)]. Operational techniques such as aircraft trajectory opti­mization or tip-path-plane AoA control proposed by Schmitz (1998) may offer such benefits in changing vortex/blade miss distances at the rotor. Validation of this approach, however, will require careful predictions of the rotor wake to understand the relative changes in the
wake to flight condition. However, to be successful any approach toward noise reduction still requires a better understanding of the nature and focusing characteristics of the critical sound sources generated by the rotor. The problem of rotor noise reduction continues to be a prime opportunity for future research in helicopter aeroacoustics – see Brentner (1997a, b) and Edwards & Cox (2002).