(19) TZZ _T (11) EP 2 568 468 B1 (12) EUROPEAN PATENT SPECIFICATION (45) Date of publication and mention (51) Int Cl.: of the grant of the patent: G10K 11/175 (2006.01) 08.01.2014 Bulletin 2014/02 (21) Application number: 11189425.9 (22) Date of filing: 16.11.2011 (54) Rijke tube cancellation device for helicopters Rijke-Rohr-Unterdrückungsvorrichtung für Helikopter Dispositif pour l’annulation du tuyau de Rijke pour des hélicoptères (84) Designated Contracting States: (56) References cited: AL AT BE BG CH CY CZ DE DK EE ES FI FR GB US-A- 3 685 610 GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR • CARVALHO J A ET AL: "Definition of heater location to drive maximum amplitude acoustic (30) Priority: 07.09.2011 US 201113227231 oscillations in a Rijke tube", COMBUSTION AND FLAME, ELSEVIER SCIENCE PUBLISHING CO., (43) Date of publication of application: INC., NEW YORK, NY.; US, AMSTERDAM, NL, vol. 13.03.2013 Bulletin 2013/11 76, no. 1, 1 April 1989 (1989-04-01), pages 17-27, XP023608572, ISSN: 0010-2180, DOI: (73) Proprietor: Bell Helicopter Textron Inc. 10.1016/0010-2180(89)90073-4 [retrieved on Fort Worth, TX 76101 (US) 1989-04-01] • CHATTERJEE P ET AL: "On the spectral (72) Inventor: Sparks, David characteristics of a self-excited Rijke tube Forth Worth, TX 76118 (US) combustor-numerical simulation and experimental measurements", JOURNAL OF (74) Representative: Lawrence, John SOUND & VIBRATION, LONDON, GB, vol. 283, no. Barker Brettell LLP 3-5, 20 May 2005 (2005-05-20), pages 573-588, 100 Hagley Road XP004830542, ISSN: 0022-460X, DOI: Edgbaston 10.1016/J.JSV.2004.04.019 Birmingham • COLLYER A A ET AL: "Generation of harmonics B16 8QQ (GB) in a Rijke tube by using a single heating element", JOURNAL OF SOUND & VIBRATION, LONDON, GB, vol. 27, no. 2, 22 March 1973 (1973-03-22) , pages 275-277, XP024196638, ISSN: 0022-460X, DOI: 10.1016/0022-460X(73)90069-2 [retrieved on 1973-03-22] Note: Within nine months of the publication of the mention of the grant of the European patent in the European Patent Bulletin, any person may give notice to the European Patent Office of opposition to that patent, in accordance with the Implementing Regulations. Notice of opposition shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). EP 2 568 468 B1 Printed by Jouve, 75001 PARIS (FR) 1 EP 2 568 468 B1 2 Description Figure 2 is the acoustic signature reduction system of Figure 1; BACKGROUND Figure 3 is a chart showing the amplitude and fre- 1. Field of the Invention 5 quency of rotor blade noise according to the pre- ferred embodiment of the present application; [0001] The presentapplication relates in general to hel- icopter acoustics, in particular, to the reduction of a hel- Figure 4 is a chart showing the amplitude and fre- icopter acoustic signature. quency of a thermo-acoustic tube such as a Rijke 10 tube according to the preferred embodiment of the 2. Description of Related Art present application; [0002] Efforts to curtail the sound produced by aircraft, Figure 5 is an oblique view of the helicopter of Figure such as helicopters, has been a focus for many years. 1 having multiple thermo-acoustic tubes coupled to Helicopters produce sound from the engine and trans- 15 the helicopter; mission as well as sound from compression waves gen- erated by the passing of each rotor blade. Figure 6 is a side view of the thermo- acoustic tube [0003] Efforts to address the sound of helicopters have as seen in Figure 2 having one or more bends; typically been in one of two areas. First, efforts regarding noise cancellation have been directed to the cabin of the 20 Figure 7 is a section view of the inside the thermo- helicopter. This would typically involve the use of sound acoustic tube of Figure 2 showing a heating element; deadening materials and insulation layers. Such efforts generally look to insulate cabin passengers from rotor Figure 8 is a section view inside the thermo- acoustic blade noise rather than reducing helicopter acoustic sig- tube of Figure 2 showing a different embodiment of nature. 25 the heating element; [0004] Secondly, efforts have been made in the area of helicopter noise reduction. Noise reduction has typi- Figure 9 is a breakout view of the in thermo- acoustic cally come via advancements in blade design by mini- tube of Figure 2 in an alternate embodiment having mizing main or tail rotor tip speed, for example. Other multiple heating elements; efforts have included ducted tail rotors or other blade 30 symmetry alterations. These particular techniques often Figure 10 is a breakout view of the thermo- acoustic require overall design changes to rotor geometry, power, tube of Figure 2 in an alternate embodiment wherein avionics, and transmission, and generally cannot be a moveable apparatus translates the heating ele- made after the helicopter has completed production. Al- ment along the axis of the thermo- acoustic tube; and so, such efforts are primarily concerned with noise re- 35 duction rather than noise cancellation. Figures 11 and 12 illustrate a cancellation area cre- None of these methods or efforts fully addresses cancel- ated by the acoustic signature reduction system of lation of the acoustic signature of a helicopter, therefore Figure 2. considerable shortcomings remain. [0005] US 3,685,610 discloses a device for reducing 40 [0007] While the system and method of the present noise produced by propellers, in which anti-sound gen- application is susceptible to various modifications and erators are placed in the region of noise generation at a alternative forms, specific embodiments thereof have spacing such that a zone of noise cancellation is provided been shown by way of example in the drawings and are with respect to the point of noise generation. herein described in detail. It should be understood, how- 45 ever, that the description herein of specific embodiments DESCRIPTION OF THE DRAWINGS is not intended to limit the application to the particular embodiment disclosed, but on the contrary, the intention [0006] The novel features believed characteristic of the is to cover all modifications, equivalents, and alternatives application are set forth in the appended claims. Howev- falling within the scope of the process of the present ap- er, the application itself, as well as a preferred mode of 50 plication as defined by the appended claims. use, and further objectives and advantages thereof, will best be understood by reference to the following detailed DETAILED DESCRIPTION OF THE PREFERRED EM- description when read in conjunction with the accompa- BODIMENT nying drawings, wherein: 55 [0008] Illustrative embodiments of the preferred em- Figure 1 is an oblique view of a helicopter with an bodiment are described below. In the interest of clarity, acoustic signature reduction system according to the not all features of an actual implementation are described preferred embodiment of the present application; in this specification. It will of course be appreciated that 2 3 EP 2 568 468 B1 4 in the development of any such actual embodiment, nu- acoustic signature of helicopter 201, and in particular ro- merousimplementation- specific decisions must be made tor blades 207, 211. to achieve the developer’s specific goals, such as com- [0013] Thermo-acoustics typically refers to the crea- pliance with system-related and business-related con- tion of sound in a device due to the transfer of energy straints, which will vary from one implementation to an- 5 from a thermal energy source. Acoustic signature reduc- other. Moreover, it will be appreciated that such a devel- tion system 101 is configured to generate a cancellation opment effort might be complex and time- consuming but noise of a selected frequency and amplitude. The ampli- would nevertheless be a routine undertaking for those of tude and frequency is chosen based on the amplitude ordinary skill in the art having the benefit of this disclo- and frequency of a compression noise generated by rotor sure. 10 blades 207, 211 while rotating. The compression noise [0009] In the specification, reference may be made to is generally the first noise heard by an observer of an the spatial relationships between various components approaching helicopter. Acoustic signature reduction and to the spatial orientation of various aspects of com- system 101 creates out- of-phase "anti-noise", or cancel- ponents as the devices are depicted in the attached draw- lation noise, through thermo- acoustic tube 103. This "an- ings. However, as will be recognized by those skilled in 15 ti-noise" is used to cancel out or significantly reduce the the art after a complete reading of the present application, fundamental frequencies and the associated harmonics the devices,members, apparatuses, etc. described here- of the compression noise. In practice, the cancellation in may be positioned in any desired orientation. Thus, noise must be of the same amplitude but with an inverted the use of terms to describe a spatial relationship be- phase, thereby creating a phase cancellation effect. tween various components or to describe the spatial ori- 20 Where the phase is inverted but the amplitude is not entation of aspects of such components should be un- equal, a reduced cancellation effect is generally ob- derstood to describe a relative relationship between the served. Although described as canceling out the com- components or a spatial orientation of aspects of such pression noise, it is understood that typically the cancel- components, respectively, as the device described here- lation noise generated by acoustic signature reduction in may be oriented in any desired direction.