Rijke Tube Cancellation Device for Helicopters

(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 preferred embodiment of the present application; [0001] The presentapplication relates in general to helicopter 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 generated 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. 25 system 101 is generally sufficient to reduce the compres- [0010] Referring toFigure 1 in thedrawings, an aircraft, sion noise to a sound level relatively equal to that of the such as a helicopter 201, having an acoustic signature engine and transmission rather than completely cance- reduction system 101 is illustrated. Helicopter 201 has a ling out the compression noise. However it is understood body 203 and a main rotor assembly 205, including main that acoustic signature reduction system 101 is capable rotor blades 207 and a main rotor shaft 208. Helicopter 30 of generating cancellation noises of any amplitude and 201 has a tail rotor assembly 209, including tail rotor frequency to produce a desired cancellation effect. In do- blades 211 and a tail rotor shaft 210. Main rotor blades ing so, acoustic signature reduction system 101 primarily 207 generally rotate about a longitudinal axis 206 of main operates with very low and defined frequencies rather rotor shaft 208. Tail rotor blades 211 generally rotate than broadband frequencies. about a longitudinal axis 212 of tail rotor shaft 210. Hel- 35 [0014] Examples of thermo-acoustic tube 103 are a icopter 201 also includes acoustic signature reduction Rijke tube or a Sondhauss tube; to name a few. For pur- system 101 according to the present disclosure for can- poses of this application, discussion of thermo-acoustic celing the acoustic signature generated by main rotor tube 103 will revolve around the use of a Rijke tube. blades 207 and tail rotor blades 211. Though a Rijke tube is used, it is understood that other [0011] Referring now also to Figure 2 in the drawings, 40 thermo-acoustic tubes may be applied and used in an acoustic signature reduction system 101 of the acoustic signature reduction system 101. Thermo- present application is illustrated. Acoustic signature re- acoustic tube 103 typically includes a strait hollow cylin- duction system 101 contains a number of devices such drical pipe portion or pipe 104 having a length L. Pipe as a thermo-acoustic tube 103, a power supply 105, and 104 has a forward end 109 and an aft end 111. Thermo- a controller 107. In alternate embodiments, acoustic sig- 45 acoustic tube 103 also includes a heating element 113. nature reduction system 101 may also include the follow- Forward end 109 is typically upstream from aft end 111. ing devices: a mechanical damping valve 115 and/or a Both forward end 109 and aft end 111 are typically open forced air unit 117. Wires 119 are coupled to the above so as to allow air to flow through pipe 104. When air flows mentioned devices and serve to provide electrical power through thermo-acoustic tube 103, the air is heated by and operational control throughout acoustic signature re- 50 heating element 113, thereby creating an acoustic insta- duction system 101. bility. Large pressure amplitudes at selected frequencies [0012] Acoustic signature reduction system 101 is are generated. Although pipe 104 is described as having used to reduce the acoustic signature of aircraft prefer- two open ends, it is understood that thermo- acoustic tube ably having well defined low frequency noise that is pro- 103 may have one or more ends closed. duced while the aircraft is in operation. Such aircraft may 55 [0015] Referring now also to Figures 3 and 4 in the be a plane, a helicopter, a tilt rotor, or an unmanned aerial drawings, charts depicting the frequency spectrum of hel- vehicle, for example. For purposes of this application, icopter 201 and a Rijke tube respectively are illustrated. the preferred embodiment will involve reducing the Chart 151 shows the sound characteristics generated by

3 5 EP 2 568 468 B1 6 helicopter 201 while blades 207, 211 are rotating. Chart tubes 103 and, in general, the features of acoustic sig- 151 compares the frequency of the compression wave nature reduction system 101. to the sound pressure in decibels (dB). Chart 161 likewise [0019] Although described as being coupled externally compares the same parameters as in chart 151, but with tohelicopter 201, it is understood thatother embodiments regard to the sound characteristics of a Rijke tube. Chart 5 can couple thermo-acoustic tube 103 to helicopter 201 151 and chart 161 illustrate that a Rijke tube, or thermo- such that a portion of thermo- acoustic tube 103 is located acoustic tube 103, can produce harmonic frequencies of internally to helicopter 201. For example, thermo- acous- similar amplitude and frequency to that of rotor blades tic tube 103 may be located internally within body 203 as 207, 211. The harmonic frequencies are denoted by the seen with thermo-acoustic tube 103’. Thermo-acoustic spikes in decibels particularly at low frequencies. The 10 tube 103’ has a forward end 109’ and an aft end 111’ distinct low frequency and high amplitude noise is being protruding externally to body 203. All other portions of referred to as a harmonic frequency. thermo-acoustic tube 103’ are illustrated internally to [0016] The number of harmonic frequencies produced body 203. by helicopter 201 and a Rijke tube are different. As seen [0020] Thermo-acoustic tube 103 may be coupled to from chart 151 for example, three pressure spikes above 15 helicopter 201 by multiple methods. For example, ther- 70 decibels were generated whereas chart 161 shows mo-acoustic tube 103 may be coupled to helicopter 201 only one was generated by the Rijke tube. The number by the use of fasteners such as clamps, threaded fasten- of harmonic frequencies produced by a Rijke tube above ers, clips, or pins to name a few. Furthermore, welding 40 decibels is fewer than that produced by helicopter or riveting may be used. Additionally, in the preferred 201. Therefore, to counter the many harmonics generat- 20 embodiment, thermo-acoustic tube 103 is typically ori- ed by rotor blade 207, 211 compression noise, a series ented such that the plane of forward end 109 is perpen- of thermo-acoustic tubes 103 will typically be required. dicular to the front of helicopter 201. It is understood that An object of the present application will be to reduce the forward end 109 and aft end 111 are not limited to being noise generated by rotor blades 207, 211 to a level com- oriented in such a way. In other embodiments, forward parable to that of the frequency and amplitude levels pro- 25 end 109 and aft end 111 may be oriented such that the duced by the engine, transmission, and other workings plane of forward end 109 or aft end 111 is not perpen- of the aircraft. Additionally, in order to increase the am- dicular to the front of helicopter 201. Furthermore, other plitude of thermo-acoustic tube 103, it can be necessary embodiments may permit thermo-acoustic tubes 103 to to stack or bunch multiple thermo- acoustic tubes 103 to- swivel or translate on or within helicopter 201. gether as seen in Figure 5. 30 [0021] Although pipe 104 has been described as hav- [0017] Thermo-acoustic tube 103 can operate much ing a circular cross- sectional shape, it is understood that like a musical instrument wherein the combination of sev- pipe 104 can have any profile shape, such as circular, eral factors can adjust the frequency and amplitude of square, or octagonal to name a few. Furthermore, al- the sound generated. For instance, the amount of air flow though pipe 104 has been described as being strait, it and the temperature of heating element 113 can affect 35 should be understood that pipe 104 may have one or the amplitude. Likewise, typically the location of heating more curves or bends along the longitudinal axis. element 113 within thermo-acoustic tube 103 and the [0022] Referring now also to Figure 6 in the drawings, length and diameter of pipe 104 can affect the frequency pipe 104 of Figure 2 is illustrated with a curved shape produced. Much like a musical instrument, thermo- having one or more bends along the axial length. As stat- acoustic tube 103 can typically "play" a selected set of 40 ed above, pipe 104 of thermo- acoustic tube 103 can vary harmonic frequencies depending on the arrangement in length and diameter in order to play certain harmonic and size of thermo-acoustic tube 103. frequencies. Depending on the frequency and amplitude, [0018] Referring now also to Figure 5 in the drawings, pipe 104 may have a diameter of one or two inches (25.4 thermo-acoustic tube 103 of the present application is or 50.8 mm) and a length up to 23 feet (7 m), for example. illustrated in multiple locations on helicopter 201. Heli- 45 The size of thermo-acoustic tube 103 can limit suitable copter 201 has a landing strut 202, a skid 204, and a locations to secure thermo- acoustic tube 103 to helicop- body 203. Body 203 typically includes a fuselage 213, ter 201, thereby resulting in acoustic signature reduction an engine cowl 215, an empennage 217, and a wing (not system 101 being limited to a narrower range of machin- shown), for example. It should be understood that body ery. Therefore, an alternate embodiment of pipe 104 may 203 is not limited to only those parts of helicopter 201 50 have a curved shape with one or more bends. By design- listed. Thermo-acoustic tube 103 is typically coupled to ing pipe 104 with a curved shape, the relative length of some external portion of helicopter 201. For example, pipe 104 is generally maintained but the effective size thermo-acoustic tube 103 may be coupled to a landing can be substantially smaller, thereby fitting a broader strut 202 or externally to a bottom portion 219 of fuselage range of aircraft. 213. Acoustic signature reduction system 101 is config- 55 [0023] This curved shape allows for thermo-acoustic ured to be easily installed on aircraft during production tube 103 to couple to helicopter 201 in a greater number or after production as a retrofit, for example. The time of of locations. For example, thermo- acoustic tube 103 can installation can affect the location of thermo-acoustic be located within and follow the contour of body 203 as

4 7 EP 2 568 468 B1 8 shown in Figure 5. Thermo- acoustic tube 103 may even from controller107 to fluctuate thetemperature of heating be incorporated into existing parts of helicopter 201. For element 113. By changing the temperature of heating example, skids 204 or landing struts 202 are typically element 113, the amplitude of the sound produced can hollow tubes. Thermo-acoustic tube 103 may be formed be altered. Although wires are depicted in Figure 2 as by creating openings, forward end 109 and aft end 111, 5 connecting to heating element 113 outside of pipe 104, to allow air to flow through skid 204. Heating element 113 it is understood that wires 119 may be located on or can then be located inside skid 204. In addition, although around any portion of pipe 104. For example, wires 119 thermo-acoustic tube103 hasbeen described as coupled may travel and be coupled to internal surface 601. to helicopter201, it isunderstood that other embodiments [0027] Heating element 113 may take any number of may permit thermo- acoustic tube 103 to be rotatably cou- 10 shapes and sizes. In the preferred embodiment, heating pled to helicopter 201 allowing thermo- acoustic tube 103 element 113 is a metallic wire mesh 114 as seen in Figure to rotate and/or swivel in relation to helicopter 201 as 7. However, other embodiments may shape heating el- mentioned previously. Although described in certain lo- ement 113 as a metallic coil 116 as seen in Figure 8, for cations and embodiments, it is understood that thermo- example. The shape of heating element 113 is not limited acoustic tube 103 may be coupled to helicopter 201 in 15 to the examples presented. It is understood that other multiple other locations not described herein. shapes can be used and create a functioning thermo- [0024] Referring now also to Figures 7 and 8 in the acoustic tube 103. Furthermore, heating element 113 is drawings, a cross sectional view of pipe 104 showing not limited to metallic materials. It is understood that any heating element 113 coupled to pipe 104 is illustrated material may be used that permits for relatively quick and without wires 119. Heating element 113 is typically a re- 20 controlled temperature changes. sistor coupled to pipe 104 by the use of fasteners 602. [0028] Furthermore,although heating element 113 has When an electrical current is received, heating element been described as being located internally to pipe 104 in 113 converts the electrical current to heat. However, a fixed location by use of fasteners 602, it should be un- heating element 113 is not limited to just using electrical derstood that heating element 113 may be oriented and energy to create heat. Other methods of generating heat 25 located in a multitude of positions with respect to pipe are understood and permissible so long as the functions 104. For example, heating element 113 may be formed of thermo-acoustic tube 103 are retained, namely gen- like a blanket wrapped around surface 601, 603 of pipe erating sound. As air passes through pipe 104, heating 104. element 113 is configured to heat the air. As heated air [0029] Referring now also to Figure 9 in the drawings, travels from heating element 113 and exits aft end 111, 30 a breakout view of thermo- acoustic tube 103 having mul- a sound wave is produced resulting in a cancellation tiple heating elements inside pipe 104 is illustrated. As noiseof a certain amplitudeand frequency. As mentioned stated previously, the location of heating element 113 previously, each thermo- acoustic tube 103 generally has partially determines the frequency of the sound pro- a set of harmonic frequencies. The location of heating duced. In the preferred embodiment, one heating ele- element 113 helps determine which harmonic frequency 35 ment 113 is used inside each pipe 104. However, in an is generated. alternate embodiment, more than one heating element [0025] Typically heating element 113 is located a pre- 113 may be used in pipe 104. Each heating element 113 determined distance along the axis of pipe 104 from for- is located in a different location within pipe 104, thereby ward end 109. The distance is generally between L/ 4 to producing multiple harmonic frequencies. Where multi- L/3 where L refers to the length of pipe 104. Heating40 ple heating elements 113 are used, multiple frequencies element 113 is generally positioned having at least a por- may be played simultaneously. tion of heating element 113 located inside pipe 104 and [0030] Referring now also to Figure 10 in the drawings, oriented such that the plane of heating element 113 is thermo-acoustic tube 103 having a moveable apparatus relatively perpendicular to the flow of air. Heating element 605 coupled to heating element 113 is illustrated. Al- 113 is coupled to pipe 104 by use of fasteners 602 such 45 though the preferred embodiment prevents axial trans- as clamps, threaded fasteners, clips, or rivets; to name lation of heating elements 113, it is understood that an a few. In the preferred embodiment, heating element pro- alternate embodiment of thermo-acoustic tube 103 may trudes through an aperture (not shown) in pipe 104 at include moveable apparatus 605 that permits the axial some defined location and is coupled to an internal sur- translation of heating element 113 inside pipe 104. In face 601 and an external surface 603 of pipe 104. In the 50 such an embodiment, moveable apparatus 605 is cou- preferred embodiment, rotational and translational pled to pipe 104. Heating element 113 is then coupled movement of heating element 113 is restricted. Where to moveable apparatus 605 in a manner that permits pipe 104 has an aperture (not shown) produced from movement of heating element 113. Such a configuration heating element 113 protruding through pipe 104, typi- results in an adjustable heating element 113. Moveable cally a sealant (not shown) is used to ensure no air leaks 55 apparatus 605 may be a motorized track or a solenoid, through the aperture. for example. The ability to translate within pipe 104 allows [0026] Wires 119 are coupled to heating element 113 a singleheating element 113 toproduce multiple frequen- as seen in Figure 2. Wires 119 carry an electrical current cies. However, a single heating element 113 could typi-

5 9 EP 2 568 468 B1 10 cally play one frequency at a time. Thermo- acoustic tube with user interface 108 may be termed a user of user 103 may incorporate the use of one or more fixed and/or interface 108 whether the person is the pilot, a crew mem- adjustable heating elements 113 within thermo- acoustic ber, or a remote person not on helicopter 201. tube 103. [0035] User interface 108 transmits a set of user com- [0031] Referring back to Figure 2 in the drawings,5 mands from the pilot, typically via wires 119, to opera- where controller 107 is illustrated. Controller 107 typically tional computer 110. Operational computer 110 simulta- incorporates an operational computer 110 and a user neously analyzes inputs 106 and the user commands interface 108. Controller 107 is operably connected to from user interface 108. Operational computer 110 then the various devices within acoustic signature reduction transmits system commands to the various devices in system 101 by wires 119. 10 acoustic signature reduction system 101 to generate a [0032] Operational computer 110 receives multiple in- cancellation noise of selected amplitude, frequency, and puts. Operational computer 110 receives operational and phase needed to cancel out the compression noise rel- environmental inputs 106 typically via existing systems ative to helicopter 201. Although wires 119 are described within helicopter 201. Operational inputs can refer to hel- and the method of transmitting and communicating be- icopter 201 in particular, such as rotor blade pitch, heli- 15 tween devices within acoustic signature reduction sys- copter speed, torque, blade speed, and so forth. Envi- tem 101, other methods of transmitting signals such as ronmental inputs can refer to general environmental con- wireless communications are possible. ditions such as air temperature, air density, elevation, [0036] In the preferred embodiment, operational com- and so forth. Inputs 106 are continuously transmitted to puter 110 and/or user interface 108 is integrated within operational controller 110. Operational computer 110 us- 20 existing computers on helicopter 201 thereby reducing es inputs 106 to aid in operating acoustic signature re- the weight required to install system 101 on helicopter duction system 101. 201. Likewise, inputs 106 are typically generated by ex- [0033] Operational computer 110 also receives user isting sensors and software on helicopter 201 so as to inputs typically from a pilot (not shown) via a user inter- decrease the weight and space required to implement face 108. User interface 108 permits a user, such as a 25 acoustic signature reduction system 101. Although de- pilot to adjust acoustic signature reduction system 101. scribed as being integrated within existing systems on User interface 108 is typically an interactive digital de- helicopter 201, it is understood that other embodiments vice, such as a touch screen, for example, that provides permit operational computer 110 and/or user interface a graphical view concerning the location of the aircraft in 108 to be a separate unit located on or off helicopter 201. relation to other objects such as terrain, aircraft, struc- 30 For example, operational computer 110 and/or user in- tures, vehicles, and so forth. Typically, some of the fea- terface 108 may be located remote to helicopter 201, tures of user interface 108 may include a mapping func- such as on another aircraft, ground vehicle, structure, or tion to illustrate these objects in relation to helicopter 201, ship, for example. In addition, acoustic signature reduc- the ability to zoom in and out on the screen, and the ability tion system 101 may also use additional sensors to gath- to select a "quiet zone" or a cancellation area 403 (see 35 er inputs 106. By being independent and separate from Figures 11 and 12) relative to helicopter 201. Cancella- existing systems on helicopter 201, acoustic signature tion area 403 can be selected to pertain to a specific reduction system 101 is adapted to be retrofitted to ex- location or to a specific object. Therefore, cancellation isting aircraft. area 403 can be stationary or mobile. Controller 107 au- [0037] In embodiments where wireless connections tomatically adjusts the phase, amplitude, and frequency 40 are used, a user can be a remote person located remote of the cancellation noise to compensate for relative mo- to helicopter 201 may access and control any portion of tion between the aircraft and cancellation area 403. acoustic signature reduction system 101. Typically, con- [0034] It is understood that user interface is not limited trol from a remote location would occur in the use of re- to those features described above. Other features are mote flying aircraft, such as unmanned aerial vehicles, known and possible that would aid the pilot in the quick 45 for example, but are not so limited. Wireless connections detection and selection of cancellation area 403. User wherein controller 107 is remote to helicopter 201 would interface108 also communicatesto the pilot performance further help facilitate retrofitting aircraft with acoustic sig- data of acoustic signature reduction system 101, such nature reduction system 101, generally needing only to as cancellation effects, frequency, amplitude, and so update software on the existing aircraft. forth. Cancellation effects refer to the resulting sound lev- 50 [0038] Although controller 107 is described as includ- el, approximate size of cancellation area 403 given dis- ing operational computer 110 and user interface 108, it tance between cancellation area 403 and helicopter 201, is understood that either one may be removed. For ex- and so forth. Though typically a touch screen device ample, where the noise to be cancelled consists of a con- would be used, other methods of permitting pilot control stant phase, frequency, amplitude and timing; controller are possible such as mechanical dials, for example. Like- 55 107 can consist of only user interface 108 to turn the wise, though a pilot has been described as operating system on and off and select cancellation areas 403. user interface 108, any member of a crew in helicopter However, the phase, frequency, amplitude, and timing 201 may use user interface 108. Any person interacting of the compression noise generated by rotor blades 207,

6 11 EP 2 568 468 B1 12

211 are not always continuous. Rather, the compression front of helicopter 201 will hear the compression noise noise is typically intermittent. of a two-bladed helicopter 201 at different intervals than [0039] Where the sound to be canceled is continuous a second observer standing on the port side of the same to all observers, a continuous cancellation noise is typi- helicopter 201. As the observer and/or helicopter 201 cally desired. Where the sound to be canceled is inter- 5 moves in relation to one another, the phase of the com- mittent as to an observer, the cancellation noise typically pression noise can also change with respect to the ob- needs to be intermittent as well. As each blade 207, 211 server. This results in compression noise that is location rotates past an observer, a distinct compression noise is dependent. heard. The per-revolution timing of the compression [0044] Acoustic signature reduction system 101 typi- noise is a function of the number of rotor blades 207, 211 10 cally generates a cancellation noise in a set phase, or on helicopter 201. with certain timing, by using damping valve 115. The [0040] The pressure amplitudes generated by thermo- phase of the cancellation noise must be inverted and of acoustic tube 103 are typically continuous as long as air equal amplitude to the compression noise in order to pro- flows through pipe 104. Damping valve 115 is used to duce a phase cancellation. For signals to be inverted, synchronize the cancellation noise generated by thermo- 15 the signals must be out of phase 180 degrees from the acoustic tube 103 with that of the compression noise as other signal. If the amplitudes are also equal, the ampli- heard by an observer relative to helicopter 201. Opera- tudes combine to cancel each other out. Acoustic signa- tional computer 110 controls damping valve 115 depend- ture reduction system 101 generates a cancellation noise ing on signals from user interface 108 and inputs 106. In that is relatively 180 degrees out- of-phase with the com- the preferred embodiment, damping valve 115 is typically 20 pression noise and of relatively equal amplitude, thereby threadedly coupled about aft end 111 of thermo- acoustic reducing or canceling the acoustic signature relative to tube 103. Thermo-acoustic tube 103 and damping valve the compression noise. Because the compression noise 115 are secured by interference fit. However, it is under- islocation dependent, the cancellation noise creates can- stood that other methods of attaching damping valve 115 cellation area 403 where the phase, amplitude, and fre- may be used such as fasteners, welding, or adhesive, 25 quency of the cancellation noise and compression noise for example. Damping valve 115 is configured to alter operate to cancel each other out. the rate of air passing through thermo- acoustic tube 103 [0045] Chart 170 and chart 171 illustrate an example by opening and/or closing aft end 111 of pipe 104. of variations in noise cancellation effects emanating from [0041] By altering the air flow rate, damping valve 115 a single reference location 401 as seen in two views. decreases the noise generated by thermo- acoustic tube 30 Chart 171 islooking down on reference location 401 while 103 to a level at or below the noise level generated by chart 170 is looking at the side of reference location 401. other parts of helicopter 201 such as the engine and Reference location 401 is representative of helicopter transmission. By repeatedly opening and closing damp- 201 as seen in chart 170. Two signals will be used to ing valve 115, noise similar to that of rotor compression describe the cancellation effect. The two signals are the noise can be simulated. Damping valve 115 can there- 35 compression noise from rotor blades 207, 211 and the fore create an intermittent cancellation noise to match cancellation noise from acoustic signature reduction sys- the per-revolution noise much like an observer would tem 101. Because the timing, or phase, of the compres- hear. Decreasing the cancellation noise between pass- sion noise is location dependent, some locations around ing rotor blades 207, 211 prevents acoustic signature helicopter 201 experience a decrease in noise while oth- reduction system 101 from adding to the overall acoustic 40 ers experience an increase in noise. As the phase of two signature of helicopter 201. signals moves away from 180 degrees out-of-phase, a [0042] Damping valve 115 can use one or more devic- partial reduction in noise or even an increase in noise es to alter the flow rate of air through thermo-acoustic will result. tube 103 such as flaps, shutters, or nozzles to name a [0046] Chart 171 illustrates the cancellation noise at few. Although damping valve 115 is located about aft end 45 50 Hertz (Hz) in a side by side configuration. For purpos- 111 of thermo-acoustic tube 103, it is understood that es of illustration, it is assumed that the two signals are damping valve 115 may be located anywhere along pipe of equal amplitude and frequency. In cancellation area 104. Furthermore, for aircraft having continuous ampli- 403, the two signals are out-of-phase by 180 degrees tudes or frequencies to be canceled by acoustic signature thereby creating a complete cancellation of the sound. A reduction system 101, damping valve 115 may be re- 50 reduction area 405 is shown on either side of cancellation moved. area 403. Reduction area 405 results from having the [0043] Referring now also to Figures 11 and 12 in the two signals be slightly less than or greater than 180 de- drawings, charts showing the noise cancellation effects grees out-of-phase. In reduction area 405, the net effect of acoustic signature reduction system 101 are illustrat- of the two signals is a slight reduction of noise. A neutral ed. Where multiple observers are positioned in different 55 area 407 is shown further away from cancellation area locations with respect to helicopter 201, the per-revolu- 403. Neutral area 407 occurs where the phase of the two tion timing, or phase of the compression noise is different signals combine to result in a net change of zero decibels. between observers. For example, an observer located in Beyond neutral area 407 is an increased area 409. In-

7 13 EP 2 568 468 B1 14 creased area 409 is the area in which the phase of the figured to operate with helicopter 201 to allow the pilot two signals is predominantly in phase with one another to designate a fixed or moving cancellation area 403. The thereby resulting in a net increase in noise. pilot positions cancellation area 403 via user interface [0047] Cancellation effects vary in size the farther the 108. Operational computer 110 then controls the phase sound travels from reference location 401 as seen in Fig- 5 and amplitude of the cancellation noise via damping ure 12. Another feature of user interface 108 is the ability valve 115 and forced air unit 117 to ensure that cancel- to allow the user to designate the size of cancellation lation area 403 remains fixed as helicopter 201 moves. area 403. Operational computer 110 is configured to dis- Furthermore, it is understood that acoustic signature re- play selected altitude and position data for helicopter 201 duction system 101 has the ability to permit a moving on user interface 108 to facilitate the required size of 10 cancellation zone 403 as well. A moving cancellation are cancellation area 403. The pilot may then manoeuvre 403 is where cancellation area 403 independently moves helicopter 201 to comply. In doing so, controller 107 per- with respect to helicopter 201. mits flight plans to be created and/or modified to optimize [0052] Although the preferred embodiment illustrates flight paths while maintaining quiet operations with re- power supply 105 as being wired to operational computer spect to cancellation area 403. Furthermore, controller 15 110, it is understood that power supply 105 may be cou- 107 can communicate with the flight control computer of pled to any device in acoustic signature reduction system helicopter 201 such that the controller and flight control 101 directly by using wires 119. It is further understood computer can alter the flight path of the aircraft without that alternate means of power may be used. In the pre- input from a pilot. For example, such an embodiment can ferred embodiment, power supply 105 is part of the ex- be used with auto- pilot systems on helicopter 201 or with 20 isting systems located on helicopter 201. Power supply unmanned aerial vehicles, to name a few. 105 may be independent from existing systems. Further- [0048] Referring back to Figure 2 in the drawings, a more, one or more power supplies 105 may be used. forced air unit 117 is illustrated in acoustic signature re- Alternate sources of power may be used such as solar duction system 101. In order to change the direction of power, for example. cancellation area 403, the phase of the cancellation noise 25 [0053] A screen 121 can be placed at any location with- would typically need to experience a phase shift. This in pipe 104 to prevent dirt, debris, and/or foreign objects phase shift could be done using forced air unit 117. from entering thermo-acoustic tube 103. Screen 121 Forced air unit 117 would be used to send bursts of air would typically be placed at forward end 109 and/or aft into thermo-acoustic tube 103 to adjust the phase of the end 111 but may be located in any location with respect cancellation noise. Operational computer 110 controls 30 to pipe 104. Screen 121 may be coupled to pipe 104 as forced air unit 117 depending on signals from user inter- a separate unit or in conjunction with that of forced air face 108 and inputs 106. Forced air unit 117 can also be unit 117 or damping valve 115. For example, screen 121 used toforce air into thermo- acoustictube 103 if sufficient could be placed around forward end 109 and be coupled air is not entering thermo-acoustic tube 103. For exam- to pipe 104 by threadedly connecting forced air unit 117 ple, slow forward movement of helicopter 201 may not 35 to forward end 109. allow sufficient air flow to reach the necessary amplitude [0054] The present application provides significant ad- or frequency required to cancel the compression noises. vantages, including: (1) the ability to create high decibel Furthermore, thermo-acoustic tube 103 may be oriented and very-low frequency noises; (2) the ability to synchro- such that forward end 109 is not perpendicular to the flow nize rotor blade compression noise with a cancellation of air during flight. Forced air unit 117 allows acoustic 40 noise device; (3) the ability to move a cancellation area signature reduction system 101 to operate whether hel- around the helicopter; (4) system can be integrated into icopter 201 is flying at any speed or is resting on the existing flight systems on an aircraft to save weight; and ground. Forced air unit 117 and damping valve 115 op- (5) acoustic signature reduction system can be installed erate in conjunction to ensure proper air flow through in retrofit installations. thermo-acoustic tube 103. 45 [0055] While the preferred embodiment has been de- [0049] Forced air unit 117 may be coupled to pipe 104 scribed with reference to an illustrative embodiment, this much the same was as described with damping valve description is not intended to be construed in a limiting 115. Furthermore, the location of forced air unit 117 is sense. Various modifications and other embodiments of depicted as being coupled to forward end 109 of pipe the invention will be apparent to persons skilled in the art 104 but it is understood that forced air unit 117 may be 50 upon reference to the description. located at any location relative to pipe 104. [0056] The particular embodiments disclosed above [0050] Another method of changing the direction of are illustrative only, as the application may be modified cancellation area 403 is to use multiple sets of thermo- and practiced in different but equivalent manners appar- acoustic tubes 103. Each set would be configured to ent to those skilled in the art having the benefit of the "play" only in selected phases. In such a configuration, 55 teachings herein. It is therefore evident that the particular forced air unit 117 may not be required. However, this embodiments disclosed above may be altered or modi- configuration would add more weight to helicopter 201. fied, and all such variations are considered within the [0051] Acoustic signature reduction system 101 is con- scope of the application. Accordingly, the protection

8 15 EP 2 568 468 B1 16 sought herein is as set forth in the description. It is ap- helicopter (201), tilt rotor aircraft, or unmanned aerial parent that an application with significant advantages has vehicle. been described and illustrated. Although the present application is shown in a limited number of forms, it is not 4. The acoustic signature reduction system (101) of limited to just these forms, but is amenable to various 5 claim1 or of claim 2or of claim 3, wherein the thermo- changes and modifications without departing from the acoustic tube (103) has one or more bends. scope of the invention, which is defined by the appended claims. 5. Theacoustic signature reduction system (101) of any preceding claim, wherein the thermo-acoustic tube 10 (103) is coupled externally or internally to the aircraft Claims (201).

1. An acoustic signature reduction system (101) for an 6. Theacoustic signature reduction system (101) of any aircraft (201) having a rotor blade compression preceding claim, wherein the thermo-acoustic tube noise, the system (101) characterised by compris- 15 (103) is rotatably coupled to the aircraft (201), and/or ing: wherein the thermo-acoustic tube (103) has one or more open ends. a thermo-acoustic tube (103) coupled to the aircraft (201), the thermo- acoustic tube (103) hav- 7. Theacoustic signature reduction system (101) of any ing a pipe portion (104) and one or more heating 20 preceding claim, wherein the heating element (113) elements(113) coupledto thepipe portion (104), is moveable relative to the pipe portion (104). each heating element (113) being configured to heat air as the air flows through the pipe portion 8. Theacoustic signature reduction system (101) of any (104), thereby generating a cancellation noise; preceding claim, wherein the controller (107) uses and 25 wireless communications to control the thermo- a controller (107) operably connected to the acoustic tube (103), and optionally or preferably, thermo-acoustic tube (103) for controlling the wherein the controller (107) is located remote to the heating elements (113), such that the cancella- aircraft (201), such that a person may access and tion noise cancels the rotor blade compression control the thermo- acoustic tube (103) without being noise at a selected location (403) relative to the 30 on the aircraft (201). aircraft (201). 9. The acoustic signature reduction system (101) of 2. The acoustic signature reduction system (101) of claim 2 wherein the forced air unit (117) is configured claim 1 comprising: to send bursts of air into the thermo-acoustic tube 35 (103) to adjust the phase of the cancellation noise. a damping valve (115) coupled to the thermo- acoustic tube (103) for synchronizing the can- 10. Theacoustic signature reduction system (101) of any cellation noise generated by the thermo- acous- preceding claim, further comprising: tic tube (103) with that of rotor blade compression noises as heard by an observer relative to 40 a screen (121) coupled to the thermo-acoustic the aircraft (201); and tube (103) for preventing dirt, debris, and foreign a forced air unit (109) coupled to the thermo- objects from entering the thermo-acoustic tube acoustic tube (103) for adjusting the phase of (103). the cancellation noise, wherein the controller (107) comprises a user 45 11. The acoustic signature reduction system (101) of interface (108) in communication with the ther- claim 2, wherein the user interface (108) is an inter- mo-acoustic tube (103), the damping valve active digital device that enables the pilot to graph- (115), and the forced air unit (117), such that ically see the location of the aircraft (201) in relation one or more of the phase, amplitude, and fre- to other objects, so as to select the cancellation area quency of the cancellation noise can be adjust- 50 (403), and optionally or preferably, ed; and wherein the controller (107) automatically adjusts wherein the cancellation noise and rotor blade one or more of the phase, amplitude, and frequency compression noise combine to produce a can- of the cancellation noise to compensate for relative cellation area wherein the rotor blade compres- motion between the aircraft (201) and the cancella- sion noise as heard by an observer is reduced. 55 tion area (403).

3. The acoustic signature reduction system (101) of 12. The acoustic signature reduction system (101) of claim 1 or of claim 2, wherein the aircraft is a plane, claim 2 or claim 11, wherein the controller (107) per-

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mits flight plans to be created and modified to opti- werden, zu synchronisieren; und mize flight paths, while maintaining a reduced acous- eine Gebläseeinheit (109), die an die thermo- tic signature with respect to the cancellation area akustische Röhre (103) gekoppelt ist, um die (403). Phase des Unterdrückungsgeräusches anzu- 5 passen, 13. A method of reducing an acoustic signature of an wobei die Steuerung (107) eine Bedienoberflä- aircraft (201) having a rotor blade compression che (108) im Informationsaustausch mit der noise, characterised by the aircraft (201) having a thermoakustischen Röhre (103), dem Dämp- thermo-acoustic tube (103) coupled thereto, the fungsventil (115) und der Gebläseeinheit (117) thermo-acoustic tube (103) having a pipe portion 10 umfasst, sodass eine oder mehrere der Phase, (104) and one or more heating elements (113) cou- Amplitude und Frequenz des Unterdrükkungs- pled to the pipe portion (104), a controller (107) being geräusches angepasst werden können; und operably connected to the thermo-acoustic tube wobei das Unterdrückungsgeräusch und das (103) for controlling the one or more heating ele- Rotorblatt-Kompressionsgeräusch kombiniert ments (113), the method comprising controlling the 15 werden, um einen Unterdrückungsbereich zu one or more heating elements (113) by the controller erzeugen, worin das Rotorblatt-Kompressions- (107) to heat air as the air flows through the pipe geräusch, das vom Beobachter gehört wird, re- portion (104), thereby generating a cancellation duziert wird. noise to cancel the rotor blade compression noise at a selected location (403) relative to the aircraft 20 3. Das System zur Reduktion der akustischen Signatur (201). (101) von Anspruch 1 oder Anspruch 2, wobei das Luftfahrzeug ein Flugzeug, ein Hubschrauber (201), ein Kipprotor-Luftfahrzeug oder ein unbemanntes Patentansprüche Luftahrzeug ist. 25 1. Ein System zur Reduktion der akustischen Signatur 4. Das System zur Reduktion der akustischen Signatur (101) für ein Luftfahrzeug (201), das ein Rotorblatt- (101) von Anspruch 1 oder Anspruch 2 oder An- Kompressionsgeräusch aufweist, wobei das System spruch 3, wobei die thermoakustische Röhre (103) (101) dadurch gekennzeichnet ist, dass es Fol- eine oder mehrere Krümmungen aufweist. gendes umfasst: 30 5. Das System zur Reduktion der akustischen Signatur eine thermoakustische Röhre (103), die an das (101) von einem vorhergehenden Anspruch, wobei Luftfahrzeug (201) gekoppelt ist, wobei die ther- die thermoakustische Röhre (103) extern oder intern moakustische Röhre (103) einen Rohrabschnitt an das Luftfahrzeug (201) gekoppelt ist. (104) aufweist und ein oder mehrere Heizele- 35 mente (113) an den Rohrabschnitt (104) gekop- 6. Das System zur Reduktion der akustischen Signatur pelt sind, und wobei jedes Heizelement (113) (101) von einem vorhergehenden Anspruch, wobei konfiguriert ist, um Luft zu erwärmen, während die thermoakustische Röhre (103) rotierbar an das die Luft durch den Rohrabschnitt (104) fließt, Luftfahrzeug (201) gekoppelt ist, und/oder wodurch ein Unterdrückungsgeräusch erzeugt 40 wobei die thermoakustische Röhre (103) ein oder wird; und mehr offene Enden aufweist. eine Steuerung (107), die funktionsfähig mit der thermoakustischen Röhre (103) zur Steuerung 7. Das System zur Reduktion der akustischen Signatur der Heizelemente (113) verbunden ist, sodass (101) von einem vorhergehenden Anspruch, wobei das Unterdrückungsgeräusch das Rotorblatt- 45 das Heizelement (113) relativ zum Rohrabschnitt Kompressionsgeräusch an einer ausgewählten (104) bewegbar ist. Stelle (403) relativ zum Luftfahrzeug (201) un- terdrückt. 8. Das System zur Reduktion der akustischen Signatur (101) eines vorhergehenden Anspruchs, wobei die 2. Das System zur Reduktion der akustischen Signatur 50 Steuerung (107) drahtlose Kommunikation verwen- (101) von Anspruch 1, umfassend: det, um die thermoakustische Röhre (103) zu steuern, und ein Dämpfungsventil (115), das an die thermo- wobei die Steuerung (107) sich wahlweise und be- akustische Röhre (103) gekoppelt ist, um das vorzugt vom Luftfahrzeug (201) entfernt befindet, so- von der thermoakustischen Röhre (103) erzeug- 55 dass eine Person auf die akustische Röhre (103) te Unterdrückungsgeräusch mit jenem der Ro- zugreifen und diese steuern kann, ohne dass sie sich torblatt-Kompressionsgeräusche, die vom Be- im Luftfahrzeug (201) befindet. obachter relativ zum Luftfahrzeug (201) gehört

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9. Das System zur Reduktion der akustischen Signatur Revendications (101) von Anspruch 2, wobei die Gebläseeinheit (117) konfiguriert ist, um Luftstöße in die thermoaku- 1. Système de réduction de la signature acoustique stische Röhre (103) zu schicken, sodass die Phase (101) pour un aéronef (201) ayant un bruit de com- des Unterdrückungsgeräusches angepasst wird. 5 pression des pales du rotor, le système (101) carac- térisé en ce qu’il comprend : 10. Das System zur Reduktion der akustischen Signatur (101) eines vorhergehenden Anspruchs, überdies un tube thermoacoustique (103) couplé à l’aé- umfassend: ronef (201), le tube thermoacoustique (103) 10 ayant une partie de tube (104) et un ou plusieurs ein Sieb (121), gekoppelt an die thermoakusti- éléments chauffants (113) couplé à la partie de sche Röhre (103), um das Eindringen von tube (104), chaque élément chauffant (113) Schmutz, Ablagerungen und Fremdkörpern in étant configuré pour chauffer de l’air lorsque l’air die thermoakustische Röhre (103) zu verhin- s’écoule à travers la partie de tube (104), géné- dern. 15 rant de ce fait un bruit neutralisant, et un contrôleur (107) relié de manière opération- 11. Das System zur Reduktion der akustischen Signatur nelle au tube thermoacoustique (103) pour com- (101) von Anspruch 2, wobei die Bedienoberfläche mander les éléments de chauffage (113), de tel- (108) ein interaktives digitales Gerät ist, das den Pi- le sorte que le bruit neutralisant annule le bruit loten befähigt, grafisch die Position des Luftfahr-20 de compression des pales du rotor à un empla- zeugs (201) im Bezug auf andere Objekte zu sehen, cement sélectionné (403) par rapport à l’aéronef um den Unterdrükkungsbereich (403) auszuwählen, (201). und wobei die Steuerung (107) wahlweise und bevorzugt 2. Système de réduction de la signature acoustique automatisch eine oder mehrere der Phase, Amplitu- 25 (101) selon la revendication 1, comprenant : de und Frequenz des Unterdrückungsgeräusches anpasst, um die relative Bewegung zwischen dem une soupape d’amortissement (115) couplée au Luftfahrzeug (201) und dem Unterdrückungsbereich tube thermoacoustique (103) pour synchroniser (403) auszugleichen. le bruit neutralisant généré par le tube thermoa- 30 coustique (103) avec celui du bruit de compres- 12. Das System zur Reduktion der akustischen Signatur sion des pales de rotor tel qu’il est perçu par un (101) von Anspruch 2 oder Anspruch 11, wobei die observateur par rapport à l’aéronef (201) et Steuerung (107) die Erstellung und Modifizierung une unité à air pulsé (109) couplée au tube ther- von Flugplänen zur Optimierung von Flugbahnen er- moacoustique (103) pour ajuster la phase du möglicht, während gleichzeitig eine reduzierte aku- 35 bruit neutralisant, stische Signatur im Hinblick auf den Unterdrük- dans lequel le contrôleur (107) comprend une kungsbereich (403) erhalten bleibt. interface utilisateur (108) en communication avec le tube thermoacoustique (103), la soupa- 13. Ein Verfahren zu Reduktion einer akustischen Si- pe d’amortissement (115), et l’unité à air pulsé gnatur eines Luftfahrzeugs (201), das ein Rotorblatt- 40 (117), de telle sorte que l’un ou plusieurs de la Kompressionsgeräusch aufweist, dadurch ge- phase, l’amplitude et la fréquence du bruit neu- kennzeichnet, dass das Luftfahrzeug (201) über ei- tralisant peuvent être ajustées, et ne thermoakustische Röhre (103) verfügt, die daran dans lequel le bruit neutralisant et le bruit de gekoppelt ist, wobei die thermoakustische Röhre compression des pales du rotor se combinent (103) einen Rohrabschnitt (104) und ein oder meh- 45 pour produire une zone d’annulation du bruit, rere Heizelemente (113) aufweist, die mit dem Rohr- dans lequel le bruit de compression des pales abschnitt (104) verkoppelt sind, wobei eine Steue- du rotor entendu par un observateur est réduit. rung (107) funktionsfähig mit der thermoakustischen Röhre (103) verbunden ist, um das eine oder die 3. Système de réduction de la signature acoustique mehreren Heizelemente (113) zu steuern, wobei das 50 (101) selon la revendication 1 ou la revendication 2, Verfahren die Steuerung des einen oder der mehre- dans lequel l’aéronef est un avion, un hélicoptère ren Heizelemente (113) durch die Steuerung (107) (201), un aéronef à rotors basculants, ou un véhicule umfasst, um die Luft zu erwärmen, während sie aérien sans pilote. durch den Rohrabschnitt (104) fließt, wodurch ein Unterdrückungsgeräusch erzeugt wird, um das Ro- 55 4. Système de réduction de la signature acoustique torblatt-Kompressionsgeräusch an einer ausge- (101) selon la revendication 1 ou la revendication 2 wählten Stelle (403) relativ zum Luftfahrzeug (201) ou la revendication 3, dans lequel le tube thermoa- zu unterdrücken. coustique (103) présente un ou plusieurs coudes.

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5. Système de réduction de la signature acoustique (101) selon la revendication 2 ou la revendication (101) selon l’une quelconque des revendications 11, dans lequel le contrôleur (107) permet de créer précédentes, dans lequel le tube thermoacoustique et de modifier un plan de vol pour optimiser les tra- (103) est couplé extérieurement ou intérieurement jectoires de vol, tout en maintenant une signature à l’aéronef (201). 5 acoustique réduite par rapport à la zone d’annulation (403). 6. Système de réduction de la signature acoustique (101) selon l’une quelconque des revendications 13. Procédé de réduction de la signature acoustique précédentes, dans lequel le tube thermoacoustique d’un aéronef (201) ayant un bruit de compression (103) est couplé en rotation à l’aéronef (201), et/ou 10 des pales du rotor, caractérisé par l’aéronef (201) dans lequel le tube thermoacoustique (103) compor- comportant un tube thermoacoustique (103) qui lui te une ou plusieurs extrémités ouvertes. est couplé, le tube thermoacoustique (103) comportant une partie de tube (104) et un ou plusieurs élé- 7. Système de réduction de la signature acoustique ments chauffants (113) couplés à la partie de tube (101) selon l’une quelconque des revendications15 (104), un contrôleur (107) étant connecté de manière précédentes, dans lequel l’élément chauffant (113) fonctionnelle au tube thermoacoustique (103) pour est mobile par rapport à la partie de tube (104). commander le ou les éléments chauffants (113), le procédé comprenant la commande de l’un ou plu- 8. Système de réduction de la signature acoustique sieurs des éléments chauffants (113) par le contrô- (101) selon l’une quelconque des revendications20 leur (107) pour chauffer l’air lorsque l’air s’écoule à précédentes, dans lequel le contrôleur (107) utilise travers la partie de tube (104), générant de ce fait des communications sans fil pour commander le tu- un bruit neutralisant pour annuler le bruit de com- be thermoacoustique (103), et éventuellement ou, pression des pales du rotor à un emplacement sé- de préférence, dans lequel le contrôleur (107) est lectionné (403) par rapport à l’aéronef (201). situé à distance de l’aéronef (201), de telle sorte25 qu’une personne peut accéder et contrôler le tube thermoacoustique (103) sans être sur l’aéronef (201).

9. Système de réduction de la signature acoustique 30 (101) selon la revendication 2, dans lequel l’unité à air pulsé (117) est configurée pour envoyer des ra- fales d’air dans le tube thermoacoustique (103) pour ajuster la phase du bruit neutralisant. 35 10. Système de réduction de la signature acoustique (101) selon l’une quelconque des revendications précédentes, comprenant en outre :

un écran (121) couplé au tube thermoacousti- 40 que (103) pour empêcher la saleté, les débris et les corps étrangers de pénétrer dans le tube thermoacoustique (103).

11. Système de réduction de la signature acoustique 45 (101) selon la revendication 2, dans lequel l’interface utilisateur (108) est un dispositif numérique interactif qui permet au pilote de voir graphiquement l’emplacement de l’aéronef (201) par rapport à d’autres ob- jets, de manière à sélectionner la zone d’annulation 50 (403), et éventuellement ou, de préférence, dans lequel le contrôleur (107) ajuste automatique- ment l’un ou plusieurs de la phase, l’amplitude et la fréquence du bruit neutralisant pour compenser le mouvement relatif entre l’appareil (201) et la zone 55 d’annulation (403).

12. Système de réduction de la signature acoustique

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REFERENCES CITED IN THE DESCRIPTION

This list of references cited by the applicant is for the reader’s convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description

• US 3685610 A [0005]