Aircraft rotor system

Abstract

The present invention relates to a rotor system (10) for an aircraft (1), the rotor system (10) comprising a rotor (20) configured to be driven, the rotor (20) comprising a plurality of rotor blades (21) arranged substantially radially around a rotation axis (R) of the rotor (20), a plane perpendicular to the rotation axis (R) and passing radially through the rotor blades (21) forming a rotor plane (RA), and a rotor shell (40) surrounding the rotor (20) in a circumferential direction relative to the rotation axis (R) and enclosing an air duct (40) of the rotor (20) extending in the axial direction of the rotation axis (R). The rotor shroud (30) forms a hollow structure (31) extending in a circumferential direction relative to the rotation axis (R), the hollow structure (31) having a gas permeable region (32a) on at least a portion of its peripheral surface (32) facing the rotor (20) in the radial direction, the rotor plane (RA) intersecting the gas permeable region (32a), and the hollow structure (31) is configured such that sound waves of at least one frequency that penetrate into the hollow structure (31) through the gas permeable region (32a) are at least partially absorbed by the hollow structure (31).

Description

Technical Field

[0001] The present invention relates to a rotor system for an aircraft.

Background Art

[0002] The operation of a rotor system causes acoustic radiation that can be perceived as a nuisance both in terms of volume and frequency. In the case of an aircraft rotor system operating near a residential area, accordingly, measures are taken to reduce the sound level emitted into the environment, either itself or at least at specific frequencies.

Summary of the Invention

Problems to be Solved by the Invention

[0003] In order to reduce the acoustic radiation of a rotor system, for example, it is known to provide an aerodynamic acoustic liner and an aerodynamic liner. The aerodynamic acoustic liner generally reduces the sound generated by the rotor, while the aerodynamic liner is intended to reduce the acoustic radiation induced by the vortices at the tips of the rotor blades.

[0004] For this purpose, European Patent Application Publication No. 2913269 proposes, with respect to the tail rotor of a helicopter, arranging an aerodynamic liner in the form of a separate channel in the region of the rotor plane. The aerodynamic acoustic liner is also arranged on the exhaust side of the rotor and is formed by a connected or nested hollow structure. However, this prior art has several drawbacks. For example, the arrangement of the aerodynamic acoustic liner adjacent to the aerodynamic liner requires an installation space corresponding axially with respect to the rotation axis of the rotor. In addition, the effect of the aerodynamic acoustic liner is limited to the region on the exhaust side, and as a result, the acoustic radiation on the intake side of the rotor is not reduced or is only reduced to a limited extent. In particular, since the stator or support strut of the rotor is arranged in the region of the aerodynamic acoustic liner, the aerodynamic acoustic liner cannot be continuously implemented. However, ultimately, due to the additional structure introduced to form separate channels for the aerodynamic liner and the interconnected or individual nested hollow structures of the aerodynamic acoustic liner, the weight of the tail rotor system also increases.

[0005] In view of the drawbacks associated with the prior art, an object of the present invention is to provide a rotor system for an aircraft, which rotor system includes an aerodynamic liner and a compactly designed aerodynamic acoustic liner for reducing acoustic radiation.

Means for Solving the Problem

[0006] The underlying object of the present invention is solved by the rotor system for an aircraft according to claim 1. Further advantageous aspects of the present invention will become apparent from the dependent claims.

[0007] Among them, the present invention is generally applicable to a rotor system for an aircraft. In particular, the present invention can be applied to the tail rotor of a helicopter.

[0008] According to the present invention, an aircraft rotor system comprises a rotor configured to be driven, the rotor having a plurality of rotor blades substantially radially arranged around the rotation axis of the rotor, and a plane perpendicular to the rotation axis and passing through the rotor blades in the radial direction forms a rotor plane; and a rotor shroud surrounding the rotor in the circumferential direction with respect to the rotation axis and confining an air channel of the rotor extending in the axial direction of the rotation axis. The rotor shroud forms a hollow structure extending in the circumferential direction with respect to the rotation axis, and the hollow structure has a gas-permeable region at least in part on its circumferential surface facing the rotor in the radial direction. The rotor plane intersects the gas-permeable region, and the hollow structure is configured such that sound waves of at least one frequency entering the hollow structure through the gas-permeable region are at least partially absorbed by the hollow structure.

[0009] The phrase "substantially radially" regarding the arrangement of the rotor blades refers to the base direction of the rotor blades. However, it is not essential that they are formed exactly radially. For example, the rotor blades can have an angle of attack with respect to the geometric radius, and the base direction is still radial in the sense of the radial arrangement of the rotor blades around the rotation axis. Further, the rotor blades do not have to be arranged at equal distances, but can have different distances from each other, for example, to transmit acoustic energy at multiple frequencies.

[0010] The rotor shroud forms an aerodynamic liner and an aeroacoustic liner via a hollow structure in cooperation with a gas permeable region formed at least in part thereof. In other words, the aerodynamic liner and the aeroacoustic liner are formed by the primary structure of the rotor shroud without the need for additional separate elements of the shroud. The term "primary structure" is used herein to clarify that the rotor shroud used to form the hollow structure refers not to other additional structural elements that do not belong to the rotor shroud itself, but to the elements of the outer shell intended to actually form the shroud. Thus, the hollow structure is formed by an inner surface disposed opposite the corresponding outer surface of the shroud. In other words, the hollow structure is formed by the elements of the shroud that make up the outer shell of the rotor shroud. Thereby, sound waves passing through the gas permeable region are each guided to a space extending radially with respect to the axis of rotation to the inside on the opposite side of the rotor shroud or the primary structure. Considering the rotor shroud extending circumferentially with respect to the axis of rotation, it can be seen that the hollow structure also extends circumferentially. This arrangement is continuous without adding further reinforcing elements and / or hollow space elements as described below. As a result, the hollow structure itself formed by the rotor shroud provides a hollow structure that acts as a whole. In other words, the hollow space formed by the hollow structure extends continuously in the circumferential direction. This can be adapted to a locally acting hollow structure, for example, by referring to the reinforcing elements and / or hollow space elements described above and below. However, this relates to a particular embodiment of the present invention. Even considering additional structural elements arranged in addition to the rotor shroud, the absorption characteristics regarding sound waves passing through the gas permeable region are determined by the rotor shroud itself.

[0011] The gas-permeable region is preferably not limited to only a part of the circumferential direction. Rather, it is formed over the entire circumferential direction so as to cooperate with the hollow structure over the entire circumference. This already results from the rotation of only the rotor blade that does not first exhibit a local sound emission and / or a maximum of the vortices at the rotor blade tip in response to the movement of the rotor blade. However, in the interaction with other structural components of the rotor system and / or in relation to the arrangement and / or angle of attack of the rotor blade, these maxima can actually occur locally, and as a result, it may be sufficient to provide one or more regions of gas permeability within the section.

[0012] Regarding the mode of action of the aerodynamic liner of the rotor shroud, the rotor blade tip vortices impinge on the gas-permeable region located in the rotor plane on the circumferential surface facing the rotor blade when the rotor is operating. The rotor blade tip vortices may penetrate the gas-permeable region and be at least partially absorbed and in particular dissipated therein. The rotor blade tip vortices that do not penetrate into the hollow structure are at least scattered. In addition to the acoustic effect, at least a partial reduction of the rotor blade tip angle caused, for example, by dissipation also affects the aerodynamic resistance and can thus increase the overall thrust generation.

[0013] However, similarly, sound waves can also penetrate into the hollow structure through the gas-permeable region, and the hollow structure absorbs sound waves of at least one frequency at least partially, and thus acts as an acoustic liner. Basically, in this context, the terms absorption and attenuation of sound waves can be used synonymously. Thereby, dissipation represents a specific form of absorption or attenuation, for example, by energy conversion. The frequency generated at a given rotational speed of the rotor is constant. However, the amplitude of each frequency can be variable depending on the current settings of the rotor system, for example, depending on the angle of attack of the rotor blades. Therefore, the hollow structure can be adapted to at least partially absorb one or more frequencies that are considered particularly complex, for example, even if they do not necessarily cause the maximum sound level. The provision of at least partial absorption characteristics of the hollow structure can be achieved geometrically, or alternatively or additionally by an appropriate selection of materials.

[0014] By arranging the combined liner of aerodynamic and aeroacoustic in the rotor plane, it becomes possible to implement it over the entire circumference of the rotor shroud without interruption.

[0015] Therefore, the above-described rotor system requires a combination of a gas-permeable region and a hollow structure to reduce the aerodynamic effect of the rotor blade tip vortices across the gas-permeable region, at least partially absorb and thus attenuate the penetrating sound waves, and have a positive impact on the efficiency of thrust generation. Since there is no need to change the outer contour of the rotor shroud, even the aerodynamic effect of the rotor shroud for thrust generation is maintained. However, depending on the frequency to be attenuated, the rotor shroud can also be adjusted taking into account the aerodynamic effect.

[0016] However, as will be described below with respect to additional elements and / or material structures, alternative or complementary means can be taken with respect to the space within the hollow structure.

[0017] In one embodiment, the circumferential surface of the hollow structure facing the gas permeable region on the side away from the rotor blade is spaced such that the gas permeable region forms a quarter-wavelength resonator with the circumferential surface facing it at least partially for at least one frequency.

[0018] The circumferential surface of the hollow structure facing the gas permeable region on the side away from the rotor blade may also be referred to as the inner circumferential surface. In contrast, the inside of the circumferential surface facing the inner circumferential surface and having the gas permeable region becomes the inner circumferential inner surface. By spacing the outer circumferential inner surface and the inner circumferential inner surface according to the quarter-wavelength resonator, a standing wave or a multiple (harmonic) of a quarter wavelength of a corresponding combination of wavelength or frequency or mode can be formed in the hollow structure. In this case, in combination with the gas permeable region representing acoustic resistance, the acoustic energy is at least partially converted into thermal energy of the corresponding frequency representing absorption. The distance may be constant in the circumferential direction, or may vary in at least a part of the circumferential direction in order to be locally adjusted to different frequencies. Such adjustment can also be provided alternatively or additionally perpendicular to the rotational direction, i.e., with respect to the axial rotation axis. This has been found to be particularly advantageous when, as may occur from different angles of attack of the rotor blade, different maximum values of a particular frequency may occur axially, as will be explained below. The spacing between the outer circumferential surface and the inner circumferential surface can be formed by appropriate arrangement of the rotor shroud material or by appropriate material shaping. The material profile can be, for example, a change in the material cross-section such that the outer circumferential inner surface and the inner circumferential inner surface can have locally different distances even though the materials are spaced parallel to each other.

[0019] Alternatively or additionally, the hollow structure at least partially forms a Helmholtz resonator for at least one frequency.

[0020] Helmholtz resonators are particularly suitable for absorbing or attenuating lower frequencies. Since medium and high frequencies are attenuated only within a limited range, the hollow structure can include the arrangement of further resonator concepts, such as at least a partial arrangement of a quarter-wavelength resonator.

[0021] In addition to the quarter-wavelength resonator, Helmholtz resonator, or combinations thereof, or geometric designs as other resonator concepts, the acoustic attenuation characteristics can also be supported by a specific choice of material and / or surface structure. For example, the structural features of a Helmholtz resonator can be used to attenuate lower frequencies, and the structural features of a quarter-wavelength resonator can be used to attenuate medium and / or higher frequencies.

[0022] According to one embodiment, the circumferential hollow structure can be designed to be fluid-permeable in the circumferential direction, at least in portions aligned parallel to the circumferential gravity.

[0023] Liquids such as precipitation or wash water can penetrate the hollow structure through the gas-permeable region or through other openings in the rotor shroud. They can also flow out again depending on the position of the respective openings. For example, during washing, water can enter through a portion of the hollow structure having a spatially oriented gas-permeable region such that the water flows out in the direction of gravity across the gas-permeable region. However, not all portions of the circumferential hollow structure provide such a drainage possibility. In this regard, portions that are circumferentially oriented parallel to gravity and thus do not have drainage even through the gas-permeable region of this portion should be designed to be fluid-permeable in particular. The fluid permeability in this case is not for the purpose of outward-directed drainage but relates to fluid conduction within the hollow structure, whereby the penetrating fluid can be guided into the hollow structure up to at least the portions enabling drainage. Since the rotor system according to the present invention can be directly formed by the primary structure, internal fluid conduction through the hollow structure can be realized in a simple manner.

[0024] However, the design of a hollow structure that is at least partially fluid-permeable also allows for the circumferential extension of the resonator space that can be effectively used. For this purpose, it can be assumed that the fluid permeability in this case also causes gas permeability. Even if a resonator cavity is not formed in the sense of standing wave formation, at least individual frequencies can be attenuated, for example, by dissipation.

[0025] The hollow structure preferably comprises at least one discharge opening. Liquid passing through the hollow structure can be selectively discharged or flowed out through the discharge opening. Thus, when the discharge opening can be selectively opened and closed, the position of the discharge can be selected to an appropriate position and / or the timing of the discharge can be preset. The discharge opening can be formed by a gas-permeable region. However, this may be insufficient in some cases, for example, because the gas-permeable region of the tail rotor of a helicopter may not form the lowest point of the hollow structure when it is in a stationary state. Thus, the gas-permeable region in this case may simply act as a drain or overflow channel, but it cannot independently discharge fluid from the hollow structure located below the gas-permeable region. Thus, a separate discharge opening may be advantageous.

[0026] In particular, the rotor blade has a variable adjustable angle of attack about an axis radial to the axis of rotation, and the gas-permeable region extends axially with respect to the axis of rotation over a region covering the position of the rotor blade that can be achieved at least by the angle of attack.

[0027] In response to the flight control being executed, the angle of attack of the rotor blade is changed. This also changes the position of the region where the rotor blade tip vortices are generated. In order to encompass all the region positions of the rotor blade tip vortices related to the angle of attack, the gas-permeable region extends circumferentially at least partially, in particular over the entire circumferentially extending surface, and axially over a region that can cover all the rotor blade positions at least according to the angle of attack of the rotor blade. Otherwise, that is, due to the axial extension of the gas-permeable region that does not cover all the adjustable rotor blade positions, the aerodynamic effect of the gas-permeable region does not exist or is at least substantially limited at all the rotor blade positions or the angle of attack of the rotor blade.

[0028] Since the propagation of the vortices at the radial rotor blade tips with respect to the axis of rotation of the rotor is not limited to the region bounded by the rotor blade tips, that is, the rotor blade tip vortices can scatter and propagate not only purely radially but also axially, the axial gas-permeable region can be designed to be larger, in particular, than the overlapping region of the rotor blade positions. Preferably, the gas-permeable region starts axially outward from the rotor plane and is larger than the overlapping region of the rotor blade positions related to this part, at least on one side, more specifically on both sides, according to the angle of attack of the rotor blade.

[0029] In one embodiment, the porosity of the gas-permeable region ranges from 5% to 90%. The gas-permeable region is basically defined as a region having a dominant gas permeability compared to other regions of the rotor shroud. The term "mainly" does not necessarily refer to a gas permeability exceeding 50%, but rather refers to the material property that can be identified as being gas-permeable. This restricts the region through the outermost gas-permeable openings such as pores where the rotor shroud transitions to the gas-impermeable material region.

[0030] Therefore, when the gas permeable region is formed by pores, the void volume with respect to the total volume of this region, that is, the ratio of the volume of all pores within this region, is 5% to 90%. When the ratio of the void volume is low, the rotor blade tip vortices are mainly reflected and are no longer introduced into the hollow structure. Therefore, the aerodynamic liner no longer operates effectively. Similarly, the acoustic bandwidth of the quarter-wavelength resonator is impaired, and at the same time, at very low frequencies that are no longer relevant to the audible range, the operating mode of the liner from the quarter-wavelength resonator shifts in the direction of a Helmholtz resonator. When the ratio exceeds 90% of the cavity volume, if the rotor blade tip vortices and / or sound waves passing through the pores can escape again without significant attenuation, the absorption or attenuation ability can be significantly reduced.

[0031] According to one embodiment, the porosity varies axially from the rotor plane with respect to the axis of rotation. In particular, the porosity increases towards at least one side in the outer direction from the rotor plane.

[0032] By varying the axial porosity, the acoustic impedance, and thus the acoustic behavior of the aerodynamic and aeroacoustic liner, can be locally adjusted. This can be particularly advantageous for rotor blades with an adjustable angle of attack since the frequency amplitude varies with the change in the angle of attack. Preferably, a greater porosity, that is, a greater void volume, should be provided in the outer region of the gas permeable region related to the effect at a greater angle of attack of the rotor blade. The change in porosity can be expressed as the porosity per unit area.

[0033] Alternatively or complementarily, the porosity varies circumferentially in the circulation of the gas permeable region. The circumferentially varying porosity enables adjustment of the impedance and thus the acoustic behavior along the circumference. Therefore, even considering the adjustable angle of attack of the rotor blade, the sound emission interaction with various structural elements along the circumference can be taken into account.

[0034] The acoustic properties can be structurally and operationally optimized by a combination of porosities that vary in the axial and circumferential directions.

[0035] In one embodiment, the gas-permeable region is formed by micro-perforations, perforated metal plates and / or wire meshes.

[0036] The micro-perforations of the rotor shroud profile or parts thereof can introduce gas permeability into the gas-permeable region without requiring structural damage. Furthermore, the distribution of the micro-perforations can be carried out accurately as required. The separate insertion of perforated plates and / or wire meshes allows for flexible adjustment of the acoustic behavior by replacing the respective inserts. Furthermore, in such cases, it is possible to utilize different material properties independent of the actual rotor shroud material.

[0037] According to a further embodiment, the hollow structures have, in their position and / or design, reinforcing elements and / or hollow structure elements that have a sound-absorbing effect or promote sound absorption.

[0038] Thus, reinforcing elements used especially for stabilization, or other hollow structure elements that function as, for example, line or pipe guides, can also be used to improve the sound-absorbing and / or sound-insulating properties of the rotor system. This also makes it possible to dispense with additional components and structures that are only intended for sound absorption.

[0039] Positioning may be performed circumferentially as a function of frequencies that are at least partially absorbed, for example, due to the natural frequency of a gas volume enclosed inside the overall structure or inside a partially acoustically separated sub-space. Alternatively or complementarily, circumferential positioning may also be performed as a function of the local interaction of acoustic radiation with the respective structural components. Thus, due to the positioning, the depth and / or volume of the hollow structure may be affected via reinforcing elements and / or hollow structure elements in order to specifically increase the at least partial absorption of at least one frequency. Thus, this also enables the conversion of an overall acoustically effective hollow structure, which could be formed by a circumferentially continuous structure without reinforcing elements and / or hollow structure elements, into a locally acoustically effective hollow structure.

[0040] In contrast, the design of the reinforcing elements and / or hollow structure elements refers to specific geometric designs such as contours or material thicknesses, the materials used, and / or various surface properties such as can be implemented by coating or surface texturing.

[0041] In particular, the hollow structure can absorb sound waves that substantially penetrate a broadband frequency range.

[0042] The broadband design is strongly related to the effect of the structure as an aerodynamic acoustic liner, which is further supported by the variable depth of the hollow structure, i.e., the radial spacing with respect to the axis of rotation. Furthermore, the appropriate selection of the perforations in the gas-permeable region can increase the bandwidth at the expense of the absolute absorption of the liner.

[0043] In one embodiment, the hollow structure at least partially absorbs sound waves that substantially penetrate a frequency range from 30 Hz to 1500 Hz.

[0044] In this frequency range, precisely, frequencies that are particularly perceived as disturbing are generated. However, in particular, this frequency range also includes frequencies that usually have clearly perceptible amplitude maxima. Thus, if an appropriate selection is made, the overall volume level can be reduced.

[0045] According to one embodiment, the rotor system comprises at least one support strut arranged on the intake side of the rotor system.

[0046] The support strut, sometimes also called a stator, serves to suspend the rotor hub. These are typically arranged on the exhaust side of the rotor system, so that the air duct formed by the rotor shroud, which is wide axially with respect to the axis of rotation, compensates for the sound emissions associated with this arrangement. However, if at least one support strut for suspending the rotor hub is arranged here on the intake side, the sound emissions caused by the air accelerated by the rotor hitting the support strut are reduced. In other words, since an additional sound source on the exhaust side is avoided, the axial width of the air duct can be reduced. The intake side is understood to be the side where air is drawn in for most of the flight operation. Similarly, the exhaust side is the side where air is discharged in most of the flight operation.

[0047] Preferably, the at least one support strut is arranged eccentrically or off - center with respect to the axis of rotation.

[0048] The eccentric arrangement supports the reduction of sound emissions resulting from the time - shifted interaction between the wake behind the support strut and the leading edge of the rotating rotor blade in the velocity profile of the incident flow caused by the shadowing effect of the support strut for normal operation, i.e., for torque compensation of the main rotor. In the case of a negative angle of attack of the rotor blade due to the operation of the aircraft, there is no simultaneous interaction. However, in this regard, the interaction between the vortices induced by the rotating rotor blade and the support strut responsible for sound generation is meant.

[0049] The features, utilities and advantages of the present invention are also described below with reference to the drawings according to embodiments.

[0050] In the drawings,

Brief Description of the Drawings

[0051]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

DETAILED DESCRIPTION OF THE INVENTION

[0052] FIG. 1 shows an aircraft 1 having a rotor system 10, which in the illustrated embodiment is used as a tail rotor system of a helicopter, in this case a helicopter. The rotor system 10 preferably comprises a rotor 20 having rotor blades 21 arranged around a rotor hub 23 supported by a plurality of support struts 22, and a rotor shroud 30. The support struts 22 are arranged on the intake side 41 (FIG. 2) in order to avoid another sound source on the exhaust side 42 (FIG. 2). Furthermore, an eccentric arrangement of the support struts 22 with respect to the axis of rotation R is provided, which also has a positive acoustic effect. The intake side 41 and the exhaust side 42 are respectively the sides where air is drawn in and discharged in most cases of flight operation. In other words, when the angle of attack of the rotor blades 21 is correspondingly negative, it is also possible to discharge air on the intake side 41, but this should only be assumed in a few cases during flight operation, and in such cases it can be ignored for the definition of the intake side 41. This also applies to the exhaust side in reverse configuration.

[0053] According to FIG. 2, the rotor shroud 30 circumferentially surrounds the rotor 20 with respect to the rotation axis R and encloses the air duct 40 of the rotor 20 extending in the axial direction of the rotation axis R. As a result of the rotation of the rotor blades 21 centered on the rotation axis R, air is conveyed from the intake side 41 to the exhaust side 42, which is also called the thrust side. The air flow direction is again indicated by the arrows inserted in FIG. 2, but this can also be reversed depending on the setting of the rotor blades. As can be further seen from FIG. 4, in the rotor plane RA formed by the rotor blades 21 perpendicular to the rotation axis R, the circumferential surface 32 of the rotor shroud 30 facing the rotor 20 intersects the rotor plane RA and has gas-permeable regions 32a extending axially on both sides of the rotor plane RA with respect to the rotation axis R. The gas-permeable regions 32a are formed by the rotor blades 21.

[0054] FIG. 3 shows in more detail a perspective cross-sectional view of the rotor system 10 according to FIGS. 1 and 2 in a cross-section parallel to the rotation axis R. In this exemplary embodiment, the gas-permeable region 32a is formed by a perforated metal plate having micro-perforations inserted and fixed in the rotor shroud 30. The porosity introduced by the micro-perforations reaches, for example, 50% and is constant in the circumferential and axial directions with respect to the rotation axis R. However, the porosity may also vary in the circumferential and axial directions with respect to the rotation axis R, and / or may be smaller or larger than 50%. The choice of porosity or its distribution can result from the respective optimization goals regarding the aerodynamic-acoustic or aerodynamic effects interacting with the respective structural design.

[0055] The gas permeable region 32a covers the radially protruding portion at the tip of the rotor blade 21 of the rotor blade. As a result, the rotor blade tip vortex generated in the gap between the rotor blade tip and the gas permeable region 32a passes through the gas permeable region 32a and is introduced into the hollow structure 31 formed by the rotor shroud 30, where it dissipates or is attenuated in other ways, achieving an aerodynamic effect. In this regard, the aerodynamic effect refers, on the one hand, to the acoustic effect achieved by eliminating or displacing the acoustic source associated with the rotor blade tip vortex to form the hollow structure 31. On the other hand, the efficiency of the rotor system 10 increases due to the reduction in drag caused by the attenuation and / or displacement of the rotor blade tip vortex. In addition, the gas permeable region 32a combined with the volume formed by the hollow structure 31 implements an aeroacoustic function in which a pure acoustic effect is also achieved for sound waves coupled to the hollow structure 31 through the gas permeable region 32a, which is also generated, for example, by at least partial absorption of at least one frequency by components other than the rotor blade tip during the operation of the rotor 20.

[0056] For this purpose, in the illustrated embodiment, the inner surface facing the rotor 20 of the circumferential surface 33 facing outward from the rotor 20, that is, the outer peripheral inner surface 33a, is spaced apart from the inner surface of the circumferential surface 32 facing the rotor 20 facing outward from the rotor 20, that is, the inner peripheral inner surface 32b, such that a quarter-wavelength resonator is formed for at least one frequency.

[0057] Accordingly, an aerodynamically and aeroacoustically combined liner is formed by positioning and sizing the gas permeable region 32a in cooperation with the volume of the hollow structure 31.

[0058] FIG. 4 again shows a schematic cross-sectional view of the rotor system 10 according to FIG. 3 and has a line of sight direction towards the cross-sectional plane in order to show the adjustable position range of the angle of attack of the rotor blade 21 and the overlap of the rotor blade tip over that range of pitch angle positions by the gas permeable region 32a. For this purpose, FIG. 4 shows the maximum reach angle position of the rotor blade 21 when rotating about the radial axis X centered on the axis of rotation R with respect to the rotor blade pitch angle. The gas permeable region 32a extends axially on both sides with respect to the axis of rotation R starting from the rotor plane RA that coincides with the radial axis X, and the gas permeable region 32a covers the maximum pitch angle position of the rotor blade tip portion. In the illustrated exemplary embodiment, the gas permeable region 32a is also enlarged with respect to the maximum position of the rotor blade tip, and the scattered rotor blade tip vortices can also be introduced into the hollow structure.

[0059] According to FIG. 5, a perspective cross-sectional view of the rotor system 10 according to FIGS. 1 and 2 is exemplarily shown in a cross-section perpendicular to the axis of rotation R. Here, the rotor shroud 30 forms a hollow structure 31 with different distances between the circumferential outer peripheral surface and the inner peripheral surface. Accordingly, locally different quarter-wavelength resonators are formed such that locally different frequencies can be at least partially absorbed, thereby reducing the overall volume and providing a broadband acoustic effect to the liner. In particular, different spacings can be provided so as to preferentially attenuate the tone components corresponding to the rotational frequency of the rotor and / or the frequencies that are particularly perceived as troublesome.

[0060] Furthermore, the hollow structure 31 has various reinforcing elements 34 and hollow structure elements 35. The reinforcing elements 34 function here, for example, as stoppers and also affect the attenuation of the sound waves introduced into and / or propagating within the hollow structure with respect to their dimensions and positioning. Similarly, the additionally introduced hollow structure elements 35 can form chambers within the hollow structure 31, for example, to form locally different resonator volumes in each case and thereby affect the attenuation of the frequency. Nevertheless, the attenuation ability here is largely determined by the primary structure of the rotor shroud 30.

[0061] Referring to FIG. 5, the rotor shroud 30 further has a discharge opening 36 that can discharge the liquid that has entered the hollow structure 31. The discharge opening 36 is located in the lower region of the rotor shroud 30 in the direction of gravity, which is the region where the liquid accumulates due to gravity. For this purpose, the hollow structure 31 is preferably partially permeable to the fluid in the circumferential direction, that is, formed as a continuous circumferential fluid channel. Even if the reinforcing element 34 and / or the hollow structure element 35 are arranged in the hollow structure 31, they should be arranged at least partially permeable to the fluid or in a position that allows the fluid to be released or discharged in some other way. The latter can be exemplified by the hollow structure element 35 shown in FIG. 5. If these hollow structure elements are not designed to be fluid permeable, at least the liquid present in the upper chamber formed by the hollow structure element 35 should be arranged in a position where it can be discharged through a gas permeable region 32a (not shown here). Therefore, in this case, the gas permeable region 32a is also fluid permeable.

[0062] The present invention is not limited to the described embodiments. In particular, specific features of possible variants or further embodiments are, in principle, also applicable to other embodiments, provided that this is not reasonably excluded. For example, even if the rotor hub 23 is supported by two support struts 22, only one support strut may be provided. Similarly, it is possible to use three or more support struts. However, in particular, the use of the rotor system 10 is not limited to the tail rotor system of a helicopter and can also be used in other aircraft such as drones or air cabs.

[0063] List of reference signs 1 Aircraft 10 Rotor system 20 Rotor 21 Rotor blade 22 Strut 23 Rotor hub 30 Rotor shroud 31 Hollow structure 32 Peripheral surface (rotor side) 32a Gas permeable region 32b Inner peripheral surface 33 Peripheral surface (side away from the rotor) 33a Outer peripheral surface 34 Reinforcing element 35 Hollow structure element 36 Discharge opening 40 Air duct 41 Intake side 42 Exhaust side R Rotation axis RA Rotor plane X Radial axis (rotor blade pitch angle position)

Claims

1. A rotor (20) configured to be driven, having a plurality of rotor blades (21) substantially radially arranged around the rotation axis (R) of the rotor (20), and a plane perpendicular to the rotation axis (R) and radially penetrating the rotor blades (21) forms a rotor plane (RA), the rotor (20), A rotor shroud (30) that surrounds the rotor (20) in the circumferential direction with respect to the rotation axis (R) and confines an air duct (40) of the rotor (20) extending in the axial direction of the rotation axis (R) A rotor system (10) for an aircraft (1), comprising: The rotor shroud (30) forms a hollow structure (31) extending in the circumferential direction with respect to the rotation axis (R), and the hollow structure (31) has a gas permeable region (32a) at least partially on its circumferential surface (32) facing the rotor (20) in the radial direction, and the rotor plane (RA) intersects the gas permeable region (32a), The hollow structure (31) is configured such that sound waves of at least one frequency entering the hollow structure (31) through the gas permeable region (32a) are at least partially absorbed by the hollow structure (31), The rotor shroud comprises a primary structure formed by a hollow structure in cooperation with a gas permeable region formed at least in part thereof, the primary structure forming an aerodynamic liner and an aerodynamic acoustic liner and not requiring further separate elements of the shroud, The hollow structure (31) comprises at least one discharge opening (36), a rotor system (10) for an aircraft (1).

2. The circumferential surface (33) of the hollow structure (31) facing the gas permeable region (32a) on the side away from the rotor blade (21) is spaced such that the gas permeable region (32a) forms a quarter-wavelength resonator with the opposing circumferential surface (33) at least partially for the at least one frequency, the rotor system (10) according to claim 1.

3. The rotor system (10) according to claim 1 or 2, wherein the hollow structure (31) at least partially forms a Helmholtz resonator for the at least one frequency.

4. The rotor system (10) according to claim 1 or 2, wherein the hollow structure (31) is designed to be fluid-permeable in the circumferential direction, at least in a portion aligned parallel to the circumferential gravity.

5. The rotor blade (21) has an angle of attack that is variably adjustable around an axis (X) radial to the rotation axis (R), and the gas-permeable region (32a) extends axially with respect to the rotation axis (R) over a region covering at least the positions of the rotor blade that can be achieved by the angle of attack. The rotor system (10) according to claim 1 or 2.

6. The rotor system (10) according to claim 1, wherein the porosity of the gas-permeable region (32a) is in the range of 5% to 90%.

7. The rotor system (10) according to claim 6, wherein the porosity varies in the axial direction with respect to the rotation axis (R) from the rotor plane (RA).

8. The rotor system (10) according to claim 7, wherein the porosity increases towards at least one side in the outer direction from the rotor plane (RA).

9. The rotor system (10) according to claim 6 or 7, wherein the porosity varies in the circumferential direction of the gas-permeable region (32a).

10. The rotor system (10) according to claim 1 or 2, wherein the gas-permeable region (32a) is formed by micro-perforations, perforated metal plates, and / or wire meshes.

11. The rotor system (10) according to claim 1 or 2, wherein the hollow structure (31) has reinforcing elements (34) and / or hollow structure elements (35) that have a sound absorption effect or promote sound absorption in terms of their positions and / or designs. **Claim 12** The rotor system (10) according to claim 1 or 2, wherein the hollow structure (31) absorbs sound waves that substantially penetrate in a broadband frequency range. **Claim 13** The rotor system (10) according to claim 1 or 2, wherein the hollow structure (31) at least partially absorbs sound waves that substantially penetrate in a frequency range of 30 Hz to 1500 Hz. **Claim 14** The rotor system (10) according to claim 1 or 2, wherein the rotor system (10) comprises at least one support strut (22) arranged on the intake side (41) of the rotor system (10). **Claim 15** The rotor system (10) according to claim 14, wherein the at least one support strut (22) is arranged eccentrically with respect to the rotation axis (R).

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