Tail rotor for rotor craft

Abstract: A ducted fan-type tail rotor of a rotor craft is surrounded by a tubular duct and includes at least six rotor blades (6) and pitch horns (11), one mounted to each rotor blade. The pitch horns (11) are swingable to change the pitch angle of the rotor blade (6). About one-half of the total number of pitch horns (11) are mounted to first type rotor blades (6) at a mounting angle larger than a reference mounting angle required to obtain a predetermined thrust, and the remaining pitch horns (11) are mounted to second type rotor blades (6) at a mounting angle smaller than the reference mounting angle.

Description: Field of the Invention and Related Art Statement The invention relates generally to a tail rotor of a rotor craft, and more particularly to a ducted fan-type tail rotor of the rotor craft. The ducted fan-type tail rotor comprises a rotor assembly including a plurality of rotor blades, and a tubular duct for surrounding the rotor assembly. The tail rotor of the ducted fan type, unlike an ordinary propeller-type tail rotor, can prevent mechanics or passengers on the ground from accidentally contacting the rotor blades since the rotor assembly is covered by the duct. Further, during a flight near the ground; the rotor blades are prevented from accidentally contacting trees or buildings. Therefore, the tail rotor of the ducted fan-type is quite safe. With respect to the performance of the tail rotor of the ducted fan-type, it has great advantages that a blow-out speed is high, and a thrust drop called "a vortex ring state" can be prevented due to the fact that the tail rotor is surrounded by the duct. In the tail rotor of the ducted fan-type, in order to decrease the axial length of the duct to reduce its head resistance area, it is required to shorten the chord of the rotor blade. Therefore, in order to obtain a required total area of the rotor blades determined by the horsepower absorbing capacity of the tail rotor, the number of the rotor blades is necessarily increased. Conventionally, the number of the rotor blades of the ducted fan-type tail rotor of the rotor craft is generally 8 to 13. On the other hand, the higher the peripheral tip speed of the blade becomes, the better becomes the performance. Therefore, the tip speed of the rotor blade of the tail rotor is generally set to be equal to that of a rotor blade of a main rotor, namely about 180 m/sec to about 220 m/sec. The radius of the tail rotor for small- and medium-size helicopters is generally 1 meter to generally 2 meters, and therefore the number of revolutions is 2,500 r.p.m. to 5,000 r.p.m. In the case where the rotor blades are equiangularly arranged, noises generated from the tail rotor of the ducted fan-type which rotates at such high speed are composed mainly of a frequency equal to the product of the revolution number and the blade number. Therefore, high-frequency noises of 300 Hz to 900 Hz are generated. The sound pressure level of this high-frequency noise is lower than that of the noise from the main rotor. However, the noise from the main rotor is a low-frequency one, and therefore is not so irritating. The noise from the tail rotor is considered the main source of the noise in the rotor craft. There are two methods of reducing the noise from the tail rotor. One is to reduce the tip speed, and the other is to arrange the rotor blades non-equiangularly. (1) Tip Speed Reduction Method When the tip speed is reduced, the noise is greatly lowered. However, at the same time, the performance is also lowered, and therefore it becomes necessary to increase the diameter of the rotor, which results in the disadvantage that the weight and the cost are increased. (2) Non-equiangular Blades Arrangement Method With an equiangular arrangement of the rotor blades, the sound pressure peak of high level is generated at a high-frequency band, as described later. However, with a non-equiangular arrangement of the rotor blades, the sound pressure is at a low level, and the peak of the sound pressure is generated at many frequency bands. Such sound is much like sound produced from a musical instrument; it is not offensive to the ear and is heard as a low sound. Such a non-equiangular arrangement is quite effective in reducing the noise. However, since the rotor blade mounting portions are disposed very close to one another they are likely to interfere with one another. Therefore, the non-equiangular arrangement is not practical from a structural point of view. Even if the equiangular arrangement is slightly modified to be non-equiangular to such an extent that adjacent two rotor blades are close to each other without interfering, such modification is not enough to reduce the noise. FR-A-1 531 536 discloses a tail rotor for a rotor craft which was the features included in the first part of claim 1. The known rotor also has a mechanism for simultaneously changing the attitude of all rotor blades by twisting a lower end of each blade about the blade pitch axis. Object and Summary of the Invention It is an object of the present invention to provide a tail rotor of a rotor craft which can sufficiently reduce a noise level without reduce of a blade tip speed (that is, without deteriorating the performance) and without inviting a structural problem with the mounting of rotor blades, and besides a rasping high-frequency sound is converged into a low-frequency sound. This object is accomplished by the present invention as defined in claim 1. Brief Description of the Drawings Fig. 1 is a perspective view of an overall construction of a rotor craft; Fig. 2 a plan view of a tail rotor of the rotor craft; Fig. 3 is a cross-sectional view taken along the line III-III of Fig. 2; Fig. 4 is an exploded perspective view of the tail rotor; Fig. 5 is a graph showing results of analysis of frequency of noise generated from the tail rotor; Figs. 6A to 6D are diagrams showing the noise from the rotor blades and the directivity thereof; Fig.7 is a schematic view showing a tail rotor having rotor blades arranged equiangularly; Fig. 8 is a graph showing results of analysis of the frequency of the noise in the rotor blades of Fig. 7, Fig. 9 is a schematic view showing a tail rotor having rotor blades arranged unequiangularly; Fig. 10 is a graph showing results of analysis of the frequency of the noise in the rotor blades of Fig. 9; Fig. 11 is a cross-sectional view taken along the line XI-XI of Fig. 4, showing the mounting of the rotor blade on a spider; Fig. 12 is a cross-sectional view showing the another mounting of a rotor blade on a spider; Fig. 13 is a graph showing the relation between the amount of movement of the spider and a rotation angle; Fig. 14 is a graph showing the relation between the spider movement rate and the pitch angle; Fig. 15 is a cross-sectional view showing the relation between the pitch angle of the rotor blade and the angle of attack; Fig. 16 is a graph showing the relation between the spider movement race and the thrust; Fig. 17 is a graph showing the relation between the speed of the ducted fan-type tail rotor and a required horsepower; Fig. 18 is a cross-sectional view showing the still another mounting of a rotor blade on a spider; and Figs. 19 to 21 are views showing another embodiments of the present invention, respectively. Detailed Description of the Preferred Embodiments As shown in Fig. 1, a rotor craft 1 comprises a tail rotor 2 and a main rotor 3. The tail rotor 2 comprises a tail rotor assembly 4, and a tubular duct 5 surrounding the tail rotor assembly 4. As shown in Figs. 2 and 3, the tail rotor assembly 4 comprises eight (8) rotor blades 6, a hub assembly 8 for mounting the rotor blades 6 on a rotation shaft 7, a control rod 9 for changing the pitch angle of the rotor blades 6 in accordance with the operation of the pilot, and a spider 10. The movement of the spider 10 is transmitted to pitch horns 11 mounted respectively to the rotor blades 6 at proximal end portions thereof, so that each rotor blade 6, supported by two bearings 12 and 13 shown in Fig. 4, is swung around a pitch axis 14 to provide the required pitch angle according to the movement of the pitch horn 11. As shown in Fig. 5, the noise generated by the rotor system consists of so-called rotation noise components including frequencies which are equal to integral multiples of the revolution number, and so-called wide-band noise components including frequencies much higher than those of the rotation noise. The wide-band noise is generated due to a vortex produced at the tip and the trailing edge of the rotor blade. However, in the case of the ducted fan-type tail rotor, since the blade tips are quite close to the duct 5, the tip vortex is not produced, and therefore the wide-band noise is of no problem. The rotation noise of the rotor blades can be broadly classified into the noise (Fig. 6A) due to the blade tip vortex, the noise (Fig. 6B) due to the thrust, the noise (Fig. 6C) due to the rotation torque, and the noise (Fig. 6D) due to the thickness of the blade. In the drawings, the abscissa axis represents the direction of the plane of rotation, and the ordinate axis represents the direction of the axis of rotation. In the case of the ducted fan-type tail rotor, the noise due to the blade tip vortex does not pose any problem since there is generated no tip vortex, as described above. With respect to the noise due to the rotation torque and the noise due to the blade thickness, any recogniseable components are in the direction of the plane of rotation plane. However, since the blades are covered by the duct in this direction, these noises can be ignored. After all, the noise from the ducted fan-type tail rotor is mainly the noise due to the thrust. The rotation noise, such as the noise due to the thrust, is constituted by components of frequencies which are integer times the revolutional frequency of the rotor blades. When the eight rotor blades 6 are arranged equiangularly, these rotor blades are 45° different in phase from one another, as shown in Fig. 7. Therefore, when the noises from these rotor blades 6 are combined together, there are remained only components which are 8 times and 16 times the revolutional frequency of the rotor blades 6, as shown in Fig. 8. The other components cancel out one another. The lower the frequency is, the higher is the sound pressure level, and therefore in the case of the arrangement of Fig. 7, only the frequency component of 8 times the revolution number is very strongly audible. In the case of the ducted fan-type actual tail rotor, the frequency of this 8 times sound is close to 1000 Hz. In Fig. 9, eight rotor blades 6 are arranged, for example, at point symmetry with respect to the centre of rotation at angular intervals of 23°, 36°, 57° and 64°. In this case, the noise is constituted by components, frequencies of which are integer times twice the revolutional frequency of the rotor blades. The sound having a frequency which is twice the revolutional frequency of the rotor blades, is mainly audible, and besides the sound pressure level is low. However, it is difficult to arrange the rotor blades close to each other, as described above. The rotor blades interfere with each other, or the pitch horns 11 interfere with each other, and therefore it is difficult to obtain the arrangement shown in Fig. 9. On the contrary, if the interval between the rotor blades 6 is increased to a structurally-allowable degree, a marked noise reduction effect as in Fig. 10 can not be obtained. In embodiments of the present invention, two types of blade constructions (later described) are employed. In order to obtain the different blade constructions, according to the first means, as shown in Figs. 11 and 12, the pitch horns 11 and the slide bearings 15 are employed, while according to the second means, as shown in Fig. 18, a pitch link 18 is employed. 1. First Means As shown in Fig. 11, a spider 10 is connected to pitch horns 11 through slide bearings 15. When the spider 10 moves in a direction P perpendicular to a direction R of rotation of rotor blades, the pitch horn 11 swings about a pitch axis 14 in response to this movement of the spider 10. Since the pitch horn 11 and the rotor blade 6 are integrally connected together, the rotor blade 6 is shifted between a position indicated in solid lines and a position indicated in two-dot lines. The pitch horn 11 is mounted to the rotor blade 6 in such a manner that the pitch horn 11 and the direction R define therebetween a mounting angle ψ. The spider 10 can be moved by a displacement δ between a position A and a position A' in the direction P through the operation of the pilot. A relation between the normal displacement δ of the spider 10 from a plane through the direction R of rotation of rotor blades and an angle γ defined between the pitch horn 11 and the direction R is varied within a range of a line portion A - A' of the curve indicated by a dot-and-dash line in Fig. 13, or y=a·tan γ (a: constant). If the mounting angle ψ' is made small as shown in Fig. 12, the operative range of the pitch angle  is somewhat shifted to a range of a line portion B - B' of the curve indicated by a dot-and-dash line in Fig. 13. The relations between δ/δmax and  in both constructions of Figs. 11 and 12 are shown in Fig. 14. In this case, the construction of Fig. 11 is called "type I", and the construction of Fig. 12 is called "type II". The initial pitch angles  0 of both types at a point where δ is zero are equal to each other. The pitch angles  become different from each other at a middle portion of the ratio δ/δmax. The absolute value of the angle  of the pitch horn 11 with respect to the direction R at the maximum pitch position B' in the type II is set to be equal to the absolute value of the angle ψ of the pitch horn 11 with respect of the direction R at the initial pitch position A in the type I, namely  = -ψ . Therefore, the pitch angles of the type I and the type II coincide with each other at the maximum pitch position Pmax in Fig. 14. At the maximum pitch position, the rotor blade 6 is rotating in an almost stalled condition, and therefore it is preferred that the pitch angles in both types are the same at such position. In this case, at the low pitch angle (that is,the maximum thrust) and at the high pitch angle (that is, the maximum reverse thrust), all rotor blades 6 of both types operate in cooperation with one another to achieve the maximum performance. In the normal operation (that is, at a middle range of the pitch angles), because of the different blade pitch angles, the thrust is produced substantially by a half of the rotor blades 6, or four rotor blades 6. Therefore, the rotation noise is close to a quadruple sound, and the frequency of the noise is lowered. In the normal operating condition near an airport, the ratio δ/δmax is generally 0.3 to 0.7. In this range, the difference in the pitch angle between the type I and the type II is about 5°. On the other hand, as shown in Fig. 15, the relative wind against the rotor blade 6 is the resultant of a component induced by the tail rotor in the direction P and a component of the wind acting in the direction R , and the angle of attack of the rotor is α. In the above normal operating condition, the angle of attack is about 5°. Therefore, the thrusts produced respectively by the rotor blades of the type I and the rotor blades of the type II vary as indicated in solid lines in Fig. 16. For most values of the ratio δ/δmax, the rotor blades of the type II mainly contribute to the thrust generation. At the point of the maximum thrust, all of the rotor blades of the type I and the type II generate their maximum thrusts which are equal with each other. Therefore, the maximum thrust performance is in no way inferior to that obtained with a conventional propeller-type tail rotor in which each of blades produces the same thrust. In the rotor craft with the ducted fan-type tail rotor, as shown in Fig. 17, during a forward flight, a vertical tail produces a counter torque, and then the tail rotor 2 almost idles, and therefore the noise is substantially reduced. Therefore, the noise of the tail rotor becomes a problem when the rotor craft is in a low-speed flight and a hovering. Such manoeuvres are normally carried out near the ground and at around δ/δmax = 0.5. Therefore, as apparent from Fig. 16, it will be appreciated that the tail rotor including the types I and II blades is operated effectively. 2. Second Means In second means, a pitch link 18 connects the spider 10 with the pitch horn 11, as shown in Fig. 18. The spider 10 and the pitch horn 11 are connected through spherical joints 19 provided at opposite ends of the pitch link 18. In accordance with the movement of the spider 10 in the direction P , the pitch horn 11 can be swung about the pitch axis 14 between a position indicated in a solid line and a position indicated in two-dot line. In this case, just as in the above embodiment, the pitch horn 11 is mounted to the rotor blade 6 in such a manner that the pitch horn 11 and the direction R define therebetween a mounting angle ψ (Fig. 18). The relation between the displacement δ of the spider 10 and the pitch angle  of the rotor blade 6 is varied along a curve indicated by a two-dot line in Fig. 13, or δ=b·sin  (b: constant). By using two kinds of rotor blade groups different in the mounting angle ψ of the pitch horn 11, the thrust produced by each group of rotor blades 6 can be varied as in the above embodiment. In this second means, since the pitch link 18 is of a turnbuckle construction, an unbalance of the thrust among the rotor blades 6 can be finely adjusted by changing the length of the pitch link 18. Further, in order to obtain the two blade constructions, the first means and the second means may be used in combination. In a first embodiment, as shown in Fig. 19, eight rotor blades 6a to 6h are disposed equiangularly, and the type I rotor blades and type II rotor blades are arranged alternately. In normal operation, the type I rotor blades 6a, 6c, 6e and 6g, which are hatched in the drawings, generate the trhust, while non-hatched type II rotor blades 6b, 6d, 6f and 6h idle. According this, the noise generated mainly includes components, the frequencies of which one-half those generated when all the blades uniformly produce the thrust. As described above, the rotor blades and the spider are connected together by the first means, or the second means, or a combination of them. In a second embodiment, as shown in Fig. 20, eight rotor blades 6a to 6h are disposed equiangularly. A first pair of adjacent rotor blades 6a and 6b, and a second pair of adjacent rotor blades 6e and 6f, which are disposed opposite to the first pair, are all type II rotor blades and generate the thrust. The remainder rotor blades 6c, 6d, 6g and 6h are type I rotor blades and idle. In this case, the noise generated mainly includes components, the frequencies of which are further reduced as compared with the first embodiment, and specifically the frequencies of which one-quarter those generated when all the blades uniformly produce the thrust. Therefore, noise is hardly heard. In the first and the second embodiments, the rotor blades 6a to 6h are disposed equiangularly, and therefore any structural problem with the blade mounting portion does not arise, and problems with the blade balance and vibration are not serious. In a third embodiment, as shown in Fig. 21, the rotor blades 6a to 6h are disposed non-equiangularly so that adjacent rotor blades do not interfere with each other. A first pair of adjacent rotor blades 6a and 6b, and a second pair of adjacent rotor blades 6e and 6f, which are disposed opposite to the first pair, are type II rotor blades and generate the thrust. The remainder rotor blades 6c, 6d, 6g and 6h are type I rotor blades and idle. In this case, the high-frequency component of the noise is further reduced, so that the noise is reduced. As described above, according to the present invention, in the tail rotor of the rotor craft, differences in the blade pitch angle are generated among the rotor blades in the normal operating range of the rotor craft. Accordingly the sound pressure of the noise generated from the tail rotor is reduced, and jarring high-frequency noise is converted into a low-frequency noise which is not so jarring. Further, the noise is reduced without tip speed reduction, which would otherwise lower the performance and cause the vibration problem, so that the performance remains high. Further, even if rotor blades whose pitch angle is large stall, as a result of increasing the angle of attack due to an abrupt blade control, the group of rotor blades whose pitch angle is small remain unstalled, and therefore the rotor craft is not brought into a dangerous condition such as a totally-stalled condition, and a higher safety is achieved. In the tail rotors according to the present invention, the conventional pitch horn is modified, and then the number of the components and the cost are not increased. Further, since the ducted fan-type tail rotor is employed, there is no increased danger of collision with persons or obstacles, and there is no influence by the vertical tail or cross wind. Thus, a high safety and performance can be obtained without a noise problem which is inevitably posed by the ducted fan-type tail rotor.

Claims: A tail rotor (2) of a rotor craft (1), which is surrounded by a tubular duct (5) and comprises at least six rotor blades (6), wherein a pitch horn (11) is mounted to each rotor blade (6) and is swingable about a pitch axis (14) of its rotor blade to change the pitch angle of said rotor blade, characterised in that, for a position of the blades corresponding to the initial pitch ( 0 ) of the blades, substantially one-half of the total number of said pitch horns are mounted to first set of rotor blades at a first predetermined mounting angle (ψ) with respect to the plane of rotation of the blades, and the remaining pitch horns are mounted to second set of rotor blades at a second mounting angle (ψ') smaller than that first mounting angle (ψ). The tail rotor of Claim 1, wherein said first-type and second-type rotor blades (6) are alternately disposed in the direction of rotation of said rotor (2). The tail rotor of Claim 1, wherein pairs of adjacent first-type rotor blades and pairs of adjacent second-type rotor blades are alternately disposed in the direction of rotation of said rotor (2). The tail rotor of Claim 2 or 3, wherein said rotor blades are disposed equiangularly or non-equiangularly. The tail rotor of Claim 1, wherein said pitch horns (11) are so mounted to said rotor blades (6) that when the angle between a direction (R) of rotation of the rotor blades and the pitch horn mounted to a first-type rotor blade is ψ, the angle between the direction (R) of rotation of the rotor blades and the pitch horn mounted to the second-type rotor blade is -ψ. The tail rotor of any one of Claims 1 to 5, including a spider (10) connected to said pitch horns (11) through slide bearings (15), wherein said pitch angle is adapted to be changed by moving said spider. The tail rotor of any one of Claims 1 to 5, including a spider (10) connected to said pitch horns (11) through spherical joints (19) provided at opposite ends of a pitch link (18), wherein said pitch angle is adapted to be changed by moving said spider. The tail rotor of any one of Claims 1 to 5, including a spider (10) connected to some of said pitch horns (11) through slide bearings (15) and to the remainder of said pitch horns through pitch links (18), each pitch link including spherical joints (19) at its opposite ends, wherein said pitch angle is adapted to be changed by moving said spider.

Inventors: BANDOH SHUNICHI, EGAMI KOICHI, WATANABE TAKASHI, BANDOH, SHUNICHI, EGAMI, KOICHI, WATANABE, TAKASHI
Applicants: KAWASAKI HEAVY IND LTD, KAWASAKI JUKOGYO KABUSHIKI KAISHA
Application year: 1993
Main classification: B64C27
IPC codes: B64C11, B64C27