RU2799170C1 - Screw for hovering aircraft - Google Patents

Abstract

FIELD: hovering aircrafts.

SUBSTANCE: screw (3) for hovering aircraft (1) contains a bushing (5) rotating around the axis (A), containing a group (9) of blades, a main rotor shaft (6) configured to be connected to the power unit of the aircraft (1) and functionally connected to the sleeve (5) to rotate the sleeve (5) around the axis (A), and the absorbing device (15) to absorb the transmission of vibrations from the main rotor shaft (6) to the aircraft (1) parallel to the specified axis (A). The absorbing device (15) contains a casing (20), the first weight (21) free to oscillate parallel to the specified axis (A) relative to the casing (20) and resiliently connected to the casing (20), the second weight (25) free to oscillate parallel to the specified axis (A), connecting means (30) configured to move the first and second weights (21, 25) in one piece along said axis (A) when the angular velocity (Ω) of the shaft (6) of the main rotor takes on the first value (Ω1), and actuators (35) for separating the first and second loads (21, 25) when the angular velocity (Ω) of the shaft (6) of the main rotor takes on the second value (Ω2) different from the first value (Ω1).

EFFECT: possibility of absorption of axial vibrations of the propeller at different angular velocities by passive absorbers is provided.

17 cl, 5 dwg

Description

Cross-reference to related applications

This patent application takes precedence over European Patent Application No. 19180113.3 filed on 06/13/2019, the full disclosure of which is hereby incorporated by reference.

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The present invention relates to a propeller for a hovering aircraft such as, for example, a helicopter, tiltrotor or hybrid helicopter.

In particular, the present invention relates to a helicopter propeller.

BACKGROUND OF THE INVENTION

As is known, helicopters generally comprise a fuselage, a main rotor located on the top of the fuselage and configured to rotate about its own axis, and a tail rotor located at the end of the fuselage.

In more detail, each main or tail rotor, in turn, mainly contains:

- power block;

- a sleeve rotatable about the aforementioned axis, and equipped with a group of blades radially attached to and protruding from the aforementioned sleeve; And

- the main rotor shaft connected to the power unit and functionally connected to the sleeve for rotation.

In addition, each main rotor contains a transmission unit located between the power unit and the main rotor shaft, and equipped with an auxiliary box, fixed relative to the axis of rotation of the main rotor shaft and held on the helicopter fuselage.

In particular, the auxiliary transmission unit box is held on the fuselage by a group of rods that are inclined relative to the axis of rotation of the main rotor shaft, and a beam, known in the industry as a “rudder beam”.

The operation of the main rotor represents the most significant source of vibration transmitted to the helicopter essentially due to the cyclic change in the aerodynamic load acting on each blade.

In fact, during each complete rotation of the main rotor shaft, each rotor blade is subjected, in a rotating frame of reference, to periodic loads at frequencies Ω and their multiples (2*Ω, 3*Ω, etc.), where Ω is the speed rotor shaft rotation. In turn, the loads to which the blade is subjected cause variable inertial and structural loads, which are also periodic.

More specifically, the aforementioned loads cause forces and moments that vary with time, which are transmitted to the fuselage through the rotor hub and support structure, degrading the comfort of passengers in the fuselage.

Vibration loads act on the hub and the main rotor shaft, both in the axial direction and orthogonally to the axis of rotation of the main rotor shaft.

It is known in the industry that axial vibration loads acting on a propeller blade are transmitted only to the hub, and therefore to the fuselage, with angular frequencies N*Ω and their relative multiples, where Ω is the speed of rotation of the rotor shaft, and N represents the number of propeller blades.

Conversely, for vibration loads orthogonal to the axis of rotation of the main rotor shaft in the rotation system, only harmonics with angular frequencies (N+1)*Ω, (N-1)*Ω and multiples of them are transmitted to the fixed system through the hub, but they are also received sleeve, and, therefore, are perceived on the fuselage with angular frequencies N*Ω and related multiples of 2*N*Ω, 3*N*Ω, etc.

It follows from the above that there is a clear need in the industry to limit the transmission from the main rotor shaft to the fuselage of vibrations with the above-mentioned angular frequency equal to the product of the rotation speed of the main rotor shaft and the number of propeller blades, i.e. N*Ω, which is by far the harmonic with the largest amplitude.

For this purpose, passive and active damping devices are known which are adapted to absorb the transmission of vibrations to the fuselage.

Active dampers are essentially actuators that exert a harmonic force on the rotor hub or shaft that counteracts the force that creates vibration.

An example of these active absorbing devices is illustrated in patent application EP-A-2857313, in the name of the same applicant.

This patent application describes the use of a pair of actuators operatively coupled to the main rotor shaft and controlled to create appropriate counter forces on the main rotor shaft having components in a plane orthogonal to the main rotor shaft's axis of rotation.

Additional examples of active absorbing devices are described in US-A-2016/0325828 and US-B-8,435,200.

In addition, US-A1-2013/0011260 describes a vibration reduction device comprising: an elastic body; dynamic load; and controlled cargo. The dynamic load is supported by a vibration reduction object through an elastic body. The actuator causes the controlled load to move relative to the dynamic load. Such a vibration reduction device can change the frequency and amplitude at which the dynamic weight vibrates relative to the vibration reduction object, causing the controlled weight to move relative to the dynamic weight accordingly, thereby allowing the vibration of the object to be more reliably reduced. In addition, such a vibration reduction device can reduce the vibration of a certain frequency of the vibration reduction object even when the controlled weight is fixed relative to the dynamic weight.

Active damping devices have the advantage that they are able to change their properties in accordance with changes in the vibration conditions of the rotor hub and shaft.

Passive draft devices mainly contain resonating systems equipped with weights suspended from the main rotor shaft or hub by an elastic system. The vibration of these suspended weights makes it possible to at least partially absorb the transmission of vibrations to the fuselage. These absorbers are usually tuned to the exact frequency of the vibrations to be absorbed by a suitable choice of the value of the suspended load and the stiffness of the elastic system.

In some types of helicopters, it is possible to selectively change the angular speed of rotation of the main rotor shaft between:

- the first nominal value, which is used in normal operating conditions of the helicopter; And

- a second value greater than the first value, which is used in the special operating conditions of the helicopter.

As an example, the second value is 102% of the first value.

This change in angular velocity shifts the angular frequency of the vibration loads transmitted to the fuselage, making passive resonant absorbers partially ineffective, which are only effective in a very normal frequency range centered on the resonant frequency.

This is because passive absorbers are typically set to the first nominal value of the main rotor shaft angular velocity, and therefore are not fully optimized to absorb the transmission of vibrations to the fuselage when the main rotor shaft rotates at the second angular velocity.

There is an industry awareness of the need to absorb the transmission of axial vibrations to the fuselage in an optimal way, both at the first angular velocity and at the second angular velocity of the main rotor shaft, as well as at additional angular velocities, while maintaining the design and implementation simplicity of passive absorbers.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a propeller for a hovering aircraft which can satisfy the above need in a simple and inexpensive manner.

The above object is achieved by the present invention, as far as it relates to a propeller for a hovering aircraft as defined in claim 1 of the claims.

Brief description of the drawings

For a better understanding of the present invention, a preferred embodiment is described below, solely by way of non-limiting example and with reference to the accompanying drawings, in which:

- Figure 1 is a front view of a helicopter containing a propeller according to an embodiment of the present invention, with parts not fully shown for clarity;

- Figure 2 is a front view, on an enlarged scale, of the transmission casing of the screw in Figure 1;

- Figure 3 shows, on an enlarged scale, the absorbing device of Figure 2; And

- Figures 4 and 5 show, on a further enlarged scale, the draft device of Figures 2 and 3 in the respective operating configurations used when the angular velocity of the main rotor shaft takes, respectively, the first and second values different from each other.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to Figure 1, reference numeral 1 indicates a hover-capable aircraft, in particular a helicopter, mainly comprising a fuselage 2, a main rotor 3 located at the top of the fuselage 2 and rotating about an axis A, and a tail rotor 4 located at the end of the fuselage 2 and rotating around its own axis transverse to axis A.

In more detail, the propeller 3 comprises a hollow hub 5 with an axis A carrying a group of cantilevered blades 9 which extend radially to the axis A.

Referring to Figure 2, propeller 3 also comprises a main rotor shaft 6 rotating about axis A with an angular velocity Ω relative to a frame of reference, integral with fuselage 2, angularly integral with hub 5, and connected in a manner not shown , with a power unit, such as a turbine, carried by helicopter 1.

The propeller 3 also includes a transmission unit 7 configured to transmit the driving force from the power unit to the main rotor shaft 6 .

In detail, the transmission unit 7 comprises, in particular, a stator 8 which is stationary with respect to the axis A.

The stator 8 is connected to the fuselage 2 in a known way, for example, by a steering beam and a group of rods inclined relative to the axis A.

In addition, the power unit is selectively controllable to drive the main rotor shaft 6, the hub 5, and the blades 9 around axis A:

- with an angular velocity, Ω, equal to the nominal value Ω1, under normal operating conditions of helicopter 1; or

- with an angular velocity Ω equal to the value Ω2 different from the nominal value Ω1, in the special operating conditions of helicopter 1.

In the case shown, the value of Ω2 is greater than the nominal value of Ω1.

More specifically, the angular velocity Ω2 is approximately 102% of the angular velocity Ω1.

The main rotor shaft 6 is partially housed inside the sleeve 5 and is angularly made in one piece with the sleeve 5 (Figure 2).

In particular, the rotor shaft 6 is hollow.

The propeller 3 also includes a vibration absorbing device 15 for absorbing the transmission of vibrations to the fuselage 2 parallel to the axis A. In other words, the absorbing device 15 absorbs axial vibrations on the rotor shaft 6 .

In the case shown, the absorbing device 15 is passive and tunable.

Draft device 15 is also coaxial with axis A.

Absorbing device 15 mainly contains (Figures 3-5):

- casing 20, continuing along the axis A;

- weight 21, made with the possibility of sliding inside the casing 20 parallel to the axis A; And

- elastic means 22 for supporting the load 21 in a sliding manner along the axis A inside the casing 20.

The elastic means 22 are placed between the casing 20 and the load 21 and have a total stiffness k parallel to the axis A.

More specifically, the casing 20 includes:

a pair of axial end walls 50 and 51 substantially orthogonal to axis A; And

a cylindrical wall 52 extending between walls 50 and 51.

The elastic means 22 contain a pair of springs 23 located on the respective mutually opposite axial sides of the load 21, between the load 21 and the casing 20.

In the case shown, the springs 23 are coil springs in a "parallel" configuration, i. they undergo the same deformation during oscillatory motion. It follows from this that each spring 23 has a stiffness equal to k/2.

One of the springs 23 is placed between the load 21 and the wall 50 of the casing 20, and the other spring 23 is placed between the load 21 and the wall 51 of the casing 20.

Preferably, absorbent device 15 also comprises:

- additional weight 25, also sliding inside the casing 20 parallel to the axis A;

- the connecting element 30, which makes the weights 21 and 25 in one piece movable along the axis A, when the angular velocity Ω of the rotor shaft 6 takes on the value Ω1 and

- an actuator 35 activated to separate the weights 21 and 25 relative to the translational movement along the axis A, when the angular velocity Ω of the main rotor shaft 6 takes on the value Ω2.

In the case shown, the casing 20 is connected to the stator 8.

Alternatively, the shroud 20 is housed within the rotor shaft 6, as shown by the dotted line in Figure 2. In this embodiment, the damper 15 also includes a slip ring configured to connect a power source located on board the fuselage 2 to an actuator 35.

When the angular velocity Ω of the rotor shaft 6 takes on the value Ω1, the draft gear 15 behaves like a tuned load absorber having a load equal to the sum of the weights 21 and 25 and a stiffness k along axis A. These load and stiffness values are determined in such a way that the absorber device 15 is tuned to an angular frequency equal to Ω1 and therefore absorbs the transmission of vibrations along axis A with this angular frequency Ω1 to the fuselage 2.

Conversely, when the angular velocity Ω of the rotor shaft 6 takes on the value Ω2, the draft gear 15 behaves like a tuned load absorber having a weight equal to the weight 21 and a stiffness k along the A axis. device 15 is tuned to an angular frequency Ω2 and therefore absorbs the transmission of vibrations along axis A with this angular frequency Ω2 to the fuselage 2.

In more detail, the connecting element 30 is magnetic. More specifically, weights 21 and 25 are made of a magnetizable material, preferably ferromagnetic.

The connecting element 30 is a permanent magnet attached to the load 21.

In the case shown, the connecting element 30 is configured to create a first magnetic field oriented parallel to axis A.

The connecting element 30 also contains a pair of discs 31 and 32 coaxial with respect to axis A and attached to the weight 21.

Disks 31 and 32 define respective mutually opposite poles of the permanent magnet.

The load 21 comprises (Figures 4 and 5) an axial end 26 located on the side near the wall 50, receiving the connecting element 30 in a fixed manner, and defining a shoulder 34, with the shape of a truncated cone in the case shown.

The load 25 contains an axial end 24 opposite the wall 50 and facing the axial end 26 of the load 21.

More specifically, the axial end 24 contains:

- arm 28 configured to abut against arm 34 when the angular velocity Ω of the rotor shaft 6 is equal to the value Ω1; And

- arm 29, which is axially opposite to arm 28.

In the case shown, the shoulder 28 is in the form of a truncated cone, and the shoulder 29 is in the form of a flat ring.

In the configuration shown in Figures 4 and 5, the connector 30 exerts an attractive force Fm on the weight 25 which is parallel to axis A and oriented in the same direction as the gravity force from wall 50 to wall 51 (i.e. downward in Figures 2-5).

Subsequently, when the angular velocity Ω of the rotor shaft 6 becomes Ω1, the load 25 is subjected to a weight force, a force Fm, and an internal force. The connector 30 and the weight 25 are sized such that the resultant of the aforementioned forces leaves the weight 25 held onto the weight 21.

Actuator 35 is selectively controllable to exert a force Fa on load 25 that is parallel to axis A and oriented in the opposite direction to the force of gravity, from wall 51 to wall 50 (ie, upwards in Figures 4 and 5).

Subsequently, when the angular velocity Ω of the rotor shaft 6 becomes Ω2, the load 25 is subjected to a weight force, a force Fm, an internal force, and an additional force Fa.

The actuating device 35 is sized to generate a force Fa of such a value that the resultant of the aforementioned forces exerted on the load 25 is directed from the wall 51 to the wall 50 and separates the load 25 from the load 21 in the axial direction.

In more detail, the actuator 35 is electromagnetic.

In more detail, the actuator 35 includes:

- a current generator, which is not shown;

- a group of toroidal electrical windings 36 (shown only schematically) having an annular shape around axis A; And

- a magnetic field generator 27 configured to create a second magnetic field oriented radially to axis A and integrally determined with the load 25.

In more detail, the toroidal electrical coils 36 are housed within a metal armature 49 of ferromagnetic material attached to the housing 20 and having the function of orienting the magnetic field generated by the generator 27.

Generator 27, on the other hand, is movable with respect to casing 20, parallel to axis A.

More specifically, the generator 27 is located on the axially opposite part of the load 25 relative to the end 28 of the load 25.

Activation of the electrical generator causes a current to circulate in the electrical windings 36. This electrical current circulates in an annular fashion about axis A and is immersed in a second magnetic field, radial to axis A, generated by the generator 27 and associated with the ferromagnetic material of the armature 49.

Due to the action of the Lorentz force and the fact that the electrical windings 36 are stationary relative to the axis A, the force Fa is generated on the load 25.

The draft device 15 also includes a sensor 39 (shown only schematically in Figures 4 and 5) configured to produce a signal associated with the angular velocity Ω of the main rotor shaft 6 and operatively connected to the actuator 35.

In the case shown, the sensor 39 is attached to the wall 50 of the casing 20.

The absorbing device 15 also includes a control unit 60 operatively connected to the sensor 39 and the actuator 35, as shown schematically in Figure 3.

The control unit 60, in more detail, is programmed to:

- deactivation of the actuator 35 when the sensor 39 detects that the angular velocity Ω of the rotor shaft 6 takes on the value Ω1 (Figure 4) in order to keep the weights 21 and 25 in one piece with each other; And

- activation of the actuator 35 when the sensor 39 detects that the angular velocity Ω of the rotor shaft 6 takes on the value Ω2 (Figure 5) in order to separate and separate the weights 21 and 25.

Absorbing device 15 also contains:

- a pair of ends of the moving elements 40 and 41, fixed relative to the casing 20 and located on the respective opposite sides of the weights 21 and 25, and configured to determine the respective mutually opposite end stops for the oscillatory movement of the weights 21 and 25; And

- a holding element 45 (Figure 3) placed between the casing 20 and the load 21, and designed to prevent the rotation of the load 21 around the axis A, following the twisting of the springs 23 around the axis A.

In particular, the end of the moving elements 40 and 41, respectively, is located on the side of the walls 50 and 51 relative to the load 21.

When the angular velocity Ω of the rotor shaft 6 is equal to the value Ω1, and the actuator 35 is thus deactivated, the end of the moving elements 40 and 41 limits the joint oscillation of the weights 21 and 25 parallel to the axis A (Figure 4).

When the angular velocity Ω of the rotor shaft 6 is equal to the value Ω2, and the actuator 35 is thus activated, the end of the moving element 40 abuts against the arm 29 of the load 25, stopping the movement from the load 21 in a direction parallel to axis A (Figure 5).

When used, the power unit rotates the sleeve 5, the main rotor shaft 6 and the blades 9 around the axis A through the transmission unit 7.

The operation of the helicopter 1 is described, starting from the condition in which the main rotor shaft 6 rotates around the axis A with an angular velocity Ω equal to the nominal value Ω1, with reference to a frame of reference made in one piece with the fuselage 2.

The rotation of the sleeve 5 and the blades 9 creates vibrations that are transmitted to the shaft 6 of the main rotor, and from there to the fuselage 2 of the helicopter 1.

These vibrations mainly have angular frequencies equal to N*Ω1 relative to the fixed fuselage system.

The transmission of these vibrations to the fuselage 2 is absorbed by the absorbing device 15.

In more detail, the sensor 39 detects the angular velocity Ω as equal to the value Ω1 of the rotor shaft 6, and the control unit 60 keeps the actuator 35 deactivated. Under these circumstances, no electric current circulates in the windings 36.

The weight 25 is held in contact with the weight 21 by the permanent magnet of the connector 30.

More specifically, weight 25 is subjected to a weight force, a force Fm exerted by the magnet of coupling 30, and an internal force due to its own oscillatory motion along axis A. These forces have a net force oriented so as to keep weights 21 and 25 in contact with each other. .

Under these conditions, the weights 21 and 25 and the generator 27 oscillate in one piece with each other along the axis A and are elastically supported by the springs 23 in their oscillatory movement along the axis A.

Subsequently, the draft gear 15 behaves like a tuned load absorber having a load equal to the sum of the weights 21 and 25 and a stiffness k along axis A. These load and stiffness values are determined such that the draft gear 15 is tuned to an angular frequency equal to Ω1. The absorber 15 thus effectively absorbs the transmission of vibrations along the axis A with this angular frequency Ω1 to the fuselage 2.

The oscillation of the weights 21 and 25 along axis A is limited by the end of the moving elements 40 and 41 on the side of the wall 50 and the wall 51, respectively.

Accidental rotation of weights 21 and 25 about axis A is prevented by holding element 45.

Under special flight conditions of the helicopter 1, the power unit rotates the main rotor shaft 6 around axis A with an angular velocity Ω equal to the value Ω2, with reference to the frame of reference constituting one with the fuselage 2.

Vibrations created by mechanical and aerodynamic loads associated with the rotation of the shaft 6 of the main rotor and the hub 5, mainly have angular frequencies equal to N*Ω2, relative to the fixed frame of the fuselage.

Under these conditions, the sensor 39 detects an angular velocity Ω equal to the value Ω2 of the main rotor shaft 6, and the control unit 60 activates the actuator 35 so as to cause the circulation of electric current in the windings 36 and thereby create a force Fa on the load 25.

More specifically, the electric current circulating in the windings 36 in an annular manner towards axis A interacts with the second magnetic field generated by the permanent magnet of the generator 27.

Since this magnetic field is oriented radially to the axis A, a Lorentz force equal to the force Fa between the windings 36 and the load 25 carrying the permanent magnet of the generator 27 is created parallel to the axis A.

Thus, the load 25 is subjected to a weight force, a force Fm exerted by the magnet of the connecting member 30, and a force Fa.

The resultant of these forces causes the load 25 to move from the load 21 parallel to the axis A until the condition (Figure 5) is reached, in which the shoulder 29 of the load 25 rests against the end of the moving element 40.

These forces have a resultant oriented from wall 51 to wall 50 in the direction opposite to the gravitational force. Subsequently, this resultant keeps the weights 21 and 25 separated from each other and keeps the weight 21 in contact with the actuator 35.

The draft gear 15 thus behaves like a tuned load absorber having a weight equal to the weight 21 and a stiffness k along axis A. These load and stiffness values are determined such that the draft gear 15 is tuned to an angular frequency equal to Ω2 and therefore absorbs the transmission of vibrations along the axis A with an angular frequency Ω2 to the fuselage 2.

The oscillation of the load 21 along the axis A is limited by the shoulder 28 of the load 25 on the side wall 50 and the end of the moving element 41 on the side of the wall 51.

The retaining element 45 ensures that the load 21 cannot rotate about axis A during oscillatory movement along axis A, for example, due to the accidental twisting of springs 23.

From the study of the properties of the screw 3 according to the present invention, the advantages that can be achieved with it are obvious.

In particular, the load 25 of the draft gear 15 is integrally movable with the load 21 in an oscillatory motion along the axis A, when the angular velocity Ω of the rotor shaft 6 is equal to the nominal value Ω1. Conversely, the weight 25 of the draft gear 15 is separated and disconnected from the weight 21 when the angular velocity Ω of the rotor shaft 6 is equal to a value Ω2 different from the nominal value Ω1.

Subsequently, the draft device 15 behaves like a tuned load absorber having a load equal to the sum of the weights 21 and 25 and a stiffness k along the axis A when the rotor shaft 6 and the blades 9 rotate at an angular velocity Ω1 equal to the nominal value. In this condition, the absorbing device 15 is tuned to the angular frequency Ω1, and therefore is able to effectively absorb the transmission of vibrations with the angular frequency Ω1 to the fuselage 2, improving passenger comfort.

Conversely, the draft gear 15 behaves like a tuned load absorber having a load equal only to the weight 21 and a stiffness k along the axis A when the rotor shaft 6 and the blades 9 rotate at an angular velocity Ω2 different from the nominal value. In this condition, the absorbing device 15 is tuned to the angular frequency Ω2 and therefore is able to effectively absorb the transmission of vibrations with the angular frequency Ω2 to the fuselage 2, improving passenger comfort even when the propeller 3 rotates at an angular velocity Ω2 larger than the nominal value.

The connecting member 30 effectively holds the weights 21 and 25 held together in relation to oscillation along axis A when the rotor shaft 6 and blades 9 rotate at an angular velocity Ω1 equal to the nominal value.

The actuator 35 is effective in applying a force Fa to the weight 25 to overcome the force Fm exerted by the coupler 30 when the sensor 39 detects that the main rotor shaft 6 and the blades 9 are rotating at an angular velocity Ω2 different from the nominal value.

The end of the moving elements 40 and 41 defines the respective mutually opposite stop surfaces for the oscillating movement of the weights 21 and 25 when the angular velocity Ω is equal to the value Ω1 and the actuating device 35 is deactivated.

The axial end of the moving member 40 holds the weight 25 and substantially prevents axial oscillation when the angular velocity Ω is equal to the value Ω1 and the actuator 35 is deactivated.

The retaining element 45 effectively prevents the rotation of the weight 21 about the axis A caused, for example, by the twisting of the springs 23.

Finally, it is clear that modifications and variations can be made with respect to the screw 3 described and illustrated herein without deviating from the scope defined by the claims.

In particular, the propeller 3 may also comprise a group of additional weights 25 selectively configured to be connected to the weight 21 when the angular velocity Ω of the main rotor shaft 6 is equal to additional appropriate values in order to adjust the draft gear 15 to the appropriate additional values.

Instead of being used in helicopter 1, propeller 3 can be used in a tiltrotor or rotary wing or combination helicopter.

In addition, the propeller 3 may include an additional vibration absorbing device 15 for absorbing the transmission of vibrations to the fuselage 2 in a plane orthogonal to axis A, i.e. associated with bending vibrations of the rotor shaft 6.

Finally, the propeller according to the present invention may be the tail rotor 4 of the helicopter 1 instead of the main rotor 3.

Claims (46)

1. Screw (3) for hovering aircraft (1), including a sleeve (5) rotating around an axis (A) and, in turn, including a group of blades (9); a main rotor shaft (6) configured to be connected to the power unit of said aircraft (1) and operatively connected to said bushing (5) to rotate said bushing (5) about said axis (A); And an absorbing device (15) for absorbing vibration transmission from said main rotor shaft (6) to said aircraft (1) parallel to said axis (A); moreover, said absorbing device (15), in turn, contains: casing (20); the first load (21), freely oscillating parallel to the specified axis (A) relative to the specified casing (20) and resiliently connected to the specified casing (20); characterized in that said absorbing device (15) is passive and additionally contains second load (25); connecting means (30) configured to move said first and second weights (21, 25) in one piece along said axis (A) when the angular velocity (Ω) of said main rotor shaft (6) takes on the first value (Ω1) ; And actuator means (35) activated to disconnect and separate said first and second weights (21, 25) relative to translational movement along said axis (A) when said angular velocity (Ω) of said main rotor shaft (6) takes on a second value ( Ω2) other than the specified first value (Ω1). 2. A screw according to claim 1, characterized in that said connecting means (30) are magnetic type connecting means and are configured to connect said first and second weights (21, 25) together with a first value of the first force (Fm). 3. Screw according to claim 2, characterized in that said connecting means (30) include a permanent magnet held on said first weight (21). 4. A screw according to any one of the preceding claims, characterized in that said actuator means (35) comprise an electromagnetic actuator that is controlled to exert a second value (Fa) of a second force on said second weight (25) when said angular velocity ( Ω) of the specified main rotor shaft (6) takes on the specified second value (Ω2). 5. Screw according to claim 4, characterized in that said first and second forces are opposite to each other; wherein said first and second values (Fm, Fa) are such as to support, in use, said second weight (25) integrally with said first weight (21) when said angular velocity (Ω) takes on said first value (Ω1) , and separate, in use, said second weight (25) from said first weight (21) when said angular velocity (Ω) takes on said second value (Ω2). 6. Screw according to claim 4 or 5, characterized in that said actuator (35) includes a group of electrical conductors (36) arranged in an annular manner around said axis (A) and within which an electric current can selectively circulate; And a generator (27) configured to generate a magnetic field radial to said axis (A), in particular a permanent magnet; wherein said second weight (25) defines said generator (27). 7. Screw according to any one of the preceding paragraphs, characterized in that said absorbing device (15) further includes a sensor (39) configured to generate a signal associated with the speed of rotation of said rotor shaft (6); moreover, the specified means (35) of the actuator is configured to be controlled based on the specified signal. 8. A screw according to any one of the preceding claims, characterized in that said absorbing device (15) additionally includes at least one spring (23) placed between said casing (21) and said first weight (21); And an anti-rotation device (45) placed between said spring (23) and said first weight (21) and configured to prevent rotation of said first weight (21) around said axis (A). 9. A screw according to any of the preceding claims, characterized in that it includes a first end (40) of the moving device and a second end (41) of the moving device, made with the ability to respectively limit the movement of the specified first load (21) and second load (25) for one with each other when the angular velocity (Ω) of said rotor shaft (6) takes, when used, said first value (Ω1); moreover, the specified first end of the moving device (40) is located against the specified second load (25), axially spaced apart from the specified first load (21), when the angular velocity (Ω) of the specified shaft (6) of the main rotor takes, when used, specified second value (Ω2). 10. A screw according to any one of the preceding claims, characterized in that said absorbing device (15) is angularly integral with said main rotor shaft (6). 11. A screw according to claim 10, characterized in that said main rotor shaft (6) is hollow and that said absorbing device (15) is housed inside said main rotor shaft (6). 12. Screw according to any one of paragraphs. 1-9, characterized in that said absorbing device (15) is angularly stationary with respect to said axis (A). 13. A screw according to claim 12, characterized in that it includes a stator (8) supporting said main rotor shaft (6) in a rotating manner around said axis (A); wherein said absorbing device (15) is attached to said stator (8). 14. A screw according to any one of the preceding paragraphs, characterized in that said first and second weights (21, 25) include respective axial ends (26, 28) facing each other and matching in shape; wherein said axial end (26) of said first weight (21) additionally accommodates said connecting means (30). 15. A screw according to any one of the preceding claims, characterized in that said casing (20) includes a first end wall (50) and a second end wall (51), which are substantially orthogonal to said axis (A), and a cylindrical wall (52), continuing between said first end wall (50) and said second end wall (51); moreover, the specified first weight (21) contains the first axial end (26), located on the side next to the specified first end wall (50), receiving the specified connecting means (30) in a fixed way, and defining the shoulder (34); wherein said second weight (25) comprises a second axial end (24) opposite said first end wall (50) and facing the first axial end (26) of said first weight (21). 16. Screw according to paragraphs. 8-15, characterized in that said absorbing device (15) additionally includes the first end of the moving device (40) and the second end of the moving device (41), configured to respectively limit the movement of the specified first weight (21) and the specified second weight (25) in one piece with each other, when the angular velocity (Ω) of the specified shaft (6) the main rotor assumes, when used, the specified first value (Ω1); moreover, the specified first end of the moving device (40) is located against the specified second load (25), axially spaced apart from the specified first load (21), when the angular velocity (Ω) of the specified shaft (6) of the main rotor takes, when used, specified second value (Ω2); moreover, the specified first and second ends of the moving devices (40, 41) are fixed relative to the specified casing (20) and are located on the respective opposite sides of the specified first and second weights (21, 25); wherein said anti-rotation device (45) is additionally configured to prevent rotation of said second weight (25) around said axis (A). 17. Capable of hovering aircraft (1), characterized in that it includes fuselage (2); And screw (3) according to any of the previous paragraphs; moreover, the specified fuselage (2) is connected directly or indirectly with the specified casing (20) of the specified absorbing device (15).

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