METHOD

PRIORITY CLAIM

Thi s application claims priority from European Patent Application No . 16207538.6 filed on December 30, 2016, the disclosure of which is incorporated by reference .

TECHNICAL FIELD

The present invention relates to a rotor for an aircraft capable of hovering, in particular a helicopter or a convertiplane . The present invention also relates to a method of operating a rotor for an aircraft capable of hovering, in particular a helicopter or a convertiplane .

BACKGROUND ART

Known hel icopters comprise a fuselage , a main rotor upwardly protruding from the fuselage and a tail rotor which is arranged at a tail of the fuselage . Furthermore, known hel icopters comprise a turbine , a main transmission group which transmits the motion from the turbine to the main rotor, and. an addi tional transmission group which transmits the motion from the main transmission group to the tail rotor .

Main rotor and tail rotor comprise, each , :

- a stationary case ;

- a mast which is driven in rotation by the main or the additional transmission group about its own axis;

- a hub driven in rotation by the mast ; and

- a plurality of blades which are articulated with respect to the hub .

A need is felt in the art to provide the rotating components of the main and tail rotor , i.e. the mast , the hub and the blades , with electric power . For example , the electric power can be used for activating a de-icing or anti-icing system formed by a plurality of electric conductors embedded in the blades and adapted to heat the relative blades by Joule effect or for activating some movable surfaces on the blades . In order to provide the rotating components of the main rotor with the requi red electric power, known helicopters normally comprise an electric generator operated by a sha ft electrically connected to the main transmiss ion group and a si ip-ring . The slip-ring transmits the electric power by creating a rubbing contact from the stationary conductors electrically connected to the generator to the rotating conductors of the main or tail rotor . Even if well performing, the previously described solutions lea e room for improvements . As a matter of fact, the si ip-ring is complex to manufacture and. maintain, and is easily subj ected to wear effect . This drawback is exacerbated especially in anti-torque tail rotors , which rotate at higher speed than main rotor . A need is therefore felt within the industry to transmit the electric power to the rotating parts of the main or tai 1 rotor, whi le eliminating the aforesaid drawbacks in a straightforward, low-cost manner . Still more precisely, a need is felt to monitor the operative status of the acces sory components for reasons of safety, while avoiding the previously identi fied drawback . For example, a need is felt to monitor whether or not they are electrically fed, the level of electric energy with which they are fed, the presence of short- circuits . US 2016/32977, US 2014/248168; US-B-S, 851, 415; and US-A-2012 /229428 disclose known rotor solutions .

DISCLOSURE OF INVENTION

11 is an obj ect of the present invention to provide a rotor for an aircraft capable of hovering, which meets at least one of the above requirements .

The aforementioned obj ect is achieved by the present invention as it relates to a rotor for an aircraft capable of hovering , as claimed in claim 1.

The invention also relates to a method for operating a rotor for an aircraft capable of hovering, as claimed in claim 14.

BRIEF DESCRIPTION OF THE DRAWINGS

Five preferred embodiments are hereinafter disclosed for a better understanding of the present invention, by way of non- limitative example and with reference to the accompanying drawings , in which :

Figure 1 is a schematic view of a helicopter comprising a main and a tail rotor in accordance to the present invention ;

- Figure 2 is a transversal sect ion of the main rotor of Figure 1 in accordance to a first embodiment of the present invention;

- Figure 3 is a schematic view of electric circuits of the rotor of Figure 2;

- Figure 4 is a transversal section of the tail rotor of Figure 2 in accordance to a second embodiment of the present invention ;

Figure 5 is a schematic view of an electric circuit of the rotor of Figure 4;

- Figure 6 is schematic view of an electric circuit of a tail rotor in accordance to a thi rd embodiment of the present invention ;

- Figure 7 is a schematic view an electric, circuit of a tail rotor in accordance to a fourth embodiment of the present invention ;

- Figure 8 is a schemat ic view of an additional electrical circuit of the rotor of Figures 1 to 7 ;

- Figure 9 is a graphical plot of some electric quantit ies of the electric circuits of the rotor of Figures 1 to 7 with respect to time ; and

- Figures 10 and 11 show, in enlarged view, some components of further electric circuits of the rotors of Figures 2 and 8, with parts removed for clarity .

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to Figure 1 , numeral 1 indicates an aircraft capable of hovering, in particular a hel icopter .

Helicopter 1 essentially comprises ( Figure 1 ) a fuselage 2 with a nose 5 ; a main rotor 3 fitted to the top of fuselage 2 and rotatable about an axis A; and an anti-torque tail rotor 4 fitted to a fin proj ecting from fuselage 2 at the oppos ite end to nose 5 and rotatable about an axis B transversal to axis A .

More specifically, main rotor 3 provides helicopter 1 with the lift to raise it , and the thrust to move it forward, whi le rotor 4 exerts force on the fin to generate a straightening torque on fuselage 2. The straightening torque balances the torque exerted on fuselage 2 by main rotor 3 , and which would otherwise rotate fuselage 2 about axis A .

Helicopter 1 also comprises :

- a pair of turbines 6 (only one of which is shown) ;

- a main transmission group 7 , which transmits the motion from turbine 6 ; and

- an additional transmission group 8, which transmits the motion from main transmission group 7 to tail rotor 4.

With reference to Figure 2, rotor 3 substantially comprises :

- a stator 10, which is fixed to fuselage 2;

- a mast 11 , which is rotatable about an axis A with respect to stator 10;

- a hub 12 , which is rotationally integral to mast 11; and

- a plural ity of blades 13 (only two of which are shown in Figure 2), which are articulated onto hub 12.

In the embodiment shown , stator 10 is stationary with respect to axis A . Furthermore , stator 10 , mast 11 and hub 12 are hoi low . Rotor 3 also comprises an epicyclic gear train 17 , which transmits the motion from an end shaft (not shown) of main transmission group 7 to mast 11 and hub 12.

In detai 1 , epicyclic gear train 17 is coaxial to axi s A and comprises :

- a sun gear 20 which is driven in rotation about axis A by the end shaft of main transmiss ion group 7 and comprises a radially outer toothing ;

- a plurality of planetary gears 21 (only two shown in Figure 2 ) which comprise, each , a radially inner toothing meshing with sun gear 20 and a radial ly outer toothing meshing with a radially inner toothing defined by stator 10; and

- a carrier 22 which is rotationally integral with and connected to planetary gears 21 and to mast 11.

In particular, stator 10 acts as a stationary crown 23 of epicyclic gear train 17.

Planetary gears 21 rotate about relative axes E parallel to axis A and revolve about axis A.

Advantageously, rotor 3 comprises (Figures 2 and

3) :

- a support element 36, which supports a source 30 of magnetic field B _s and is driven in rotation about axi s A with a rotational speed ωΐ ; and

an electric circuit 32 , which is operatively connected to mast 11 and is driven in rotation at a rotational speed ω2 different from first rotational speed ωΐ ; electric circuit 32 is electromagnetically coupled with said source 30 , so that an electromot ive force emf _R is magnetically induced, in use, in electric circuit 32 and an electric current i _R flow in electric circuit 32 ;

an electric circuit 65 , which is arranged on stator 10; and

a sensor 58 , which is adapted to detect a quantity associated to the back electromotive force bemfc induced on support element 36 and associated to current i _R flowing on electric ci rcuit 32.

I n this way, source 30 and electric ci rcuit 32 form an electric generator , which induces electromotive force emf in mast 11 and, therefore, in hub 12 and blades 13 , due to the differential rotational speed ω2-ω1.

In the embodiment shown , source 30 comprises a plurality of permanent magnets 81 , which are fitted to support element 36 and angularly spaced with respect to axis A. Electromotive force emf _R generates an electric current i _R in electric circuit 32. Current i _R generates a magnetic field Br which, in turn , induces , by Faraday' s law back electromotive force bemf _c on stator 10. In the embodiment shown, electric generator is an axial flux machine , in which the magnetic field generated by source 30 is mainly di rected parallel to axi s A.

Electric circuit 32 compri ses (Figure 3 ) a plurality of branches 60 ( three in the embodiment shown) , which extend partially inside respective blades 13 and are electrical ly connected to one another in a common knot 61. Each branch 60 comprises a resistive load 62 , which is arranged on relative blade 13. The electric current flowing in branches 60 is indicated in Figure 3 as i _R . Loads 62 can be determined by an electric circuit embedded inside blades 13 themselves and fed with electric current i _H . This electric circuit operates as an anti-icing system or a de-icing system . In another embodiment , loads 62 can be determined by respective actuators , which are fed with electric current i _R .

With reference to Figures 10 and 11 , electric circuit 32 comprises, for each branch 60, a plurality, two in the embodiment shown , of windings 68 and respective switches 69. Windings 68 and switches 69 of each branch 60 are interposed between knot 61 and relative load 62. Each branch 60 of electric circuit 32 also comprise, in one embodiment shown in Figure 11 , a return portion 66 , which extends from relative load 62 and knot 61 and is arranged on the opposite side of load 62 with respect to windings 68. Branch 60 also comprises a switch 65 arranged on relative portion 66. Alternatively, electric circuit 32 comprises only a return portion 71 common to all branches 60 , which electrically connects all loads 62 to knot 61 and along which switch 66 is interposed (Figure 10 ) .

Thus , a certain degree of redundancy is ensured, in case of fault of one of windings 68 or short-circuits of some of windings 68. Switches 65, 69 and portion 71 are not shown in Figures 3 and 5 to 7.

Each branch 60 has an equivalent resistance R _R and inductance L _R which in Figure 3 is modelled with a resistor and inductor respectively . Furthermore, in figure 3 , the electromotive forces emf _k acting on respective branches 60 are model led with an alternate voltage generator . Each branch 60 also compri ses a portion 87 (Figure 2 ) which extends between hub 12 and relative blade 13. Source 30 comprises a plurality of angularly spaced permanent magnets 81 (only one of which is shown in Figure 2), which generate a magnet ic field Bss parallel to axis A.

Electric circuit 65 is, in the embodiment shown, an open coi 1 67 , which is electromagnetical ly coupled with electric circuit 32. Electric circuit 65 has an equivalent electric resistance R _c and an inductance L _c , which in Figure 3 are model led with a resistor and inductor respectively . Furthermore , in figure 3 , back electromotive force bemf _c acting on electric circuit 65 is model led with an alternate voltage generator . Sensor 58 is a voltage sensor and is configured to detect voltage V _c across electric circuit 65 and to generate a signal associated to voltage V _c .

Rotor 3 further comprises , an electric ci rcuit 80 (only schematically shown in Figure 8), which receives in input the signal generated by sensor 58 and outputs peak value V _Cmax of voltage V _c and, therefore, of back electro-motive force befm _c . Peak value V _C max of voltage V _c is associated to peak value of current i _R flowing inside loads 62. In particular, on the basis of peak value V _Cmax of voltage V _c and, therefore, of the peak value of current i _H , it is poss ible to recognize the fol lowing operative configurations of electric circuit 32 :

- peak value of current i _R is null; this condition corresponds to the fact that loads 62 are not electrically fed and, e.g., the anti-icing system is not operative;

peak value of current i _R is lower than the maximum value; this condition corresponds to the fault of one or some of windings 68 ; and

- peak value current i _R is higher than the maximum value; this condition corresponds to the short-circuit of windings 68 electrically connected to same load 62.

Rotor 3 further comprises a control unit 200 ( Figure 8), which receives peak value V _Cmax of voltage V _c from electrical circuit 80 and is configured to accordingly control s switches 65, 69 or generate a warning signal . For example , in case of short-circuit of windings 68 connected to same load 62, control unit 200 is programmed to set relative switch 65 in the open position . In case of fault of one windings 68 connected to a load 62, control unit 200 is programmed to set relative switch 68 in the open position . In case of fault of load 62 , control unit 200 generates a warning a1arm for the pilot .

Rotor 3 further comprises :

- a shaft 35, which is elongated paral lei to axis A and is fixed to stator 10;

- support element 36; and - an epicycl ic gear train 37.

Support element 36 comprises :

- a shaft 38 elongated about axis A;

- a pai r of disks 39 protruding from shaft 38 orthogonal ly to axis A.

Disks 39 comprise respective faces 40 which face with one another along axis A and to which permanent magnets 81 are fitted .

Epicyclic gear train 37 substantially comprises :

- a radially outer toothing defined by an axia11y end gear 45 of shaft 38 , which is arranged on the axial side of stator 10 ;

- a plurality of planetary gears 46, which extend about respective axes F parallel to and staggered from axi s A and have, each, a radially outer toothing with respect to relative axis F meshing with radial ly outer toothing of end gear 45 ;

- a carrier 47 , which is rotational ly integral and connected, to planetary gears 46 on one axial side and is connected to shaft 35 on the other axial side; and

an annular ring 48 , which connected to and rotational ly integral with hub 12 and which comprises a radially inner toothing with respect to axis A meshing with radially outer toothing of planetary gears 46.

Planetary gears 46 rotate about respective axes F revolute about axis A .

Hub 12 comprises :

- a main tubular body 51; and

- a pair of rings 52 , 53 which extend from body 51 towards axis A and orthogonally to axis A .

Ring 52 bounds hub 12 on the axial side of stator 10. Ring 53 is axially interposed between rings 52 , 48. Furthermore, ring 48 extends from body 51 towards axis A and orthogonally to axis A. Ring 53 supports electric circuit 32. Ring 52 is axially interposed between disks 39 of support element 26. Rings 52 , 53 surround shaft 38 and shaft 35 respectively with the interposition of a radial gap .

11 is therefore possible to identi fy three assembl ies ins ide rotor 3 , which have relat ive rotational speed about axis A :

- stator 10 and shaft 35 , which are stationary about axis A;

- support element 36 and source 30 , which rotate with rotational speed ωΐ in a first direction about axis A; and

- mast 1 i , hub 12 with electric circuit 32 , which rotate with rotat ional speed ω2 in a second direction, opposite to first direction, about axi s A .

Rotor 3 also comprises a hollow flow deflector 85 which is connected to an axial end of hub 12 and is rotational ly integral with hub 12. Flow deflector 85 bounds rotor 3 on the opposite axial side of stator 10. Flow deflector 85 houses one disk 39 , a top axial end of sha ft 38 opposite to shaft 46 , and disk 52. Furthermore, flow deflector 85 houses an electronic control unit 86 for controlling permanent magnets 81 and electric circuit 32. Preferably, flow deflector 85 is provided with an electric power storage device 89, which is charged by the electric current flowing inside electric conductive element 32. Flow deflector 85 is made of metal and comprises a plurality of thermally conductive rings 90 which are connected to ring 53 .

Rotor 3 further comprises , with respect to axis A, : a bearing 100, which is radially interposed between shaft 38 and hub 12 , with respect to axis A; and

- a pair of axially spaced bearings 101 which are radial ly interposed between shaft 35 and mast 11 and hub 12 , with respect to axis A.

In use , the end shaft of main transmiss ion group 7 drives in rotation sun gear 20 of epicycl ic gear train 17 about axis A . Accordingly, also planetary gear 21 and carrier 22 rotate about axis A, thus driving in rotation mast 11 , hub 12 and blades 13 about same axis A . Blades 13 are driven in rotation by hub 12 about axis A and can move with respect to hub 12 in a known manner . Hub 12 , ring 48 and therefore electric conductive element 32 rotate about axis with rotat ional speed ω2 about axis A . In the meanwhile, epicyclic gear train 37 receives the motion from ring 48 rotating with rotational speed ω2 about axis A and drives in rotation support element 36, therefore, source 30 and permanent magnets 81 with a rotational speed, ωΐ about axis A . In particular, ring 48 integral with hub 12 meshes with planetary gears 46 stationary about axis A, and planetary gears 46 mesh with gear 45 rotationally integral with support element 36 and source 30. As a result , source 30 rotates with a rotational speed ωΐ, electric circuit 32 rotates with a rotational speed ω2 di fferent from rotational speed ωΐ , and source 30 and conductive element 32 face with one another along axis A . Thus , magnetic field B _s is generated by source 30 and electromotive force emf _K is magnetically induced, by means of Faraday' s law, in branches 60 rotating integrally with hub 12. Electromotive forces emf _K cause the flowing of electric currents i _R in branches 60 and loads 62. In particular, when switches 65, 69 are closed, electrical current i _R flow inside relative windings 68 and electrically feed loads 62. Current i _R is used for several purposes . For example , it can be used for feeding electric ci rcuits ins ide blades 13 and providing de-icing or anti -icing function . Alternatively or in combination, the electromotive force can be used for operating the actuators fitted to blades 13. Being currents i _R variable in the time , they generate a magnetic field B _R , which is variable in time . Time-variable magnetic field B _R induces , by means of Faraday' s law, a back electromotive force bemfc on electric ci rcuit 65.

Sensor 58 senses voltage V _c across electric circuit 65. Being voltage V _c generated by an alternate current, the signal generated by sensor 58 has a characteristic profile (an example of which is shown in Figure 9) , which is modulated by electric current iR . In particular , the amplitude and the frequency of current i _R and voltage V _c depend with rotational speed ω2 of mast 11 and hub 12. For a given load 62 and rotational speed ω2, the signal is proportional to current i _R and periodic with a frequency depending on rotational speed ω2 of mast 11 and hub 12. Thus , the s ignal contain useful information on the operation of loads 62. Preferably, electric ci rcui t 80 receives in input the signal generated by sensor 58 and outputs peak value Vcmax of voltage V _c and, therefore, of back electromoti e force befm _c . Peak value V _Cmax of voltage V _c is proportional to peak value of current i _R flowing ins ide loads 62. In particular, on the basi s of peak of value Vcmax of voltage V _c and, therefore , of current i _K , it is possible to recognize the foi lowing operative configuration of electric circuit 32 and of loads 62:

- peak value of current i _R is null ; this condition corresponds to the fact that loads 62 are not electrically fed and, e.g., the anti-icing system is not operative; and

peak value of current i _R is lower than the maximum value; this condition corresponds to the fault of one or some of windings 68 ; and

- peak value current i _K is higher than the maximum value; this condition corresponds to the short-circuit of some of windings 68.

With reference to Figures 4 and 5 , reference number 4 indicates , as a whole, an anti-torque tail rotor according to a second embodiment of the present invention .

Rotor 4 is similar to rotor 3 and will be described hereinafter only as far as it di ffers therefrom; corresponding or equivalent parts of rotors 4 , 3 will be indicated where possible by the same reference numbers .

In particular, rotor 4 substantial ly comprises (Figure 4 ) :

- a hollow housing 150 which is fitted to fuselage

2;

- a hollow mast 151, which rotatabie about an axis B transversal to axis A with a rotation speed ω2 and is connected to an end shaft 157 of additional transmission group 8 by means of a bevel gear 152 arranged at an axial end of mast 151;

- a hub 153 which is rotational ly integral with and connected to mast 151 ; and

- a plurality of blades 154 (only two of which are shown in Figure 4) , which are articulated onto hub 153 and extend along respective longitudinal axes D .

Bevel gear 152 and mast 151 are contained ins ide housing 150. Hub 153 and blades 154 extend outs ide housing 150.

Rotor 4 also comprises a control rod 155, which extends along axis B and is slidable along axis B with respect to mast 151. Control rod 155 comprises an axial end on the opposite side of gear 152 which is fixed to lever 156. Lever 156 extends t ransversally to axis B and is connected to blades 154 eccentrically to relative axes D . In this way, the movement of rod 155 along axis B causes the rotation of blades 154 along relative axes D and the adj ustment of relative pitch angles .

In greater detai 1 , hous ing 150 comprises :

- a main body 160 elongated along axis B; and

a pair of annular disks 161, which radially protrude from body 160 on the opposite side of axis B and lie on relative planes orthogonal to axis B.

Disks 161 comprise relative surface 162, which axially face with one another and to which respective permanent magnets 164 are fitted. Permanent magnets 164 generate magnetic field Bs parallel to axis B.

Hub 153 comprises :

- a body 165, which is connected to mast 151, is arranged in front of an axial end of housing 150 on the axial opposite side with respect to be el gear 152 ; and

- a body 166, which is connected to body 165 and. surrounds an open axial end of housing 150 opposite to bevel gear 152.

In greater detail , blades 154 are articulated onto body 165. Body 166 comprises , proceeding along axis B from body 165 towards bevel gear 152: - an annular ring 167 connected to body 165;

- an annular ring 168 onto which an electric circuit 169 is fixed; and

an annular ring 170 onto which an electronic control unit 171 for controlling electric circuit 169 is fitted.

Ring 168 is axially interposed between disks 161.

Electric circuit 169 is axially interposed between permanent magnets 164.

In this way, electric circuit 169 rotating at rotational speed ω2 is magnetically coupled with source 163 of magnetic field Bs rotating at rotational speed ω1=0 , i.e. stationary about axis B. Accordingly, permanent magnets 164 and electric circuit 169 form an electric generator, which induces by Faraday' s law an electromotive force emf _R in mast 151 and, therefore , in hub 152 and blades 154 , due to the di fferential rotational speed ω2-ω1=ω2. Electric circuit 169 is electrically connected to blades 154 by means of electric wires 172. In this way, electric current i _R is available to blades 154.

Finally, rotor 4 comprises , with respect to axis

B, :

- a plurality of bearings 180 radially interposed between control rod 155 and mast 152;

- bearings 181 radial ly interposed between mast 152 and a radially inner surface of housing 150; and

bearings 183 radially interposed between a radial ly outer surface of housing 150 and relative disks 167, 170.

The operat ion of rotor 4 is similar to rotor 3 and is described only insofar as it di ffers from that of rotor 3.

In particular, end shaft 157 of additional transmission group 8 drives in rotation bevel gear 152 about axis B at rotational speed ω2. Accordingly, also hub 152 and blades 154 and electric circuit 169 are driven in rotation about axis B with rotational speed 0)2. Blades 154 are driven in rotation by hub 152 about axis B and can move with respect to hub 152 in a known manner . Furthermore, the pitch angles with respect to relative axes D of blades 154 can be adj usted by sliding movement of control rod 155 along axis B.

Source 163 and permanent magnets 164 are fitted to housing 150 and is stationary about axis B, i.e. source 163 and permanent magnets 164 can be seen as rotating with a rotational speed ω1=0 about axis B . Permanent magnets 164 generate magnetic field B _s . Thanks to the di fferent rotational speed between electric circuit 169 and source 163, electromotive force emf _R is magnetical ly induced, by means of Faraday' s law, in electric circuit 169 rotating integral ly with hub 152.

With reference to Figure 5 , being electric currents i _R variable in the time, they generate a magnetic field B _K , which is variable in time .

ime-variable magnetic field B _R induces , by means of Faraday' s law, a back electro-motive force bemf<■ on electric circuit 65.

Sensor 58 senses vol tage V _c across electric circuit

65.

With reference to Figure 6, 4 ' indicates , as a whole, a tail rotor according to a thi rd embodiment of the present invention .

Rotor 4 ' is similar to rotor 4 and will be described hereinafter only as far as it differs therefrom; corresponding or equivalent parts of rotors 4 , 4 ' will be indicated where possible by the same reference numbers .

In particular, rotor 4 ' di ffers for rotor 4 in that source 163 comprises , instead of spaced permanent magnets 164 , an electric circuit 70 which generates magnetic field B _s di rected along axis B. Electric circuit 70 is electromagnetically coupled with electric circuit 169. Electric circuit 70 compri ses a voltage generator 71 generating a voltage V _s and has an equivalent electric resistance R _s and an inductance L _s , which in Figure 6 are model led with a resistor and inductor respectively. Voltage generator 71 causes the flow of an electric current i _s inside electric circuit 70. Electric current i _s generates , in turn , magnetic field B _s . Preferably, vol tage generator 71 is a direct voltage generator . Furthermore, in figure 6 , back electromotive force bemf _s acting on electric circuit 70 is model led with an alternate vol tage generator . Sensor 58 is a voltage sensor and is configured to detect voltage V _c across electric circuit 70.

The operation of rotor 4 ' is similar to rotor 4 and is described only insofar as it di ffers from that of rotor 4. In particular, the operation of rotor 4 ' di ffers from rotor 4 in that magnetic field B _s is generated by electric circuit 70 of housing 150 and in that back electromotor force bemf _s is induced by magnetic field Br on electric circuit 70.

With reference to Figure 7 , 4 ' ' indicates , as a whole, a tail rotor according to a fourth embodiment of the present invention . Rotor 4 ' ' is simi lar to rotor 4 and will be described hereinafter only as far as it differs therefrom; corresponding or equivalent parts of rotors 4 , 4 ' ' will be indicated where possible by the same reference numbers . In particular, rotor 4 ' ' differs for rotor 4 ' for comprising, in addition to electric circuit 70, an electric circuit 90. Electric circuit 90 is elect romagnetically coupled with electric circuit 169. Electric circuit 90 is , in the embodiment shown, an open coi 1 170, which is electromagnetical ly coupled with electric circuit 169. Electric circuit 90 has an equivalent electric resistance R _c and an inductance L _c , which in Figure 7 are modelled with a resistor and inductor respecti ely . Furthermore , in figure 6 , back electromotive force Bemf _c act ing on coil 170 is modelled with an alternate voltage generator . Sensor 58 is a voltage sensor and is configured to detect the voltage V _c across electric circuit 90.

The operation of rotor 4 ' ' is similar to rotor 4 and is described only insofar as it di ffers from that of rotor 4. In particular, the operation of rotor 4 ' ' di ffers from the one of rotor 4 in that back electromotor force bemf _c is induced by magnetic field B _R on electric circuit 90.

The advantages of rotor 3 , 4 , 4 ' , 4 ' ' and the method according to the present invention will be clear from the foregoing description .

In particular, rotor 3 , 4 , 4 ' , 4 ' ' comprises sensor 58 , which generates a signal associated to the back electromotive force bemf _c , bemf _;; induced on electric circuit 65 , 70 , 90 by variable current i _R flowing inside electric circuit 32.

This signal allows to recognize some features of the temporal variation of i _R , which are associated to several operative configurations of loads 62 and windings 68.

In particular, :

- in case electrical current i _R is null , loads 62 are not electrically fed and, e.g., the anti-icing system is not operative;

- in case electrical current i _R is lower than the maximum value, there is a fault in one or some of windings 68 ; and

- in case peak value of electrical current i _R is higher than the maximum value, there is a short-circuit of windings 68 electrically connected to same load 62.

I t is therefore possible recognize potentially dangerous for rotor 3 , 4, 4 ' , 4 ' ' , as for example the fact that the anti-icing system is not operative or the presence of short-circuits of windings 68 which could lead to an excess in the torque acting on mast 11 and hub 12.

Furthermore ( Figures 10 and. 11), switches 69 interposed between each load 62 and relative windings 68 allow to exclude one of windings 68 in case of not proper operation thereof .

Switches 65 interposed between knot 61 and relative load 62 allow to exclude both relative windings 68 in case of short-circuit thereof . Thus , the signal generated by sensor 58 provides highly relevant information on the operative status of loads 62 and therefore of corresponding accessory components , without requiring any physical connection between support element 36 or housing 150 and hub 11. This informat ion can be provided inside fuselage 2. As regards to rotor 3 shown in Figures 3 and 4, electric circuit 65 is fitted to stator 10. Thus , even if permanent magnets 81 are angularly integral with support element 36 rotating at rotational speed ωΐ , the signal provided by sensor 58 is avai lable at stator 10 and, therefore, at fuselage 2 of helicopter 1. Accordingly, rotor 3 allows to increase the di fferential rotational speed ω2-ω1 between electrical conductive element 32 and source 30 of magnetic field, with a reduced axial size . Thus , support element 32 and hub 12 , can be made smaller and more weight efficient , because the higher the di fferential rotational speed ω2-ω1, the lower is the torque requi red for a given value of the electromotive force induced in electric conductive element 32. The less the torque , the smal ler the diameter and, therefore, the weight of support element 32 and hub 12 , with evident advantages on the payload . Thi s is particularly advantageous , when rotational speed co2 of hub 12 is necessarily slow as in main rotor 3.

Furthermore, the electric generator formed by source 30, 163 and electric circuit 32 , 169 as well as electric circuit 65 , 70, 90 and sensor 58 has a contained axial size and can be, therefore, easily integrated in the customary size of rotor 3 , 4 , 4 ' , 4 ' ' , without requiring any re-designing thereof .

For the same reasons, the electric generator formed by source 30 , 163 and electric conductive element 32 , 169 can be easily retrofitted inside an already existing rotor 3, 4 , 4' , 4 ' ' .

Clearly, changes may be made to rotor 3 , 4 , 4 ' , 4 ' ' and the method according to the present invent ion without , however, depart ing from the scope as defined in the accompanying Claims .

In particular , the electric generator formed by source 30, 163 and electric circuit 32 , 169 could be a radial flux machine, in which the magnetic field generated by source 30, 163 is mainly directed radially axis A, B.

Furthermore , electric circuit 65 of rotor 3 could be arranged on support element 36 and rotate with rotational speed ωΐ about axis A.

Source 30 of magnetic field B _s in rotor 3 could be, instead of permanent magnets 81, an electric circuit similar to electrical circuit 70 of rotor 4 ' and arranged on support element 36.

Rotor 3 , 4, 4 ' , 4 ' ' could comprise, instead of mechanical main and additionally transmission group 7, 8 , an electrical motor for driving rotor 3 , 4 , 4 ' , 4 ' ' . In this case, rotor 3 , 4 ' , 4 ' ' , 4 ' ' ' would comprise a stator to which source 30, 163 would be fitted and a rotor to which electrical circuit 32 , 169 would be fitted.

Aircraft 1 could be a convertiplane instead of a hel icopter .

Ai rcraft 1 could comprise rotor 3 , 4 ' , 4 ' ' , 4 ' ' ' in accordance with the invention and a conventional anti- torque tail rotor, or a conventional main rotor and anti-torque tail rotor 4 in accordance with the invention .