METHOD FOR ESTIMATING THE ROTOR TORQUES OF AN AIRCRAFT CAPABLE OF HOVERING AND CONTROL UNIT FOR AN AIRCRAFT CAPABLE OF HOVERING

Abstract

A method for estimating rotor torques of an aircraft capable of hovering and comprising a plurality of rotors, which are rotatable under the action of respective rotor torques; and an engine, which is operatively connected to the rotors to provide them with an engine torque. Each rotor comprises a hub and a plurality of blades articulated on the respective hub in such a way that respective collective pitch angles are adjustable. The method comprises the steps of i) calculating a symmetric component on the basis of the engine torque; ii) receiving a signal associated with collective pitch angles; iii) calculating an asymmetric component on the basis of a pitch angle difference between the collective pitch angles; and iv) calculating each rotor torque as the algebraic sum of the symmetric component and the respective asymmetric component.

Claims

1. A method for estimating rotor torques of an aircraft capable of hovering; said aircraft comprising: a plurality of rotors, which are operatively connected to each other and rotatable relative to respective rotational axes under the action of respective rotor torques; and at least one first engine, which is operatively connected to said rotors and is adapted to provide said rotors with an engine torque; said rotors each comprising a hub and a plurality of blades articulated on said respective hub in such a way that respective collective pitch angles of said plurality of blades relative to the respective rotational axis are adjustable; said method comprising the steps of: i) calculating a symmetric component of said rotor torques on the basis of said engine torque; said symmetric component being equal to said rotor torques, when said collective pitch angles are equal to each other; characterized in that it comprises the further steps of: ii) receiving a signal associated with said collective pitch angles; iii) calculating an asymmetric component of each said rotor torques on the basis of at least a pitch angle difference between said collective pitch angles; and iv) calculating each said rotor torque as the algebraic sum of said symmetric component and said respective asymmetric component.

2. The method according to claim 1, wherein said rotors are associated with respective pitch angle differences; characterized in that said step iii) of calculating said asymmetric component of each said rotor torques comprises the steps of: v) determining a parameter associated with a variation of each said rotor torque with respect to a variation of said collective pitch angle of the same rotor; and vi) multiplying each said parameter by said respective pitch angle difference of the respective rotor.

3. The method according to claim 2, characterized in that said step v) comprises the step vii) of determining said parameters on the basis of at least two independent variables associated with the flight conditions of said aircraft.

4. The method according to claim 3, characterized in that said independent variables include: an angle corresponding to an orientation of said rotational axes of said rotors with respect to a reference system fixed to said aircraft; the airspeed of said aircraft; and/or said symmetric component calculated at said step i).

5. The method according to claim 4, characterized in that said step vii) comprises the further steps of: viii) determining each said parameter on the basis of said airspeed, if the angle of the respective rotor is lower than or equal to a threshold value; and ix) determining each said parameter on the basis of said symmetric component, if the angle of the respective rotor is greater than said threshold value.

6. The method according to claim 1, wherein said aircraft further comprises a second engine, which is operatively connected to said rotors and is adapted to provide said rotors with a further engine torque; characterized in that said step i) comprises the steps of: x) calculating a total torque produced by said first engine and second engine as the sum of said engine torque and said further engine torque; xi) subtracting from said total torque a first subtrahend term corresponding to the transmission losses due to the transmission of said engine torque and said further engine torque from said first engine and said second engine to said rotors; and/or subtracting from said total torque a second subtrahend term corresponding to loads imparted by said first engine and/or said second engine to accessories of said aircraft; and xii) dividing said sum obtained after said step xi) by the number of rotors of said aircraft.

7. The method according to claim 2, characterized in that the pitch angle difference of each said rotor is a difference between said respective collective pitch angle of the same rotor and a symmetric collective pitch angle; said symmetric collective pitch angle being the sum of collective pitch angles of all rotors divided by the number of said rotors.

8. Control unit for an aircraft capable of hovering; said control unit being programmed to: calculate a symmetric component of rotor torques of rotors of said aircraft on the basis of an engine torque provided to said rotors by at least one first engine of said aircraft; said symmetric component being equal to said rotor torques, when collective pitch angles of respective rotors are, in use, equal to each other; said control unit being characterized in that it is further programmed to: receive a signal associated with said collective pitch angles; calculate an asymmetric component of each said rotor torque on the basis of at least a pitch angle difference of the respective rotor; and calculate each said rotor torque as the algebraic sum of said symmetric component and said respective asymmetric component.

9. Control unit according to claim 8, characterized in that it is configured to: determine a parameter associated with a variation of each said rotor torque with respect to a variation of said collective pitch angle of the same rotor; multiply each said parameter by said respective pitch angle difference.

10. Control unit according to claim 9, characterized in that it is configured to calculate each said parameter on the basis of at least two independent variables associated with the flight conditions of said aircraft.

11. Control unit according to claim 10, characterized in that said independent variables include: an angle corresponding to an orientation of said rotational axes of each said rotors with respect to a reference system of said aircraft; airspeed of said aircraft; and/or said symmetric component.

12. Control unit according to claim 11, characterized in that it is configured to: calculate each said parameter on the basis of said airspeed, if said respective angle is lower than a threshold value; and calculate each said parameter on the basis of said symmetric component, if said respective angle is equal to or greater than said threshold value.

13. Control unit according to claim 12, characterized in that it is configured to: calculate a total torque produced by said first engine and at least one second engine as the sum of said engine torque and a further engine torque of said second engine; subtract from said total torque a first subtrahend term corresponding to the transmission losses due to the transmission of said engine torques from said first and second engines to said rotors; and/or subtract from said total torque a second subtrahend term corresponding to loads imparted by said first and/or second engines to accessories of said aircraft; and divide said difference obtained after the subtraction of said first and/or said second subtrahend term from said total torque by the number of rotors of said aircraft.

14. Control unit according to claim 9, characterized in that it comprises a computational unit, a memory, and at least one interface unit electrically and operatively connectable to sensor means of said aircraft; said memory comprising, in turn, a database storing data correlating said parameter with a plurality of variables associated with the flight conditions of said aircraft.

15. Aircraft capable of hovering, comprising: a plurality of rotors; said rotors being rotatable relative to respective rotational axes under the action of respective rotor torques; said rotors being operatively connected to each other; said rotors comprising each a hub and a plurality of blades articulated on said respective hub in such a way that respective collective pitch angles of said plurality of blades relative to the respective rotational axis are adjustable; at least one first engine, which is operatively connected to said rotors and is adapted to provide said rotors with an engine torque; first sensor means configured to measure said collective pitch angles; and second sensor means configured to measure said engine torque generated, in use, by said first engine; characterized in that it comprises a control unit according to claim 8, which is operatively connected to said first and second sensor means.

16. Aircraft according to claim 15, characterized in that it comprises: third sensor means configured to measure the angles corresponding to an orientation of said rotational axes of said rotors with respect to a reference system of said aircraft and operatively connected to said control unit; and/or fourth sensor means configured to measure an airspeed of said aircraft and operatively connected to said control unit; and/or characterized in that it is a convertiplane or a helicopter; and/or characterized by comprising a second engine operatively connected to said rotors and adapted to provide said rotors with a further engine torque.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Three embodiments are described below for a better understanding of the present invention, provided by way of non-limiting example with reference to the accompanying drawings, wherein:

[0028] FIG. 1 shows a top view of a first embodiment of an aircraft capable of hovering, in particular a convertiplane, comprising two rotors;

[0029] FIG. 2 is a block diagram related to a method according to the present invention for estimating the torques of the aircraft of FIG. 1;

[0030] FIG. 3 is a detail of the block diagram of FIG. 2;

[0031] FIG. 4 is a schematic representation of some components of the aircraft of FIG. 1, with parts removed for clarity;

[0032] FIG. 5 shows a top view of a second embodiment of an aircraft capable of hovering comprising four rotors;

[0033] FIG. 6 is a block diagram related to the method according to the present invention for estimating the torques of the aircraft of FIG. 5;

[0034] FIG. 7 is a schematic representation of some components of the aircraft of FIG. 5, with parts removed for clarity; and

[0035] FIG. 8 is a block diagram related to the method according to the present invention for estimating the torques of a third embodiment of an aircraft capable of hovering comprising any number of rotors.

DESCRIPTION OF EMBODIMENTS

[0036] With reference to FIG. 1, number 1 denotes an aircraft capable of hovering, in the shown case a convertiplane.

[0037] It should be noted that the terms front, longitudinal, lateral, above and below and the like used in this description refer to a normal direction of advancement of convertiplane 1.

[0038] The convertiplane 1 essentially comprises: [0039] a fuselage 2 having an axis A of longitudinal extension; [0040] a pair of half-wings 3 extending cantilevered from respective parts opposite one another of fuselage 2 and transversely to axis A; [0041] a pair of nacelles 11, 12 housing at least partially respective engines 21, 22 and attached to respective half-wings 3; [0042] a pair of rotors 31, 32 operatively connected with the respective engines 21, 22; and [0043] a control unit 5.

[0044] In detail, control unit 5 comprises in a known manner a computational unit, a memory, and one or more interface units for the electrical and operational connection to sensor means 40, 41, 42, 43 of aircraft 1, which will be described in detail in the following (FIG. 4).

[0045] Each rotor 31, 32 essentially comprises a hub 7 and a plurality of blades 8.

[0046] Convertiplane 1 further comprises two transmission units 6, which are each operatively connected to a respective engine 21, 22 and the relative hub 7, to drive it in rotation. In particular, rotors 31, 32 are rotatable about respective axes B with respective angular speeds 1, 2. Preferably, angular speeds 1, 2 are constant over time.

[0047] Furthermore, blades 8 are articulated on the respective hub 7 in such a way that respective angles of attack 1.sub.COLL, 2.sub.COLL are collectively adjustable relative to respective axes B.

[0048] In the industry, angles of attack 1.sub.COLL, 2.sub.COLL are commonly known as collective pitch angles and will therefore be referred to that way in the following.

[0049] Rotors 31, 32 are controllable independently of each other. In particular, the collective pitch angles 1.sub.COLL and 2.sub.COLL may be different from each other.

[0050] More specifically, as the collective pitch angles 1.sub.COLL, 2.sub.COLL of one rotor 31, 32 increases, the thrust exerted by the rotor 31, 32 parallel to axis B and the drag torque acting on the rotor 31, 32 correspondingly increases. As a result, angular speeds 1, 2 remain substantially constant.

[0051] Furthermore, rotors 31, 32 are identical to each other. In detail, rotors 31 and 32 have the same power required for the same input. This means, in further detail, that for a given collective input and a given boundary condition, the torque required to rotate rotors 31 and 32 is the same or substantially the same, for all inputs and operating conditions.

[0052] Convertiplane 1 further comprises sensor means 40 configured to measure collective pitch angles 1.sub.COLL, 2.sub.COLL of respective rotors 31, 32. Sensor means 40 are configured to generate a signal associated with collective pitch angles 1.sub.COLL, 2.sub.COLL and are operatively connected to control unit 5 (FIG. 4).

[0053] Furthermore, rotors 31, 32 are tiltable with respect to respective axes C relative to half-wings 3. In particular, axes C are transverse to axis A and axes B (FIG. 1).

[0054] It is important to note that axes A, B and C are fixed with respect to convertiplane 1.

[0055] Therefore, convertiplane 1 can be selectively arranged: [0056] in a helicopter configuration (not shown), wherein axes B of rotors 31, 32 are orthogonal to axis A and parallel to axes C; and [0057] in an airplane configuration (shown in FIG. 1), wherein axes B of rotors 31, 32 are parallel to axis A and orthogonal to axes C.

[0058] In detail, it is possible to define angles 1, 2 corresponding respectively to the orientation of rotational axes B with respect to axis A.

[0059] Convertiplane 1 further comprises sensor means 41 configured to measure angles 1, 2 of respective rotational axes B of rotors 31, 32 with respect to axis A. In detail, sensor means 41 are configured to generate a signal associated with angles 1, 2 and are operatively connected to control unit 5 (FIG. 4).

[0060] In the embodiment shown, rotors 31, 32 are tiltable with respect to respective axes C integrally with respective nacelles 11, 12. Therefore, sensor means 41 are also configured to detect nacelle angles of nacelles 11, 12 corresponding to the orientation of nacelles 11, 12 with respect to axis A.

[0061] Each engine 21, 22 is adapted to generate a respective engine torque TQ.sub.eng1, TQ.sub.eng2, which is transmitted at least in part to rotors 31, 32 by transmission units 6. In addition, each rotor 31, 32 is rotatable relative to the respective rotational axis B under the action of respective rotor torques TQ.sub.mast1, TQ.sub.mast2, which are correlated to engine torques TQ.sub.eng1 and/or TQ.sub.eng2.

[0062] In particular, in the event of an increase in engine torques TQ.sub.eng1, TQ.sub.eng2, collective pitch angles 1.sub.COLL, 2.sub.COLL are increased in such a way that angular speeds 1, 2 are kept constant due to the corresponding increase in the drag torques acting on rotors 31, 32.

[0063] Furthermore, rotors 31, 32 are operatively connected to each other. In particular, convertiplane 1 comprises an interconnection shaft 4, which is operatively connected to rotors 31 and 32 (FIG. 1).

[0064] Interconnection shaft 4 is adapted to allow the rotation of rotors 31, 32 in case of failure of one of engines 21, 22.

[0065] In a known manner, interconnection shaft 4 is adapted to: [0066] allow at least part of engine torques TQ.sub.eng1, TQ.sub.eng2 to be transmitted between transmission units 6 of rotors 31 and 32; and [0067] allow rotors 31, 32 to tilt with respect to the respective axis C.

[0068] Each rotor torque TQ.sub.mast1, TQ.sub.mast2 is in general different from engine torques TQ.sub.eng1, TQ.sub.eng2 for several reasons, which will be described in the following.

[0069] In particular, at least part of engine torques TQ.sub.eng1, TQ.sub.eng2 transmitted by transmission unit 6 is dissipated because of mechanical losses occurring at transmission unit 6.

[0070] In addition, convertiplane 1 may comprise a plurality of not-shown accessories, which perform various functions on board the convertiplane and require a certain amount of power to be operated.

[0071] The accessories are at least indirectly operatively connected to engines 21 and/or 22 to receive the power necessary for their operation. In particular, engines 21 and/or 22 are adapted to provide the accessories with a torque TQ.sub.acc for powering them.

[0072] Furthermore, convertiplane 1 comprises (FIG. 4): [0073] sensor means 42 configured to measure engine torques TQ.sub.eng1, TQ.sub.eng2 generated by respective engines 21, 22; and [0074] sensor means 43 configured to measure the airspeed v of convertiplane 1.

[0075] Sensor means 42 and 43 are operatively connected to control unit 5 and are adapted to generate respective signals associated with the measured values of engine torques TQ.sub.eng1, TQ.sub.eng2 and airspeed v of convertiplane 1.

[0076] Control unit 5 is configured to calculate a first component TQ.sub.mastSYM of rotor torques TQ.sub.mast1, TQ.sub.mast2 on the basis of engine torques TQ.sub.eng1, TQ.sub.eng2. In detail, first component TQ.sub.mastSYM is equal to rotor torques TQ.sub.mast1, TQ.sub.mast2 when collective pitch angles 1.sub.COLL, 2.sub.COLL are equal to each other.

[0077] For the purpose of calculating first component TQ.sub.mastSYM, control unit 5 is configured to (FIG. 2): [0078] receive from sensor means 42 the signal associated with the measured values of engine torques TQ.sub.eng1, TQ.sub.eng2; [0079] calculating a total engine torque TQ.sub.engTOT generated by engines 21 and 22 as the sum of the engine torques TQ.sub.eng1, TQ.sub.eng2 (block 50); [0080] subtracting from the total engine torque TQ.sub.engTOT a term corresponding to the transmission losses due to the transmission of engine torques TQ.sub.eng1, TQ.sub.eng2 (or part thereof) from engines 21, 22 to rotors 31, 32 (block 51) through transmission unit 6; and/or [0081] subtracting from the total engine torque TQ.sub.engTOT the torque TQ.sub.acc for powering the accessories of convertiplane 1, where present (block 52).

[0082] In particular, control unit 5 may be configured to subtract the term corresponding to the transmission losses from the total engine torque TQ.sub.engTOT by multiplying total engine torque TQ.sub.engTOT by a mechanical loss coefficient .sub.XMNS (.sub.XMNS<1).

[0083] The difference between total engine torque TQ.sub.engTOT and the transmission losses and/or torque TQ.sub.acc is the total available engine torque TQ.sub.engavail.

[0084] Control unit 5 is further configured to calculate first component TQ.sub.mastSYM by dividing the total available engine torque TQ.sub.engavail by two, which is the number of rotors 31, 32 of convertiplane 1 (block 53 in FIG. 2).

[0085] Advantageously, control unit 5 is configured to: [0086] receive the signal associated with collective pitch angles 1.sub.COLL, 2.sub.COLL of rotors 31 and 32 from sensor means 40; and [0087] calculate a second component TQ.sub.mastASYM1, TQ.sub.mastASYM2 of respective rotor torques TQ.sub.mast1, TQ.sub.mast2 on the basis of at least a respective pitch angle difference .sub.COLL1, .sub.COLL2 between collective pitch angles 1.sub.COLL and 2.sub.COLL (block 54 of FIG. 2).

[0088] In particular, second components TQ.sub.mastASYM1, TQ.sub.mastASYM2 respectively represent how much the rotor torque TQ.sub.mast1, TQ.sub.mast2 of each rotor 31, 32 deviates from the rotor torque that would act on each rotor 31, 32 if total available engine torque TQ.sub.engavail were equally apportioned between rotors 31 and 32. Accordingly, first component TQ.sub.mastSYM and second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, may be referred respectively to as a symmetric component and an asymmetric components of rotor torques TQ.sub.mast1, TQ.sub.mast2.

[0089] The following are exemplary situations in which convertiplane 1 is in a flight condition in which total available engine torque TQ.sub.engavail is not equally apportioned between rotors 31 and 32: [0090] hovering during vertical take-off and landing or during conversion of convertiplane 1 between the airplane and the helicopter configurations with steady cross wind; [0091] nacelle angle equal to 75 with steady heading sideslip relative to a direction fixed with respect to the ground; [0092] nacelle angle equal to 50 with steady heading sideslip relative to the direction fixed with respect to the ground; [0093] convertiplane 1 arranged in the airplane configuration with steady heading sideslip relative to the direction fixed with respect to the ground; [0094] bank angle different than zero and nacelle angle equal to 50 to maintain null sideslip.

[0095] Control unit 5 is further configured to calculate each rotor torque TQ.sub.mast1, TQ.sub.mast2 as the sum of first component TQ.sub.mastSYM and the respective second component TQ.sub.mastASYM1, TQ.sub.mastASYM2 (TQ.sub.mast1=TQ.sub.mastSYM+TQ.sub.mastASYM1; TQ.sub.mast2=TQ.sub.mastSYM+TQ.sub.mastASYM2) (block 55 in FIG. 2).

[0096] More specifically, for the purpose of calculating second components TQ.sub.mastASYM1, TQ.sub.mastASYM2 (block 54), control unit 5 is configured to (FIG. 2): [0097] determine parameters TQ.sub.1, TQ.sub.2, which are respectively associated with the variation of rotor torques TQ.sub.mast1, TQ.sub.mast2 as a result of a variation of collective pitch angle 1.sub.COLL, 2.sub.COLL of the same rotor 31, 32 (block 56); and [0098] multiply each parameter TQ.sub.1, TQ.sub.2 by respective pitch angle difference .sub.COLL1, .sub.COLL2 (block 57).

[0099] Pitch angle differences .sub.COLL1, .sub.COLL2 of rotors 31, 32 are respective differences between each collective pitch angle 1.sub.COLL, 2.sub.COLL and a symmetric collective pitch angle 0 (.sub.COLL1=1.sub.COLL0; .sub.COLL2=2.sub.COLL0). In detail, symmetric collective pitch angle 0 is calculated as the sum of collective pitch angles 1.sub.COLL, 2.sub.COLL divided by two, which is the number of rotors 31, 32 of aircraft 1 (0=(1.sub.COLL+2.sub.COLL)/2).In detail, each parameter TQ.sub.1, TQ.sub.2 may be expressed as a partial derivative of the relative rotor torque TQ.sub.mast1, TQ.sub.mast2 with respect to the collective pitch angle COLL (TQ.sub.1=TQ.sub.mast1/.sub.COLL; TQ.sub.2=TQ.sub.mast2/.sub.COLL). Parameters TQ.sub.1, TQ.sub.2 are variable as a function of several variables associated with the flight conditions of convertiplane 1.

[0100] In particular, the memory of control unit 5 comprises a database 9, in which data correlating parameters TQ.sub.1, TQ.sub.2 with a plurality of variables associated with the flight conditions of convertiplane 1 are stored. In detail, the data stored in database 9 may be arranged in tables and/or graphs, which are preferably multiple input tables and/or graphs (FIG. 4).

[0101] Control unit 5 is configured to: [0102] receive a plurality of signals associated with the flight conditions of convertiplane 1; [0103] access database 9 using the signals associated with the flight conditions as inputs; and [0104] determine parameters TQ.sub.1 , TQ.sub.2 on the basis of the inputs and on the basis of the data correlating parameters TQ.sub.1, TQ.sub.2 and the variables associated with the flight conditions of convertiplane 1 stored in database 9.

[0105] More specifically, the data stored in database 9 are calculated or experimentally measured in a known manner by means of statistical methods or by test on the basis of one or more sets of the following variables of convertiplane 1: [0106] airspeed; [0107] torque settings; [0108] nacelle angle; [0109] rotor speed; [0110] altitude; [0111] ambient temperature; [0112] number of operative engines 21, 22.

[0113] With reference to FIG. 3, control unit 5 is configured to calculate parameters TQ.sub.1, TQ.sub.2 on the basis of at least two independent variables associated with the performance of rotor 31 and/or 32.

[0114] In the embodiment shown in FIG. 3, each parameter TQ.sub.1, TQ.sub.2 is calculated on the basis of: [0115] the angle 1, 2 of the respective rotor 31, 32 measured by sensor means 41; and [0116] airspeed v measured by sensor means 43; and/or [0117] first component TQ.sub.mastSYM.

[0118] In detail, database 9 comprises data correlating parameters TQ.sub.1, TQ.sub.2 with respective angle 1, 2, airspeed v and first component TQ.sub.mastSYM.

[0119] In further detail, control unit 5 is configured to (FIG. 3): [0120] calculate parameter TQ.sub.1, TQ.sub.2 on the basis of airspeed v, if respective angle 1, 2 is lower than or equal to a threshold value th (block 58); and [0121] calculate parameter TQ.sub.1, TQ.sub.2 on the basis of first component TQ.sub.mastSYM, if respective angle 1, 2 is greater than threshold value th (block 59).

[0122] In particular, if the collective pitch angle 1.sub.COLL, 2.sub.COLL is the same for the two rotors 31, 32, the second components TQ.sub.mastASYM1, TQ.sub.mastASYM2 are both null and the rotor torques TQ.sub.mast1, TQ.sub.mast2 are equal to each other and to the first component TQ.sub.mastSYM (TQ.sub.mast1=TQ.sub.mast2=TQ.sub.mastSYM).

[0123] In addition, since rotors 31, 32 are identical to each other and have the same power required for the same input, if angles 1 and 2 are equal to each other, parameters TQ.sub.1, TQ.sub.2 are also identical. In other words, if angles 1 and 2 are equal to each other, the variation of rotor torque TQ.sub.mast1 as a result of a variation of collective pitch angle 1.sub.COLL is identical to the variation of rotor torque TQ.sub.mast2 as a result of a variation of collective pitch angle 2.sub.COLL.

[0124] In use, control unit 5 calculates an estimate of rotor torques TQ.sub.mast1, TQ.sub.mast2 with a sum of first component TQ.sub.mastSYM and respective second components TQ.sub.mastASYM1, TQ.sub.mastASYM2.

[0125] The following is a description of the steps required to calculate first component TQ.sub.mastSYM.

[0126] In detail, sensor means 42 periodically measure engine torques TQ.sub.eng1, TQ.sub.eng2 and send the associated signal to control unit 5 (FIG. 4). Subsequently, control unit 5 calculates total engine torque TQ.sub.engTOT as the sum of the engine torques TQ.sub.eng1, TQ.sub.eng2 measured by sensor means 42 (block 50 in FIG. 2).

[0127] Control unit 5 then calculates total available engine torque TQ.sub.engavail by subtracting from total engine torque TQ.sub.engTOT the transmission losses due to the transmission of engine torques TQ.sub.eng1, TQ.sub.eng2 through transmission unit 6 (block 51 in FIG. 2) and/or the torque TQ.sub.acc for powering the accessories of convertiplane 1, where present (block 52 in FIG. 2).

[0128] Control unit 5 then calculates first component TQ.sub.mastSYM by dividing available engine torque TQ.sub.engavail by the number of rotors 31, 32 of convertiplane 1 (block 53 in FIG. 2).

[0129] The following is a description of the steps required to calculate second components TQ.sub.mastASYM1, TQ.sub.mastASYM2.

[0130] Periodically, sensor means 40 measure collective pitch angles 1.sub.COLL, 2.sub.COLL of rotors 31 and 32, sensor means 41 measure angles 1, 2 and sensor means 43 measure airspeed v.

[0131] Control unit 5 calculates the pitch angle differences .sub.COLL1, .sub.COLL2 between collective pitch angles 1.sub.COLL and 2.sub.COLL. In addition, control unit 5 accesses database 9 using angle 1, 2, airspeed v and/or first component TQ.sub.mastSYM as inputs and determines parameters TQ.sub.1, TQ.sub.2 on the basis of the data stored in database 9.

[0132] In particular, if angle 1 is lower than or equal to threshold value th, control unit 5 calculates parameter TQ.sub.1 on the basis of angle al and airspeed v (block 58 in FIG. 3); if angle 1 is equal to or greater than threshold value th, control unit 5 calculates parameter TQ.sub.1 on the basis of angle 1 and first component TQ.sub.mastSYM (block 59 in FIG. 3). Similarly to the foregoing, if angle 2 is lower than or equal to threshold value th, control unit 5 calculates parameter TQ.sub.2 on the basis of angle 2 and airspeed v; if angle 2 is equal to or greater than threshold value th, control unit 5 calculates parameter TQ.sub.2 on the basis of angle 2 and first component TQ.sub.mastSYM.

[0133] Once parameters TQ.sub.1, TQ.sub.2 are determined, control unit 5 calculates each second component TQ.sub.mastASYM1, TQ.sub.mastASYM2 by multiplying respective parameter TQ.sub.1, TQ.sub.2 by the respective pitch angle difference .sub.COLL1, .sub.COLL2 (block 57 in FIG. 2). Control unit 5 then calculates each rotor torque TQ.sub.mast1, TQ.sub.mast2 as the sum of first component TQ.sub.mastSYM and the respective second component TQ.sub.mastASYM1, TQ.sub.mastASYM2 (block 55 in FIG. 2).

[0134] With reference to FIG. 5, 1 denotes an aircraft capable of hovering according to a second embodiment of the present invention. Aircraft 1 is similar to aircraft 1 and will be described hereinafter only insofar as it differs from the latter; equal or equivalent parts of aircrafts 1, 1 will be marked, where possible, by the same reference numerals.

[0135] Aircraft 1 differs from aircraft 1 in that it comprises: [0136] two pairs of half-wings 3, 3; the half-wings of each pair of half-wings 3, 3 extending cantilevered from respective parts opposite one another of fuselage 2 and transversely to axis A; the two pairs of half-wings 3, 3 being distanced from each other along axis A; [0137] four rotors 31, 32, 33, 34 operatively connected with respective engines 21, 22, 23, 24 (only schematically shown in FIG. 7).

[0138] Each rotor 31, 32, 33, 34 essentially comprises a hub 7 and a plurality of blades 8. In addition, rotors 31, 32, 33, 34 are rotatable about respective axes B with respective angular speeds 1, 2, 3 and 4, which are preferably constant over time.

[0139] Furthermore, blades 8 are articulated on the respective hub 7 in such a way that respective angles of attack 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL are collectively adjustable relative to respective axes B.

[0140] Rotors 31, 32, 33, 34 are controllable independently of one another. In particular, the collective pitch angles 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL may be different from one another.

[0141] Furthermore, rotors 31, 32, 33, 34 are identical to one another. In detail, rotors 31, 32, 33, 34 have the same power required for the same input.

[0142] In addition, rotors 31, 32 are tiltable with respect to respective axes C (not-shown) relative to half-wings 3 and rotors 33, 34 are tiltable with respect to respective axes C (not-shown) relative to half-wings 3. In detail, aircraft l is a quadcopter.

[0143] Aircraft 1 further comprises (FIG. 7): [0144] sensor means 40 configured to measure collective pitch angles 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL; [0145] sensor means 41 configured to measure angles 1, 2, 3, 4 of respective rotational axes B of rotors 31, 32, 33, 34 with respect to axis A.

[0146] Each engine 21, 22, 23, 24 is adapted to generate a respective engine torque TQ.sub.eng1, TQ.sub.eng2, TQ.sub.eng3, TQ.sub.eng4, which is transmitted at least in part to rotors 31, 32, 33, 34 by respective transmission units 6. In addition, each rotor 31, 32, 33, 34 is rotatable relative to the respective rotational axis B under the action of respective rotor torques TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4, which are correlated to engine torques TQ.sub.eng1 and/or TQ.sub.eng2 and/or TQ.sub.eng3 and/or TQ.sub.eng4.

[0147] Furthermore, rotors 31, 32, 33, 34 are operatively connected to one another. In particular, convertiplane 1 comprises an interconnection mechanism 4, which is operatively connected to rotors 31, 32, 33, 34 (FIG. 5).

[0148] Interconnection mechanism 4 is adapted to allow the rotation of rotors 31, 32, 33, 34 in case of failure of one of engines 21, 22, 23, 24.

[0149] In detail, each engine 21, 22, 23, 24 is coaxial to the respective rotor 31, 32, 33, 34 and interconnection mechanism 4 comprises (FIG. 5): [0150] an interconnection shaft 4a, which operatively connects engine 21 to engine 22; [0151] an interconnection shaft 4b, which operatively connects engine 23 to engine 24; and [0152] an interconnection shaft 4c, which operatively connects interconnection shaft 4a to interconnection shaft 4b.

[0153] Aircraft 1 further comprises (FIG. 7): [0154] sensor means 42 configured to measure engine torques TQ.sub.eng1, TQ.sub.eng2, TQ.sub.eng3, TQ.sub.eng4 generated by respective engines 21, 22, 23, 24; and [0155] sensor means 43 configured to measure the airspeed v of aircraft 1.

[0156] Control unit 5 is configured to calculate a first component TQ.sub.mastSYM of rotor torques TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4 on the basis of engine torques TQ.sub.eng1, TQ.sub.eng2, TQ.sub.eng3, TQ.sub.eng4. In detail, first component TQ.sub.mastSYM is equal to rotor torques TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4 when collective pitch angles 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL are equal to one another.

[0157] For the purpose of calculating first component TQ.sub.mastSYM, control unit 5 is configured to (FIG. 6): [0158] receive from sensor means 42 the signal associated with the measured values of engine torques TQ.sub.eng1, TQ.sub.eng2, TQ.sub.eng3, TQ.sub.eng4; [0159] calculating a total engine torque TQ.sub.engTOT generated by engines 21, 22, 23 and 24 as the sum of the engine torques TQ.sub.eng1, TQ.sub.eng2, TQ.sub.eng3, TQ.sub.eng4 (block 50); [0160] subtracting from the total engine torque TQ.sub.engTOT a term corresponding to the transmission losses due to the transmission of engine torques TQ.sub.eng1, TQ.sub.eng2, TQ.sub.eng3, TQ.sub.eng4 (or part thereof) from engines 21, 22, 23, 24 to rotors 31, 32, 33, 34 (block 51) through transmission units 6; and/or [0161] subtracting from the total engine torque TQ.sub.engTOT the torque TQ.sub.acc for powering the accessories of aircraft 1, where present (block 52).

[0162] The difference between total engine torque TQ.sub.engTOT and the transmission losses and/or torque TQ.sub.acc is the total available engine torque TQ.sub.engavail.

[0163] Control unit 5 is further configured to calculate first component TQ.sub.mastSYM by dividing the total available engine torque TQ.sub.engavail by four, which is the number of rotors 31, 32, 33, 34 of aircraft 1 (block 53 in FIG. 6).

[0164] Advantageously, control unit 5 is configured to: [0165] receive the signal associated with collective pitch angles 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL of rotors 31, 32, 33, 34 from sensor means 40; and [0166] calculate a second component TQ.sub.mastASYM1, TQ.sub.mastASYM2, TQ.sub.mastASYM3, TQ.sub.mastASYM4 of each rotor torque TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4 on the basis of at least pitch angle differences .sub.COLL1, .sub.COLL2, .sub.COLL3, .sub.COLL4 between collective pitch angles 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL (block 54 of FIG. 6).

[0167] In particular, second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, TQ.sub.mastASYM3, TQ.sub.mastASYM4 represent how much the rotor torque TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4 of each rotor 31, 32, 33, 34 deviates from the rotor torque that would act on each rotor 31, 32, 33, 34 if total available engine torque TQ.sub.engavail were equally apportioned between rotors 31, 32, 33, 34.

[0168] Control unit 5 is further configured to calculate rotor torques TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4 as an algebraic sum of first component TQ.sub.mastSYM (which is common to all rotors 31, 32, 33, 34) and the respective second component TQ.sub.mastASYM1, TQ.sub.mastASYM2, TQ.sub.mastASYM3, TQ.sub.mastASYM4 (TQ.sub.mast1=TQ.sub.mastSYM+TQ.sub.mastASYM1; TQ.sub.mast2=TQ.sub.mastSYM+TQ.sub.mastASYM2; TQ.sub.mast3=TQ.sub.mastSYM+TQ.sub.mastASYM3; TQ.sub.mast4=TQ.sub.mastSYM+TQ.sub.mastASYM4;) (block 55 in FIG. 6).

[0169] In particular, if the collective pitch angle 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL is the same for the four rotors, the second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, TQ.sub.mastASYM3, TQ.sub.mastASYM4 are null and the rotor torques TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4 are equal to one another and to the first component TQ.sub.mastSYM (TQ.sub.mast1=TQ.sub.mast2=TQ.sub.mast3=TQ.sub.mast4=TQ.sub.mastSYM).

[0170] More specifically, for the purpose of calculating the second component TQ.sub.mastASYM1, TQ.sub.mastASYM2, TQ.sub.mastASYM3, TQ.sub.mastASYM4 of each rotor 31, 32, 33, 34 (block 54), control unit 5 is configured to (FIG. 6): [0171] determine parameters TQ.sub.1, TQ.sub.2, TQ.sub.3, TQ.sub.4, which are respectively associated with the variation of the respective rotor torque TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4 as a result of a variation of collective pitch angle 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL of the same rotor 31, 32, 33, 34 (block 56); and [0172] multiply each parameter TQ.sub.1, TQ.sub.2, TQ.sub.3, TQ.sub.4 by the pitch angle difference .sub.COLL1, .sub.COLL2, .sub.COLL3, .sub.COLL4 of the respective rotor 31, 32, 33, 34 (block 57).

[0173] Control unit 5 is configured to calculate each parameter TQ.sub.1, TQ.sub.2, TQ.sub.3, TQ.sub.4 on the basis of at least two independent variables associated with the performance of the respective rotor 31, 32, 33, 34. In detail, each parameter TQ.sub.1, TQ.sub.2, TQ.sub.3, TQ.sub.4 is calculated on the basis of: [0174] the respective angle 1, 2, 3, 4 measured by sensor means 41; and [0175] airspeed v measured by sensor means 43; and/or [0176] first component TQ.sub.mastSYM.

[0177] In detail, database 9 comprises data correlating parameter TQ with angle 1, 2, 3, 4, airspeed v and first component TQ.sub.mastSYM.

[0178] In further detail, control unit 5 is configured to: [0179] calculate each parameter TQ.sub.1, TQ.sub.2, TQ.sub.3, TQ.sub.4 on the basis of airspeed v, if the respective angle 1, 2, 3, 4 is lower than or equal to a threshold value th; and [0180] calculate each parameter TQ.sub.1, TQ.sub.2, TQ.sub.3, TQ.sub.4 on the basis of first component TQ.sub.mastSYM, if the respective angle 1, 2, 3, 4 is greater than threshold value th.

[0181] Since rotors 31, 32, 33, 34 are identical to one another, if angles 1, 2, 3, 4 are equal to one another, parameters TQ.sub.1, TQ.sub.2, TQ.sub.3, TQ.sub.4 are also equal to one another.

[0182] Pitch angle differences .sub.COLL1, .sub.COLL2, .sub.COLL3, .sub.COLL4 of rotors 31, 32, 33, 34 are respective differences between each collective pitch angle 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL and a symmetric collective pitch angle 0 (.sub.COLL1=1.sub.COLL0; .sub.COLL2=2.sub.COLL0; .sub.COLL3=3.sub.COLL0; .sub.COLL4=4.sub.COLL0). In detail, symmetric collective pitch angle 0 is calculated as the sum of collective pitch angles 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL divided by four, which is the number of rotors 31, 32, 33, 34 of aircraft 1 (0=(1.sub.COLL+2.sub.COLL+3.sub.COLL+4.sub.COLL)/4).

[0183] In use, control unit 5 calculates an estimate of rotor torques TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4 as an algebraic sum of first component TQ.sub.mastSYM and respective second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, TQ.sub.mastASYM3, TQ.sub.mastASYM4.

[0184] The description of the steps required to calculate first component TQ.sub.mastSYM of rotor torques TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4 of aircraft 1 is similar to the description of the steps required to calculate first component TQ.sub.mastSYM of rotor torques TQ.sub.mast1, TQ.sub.mast2 of aircraft 1 and will be omitted for the sake of brevity.

[0185] The following is a description of the steps required to calculate second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, TQ.sub.mastASYM3, TQ.sub.mastASYM4.

[0186] Periodically, sensor means 40 measure collective pitch angles 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL of rotors 31, 32, 33, 34, sensor means 41 measure angles 1, 2, 3, 4 and sensor means 43 measure airspeed v.

[0187] Control unit 5 calculates pitch angle differences .sub.COLL1, .sub.COLL2, .sub.COLL3, .sub.COLL4 between collective pitch angles 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL. In addition, control unit 5 accesses database 9 using angle 1, 2, 3, 4, airspeed v and/or first component TQ.sub.mastSYM as inputs and determines parameters TQ.sub.1, TQ.sub.2, TQ.sub.3, TQ.sub.4 on the basis of the data stored in database 9.

[0188] In detail, each pitch angle difference .sub.COLL1, .sub.COLL2, .sub.COLL3, .sub.COLL4 is calculated by subtracting symmetric collective pitch angle 0 from collective pitch angles 1.sub.COLL, 2.sub.COLL, 3.sub.COLL, 4.sub.COLL of respective rotors 31, 32, 33, 34.

[0189] Subsequently, control unit 5 calculates second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, TQ.sub.mastASYM3, TQ.sub.mastASYM4 as multiplications of respective parameters TQ.sub.1, TQ.sub.2, TQ.sub.3, TQ.sub.4 by respective pitch angle differences .sub.COLL1, .sub.COLL2, .sub.COLL3, .sub.COLL4.

[0190] Control unit 5 then calculates each rotor torque TQ.sub.mast1, TQ.sub.mast2, TQ.sub.mast3, TQ.sub.mast4 as the sum of first component TQ.sub.mastSYM and the respective second component TQ.sub.mastASYM1, TQ.sub.mastASYM2, TQ.sub.mastASYM3, TQ.sub.mastASYM4.

[0191] FIG. 8 illustrates a block diagram related to a method for estimating the rotor torques of an aircraft 1 capable of hovering (not shown) according to a second embodiment of the present invention comprising N operatively interconnected and identical rotors. In detail, N is a natural number greater than one. In addition, N may be an even or an odd number.

[0192] Aircraft 1 further comprises M engines operatively connected to the N rotors. In detail, M is a natural number greater than or equal to one; M might be equal to or different from N.

[0193] Aircraft 1 is a generalization of aircrafts 1 and 1. Accordingly, equal or equivalent parts of aircrafts 1, 1, 1 will be marked, where possible, by the same reference numerals. In detail, aircraft 1 is a multicopter.

[0194] According to the estimation method shown in FIG. 8, control unit 5 of aircraft 1 is configured to calculate a first component TQ.sub.mastSYM of rotor torques TQ.sub.mast1, TQ.sub.mast2, . . . , TQ.sub.mastN on the basis of engine torques TQ.sub.eng1, TQ.sub.eng2, . . . , TQ.sub.engM. In detail, first component TQ.sub.mastSYM is equal to rotor torques TQ.sub.mast1, TQ.sub.mast2, . . . , TQ.sub.mastN when collective pitch angles 1.sub.COLL, 2.sub.COLL, . . . , N.sub.COLL are equal to one another.

[0195] For the purpose of calculating first component TQ.sub.mastSYM, control unit 5 is configured to (FIG. 8): [0196] receive from sensor means 42 the signal associated with the measured values of engine torques TQ.sub.eng1, TQ.sub.eng2, . . . , TQ.sub.engM; [0197] calculating a total engine torque TQ.sub.engTOT generated by the engines operatively connected to the rotors of aircraft 1 as the sum of the engine torques TQ.sub.eng1, TQ.sub.eng2, . . . , TQ.sub.engM (block 50); [0198] subtracting from the total engine torque TQ.sub.engTOT a term corresponding to the transmission losses due to the transmission of engine torques TQ.sub.eng1, TQ.sub.eng2, . . . , TQ.sub.engM (or part thereof) from the M engines (block 51) through transmission units 6; and/or [0199] subtracting from the total engine torque TQ.sub.engTOT the torque TQ.sub.acc for powering the accessories of aircraft 1, where present (block 52).

[0200] The difference between total engine torque TQ.sub.engTOT and the transmission losses and/or torque TQ.sub.acc is the total available engine torque TQ.sub.engavail.

[0201] Control unit 5 is further configured to calculate first component TQ.sub.mastSYM by dividing the total available engine torque TQ.sub.engavail by the number of rotors N of aircraft 1 (block 53 in FIG. 8, formula a)):

[00001]$\begin{array}{cc}a)& \\ {\mathrm{TQ}}_{\mathrm{mastSYM}}=\frac{{\mathrm{TQeng}}_{\mathrm{avail}}}{N}& \end{array}$

[0202] Advantageously, control unit 5 is configured to: [0203] receive the signal associated with collective pitch angles 1.sub.COLL, 2.sub.COLL, . . . , N.sub.COLL of the N rotors from sensor means 40; and [0204] calculate a second component TQ.sub.mastASYM1, TQ.sub.mastASYM2, . . . , TQ.sub.mastASYMN of each rotor torque TQ.sub.mast1, TQ.sub.mast2, . . . , TQ.sub.mastN on the basis of at least pitch angle differences .sub.COLL1, .sub.COLL2, . . . , .sub.COLLN between collective pitch angles 1.sub.COLL, 2.sub.COLL, . . . , N.sub.COLL (block 54 of FIG. 8).

[0205] In particular, second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, . . . , TQ.sub.mastASYMN represent how much the rotor torque TQ.sub.mast1, TQ.sub.mast2, . . . , TQ.sub.mastN of each rotor deviates from the rotor torque that would act on each of the rotors if total available engine torque TQ.sub.engavail were equally apportioned between the rotors.

[0206] Control unit 5 is further configured to calculate rotor torques TQ.sub.mast1, TQ.sub.mast2, . . . , TQ.sub.mastN as an algebraic sum of first component TQ.sub.mastSYM (which is common to all rotors of aircraft 1) and the respective second component TQ.sub.mastASYM1, TQ.sub.mastASYM2, . . . , TQ.sub.mastASYMN (block 55 in FIG. 8, formula b)):

[00002]$\begin{array}{cc}b)& \\ {\mathrm{TQ}}_{\mathrm{masti}}={\mathrm{TQ}}_{{\mathrm{mast}}_{\mathrm{SYM}}}+{\mathrm{TQ}}_{{\mathrm{mast}}_{{\mathrm{ASYM}}_i}},i=1:N& \end{array}$

[0207] In particular, if the collective pitch angle 1.sub.COLL, 2.sub.COLL, . . . , N.sub.COLL is the same for the N rotors, the second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, . . . , TQ.sub.mastASYMN are null and the rotor torques TQ.sub.mast1, TQ.sub.mast2, . . . , TQ.sub.mastN are equal to one another and to the first component TQ.sub.mastSYM (TQ.sub.mast1=TQ.sub.mast2= . . . =TQ.sub.mastN=TQ.sub.mastSYM).

[0208] More specifically, for the purpose of calculating second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, . . . , TQ.sub.mastASYMN, control unit 5 is configured to (FIG. 8): [0209] determine parameters TQ.sub.1, TQ.sub.2, . . . ,TQ.sub.N, which are respectively associated with the variation of the respective rotor torque TQ.sub.mast1, TQ.sub.mast2, . . . , TQ.sub.mastN as a result of a variation of collective pitch angle 1.sub.COLL, 2.sub.COLL, . . . , N.sub.COLL of the same rotor (block 56); and [0210] multiply each parameter TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N by the respective pitch angle difference .sub.COLL1, .sub.COLL2, . . . , .sub.COLLN (block 57, formula c)):

[00003]$\begin{array}{cc}c)& \\ {\mathrm{TQ}}_{{\mathrm{mast}}_{{\mathrm{ASYM}}_i}}={}_{\mathrm{COLLi}}.Math.{\mathrm{TQ}}_i^{},i=1:N& \end{array}$

[0211] Control unit 5 is configured to calculate each parameter TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N on the basis of at least two independent variables associated with the performance of the respective rotor. In detail, each parameter TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N is calculated on the basis of: [0212] the angle 1, 2, . . . , N of the respective rotor measured by sensor means 41; and [0213] airspeed v measured by sensor means 43; and/or [0214] first component TQ.sub.mastSYM.

[0215] In detail, database 9 comprises data correlating parameters TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N with angles 1, 2, . . . , N, airspeed v and first component TQ.sub.mastSYM.

[0216] In further detail, control unit 5 is configured to: [0217] calculate each parameter TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N on the basis of airspeed v, if the respective angle 1, 2, . . . , N is lower than or equal to a threshold value th; and [0218] calculate each parameter TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N on the basis of first component TQ.sub.mastSYM, if the respective angle 1, 2, . . . , N is greater than threshold value th.

[0219] Since rotors N are identical to one another, if angles 1, 2, . . . , N are equal to one another, parameters TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N are also equal to one another.

[0220] Pitch angle differences .sub.COLL1, .sub.COLL2, . . . , .sub.COLLN are respective differences between each collective pitch angle 1.sub.COLL, 2.sub.COLL, . . . , 4.sub.COLL and a symmetric collective pitch angle 0 (formula d)):

[00004]$\begin{array}{cc}d)& \\ {}_{\mathrm{COLLi}}={}_{\mathrm{iCOLL}}-0,i=1:N& \end{array}$

[0221] In detail, symmetric collective pitch angle 0 is calculated as the sum of collective pitch angles 1.sub.COLL, 2.sub.COLL, . . . , N.sub.COLL divided by the number N of rotors (formula e)):

[00005]$\begin{array}{cc}e)& \\ 0=\frac{1}{N}.Math.{.Math.}_{i=1}^N{}_{\mathrm{iCOLL}}& \end{array}$

[0222] From an examination of the characteristics of the method for estimating the rotor torques TQ.sub.mast1, TQ.sub.mast2, . . . , TQ.sub.mastN, control unit 5 and aircraft 1, 1, 1 according to the present invention, the advantages they allow obtaining are evident.

[0223] In particular, rotor torques TQ.sub.mast1, TQ.sub.mast2 of convertiplane 1 are calculated as the algebraic sum of first component TQ.sub.mastSYM, which is equal to rotor torques TQ.sub.mast1, TQ.sub.mast2 when collective pitch angles 1.sub.COLL, 2.sub.COLL are equal to each other, and respective second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, which represents how much the rotor torque TQ.sub.mast1, TQ.sub.mast2 of each rotor 31, 32 deviates from the rotor torque that would act on each rotor 31, 32 if total available engine torque TQ.sub.engavail were equally apportioned between rotors 31 and 32. Therefore, the rotor torques TQ.sub.mast1, TQ.sub.mast2 of rotors 31, 32, which are independently controllable from each other, can be efficiently and reliably estimated even in situations in which collective pitch angles 1.sub.COLL, 2.sub.COLL are different from each other and without any physical sensor for directly measuring the rotor torques TQ.sub.mast1, TQ.sub.mast2.

[0224] Indeed, it has been observed that the rotor torques TQ.sub.mast1, TQ.sub.mast2 estimated by combining first and second components TQ.sub.mastSYM, TQ.sub.mastASYM1, TQ.sub.mastASYM2 are comparable to and more reliable than the rotor torques directly measured by the physical sensors mentioned in the introductory part of the description.

[0225] The estimation method according to the present invention also allows the rotor torques of an aircraft comprising any number of rotors to be efficiently estimated. Furthermore, control unit 5 comprises database 9, in which data correlating parameters TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N with variables associated with the flight conditions of convertiplane 1 are stored. Therefore, parameters TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N and second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, . . . , TQ.sub.mastASYMN are calculable on the basis of a limited number of independent variables.

[0226] The fact that parameters TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N can be determined on the basis of a limited number of variables allows to improve the robustness of the estimation method, because fewer errors due to false dependencies on other variables are possible.

[0227] In addition, since angles 1, 2, . . . , N corresponding to the orientation of axes B of the respective rotors are taken into account when determining parameters TQ.sub.1, TQ.sub.2, . . . , TQ.sub.N, second components TQ.sub.mastASYM1, TQ.sub.mastASYM2, . . . , TQ.sub.mastASYMN are calculated regardless of whether the M engines are tilted integrally with the N rotors or are fixed with respect to fuselage 2.

[0228] Finally, it is clear that modifications and variations can be made to the method, the control unit 5 and the aircraft 1, 1, 1 previously described without thereby departing from the scope of protection of the present invention.

[0229] In particular, the method according to the present invention may be implemented for estimating the rotor torques of a helicopter comprising at least two rotors, which are operatively connected to one another and have respective collective pitch angles independently controllable from one another.

[0230] Furthermore, the method according to the present invention may be implemented for estimating the rotor torques of an aircraft capable of hovering comprising at least two operatively connected and independently controllable rotors and with hybrid propulsion or full electric propulsion. In detail, the aircraft may comprise one engine or more than one engines and/or one or more electric motors.

[0231] Finally, nacelles 11, 12 and respective engines 21, 22 might be rotationally fixed with respect to axis B, being only rotors 31, 32 tiltable around axis B.