System and method for controlling the modification of the pitch of the blades of a turbine engine

A system for controlling the modification of the pitch of the blades of a fan of a turbine engine, in particular for an aircraft. The turbine engine comprising blades mounted radially in a drive shaft and a setting device configured to modify the pitch of the blades on the basis of an axial force applied to said setting device. The control system comprises a hydraulic actuator, a hydraulic pump, a connecting ring connected mechanically to the hydraulic pump such that the flow rate of the hydraulic pump is proportional to the relative speed between the drive shaft and the connecting ring, and a magnetic coupling device designed to control the drive speed of the connecting ring in order to control the pitch of the blades independently of the speed of the drive shaft.

TECHNICAL FIELD

The present invention relates to a system for controlling the pitch modification of the blades of a turbomachine comprising a plurality of blades with variable pitch angle. By blades, it is understood to mean both blades of a propeller of a turbopropeller and blades of a turbojet fan.

For example, to increase performance and improve efficiency of a turbomachine during all phases of its operation, from take-off to landing, it is known to vary the pitch of the blades. This variable pitch allows the speed of the propeller or fan to be varied in order especially to improve the propulsion efficiency of the propeller, without modifying that of the turbine which is generally set to its continuous maximum speed. In addition, blade pitching during landing reverses thrust, eliminating the need for complex and heavy conventional thrust reversal systems.

The difficulty of a pitch modification control system is that the blades belong to a rotating reference frame and that the modification in blade angle requires a large amount of energy to be transmitted in this rotating reference frame.

Current pitch modification control systems typically comprise hydraulic actuators that provide the necessary force to place the pitch to the desired angle (pitching). Such hydraulic actuators belong to the rotating reference frame and are supplied by a hydraulic pump belonging to the fixed reference frame. These systems are particularly complex, of large overall size, and have significant reliability issues, especially due to the use of rotary joints or connections to transmit hydraulic fluid between the fixed reference frame and rotating reference frame. It is important to minimize leakage from the rotating connections as the pressure drop generated has be compensated for by increasing pumping power, which impacts the mass and efficiency of the actuation system

Another solution is to perform an electrical command of an electric actuator with electrical contacts that are rotating, but these have the drawback of wearing out early, which is problematic when they are difficult to access. Another solution is to perform an electrical command via an electrical transformer connected to an electric actuator, which forces to provide an electrical transformer with dimensions similar to the electric motor, which is heavy, of large overall size and expensive. Incidentally, in prior art from patent application FR2831225A1, an electrohydraulic device comprising a hydraulic cylinder supplied by a hydraulic pump controlled by an electric motor, all of which belong to the rotating reference frame, is known. The electric motor, belonging to the rotating reference frame, is controlled and supplied by induction via a control circuit belonging to the fixed reference frame. The electric motor has to be powerful enough to supply the hydraulic pump. Such an electric motor increases the mass in the rotating reference frame, which is a drawback. In addition to the difficulties in transmitting power in the rotating reference frame, another difficulty is to manage failure cases during this power transfer. Indeed, during a failure, it is necessary to put the propeller in a safe position to limit drag, a failure leads to both the loss of actuation and the inability to secure the propeller. Full redundancy of the rotating transfer is severely detrimental to the mass.

A system for controlling and changing the pitch of the blades of an airplane propeller is also known in prior art from patent application U.S. Pat. No. 2,699,220A.

Thus, the invention aims to eliminate at least some of these drawbacks.

SUMMARY

The invention relates to a system for controlling the pitch modification of the blades of a fan of a turbomachine, in particular for an aircraft, the turbomachine comprising a drive shaft extending along a longitudinal axis and adapted to be rotatably driven about said longitudinal axis relative to a fixed structure of said turbomachine, blades radially mounted with respect to the drive shaft and adapted to be oriented at a blade pitch and an orientation device configured to modify the pitch of the blades as a function of an axial load applied to said orientation device, the control system comprising a hydraulic actuator, rotatably integral with the drive shaft, in order to apply the axial load to said orientation device and a hydraulic pump, rotatably integral with the drive shaft, supplying power to the hydraulic actuator in order to apply the axial load to said orientation device as a function of the flow rate of the hydraulic pump.

The invention is remarkable in that the control system includes a connecting ring gear rotatably mounted about said longitudinal axis relative to the fixed structure of said turbomachine, said connecting ring gear being mechanically connected to the hydraulic pump in such a way that the flow rate of the hydraulic pump is proportional to the relative speed between the drive shaft and the connecting ring gear, and a magnetic coupling device configured to pilot the driving speed of the connecting ring gear in order to control the pitch of the blades independently of the speed of the drive shaft.

By virtue of the invention, energy fed to the pump is taken from the drive shaft, which avoids the need for high electrical power transmission as in prior art. Advantageously, the blade angle can be adjusted simply by braking the connecting ring gear. The magnetic coupling device is easy to maintain and is reliable as there is no contact. Advantageously, the fluid circuit is closed (without power supply) and belongs only to the rotating reference frame.

Preferably, the hydraulic actuator comprises an actuator body comprising a first chamber and a second chamber configured to receive a hydraulic fluid so as to translationally move the actuator body along the longitudinal axis as a function the pressure in the chambers. Preferably, the hydraulic pump is configured to supply the first chamber. Preferably, the hydraulic pump is configured to collect hydraulic fluid in the second chamber.

Preferably, the hydraulic pump being configured to move the hydraulic actuator in a first direction, in particular downstream, the control system comprises a return member configured to exert an axial return load in a second direction opposite to the first direction, in particular upstream. Advantageously, the return device is configured to reduce the pitch of the blades to the safe position. The return member thus performs a safety function by automatically reducing the pitch in the event of a lack of hydraulic pressure. Advantageously, securing is passive. The return member is preferably in the form of a spring.

Preferably, in the absence of magnetic coupling, the connecting ring gear is rotatably driven by the hydraulic pump, in particular, at the same rotational speed. Thus, in order to create a speed differential, the coupling device should slow down the connecting ring gear, which requires little energy and also allows its collection.

Preferably, the magnetic coupling device comprises a permanent magnet electric machine comprising stator members integral with the fixed structure and rotor members, integral with the connecting ring, which are magnetically coupled with the stator members so as to magnetically brake the connecting ring gear.

According to one aspect of the invention, the magnetic coupling device further comprises a control unit configured to provide a control current to the stator members in order to vary the braking force.

Preferably, the control unit is integral with the fixed structure. It can advantageously be housed in a compartment with favorable temperature and pressure conditions, which increases reliability and reduces cost.

According to a preferred aspect, the control unit comprises at least one variable resistor. This makes it possible to conveniently set the braking force and to collect heat and electrical energy when braking.

According to one aspect of the invention, the control system comprises at least two hydraulic pumps, the hydraulic pumps in particular being connected in series or in parallel. Such hydraulic pumps provide redundancy and balancing of the whole.

According to one aspect of the invention, the control system comprises at least one overspeed management system configured to modify the power supply to the hydraulic actuator in the event of detection of an overspeed. This ensures safety.

The invention also relates to a turbomachine fan module comprising a control system as previously set forth.

The invention also relates to a turbomachine, in particular for an aircraft, comprising a drive shaft extending along a longitudinal axis and adapted to be rotatably driven about said longitudinal axis with respect to a fixed structure of said turbomachine, blades radially mounted in said drive shaft and adapted to be oriented at a blade pitch and an orientation device configured to modify the pitch of the blades as a function of an axial load applied to said orientation device, the turbomachine comprising a control system as previously set forth to apply the axial load to the orientation device.

The invention also relates to a method for controlling the pitch modification of the blades of a turbomachine fan by means of a control system as previously set forth, the method comprising:a step of modifying the drive speed of the connecting ring gear by the magnetic coupling device,a step of driving the hydraulic pump, by the connecting ring gear, in order to activate the hydraulic actuator and apply an axial load to the orientation device anda step of modifying the pitch of the blades as a function of the axial load applied to the orientation device.

It should be noted that the figures set out the invention in detail in order to implement the invention, said figures may of course be used to better define the invention if necessary.

DETAILED DESCRIPTION

The invention will be set forth for a turbopropeller but the invention applies to any turbomachine having rotating blades whose pitch can be modified, in particular, a turbojet having a fan with rotating blades or a turbopropeller having a propeller with rotating blades. Subsequently, for the sake of brevity, the term “fan” will be used to refer to both a turbojet fan and a propeller of a turbopropeller.

Subsequently, the invention will be set forth in connection with a turbopropeller but the invention also applies to a turbojet.

With reference toFIG.1, a partial cross-section view of a turbopropeller10according to one embodiment of the invention is represented.

The turbomachine10comprises a drive shaft11extending along a longitudinal axis X and adapted to be rotatably driven about said longitudinal axis X relative to a fixed structure12of said turbomachine10, in particular, via bearings111. Subsequently, the terms “downstream” and “upstream” are determined in relation to the longitudinal axis X, which is oriented from downstream to upstream inFIG.1.

The turbomachine10comprises a fan H comprising blades13radially mounted with respect to said drive shaft11, in particular via a hub, and adapted to be oriented at a pitch θ with respect to a radial axis R which is orthogonal to the longitudinal axis X. Subsequently, the pitch θ varies between a minimum pitch, where blades13are ‘feathered’ and a maximum pitch.

Still with reference toFIG.1, the turbomachine10comprises an orientation device14configured to modify the pitch θ of the blades13as a function of an axial load F applied to said orientation device14. Such an orientation device14is known to those skilled in the art, in particular from patent application FR3036093. Preferably, the orientation device14includes crank pins off-centered with respect to the radial axis R.

In order to modify the pitch of the blades13, the turbomachine10comprises a control system which comprises a hydraulic actuator3, a hydraulic pump4, a connecting ring gear5and a magnetic coupling device6which will be set forth in detail.

The hydraulic actuator3is configured to apply an axial load F to said orientation device14. In this example, the hydraulic actuator3is in the form of a hydraulic cylinder mounted externally to the drive shaft11and rotatably integral with it. In other words, the hydraulic actuator3belongs to the rotating reference frame.

In this example, the hydraulic actuator3conducts oil, but it goes without saying that other hydraulic fluids could be contemplated.

With reference toFIG.1, the hydraulic actuator3comprises an actuator body30comprising a first chamber31and a second chamber32configured to receive hydraulic fluid so as to translationally move the actuator body30along the longitudinal axis X as a function of the pressure in the chambers31,32. In this embodiment, the first chamber31is downstream of the second chamber32. In this example, as will be set forth later, when the pressure in the first chamber31is increased, the actuator body30moves downstream. The hydraulic actuator3further comprises a calibrated opening33which allows fluid to circulate from the first chamber31to the second chamber32with a controlled flow rate. The chambers31and32are separated by a wall belonging to the drive shaft11.

In this example, the first chamber31is a supply chamber by the hydraulic pump4while the second chamber32is a collection chamber of the hydraulic pump4. The hydraulic pump4thus allows the actuator body30to be moved axially downstream.

In this example, the actuator body30moves between an end upstream position that corresponds to the minimum pitch and an end downstream position that corresponds to the maximum pitch.

Preferably, the hydraulic actuator3has sensors configured to detect position of the hydraulic actuator3and compare it with its setpoint position as will be set forth later.

Still referring toFIG.1, the turbomachine10further comprises a return member7configured to exert an axial return load directed upstream on the orientation device14, that is in the opposite direction to that of drive by the hydraulic pump4.

In this example, with reference toFIG.1, the return member7is mounted about the drive shaft11between a stop member112of the drive shaft11and the actuator body30in order to exert a stress directly upstream on the actuator body30.

Thus, when the hydraulic pressure increases in the first chamber31, the return member7makes it possible to oppose the downstream movement. When the first chamber31is no longer under pressure, the return member7allows the actuator body30to be returned upstream to the safety position by forcing fluid circulation from the first chamber31to the second chamber32via the calibrated opening33. In other words, the return member7enables the blades13to be passively feathered.

As will be set forth later, the return member7advantageously improves safety by reducing pitch of the blade13in the event of a control system malfunction. The blades13are advantageously brought back to a safe position (feathered blades) in the absence of hydraulic pressure supplied by the hydraulic pump4.

Preferably, the return member7is in the form of a spring, but it goes without saying that it could be in other forms, for example, a flexible rod or a counterweight.

According to one aspect of the invention, the magnetic coupling device6is configured to rotate the connecting ring gear5in both directions and thus dynamically control axial position of the hydraulic actuator3. This means that a distinct return member is not required.

Still with reference toFIG.1, the hydraulic pump4is rotatably integral with the drive shaft11and supplies the hydraulic actuator3in order to apply the axial load F to said orientation device14as a function of the flow rate of the hydraulic pump4. Thus, the hydraulic pump4and the hydraulic actuator3belong to the same rotating reference frame. The hydraulic pump4is connected to the first chamber31by a first supply channel41and to the second chamber32by a second collection channel42. The hydraulic pump4is configured to increase the hydraulic pressure in the first chamber31. In other words, the hydraulic pump4is configured to supply the first chamber31with fluid via the first supply channel41. The fluid circulates into the second collection chamber32via the calibrated opening33before being collected by the hydraulic pump4via the second collection channel42. Thus, the fluid circuit only belongs to the rotating reference frame, which limits the risk of leakage in comparison with prior art, which required rotary joints to link equipment of the fixed reference frame and of the rotating reference frame.

As the channels41,42are rotary, the fluid circuit loop configuration advantageously avoids the appearance of a pressure difference which would modify the equilibrium position of the hydraulic actuator3. In practice, when the pressures are summed up over the entire fluid circuit loop, the “rising” pressure and the “descending” pressure in relation to the axis of rotation X are summed up, resulting in a total zero value.

Preferably, the hydraulic pump4is a fixed volume pump, in particular a gear pump.

With reference toFIG.1, the hydraulic pump4includes a mechanical input43configured to receive an input torque. Depending on the input torque applied to the mechanical input43, the hydraulic pump4applies a different flow rate and therefore a different pressure in the first chamber31. In other words, the hydraulic pump4makes it possible to convert the input torque into a flow rate setpoint, which advantageously modifies the axial load F.

In this example, when the input torque is high, the hydraulic pump4increases the pressure in the first chamber31, which moves the actuator body30downstream and compresses the return member7as illustrated inFIG.2. Conversely, when the input torque is low or zero, the hydraulic pump4does not supply any flow rate and the pressure between the chambers31,32is balanced by the return member7as illustrated inFIG.3. The return member7controls a small pitch angle and the blades13are in a safe position. This means that in the event of a malfunction of the hydraulic pump4, safety is ensured by the return member7.

Preferably, the mechanical input43is in the form of a drive pinion which meshes with the connecting ring gear5. Advantageously, the connecting ring gear5is only coupled to the hydraulic pump4so that, in the absence of magnetic coupling, the hydraulic pump4does not (or only slightly) supply the hydraulic actuator3. The mechanical input43and the hydraulic pump4mechanically cooperate so that the mechanical input43rotatably drives the connecting ring gear5in the absence of magnetic coupling. Indeed, the hydraulic pump4belongs to the rotating reference frame and is rotatably driven. As will be set forth below, the magnetic coupling device6makes it possible to magnetically brake the connecting ring gear5to generate a speed difference between the connecting ring gear5and the hydraulic pump4.

The connecting ring gear5is rotatably mounted about said longitudinal axis X with respect to the fixed structure12of said turbomachine10and is configured to provide the input torque applied to the mechanical input43of the hydraulic pump4. As illustrated inFIG.1, the connecting ring gear5is connected to the fixed structure12via bearings51.

The connecting ring gear5is mechanically connected to the hydraulic pump4so that the flow rate of the hydraulic pump4is proportional to the relative speed between the drive shaft11and the connecting ring gear5. As explained previously, the hydraulic pump4is rotatably integral with the drive shaft11. Since the rotational speed of the drive shaft11is known, adjusting the rotational speed of the connecting ring gear5is sufficient to adjust the relative speed between the drive shaft11and the connecting ring gear5to modify the flow rate of the hydraulic pump4. As the connecting ring gear5is rotatably driven by the hydraulic pump4, it is sufficient to brake the connecting ring gear5to adjust the relative speed between the drive shaft11and the connecting ring gear5.

The magnetic coupling device6is configured to modify the drive speed of the connecting ring gear5about said longitudinal axis X in order to indirectly control the pitch of the blades13. In particular, the magnetic coupling device6performs a magnetic braking function for the connecting ring gear5.

With reference toFIG.1, the magnetic coupling device6comprises a permanent magnet electric machine60comprising stator members60A integral with the fixed structure12and rotor members60B, integral with the connecting ring gear5, which are magnetically coupled with the stator members60A. The magnetic coupling device6further comprises a control unit61, belonging to the fixed reference frame, configured to provide a control current to the stator members60A in order to vary the rotational speed of the connecting ring gear5.

Thus, in summary, the control unit61can conveniently control the pitch θ of the blades13. The contactless magnetic connection between the fixed structure12and the connecting ring gear5reduces complexity by keeping the hydraulic members (hydraulic actuator3and hydraulic pump4) only in the rotating reference frame. In addition, there is no need for an electric motor to control the hydraulic pump4. The energy required for activation is taken directly from the drive shaft11via the connecting ring gear5. Power transfer is simplified, which also makes maintenance easier.

Preferably, in order to facilitate maintenance, the magnetic coupling device6is configured to generate a rotating magnetic field in order to control the hydraulic actuator3when the turbomachine10is stopped. According to one preferred aspect, as mentioned previously, the magnetic coupling device6is configured to generate a magnetic field rotating in both directions to dynamically control position of the hydraulic actuator3upstream and downstream without a return member7.

Alternatively, when the aircraft is on the ground, an auxiliary device may be connected to the magnetic coupling device6to generate a rotating magnetic field in order to control the hydraulic actuator3. In other words, the auxiliary device can be used during maintenance to check the operation of the control system.

A method for controlling pitch modification of the blades13will now be set forth. In this example of implementation, the permanent magnet electric machine60is initially deactivated and the connecting ring gear5is rotatably integral with the hydraulic pump4, which itself is rotatably integral with the drive shaft11. The hydraulic pump4is not supplied and there is no overpressure in the first chamber31of the hydraulic actuator3. The body of the actuator30is restricted in the upstream position by the return member7as illustrated inFIG.3, the pitch θ then being minimal.

In this example, the control unit61receives a command for modifying the pitch of the blades13from, for example, a calculator of the turbomachine.

With reference toFIG.4, the method comprises a step of modifying E1the electromotive force FEM in the permanent magnet electric machine60. The result is a step of modifying E2the drive speed of the connecting ring gear5by the magnetic coupling device6. In this example, the permanent magnet electric machine60brakes the connecting ring gear5. The relative speed between the hydraulic pump4and the connecting ring gear5increases.

The method comprises a step of driving E3the hydraulic pump4, via the connecting ring gear5, in order to activate the hydraulic actuator3and apply an axial load F to the orientation device14. In practice, the relative speed activates the hydraulic pump4which supplies the first chamber31of the hydraulic actuator3. The actuator body30moves downstream against the return member7and applies an axial load F to the orientation device14as illustrated inFIG.2.

This results in a step of modifying E4the pitch θ of the blades13as a function of the axial load F applied to said orientation device14. In this case, the pitch θ of the blades13increases to flatten the blades13.

Thus, by setting the electromotive force FEM, the pitch θ of the blades13can be conveniently controlled. Advantageously, the activation force of the hydraulic pump4is taken from the drive shaft11, which improves efficiency.

When the electromotive force FEM is stopped, the return force Fr of the return member7moves the hydraulic actuator3upstream, which modifies the pitch θ of the blades13to the safety position as illustrated inFIG.3. This ensures safety even if the magnetic coupling6, the connecting ring gear5or the hydraulic pump4fail.

According to one aspect of the invention, with reference toFIG.5, the control unit61comprises a variable resistor62so as to vary the current flowing in the stator members60A and thus vary the rotational speed of the connecting ring gear5to control the pitch θ of the blades13.

Advantageously, when the hydraulic pump4is supplied, permanent braking of the connecting ring gear5generates current in the variable resistor62, which can be collected and subsequently used. In practice, electricity generation is more important at a very high pitch θ, that is at cruise power. Conversely, only the feathered position (minimum pitch) produces no energy. An electric battery may be provided to store electric energy from the variable resistor62and distribute it to other equipment, for example the fuel pump. Still preferably, a battery may also be provided to store heat energy from the variable resistor62.

Preferably, the control unit61is positioned in a mild thermal environment, for example, in proximity to the turbomachine regulation calculator. By way of example, with reference toFIG.6, the control unit61may be positioned at different positions of the turbomachine, in particular, in the nacelle (position P1) or downstream in the “core” zone (position P2), which is located between the inner wall of the secondary stream and the outer wall of the primary stream.

According to one aspect of the invention, with reference toFIG.7, a first example of the “series” type redundancy is represented. In this figure, the turbomachine10comprises a first hydraulic pump4aconnected to a first connecting ring gear5aand to a first magnetic coupling system6a. Similarly, a second hydraulic pump4bconnected to a second connecting ring gear5band to a second magnetic coupling system6bin order to provide redundancy is represented.

The first chamber31of the hydraulic actuator3is supplied by a first supply channel41supplied, on the one hand, by a first elementary supply channel41aconnected to the first hydraulic pump4aand, on the other hand, by a second elementary supply channel41bconnected to the second hydraulic pump4b.

The second chamber32of the hydraulic actuator3is connected to a second collection channel42connected, on the one hand, to a first elementary return collection channel42aconnected to the first hydraulic pump4aand, on the other hand, to a second elementary collection channel42bconnected to the second hydraulic pump4b.

In nominal operation, the first hydraulic pump4aand the second hydraulic pump4bboth supply the first chamber31of the hydraulic actuator3. Each magnetic coupling system6a,6bprovides calibrated braking.

In the event of a failure, for example due to a short circuit of the second magnetic coupling system6bthat increases the electromotive force FEM, the second hydraulic pump4bis actuated in the absence of a modification order. Such a modification leads to a downstream movement of the hydraulic actuator3and to flattening of the blades13, that is in contrast to “feathering” safety.

When it is detected that the position of the hydraulic actuator3is different from the position setpoint, the first magnetic coupling system6areduces its electromotive force as much as possible to reduce the flow rate of the first hydraulic pump4aand thus compensates for the increase caused by the second hydraulic pump4b. This eliminates unwanted movement of the hydraulic actuator3. Advantageously, failure of one of the hydraulic pumps4a,4bcan be compensated for by using two hydraulic pumps4a,4bin series.

Alternatively, in order to prevent one of the hydraulic pumps4a,4bfrom providing a supply flow rate, a check valve may be provided on one of the hydraulic pumps4a,4b, which allows the hydraulic actuator3to be actuated independently, the actuation speed corresponding to the sum of the two hydraulic pumps4a,4b. Optionally, in order to prevent one of the hydraulic pumps4a,4bfrom controlling the hydraulic actuator3towards large pitches θ, an overspeed management mechanism known to those skilled in the art as the term “Overspeed Governor” for turbopropellers can be provided. If the pitch θ becomes too low, the speed increases and can be detected. These aspects will be set forth in detail later.

According to another aspect of the invention, with reference toFIG.8, a second example of “parallel” type redundancy is represented.

In this figure, analogously to previously, the turbomachine10comprises a first hydraulic pump4aconnected to a first connecting ring gear5aand to a first magnetic coupling system6a. Similarly, a second hydraulic pump4bconnected to a second connecting ring gear5band to a second magnetic coupling system6bin order to provide redundancy is represented.

The first chamber31of the hydraulic actuator3is supplied by a first supply channel41supplied, on the one hand, by a first elementary supply channel41aconnected to the first hydraulic pump4aand, on the other hand, by a second elementary supply channel41bconnected to the second hydraulic pump4b.

The second chamber32of the hydraulic actuator3is connected to a second collection channel42connected, on the one hand, to a first elementary collection channel42aconnected to the first hydraulic pump4aand, on the other hand, to a second elementary collection channel42bconnected to the second hydraulic pump4b. In this example, a first check valve81is mounted in the first elementary collection channel42aand a second check valve82is mounted in the second elementary collection channel42b. Such check valves81,82allow the two hydraulic pumps4a,4bto be used alternately in the event of a pressure loss in one of the hydraulic pumps4a,4b.

In nominal operation, the first hydraulic pump4aand the second hydraulic pump4bboth supply the first chamber31of the hydraulic actuator3.

Still with reference toFIG.8, an overspeed management mechanism, comprising a first member91and a second member92, is also present. The first member91is mounted in the first elementary collection channel42ain parallel with the first check valve81. The second member92of the overspeed management mechanism is mounted in the second elementary collection channel42bin series with the second check valve82. The first member91and the second member92of the overspeed management mechanism are both connected to the first elementary supply channel41aas illustrated inFIG.8.

If one of the hydraulic pumps4a,4bfails, the overspeed management mechanism comprises a speed detection member93which, above a predetermined speed threshold, controls the first member91and/or the second member92to divert supply to the first chamber31and place the blades13in the safety position. Preferably, the speed detection member93comprises a centrifugal force-sensitive counterweight mounted with a return member in order to determine the speed threshold. Preferably, each member91,92is in the form of a sliding spool so as to directly connect an inlet channel and a return channel of a hydraulic pump4a,4bin the event of overspeed. In other words, each member91,92comprises at least two positions (a pass position and a safety deflection position) which are controlled by the speed detection member93. This prevents any unwanted modification to the pitch θ. Of course, several counterweights could be used on several spools.

When two hydraulic pumps4a,4bare used, they are preferably positioned diametrically opposite to each other in order to reduce imbalance.