Brake system for aircraft undercarriage

A braking system for an aircraft provided with undercarriage, wherein an axial-flux reversible electrical machine is associated to at least one wheel of the undercarriage and is set in rotation by the rotation of the wheel. Current-dissipator is provided, which can be connected to the windings of the axial-flux reversible electrical machine during rotation of the wheel in the landing phase for dissipating in the current-dissipator the induced currents generated by the machine, which behaves as current generator, and producing a braking effect that slows down the movement of the wheel. An epicyclic reducer is provided between the wheel of the undercarriage and the rotor of the reversible electrical machine. The epicyclic reducer provides a transmission ratio T in such a way that the rotor will turn at a velocity Tω with respect to the velocity of rotation ω of the wheel.

The present invention relates to a braking system for aircraft undercarriage.

BACKGROUND OF THE INVENTION

As is known, during the operations of landing of aircraft, the latter have to be braked in order to reduce their speed and terminate the trip safely within the landing strip. Braking of the aircraft occurs by the action of brakes of an aerodynamic type and by the action of mechanical brakes coupled to the wheels of the undercarriage of the aircraft.

Currently, undercarriages of aircraft are provided with mechanical disk brakes, which are operated by oil under pressure coming from a hydraulic circuit. In particular, the braking action is modulated manually by the pilot by action on a brake pedal that acts on valves of the hydraulic circuit.

As is known, disk brakes are subject to a rapid wear on account of the high value of kinetic energy that is to be dissipated during braking of the aircraft.

Disk brakes moreover reach very high temperatures, which can jeopardize the efficiency thereof and drastically reduce their service life.

There have moreover been proposed braking systems of an electrical type, which use reversible electrical machines, directly coupled to the undercarriage, which are designed to provide a braking action of a “totally electrical” type.

For example, the patent application No. PCT WO 2005/102839 describes an axial-flux machine directly coupled to the wheels of an undercarriage of an aircraft in order to provide a plurality of functions, amongst which:prior to the landing phase the electric motor is supplied so as to set the wheels of the undercarriage in rotation and favour landing, moreover reducing the wear of the tyres due to the effect of friction on the landing strip;following upon contact of the wheels of the undercarriage with the landing strip, the reversible electrical machine behaves as a generator, producing energy and thus exerting a braking action—the electrical energy is dissipated in resistors or is supplied to sections of the motor that provide a braking action opposite to the direction of rotation of the generator;part of the energy is stored in an on-board system for being re-used subsequently; andfollowing upon completion of the operations of landing of the aircraft, the reversible electrical machine can be supplied and used on the runway for moving the aircraft in opposite directions.

The applicant of the patent application has found how, notwithstanding the fact that the solution disclosed in the patent application No. PCT WO 2005/102839 referred to above can be acknowledged absolute theoretical validity, it cannot be implemented on any commercial aircraft in operation. In fact, from an analysis of the dimensions of the rims of the wheels for aircraft undercarriage and of the values of the torques necessary to obtain safe braking (i.e., in the times and in the ways required by current certification standards), it emerges that at present the rims of the undercarriage wheels do not enable in any way integral housing inside them of machines capable of generating adequate braking torques in so far as the radial dimensions are limited by the internal diameter of the rim of the wheels of the undercarriage within which the reversible electrical machine is to be integrally housed.

SUMMARY OF THE INVENTION

The aim of the present invention is instead to provide a braking system for the wheels of an undercarriage of an aircraft that will solve the problems referred to above typical of known “totally electrical” braking systems. In particular, in order also to limit the weight and overall dimensions of the machine, it is envisaged that the coupling between the wheel of the undercarriage and the axial-flux machine cannot be of the direct type but must necessarily envisage the use of a reducer, for example an epicyclic reducer, thus enabling the machine to generate the torque necessary for braking.

The above aim is achieved by the present invention in so far as it regards a braking system for an aircraft provided with undercarriage in which an axial-flux reversible electrical machine is associated to at least one wheel of the main undercarriage and is set in rotation by the rotation of the wheel. Since current-dissipator means are provided, which can be connected to the windings of said axial-flux reversible electrical machine, during rotation of said wheel in the landing phase induced currents are produced in the dissipator means that are generated by the machine, which behaves as electric generator, thus producing a braking effect that slows down the movement of said wheel. Said braking system is characterized in that it comprises an epicyclic reducer set between the wheel of the undercarriage and the rotor of said reversible electrical machine, said epicyclic reducer providing a transmission ratio T in such a way that the rotor will turn at a velocity Tω with respect to the velocity of rotation ω of the wheel.

DETAILED DESCRIPTION OF THE INVENTION

Designated as a whole by1inFIG. 1is a braking system for an aircraft2(for example, an aircraft for regional transport—represented schematically) comprising a fuselage3, two side wings4, a front undercarriage5carried by the fuselage3and two lateral undercarriages7, each carried in the example of embodiment, by a corresponding wing4.

Each of the undercarriages5,7comprises a respective frame8, a top terminal portion of which is coupled to a corresponding load-bearing structure (not illustrated) of the aircraft2, and an opposite bottom terminal portion of which carries hinged, in the particular example described, one or more pairs of wheels10hinged to the frame8so as to turn about a corresponding hinge axis11.

Each wheel10(FIG. 3) comprises a metal rim12, which carries at least one tyre13, in turn comprising a tread14and two side walls15.

In the particular example described, the undercarriages5and7are of a retractable type and for this reason are associated to respective movement assemblies, in themselves known and not described in detail, each designed to displace the corresponding undercarriage5,7between a retracted resting position (not illustrated), in which the undercarriages5,7are completely housed in a seat of the fuselage3and, respectively, of the wings4, and an operative extracted position (illustrated inFIG. 1), in which the undercarriages5,7extend downwards from the fuselage3and from the wings4.

An axial-flux reversible electrical machine20of a known type (FIG. 2) is coupled to the wheel10of an undercarriage5,7in such a way that the rotor of the electrical machine20is set in rotation following upon the angular movement of the wheel10of the undercarriage5,7with the stator of the machine20fixed with respect to the frame8.

According to the invention, an epicyclic reducer21is set between the wheel10of the undercarriage and the rotor of the reversible electrical machine20; the epicyclic reducer21provides a transmission ratio T in such a way that the rotor turns at a velocity Tω with respect to the velocity of rotation ω of the wheel10(with Tω>ω).

In greater detail, the epicyclic reducer21comprises:a ring gear22fixed with respect to an end portion of the rim12of the wheel10;planetary gears or satellites24(three in the example), which mesh on the ring gear22and are carried by a planetary-bearing disk (not illustrated for reasons of simplicity and fixed with respect to the frame8); anda sun gear25, which meshes with the planetary gears or satellites24and is angularly fixed with respect to the rotor of the reversible electrical machine20.

In the example of embodiment illustrated, the axial-flux reversible electrical machine20comprises a first rotor32a, a second rotor32b, and a third rotor32ccarried by a tubular body33fixed with respect to the sun gear25and mounted by interposition of bearings (not illustrated) on a wheel-bearing shaft34rof the frame8.

Each of the rotors32a,32band32c(of a known type) is formed by a plane metal wall shaped like an annulus provided with a plurality of permanent magnets M angularly spaced apart along a circular path. Typically, the permanent magnets M, of a plane type, have a trapezoidal shape in plan view.

The axial-flux electrical machine20comprises two stators34a,34bangularly fixed with respect to the shaft34r. Each stator34is set between two rotors32set facing opposite faces of the stator34.

Each of the stators34aand34b(of a known type) comprises a toroidal core made of ferromagnetic material (not illustrated) provided in which is a plurality of slots that house insulated electrical conductors wound around the toroidal core to provide a first winding36a, a second winding36b, and a third winding36c, which have first terminals connected to one another and second terminals connected to a first electric line37a, a second electric line37b, and a third electric line37c(FIG. 2), respectively.

Each electric line37a,37b,37ccommunicates with respective first terminals of a single three-phase switch (for example, a static switch), designed to close/open three contacts38a,38b,38c; the three-phase switch has second terminals connected, respectively, to a first terminal of a variable resistor40a,40b,40chaving a second common connection terminal.

The value of resistance R(f) provided by the variable resistor40a,40b,40cis modifiable on the basis of a command signal set from a control block43under manual action of the pilot, who can act on a brake pedal (not illustrated).

In this a way, by closing each contact38a,38b,38cclosing of the first, second, and third windings36a,36b,36con a respective variable resistor40a,40b,40cis obtained.

Each first terminal of the static three-phase switch is connected to one end of an electric line44a,44b,44c, which communicates with a device46(of a known type), in which electrical charge can be accumulated through an AC-DC converter47.

Switching of the three-phase switch and operation of the converter47and of the device46is controlled by an electronic unit50that carries out braking of the aircraft2with modalities that will be clarified hereinafter.

Also present on the aircraft2is a three-phase electrical network55supplied by a current generator57operated by one of the engines and/or by an auxiliary turbine59(APU).

The same generator57is also coupled with an AC-AC converter47, which interfaces with the on-board three-phase electrical network55.

The electronic unit50controls, with the modalities that will be clarified hereinafter, communication of the three-phase electrical network55with the electrical lines37a,37b,37cthrough the section of AC-AC conversion of electrical power according to techniques of a known type that will not be described in further detail.

The electronic unit50moreover communicates with the block43for implementation of the manual braking command by means of a pedal.

In use, during landing of the aircraft2, following upon contact between the undercarriages5,7and the runway, the wheels10are set in rapid rotation. The presence of an epicyclic converter with transmission ratio T means that the rotors32a,32b,32cmove at a velocity Tω higher than the velocity of rotation ω of the wheel.

In this way, the rotors turn at a high velocity and consequently high electromotive forces are induced on the windings36a,36b,36cin so far as the axial-flux reversible electrical machine20behaves as a current generator.

The electronic unit50then governs closing of the contacts38a,38b,38cin such a way that the induced currents generated by the current generator20close on the resistors40a,40b,40c, where the electrical energy is converted into heat by the Joule effect.

The induced currents have a direction that opposes the cause that has generated them, i.e., the movement of the rotors32a,32b,32cwithin the magnetic field of the stator34.

Consequently, a braking effect is produced, which slows down the movement of the rotor32and hence of the wheel10given the same braking power.

The braking effect is all the more intense the higher the velocity of the rotor32with respect to the stator34; the presence of an epicyclic converter ensures reaching of a velocity of the rotor32that guarantees a high braking effect.

For this principle of operation, the braking action is maximum at the moment of contact of the aircraft2with the landing strip and decreases with the reduction of the speed of the aircraft2.

Furthermore, by means of the control block43the pilot can modify the value of resistance R(f) and hence the value of the current that is dissipated by the resistors and modulates the braking force as a function of the velocity of the wheel10. In other words, the degree of the braking action is given by the value of torque that is imposed by the armature current of the machine (induced on the windings of the stator elements36a,36b,36c) and is determined by the value of the three-phase resistance R(f) due to the action of the pilot on the brake pedal.

In this way, unlike the majority of mechanical brakes that function by exploiting forces of friction, the principle of operation of the braking system of the present invention does not envisage parts subject to wear.

For each pair of wheels10the system in question subsequently enables actuation of an intrinsic anti-skid control (ASK) (of the ABS type) capable of modulating the braking action following upon a non-uniform deceleration of the wheels10. In particular, in the case of blocking of a wheel10due, for example, to skidding phenomena, the system automatically blocks its braking action in so far as it no longer receives energy for developing the opposing resisting torque.

The amount of electrical power not used for braking is transferred, through the electrical lines44a,44b,44cand the converter AC/DC47, into the device46where the electrical charges accumulate in an accumulation system of a super-capacitive type.

When the velocity of rotation of the wheel10drops below a first threshold value such that the amount of the induced electromotive force, notwithstanding the presence of the epicyclic converter21, would in turn determine an insufficient braking, the electronic unit50governs a gradual reduction of the value of resistance (up to short-circuiting) so as to keep the currents in the stator windings high. When the velocity of the wheel drops below a value such that the braking action by induction becomes negligible, the electronic unit50governs transfer of charge from the device46, which releases the accumulated charge.

When the velocity of rotation of the wheel10drops below a second threshold value lower than the first, the on-board electrical network55intervenes. In particular, the electronic unit50is able to detect the angular velocity of the wheel10(i.e., of the rotors32a,32b,32c) and simultaneously its instantaneous derivative (amount of deceleration) by governing, through the converter47, the on-board electrical network55to impose upon the machine an appropriate armature current such as to maintain the opposing braking torque required by the pilot through the action on the brake pedal.

In this case, by using the on-board electrical network55, the electrical machine20supplies further braking power.

Consequently, in this way, the definitive arrest of the aircraft2can be obtained in a “totally electrical” way, without using any brake of a mechanical type to enable definitive arrest of the means.

The aircraft2can be provided with a parking brake with mechanical blocking (by means of a pawl-and-ratchet mechanism of a known type, not illustrated) activated and de-activated electrically.

The electronic unit50can also be configured in such a way that the on-board electrical network55will supply through the converter47the electrical machine20with a current having a direction such as to obtain rotation of the reversible electrical machine and displacement of the aircraft2on the runway.

The reversibility of the machine20determines in fact the possibility of using the system1described also for the operations of taxiing and towing of the aircraft2on the runway. In this case, the power for supply of the system, necessary to obtain the static torque useful for movement of the aircraft, is detected directly by the on-board electrical network55without the need to turn on the main engines but by exploiting, for example, the generator59(APU) (already in itself operative during the step of loading on the ground).

During the taxiing step, there is subsequently envisaged a further control of a differential type capable of processing the information received from the front steering wheels (angle and direction of rotation) so as to change the velocity of rotation of each pair of wheels following upon non-rectilinear paths.

In the braking system described above, where the reversible electrical machines are of a three-phase type, there exists a direct proportionality between the opposing braking torque and the radius r of the wheel10(arm of the torque) that is of a cubic type, i.e., C=f(r3).

However, since the radius of the wheel10is a fixed quantity and the quantities involved are of a sufficiently high value, an increase of the braking torque can be obtained using an electrical machine of a “six-phase” type (not illustrated).

On this hypothesis, the increase of the number of phases (corresponding to a reduction of the polar pitch of the machine) determines an increase in the induced counter-electromotive force (i.e., of the resistant torque acting on the wheel10) given the same size and velocity of rotation of the rotor24(and hence of intensity of the current induced on each single phase).