Patent Description:
The invention is applicable to any type of aircraft and is particularly advantageously applied in vertical take-off aircraft. This specific technical field will be referred to directly, but not in a limited way, in the following description.

In recent years, many vertical take-off aircraft have been studied and developed to allow for the advancement of mobility in large metropolises and in areas without airports and other communication means.

In some cases the same engines are used both during take-off and for conventional flight where the weight of the aircraft is supported by the wing lift and the required thrust is significantly reduced.

In the first phase, for take-off and vertical flight, it is necessary to use propellers of large diameter and very small pitch, thus achieving high static thrusts. These propellers cannot be used for translated flight, as the blade pitch could generate resistance rather than thrust.

This is problematic in a vertical take-off aircraft commonly referred to as a tiltrotor, where the same propellers are used for both take-off and vertical flight and for cruise flight, by means of propeller axis tilting means. Depending on the type of these tilting means, tiltrotors are divided into three categories: a first category provides for the rotation of the entire aircraft and is therefore identified with the term "tailsitter"; a second category provides for the rotation of the engine unit only; a third category provides for the rotation of the aerodynamic surface to which the engine is connected.

In such aircraft, regardless of the category to which they belong, variable pitch propellers must be used, since the same propellers are used for both flight modes. However, this solution increases the complexity, cost and weight of the engines. In addition, even it the propellers designed for vertical take-off provide variable pitch control, they cannot be optimized for horizontal flight. This is because they have a completely incorrect pitch extension for cruise flight, which leads to very low yields. Document <CIT> discloses a system with a first propeller attached to a driveshaft through a first one-way bearing for allowing rotation in first direction and resist rotation in second direction that is opposite the first direction and a second propeller attached to the driveshaft through a second one-way bearing for allowing rotation in the second direction and resists rotation in the first direction.

The present invention aims to overcome this limit of the currently known propulsion devices with a device as described at the beginning, in which said propellers further have a fixed and different pitch therebetween, having a first said propeller with a smaller pitch and having a second said propeller with a larger pitch.

In the case of vertical take-off aircraft, the first propeller may then be used to exert a thrust during take-off and vertical flight and the second propeller to exert a thrust during cruise flight and also during take-off and vertical flight.

In this way the first propeller has the pitch optimized for the take-off phase and for vertical flight while the second propeller has the pitch necessary for cruise flight. Both propellers are used during the take-off phase and, being counter-rotating, provide a much greater thrust than that of the first propeller alone. During translated flight, however, only the second propeller is used.

In the present invention, the first propeller is positioned in front with respect to the second propeller, in the forward direction of the aircraft.

This is particularly advantageous because the second propeller, having optimized pitch for cruise flight, operates more efficiently when it encounters moving air. In this way, therefore, the second propeller, when operational, always encounters moving air: in the take-off and vertical flight phase the air is moved by the first propeller, which precedes the second propeller; in the cruise flight phase, in which the first propeller is stopped, the air still has a relative speed due to the forward movement of the aircraft.

In one embodiment, the first propeller is provided with means for feathering the blades, i.e. oriented to lie on one or more planes parallel to the movement direction of the aircraft.

Such means may be of any currently known type and preferably are passive and such that they automatically allow for feathering during translated flight when the propeller is stopped. In this way the first propeller may be stopped in the initial cruise phase of the flight to allow the thrust of the second propeller only and may further be put in feathered position so as not to generate resistance.

According to the present invention, the device is provided with one or more electric motors to power the propellers. The propulsion device may then be advantageously used for electric air mobility, for example in passenger or cargo drones, reducing complexity, weight and costs.

In a first embodiment, the first propeller is driven by a first electric motor and the second propeller is driven by a second electric motor.

This allows the rotational speed and torque to the first and second propellers to be adjusted completely independently.

In addition, to stop the first propeller, it is sufficient to stop the relative first electric motor which drives it.

In a second embodiment, the propellers are driven by a single electric motor, the first propeller being connected to a first rotor of said electric motor and the second propeller being connected to a second rotor of said electric motor.

This second variant has the great advantage of providing a single electric motor in which, instead of having a rotor and a stator, where the rotor rotates within a stationary stator, the stator is used as a second rotor and rotates in the opposite direction to the first rotor.

According to a refinement, a brake is provided for stopping the first propeller.

In this way, at the beginning of the cruise flight phase it is possible to stop the first propeller, preferably in a desired position, bringing the relative rotor into stator condition and allowing only the rotor relative to the second propeller to move.

Both of the embodiments described above are particularly advantageous because they relate to a "fail-safe" configuration, in which each propeller may operate independently of the other. If one of the propellers stops accidentally, the other propeller is able to exert the necessary thrust for the two different flight phases, also for the flight phase for which it is not optimized.

In a further embodiment an outer propeller containment conduit is provided, normally referred to as an intubated propeller.

This improves performance and reduces noise. Such a conduit may be similar to that used in aircraft commonly referred to as "turbofan".

Alternatively, the propellers may be provided with the ends of the blades free in the air.

An object of the present invention is also an aircraft provided with carrier wings for cruise flight, comprising at least one propulsion device as described above.

In a preferred embodiment, the aircraft is of the vertical take-off type and may be provided with means for tilting the axis of rotation of said propellers from a vertical position for take-off and vertical flight to a horizontal or substantially horizontal position for cruise flight.

In an embodiment, at least one said propeller device is positioned behind a wing, the first propeller being provided with two blades and first propeller locking means being provided in a cruise condition, in which the blades of the first propeller are feathered and lie on the plane of the relative wing.

In this way, the first propeller is aligned and "hidden" behind the wing profile, and minimizes the aerodynamic resistance of the aircraft.

The invention also relates to a propulsion process according to claim <NUM>.

In a preferred embodiment of the method, the aircraft is of the vertical take-off type and, prior to take-off, a positioning of the rotation axis of the propellers along a vertical direction is provided, and, after take-off, a tilting of the rotation axis of the propellers from a vertical position to a horizontal or substantially horizontal position is provided.

According to an embodiment of the procedure, after the first propeller is stopped, the blades of such first propeller are feathered.

These and other features and advantages of the present invention will become clearer from the following description of some non-limiting exemplary embodiments illustrated in the attached drawings in which:.

<FIG> shows a first embodiment variant of the aeronautical propulsion device for a vertical take-off aircraft according to the present invention.

The propulsion device comprises at least two coaxial fixed pitch propellers counter-rotating therebetween, of which a first propeller <NUM> with smaller pitch to exert a thrust during take-off and vertical flight and a second propeller <NUM> with larger pitch to exert a thrust both during take-off and vertical flight and during cruise flight.

The aircraft is also provided with rotation axis tilting means of the propellers from a vertical position for take-off and vertical flight to a horizontal position for cruise flight. The tilting means are preferably of currently known type and are not illustrated in the figures.

The first propeller <NUM> is positioned in front of the second propeller <NUM>, in the forward direction of the aircraft. The propellers <NUM> and <NUM> may have any number of blades <NUM> and <NUM>. Preferably both the first propeller <NUM> and the second propeller <NUM> are each provided with a pair of first blades <NUM> and second blades <NUM> respectively, each pair of blades being arranged at <NUM>° to each other. Compared to the second propeller <NUM>, the first propeller <NUM> not only has a smaller pitch, optimized for vertical take-off, but also has a larger diameter, to ensure the necessary thrust.

The first propeller <NUM> is provided with feathering means <NUM> of both the first blades <NUM>. Such means <NUM> may be of any currently known type and preferably are passive and such that they automatically allow the feathering of the first blades <NUM> during translated flight when the first propeller <NUM> is stopped, as illustrated in <FIG>.

In the embodiment variant of <FIG>, the propellers <NUM> and <NUM> are driven by a single electric motor <NUM>. The first propeller <NUM> is connected to a first rotor of the electric motor <NUM> and the second propeller <NUM> is connected to a second rotor of the electric motor <NUM>. In this variant, since there are no static parts of the motor <NUM>, it is necessary to provide a rotating manifold <NUM> for the power supply of the motor <NUM>.

A brake <NUM> is also provided for stopping the first propeller <NUM>, such that at the beginning of the cruise flight phase it is possible to stop the first propeller <NUM>, bringing the relative rotor to stator condition and allowing only the rotor relative to the second propeller <NUM> to move.

In the embodiment of <FIG> the first propeller <NUM> is driven by a first electric motor <NUM>' and the second propeller <NUM> is driven by a second electric motor <NUM>".

In this case, to stop the first propeller <NUM> it is sufficient to stop the relative first electric motor <NUM>' which drives it. A first motor <NUM>' of the type such that it is possible to stop the first propeller <NUM> at a desired position may be provided.

<FIG> and <FIG> both illustrate a configuration with the propulsion device positioned in front of the wing of the aircraft along the forward direction. However, the device may be provided in other suitable positions on the aircraft, in particular also behind the wing. In the case of the propulsion device positioned behind of the wing, if the first propeller <NUM> is provided with two first blades <NUM> as in the figures, it is possible to provide for locking means of the first propeller <NUM> in a cruise condition, in which the first blades <NUM> of the first propeller <NUM> are feathered and lie on the plane of the relative wing. The locking means of the first propeller <NUM> may for example be formed by the brake <NUM> in the variant of <FIG>. Alternatively, the locking means may be formed by the first motor <NUM>' in the variant of <FIG>.

The propellers <NUM> and <NUM> may be provided with the ends of the blades <NUM> and <NUM> free in the air or alternatively an outer propeller containment conduit <NUM> and <NUM>, not illustrated in the figure, may be provided.

<FIG> illustrates a front view of the device, in which the first propeller <NUM> and the second propeller <NUM> rotate in a reverse direction relative to each other, as indicated by the arrows. This operating arrangement corresponds to the take-off and vertical flight phase, in which both propellers <NUM> and <NUM> are operated.

In <FIG>, instead, the first propeller <NUM> is stopped and placed with the first blades <NUM> in the feathered position. This further operating arrangement corresponds therefore to the cruise flight phase, in which the first propeller is stopped and only the second propeller <NUM> is operated.

<FIG> shows a diagram of the propulsion process of a vertical take-off aircraft object of the present invention.

The process uses the propulsion device described above and includes the following steps.

The propulsion device is initially positioned at step <NUM> in a position adapted to vertical take-off, i.e. with the rotation axis of the first propeller <NUM> and the second propeller <NUM> along a vertical direction.

Both propellers <NUM> and <NUM> are then driven in rotation in step <NUM> and, at the right thrust, this causes the aircraft to take off (step <NUM>).

The propulsion device is then rotated at step <NUM>, causing a tilting of the rotation axis of the propellers <NUM> and <NUM> from a vertical position to a horizontal position.

Then the first propeller <NUM> is stopped (step <NUM>) and with the feathering of the first blades <NUM> of this first propeller <NUM> (step <NUM>).

Claim 1:
An aeronautical propulsion device for an aircraft provided with carrier wings for cruise flight, the propulsion device comprising at least two coaxial propellers (<NUM>, <NUM>), which propellers (<NUM>, <NUM>) have a fixed and different pitch therebetween, having a first said propeller (<NUM>) smaller pitch and having a second said propeller (<NUM>) larger pitch, being the first propeller (<NUM>) placed in front of the second propeller (<NUM>), in the forward direction of the aircraft
characterized in that
said propellers are connected to one or more electric motors so as to be driven at least temporarily simultaneously in a counter-rotating way therebetween.