WO2015056124A1

WO2015056124A1 - Propulsion system for vertical or substantially vertical takeoff aircraft

Info

Publication number: WO2015056124A1
Application number: PCT/IB2014/064961
Authority: WO
Inventors: Mario Burigo; Marcello Vignoli; Marco MATTEONI; Gianni SANTARELLI; Fabrizia DI MASCIO
Original assignee: Navis SRL
Current assignee: Navis SRL
Priority date: 2013-10-14
Filing date: 2014-09-30
Publication date: 2015-04-23
Anticipated expiration: 2016-04-14

Abstract

The present invention relates to a propulsion system for aircraft, comprising: • - a primary propulsion system (PI) for generating horizontal thrust needed for cruise; • - a secondary propulsion system (RAPS) for generating vertical thrust so that the aircraft is supported and/or lifted in a configuration other than the cruise one; • - a control system (RCU) intended to control the primary and secondary propulsion system. The secondary propulsion system (RAPS) comprises a plurality of additional thrust generators (RTG), in particular propellers and/or fans (100/103/104), powered by rechargeable energy storage devices RRSU). The energy storage devices (RRSU) and the secondary propulsion system (RAPS) are dimensioned so that they can lift the aircraft without other means.

Description

PROPULSION SYSTEM FOR VERTICAL OR SUBSTANTIALLY VERTICAL TAKEOFF AIRCRAFT

DESCRIPTION TECHNICAL FIELD

The present invention relates to the field of propulsion systems for aircraft and in particular to propulsion systems for Vertical Take-Off and Landing aircraft (VTOL) or Vertical and /or Short Take- Off and Landing aircraft (VSTOL).

In particular, the present invention is preferably applied in the field of vertical or nearly vertical takeoff aircraft, which use a main propulsion system for cruise and an auxiliary propulsion system, which intervenes in the takeoff /landing phase.

In particular, the invention relates to a propulsion system according to the preamble of claim 1 and to a respective aircraft vehicle.

STATE OF THE ART

It is known that one of the main limits of airplanes (whether they are manned or Unmanned Aerial Vehicles, UAV) concerns the takeoff/ landing phases, which can be carried out only on large takeoff/landing surfaces at least some hundreds meters long. It is also known that there exist aircraft able to overcome this limit as they can takeoff /land vertically or almost vertically,

Examples of these aircrafts are helicopters, "Harrier" type aircraft and convertiplanes.

However, vertical or nearly vertical takeoff aircraft have some drawbacks as well, that have limited their use in time. One of this drawbacks in common with the most part or almost with all the vertical or vertical and short takeoff aircrafts is that, for takeoff, all these aircrafts need very high thrust (higher than the aircraft weight) which is obtained by means of very powerful engines, which during cruise (horizontal flight) are little used.

In order to overcome this drawback, many ideas have been proposed, for example known from the following US patents and patent applications: US 4,125,232, US 4,828,203, US 7,267,300, US 2006/0226281, US 7,472,863, US 2004/094662, US 7,461,811, US 2003/062442, US 6,892,979, US 2012/0012692 Al, US 6,464,166, US 2003/080242, US 2007/0057113, US 2008/0054121, US 2002/113165, US 6,488,232. In particular, the US patent US 7,857,254 relates to a V/STOL vehicle (Vertical /Short Take Off and Landing) equipped with a main internal combustion engine which can be rotated between a horizontal position for cruise and a substantially vertical one for takeoff and landing. The vehicle is further provided with a second propulsion system which is used only for the vertical takeoff and landing phases. This second propulsion system is provided with fans which are driven by electric engines and provide vertical thrust which is added up to the main engine thrust for allowing takeoff and landing. The electric engines driving the fans are powered by an electric generator, which in turn is powered by the main thermal engine. In an embodiment of the invention, there are batteries provided to give energy to the electric engines, but for takeoff and landing it is always needed also the thrust provided by the main internal combustion engine.

Even if efficient, the solution proposed by US 7,857,254 has the drawback that for takeoff and landing it is always needed also the thrust provided by the main internal combustion engine which has to be rotated in vertical position, which is a complex operation. This because the flow areas for vertical thrust are limited resulting in a high value of the power to aircraft weight ratio, which has to be supplied mainly by the main engine. Moreover, the electric batteries, as the litliium ones there described, have a quite limited duration: generally, they have to be substituted after 100-200 recharge cycles.

From the patent applications WO2014/021798 and WO2006/ 13877 there are known vertical takeoff aircraft equipped with a primary propulsion system, which gives the horizontal thrust during the flight, and a secondary propulsion system for lifting the aircraft vertically, In both cases, vertical takeoff energy is taken from the engine of the primary propulsion system, which has to be over-dimensioned to provide energy both for vertical takeoff and horizontal thrust.

OBJECTS AND SUMMARY OF THE INVENTION

Aim of the present invention is to provide a propulsion system for aircraft and a respective aircraft using such propulsion system, which overcomes some drawbacks of the prior art.

In particular, aim of the present invention is to provide a vertical or almost vertical takeoff and landing aircraft with low realization costs.

These and other aims are reached by means of a propulsion system for aircraft, which is provided with the features of the appended claims, which are integral part of the present description.

The basic idea of the present invention is to provide an aircraft with a propulsion system and an aerodynamic configuration intended for horizontal translation, and a propulsion system and an aerodynamic configuration intended for takeoff and landing, wherein this latter propulsion system is powered only by the energy stored in suitable energy storage devices.

This allows to optimize the propulsion system dimensioning which is being used during cruise since the energy needed for takeoff is obtained only by storage devices, as for example batteries, which can be possibly recharged at ground.

In an embodiment, the present invention relates to an auxiliary propulsion system for manned or UAV airplanes (Unmanned Aerial Vehicle) with a primary thermal engine based on one or more rechargeable energy storage systems able to provide high specific power for a sufficiently long discharge time interval to be used with dedicated thrust generators, which give vertical thrust levels higher than the aircraft weight, so that it can take off and land vertically or almost vertically. In such a great meaning scope, the whole aircraft power system is here defined "hybrid" since it is made up of at least a primary aeronautical engine and additional engines (not necessarily internal combustion engines), which use energy stored in kinetic, electrochemical and electric form.

In an embodiment of the invention, the propulsion system for aircraft comprises:

- a primary propulsion system for generating at least horizontal thrust for the aircraft to fly in cruise configuration,

- a secondary propulsion system intended to generate vertical thrust needed to support and/or lift the aircraft in a takeoff /landing configuration other than the aircraft cruise configuration,

- a control system for controlling the primary and secondary propulsion system in order to provide the needed thrust for the aircraft to fly.

The secondary propulsion system comprises a plurality of additional thrust generators, in particular propellers and/ or fans, which generate vertical thrust for lifting the aircraft. The additional thrust generators are driven by one or more engines powered by rechargeable energy storage devices, which are dimensioned so that they allow the aircraft to be lifted by only the additional thrust generators. Such a system has never been realized since it has never been thought to combine an aircraft with a propulsive configuration variable between takeoff and landing with great flow areas for vertical propulsion, and a cruise one with good aerodynamic efficiency for horizontal flight, with particular storage systems suitably designed for providing great power to energy ratios. Therefore, it is useful to make some considerations about the secondary propulsion system dimensioning, in particular concerning the flow areas during takeoff and landing and the power and capacity of the storage devices for possible takeoff and landing trajectories.

The helicopters, which among vertical takeoff aircraft are the ones with the highest propulsive efficiency thanks to the very high flow area of the rotor disk (or disks) (the total propulsive flow area to the maximum takeoff weight ratio is about 0,025 mq/kg) have an engine or engines installed power ratio which rarely goes under the value of 0,30 kW/kg. For the convertiplanes which have smaller flow areas (in this case the total flow area to the maximum takeoff weight ratio is about 0,008 mq/kg) the installed power to weight ratios is about 0,35 kW/kg. Naturally, the installed power values on helicopters and convertiplanes are higher than the ones strictly needed for vertical takeoff and landing. For the present invention, as yet said, it is not realistic to have high flow areas as for the helicopters ones, but by providing more than one propulsive, yet deployable, unit the convertiplane values can be approached (total flow areas to maximum takeoff weight ratios of about 0,008 mq/kg) and it is possible to have ratios between the power which will be supplied to the propulsion units of the present invention and the maximum takeoff weight of the aircraft similar to the convertiplanes ones. Moreover such power will have to be provided for a sufficient interval time for ensuring vertical or almost vertical takeoff and landing operations with a certain margin.

The simulations carried out for possible takeoff and landing trajectories, as indicated in figures 2, 3, 4 and 5, show that the needed interval time for takeoff and landing is about 20 seconds,

Therefore the result is that the storage devices power to aircraft maximum takeoff weight ratio is roughly 0,35 kW/kg for the realistically hypothetical flow areas and that the energy needed to obtain the takeoff expressed as the stored energy to aircraft maximum takeoff weight ratio is roughly about 0,00145 kWh/kg.

So, in an embodiment of the invention, the energy storage devices have a very high power (kW)/capacity (kWh) ratio (higher than 100 kW/kWh) and have to be suitably dimensioned for this application to have weights compatible with aeronautical applications. Devices of this type are very high speed flywheel devices of new generation or some storage devices exploiting combinations of special polymer electrochemical batteries and /or capacitors which have to be necessarily built by combining few volts elementary cells so that the very high power /capacity ratio desired is obtained to be able to maintain the weights at values compatible with the aeronautical applications. The conventional batteries have very lower power to capacity ratios and if energy of such systems is extracted in very short times, i.e, with great power as requested in takeoff and landing phases, there would be a great stress and heating which would reduce the duration to few cycles. However, the weight limits for the aeronautical usage would not allow to install great systems of such batteries to obtain the needed powers.

In an embodiment of the invention, the storage devices are of flywheel type and comprise a couple of flywheels intended to rotate in opposed direction. In this embodiment, the control system is configured to receive an attitude signal relating to the aircraft attitude (for example by one or more aircraft sensors). In response to receiving said attitude signal, the control system controls the rotation speed of the couple of flywheels, so that it is generated a moment needed to control the aircraft attitude.

Therefore, this solution allows the storage systems to be exploited both for energy storage and release needed for takeoff/ landing, and for controlling the aircraft attitude.

In an embodiment of the invention, the energy storage devices are of flywheel type and the auxiliary propulsion system comprises at least a dynamo connected to the flywheel so that the mechanical motion of the flywheel is converted in electric energy and the electric engine, which actuates at least one of the additional thrust generators, is supplied.

In a further embodiment of the invention, the secondary propulsion system comprises an energy recovery system, in particular kinetic and /or thermal energy dissipated by at least a primary engine of the primary propulsion system. The energy recovery system is operatively connected to the energy storage devices so that the energy storage devices are charged by means of energy recovered by the engine of the primary propulsion system.

Advantageously, in an embodiment of the invention, for each thrust generator it is provided at least a respective energy storage device which supplies it and which is positioned near the thrust generator.

This solution allows to reduce the weight of the wires needed to bring energy from the storage devices to the additional thrust generators. Considering the values of power and the realistically usable voltages, the wires can have great sections and can be heavy.

In an embodiment of the invention, the thrust generators can be moved between a first takeoff configtiration, in which they generate vertical thrust on the aircraft, and a second cruise configuration in which they allow the airplane to maintain aerodynamic efficiency needed for the cruise flight.

Moreover, the control system is preferably intended to control the position of the thrust generators by setting them up so that the thrust of such thrust generators contributes to speed up or slow down the aircraft.

In an embodiment of the invention, the propulsion system comprises a recharge system of at least one of the energy storage devices; such recharge system comprises a connection for connecting to an electric or kinetic energy generator outside the aircraft, and is operatively connected to one of the energy storage devices and it is intended to transfer and/or convert the energy absorbed by the generator outside the aircraft.

This solution is advantageous in that it allows the recharge of the storage devices at ground.

According to an embodiment of the invention, the energy storage devices can be mechanically or electrically connected to a primary engine of the primary propulsion system, so that the mechanical or electric energy directly or indirectly generated by said energy storage devices is exploited to operate the primary propulsion system. This solution allows, in case of failure of an engine of the primary propulsion system, to take energy for continuing the cruise flight from the storage systems. The invention further relates to an aircraft comprising a propulsion system provided with one or more features described or claimed in the following.

The invention further relates to a method of takeoff and a landing one of aircraft provided with a propulsion system comprising one or more features described or claimed in the following.

In particular, object of the present invention is a method for carrying out aircraft takeoff, comprising the phases of:

a. recharging energy storage devices at ground;

b. arranging aircraft takeoff configuration with additional thrust generators configured for generating vertical thrust of the aircraft;

c. driving the additional thrust generators using the energy available in the storage devices for generating vertical thrust higher than the aircraft weight up to reach height compatible with the transition maneuver to cruise flight, suitably controlling the different propulsive flows generated by the additional thrust generators to maintain the desired attitude and trajectory;

d. activating the primary propulsion system to be able to reach the translational speed needed to generate sufficient wings lift; e. deactivating the secondary propulsion system and transition to the cruise configuration;

f. continuing the flight using the primary propulsion system;

g. recharging the storage devices deriving energy from the primary engine of the aircraft and /or from auxiliary generators on board, in particular thermal energy recovery units or photovoltaic cells.

Preferably, during takeoff the thrust direction of the additional thrusters is not kept constant but, when the primary propulsion system is actuated, it is gradually oriented or deviated in horizontal direction so to contribute to increase the translational speed of the airplane, thus reducing at the same time progressively its vertical component as the overall wing lifts increases.

Still, another object of the present invention is a method for carrying out landing of an aircraft comprising a propulsion system according to any one of the appended claims, comprising the steps of:

a. checking the charge state of the energy storage devices;

b. if needed, recharging the energy storage devices, deriving energy from the primary engine of the aircraft and/or from the auxiliary generators on board, in particular thermal energy recovery units or photovoltaic cells;

c. beginning descent towards height compatible with the transition maneuver from the cruise flight to the vertical one, slowing down the speed of the airplane at the same time up to a safe condition of slow flight;

d. arranging the landing configuration with the additional thrust generators configured to provide thrust with such value and direction that a desired flight trajectory is realized;

e. deactivating the primary propulsion system;

f. gradual reducing the vertical thrust of the additional thrust generators, so that the aircraft is allowed to descend following a desired trajectory and descent speed, controlling attitude and trajectory by means of orientation and flow control of each thrust generator;

g. once landing is ended, deactivating the secondary propulsion system. Other advantageous features of the present invention are object of the dependent appended claims, which are intended to be integral part of the present description. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following with reference to not limiting examples provided as explanatory purpose and not limitative of the appended drawings. These drawings show various aspects and embodiments of the present invention and, where appropriate, reference numbers showing structures, components, materials and/or elements similar throughout the various figures are indicated with the same reference number.

Fig. 1 shows a scheme of a propulsion system according to the present invention.

Fig. 2 shows the takeoff trajectory of an aircraft of 500 kg weight, provided with a 1000 N primary thruster and with a secondary propulsion system made up of two 120 kW/33s storage units, up to reach horizontal speed of 100 km/h.

Fig. 3 shows the takeoff trajectory of an aircraft of 3000 kg weight, provided with a 7500 N primary thruster and with a secondary propulsion system made up of two 360 kW/33s storage units, up to reach speed of 150 km/h.

Fig. 4 shows the landing trajectory of an aircraft travelling at speed 100 km/h, of 500 kg weight, provided with a 1000 N primary thruster and with a secondary propulsion system made up of two 120 kW / 33s storage units.

Fig. 5 shows the landing trajectory of an aircraft travelling at speed 150 km/h, of 3000 kg weight, provided with a 7000 N primary thruster and with a secondary propulsion system made up of four 360 kW/33s storage units.

Fig. 6 shows an embodiment of the invention with kinetic type storage units in quantity equal to the mechanically actuated additional thrust generators.

Figs, 7a and 7b show a possible installation of the invention using foldable and swinging arms in the takeoff /landing configuration and the cruise one, respectively.

Figs. 8a and 8b show a possible installation of the invention for canard aircraft in the takeoff/landing configuration and the cruise one, respectively.

Figs. 9a and 9b show another possible installation of the invention.

Fig. 10 shows a possible installation of the invention with the fixed additional thrust generators integrated in the aircraft structure.

Figs. 11a, lib, 11c show a possible lower cover mechanism of the additional thrust generators in the open position, partially open to deviate the air flow, and in the closed one.

Fig. 12 shows an embodiment of the invention with kinetic type storage units in quantity equal to the electrically actuated additional thrust generators.

Fig. 13 shows an embodiment of the invention with centralized kinetic type storage units and electrically actuated additional thrust generators.

Fig. 14 shows an embodiment of the invention with centralized electrochemical/electric type storage units and electrically actuated additional thrust generators.

Fig. 15 shows a particular type of thrust generator which can be used in the present invention.

Fig. 16 shows an embodiment of the invention with both centralized kinetic and electrochemical /electric storage units and electrically actuated additional thrust generators.

Fig, 17a shows a takeoff operation using the claimed system.

Fig. 17b shows the execution of the takeoff operation controlling also the aircraft attitude to direct the propulsive flows.

Fig. 18a shows the execution of the landing operation using the claimed system.

Fig. 18b shows the execution of the landing operation controlling also the attitude of the aircraft to direct the propulsive flows.

DETAILED DESCRIPTION OF THE INVENTION

While the invention is susceptible of many modifications and alternative constructions, some preferred embodiments thereof are shown in the drawings and will be described in detail in the following. However, it is to be intended that the present invention is not limited to the shown embodiment, but on the contrary, the invention is intended to cover all the modifications, alternative constructions and equivalents in the scope of the invention as claimed,

The word or phrase "for example", "etc.", "or" indicates not exclusive alternatives without limitation, unless otherwise stated. The word "comprises" means "comprises but not limited to", unless otherwise stated.

The word "aircraft" is intended to comprise any vehicle able to fly.

The word "cruise flight" is intended to refer to substantially horizontal flight of the aircraft, with possible alternating ascending and descending phases obtained only by varying the aircraft lift, for example by acting on the aircraft speed or on the wing profile, without the vertical thrust generators contribute thereto.

In the following it will be described a propulsion system for manned or UAV airplanes eqwuipped with a primary propulsion system for the cruise flight and a secondary propulsion system, called RAPS (Rechargeable Auxiliary Propulsion System) mainly based on high power density rechargeable energy storage devices for weight unit. While the example is applied to an airplane, such propulsion system can be applied to any aircraft.

In an embodiment of the invention, schematized in fig. 1, the propulsion system of the aircraft comprises a primary propulsion system (PI) used for cruise flight, and a secondary propulsion system (RAPS) comprising:

- a storage section (P2) with one or more rechargeable energy storage devices (called RRSU, acronym of RAPS Rechargeable Storage Unit) both in kinetic (for example by means of flywheels) and in electrochemical or electric form (batteries and /or su er- capacitors). A storage section can be made up of a combination of the various types of storage devices as well. In the preferred embodiment here described, RAPS is provided with flywheel storage systems.

- A recharge section (P3) comprising one or more RRU devices (RAPS Recharge Units) able to derive energy from different sources arranged on board of the aircraft, for example mechanical energy from the aircraft engine or from auxiliary turbines (as RAT), electric energy from the aircraft engine, outer generators, photovoltaic cells, on board electricity or from the thermal energy recovery from the primary thermal engine.

- A thrust section (P4) comprising one or more RTG thrust generators (RAPS Thrust Generator) capable of transforming the energy provided thereto in an acceleration of a stream of air to obtain total thrust higher than the aircraft weight. The RTG units are substantially propellers and/or fans, ducted and/or counter-rotating as well and have flow areas sufficiently high to ensure good propulsive efficiency. The RTG units are arranged in the most convenient positions of the aircraft and can be both fixed and deployable in takeoff and landing phases to be refolded and/or covered by suitable ports at the end of these phases in order not to penalize the aircraft aerodynamics during the normal flight phases. In particular, the deployable RTGs allow to obtain higher flow areas ensuring thereby greater thrust efficiency. Therefore, the aircraft is provided with a configuration for takeoff and landing with open and/or deployed RTGs and a configuration for cruise with closed and/or folded RTGs. The propulsive flow of each RTG unit can be directed around axes parallel to the aircraft pitch axis and/or modulated in module to be able to generate the desired total thrust and the rotation torques around the pitch, roll and yaw axes to control the aircraft attitude and its trajectory. Given the lightness and compactness of its components, RTGs can be driven by a double engine and can be possibly counter-rotating, and connected to engines so that failure of an engine is admitted and have negligible reaction torques. The RAPS system can allow also to interrupt the takeoff and landing maneuvers at any time, and to bring back the aircraft to safe operative conditions. The thrust generators can be both of mechanical type driven by a driving shaft and of electric type and therefore driven by a dedicated electric engine.

- A control section P5, called RCU (RAPS Control Unit), which in the example of fig, 1, is made up of all the elements needed to manage the aircraft propulsion system (both the primary one, PI, and the secondary one, RAPS) in total autonomy and/or partial and /or total direct control of the pilot, which comprise sensors and actuators for:

- transition from the cruise configuration to the takeoff and landing configuration,

- controlling the aircraft attitude and its trajectory during takeoff and landing,

- transition from the takeoff and landing configuration to the cruise configuration, - monitoring and charging the storage devices of RAPS, In an embodiment of the invention, RCU comprises a memory area in which it is stored a software APTOL (Assisted Precision Take Off and Landing). When executed, APTOL allows vertical takeoff and landing automatically; RCU considers the signals received from ground (for example transmitted by a control tower) and the ones received by the sensors on board and coordinates the RTGs thrust to carry out takeoff and landing. Preferably, the pilot can make slight changes or corrections to the operation carried out by the RCU according to APTOL, and if necessary, he cannot choose the assisted operation. The supporting surfaces of the aircraft can be provided with suitable flaps so that the transition from the vertical flight to the horizontal one and vice versa is made easier.

RTGs are powered by energy taken only from the storage devices in the storage section,

The storage section comprises at least a rechargeable energy storage device (RRSU) with very higher capacity both in terms of available stored energy and power than the one usually provided in aircraft (to start engines and some on board functions), in particular the RRSU devices have to be preferably provided with a stored energy capacity higher than 0,001 kWh per kg of aircraft weight, a usable power higher than 0,20 kW per kg of the aircraft and a power to capacity ratio of at least 100 kW/kWh. These values of capacity and power derive from the considerations in the following. The helicopters, which among the vertical takeoff aircraft are the ones with the greatest propulsive efficiency thanks to the very large area of the rotor disk (or disks) (the total propulsive flow area to the maximum takeoff weight ratio is about 0,025 mq/kg) have a ratio of installed power of the engine or engines to the helicopter weight which rarely goes under the value of 0,30 kW/kg. The convertiplanes with smaller flow areas (in this case the total flow area to the maximum takeoff weight ratio is about 0,008 mq/kg) have installed power to weight ratios of about 0,35 kW/kg. The installed power values on helicopters and convertiplanes are higher than the ones strictly needed for vertical takeoff and landing. For an aircraft, as the one described in the following, it is not realistic to have large flow areas as for the helicopters ones, but by providing more than one additional thrust generators, yet deployable, the convertiplane values can be approached (total flow areas to maximum takeoff weight ratios of about 0,008 mq/kg) and it is possible to have ratios between the power which is going to supply the RTG propulsion units and the maximum aircraft takeoff weight similar to the convertiplanes ones. Moreover such power will have to be provided for a sufficient interval time for ensuring vertical or almost vertical takeoff and landing maneuvers with a certain margin.

By using RAPS there can be considered three takeoff phases:

a. a first vertical or almost vertical flight phase up to certain height, which depends on the surrounding obstacles present, on the weather conditions (in particular wind speed) and on possible air traffic, during which the RAPS is operating to provide a resultant thrust, mainly directed upwards, controlled in magnitude and direction. b, a following transition phase in which, reached a certain height, while keeping RAPS operative, it is also used the aircraft primary propulsive system to accelerate the aircraft up to reach translation speed sufficient to have the needed wings lift to sustain the aircraft. During this phase the total thrust provided by RAPS can be progressively directed from the vertical direction to the horizontal one since the progressive increase in wing lift will reduce progressively the need of the vertical thrust generated by RAPS. As an alternative, the thrust module of RAPS can be reduced while maintaining the mainly upwards thrust direction.

c. a cruise flight phase only with the primary propulsion system.

Concerning the reactions and the inertial effects of RAPS system on the aircraft, they depend on the number of flywheel and rotors available, their arrangement and on rotating/ counter-rotating pairs which are provided and which can be managed at advantage of the maneuverability and stability of the same aircraft.

By providing at least a couple of flywheels intended to rotate in opposed direction and controllable by the control system RCU of RAPS, it is possible to exploit the storage section to control the aircraft attitude. Therefore, the control system RCU is configured to receive an attitude signal relating to the aircraft attitude (for example a signal emitted by a suitable sensor) and, in response to receiving said attitude signal, to set up the rotation speed of the flywheel couple, so that a moment needed to set up the aircraft attitude is generated.

The amount of energy to be stored can be obtained by using, before takeoff, the aircraft engine for a certain time interval (for example 30 kW for 5 min, or 50 kW for 3 min) and/or recovering part of heat produced by combustion, for example by means of units as TERS (Thermal Energy Recovery System) or MGUH (Motor Generator Unit - Heat), recently developed for the automobile races, The energy needed to charge the accumulators before takeoff can be also obtained by using an outer charge device.

Figs. 2 and 3 show two possible takeoff trajectories, for an aircraft provided with RAPS, weighing about 500 kg, with primary thruster of 1000 N and minimum safe speed of 100 km/h (fig. 2) and for an aircraft provided with RAPS, weighing 3000 kg, with traditional thrust of 7500 N and minimum safe speed of 150 km/h (fig. 3), respectively,

The takeoff trajectories chosen in these examples are more extreme and limiting than the one usually followed by helicopters and this is to highlight the advantages resulting by using RAPS.

The trajectories of examples of figs. 2 and 3 are obtained by dimensioning RAPS to obtain a value of power to weight ratio of about 0,50 kW/kg, very higher than the one needed to lift the helicopters and the convertiplanes. Therefore for the aircraft of 500 kg it is considered, in the analyses, a RAPS comprising 2 120 kW/33s storage units (i.e. units able to provide 120 kW per 33s) to give a maximum additional propulsive thrust of 6000 N and for the one of 3000 kg a RAPS comprising 4 360 kW/33s units to obtain a maximum additional propulsive thrust of 36000 N,

Once in flight and before landing the units can be recharged in two ways:

- by means of the engine of the primary propulsion system, by using part of its available power (the engines are generally provided with higher power than the ones needed for cruise) and/or by recovering part of thermal /kinetic energy produced by the engine by means of a suitable energy recovery system, for example MGUH unit.

- by means of on board generators, as for example turbines suitably mounted as RAT (Ram Air Turbine)_/ known and mounted on large aircraft to be used as emergency power sources in case of total failure of engines. RATs are usually extracted from the fuselage only in case their usage is required, thus generating energy thanks to the air flow due to aircraft speed. In alternative or in addition to RATs, as on board generators there can be also used photovoltaic cells mounted on the aircraft; this solution seems to be advantageous mainly in some types of UAV.

The power used to recharge the accumulators while in flight has to be compatible with the flight time and will depend on the accumulators residual charge level after takeoff.

Similarly, concerning landing, three steps can be considered:

a. the first cruise flight phase in which it is however needed to go down to certain height and speed compatible with the aircraft features, with possible local air traffic and surrounding obstacles;

b. a following phase in which the secondary propulsion system is switched to the takeoff /landing configuration and then activated to provide, by modulating its value, additional thrust such that the desired flight trajectory in deceleration up to the stationary flight is achieved thus realizing the desired rate of descent, possibly also deriving energy from the primary engine. During the transition from the cruise configuration to the takeoff/ landing one according to the chosen solutions the lift and drag of the aircraft will change and have to be properly considered in order to maintain a safe flight during this transition;

c. a final phase in which the primary propulsion system is deactivated and RAPS additional thrust is gradually reduced so that the aircraft goes down following the trajectory with desired descending speed, while controlling the attitude and direction by means of propulsive flows generated by each RTG.

Figs. 4 and 5 show two possible landing trajectories of two aircrafts provided with RAPS; figure 4 refers to an aircraft of 500 kg which reaches in normal flight the height of 100 m with speed of 100 km/h, while figure 5 refers to an aircraft of 3000 kg which similarly reaches 150 m with speed of 150 km/h.

Also in this case, the chosen landing trajectories are much more demanding than the ones usually followed by helicopters to highlight the advantages of RAPS.

RAPS system will allow also to interrupt the landing or takeoff maneuvers at any time and to bring back the aircraft to safe operative conditions.

In the following, as a way of describing and not as limiting example, there is described an aircraft with four or six RTG units suitably arranged so that an easy controlling of the aircraft is possible during takeoff and landing without strongly affecting the constructive complexity.

In a first embodiment of the invention, the aircraft is equipped with:

- a primary propulsion system for generating only horizontal thrust and for the aircraft to fly in cruise configuration;

- a secondary propulsion system intended to generate vertical thrust needed to support and /or lift the aircraft,

- a control system (RCU) intended to control the primary propulsion system and the secondary propulsion system to provide the thrust needed for the aircraft to fly. The secondary propulsion system comprises a plurality of additional thrust generators (RTG, also of deploy able type), in particular propellers and /or fans, intended to generate vertical thrust of the aircraft, and wherein the additional thrust generators are driven by one or more engines powered by rechargeable energy storage devices (RRSU) by means of a recharge section (RRU).

The energy storage devices and the secondary propulsion system are dimensioned so that they, without other means, allow the aircraft to be lifted.

In an embodiment of the invention, the aircraft comprises a number of kinetic type RRSU, for example flywheels, equal to the one of RTG additional thrust generators. In this embodiment of the invention, RRSUs are preferably positioned in the immediate vicinity of the thrust generators (RTG), so that they are actuated by means of a CVT variable transmission (Continuously Variable Transmission).

The RRSU charge occurs by drawing electric energy, coming from RRUs, by means of electric engines which transform it in kinetic energy: in practice the electric current of RRUSs produces the rotation of the CVT mounted on the engine rotor. The CVT is connected to a kinetic RRSU which stores the kinetic energy generated by the rotating motion of CVT.

In discharge phase, the kinetic type RRSUs make the CVTs rotate, which in turn make the RTG propellers and fans rotate. The control of the same speed is carried out controlling the mechanic torque exchanged between the CVT inlet and outlet. In another embodiment of the invention_/ the aircraft is provided with an electric type RTG (i.e. comprising a respective electric engine) and with a kinetic type RRSU moved by electric engines (MGU - Motor Generator Unit) in equal number to the one of RTGs.

The RRSUs, which for example, comprise flywheel type energy storage devices, are positioned in the immediate vicinity of additional thrust generators (RTG).

In this embodiment of the invention, the electric energy coming from RRU is transformed by MGUs in kinetic energy which is stored in kinetic type RRSUs. For example, the electric energy drawn by MGUs makes the MGU rotor rotate, which in turn, makes a flywheel rotate which stores the energy in kinetic energy form.

When RTG additional thrust generators are driven, RRSUs make the MGU unit rotors rotate, which generate a corresponding electric energy which is sent to the power management units. These latter set up the voltage and current coming from MGUs to provide the additional RTG thrust generators with suitable values of voltage and current.

In this variant, the energy distribution is carried out by means of electric wires, which allows the thrust generators to be distributed in the most suitable positions, since their connection is extremely easy.

In another variant, the aircraft is provided with centralized RRSUs and MGUs and a plurality of RTGs distributed in suitable positions of the aircraft.

In this embodiment of the invention, RRSUs (for example a flywheel, or a bank of flywheels) are preferably arranged near RRU recharge units.

An MGU unit transforms the electric energy coming from RRU recharge region in mechanic energy which is stored in the kinetic type RRSUs.

When it is needed to actuate RTGs, RCU controls MGU, which draws kinetic energy from RRSUs, in particular from the flywheel (or flywheels), and converts it in electric energy. For example, a flywheel makes an MGU rotor rotate, which then generates electric energy.

The electric energy generated by MGU is input in a supply network accessible by the various RTGs by means of one or more power management units, which adapt the voltage and current parameters present on the network to the one needed for RTGs. In another variant, the aircraft is provided with one or more electrochemical or electric type RRSU which supply one or more electric RTGs. Also in this case, it is possible to provide one or more management power units which set up the electric energy sent to the additional thrust generators.

In another variant, RRSUs comprise kinetic type units (for example flywheels) and electrochemical/ electric type units to supply one or more electric RTGs.

Therefore, during charging, the electric energy of RRUs is partially or totally converted in mechanical one by MGU units integrated with the flywheels so that they are recharged, and partially or totally adapted by power management devices to be stored in electrochemical type storage units. The energy distribution between RRSUs is decided by the user or by the same control unit according to predeBned preferred parameters, the capacity available of the single RRSUs, etc.

In order to actuate electric RTGs:

- MGU draws kinetic energy from flywheels, transforms it in electric energy, possibly suitably adjusted by power management units , and sends it to the additional thrust generators (RTG);

- the electrochemical accumulators provide directly electric energy which, possibly adjusted by power management units, is sent to the additional thrust generators. With reference to fig. 6, it is shown a scheme of the RAPS secondary propulsion system, which provides a kinetic type energy storage. The system comprises flywheel type storage devices (in the following simply shortened with "flywheels") 100 in number equal to the one of mechanic actuated additional thrust generators 106. Moreover, the system comprises suitable devices for energy conversion and for controlling the thrust generated by the generators 106.

In the charge phase, RRU recharge unit (units) provides electric energy in one or more of the following ways;

- drawing electric energy from aircraft on board devices, such for example the aircraft primary propulsion system, electric generators or photovoltaic cells;

- conversion of thermal energy generated from aircraft primary thruster in electric energy;

- conversion of mechanical energy drawn from the primary propulsion system or auxiliary turbines, for example RATs, in electric energy; in this case, conversion can be obtained by means of a generator 102.

The electric energy drawn from RRUs is distributed in a supply network 105 used by suitable electric engines 103, whose function is to convert the electric energy in mechanical energy which is used to recharge the flywheels 100 by means of a continuous speed variator CVT 104; the variator has to allow the coupling between a flywheel 100, which rotates at a speed proportional to the stored energy, and the electric driving shaft 103 which can rotate at definitely lower speed.

When it is needed to actuate the additional thrust generators 106, energy is drawn from the flywheels 100 by means of the continuous speed variator 104 and it is sent directly to the respective thrust generators 106, while controlling the power sent by means of the supplied torque control. In other words, the flywheel 100 makes a respective CVT 104 rotate, which in turn makes the propellers /fans of the additional thrust generator 106 rotate.

The charge and discharge steps of the storage devices are controlled by RCU, which is operatively connected to the engines 103, to the speed variators 104, to the storage devices 100 and the thrust generators 106, so that the energy flows from the engines 103 to the flywheels and the ones from the flywheels to the additional thrust generators 106 are set up.

Therefore, RCU is provided with sensors able to detect the charge state of the storage devices and actuators (whose actuating depends on the signals received by the sensors and/or commands of the aircraft pilot) needed to connect dynamically the engine 103 to the speed variators 104, CVTs to the storage devices 100 and CVTs to the additional thrust generators 106. As previously said, there are many alternatives to the arrangement of the additional thrust generators and of the other system elements. They depend on their number, structure and type of the airplane in addition to the chosen system configuration. Only as a way of example and with no specificity, there will be shown some thereof. In fig. 7a, it is shown a first arrangement of the main elements, which constitute a RAPS system in an airplane 200. In this case, the additional thrust generators 106 are mounted on suitable deployable opposed arms 201, which can rotate around axes parallel to the y axis (roll) to go from the takeoff/ landing configuration shown in fig. 7a to the cruise configuration shown in fig. 7b.

In the example of figs. 7a and 7b, the additional thrust generators 106 are ducted propellers, whose rotation axis is connected to a CVT 104 which sets up its speed and so the generated thrust. To this aim, CVT is electrically connected to RCU which controls its rotation speed. The energy to move CVT in takeoff or landing step is taken only by the energy storage devices 100, which in the example of figs. 7a-7b are flywheels which are mechanically coupled to CVT and which make it rotate, thus transferring kinetic energy there stored to the respective CVT.

In the example of figs, 7a-7b for each thrust device 106 it is provided a respective CVT 104 and a respective storage system 104, made up of one or more flywheels connected in series.

The recharge of the storage devices 100 is ensured by RRU recharge units, not shown in figure, which provide electric energy in a supply line to which there are connected electric engines 103, according to the scheme of fig. 6. When powered, the electric engines 103 make the output shaft, to which a CVT 104 is connected, rotate. The connection between CVT 104 and the driving shaft 103 is preferably removable, so that it is provided only when it is desired to charge the storage device 100.

CVT 104 is connected mechanically to the storage device 100, to which it transfers its own energy.

CVT can be then connected, in a way controlled by RCU, to a thrust generator 106, in this way, when the thrust generators 106 are to be actuated a CVT 104 is connected to a respective thrust generator and drives it, for example CVT makes a respective ducted propeller rotate.

In this embodiment of the invention, each arm 201 supports an electric engine 103, a storage device 100, a CVT 104 and a thrust generator 106.

During cruise flight, the arms 201 are retracted along the side of the fuselage or inside suitable housings provided on the same fuselage. In the first case, in order to improve aerodynamic efficiency the ducts housing the propellers can be closed again using suitable ports. In the second case, along the fuselage there are provided closable ports, for example of the type used for landing gears so that good aerodynamic efficiency of the aircraft is kept.

In the example of figs. 8a and 8b, the additional thrust generators 106 are mounted on suitable deployable arms 201, which can rotate around axes parallel to the z axis to go from the takeoff/landing configuration shown in fig. 8a to the cruise configuration shown in fig. 8b with the additional propulsion devices in a rest position which allows good aerodynamic efficiency of the aircraft to be kept. According to this embodiment, the arms 201 are folded so that the thrust generators 106 are positioned under the fuselage or possibly inside the housings provided on the belly of the airplane 200.

In the example of figs. 8a~8b, the RAPS auxiliary propulsion system is realized according to the scheme of fig. 6, therefore as for fig. 7a, also in this embodiment of the invention, each arm 201 supports a storage device 100, a CVT transmission 104 and an engine 103 which converts electric energy, provided by RRU recharge units, in mechanic energy which actuates a respective CVT 104.

In a different embodiment shown in fig. 9a, the additional thrust generators 106 are mounted on movable arms 201, hinged to the wings 202 with the possibility to rotate along the x axis (pitch). This allows them, in cruise configuration, to be positioned inside or under the same wings so that the aircraft aerodynamics is not invalidated, possibly by means of closable ports (fig. 9b), There can be also available small tail additional thrust generators 106.

Even if not shown in detail, also in this embodiment the storage devices 100 can be arranged on the arms 201 as well as for the configurations of figs. 7a-7b. In all the above described embodiments of the invention with reference to figs, 7a- 7b, 8a-8b and 9a-9b, RCU is configured to control, during takeoff/ landing steps, the rotation of the arms 201 around the x axis (pitch) and the speed of the fans 106 (independently for each one) to control the airplane attitude. In addition or alternatively to the control of the position of the arms 201, the control of the aircraft attitude can be obtained by rotating the thrust generators 106 around the arms 201 or, in case of flywheel type storage devices the attitude can be controlled by adjusting the flywheel speed. In detail, in an embodiment of the invention the storage devices comprise at least a couple of flywheels intended to rotate in opposed direction, and RCU control system is configured for:

- receiving an attitude signal relating to the aircraft attitude (for example a signal emitted by a respective attitude sensor) and

- in response to receiving said attitude signal, adjusting the rotation speed of the couple of flywheels for generating a moment needed to set up the aircraft attitude. In an embodiment of the invention, for each thrust generator 106 it is provided a couple of flywheels intended to rotate in opposed direction. In this way, it is possible to generate a different moment at each thrust generator.

h the example of fig. 10, the thrust generators 106 are integrated in the same structure of the airplane 200 and are fixed (in particular arranged on the wings 202 and on the fuselage 203). In the example of fig. 10, the thrust generators 106 are propellers/fans arranged inside the openings 210 of the wings or fuselage. The openings 210 are provided with suitable closing mechanisms able to close hermetically such openings for cruise flight. In takeoff/landing steps of the aircraft, such closing mechanisms are maintained in such a configuration that a passage of air through the opening 201 is possible. In an embodiment of the invention, such closing mechanisms, at least in the air outlet region, have elements (in particular tabs) which can direct the air flows for the aircraft control. Fig. 11a shows a side section of a wing profile of the aircraft of fig. 10 at a thrust generator; the cover mechanism 204 is completely opened, with tabs 204a thereof arranged vertically. Therefore, the air flow 204b generated by the blades is directed perpendicularly to the wing. By varying the angle of the tabs 204a the direction of the air flow can be changed, as it is shown in fig. lib. When the airplane is in cruise flight (or however when it is not required the use of additional thrust generators) the tabs 204a are brought horizontally, thus closing the opening (fig. 11c).

Also in the solution of fig. 10, the storage devices 100, with their provision of engines 103 and CVT 104, given their limited dimensions, are mounted immediately on the additional thrust generators 106.

In the example of fig. 10, the control unit 109 is arranged in the cockpit to integrate the equipment on board. By means of this unit it is possible to carry out the passage from the takeoff/ landing configuration to the cruise one, to control the charge state and the charge of accumulators 100 and to control the rotation speed and the flow direction of each thrust generator 106.

In fig. 12 it is shown a second embodiment of the RAPS auxiliary propulsion system, which provides kinetic type storage devices (for example flywheels 100) in number equal to the additional thrust generators 106; these latter are electrically actuated, therefore with respect to the above described example with reference to fig. 6, it is not to interact with the flywheels by means of a speed variator but by means of engine /generator units (MGU, engine /generator unit) 107.

An MGU unit is a reversible electric machine able to function both as electric engine (and so to transform electric energy in mechanical energy) and as generator (to transform mechanical energy in electric energy), MGU can be arranged in the fan hub or can be connected to this one by means of a reduction unit. In this case, the position of MGU will be corresponding to the position of flywheels of fig. 7a.

In charge phase of the storage devices 100, RRU recharge unit (units) functions exactly as in the previous case, generating electric energy (for example by means of conversion of mechanical energy in electric energy by means of a generator 102) and providing it to a supply network 105 where MGUs are connected. The electric energy produced by RRU recharge units is converted in mechanical energy by the single MGUs 107 and stored in the flywheels connected to the latter ones.

In discharge step, MGUs 107 take kinetic energy from respective flywheels and convert it in electric energy, for example, the flywheel makes the MGU rotor rotate, which generates electric current as a consequence of the windings arrangement on the stator.

Each MGU is provided in output with an electronic power unit (power management) 108 which sets up suitably parameters of voltage and current generated by MGU so that they are compatible with the charge, The unit 108 provides for example to adapt voltage and current levels, to level the wave forms etc.

Also this embodiment of the invention provides an arrangement of the elements similar to the first one, with the storage units 100 (comprising MGUs 107 and power electronics 108) in the immediate vicinity of the additional thrust generators 106. As yet said, also for this variant, the additional thrust generators and the other components of the system can be arranged in the most convenient positions. The chosen configuration depends on the number of the units, the structure and the airplane type.

In another embodiment of the invention shown with reference to fig. 13, the RAPS secondary propulsion system is provided with a flywheel (or bank of flywheels) arranged in the immediate vicinity (or however, the nearest possible to) of the recharge region, while the additional thrust generators are electrically powered (i.e. they are fans comprising a respective electric engine).

In charge phase of the storage devices, the electric energy coming from the recharge region, is converted in mechanical energy by MGU 107 and stored in the flywheel or flywheels 100.

In discharge step of the storage devices, MGU 107 draws mechanical energy from the flywheel 100, to send it, by means of the supply network 105, to the additional thrust generators 106, to which it arrives after being suitably processed by the power electronics 108.

In the example of fig. 14 it is shown a solution which uses an electrochemical /electric type storage section (for example batteries or ultra-capacitors) while the additional thrust generators are electrically actuated. The scheme is shown in fig. 14: in charge phase, the recharge section P3, with possible generator 102, functions similarly to the previous variants generating electric energy. The electric energy coming from the recharge section is properly adjusted (for example by acting on the voltage and current amplitude or on frequency) by dedicated power management devices 108a. The devices 108a allow to set up the power provided to the elech'ic/ electrochemical devices, in such a way that the charge follows optimal charge curves,

In the example of fig. 14, it is shown a variant of the aircraft propulsion system, in which the electric energy thus stored in the electrochemical /electric storage devices 100a is then released in a supply network 105 to which there are connected various power management units 108 intended to adjust the output power which is provided to the additional thrust generators 106. Clearly it is also possible to provide various electric/ electrochemical batteries, which power respective thrust generators 106 directly.

In discharge phase of storage device /devices 100a, this/these powers/power the additional thrust generators 106 with electric energy, which arrives thereto upon treatment in the power management unit 108 (present in number equal to the propellers one).

As in the previously described examples, also in case of electric/ electrochemical batteries the additional thrust generators 106 and the other components of the propulsion system can be arranged in the most convenient positions of the aircraft, for example there can be provided both fixed and deployable thrust generators as shown in figs. 7, 8, 9, 10, the batteries can be concentrated in only one position or can be distributed, for example can be positioned coaxially to a respective thrust generator.

A preferred solution of a thrust generator thatcanbe used in any one of the above described propulsion system is shown in fig. 15.

In this solution, the thrust generator comprises a propeller or fan 203 with a certain number of blades 203a, arranged in a circular space which can be fixed in the aircraft or mounted on deployable supports. Inside this space, there is a stator winding 203b, while the blade bits are connected with respect to each other by means of a metal ring 203c on which permanent magnets 203d are fixed. Therefore, an outer electric engine is provided which can allow to spare weight and which can reach high blade rotation speeds.

In another embodiment of the secondary propulsion system for aircrafts, described in the following with reference to fig. 16, the storage section P2 comprises both kinetic accumulators and electrochemical /electric accumulators,

In charge step, a switch 110, controlled by RCU, transmits electric energy coming from the recharge section P3 to a kinetic energy storage section SI or to an electric energy storage section S2.

RCU controls the switch 110 so that the energy is conveyed to the various storage sections SI and S2, in fixed and prefixed proportions (for example pre-set up by the pilot) or in proportions dynamically calculated on the basis of parameters as for example the residual charge of the single storage devices.

Concerning the kinetic storage, the MGU unit 107 has to convert the electric energy to be able to store it in the flywheel (or flywheels) 100.

Concerning the electrochemical/electric storage, in the example of fig. 16 it is provided a power management unit 108a, which modifies parameters of inlet voltage and current to charge electrochemical /electric storage devices (batteries and /or capacitors) according to preferred charge curves intended to optimize the patent life.

In discharge step, energy is required by one or both the storage sections SI and S2; to that aim, the storage sections SI and S2 are connected to the network 120 (by which the thrust generators 106 are supplied) by means of suitable switches (represented with block 111) controlled by RCU.

The kinetic energy stored in the flywheels 100 is reconverted in electric energy by the MGU unit 107, which thus provides the network 120 with the electric energy needed for the additional thrust generators 106 to function.

In the example of fig. 16, the additional thrust generators 106 are motorized fans connected to the supply network 120 by means of a respective power management unit 108 which provides the electric engines of the generators 106 with the correct supply voltage,

With reference to figs. 17a and 17b, there are shown various takeoff methods of an aircraft using the above described propulsion system.

In fig. 17a, it is shown a takeoff step using the system object of the present invention. The takeoff method provides to bring the airplane 200 in the takeoff point. In this position, the secondary propulsion system is arranged in takeoff /landing configuration (1000), this is to say for example to open the ports of the additional thrust generators 106 and to bring these latter in a correct takeoff position, i.e. with the generators oriented to generate vertical thrust.

Once positioned, the secondary propulsion system is activated, generating thrust 111a higher than the same aircraft weight (1001) obtaining thereby its lifting along the perpendicular to ground up to height hi (position 1002) compatible with the transition maneuver to cruise flight and dependent also on possible obstacles or air traffic present in the area. The lifting is obtained by means of the sole energy recovered by RRSU storage section. The storage section and the thrust generators are in fact dimensioned so that the thrust needed for lifting the aircraft up to reach the cruise flight height is ensured.

Once the height hi is reached, the primary propulsion system is actuated, which provides horizontal force 111c aiming at increasing the translational speed of the aircraft and so the wings lift (position 1002). During this whole phase, the secondary propulsion system thrust always remains oriented upwards even if it can be reduced in module. When the speed along the horizontal axis Y is such that the auto- supporting condition can be reached, it begins the normal cruise step by means of the sole primary propulsion system (position 1004), while the additional one is deactivated and brought back to cruise configuration.

Still in fig. 17a, it is shown another possibility provided by the propulsion system here described: once height hi (position 1002) is reached and the primary propulsion system is actuated, while horizontal speed increases and so wings lift as well, the additional system thrust can be gradually directed forward and positioned as in 111b (i.e. forming an angle a with the vertical). In such a way, it provides two components: a vertical one which is approximately equal to the difference between aircraft weight and wings lift at that time and a horinzontal one which is added to the one of the primary propulsion system and so increases the horizontal translation speed, thus allowing the auto-supporting to be reached in shorter time.

In fig. 17b it is still represented a takeoff maneuver in which it is highlighted how by varying the various propulsive flows (position 1005) it is generated a torque around the pitch axis to vary the airplane attitude. In this case, it is possible to obtain a component of the propulsive flows along the horizontal without mechanisms to direct RTGs with respect to the airplane.

In both cases of fig. 17a and 17b, for takeoff it is necessary that the storage devices are sufficiently charged to allow the aircraft takeoff. Therefore, it is provided to recharge the storage devices on ground. In an embodiment of the invention, this can be implemented with the RRU recharge section which recovers energy from the main engine when this one is actuated for moving the aircraft at ground. Alternatively, RRU comprises a coupling for connecting to an electric or kinetic energy generator outside the aircraft, and it is configured to transfer energy (possibly by converting it from kinetic or thermic in electric energy or vice versa) absorbed by the generator outside the aircraft, towards the RRSU storage section.

It is then clear how an aircraft provided with a th uster of the above described type is able to implement a takeoff method comprising the phases of:

a. recharging the energy storage devices at ground;

b. arranging the aircraft takeoff configuration with additional thrust generators configured for generating vertical thrust of the aircraft;

c. powering the additional thrust generators using the energy available in the storage devices for generating vertical thrust higher than the aircraft weight up to reach height compatible with the transition maneuver to cruise flight, suitably controlling the different propulsive flows generated by the additional thrust generators to maintain the desired attitude and trajectory;

d. activating the primary propulsion system to be able to reach the translational speed needed to generate sufficient wings lift; e. deactivating the secondary propulsion system and transition from the takeoff /landing configuration to the cruise one;

f . continuing the flight using the primary propulsion system;

In fig. 18a it is shown a landing phase of an aircraft, which uses a propulsion system object of the present invention.

While the airplane 200 is in flight moved by the force 111c of the sole primary propulsive system it is controlled that the storage devices are sufficiently charged and the secondary propulsion system is arranged in the takeoff /landing configuration (position 1100), for example by deploying the additional thrust generators 106 (for example as explained with reference to figs. 7a and 7b) or opening the ports of possible generators fixed in the fuselage (as described with reference to example of fig. 10).

The airplane begins its descent to height h2 compatible with the transition maneuver from cruise flight, with possible local air traffic and with the surrounding obstacles, slowing down the speed at the same time (position 1101) up to a safe condition of slow flight. At this point (position 1102) the secondary propulsion system is powered to generate thrust 111a. The intensity of this thrust 111a is modulated (position 1103) to allow to descend at the desired speed. Once at ground (position 1104) the secondary propulsion system is deactivated and brought back in the cruise configuration (rest position).

Still in fig. 18a, it is shown another possibility of landing; in the first step of the landing maneuver, i.e. during the slowing down step, the secondary propulsion system can be powered and directed so that it generates thrust with horizontal component in opposed direction to the advancement one and so to contribute to the aircraft slowing down (position 1101, arrow 111b). By reducing the horizontal speed, the thrust 111b of the additional thrust generators 106 is gradually directed upwards, thus forming an angle a with the vertical (position 1102, arrow 111b), to contribute to the means supporting, and then to proceed to letdown at controlled speed. During this last descent phase, the flows produced by the secondary propulsion system are not compulsory vertical but they can be directed according to needs to maintain the right aircraft attitude (position 1103, arrow 111b).

In fig, 18b, it is still shown a landing maneuver in which it is shown how it is possible to change the aircraft attitude by modifying the various thrusts. As previously indicated in fig. 17b, also in this case there can be propulsion flows with horizontal thrust component which do not require mechanisms in order to direct them with respect to the aircraft (by orienting the same generators in case they are deployable or by deviating the flow in case of fixed thrust generators) but the propulsive flows orienting from the vertical can be obtained by varying the same attitude of the aircraft.

It is to be precised that for safety reasons, before landing the charge state of the energy storage devices is to be checked and, if needed, they are to be recharged by RRU, for example deriving energy from the primary engine of the aircraft and/or auxiliary generators on board, in particular thermal energy recovery units or photovoltaic cells or RAT turbines. Such check is carried out thanks to suitable sensors, for example voltage sensors, which allows to detect the charge of the storage devices and signal it to pilot or RCU.

Therefore, it is clear how the above described propulsion system allows to implement a method for carrying out landing of an aircraft comprising the steps of: a. checking the charge state of the energy storage devices;

b. if needed, recharging the energy storage devices, deriving energy from the primary engine of the aircraft and/or from the auxiliary generators on board, in particular thermic energy recovery units or photovoltaic cells;

c. beginning the descent towards height compatible with the transition maneuver from the cruise flight to the vertical one, slowing down the speed of the airplane at the same time up to a safe condition of slow flight;

e. deactivating the primary propulsion system;

f. gradual reducing the vertical thrust of the additional thrust generators, so that the aircraft is allowed to descend following a desired trajectory and descent speed, controlling attitude and trajectory by controlling each propulsive flow;

g. once landing is ended, deactivating the secondary propulsion system.

Therefore, the above described propulsion system allows to reach the prefixed aims and advantages.

It is also clear that many variants can be adopted to modify the propulsion system and /or the aircraft using it without departing from the protection scope of the invention as defined in the appended claims.

For example, it is possible to realize an aircraft in which the arrangement of the RTG additional thrust devices is the one shown in figs. 7a-7b, 8a-8b, 9a-9b, while the scheme of the RAPS propulsion system is other than the one of fig. 6, for example it is the one of fig. 12 or fig. 13 or fig. 14.

While the present invention has been described with particular reference to an aircraft, it is clear that the RAPS system or some of the components thereof can be usefully applied on other types of aircraft, For example, the recharge sections and the RAPS system storage ones can be provided on an helicopter and used as backup system in case of failure of the primary internal combustion engine, i.e. of the engine actuating the rotor, In this case, the storage sections can be connected to the rotor of the helicopter in similar way to one of the above described configurations for connecting the storage sections to RTG of RAPS. Therefore, in case of failure, the storage section and the various RRSU provide energy to rotor thus allowing the aircraft to land.

Claims

1. Propulsion system for aircraft_/ comprising:

- a primary propulsion system (PI) for generating at least horizontal thrust to allow the aircraft to fly in cruise configuration,

- a secondary propulsion system (RAPS) intended to generate vertical thrust needed to sustain and /or lift the aircraft in a takeoff/ landing configuration other than the cruise configuration,

- a control system (RCU) intended to control the primary propulsion system and the secondary propulsion system in order to generate the thrust needed for the aircraft to fly,

wherein the secondary propulsion system (RAPS) comprises a plurality of additional thrust generators (RTG), which function thanks to the energy received from the rechargeable energy storage devices (RRSU),

characterized in that

the primary propulsion system (PI) is fixed and generates only horizontal thrust, and in that the energy storage devices (RRSU) and the secondary propulsion system (RAPS) are dimensioned so that they alone allow the aircraft to be lifted with energy drawn only from the energy storage devices (RRSU).

2. Propulsion system according to claim \, wherein said energy storage devices are of flywheel type.

3. Propulsion system according to claim 2, wherein said flywheel type storage devices comprise at least a couple of flywheels intended to rotate in opposed direction, and wherein the control system is intended

- to receive an attitude signal relating to the aircraft attitude and

- in response to receiving said attitude signal, to adjust the rotation speed of the couple of flywheels for generating a moment needed to adjust the aircraft attitude.

4. Propulsion system according to claim 2 or 3, wherein a thrust generator is a propeller or a fan, and wherein a rotation axis of a flywheel of said devices can be connected mechanically to said propeller or fan by means of a continuous speed variator.

5. Propulsion system according to claim 1, wherein said storage systems are of electrochemical and electric type,

6. Propulsion system according to any one of the preceding claims, wherein the secondary propulsion system comprises a recharge section intended to recover energy, in particular kinetic and/or thermal energy, dissipated by at least a primary engine of the primary propulsion system, and wherein said recharge section is operatively connected to said energy storage devices so that the energy storage devices are charged by means of energy recovered by said engine of the primary propulsion system.

7. Propulsion system according to claim 6, the recharge section comprising a coupling for connecting to an electric or kinetic energy generator outside the aircraft, and wherein said recharge system is intended to transfer and/or convert the energy collected by the generator outside the aircraft.

8. Propulsion system according to any one of the preceding claims, wherein an energy storage device is connected to a respective additional thrust generator and wherein said storage device and said respective thrust generator are supported by the same mechanical arm.

9. Propulsion system according to any one of the preceding claims, wherein said energy storage devices comprise flywheel type storage devices in which the auxiliary propulsion system comprises at least a dynamo connected to a flywheel of said energy storage devices so that the mechanical movement of the flywheel is converted in electric energy, and in which at least one of said thrust generators is driven by an electric engine powered by said dynamo.

10. Propulsion system according to any one of the preceding claims, wherein said thrust generators are movable from a first takeoff position in which they generate vertical thrust for the aircraft to a second cruise position in which they allow the aircraft to maintain aerodynamic efficiency needed for cruise flight.

11. Propulsion system according to claim 10, wherein the control system is intended to control the position of the thrust generators by adjusting them so that the thrust of said thrust generators contributes to speed up or slow down the aircraft.

12. Propulsion system according to any one of the preceding claims, wherein the energy storage devices can be mechanically or electrically connected to a primary engine of the primary propulsion system, so that the mechanical or electric energy directly or indirectly generated by said energy storage devices is exploited to operate the primary propulsion system.

13, Aircraft comprising a propulsion system according to any one of claims 1 to 12.

14, Method for carrying out takeoff of an aircraft comprising a propulsion system according to any one of claims 1 to 12, comprising the steps of:

a. recharging the energy storage devices at ground;

b. set up the aircraft takeoff configuration with additional thrust generators configured for generating vertical thrust of the aircraft;

c. powering the additional thrust generators using only the energy available in the storage devices for generating vertical thrust higher than the aircraft weight up to reach height compatible with the transition maneuver to cruise flight, suitably controlling the different propulsive flows generated by the additional thrust generators to maintain the desired attitude and trajectory;

d. activating the primary propulsion system to be able to reach the translational speed needed to generate sufficient wings lift;

e, deactivating the secondary propulsion system and transition from the takeoff configuration to the cruise one;

f. continuing the flight using the primary propulsion system;

g. recharging the storage devices deriving energy from the primary engine of the aircraft and/or from auxiliary generators on board, in particular thermal energy recovery units or photovoltaic cells.

15. Method according to claim 14 when dependent on claim 6 or 7, wherein the recharge of the energy storage devices occurs by using the recharge section which takes energy from the main engine or from an outer generator connected to the recharge section,

16, Method according to claim 14 or 15, wherein in takeoff step the thrust direction of the additional thrusters is not kept constant but, when the primary propulsion system is activated, it is gradually oriented or deviated in horizontal direction so that the translational speed of the airplane is increased, thus reducing at the same time the vertical portion while the whole wing lifts increases.

17, Method for carrying out landing of an aircraft comprising a propulsion system according to any one of claims 1 to 12, comprising the steps of:

a. checking the charge state of the energy storage devices;

e. deactivating the primary propulsion system;

f. gradual reducing the vertical thrust of the additional thrust generators, so that the aircraft is allowed to descend following a desired trajectory and descent speed, controlling attitude and trajectory by controlling each propulsive flow of the additional thrust generators;

g. once landing is ended, deactivating the secondary propulsion system.

18. Method according to claim 17, wherein in landing phase, the thrust of the additional thrust generators is initially directed forward to contribute to the aircraft slowing down, and then it is gradually oriented downwards up to become vertical when the vertical descent has to begin.