This invention relates to an aircraft, and a method of transitioning an aircraft between two flight modes.

Prior art aircraft capable of vertical take off and landing (VTOL) are known in the art, the most well-known of these being the helicopter. Aircraft with VTOL capability are particularly useful for operations in areas where it is impractical to provide a runway for an aircraft, e.g. urban environments.

A drawback with prior art VTOL capable aircrafts is that they are fuel inefficient for long periods of horizontal flight, which can limit range, and / or they have a low top speed in horizontal flight. To overcome these drawbacks a number of VTOL capable aircrafts have been developed capable of two dedicated flight modes, e.g. a vertical flight mode and a horizontal flight mode, such as the Bell Boeing V-22 Osprey.

However, in prior art VTOL capable aircrafts capable of two dedicated flight modes, the aircraft transitions between the two flight modes by operating a rotation mechanism between the thrust means (e.g. propeller or jet) and the aircraft wing. This presents a safety risk, as the aircraft can lose control if the rotation mechanism undergoes a mechanical jam during the transition.

It is an aim of the present invention to provide an aircraft capable of two dedicated flight modes, in which the thrust means does not need to be rotated with respect to the aircraft wing.

This aim is achieved by providing an aircraft in which the transition between vertical and horizontal flight modes is effected only by differential thrust between fore and aft thrust means of the aircraft. In accordance with a first aspect of the present invention there is provided an aircraft comprising:

a fore thrust means arranged to produce thrust along a first thrust axis; and an aft thrust means arranged to produce thrust along a second thrust axis, wherein the fore thrust means is associated with a fore fixed wing,

wherein the aft thrust means is associated with an aft fixed wing, and

wherein the first and second thrust axes are offset, such that differential thrust between the fore and aft thrust means causes rotation of the fore and aft thrust means about a lateral axis of the aircraft.

The fore and aft thrust means may be connected via one or more connecting arms. A direct connection between the thrust means allows the efficient transmission of forces between the thrust means, which allows a responsive rotation of the thrust means about the aircraft lateral axis when differential thrust is applied.

The aircraft may further comprise a passenger compartment. The passenger compartment may be suspended from the one or more connecting arm and a rotation mechanism may be between the passenger compartment and the one or more connecting arm. This arrangement allows the absolute orientation of the passenger compartment to be varied by operating the rotation mechanism and rotating the passenger compartment relative to the connecting arm.

The rotation mechanism may be operable to maintain the passenger compartment in a constant orientation with respect to the aircraft lateral axis as the fore and aft thrust means rotate about the aircraft lateral axis. This arrangement increases the comfort of a passenger within the passenger compartment as they can remain in the same absolute orientation / posture regardless of the angle of the fore and aft thrust means.

Alternatively, the passenger compartment could contain a seat for a user, wherein the seat is arranged to rotate relative to the passenger compartment about the aircraft lateral axis. This allows for the same effect set out above but without the need for a rotation mechanism between the passenger compartment and the connecting arm.

The fore thrust means may comprise a plurality of propellers spaced apart in the direction of the aircraft lateral axis and / or the aft thrust means may comprise a plurality of propellers spaced apart in the direction of the aircraft lateral axis. This arrangement allows the aircraft pitch, roll and yaw to be varied merely by varying the rotation speed of the various propellers (as will be described with reference to the Figures below) without the need for any conventional control surfaces. Alternatively the fore / aft thrust means may comprise a plurality of jets spaced apart in the direction of the aircraft lateral axis or a plurality of ducted fans spaced apart in the direction of the aircraft lateral axis, or a mixture of propellers / jets / ducted fans.

The aircraft could comprise at least one control surface on one of the fore fixed wing and the aft fixed wing (or both). The aircraft could comprise at least one control surface on the connecting arm. The aircraft could comprise at least one control surface on the passenger compartment. While the aircraft may be manoeuvred using only the thrust means (as will be described with reference to the Figures below) without the need for any conventional control surfaces, it may be desirable to include them where possible for redundancy and / or to improve manoeuvrability.

In accordance with a first aspect of the present invention there is provided a method of transitioning an aircraft between two flight modes, the aircraft comprising a fore thrust means arranged to produce thrust along a first thrust axis, and an aft thrust means arranged to produce thrust along a second thrust axis, wherein the fore thrust means is associated with a fore fixed wing, wherein the aft thrust means is associated with an aft fixed wing, and wherein the first and second thrust axes are offset, the method comprising the steps of:

applying substantially the same amount of thrust to the fore and aft thrust means to move the aircraft in a first direction corresponding to a first flight mode; applying a differential amount of thrust to the fore and aft thrust means to rotate the fore and aft thrust means about a lateral axis of the aircraft; and

applying substantially the same amount of thrust to the fore and aft thrust means to move the aircraft in a second direction corresponding to a second flight mode.

The first direction may be vertical and the second direction may be horizontal.

Detailed description The invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 schematically shows a side view of an aircraft in accordance with the present invention;

Fig. 2 schematically shows the aircraft shown in Fig. 1 transitioning from a first flight mode to a second flight mode;

Fig. 3 schematically shows a plan view of an aircraft in accordance with the present invention;

Figs. 4a-d schematically show various views of an aircraft in accordance with the present invention illustrating control of the aircraft during a first (vertical) flight mode; and

Figs. 5a-d schematically show various views of an aircraft in accordance with the present invention illustrating control of the aircraft during a second (horizontal) flight mode.

Fig. 1 shows an aircraft 10 comprising a fore thrust means 12 and an aft thrust means 14. In this embodiment both the fore and aft thrust means 12, 14 comprise propellers. However, in practice the thrust means 12, 14 may comprise any suitable thrust means (e.g. jet engines, ducted fans). Dissimilar thrust means could also be used. In this specification the terms“fore” and“aft” are used as per their normal and well- understood definitions in the field of aerospace to denote, respectively, towards the front of the aircraft (towards the normal direction of travel) and towards the rear of the aircraft (away from the normal direction of travel). As illustrated in Fig. 1 , the aircraft need not have a substantially elongate body to have fore and aft sections.

The fore thrust means 12 generates thrust along a first thrust axis A. The aft thrust means 12 generates thrust along a second thrust axis B. The first and second axes A, B are offset from one another as shown. This arrangement allows the rotation of the thrust means 12, 14 merely by virtue of the relative amounts of thrust generated by the fore thrust means 12 and the aft thrust means 14. That is to say, if the same amount of thrust is generated by both the fore thrust means 12 and the aft thrust means 14, no rotation will occur and the aircraft will move in a linear direction, i.e. vertically as shown in Fig. 1. However, if the fore thrust means 12 generates more thrust than the aft thrust means 14, both the fore thrust means 12 and aft thrust means 14 will rotate in a first direction (anti-clockwise as shown in Fig. 1 ), and if more the aft thrust means 14 generates more thrust than the fore thrust means 12, both the fore thrust means 12 and aft thrust means 14 will rotate in a second direction (clockwise as shown in Fig. 1). The fore thrust means 12 is connected to the aft thrust means 14 via a connecting arm 16. The aircraft 10 comprises a passenger compartment 18 which is attached to the connecting arm 16 via rotation mechanism 20.

In the embodiment shown in Fig. 1 the passenger compartment 18 is located between the fore thrust means 12 and the aft thrust means 14. This is a preferable arrangement as the centre of mass of the aircraft is located near to the point about which the thrust means rotate when transitioning between the first and second flight modes. Other embodiments, in which the passenger compartment 18 is not located between the fore and aft thrust means are also envisaged, but would likely require a greater thrust differential to effect transition and would also likely be less aerodynamic and fuel efficient. As also shown in the embodiment of Fig. 1 , the planes of the thrust means are offset as well as the thrust axes, such that in the first flight mode the fore thrust means 12 also represents an‘upper’ thrust means and aft thrust means 14 also represents a‘lower’ thrust means. While alternative embodiments are possible in which the planes of the thrust means are not offset, the arrangement shown in Fig. 1 is preferable as the fore and aft thrust means are located diametrically opposite the centre of mass and the point about which the thrust means rotate to effect transition. This gives and efficient imparting of moment to the thrust means when differential thrust is applied.

Fig. 2 schematically shows the aircraft 10 shown in Fig. 1 transitioning from a first flight mode to a second flight mode.

The first flight mode is represented by section C. In this flight mode substantially the same amount of thrust is being generated by both the fore thrust means 12 and the aft thrust means 14 and the aircraft is moving substantially vertically.

Section D represents the transition from the first flight mode to the second flight mode. In section D, the aft thrust means 14 is generating a larger amount of thrust than the fore thrust means 12. As a result, both the fore thrust means 12 and the aft thrust means 14 are rotating, i.e. in a clockwise direction as shown in Fig. 2. The aircraft 10 is also moving slightly forward during this transition phase.

The second flight mode is represented by section E. Similarly to section C, in this flight mode substantially the same amount of thrust is being generated by both the fore thrust means 12 and the aft thrust means 14. However, in section E the fore and aft thrust means 12, 14 have been rotated to a horizontal orientation and so applying the same thrust to the fore and aft thrust means 12, 14 at this point moves the aircraft substantially horizontally. Fig. 3 schematically shows a plan view of an aircraft 100 in accordance with the present invention.

The aircraft 100 comprises four fore propellers 102, 112, 122, 132 driven by respective motors 104, 114, 124, 134 which in turn are controlled by respective motor control units 106, 116, 126, 136. The four fore propellers, associated motors and motor control units are associated with a fore fixed wing 190 and collectively comprise a fore thrust means.

The aircraft 100 further comprises four aft propellers 142, 152, 162, 172 driven by respective motors 144, 154, 164, 174 which in turn are controlled by respective motor control units 146, 156, 166, 176. The four aft propellers, associated motors and motor control units are associated with an aft fixed wing 192 and collectively comprise an aft thrust means. The fore wing 190 is connected to the rear fixed wing 192 via a pair of connecting arms 194, 196. A passenger compartment 198 is disposed between the fore wing 190 and the rear fixed wing 192 and is mounted to the connecting arms 194, 196 via a pair of rotation mechanisms 200, 202. It should be noted that the four fore propellers 102, 112, 122, 132 are maintained in a fixed orientation with respect to the fore fixed wing 190 and the four aft propellers 142, 152, 162, 172 are maintained in a fixed orientation with respect to the aft fixed wing 192. Unlike prior art aircraft capable of transitioning between vertical and horizontal flight modes there is no rotation mechanism between the propellers and the wings. This gives a simpler design with fewer failure points. While there may be a rotation mechanism between the passenger compartment 198 and the connecting arms 194, 196 this is only present to improve passenger comfort. The failure of this mechanism will not prevent the aircraft from continuing to fly or from landing safely, and the rotation mechanism can be omitted entirely in some embodiments. A first battery 184 is located within the passenger compartment 198. The first battery 184 supplies power to motor control units 116, 126, 156 and 166. A second battery 186 is located within the passenger compartment 198. The second battery 186 supplies power to motor control units 106, 136, 146 and 176. This arrangement provides a redundant system architecture, whereby in the event of the failure of either the first or second battery, at least two propellers on the fore wing 190 and two propellers on the aft wing 192 will continue to function. This ensures that the fore and aft thrust means still provide sufficient thrust for the aircraft 100 to perform a controlled landing in the event of the failure of the first or second battery.

A first emergency battery 180 is located within the fore wing 190, and is coupled to the motor control units 116 and 126. A second emergency battery 182 is located within the aft wing 192, and is coupled to the motor control units 156 and 166. The first and second emergency batteries 180, 182 are kept fully charged and are not used in normal operation of the aircraft 100. The emergency batteries 180, 182 are located in a separate location to the normal batteries 184, 186 so that potential events disrupting power supply from the normal batteries 184, 186 (e.g. a fire in the passenger compartment) will not also affect the emergency batteries 180, 182 In the event of the failure of both the first and second batteries 184, 186 the first emergency battery 180 is arranged to provide power to motor control units 116 and 126 and the second emergency battery 182 is arranged to provide power to motor control units 156 and 166. This ensures that in a failure of both of the first and second batteries 184, 186 the fore and aft thrust means still provide sufficient thrust for the aircraft 100 to perform a controlled landing. As a last resort safety feature, a separate parachute (not shown) may also be present on the aircraft. The parachute may be deployed ballistically. Such parachutes are well-known in the art.

Figs. 4a-d schematically show various views of an aircraft 300 in accordance with the present invention illustrating control of the aircraft during a first (vertical) flight mode. In the embodiment shown in Fig. 4a the aircraft comprises four fore propellers 1 , 2, 3, 4 which collectively comprise a fore thrust means, and four aft propellers 5, 6, 7, 8 which collectively comprise an aft thrust means.

Fig. 4a is a plan view of the aircraft 300. As shown in Fig. 4a, two of the fore propellers 1 , 2 rotate in a first direction (i.e. clockwise as shown in Fig. 4a) and two of the fore propellers 3, 4 rotate in a second direction opposite to the first direction (i.e. anti- clockwise as shown in Fig. 4a).

As also shown in Fig. 4a, two of the aft propellers 7, 8 rotate in a first direction (i.e. clockwise as shown in Fig. 4a) and two of the fore propellers 5, 6 rotate in a second direction opposite to the first direction (i.e. anti-clockwise as shown in Fig. 4a).

Fig. 4b is a frontal view of the aircraft 300. The arrow in Fig. 4b indicates a roll direction of the aircraft 300. Roll of the aircraft can be effected by altering the amount of thrust of the propellers. The aircraft 300 will roll in the direction of the arrow shown is more thrust is generated by propellers 3, 4, 7 and 8 than propellers 1 , 2, 5 and 6.

Fig. 4c is a side view of the aircraft 300. The arrow in Fig. 4c indicates a pitch direction of the aircraft 300. Pitch of the aircraft can be effected by altering the amount of thrust of the propellers. The aircraft 300 will pitch in the direction of the arrow shown is more thrust is generated by propellers 5, 6, 7 and 8 than propellers 1 , 2, 3 and 4.

Fig. 4d is a plan view of the aircraft 300. The arrow in Fig. 4d indicates a yaw direction of the aircraft 300. Yaw of the aircraft can be effected by altering the rotation speed of the propellers arranged to turn in the same direction. The aircraft 300 will yaw in the direction of the arrow shown if propellers 3, 4, 5 and 6 are controlled to turn faster than propellers 1 , 2, 7 and 8.

Figs. 5a-d schematically show various views of an aircraft 300 in accordance with the present invention illustrating control of the aircraft during a second (horizontal) flight mode. Like reference numerals from Figs. 4a-d have been retained as appropriate. Fig. 5a is a frontal view of the aircraft. As in Fig. 4a, two of the aft propellers 7, 8 rotate in a first direction (i.e. clockwise as shown in Fig. 5a) and two of the fore propellers 5, 6 rotate in a second direction opposite to the first direction (i.e. anti-clockwise as shown in Fig. 5a).

Fig. 5b is a frontal view of the aircraft 300. The arrow in Fig. 5b indicates a roll direction of the aircraft 300. Roll of the aircraft can be effected by altering the rotation speed of the propellers arranged to turn in the same direction. The aircraft 300 will roll in the direction of the arrow shown if propellers 3, 4, 5 and 6 are controlled to turn faster than propellers 1 , 2, 7 and 8.

Fig. 5c is a side view of the aircraft 300. The arrow in Fig. 5c indicates a pitch direction of the aircraft 300. Pitch of the aircraft can be effected by altering the amount of thrust of the propellers. The aircraft 300 will pitch in the direction of the arrow shown is more thrust is generated by propellers 5, 6, 7 and 8 than propellers 1 , 2, 3 and 4.

Fig. 5d is a plan view of the aircraft 300. The arrow in Fig. 5d indicates a yaw direction of the aircraft 300. Yaw of the aircraft can be effected by altering the amount of thrust of the propellers. The aircraft 300 will yaw in the direction of the arrow shown is more thrust is generated by propellers 1 , 2, 5 and 6 than by propellers 3, 4, 7 and 8

As demonstrated by the above, control of the aircraft in each dimension can be effected simply by varying propeller speed. This eliminates the need for separate control surfaces to be present on the aircraft.

The above-described embodiments are exemplary only, and other possibilities and alternatives within the scope of the invention will be apparent to those skilled in the art. For example, while the passenger compartment is connected to the connecting arm via a rotation mechanism in the described embodiment, this is an optional feature to improve passenger comfort and it could be omitted in some embodiments. Alternatively, the passenger compartment may include a seat for a passenger that includes a rotation mechanism operable to adjust the orientation of the seat relative to the passenger compartment. In some embodiments the passenger compartment could be omitted entirely where the aircraft is intended to be an unmanned vehicle (e.g. a drone).

The connecting arm in each embodiment shown is substantially S-shaped, however and suitable shape may be used in practise, e.g. Z-shaped, linear, etc. In some embodiments the fore and aft thrust means may not be directly connected to one another, and may each be separately connected to an intermediate part, such as the passenger compartment.

The number of propellers shown in the Figures is exemplary only. More or fewer could be used in practice. In the limit, only a single thrust means needs to be present on fore and aft parts of the aircraft.

Various alternatives will now be discussed for each component.

Wings / thrust means

The wing design of the present invention is compatible with any of the modern wing configurations designed to enhance performance. The wing design of the above described embodiments is based on simple straight constant chord wing profile. However, any other wing configuration may be implemented, for example the wings may be tapered, swept, delta shaped, trapezoidal etc.

The above described embodiments feature a pair of backward staggered wings, however a forward staggered arrangement may be implemented and / or the number of wings may be increased to provide additional lift and/or additional control surfaces. For example, additional small wings (sometimes called canards) may be fitted to the passenger compartment to improve stability and / or act as an elevator. The above described embodiments do not feature any conventional control surfaces, instead relying on differential thrust and / or differential lift to manoeuvre the aircraft about all three axes (e.g. yaw, roll, pitch). However, the present invention is not limited to this and conventional control surfaces (e.g. ailerons, elevators, slats, flaps, rudders) may be used to provide additional controllability and / or allow the pilot to retain sufficient control over the aircraft in the event of a loss of power / thrust, for example, or to reduce the stall speed of the wings. To enhance aerodynamics, any configuration of wing end plates may be implemented (e.g. winglets, wingtips, etc). The wingtips of the fore and aft wings may join, for example to improve stiffness and reduce wingtip aerodynamic losses, in what is sometimes referred to as joined wings. The above described embodiments feature conventional propellers for simplicity and to reduce the weight of the aircraft. The above described embodiments feature eight tractor propellers (four rotating clockwise and four rotating anticlockwise) to provide redundancy and improve safety. Ideally the propellers are distributed along the wing to provide the benefits of what is known as distributed propulsion (whereby blowing on the wing increases lift, allowing for a reduced wing surface and consequently a reduced drag and structure mass).

It is possible, with the advent of modern flight controllers and auto-pilot electronics and software, to configure the aircraft with as few as three propellers (for example two on the aft wing and one of the forward wing). However, the aircraft should preferably have a minimum of four propellers to ensure optimum controllability.

Similarly, the concept may have any number of smaller propellers (for example, six on each wing for a total of twelve per aircraft), distributed along its wings to provide additional redundancy and further improve the performance of the distributed propulsion. There is also no requirement for an equal number of propellers per wings, and depending on the aerodynamic performance and configuration of each wings, the front wing may have fewer propellers than the aft wing or vice versa.

The propellers may be ducted to improve thrust and efficiency (typically by reducing propeller tip losses and via the extra thrust / lift typically generated by the duct itself).

However, a drawback of ducts is that it may add more mass and complexity to the aircraft through the use of an external fairing.

Similarly, propellers may be replaced by fans and in particular ducted fans to provide additional thrust in a smaller space envelope. In the event ducted fans would be implemented in place of propellers, it is anticipated that a greater number of thrusters may be required to generate sufficient thrust, which may increase complexity and cost.

Tractor (i.e. in front of the wings, pulling on the wing) propellers / fans/ thrusters may be replaced with pusher (i.e. behind the wing, pushing on the wing) propellers.

Alternatively, a combination of tractor and pusher propellers may also be implemented.

Generally, more thrust is required in vertical flight phases than in level flight phases. Consequently, and in the interest of efficiency, some of the motors / propellers may be switched off during level flight phases and possibly fitted with folding propeller arrangements to further reduce drag in level flight.

Similarly, a combination of different types of propellers / thrusters may be implemented to offer a compromise between vertical thrust and level flight speed. For example, a set of large pitch / small diameter propellers may be implemented to allow for fast level flight speed with reduced motor torque, and a set of larger diameter propellers may be implemented to provide high thrust during vertical flight. Both sets of propellers may work together to provide the maximum possible thrust during vertical take-off and landing but the larger diameter propellers may be switched off during level flight to allow the aircraft to travel as fast as possible using as little energy as possible. Finally, in the above described embodiment, the propellers are aligned against the wing. However, it may be possible to stagger the thrusters of each wing to provide additional controllability, in particular in vertical flight phases. For example, the propellers near to the passenger compartment may be further forward than those at the distal ends of the wings.

Aircraft control

Two options are proposed to effect a transition of the aircraft from vertical to horizontal flight:

i) Sensors fixed to the wings: this option consists of referencing (“fixing”) the flight computer sensors (e.g. compass, gyroscope, accelerometers etc.) relative to the wings of the aircraft. In this case, the flight controller“knows” that it is going to be rotated with respect to the earth referential during the transition, and it is programmed to command / control the thrusters to pitch the wings from vertical (e.g. 90deg pitch) to horizontal (e.g. Odeg pitch) whilst commanding the life cell / pod to remain quasi-level at all time (using any suitable angular/position/attitude sensor). In doing so the wings "lead" by rotating ahead of the passenger compartment, and the passenger compartment rotating mechanism "follows" the wings, and rotates relative to the wings accordingly to keep the passenger(s) in a comfortable level or quasi-level position. ii) Sensors fixed to the passenger compartment: this option consists of referencing ("fixing") the flight computer sensors (e.g. compass, gyroscope, accelerometers etc.) to the passenger compartment. In this configuration the flight computer "does not know" that it is going to be rotated with respect to the earth referential during transition. The passenger compartment is commanded to rotate which transiently causes it to be slightly out of alignment with the horizontal direction, forcing the flight controller to adjust the thrusters to cause the wings to rotate with respect to the earth referential and level the orientation of the rotating life cell. In doing so, the passenger compartment "leads" the wings which are forced to "follow" the passenger compartment rotation, and rotate with respect to the earth differential from vertical to horizontal. Ideally, in a redundant flight control architecture, both strategies would be implemented, with a redundant set of sensors and computers (fixed to the wings) and a redundant set of sensors and computers (fixed to the life cell) both in parallel controlling the aircraft attitude.

One flight controller (for example the system fixed to the passenger compartment, designated the main system) would be given authority over the other flight controller (for example the system fixed to the wing, designated the back-up system) and in the event of the main flight controller failing for malfunctioning, the back-up system would safely take over.

An implementation using dissimilar software, sensors and computers, would provide increased safety by avoiding common mode failures of both systems simultaneously, together with redundant and segregated power distribution architectures and the use of multiple redundant thrust means.

Passenger compartment Unlike conventional VTOL aircraft that rely on rotating wings and / or thrusters by mechanical means, the proposed concept in fact effect the wings rotation purely via means of differential thrust / differential lift between the forward and aft wings. For passenger comfort, but not required for safe flight, the life passenger compartment is actuated to remain level (e.g. horizontal) or quasi-level. The actuation mechanism of the life passenger compartment is non-critical and is potentially only lightly loaded as both its mass and aero moments can be balanced about its centre of rotation. Moreover, as there are no rotating masses inside the passenger compartment, its rotation does not generate any gyroscopic effect often source of instability during the transition of VTOL aircrafts. Actuation of the passenger compartment can be achieved by any suitable mean but may typically be implemented either using one of: a direct drive rotary actuator, where the output of the rotary actuator is aligned with the axis of rotation of the passenger compartment; an indirect rotary actuator, where the output of the rotary actuator is offset from the axis of the axis of rotation of the passenger compartment and a link and bell- crank connect the rotary actuator to the passenger compartment axis of rotation; or a linear actuator with its output connected to the passenger compartment axis of rotation via a bell-crank. Ground stability

On the ground, the aircraft may rest on a conventional landing gear comprising shock absorbing struts and wheels. However to reduce weight, complexity and cost simple skids, similar to the skids of a helicopter, are preferable. These could be designed with an element of flexibility in order to dampen the vertical velocity / loads of the aircraft during landing.

Alternatively, as shown in the above described embodiments, the passenger compartment could form a tripod with the aft fixed wing and act as a skid / landing gear. A set of wheels or pads may be fitted to the nose of the passenger compartment (in contact with the ground) and the rotational degree of freedom of the passenger compartment mechanism may be exploited to provide the compliance and dampening required for a comfortable landing. This may be achieved using a linear spring / damper strut if using an indirect rotary or linear actuation system or a rotary damper with its output directly connected to the passenger compartment axis of rotation.

Motor configurations

The present invention relies on a number of thrusters, at least three but ideally four, to allow controlled and stable flight. Preferably, the proposed embodiment includes eight thrusters arranged in redundant pairs to add an element of safety. The use of eight thrusters or more, distributed along the wing, enhances lift, allowing for a reduction in wing surface and a consequent reduction in drag and aircraft mass. This is known as‘distributed propulsion’ and although conventional engines, as found in existing VTOL aircraft, may be used to power each propeller / fan directly or indirectly (via gearboxes and shafts), distributed propulsion is better suited to electric propulsion, where individual electric motors power propellers or fans, with or without gearboxes.

The above described embodiments therefore feature eight variable speed electric motors connected to fixed pitch propellers for a simple and practical implementation of distributed electric propulsion.

To further enhance the safety of the aircraft, different motor technologies may be used when redundancy is implemented. For example, in an eight motors / thrusters configuration, four motors may be permanent rare earth motors (brushed or brushless) whereas the other four motors may be based on a different technology such as switch reluctance motors (not based on permanent magnets) to avoid common mode failures (for example demagnetisation of the permanent magnets due to excessive temperature) to improve safety.

Power sources

Although the present invention is compatible with traditional fossil fuel (e.g. kerosene, petrol, diesel, gas) engines (e.g. piston engines, turbines), the aircraft is better suited to electric propulsion. The aircraft therefore typically requires a source of electricity , preferably high voltage DC if using permanent magnet motors, as this will reduce current and wire gauges.

The source of electricity may be provided by batteries, fuel cells (for example hydrogen fuel cells) or a hybrid-power unit (for example an internal combustion engine coupled with an electric generator). A combination of more than one power source may be used. For example, a hybrid-power unit may provide nominal power and emergency batteries may provide reserve power for non-nominal conditions.

The aircraft would ideally be modular and designed / certified to be compatible with various, interchangeable, sources of power. A dedicated space envelope could be allocated on the aircraft to the power source.

This space envelope / compartment could be fitted with a battery module for a fully electric aircraft with reduced range, or users could elect to purchase a hybrid power module that would fit within the dedicated space envelope and provide enhanced range for users less concerned with emissions. Alternatively, a fuel cell module may be fitted for a cleaner more readily re-chargeable alternative to batteries or hybrid-power unit.

Similarly, in the interest of safety, dissimilar battery technologies may be used between normal and emergency power sources when implementing a fully electric architecture. For example, lighter / smaller lithium ion batteries may be used for normal power and heavier / larger NiMH batteries may be used for emergency power. The weight and space envelope penalty of NiMH batteries would be offset by the increased dissimilarity that would improve the safety of the aircraft and prevent common mode failures that may lead to both normal and emergency batteries failing simultaneously. An example of common mode failure could occur in the case of extreme temperatures (hot or cold) which are better tolerated by NiMH batteries.