An aircraft

Abstract

An aircraft including: a fuselage having a first side, a second side, a fore end and an aft end; there being a first plane which extends horizontally through a portion of the fuselage, a second plane, perpendicular to the first plane, positioned between the fore end and aft end, and a third plane, perpendicular to the first and second planes, positioned between the first side and second side; a first rotor hub connected directly or indirectly to the fuselage and positioned at the first side, the first rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a first axis of rotation; a second rotor hub connected directly or indirectly to the fuselage and positioned at the second side, the second rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a second axis of rotation; and wherein the first and second axes of rotation are non-perpendicular and non-parallel with the first, second and third planes.

Description

[0001] AN AIRCRAFT

[0002] FIELD

[0003] Embodiments of the present invention relate to an aircraft, and in particular to an aircraft having a plurality of rotor hubs.

[0004] BACKGROUND

[0005] Many types of aircraft utilise at least one rotor to generate thrust. Some types of aircraft may include rotors which are used principally for horizontal or forward flight, rotors which are used principally for vertical take-off and landing manoeuvrers, or rotors which may be used for both types of flight condition. Some types of aircraft may be further equipped with lifting surfaces to which the rotors may be connected to. Forothertypes of aircraft, the rotors may be connected to the fuselage directly, or via connecting arms or other support structures.

[0006] For any type of aircraft having at least one rotor, damage to the rotor, particularly destructive failure, may inhibit the aircraft’s ability to stay airborne. In particularly disastrous situations, such failure of a rotor may cause destructive failure to other parts of the aircraft which may result in failure or destruction of the craft and may put at risk the lives of the occupants of the aircraft (for manned aircraft) and those in the vicinity of the aircraft (for both manned and unmanned aircraft). The present invention has been devised to address these issues.

[0007] BRIEF DESCRIPTION OF THE INVENTION

[0008] According to a first aspect of the invention we provide an aircraft including:

[0009] a fuselage having a first side, a second side, a fore end and an aft end; there being a first plane which extends horizontally through a portion of the fuselage,

[0010] a second plane, perpendicular to the first plane, positioned between the fore end and aft end, and

[0011] a third plane, perpendicular to the first and second planes, positioned between the first side and second side;

[0012] a first rotor hub connected directly or indirectly to the fuselage and positioned at the first side, the first rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a first axis of rotation; a second rotor hub connected directly or indirectly to the fuselage and positioned at the second side, the second rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a second axis of rotation; and

[0013] H 15967 WOwherein the first and second axes of rotation are non-perpendicular and non-parallel with the first, second and third planes.

[0014] The aircraft may further include:

[0015] a third rotor hub connected directly or indirectly to the fuselage positioned at the first side, the third rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a third axis of rotation; a fourth rotor hub connected directly or indirectly to the fuselage positioned at the second side, the fourth rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a fourth axis of rotation; and wherein the third and fourth axes of rotation may be non-perpendicular and non-parallel with the horizontal plane, first vertical plane, and second vertical plane of the fuselage.

[0016] The first and second rotor hubs may be positioned generally towards the fore end of the fuselage, and the third and fourth rotor hubs may be positioned generally towards the aft end of the fuselage.

[0017] Each of the axes of rotation may intersect at a common intersection point below the lower rotors of the rotor hubs and preferably below the fuselage.

[0018] Each rotor hub may be operable to provide thrust, and wherein the resultant thrust vector of each rotor hub may be directed away from the first, second and third planes

[0019] The third plane may be positioned equidistantly between the first and second rotor hubs.

[0020] The third plane may be positioned equidistantly between the third and fourth rotor hubs, and preferably wherein the second plane may be positioned equidistantly between the first and third rotor hubs and the second and fourth rotor hubs.

[0021] For each rotor hub there may be an upper rotor plane coincident with a portion of a blade of the upper rotor and lower rotor plane coincident with a portion of a blade of the lower rotor, each of the upper and lower rotor planes may be generally perpendicular to the axis of rotation of the corresponding rotor hub, and wherein the upper and lower rotor planes of each rotor hub may not intersect with any portion of the blades of the other rotor hub(s).

[0022] The upper and lower rotor planes for each rotor hub may be non-parallel with the upper rotor and lower rotor planes of the remaining other rotor hub(s).

[0023] The upper and lower rotor planes for each rotor hub may not intersect any part of the aircraft.

[0024] H 15967 WOThe upper rotor and a lower rotor of each rotor hub may be operable to rotate in the same rotational direction about the respective axis of rotation.

[0025] The upper rotor of each rotor hub may have a first upper blade and the lower rotor of each rotor hub may a first lower blade, and wherein, during rotation, the first upper blade and the first lower blade may be always separated by an angular distance.

[0026] The upper rotor of each rotor hub may include a second upper blade, and wherein the first lower blade may be positioned or positionable at an equidistant angular distance from the first and second upper blades.

[0027] The angular distance may be:

[0028] a) fixed; or

[0029] b) adjustable.

[0030] The lower rotor of each rotor hub may be driven by a first drive source and the upper rotor of each rotor hub may be driven by a second drive source, such that the lower rotor and upper rotor of each rotor hub may be driven independently.

[0031] The first drive source may be positioned between the lower rotor and the second drive source.

[0032] Each rotor hub may include a first shaft member connecting, directly or indirectly, the first drive source to the lower rotor, and a second shaft member connecting, directly or indirectly, the second drive source to the upper rotor.

[0033] The second shaft member may extend through the first shaft member, preferably wherein the first shaft member and second shaft member may be arranged concentrically about the axis of rotation of the respective rotor hub.

[0034] The first and second shaft members of each rotor hub may be connected or connectable, directly or indirectly, by a transmission element, wherein, in use, the transmission element may enable one of the first or second drive source to drive the upper and lower rotors of the respective rotor hub.

[0035] The transmission element may be:

[0036] a) an active clutch, such as a dog clutch or friction plate clutch; or

[0037] b) a passive clutch, such as a sprag clutch or a ratchet and pawl clutch.

[0038] According to a second aspect of the invention we provide an aircraft including:

[0039] a fuselage having a first side, a second side, a fore end and an aft end; there being

[0040] H 15967 WOa horizontal plane which extends through a portion of the fuselage,

[0041] a first vertical plane positioned between the fore end and aft end;

[0042] a first rotor hub connected directly or indirectly to the fuselage positioned at the first side, the first rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a first axis of rotation;

[0043] a second rotor hub connected directly or indirectly to the fuselage positioned at the second side, the second rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a second axis of rotation;

[0044] wherein the first and second axes of rotation intersect at a common intersection point below the first and second rotor hubs, the common intersection point being coincident with the first vertical plane, and wherein the first and second axes of rotation are non-perpendicular and non-parallel with the first vertical plane.

[0045] The aircraft may further include:

[0046] a third rotor hub connected directly or indirectly to the fuselage positioned at the first side, the third rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable about a third axis of rotation;

[0047] a fourth rotor hub connected directly or indirectly to the fuselage positioned at the second side, the fourth rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable about a fourth axis of rotation;

[0048] there being a second vertical plane positioned between the fore end and aft end; and wherein the third and fourth axes of rotation may intersect at a common intersection point below the third and fourth rotor hubs, the common intersection point may be coincident with the second vertical plane, and wherein the third and fourth axes of rotation may be non-perpendicular and non-parallel with the second vertical plane.

[0049] The first and second vertical planes may be coincident, preferably wherein the first, second, third and fourth axes of rotation may all intersect at a common intersection point.

[0050] According to a third aspect of the invention we provide a method of controlling an aircraft, the method including:

[0051] detecting whether an upper or lower rotor of a rotor hub is non-operational;

[0052] and if so deactivating the corresponding upper or lower rotors of the other rotor hub(s).

[0053] According to a fourth aspect of the invention we provide a method of controlling an aircraft, the method including:

[0054] detecting whether one of the plurality of upper or lower blades of a rotor hub is non-operational;

[0055] H 15967 WOand if so increasing the power output of the other of the plurality of lower or upper blades of that rotor hub such that the power output of said rotor hub generally corresponds to the power output of the other rotor hub(s).

[0056] According to a fifth aspect of the invention we provide a method of controlling an aircraft according to any of claims 15 to 20, the method including:

[0057] detecting whether the first or second drive source of a rotor hub is non-operational; and if so deactivating the corresponding first or second drive source of the other rotor hub(s).

[0058] According to a sixth aspect of the invention we provide a method of controlling an aircraft, the method including:

[0059] detecting whether the first or second drive source of a rotor hub is non-operational; and if so activating the transmission element such that the other of the first or second drive source drives both of the upper and lower rotors.

[0060] H 15967 WOBRIEF DESCRIPTION OF THE FIGURES

[0061] In orderthat the present disclosure may be more readily understood, preferable embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0062] FIGURE 1 is a plan view of an aircraft, embodying the present disclosure;

[0063] FIGURE 2 is a side view of the aircraft of figure 1 ;

[0064] FIGURE 3 is a side view of the aircraft of figure 1 ;

[0065] FIGURE 4 is a plan view of an aircraft, embodying the present disclosure;

[0066] FIGURE 5 is a side cross-section view of a rotor hub;

[0067] FIGURE 6 is a side cross-section view of a part of a rotor hub; and

[0068] FIGURE 7 is a plan view of a rotor hub;

[0069] H 15967 WODETAILED DESCRIPTION OF THE DISCLOSURE

[0070] Referring to the figures, there is shown an aircraft 10. The aircraft 10 includes a fuselage 20, a first rotor hub 100A and a second rotor hub 100B. The aircraft 10 may include other features as described below.

[0071] Referring to figures 1 and 2, the fuselage 20 includes a fore end 200A, an aft end 200B, a first side 200C and a second side 200D. The fore end 200A may generally represent a front end of the aircraft 10, and the aft end 200B is generally opposite the fore end 200A and may represent a rear end of the aircraft 10. The first side 200C may represent either a left or right side of the aircraft 10, with said first side 200C positioned or otherwise located generally between the fore end 200A and aft end 200B, and the second side 200D may represent the other of a left or right side of the aircraft 10. It should be understood that in some embodiments of an aircraft 10, the aircraft 10 construction may be such that there is a clearly defined fore end 200A and aft end 200B (for example, the aircraft 10 has only one general orientation and direction of travel during flight). In other embodiments, the general orientation of the aircraft 10 may change during flight, for example depending on the operation of a plurality of rotors, such that the fore end 200A and aft end 200B may be determined by the specific orientation during use or may be arbitrarily designated. In some embodiments, the aircraft 10 may have several degrees of rotational symmetry, or near symmetry.

[0072] Referring to figures 1 and 2, a first plane P1 extends horizontally through a portion of the fuselage 20. The first plane P1 may extend through any appropriate portion of the fuselage. A second plane P2 is positioned between the fore end 200A and the aft end 200B, and is perpendicular to the first plane P1. In other words, the second plane P2 may be a vertical plane, with said plane positioned at any point between the fore end 200A and aft end 200B. In some embodiments, the second plane P2 may be positioned equidistantly, or generally centrally, between the fore end 200A and aft end 200B. A third plane P3 is positioned between the first side 200C and second side 200D, and is perpendicular to the first plane P1 and second plane P2. In some embodiments, the third plane P3 may be positioned equidistantly, or generally centrally, between the first end 200C and second end 200D. It should be understood that terms such as horizontal and vertical are made with reference to the general orientation of the aircraft during typical flight situations, such as cruising and vertical take-off manoeuvres, with their intent, definition and typical usage readily understood by the person skilled in the art.

[0073] The first rotor hub 100A of the aircraft 10 is connected directly or indirectly to the fuselage 20, and is positioned at the first side 200C of the fuselage 20 of the aircraft 10. In embodiments, the first rotor hub 100A may be connected directly to the fuselage 20, or it may be connected to a connecting part 22 such as a strut, pylon, arm or other feature which is also connected to the fuselage 20. The second rotor hub 100B of the aircraft 10 is connected directly or indirectly to the

[0074] H 15967 WOfuselage 20, and is positioned at the second side 200D of the fuselage 20 of the aircraft 10. In embodiments, the second rotor hub 100B may be connected directly to the fuselage 20, or it may be connected to a connecting part 22 such as a strut, pylon, arm or other feature which is also connected to the fuselage 20. In embodiments, the first and second rotor hubs 100A, 100B may be positioned generally towards the fore end 200A of the aircraft 10. As the first and second rotor hubs 100A, 100B are positioned at respective first and second sides 200C, 200D of the fuselage 20, the third plane P3 may therefore be positioned between the first and second rotor hubs 100A, 100B, and in embodiments the third plane P3 may be positioned equidistantly between the first and second rotor hubs 100A, 100B.

[0075] The first rotor hub 100A includes an upper rotor 110A including a plurality of upper blades 115A and a lower rotor 120A having a plurality of lower blades 125A. Each rotor 110A, 120A is operable to rotate about a first axis of rotation 300A. In embodiments, the upper and lower rotors 110A, 120A may separated by a distance which ensures that any of the blades of the plurality of upper blades 115A do not come into contact with any of the blades of the plurality of lower blades 125A when the rotor 100A is in use. In embodiments, the upper rotor 110A and a lower rotor 120A of the first rotor hub 100A may be operable to rotate in the same rotational direction about the first axis of rotation 300A. In other words, the rotors 110A, 120A of the first rotor hub 100A co-rotate. In other embodiments, the rotors 110A, 120A of the first rotor hub 100A may rotate in opposite rotational directions. In other words, the rotors 110A, 120A may counter- rotate. The second rotor hub 100B includes an upper rotor 110B including a plurality of upper blades 115B and a lower rotor 120B having a plurality of lower blades 125B. Each rotor 110B, 120B is operable to rotate about a second axis of rotation 300B. In embodiments, the upper rotor 110B and a lower rotor 120B of the second rotor hub 100B may be operable to rotate in the same rotational direction about the second axis of rotation 300B. In other words, the rotors 110B, 120B of the second rotor hub 100B may corotate. In other embodiments, the rotors 110B, 120B of the second rotor hub 100B may rotate in opposite rotational directions. In other words, the rotors 110B, 120B may counter- rotate.

[0076] As shown in figures 1 and 4, the first and second axes of rotation 300A, 300B are nonperpendicular and non-parallel with the first plane P1 , second plane P2 and third plane P3. As a result, the first and second axes of rotation 300A, 300B are generally tilted or canted with respect the first, second and third planes P1 , P2, P3. In some embodiments, each of the axes of rotation 300A, 300B may intersect at a common intersection point, henceforth referred to as a first intersection point IP’, below the lower rotors 120A, 120B of the rotor hubs 100A, 100B. In embodiments, the first intersection point IP’ may be generally below the fuselage 20. As a result of the orientation of the axes 300A, 300B with respect to the first, second and third planes P1 , P2, P3 in combination with the position of the first intersection point IP’ below the fuselage 20, the first and second axes of rotation 300A, 300B are tilted or canted such that they generally point away from the fuselage 20. As shown in figure 3, the first rotor hub 100A may, in use, generate thrust in a

[0077] H 15967 WOdirection shown by resultant thrust vector 400A, which is generally parallel / colinear / coincident with the first axis of rotation 300A. The resultant thrust vector 400A includes a generally horizontal thrust component which generally points horizontally away from the fuselage 20, and a generally vertical thrust component which generally points vertically upwards and away from the fuselage 20. Therefore, as the first axis of rotation 300A is tilted or canted away from the fuselage 20, the resultant thrust vector 400A generated by the first rotor hub 100A is directed generally away from the fuselage 20. Whilst a resultant thrust vector 400B is generated by the second rotor hub 100B, it is not shown in figure 3. However, it would be readily understood by a person skilled in the art that the resultant thrust vector 400B generated by the second rotor hub 100B is also directed generally away from the fuselage 20.

[0078] In embodiments, and with reference to figures 1 to 4, the aircraft 10 may further include a third rotor hub 100C and a fourth rotor hub 100D. In other embodiments, the aircraft 10 may include three rotor hubs, or may include more than four rotor hubs.

[0079] The third rotor hub 100C of the aircraft 10 may be connected directly or indirectly to the fuselage 20, and may positioned at the first side 200C of the fuselage 20 of the aircraft 10. In embodiments, the third rotor hub 100C may be connected directly to the fuselage 20, or it may be connected to a connecting part 22 such as a strut, pylon, arm or other feature which is also connected to the fuselage 20. The fourth rotor hub 100D of the aircraft 10 is connected directly or indirectly to the fuselage 20, and is positioned at the second side 200D of the fuselage 20 of the aircraft 10. In embodiments, the fourth rotor hub 100D may be connected directly to the fuselage 20, or it may be connected to a connecting part 22 such as a strut, pylon, arm or other feature which is also connected to the fuselage 20. In embodiments, the third and fourth rotor hubs 100C, 100D may be positioned generally towards the aft end 200B of the aircraft 10, and in some embodiments, the third rotor hub 100C may be positioned generally opposite the first rotor hub 100A and the fourth rotor hub 100D may be positioned generally opposite the second rotor hub 100B. As the third and fourth rotor hubs 100C, 100D are positioned at respective first and second sides 200C, 200D of the fuselage 20, the third plane P3 may therefore also be positioned between the third and fourth rotor hubs 100C, 100D, and in embodiments the third plane P3 may be positioned equidistantly between the third and fourth rotor hubs 100C, 100D. Additionally, as the first and second rotor hubs 100A, 100B are positioned at the fore end 200A and the third and fourth rotor hubs 100C, 100D, the second plane P2 may be positioned equidistantly between the first and third rotor hubs 100A, 100C and the second and fourth rotor hubs 100B, 100D.

[0080] The third rotor hub 100C includes an upper rotor 110C including a plurality of upper blades 115C and a lower rotor 120C having a plurality of lower blades 125C. Each rotor 110C, 120C is operable to rotate about a third axis of rotation 300C. In embodiments, the upper rotor 110C and a lower rotor 120C of the third rotor hub 100C may be operable to rotate in the same rotational direction

[0081] H 15967 WOabout the third axis of rotation 300C. In other words, the rotors 110C, 120C of the third rotor hub 100C co-rotate. In other embodiments, the rotors 110C, 120C of the third rotor hub 100C may rotate in opposite rotational directions. In other words, the rotors 110C, 120C may counter- rotate. The fourth rotor hub 100D includes an upper rotor 110D including a plurality of upper blades 115D and a lower rotor 120D having a plurality of lower blades 125D. Each rotor 110D, 120D is operable to rotate about a fourth axis of rotation 300D. In embodiments, the upper rotor 110D and a lower rotor 120D of the fourth rotor hub 100D may be operable to rotate in the same rotational direction about the fourth axis of rotation 300D. In other words, the rotors 110D, 120D of the fourth rotor hub 100D may co-rotate. In other embodiments, the rotors 110D, 120D of the fourth rotor hub 100D may rotate in opposite rotational directions. In other words, the rotors 110D, 120D may counterrotate.

[0082] As shown in figures 1 and 4, the third and fourth axes of rotation 300C, 300D may be nonperpendicular and non-parallel with the first plane P1 , second plane P2 and third plane P3. As a result, the third and fourth axes of rotation 300C, 300D may be generally tilted or canted with respect the first, second and third planes P1 , P2, P3, similar to the first and second axes of rotation 300A, 300B. In some embodiments, each of the axes of rotation 300B, 300D may intersect at a common intersection point, henceforth referred to as a second intersection point IP”, below the lower rotors 120C, 120D of the rotor hubs 100C, 100D. In embodiments, the second intersection point IP” may be generally below the fuselage 20. As a result of the orientation of the axes 300C, 300D with respect to the first, second and third planes P1 , P2, P3 in combination with the position of the second intersection point IP” below the fuselage 20, the third and fourth axes of rotation 300C, 300D are tilted or canted such that they generally point away from the fuselage 20. As shown in figure 3, the fourth rotor hub 100D may, in use, generate thrust in a direction shown by resultant thrust vector 400D, which is generally parallel I colinear I coincident with the fourth axis of rotation 300D. The resultant thrust vector 400D includes a generally horizontal thrust component which generally points horizontally away from the fuselage 20, and a generally vertical thrust component which generally points vertically upwards and away from the fuselage 20. Therefore, as the fourth axis of rotation 300D is tilted or canted away from the fuselage 20, the resultant thrust vector 400D generated by the fourth rotor hub 100D is directed generally away from the fuselage 20. Whilst a resultant thrust vector 400C is generated by the third rotor hub 100C, it is not shown in figure 3. However, it would be readily understood by a person skilled in the art that the resultant thrust vector 400C generated by the third rotor hub 100C is also directed generally away from the fuselage 20.

[0083] In embodiments of an aircraft 10 including first and second rotor hubs 100A, 100B, and third and fourth rotor hubs 100C, 100D, the first and second axes 300A, 300B and third and fourth axes 300C, 300D may all be tilted or canted with respect to the fuselage, such that their resultant thrust vectors 400A, 400B, 400C, 400D are all directed away from the fuselage 20. In figure 4, the

[0084] H 15967 WOaircraft 10 is arranged such that the first and second axes of rotation 300A, 300B intersect at a first intersection point IP’, and the third and fourth axes of rotation 300C, 300D, intersect at a second intersection point IP”. In such embodiments, the first intersection point IP’ is positioned below the first and second rotor hubs 100A, 100B and is coincident with a first vertical plane VP1 , with said first vertical plane VP1 being positioned between the fore end 200A and aft end 200B, and which may be parallel with the second plane P2 described above. The first intersection point IP’ is positioned coincident with the first vertical plane VP1 such that the first and second axes of rotation 300A, 300B are non-perpendicular and non-parallel with the first vertical plane VP1. As a result of the first intersection point IP’ being below the first and second rotor hubs 100A, 100B, and preferably the fuselage 20, the first and second axes of rotation 300A, 300B are thus also nonperpendicular and non-parallel with a horizontal plane HP which extends through a portion of the fuselage 20, which may be parallel with, or may be, the first plane P1 described above.

[0085] In embodiments of an aircraft 10 including four rotor hubs 100A, 100B, 100C, 100D, the second intersection point IP” may be positioned below the third and fourth rotor hubs 100C, 100D and is coincident with a second vertical plane VP2, with said second vertical plane VP2 being positioned between the fore end 200A and aft end 200B, and which may be parallel with the second plane P2 described above and the first vertical plane VP1. The second intersection point IP” may be positioned coincident with the second vertical plane VP2 such that the third and fourth axes of rotation 300C, 300D are non-perpendicular and non-parallel with the second vertical plane VP2. As a result of the second intersection point IP” being below the third and fourth rotor hubs 100C, 100D, and preferably the fuselage 20, the third and fourth axes of rotation 300C, 300D are thus also non-perpendicular and non-parallel with a horizontal plane HP which extends through a portion of the fuselage 20, which may be parallel with, or may be, the first plane P1 described above.

[0086] In the embodiment shown in figure 4, the first vertical plane VP1 and second vertical plane VP2 may be relatively parallel with each other, but not coincident. As such, the first and second vertical planes VP1 , VP2 may be separated by a distance, which is determined by the location and tilt of the rotor hubs 100A, 100B, 100C, 100D and corresponding axes of rotation 300A, 300B, 300C, 300D, and may also be separated by a distance from the second plane P2. In some embodiments, the first and second vertical planes VP1 , VP2 may be positioned equidistantly from the second plane P2. Additionally, in such embodiments, the first and I or second intersection points IP’, IP”, may also be coincident with the third plane P3. In embodiments wherein the third plane P3 is positioned equidistantly between the first and second rotors hubs 100A, 100B and I or the third and fourth rotor hubs 100C, 100D, the corresponding first and I or second intersection points IP’, IP” may also be positioned equidistantly between said rotor hubs 100A, 100B, 100C, 100D.

[0087] H 15967 WOIn other embodiments, such as the aircraft 10 shown in figure 2, the axes of rotation 300A, 300B, 300C, 300D may all intersect at a common intersection point IP, which may be positioned below the rotor hubs 100A, 100B, 100C, 100D and preferably the fuselage 20. As a result, the common intersection point IP of the axes of rotation 300A, 300B, 300C, 300D may be coincident with a single plane. In embodiments, said plane may be the second plane P2, or may be a plane parallel with, or coincident with, the second plane P2. Additionally, the common intersection point IP may be coincident with the third plane P3. In embodiments wherein the third plane P3 is positioned equidistantly between the rotors hubs 100A, 100B, 100C, 100D, the common intersection points IP may also be positioned equidistantly between said rotor hubs 100A, 100B, 100C, 100D.

[0088] In embodiments, the common intersection point IP may also be coincident with a vertical axis VA which may extend through the fuselage 20 and which is generally perpendicular to the first plane P1. Due to the tilt of the rotor hubs 100A, 100B, 100C, 100D away from the fuselage 20, the corresponding axes of rotation 300A, 300B, 300C, 300D, may be inclined or tilted relative to the vertical axis by a tilt angle a. As shown in figure 2, the first axis of rotation 300A of the first rotor hub 100A may be inclined relative to the vertical axis VA by the tilt angle a, and it should be understood that each of the axes 100A, 100B, 100C, 100D may be inclined relative to the vertical axis VA by the tilt angle a. In embodiments, the tilt angle a may be of the same magnitude for each axes 100A, 100B, 100C, 100D, and in such embodiments the vertical axis VA may extend through a geometric centre point of the fuselage 20, or of the entire aircraft 10. It should be understood that in other embodiments, the tilt angle a may differ in magnitude for at least one of the axes 100A, IOOB, 100C, 100D.

[0089] As set out above, the first and second rotor hubs 100A, 100B include an upper rotor 110A, 110B, and a lower rotor 120A, 120B. In embodiments, the third and fourth rotor hubs 100C, 100D may also include an upper rotor 110C, 110D and a lower rotor 120C, 120D. Features of the upper rotor and lower rotor will now be described with specific reference to the upper rotor 110A and lower rotor 120A of the first rotor hub 100A. It should be understood that the following features may also be included in the upper rotors 110B, 110C, 110D of the second, third and fourth rotor hubs 100B, IOOC, 100D.

[0090] In embodiments, there may be an upper rotor plane UP coincident with a portion of a blade of the plurality of upper blades 115A and a lower rotor plane LP coincident with a portion of a blade of the plurality of lower blades 125A, with each of the upper and lower rotor planes UP, LP being generally perpendicular to the axis of rotation 300A of the first rotor hub 100A. As a result of the tilting or canting of the rotor hub 100A and corresponding axis of rotation 300A, the upper and lower rotor planes UP, LP may be tilted or canted with respect to the first, second and third planes P1 , P2, P3, and accordingly may be non-perpendicular and non-parallel with respect to the first, second and third planes P1, P2, P3.

[0091] H 15967 WOIn embodiments, the upper and lower planes UP, LP of each rotor hub 100A, 100B, 100C, 100D may thus each be non-perpendicular and non-parallel with respect to the first, second and third planes P1 , P2, P3. Accordingly, the upper and lower rotor planes UP, LP of each rotor hub 100A, 100B, 100C, 100D may not intersect with any portion of the blades of the other rotor hubs 100A, 100B, 100C, 100D. Additionally, the tilting orcanting of the axes of rotation 300A, 300B, 300C, 300D of each rotor hub 100A, 100B, 100C, 100D ensures that the upper and lower rotor planes UP, LP of a respective rotor hub 100A, 100B, 100C, 100D do not intersect with any of the upper and lower rotor planes UP, LP of the remaining other rotor hubs 100A, 100B, 100C, 100D. In embodiments, such as those shown in figures 1 to 4, the upper and lower rotor planes UP, LP may be oriented such that said planes UP, LP do not intersect any part of the aircraft 10.

[0092] Advantageously, the tilting or canting of each of the first and second rotor hubs 100A, 100B, and third and fourth rotor hubs 100C, 100D in embodiments, means that in the event of failure of a rotor hub (such as a blade shedding event for an upper or lower rotor) damage to other rotor hubs and the fuselage can be minimised or prevented entirely. This is because, in the event of a blade shedding event for example, the initial trajectory of a blade shed from the hub will be generally parallel with the corresponding rotor plane UP, LP. As the rotor planes UP, LP do not intersect any of the neighbouring rotor hubs, or the fuselage 20, the shed blade will be prevented from coming into contact with said rotor hubs or fuselage 20.

[0093] As set out above, in embodiments the upper rotor and lower rotor 110A, 120A of the first rotor hub 100A (and of all other rotor hubs) may be operable to rotate in the same rotational direction, such that the rotors 110A, 120A are co-rotating rotors. The upper rotor and lower rotor 110A, 120A each include a plurality of blades 115A, 125A. The plurality of upper blades 115A may include a first upper blade 1151A and the plurality of lower blades 125A may include a first lower blade 1251A, with respective blades 1151 A and 1251 A being always separated by an angular distance 0, when the aircraft 10 is in use. The angular distance 0 may be determined by the expect in-flight requirements of the rotors, and also by the number of blades of each rotor. In embodiments, the angular distance 0 between the first upper blade 1151A and the first lower blade 1251A fixed or permanent, such that it cannot change when the aircraft 10 is in use. In other embodiments, the angular distance 0 may be adjustable when the aircraft 10 is in use, and thus may be reconfigurable such that it may adapt to flight conditions (such as cruising and vertical take-off) or to exterior conditions such as environmental factors. In embodiments, the upper rotor 110A may include a second upper blade 1152A, and the first lower blade 1251A may be positioned between the first and second upper blades 1151 A and 1152A. In embodiments, the first lower blade 1251 A may be positioned or positionable (depending on whether the angular relation is fixed or adjustable) at an equidistant angular distance 0 from each of the first and second upper blades 1151 A, 1151 B. In other embodiments, the first lower blade 1251 A may be positioned relatively closer to one of the first or second upper blades 1151 A, 1152A, such that the angular distance

[0094] H 15967 WObetween the first lower blade 1251A and the first upper blade 1151A is different to the angular distance between the first lower blade 1251 A and the second upper blade 1152A. An advantage of this angular separation between the blades is that, in the event of failure of a blade, such as a blade shedding event or a blade being deformed momentarily or permanently, the occurrence of the shed blade coming into contact with a blade of the neighbouring upper or lower rotor will be minimised, or prevented entirely.

[0095] In embodiments, and as shown in figures 5 and 6, the upper and lower rotor hubs 110A, 110B of the first rotor hub 100A (or any rotor hub) may be driven by a drive source 50. In embodiments, the drive source 50 may include a first drive source 50A and a second drive source 50B. The lower rotor 120A may be driven by the first drive source 50A, and the upper rotor 110A may be driven by the second drive source 50B. The drive sources 50A, 50B may be any suitable drive source, such as an electric motor, or an engine powered by petrol, diesel, biodiesel hydrogen, nitrogen or may be powered by a hybrid energy source or any other suitable power source. As such, the upper and lower rotor 110A, 120A may be driven independently from each other. As shown in figures 5 and 6, the first drive source 50A may be positioned between the lower rotor 120A and the second drive source 50B. In other words, the first drive source 50A may be positioned below the lower rotor 120A, and the second drive source 50B may be positioned below the first drive source 50A and the lower rotor 120A. In embodiments, the first drive source 50A and second drive source 50B may be generally in-line with each other. Each rotor hub 100A may include a first shaft member 60A connecting, directly or indirectly, the first drive source 50A to the lower rotor 120A, and a second shaft member 60B, connecting, directly or indirectly, the second drive source 50B to the upper rotor 110A. In embodiments, the first and I or second shaft member 60A, 60B may connect the corresponding drive source 50A, 50B directly to the corresponding rotor 110A, 120A, for example by connecting directly to an output shaft of the drive source 50A, 50B. In other embodiments, the first and I or second shaft member 60A, 60B may connect the corresponding rotor 110A, 120A to an intermediate transmission (such as a gearbox), or to another shaft member which is connected to the corresponding drive source 50A, 50B. Advantageously, the independent driving of the upper and lower rotors 110A, 120A means that each rotor 110A, 120A can be driven at an optimum speed, and the relative speeds between them and / or the angular distance between the blades of the upper and lower rotor 110A, 120A can be easily altered if necessary. Additionally, if one drive source 50A, 50B fails, the other of the drive sources 50A, 50B may still be able to drive the remaining rotor 110A, 120A, such that the rotor hub 100A may be at least partially operable and a level of thrust may still be generated by the rotor hub 100A.

[0096] As shown in figures 5 and 6, the first drive source 50A and second drive source 50B may be positioned generally in-line with each other, such that the second drive source 50B is positioned below the first drive source 50B. This tandem arrangement of drive sources 50A, 50B is beneficial as it may minimise the occurrence of damage to one of the drive sources 50A, 50B in the event of

[0097] H 15967 WOfailure of the other of the drive sources 50A, 50B. For example, if one of the drive sources 50A, 50B is impacted, or undergoes a catastrophic event such as an explosion, much of the debris shed by the drive source 50A, 50B may be directed away from the other drive source 50A, 50B.

[0098] The second shaft member 60B, which connects to the upper rotor 110A, may extend through the first shaft member 60A, such that at least a portion of the second shaft member 60B is circumferentially surrounded by at least a portion of the first shaft member 60A. As such, the first and second shaft members 60A, 60B may both rotate about the axis of rotation of 300A of the rotor hub 110A, and may be arranged concentrically about the axis of rotation 300A of said rotor hub 110A. In embodiments, the first and second shaft members 60A, 60B may be independently connected to the corresponding drive source 50A, 50B, such that the rotation of one shaft member 60A, 60B, does not influence the rotation of the other of the shaft members 60A, 60B in use of the aircraft 10. In some embodiments, the shaft members 60A, 60B may be connected, or connectable, directly or indirectly, to each other by a transmission element 70. In use, the transmission element 70 may enable one of the first or second drive source 50A, 50B to drive the upper and lower rotors 110A, 120A of the respective rotor hub 100A. In use, the transmission element 70 may be selectively activated such that it may drive both upper and lower rotors 110A, 120A only when necessary. In some embodiments, the transmission element 70 may be operable as an active clutch, such as, but not limited to, a dog clutch or a friction plate clutch which may be selectively actuatable such that it may drivingly connect and disconnect the drive sources 50A, 50B. In such embodiments, the transmission element 70 may be monitored and controlled by an onboard control system which forms part of the aircraft 10, or may be monitored and controlled by a user of the aircraft 10. In other embodiments, the transmission element 70 may be a passive clutch, such as, but not limited to, a sprag clutch or a ratchet and pawl clutch which may only drivingly connected and disconnect the drive sources 50A, 50B when a change of operating condition occurs, such as a change in relative speed between the two shaft members 60A, 60B. Such a passive clutch may or may not be monitored by an onboard control system, or may or may not be monitored by a user of the aircraft 10.

[0099] The first and second drive sources 50A, 50B independently driving each rotor 110A, 110B is advantageous as the rotor hubs 100A, 100B, 100C, 100D may be controlled in response to changing flight conditions, particularly those resulting from the loss of operation of a rotor 110A, 120A ora rotor hub 100A. Several methods of controlling the aircraft 10 are set out in detail below. It should be understood that where reference is made to the first rotor hub 100A and its corresponding features, the below description is applicable to any of the rotor hubs 100B, 100C, 100D, if such rotor hubs are included in an embodiment of the aircraft 10.

[0100] In one example method, the aircraft 10 may be controlled by detecting whether an upper or lower rotor 110A, 110B is non-operational. For example, the aircraft 10 may include onboard sensors

[0101] H 15967 WOwhich may detect the operation or integrity of each of the rotors 110A, 110B, or the operation may be detecting by sensing the performance of the drive sources 50A, 50B. Thus, non-operation may be considered to result from a loss of integrity of a rotor 110A, 11 OB, a shaft member 60A, 60B, and I or of the corresponding drive source 50A, 50B. In such a scenario, when it is detected that an upper or lower rotor 110A, 120A is non-operational, the corresponding upper or lower rotor(s) of the other rotor hub(s) 110B, 110C, 110D may be deactivated accordingly. A benefit of this control method is that, in the event of one rotor 110A, 120A being rendered non-operational, deactivating the equivalent rotor of the other hub(s) may serve to equalise or balance the thrust generated by all of the rotor hub(s) such that stable flight is maintained.

[0102] In other scenarios, the aircraft 10 may be controlled by detecting whether one of the blades of the plurality of upper or lower blades 115A, 125A is non-operational. For example, if the blade is damaged or is shed from the rotor 110A, 120A. When it is detected that one of, or at least one of, the upper or lower blades 115A, 125A is non-operational, the power output of the other of the plurality of blades 115A, 125A may be increased such that the overall power output of the rotor hub 110A is maintained, and generally corresponds to the power output of the other rotor hub(s) 11 OB, 110C, 110D. Additionally, the method of controlling the aircraft 10 in this scenario may also include disconnecting the compromised rotor 110A, 120A from its respective drive source 50A, 50B, such that the remaining blades may freely autorotate, or the comprised rotor 110A, 120A may be stalled entirely, such as by a brake, such that it is held in place whilst the remaining rotor 110A, 120A rotates and generates thrust.

[0103] Additionally, the aircraft 10 may be controlled by detecting whether one of the first or second drive sources 50A, 50B is non-operational. For example, non-operation may be due to an electrical fault, a lack of fuel, or mechanical failure / damage of the drive source. When it is detected that one of the drive sources 50A, 50B is non-operational, the corresponding first or second drive source of the other rotor hub(s) 100B, 100C, 100D may be deactivated, such that generally equal power output is maintained by all of the rotor hubs. Alternatively, when it is detected that one of the drive sources 50A, 50B is non-operational, the power output of other of the drive sources 50A, 50B may be increased such that the power output of the rotor hub 110A is generally equivalent to the power output of the remaining rotor hub(s) 110B, 110C, 110D. Additionally, if it is not possible to raise the power output of the rotor hub 110A to be generally equivalent to the power output of the remaining rotor hub(s), the power output of the remaining rotor hub(s) may also be decreased to match the power output of the compromised rotor hub 100A.

[0104] In another method of controlling the aircraft 10, upon detection of one of the first or second drive sources 50A, 50B being non-operational, the transmission element 70 may be activated such that the other of the first or second drive source 50A, 50B drives both of the upper and lower rotors 110A, 120A. For example, an onboard control system may be operable to activate an active clutch, such

[0105] H 15967 WOas a friction plate clutch, when non-operation of one of the drive sources 50A, 50B is detected. Alternatively, a passive clutch may be automatically activated when one of the drive sources 50A, 50B is non-operational.

[0106] Embodiments of the aircraft 10 described above provide many advantageous synergistic benefits. The tilting or canting of the rotor hubs 100A, 100B, 100C, 100D, and thus the tilting or canting of the associated rotor hubs, away from fuselage 20 may serve to minimise or prevent the occurrence of a blade shed from one rotor hub impacting the rotors of the remaining rotor hubs, and from impacting the fuselage 20. Therefore, in the instance of failure of part of a rotor hub, safe operation of the remaining hubs and the aircraft 10 more generally may be ensured. The co-rotation of the upper and lower rotors of each rotor hub, and the selection of a suitable angular distance between the blades of each rotor as described above, serves to minimise collision between neighbouring blades in the event of blade damage, deformation or shedding. Therefore, in the instance of failure of a blade of a rotor hub, the safe operation of said rotor hub may also be ensured. The provision of independent drive sources 50A, 50B is beneficial in that each rotor of a rotor hub may be independently driven, and thus the control of thrust from each rotor hub may be controlled in many different ways as described above, which enables the aircraft 10 to adapt the operation of each rotor hub when other rotor hubs are rendered non-operational. Additionally, the tandem arrangement of the drive sources for each rotor hub as described above may serve to minimise damage to one of the drive sources when the other drive source is damaged, such as by impact or explosion. Therefore, embodiments of the aircraft 10 are optimised to minimise the impact of many different types of failure, non-operativity, and damage to rotating parts of the aircraft 10, and thus safety of operation of the aircraft 10 is ensured to those who are onboard the aircraft 10 (for manned aircraft) and those who are in the vicinity of the aircraft 10 (for both manned and unmanned aircraft).

[0107] When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

[0108] The invention may also broadly consist in the parts, elements, steps, examples and / or features referred to or indicated in the specification individually or collectively in any and all combinations of two or more said parts, elements, steps, examples and / or features. In particular, one or more features in any of the embodiments described herein may be combined with one or more features from any other embodiment(s) described herein.

[0109] Protection may be sought for any features disclosed in any one or more published documents referenced herein in combination with the present disclosure.

[0110] H 15967 WOAlthough certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and / or to encompass equivalents.

[0111] H 15967 WO

Claims

CLAIMS1. An aircraft including:a fuselage having a first side, a second side, a fore end and an aft end; there being a first plane which extends horizontally through a portion of the fuselage,a second plane, perpendicular to the first plane, positioned between the fore end and aft end, anda third plane, perpendicular to the first and second planes, positioned between the first side and second side;a first rotor hub connected directly or indirectly to the fuselage and positioned at the first side, the first rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a first axis of rotation; a second rotor hub connected directly or indirectly to the fuselage and positioned at the second side, the second rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a second axis of rotation; andwherein the first and second axes of rotation are non-perpendicular and non-parallel with the first, second and third planes.

2. An aircraft according to claim 1 , further including:a third rotor hub connected directly or indirectly to the fuselage positioned at the first side, the third rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a third axis of rotation; a fourth rotor hub connected directly or indirectly to the fuselage positioned at the second side, the fourth rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a fourth axis of rotation; and wherein the third and fourth axes of rotation are non-perpendicular and non-parallel with the horizontal plane, first vertical plane, and second vertical plane of the fuselage.

3. An aircraft according to claim 2, wherein the first and second rotor hubs are positioned generally towards the fore end of the fuselage, and the third and fourth rotor hubs are positioned generally towards the aft end of the fuselage.

4. An aircraft according to any preceding claim, wherein each of the axes of rotation all intersect at a common intersection point below the lower rotors of the rotor hubs and preferably below the fuselage.H 15967 WO5. An aircraft according to any preceding claim, wherein each rotor hub is operable to provide thrust, and wherein the resultant thrust vector of each rotor hub is directed away from the first, second and third planes6. An aircraft according to any preceding claim, wherein the third plane is positioned equidistantly between the first and second rotor hubs.

7. An aircraft according to claim 6, when dependent directly or indirectly on claim 2, wherein the third plane is positioned equidistantly between the third and fourth rotor hubs, and preferably wherein the second plane is positioned equidistantly between the first and third rotor hubs and the second and fourth rotor hubs.

8. An aircraft according to any preceding claim, wherein for each rotor hub there is an upper rotor plane coincident with a portion of a blade of the upper rotor and lower rotor plane coincident with a portion of a blade of the lower rotor, each of the upper and lower rotor planes being generally perpendicular to the axis of rotation of the corresponding rotor hub, and wherein the upper and lower rotor planes of each rotor hub do not intersect with any portion of the blades of the other rotor hub(s).

9. An aircraft according to claim 8, wherein the upper and lower rotor planes for each rotor hub are non-parallel with the upper rotor and lower rotor planes of the remaining other rotor hub(s).

10. An aircraft according to claim 8 or claim 9, wherein the upper and lower rotor planes for each rotor hub do not intersect any part of the aircraft.

11. An aircraft according to any preceding claim, wherein the upper rotor and a lower rotor of each rotor hub are operable to rotate in the same rotational direction about the respective axis of rotation.

12. An aircraft according to any preceding claim, wherein the upper rotor of each rotor hub has a first upper blade and the lower rotor of each rotor hub has a first lower blade, and wherein, during rotation, the first upper blade and the first lower blade are always separated by an angular distance.

13. An aircraft according to claim 12, wherein the upper rotor of each rotor hub includes a second upper blade, and wherein the first lower blade is positioned or positionable at an equidistant angular distance from the first and second upper blades.

14. An aircraft according to claim 12 or 13, wherein the angular distance is:a) fixed; orb) adjustable.H 15967 WO15. An aircraft according to any preceding claim, wherein the lower rotor of each rotor hub is driven by a first drive source and the upper rotor of each rotor hub is driven by a second drive source, such that the lower rotor and upper rotor of each rotor hub may be driven independently.

16. An aircraft according to claim 15, wherein the first drive source is positioned between the lower rotor and the second drive source.

17. An aircraft according to claim 15 or 16, wherein each rotor hub includes a first shaft member connecting, directly or indirectly, the first drive source to the lower rotor, and a second shaft member connecting, directly or indirectly, the second drive source to the upper rotor.

18. An aircraft according to claim 17, wherein the second shaft member extends through the first shaft member, preferably wherein the first shaft member and second shaft member are arranged concentrically about the axis of rotation of the respective rotor hub.

19. An aircraft according to claim 17 or 18, wherein the first and second shaft members of each rotor hub are connected or connectable, directly or indirectly, by a transmission element, wherein, in use, the transmission element enables one of the first or second drive source to drive the upper and lower rotors of the respective rotor hub.

20. An aircraft according to claim 19, wherein the transmission element is:a) an active clutch, such as a dog clutch or friction plate clutch; orb) a passive clutch, such as a sprag clutch or a ratchet and pawl clutch.

21. An aircraft including:a fuselage having a first side, a second side, a fore end and an aft end; there beinga horizontal plane which extends through a portion of the fuselage, a first vertical plane positioned between the fore end and aft end;a first rotor hub connected directly or indirectly to the fuselage positioned at the first side, the first rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a first axis of rotation;a second rotor hub connected directly or indirectly to the fuselage positioned at the second side, the second rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable to rotate about a second axis of rotation;wherein the first and second axes of rotation intersect at a common intersection point below the first and second rotor hubs, the common intersection point being coincident with the first verticalH 15967 WOplane, and wherein the first and second axes of rotation are non-perpendicular and non-parallel with the first vertical plane.

22. An aircraft according to claim 21 , further including:a third rotor hub connected directly or indirectly to the fuselage positioned at the first side, the third rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable about a third axis of rotation;a fourth rotor hub connected directly or indirectly to the fuselage positioned at the second side, the fourth rotor hub having an upper rotor including a plurality of upper blades and a lower rotor including a plurality of lower blades, each rotor operable about a fourth axis of rotation;there being a second vertical plane positioned between the fore end and aft end; and wherein the third and fourth axes of rotation intersect at a common intersection point below the third and fourth rotor hubs, the common intersection point being coincident with the second vertical plane, and wherein the third and fourth axes of rotation are non-perpendicular and nonparallel with the second vertical plane.

23. An aircraft according to claim 22, wherein the first and second vertical planes are coincident, preferably wherein the first, second, third and fourth axes of rotation all intersect at a common intersection point.

24. A method of controlling an aircraft according to any of claims 15 to 20, the method including:detecting whether an upper or lower rotor of a rotor hub is non-operational;and if so deactivating the corresponding upper or lower rotors of the other rotor hub(s).

25. A method of controlling an aircraft according to any of claims 15 to 20, the method including:detecting whether one of the plurality of upper or lower blades of a rotor hub is non-operational;and if so increasing the power output of the other of the plurality of lower or upper blades of that rotor hub such that the power output of said rotor hub generally corresponds to the power output of the other rotor hub(s).

26. A method of controlling an aircraft according to any of claims 15 to 20, the method including:detecting whether the first or second drive source of a rotor hub is non-operational; and if so deactivating the corresponding first or second drive source of the other rotor hub(s).

27. A method of controlling an aircraft according to any of claims 19 or 20, the method including:detecting whether the first or second drive source of a rotor hub is non-operational; and if so activating the transmission element such that the other of the first or second drive source drives both of the upper and lower rotors.H 15967 WO

Images