Abstract:
An asymmetric multirotor helicopter has a structure supporting at least one main and two secondary propulsion systems. A flight control unit controls the helicopter by varying the relative speed of each of the main and secondary propulsion systems. Each main propulsion system includes at least one main motor drive and a main drive shaft that carries and propels a main differential contra-rotating transmission configured to share the power provided by the main drive shaft with two contra-rotating output shafts. Each secondary propulsion system includes at least one secondary motor drive and a secondary drive shaft that carries and propels respective secondary propulsion blades. The two contra-rotating output shafts support and propel for mutually contra-rotation motion two sets of main propulsion blades. The main drive shaft rotates in the same direction as one of the secondary drive shafts and at least one secondary drive shaft rotates in an opposite direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Israeli Application No 231811 filed on Mar. 16, 2014. 
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     Not Applicable 
     BACKGROUND 
     Field of the Invention 
     This invention relates to a helicopter with a multirotor structure and flight control. 
     Background of the Invention 
     The following definition for a multirotor aerial vehicle is: 
     “A multirotor or multicopter is a rotorcraft with more than two rotors. Multirotors often use fixed-pitch blades, whose rotor pitch does not vary as the blades rotate; control of vehicle motion is achieved by varying the relative speed of each rotor to change the thrust and torque produced by each. Due to their ease of both construction and control, multirotor aircraft are frequently used in model and radio control aircraft projects in which the names quadcopter, hexacopter and octocopter are frequently used to refer to 4-, 6- and 8-rotor helicopters, respectively. Radio controlled multirotors are increasingly used as a low-budget option to create aerial photography and videos of sites and buildings. 
     Flight control in electronically controlled multicopters is achieved using a minimum of four control channels. One channel is usually labeled throttle and increases or decreases power to all motors evenly. This causes the aircraft to ascend or descend. The other three channels, labeled aileron, elevator, and rudder, control the roll, pitch, and yaw axes respectively. These three control inputs work by causing a change in aircraft attitude (tilt or direction). For example in an X4 configuration, forward tilt or pitch is controlled by increasing the speed of the two rear motors while decreasing the speed of the two front motors. Left or right tilt or roll is controlled by differences in the speed of the two right motors vs. the two left motors. The flight direction or yaw is controlled by changing the relative speeds of adjacent, counter rotating motors.” (Wikipedia) 
     A typical multirotor propulsion system may be of direct-drive configuration i.e. without reduction gear, this being the most common configuration, or of non-direct-drive configuration i.e. with reduction gear, this being a less common configuration. As long as a low lift capacity is required, as it is in model and radio controlled aircraft projects, the multirotor configuration has advantages relating to simplicity of structure and flight control and in reduced production costs, relative to the typical single main rotor helicopter configurations (e.g. Bell 206), or to tandem main rotor configurations (e.g. Chinook). However, at high lift capacity, the complexity of the multirotor vehicle, because of the higher number of high power propulsion systems and because of the significantly larger span and weight of the vehicle&#39;s structure required to support the propulsion systems, is considered a disadvantage relative to typical helicopter configurations. 
     It would obviously be advantageous to combine the advantages of a multirotor configuration relating to simplified structure and flight control with those of a typical helicopter having fewer main propulsion systems. 
     BRIEF SUMMARY 
     It is therefore a broad object of the present invention to provide a helicopter with a multirotor structure and flight control. 
     Specifically, it is an object of the invention to provide a helicopter having one or, at most, two main rotors, and a flight control based on the structure and principles of multirotor flight control. 
     The object is realized in accordance with a broad aspect of the invention by an asymmetric helicopter with a multirotor structure and flight control having the features of the main claim. 
     In one embodiment, the asymmetric helicopter comprises:
         a structure supporting at least one main propulsion system and at least two secondary propulsion systems;   a flight control unit configured to control the helicopter by varying the relative speed of each of the main and secondary propulsion systems to change the respective thrust and the torque produced thereby;   each of said main propulsion systems including at least one main motor drive and a main drive shaft that carries and propels a main differential contra-rotating transmission;   each of said secondary propulsion systems including at least one secondary motor drive and a secondary drive shaft that carries and propels respective secondary propulsion blades;   said main differential contra-rotating transmission being configured to share the power provided by the main drive shaft with two contra-rotating output shafts so as to transfer an increased output torque to the two contra-rotating output shafts without inducing a high reactive torque on said structure;   said two contra-rotating output shafts supporting and propelling for mutually contra-rotation motion two sets of main propulsion blades;   at least one of the main drive shafts being configured to rotate in the same direction as one or more of the main or secondary drive shafts;   at least one of the secondary drive shafts being configured to rotate in an opposite direction to the direction of at least one of the main or secondary drive shaft;   said main propulsion blades being adapted to provide significantly higher thrust than the secondary propulsion blades; and   said main drive shaft generating a torque that varies in a range matching that of the secondary drive shaft.       

     The flight control unit controls the helicopter based on known principles of the multirotor flight control. Thus, the flight control unit has a throttle, aileron, elevator, and rudder configured to operate as follows:
         ascend or descend is controlled by increasing or decreasing the power to the all the propulsion systems;   forward tilt is controlled by the elevator by increasing the speed of the rear propulsion system while decreasing the speed of the front propulsion system;   rear tilt is controlled by the elevator by increasing the speed of the front propulsion system while decreasing the speed of the rear propulsion systems;   left tilt is controlled by the aileron by increasing the speed of the right propulsion system while decreasing the speed of the left propulsion system;   right tilt is controlled by the aileron by increasing the speed of the left propulsion system while decreasing the speed of the right propulsion system;   left direction is controlled by the rudder by increasing the speed of the clockwise rotating motor drives while decreasing the speed of the counterclockwise rotating motor drives; and   right direction is controlled by the rudder by increasing the speed of the counter-clockwise rotating motor drives while decreasing the speed of the clockwise rotating motor drives.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIGS. 1 to 4  are a general perspective, side, front and top views of a single main rotor helicopter with a multirotor structure and flight control of “+” configuration; and 
         FIGS. 5 to 7  are general perspective, side and top views of a two main rotors helicopter with a multirotor structure and flight control of “+” configuration; 
     
    
    
     DETAILED DESCRIPTION 
     The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments. 
       FIGS. 1 to 4  are general representations of a single main rotor helicopter with a multirotor structure and flight control of “+” configuration shown generally as  2  comprising a main structure  4  that carries a main propulsion system  6  and having three secondary arms  8 , each adapted to carry one of three secondary propulsion systems  10 ,  12  and  14 . The main propulsion system  6  is a high-power system and it is significantly more powerful than the three secondary propulsion systems  10 ,  12  and  14 . The high power main propulsion system  6  includes at least one high-speed main motor drive  18  that optionally may be coupled to a reduction transmission  20 . The main propulsion system  6  has a main drive shaft  22  that carries and propels a main differential contra-rotating transmission  24 . The differential contra-rotating transmission  24  has a first output shaft  26  rotatable in the same direction as the main drive shaft  22 , and a second output shaft  28  rotatable in the opposite direction to the main drive shaft  22 , each connectable to the main propulsion blades  30  and  32 , respectively. 
     The high speed of the main motor drive  18  is converted by the differential contra-rotating transmission  24  into a high torque at the output shafts  26  and  28 , with only moderate reactive torque on the main structure  4 . The differential contra-rotating transmission  24  can be implemented based on the prior art publications (e.g. U.S. Pat. No. 4,963,108), or based on the commonly implemented planetary gear transmissions, or based on IL 231617 filed 20 Mar. 2014 also published as WO 2015/140805 on Sep. 24, 2015. The fact that the main motor drive  18  is a high-speed motor, allows the main propulsion system  6  to provide high power via the main drive shaft  22 , while the main drive shaft  22  transfers a moderate propulsion torque. The propulsion torque transferred via the main drive shaft  22  can be moderate because of the implementation of the differential contra-rotating transmission  24 , which allows the torque at the output shafts  26  and  28  to be significantly increased, while significantly reducing the rotation speed of these shafts. The secondary propulsion systems  10 ,  12  and  14  are low-power systems and include at least one low-power secondary motor drive  34  that optionally may be coupled to a secondary reduction transmission, shown schematically as  35  in  FIG. 1 . The secondary propulsion systems  10 ,  12  and  14  each have a secondary drive shaft  36  that carries and propels the secondary propulsion blades  38 . The secondary propulsion systems  10 ,  12  and  14  provide low power via the secondary drive shaft  36 , which is rotatable at low speed but transfers a moderate propulsion torque of the same magnitude as the torque transferred by the main drive shaft  22 . 
     In  FIGS. 1 to 4 , the main motor drive  18  (and its respective differential contra-rotating transmission  24  and blades  30 ,  32 ) and the secondary motor drive  34  of the secondary propulsion system  12  (and its respective blades  38 ), constitute front and rear propulsion systems whose respective motor drives rotate in the same direction, while the secondary motor drives  34  of the secondary propulsion systems  10  and  14  (and their respective blades  38 ) rotate in the opposite direction to the motor drive  18 . 
     In  FIGS. 5 to 7 , there is shown a helicopter having a main structure  4  that carries two main propulsion systems  6  connected in tandem (one at the front and one at the rear) and two secondary arms  8 , each adapted to carry respective secondary propulsion systems  10  and  14 . The main front motor drive  18  (and its respective blades  30 ) and the main rear motor drive  18  (and its respective blades  30 ), rotate in the same direction, while the secondary motor drives  34  of the secondary propulsion systems  10  and  14  (and their respective blades  38 ) rotate in the opposite direction to the motor drive  18 . 
     The design of the helicopter  2  ensures that the center of gravity  40  is aligned with the direction of the resultant thrust force of the main and secondary propulsion blades  30 ,  32  and  38 . The center of gravity  40  ( FIGS. 1 to 4 ) will be close to the main propulsion system  6 , since the main propulsion system  6  provides a significant portion of the thrust, and therefore the center of gravity will be close to the main structure  4 . This fact makes it possible to reduce the weight of the main structure  4  and of the secondary arms  8  and thus to reduce the overall weight of the helicopter  2 . 
     The flight control unit (not shown) electronically controls the helicopter  2  using a minimum of four control channels: first channel controls the throttle (not shown), other three channels control the aileron (not shown), elevator (not shown), and rudder (not shown), which control the roll, pitch, and yaw axes, respectively. 
     The above-mentioned combined configuration, featured by one, or, at most, two main rotors driven by a main propulsion system  6 , and featured by a flight control unit which is based on the principles of multirotor flight control, can be achieved due to the torque similarity at the main drive shaft  22  and at the secondary drive shafts  36 . This similarity is a result of the implementation of a differential contra-rotating transmission  24 . It will be appreciated that the flight control unit is not a feature of the invention per se and is therefore not described. Typically, the flight control unit is a standard unit in a multirotor helicopter to which the throttle, the aileron, the elevator, and the rudder are responsively coupled in known manner. 
     For the sake of completeness, it is noted that the four control channels control the helicopter  2  of “+” configuration ( FIGS. 1 to 7 ) based on the following principles:
         the ascend or descend is controlled by the throttle by increasing or decreasing the power to the all the propulsion systems;   the forward tilt is controlled by the elevator by increasing the speed of the rear propulsion system while decreasing the speed of the front propulsion system;   the rear tilt is controlled by the elevator by increasing the speed of the front propulsion system while decreasing the speed of the rear propulsion systems;   the left tilt is controlled by the aileron by increasing the speed of the right propulsion system while decreasing the speed of the left propulsion system;   the right tilt is controlled by the aileron by increasing the speed of the left propulsion system while decreasing the speed of the right propulsion system;   the left direction is controlled by the rudder by increasing the speed of the clockwise rotating motor drives while decreasing the speed of the counter-clockwise rotating motor drives;   the right direction is controlled by the rudder by increasing the speed of the counter-clockwise rotating motor drives while decreasing the speed of the clockwise rotating motor drives.       

     We will now consider the behavior of the helicopter  2  under the above-mentioned control. As stated above, control of multirotor motion is achieved by varying the relative speed of each rotor to change the thrust and torque produced by each. The main propulsion system  6  of the helicopter  2  is significantly more powerful than the secondary propulsion systems  10 ,  12  and  14 . Assuming a linear response of the main and the secondary propulsion systems  6 ,  10 ,  12  and  14 , respectively, if the center of gravity  40  is aligned with the direction of the resultant thrust force, then motion of the helicopter  2  during ascend or descend control will not create any derived motions in the roll, pitch, and yaw axes. 
     Whenever high cruising speed is required, the thrust vector of the helicopter  2  can be adjusted for the partial horizontal thrust, just by the tilt of the rear secondary propulsion system  12 , or by the tilt of the right and left secondary propulsion systems  10  and  14 . 
     It should be stated, that the front and the rear of the helicopter  2 , as described in the detail description and in the drawings, can be interchanged such that in the forward flight the main propulsion system  6  is at the rear of the helicopter  2  and the secondary propulsion system  12  is at the front. Therefore, within the context of the appended claims the terms “front” and “rear” and “right” and “left” are not to be construed in a limiting manner but are relative to the direction of motion of the helicopter. 
     Although, for the sake of explanation, the principles of the helicopter according to the invention have been described and shown in the figures with regard to the “+” configuration, the same principles are equally applicable to other configurations as well such as 6 rotors, 8 rotors, etc. 
     It should also be noted that the secondary motor drives  34  may be of a different rated power and the secondary propulsion blades  38  may be of different diameter/shape. For example, the rear secondary propulsion system  12  may have a secondary motor drive  34  with a power rating that is higher or lower than that of the motor drives  34  of the secondary propulsion systems  10  and  14 . Accordingly, the secondary propulsion blades  38  connected to the secondary drive shaft  36  of the secondary propulsion system  12 , may be of larger or smaller diameter and/or of a different shape than the secondary propulsion blades  38  of the secondary propulsion systems  10  and  14 . 
     It will likewise be appreciated that although the invention has been described with specific reference to an aerial vehicle, this is by way of non-limiting example only and the multirotor structure and flight control according to the invention may be used in other media (e.g. as marine vehicle), configurations or applications. Likewise, while no fixed wing has been shown in the embodiments described, in other configurations the helicopter may have a structure that incorporates a fixed-wing.