Abstract:
A system and method for counteracting a rotor moment of one or more rotors of a coaxial rotor helicopter includes receiving signals with a processor indicative of a displacement command from a controller during a flight maneuver; receiving one or more signals with the processor from a sensor indicative of an airspeed and air density for the helicopter; determining a commanded rate of acceleration for the helicopter during the flight maneuver; and adjusting with one or more control servos a cyclic pitch for the one or more rotors to counteract the rotor moment during the flight maneuver.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates generally to the field of helicopters, and to control and reduction of gyroscopic rotor moments in rigid, coaxial-rotor helicopters. 
       DESCRIPTION OF RELATED ART 
       [0002]    One class of dual rotor helicopters is comprised of those with two coaxial, counter-rotating rotors, a sub-class of which includes rotors whose flapping stiffness is high enough to be considered rigid. During helicopter maneuvers, gyroscopic moments acting on each rotor are produced which are equal to the rotor mass moment of inertia, times the vector cross-product of the rotor angular velocity and the helicopter angular velocity. These rotor moments are therefore produced at a right angle to the plane formed by the aircraft angular velocity and the direction of rotor rotation. Since the rotors are counter-rotating, the rotor gyroscopic moments acting on each rotor are substantially opposing. For example, a helicopter roll rate produces opposing rotor pitch moments in the rotors; and a helicopter pitch rate produces opposing rotor roll moments in the rotor. These rotor moments lead to large bending loads along the blade that drive the blade tips together. The large bending loads fatigue the shafts that carry the loads and compromise clearance between the dual rotors. A method to compensate for these gyroscopic loads would be well received in the art. 
       BRIEF SUMMARY 
       [0003]    According to one aspect of the invention, a method for counteracting a rotor moment of one or more rotors of a coaxial rotor helicopter, includes receiving signals with a processor indicative of a displacement command from a controller during a flight maneuver; receiving one or more signals with the processor from a sensor indicative of an airspeed and air density for the helicopter; determining commanded rate for the helicopter during the flight maneuver; and adjusting with one or more control servos a cyclic pitch for the one or more rotors to counteract the rotor moment during the flight maneuver. 
         [0004]    According to another aspect of the invention, a control system for counteracting a rotor moment of one or more rotors of a coaxial rotor helicopter with one or more sensors configured to determine an airspeed and air density for the helicopter; a processor; and memory having instructions stored thereon that, when executed by the processor, cause the system to: receive signals indicative of a displacement command from a controller during a flight maneuver; receive signals indicative of the airspeed and the air density for the helicopter; determine a commanded rate for the helicopter during the flight maneuver; and adjust with one or more control servos a cyclic pitch for the one or more rotors to counteract the rotor moment during the flight maneuver. 
         [0005]    In another aspect of the invention, a helicopter having an airframe, coaxial rotors disposed concentrically at the airframe, each rotor including a plurality of rotor blades, and a control system for counteracting a rotor moment of one or more rotors of the two rotors with one or more sensors configured to determine an airspeed and air density for the helicopter; a processor; and memory having instructions stored thereon that, when executed by the processor, cause the system to: receive signals indicative of a displacement command from a controller during a flight maneuver; receive signals indicative of the airspeed and the air density for the helicopter; determine a commanded rate for the helicopter during the flight maneuver; and adjust with one or more control servos a cyclic pitch for the one or more rotors to counteract the rotor moment during the flight maneuver. 
         [0006]    In addition to one or more of the features described above, or as an alternative, further embodiments could include a commanded body roll rate. 
         [0007]    In addition to one or more of the features described above, or as an alternative, further embodiments could include a commanded body pitch rate. 
         [0008]    In addition to one or more of the features described above, or as an alternative, further embodiments could include adjusting at least one of a longitudinal cyclic pitch and a lateral cyclic pitch. 
         [0009]    In addition to one or more of the features described above, or as an alternative, further embodiments could include adjusting the longitudinal cyclic pitch as a function of a commanded body roll rate, a defined air speed, and an air density ratio. 
         [0010]    In addition to one or more of the features described above, or as an alternative, further embodiments could include adjusting the lateral cyclic pitch as a function of a commanded body pitch rate and a defined air speed and an air density ratio. 
         [0011]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining a feed-forward compensator gain as a function of the airspeed, the commanded body rates and an air density ratio. 
         [0012]    Other aspects, features, and techniques of the invention will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0013]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which like elements are numbered alike in the several FIGURES: 
           [0014]      FIG. 1  is a schematic view of an exemplary helicopter according to an embodiment of the invention; 
           [0015]      FIG. 2  is a schematic of an embodiment of a helicopter during a roll left maneuver; 
           [0016]      FIG. 3  is a schematic of an embodiment of a helicopter during a roll right maneuver; 
           [0017]      FIG. 4  is a schematic diagram of an embodiment of a control system for a helicopter; and 
           [0018]      FIG. 5  is a schematic block diagram for implementing a feed-forward compensation algorithm according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Shown in  FIG. 1  is a schematic of an embodiment of rotary-wing aircraft such as, for example, a coaxial rotor helicopter  10 . The helicopter  10  includes an airframe  60  and two rotors  12   a  and  12   b  arranged concentrically at the airframe  60  at a rotor axis  14 . The rotors  12   a  and  12   b  are counter-rotating such that, for example, when viewed from above, rotor  12   a  rotates in a counterclockwise direction and rotor  12   b  rotates in a clockwise direction. It is to be appreciated that, in other embodiments, the directions of rotation of the rotors  12   a  and  12   b  may be reversed. Each of the rotors  12   a  and  12   b  is connected to a conventional swashplate  18  so that motion of the swashplate  18  along the rotor axis  14  will cause the blades  24  to vary pitch collectively relative to a blade axis  26  and tilting of the swashplate  18  relative to the axis  14  will cause the blades  24  to pitch cyclically relative to the blade axis  26 . The swashplate  18  is driven by one or more control servos  28  to move and/or tilt the swashplate  18  with respect to the rotor axis  14 . 
         [0020]    Referring now to  FIG. 2 , when the helicopter  10  performs a maneuver in rolling to the left about a helicopter axis  30 , rotor pitch moments are induced in the rotors  12   a  and  12   b  at a right angle to the helicopter angular velocity such that rotor  12   a  tends to be pitched upwardly and the rotor  12   b  tends to be pitched downwardly. This can create a close clearance  32  at a rear portion  34  of the rotors  12   a  and  12   b . Similarly, as shown in  FIG. 3 , when the helicopter  10  rolls to the right, rotor  12   a  tends to be pitched upwardly and rotor  12   b  tends to be pitched downwardly. This results in a close clearance  32  at a forward portion  36  of the rotors  12   a  and  12   b . One of ordinary skill in the art will readily understand that other maneuvers, such as pitch maneuvers of the helicopter  10 , will result in substantially equal and opposite rotor moments in the rotors  12   a  and  12   b . Such rotor moments induce stresses in the rotors  12   a  and  12   b  due to the large gyroscopic bending loads and it is desired to minimize the bending loads and provide sufficient clearance between rotors  12   a  and  12   b.    
         [0021]    Referring to  FIG. 4 , in order to compensate/reduce the rotor moments induced by such maneuvers, a model following control system  38  implements a gyroscopic feed forward compensation algorithm  52  (hereinafter “feed forward algorithm  52 ”) in order to improve stability and robustness of the control system  38 . The feed-forward algorithm  52  uses, in an embodiment, body pitch or body roll commanded rates and a schedule of corrective cyclic inputs for these commanded body-pitch or roll rates in order to drive cyclic pitch changes in the rotors  12   a  and  12   b . These cyclic pitch changes counteract the gyroscopic rotor moments with aerodynamic forces on the blades. The feed forward algorithm  52  utilizes a feed-forward commanded body pitch or body roll rate as the actual pitch or roll rate. A schematic of a control system  38  to accomplish this is illustrated in  FIG. 4 . 
         [0022]    Pilot inputs  40  from, for example, a cyclic controller such as a pilot cyclic stick, and/or foot pedals are received by a computer  42  as commanded body pitch or roll rates. The pilot inputs  40  indicate direction of flight, for example, roll, pitch, or the like. A number of sensors  44  are located at the helicopter  10  to sense parameters of helicopter  10  flight such as pitch and/or roll angular velocities, pitch and/or roll angular accelerations, vertical acceleration, airspeed, air density, or the like. Data from the sensors  44  is provided to the computer  42  operably connected to the sensors  44 . Computer  42  compares the sensor data to control laws  46 , which define flight control commands  54  for the helicopter  10  based on a schedule of corrective lateral and longitudinal cyclic inputs as a function of airspeed and density ratio and the commanded roll and pitch rates from pilot inputs  40 . 
         [0023]    The control system  38  includes a computer  42  that determines estimated differential longitudinal and lateral cyclic pitch inputs as a function of the commanded body pitch and roll rates. The estimated inputs produce a desired aerodynamic response for the gyroscopic moments induced on the rotors  12   a - 12   b  ( FIG. 1 ). In an embodiment, computer  42  includes a memory  48 . The memory  48  stores feed forward algorithm  52  as executable instructions that is executed by a processor  50 . The instructions may be stored or organized in any manner and at any level of abstraction, such as in connection with the execution of the compensation algorithm. The processor  50  may be any type of processor (CPU), including a general purpose processor, a digital signal processor, a microcontroller, an application specific integrated circuit, a field programmable gate array, or the like. Also, in embodiments, memory  48  may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored the mixing algorithm described below. 
         [0024]      FIG. 5  illustrates a schematic block diagram of circuitry for implementing the feed forward algorithm  52  by computer  42  ( FIG. 4 ) according to an embodiment of the invention. The feed forward algorithm  52  includes pilot cyclic roll stick input signal  56  and cyclic pitch stick input signal  58 . The signals  56 ,  58  are applied to each of a roll dynamic shaping block  60  and a pitch dynamic shaping block  62 . The dynamic shaping blocks  60 ,  62  include gain constants which are maintained in one or more lookup tables in memory  48  ( FIG. 4 ) for the respective stick input signals  56 ,  58 . 
         [0025]    The dynamic shaping blocks  60 ,  62  output a respective commanded roll rate  64  and a commanded pitch rate  66  of the model-following control system  38 . These commanded roll and pitch rates  64  and  66  are applied to respective longitudinal and lateral load compensator blocks  68 ,  70  and blocks  65 ,  67 . The load compensator blocks  68 ,  70  are gyroscopic compensator blocks that include a schedule of corrective cyclic inputs as a function of, in embodiments, airspeed and air density for the commanded roll and pitch rates  64 ,  66  which are stored in one or more lookup tables in memory  48  ( FIG. 4 ). Also, load compensator blocks  68 ,  70  utilize the commanded roll and pitch rates  64 ,  66  as the actual pitch or roll rates during a determination of the corrective cyclic input. 
         [0026]    Longitudinal gyroscopic load compensator block  68  determines a signal for a longitudinal compensator gain  74  from a schedule of corrective longitudinal cyclic inputs. A corrective longitudinal cyclic input provides corrective signals as a function of air speed, air density ratio, and commanded roll rate  64 . The air density ratio is the relative density of air in flight over the density of air at sea level. Thus, the compensator gain  74  is a function of true airspeed of the helicopter  10 , scheduled gain, and air density ratio. The compensator gain  74  represents an amount of differential longitudinal cyclic pitch per degree per second of commanded roll rate that is to be applied to the off-axis in order to counteract the rotor moments with aerodynamic loads via rotor longitudinal cyclic pitch changes. Similarly, lateral gyroscopic load compensator block  70  determines a compensator gain signal  76  for a lateral compensator gain from a schedule of corrective lateral cyclic inputs. A corrective lateral cyclic input provides corrective signals as a function of air speed, air density ratio, and commanded pitch rate  66 . Air density ratio is the relative density of air in flight over the density of air at sea level. Thus, the compensator gain signal  76  is a function of the airspeed of the helicopter  10 , scheduled gain, and air density ratio. The compensator gain  76  signal represents an amount of differential lateral cyclic pitch per degree per second of commanded pitch rate that is to be applied to the off-axis in order to counteract the off-axis rotor moments with aerodynamic loads via rotor lateral cyclic pitch changes. 
         [0027]    Compensator gain signals  74 ,  76  from respective longitudinal and lateral load compensators  68 ,  70  are inputted into mixing block  78 . Additionally, output signals  69 ,  71  from respective blocks  65 ,  67  are inputted into mixing block. Signal  69  represents lateral cyclic pitch commands while signal  71  represents longitudinal cyclic pitch commands. Block  78  represents a mathematical representation of aircraft that receives the signals  69 ,  71 ,  74 , and  76  by flight control computer  42  ( FIG. 4 ) and determines displacement commands  80 ,  82 ,  84 , and  86  for displacement of servos and actuators of rotors  12   a - 12   b  ( FIG. 1 ) in order to produce desired aerodynamic response for the gyroscopic moments induced on the rotors  12   a - 12   b  ( FIG. 1 ) Mixing block  78  multiplies respective commanded rates with compensator gain to determine an actual roll rate  80 , actual pitch rate  84 , a differential longitudinal cyclic pitch command  82 , and a differential lateral cyclic pitch command  86  in order to counteract the off-axis coupling moments in the helicopter  10 . 
         [0028]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while the various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.