Abstract:
A method for controlling a differential rotor roll moment for a coaxial helicopter with rigid rotors, the method including receiving, with a processor, a signal indicative of a displacement command from a controller; receiving, with the processor via a sensor, one or more signals indicative of a longitudinal velocity, an angular velocity of one or more rotors and an air density ratio for the helicopter; determining, with the processor, a ganged collective mixing command in response to the receiving of the displacement command; determining, with the processor, a rotor advance ratio as a function of the longitudinal velocity and the angular velocity; and determining, with the processor, a corrective differential lateral cyclic command for the rigid rotors that controls the differential rotor roll moment to a desired value.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates generally to the field of helicopter control and to a steady state differential roll moment control with automated differential lateral control commands for rigid dual-rotor helicopters. 
       DESCRIPTION OF RELATED ART 
       [0002]    Rigid dual rotor helicopters are those with two coaxial, counter-rotating rotors. Each rotor is controlled by a respective independent swash plate and can be commanded with both differential and ganged collective and cyclic controls. Rigid coaxial rotor helicopters must be designed with sufficient rotor spacing to ensure that the blade tips never touch during any maneuver in the flight envelope. As forward flight speed increases, the advancing portion of the rotor produces more lift than the retreating portion because the relative wind speed is higher. This phenomenon is lift offset, the measure of where the center of lift is generated on the two rotors. The use of collective control also changes lift offset with forward airspeed. Lift offset produces opposing roll moments, thus the rotor roll moments acting on each rotor are substantially opposing. These roll moments reduce tip clearance on the rotor blades. Further, these roll moments are large and generally greater than what is needed to maneuver the helicopter. Additionally, lift offset can adversely affect tip clearance. In conventional rigid dual rotor helicopters, lift offset is manually maintained by the pilot via a differential lateral cyclic beeper or adjusting the relative control phase angle of the individual swash plates which transforms some of the ganged longitudinal cyclic into differential lateral cyclic. An advanced rotorcraft control law with an automatic differential lateral control command for managing differential roll moment would be well received in the art. 
       SUMMARY 
       [0003]    According to one exemplary embodiment, a method for controlling a differential rotor roll moment for a coaxial helicopter with rigid rotors includes receiving, with a processor, a signal indicative of a displacement command from a controller; receiving, with the processor via a sensor, one or more signals indicative of a longitudinal velocity, an angular velocity of one or more rotors and an air density ratio for the helicopter; determining, with the processor, a ganged collective mixing command in response to the receiving of the displacement command; determining, with the processor, a rotor advance ratio as a function of the longitudinal velocity and the angular velocity; and determining, with the processor, a corrective differential lateral cyclic command for the rigid rotors that controls the differential rotor roll moment to a desired value. 
         [0004]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the corrective differential lateral cyclic command with a model which is a function of the ganged collective mixing command, air density ratio, and rotor advance ratio. 
         [0005]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the corrective differential lateral cyclic command as a function of measured differential rolling moments and targeted differential rolling moments. 
         [0006]    In addition to one or more of the features described above, or as an alternative, further embodiments could include varying the targeted differential rolling moment based upon aircraft flight condition, including at a minimum longitudinal velocity. 
         [0007]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining an error value between the measured and targeted differential rolling moments. 
         [0008]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the corrective differential lateral cyclic command for rotor speeds above a predefined threshold value. 
         [0009]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the corrective differential lateral cyclic command as (i) a function of a gain of the rotor advance ratio and/or (ii) a function of upper and lower limits of differential lateral cyclic. 
         [0010]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining a quantity of corrective differential lateral cyclic command at different rotor speeds. 
         [0011]    According to one exemplary embodiment, a control system for controlling a differential rotor roll moment for a coaxial helicopter with rigid rotors includes one or more sensors configured to determine a longitudinal velocity of the helicopter, an angular velocity of one or more rotors and an air density ratio for the helicopter; one or more controllers configured to issue a displacement command during a flight maneuver; and a computer operably connected to the one or more controllers and configured to: determine a ganged collective mixing command in response to the displacement command; determine a rotor advance ratio as a function of the longitudinal velocity and the angular velocity; and determine a corrective differential lateral cyclic command for the rigid rotors to control the differential rotor roll moment to a desired value. 
         [0012]    In addition to one or more of the features described above, or as an alternative, further embodiments could include the processor configured to determine the corrective differential lateral cyclic command as a function of the ganged collective mixing command, air density ratio, and rotor advance ratio. 
         [0013]    In addition to one or more of the features described above, or as an alternative, further embodiments could include the processor configured to determine the corrective differential lateral cyclic command with a model which is a function of measured differential rolling moments and targeted differential rolling moments. 
         [0014]    In addition to one or more of the features described above, or as an alternative, further embodiments could include the processor configured to vary the targeted differential rolling moment based upon aircraft flight condition, including at a minimum longitudinal velocity. 
         [0015]    In addition to one or more of the features described above, or as an alternative, further embodiments could include the processor configured to determine an error value between the measured and targeted differential rolling moments. 
         [0016]    In addition to one or more of the features described above, or as an alternative, further embodiments could include the processor configured to determine the corrective differential lateral cyclic command for rotor speeds above a predefined threshold value. 
         [0017]    In addition to one or more of the features described above, or as an alternative, further embodiments could include the processor configured to determine the corrective differential lateral cyclic command as (i) a function of a gain of the rotor advance ratio and/or (ii) a function of upper and lower limits of differential lateral cyclic. 
         [0018]    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 
         [0019]    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: 
           [0020]      FIG. 1  is a schematic view of an exemplary helicopter according to an embodiment of the invention; 
           [0021]      FIG. 2  is a schematic diagram of an embodiment of a control system for a helicopter; 
           [0022]      FIG. 3  is a schematic block diagram of an architecture for implementing automatic control of lift offset using a model-based approach to differential lateral determination; 
           [0023]      FIG. 4  is a schematic block diagram of an architecture for implementing automatic control of lift offset using a feedback-based approach to differential lateral determination; 
           [0024]      FIG. 5  is a bock diagram of a differential lateral model that is shown in  FIG. 3  according to an embodiment of the invention; and 
           [0025]      FIG. 6  is a schematic block diagram for implementing a control algorithm according to an embodiment of the invention as shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Shown in  FIG. 1  is a schematic of an embodiment of rotary-wing aircraft such as, for example, a rigid dual-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  counter-rotate 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 its respective conventional swashplate  18 . 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 . 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, either longitudinally or laterally, of the swashplate  18  relative to the axis  14  will cause the blades  24  to pitch cyclically in respective longitudinal or lateral directions relative to the blade axis  26 . Further, the rotors  12   a  and  12   b  may be commanded to do the same thing either collectively or cyclically (referred to as ganged) or may be commanded to pitch or roll opposite to the other rotor (referred to as differential). 
         [0027]    Referring to  FIG. 2 , in order to manage the differential roll moments induced by lift offset, a model following control system  70  is implemented with a differential moment control algorithm  72  (also referred to as “control algorithm  72 ”). Control algorithm  72  determines a required amount of differential lateral control that is needed to maintain a desired amount of lift offset, thus keeping the differential roll moments that are induced by the lift offset within the design limits and improving tip clearance. Control algorithm  72  is at least based on collective commands to rotors  12   a  and  12   b  ( FIG. 1 ) in order to counteract the large differential rotor moments that are induced in the helicopter  10 . In an embodiment, control algorithm  72  utilizes scheduled gain and rotor advance ratio to determine a differential lateral cyclic command that achieves a targeted differential rolling moment for helicopter  10 . In an embodiment, control algorithm  72  determines the required amount of differential lateral control needed to manage the differential rolling moment based on the rotor advance ratio, air density ratio and commanded collective position. If any of these inputs change, an updated differential lateral control will be determined and/or commanded from control algorithm  72  in order to keep the differential rolling moment at the desired magnitude. 
         [0028]    A schematic of control system  70  to accomplish this is illustrated. Pilot inputs  74 , for example, from a controller such as a pilot collective stick and/or a pilot cyclic stick are received by flight control computer  76  (FCC  76 ) as commanded collective pitch or roll rates. These may include differential or ganged collective and cyclic inputs. Pilot inputs  74  include commands to control a direction of flight, for example, roll, pitch or the like. A number of sensors  78  are located on helicopter  10  to sense parameters during 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 sensors  78  is directed to FCC  76  operably connected to sensors  78  where it is compared to control laws  80 . Control laws  80  define flight control commands  82  for helicopter  10  based on a scheduled gain as a function of a rotor advance ratio, which is described in detail below with reference to  FIG. 6 . 
         [0029]    FCC  76  automatically determines estimated differential lateral cyclic commands as a function of the commanded collective position, rotor advance ratio, and air density ratio, in order to produce desired differential rolling moments on rotors  12   a,   12   b  ( FIG. 1 ). In an embodiment, FCC  76  includes a memory  84 . Memory  84  stores control algorithm  72  as executable instructions that are executed by processor  86 . The instructions may be stored or organized in any manner and at any level of abstraction, such as in connection with the execution of control algorithm  72 . Processor  86  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  84  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. 
         [0030]      FIG. 3  illustrates a schematic block diagram of a high level architecture  100  that is implemented by control algorithm  72  for determining differential lateral cyclic commands  124  for determining differential lateral cyclic commands via a model-based approach according to an embodiment. Initially, sensed parameters that are received from sensors  78  ( FIG. 2 ) and/or calculated or estimated parameters are received as signals  102 ,  104  that are stored in FCC  76  ( FIG. 2 ). For example, signals  102  represent an estimate of the magnitude of the helicopter&#39;s airspeed or longitudinal velocity, rotor speed, and rotor radius. In an embodiment, longitudinal velocity (or forward speed) in knots can be converted into feet per second (fps) where signal  102  represents foot longitudinal velocity in fps (or foot forward speed). Also, signals  104  represent ganged collective mixing command and air density ratio. Air density ratio is a ratio of a relative density of air in flight to the density of air at sea level. Signal  102  is inputted into advance ratio calculation block  106 , which outputs signal  108  that represents a rotor advance ratio as a function of airspeed and rotor speed. Also, signals  104 ,  108  are inputted into differential lateral model  110 . Output signal  114  represents degree or degrees of differential lateral cyclic command  124  as a function of ganged collective mixing command, air density ratio, and rotor advance ratio. A detailed description of differential lateral model  110  is provided below in reference to  FIG. 5 . Fade in/out logic block  112  receives signal  108  that represents rotor advance ratio and outputs signal  116  that represents a corrective differential lateral cyclic command. Signals  114 ,  116  are multiplied in multiplier block  118 , which outputs a multiplied signal  120 . Multiplied signal  120  is inputted into authority limiting block  122  and outputs a differential lateral cyclic command  124  as a function of upper and lower limits of differential roll moments. Architecture  100  determines the required amount of differential lateral cyclic command  124  needed to manage the differential rolling moment based on signals  104 ,  108  for air density ratio, ganged collective mixing command, and rotor advance ratio. 
         [0031]      FIG. 4  depicts a schematic block diagram of high-level architecture  150  that is implemented by control algorithm  72  for determining differential lateral cyclic commands  122  as a function of sensed differential rolling moments according to another embodiment of the invention. Architecture  150  is substantially similar to architecture  100  but uses measured hub rolling moments of rotors  12   a,    12   b  including error values between a reference signal and a measured signal for differential rolling moment loads on the rotors. Initially, sensed parameters from sensors  78  ( FIG. 2 ) and/or calculated or estimated parameters are received as signals  152 ,  162 , and  168  that are stored in FCC  76  ( FIG. 2 ). For example, signal  152  represents an estimate of the magnitude of helicopter  10  airspeed or longitudinal velocity, rotor speed, and rotor radius; signal  162  represents an estimate of the magnitude of helicopter  10  airspeed; and signal  168  represents measured differential hub rolling moments for rotors  12   a,    12   b  ( FIG. 1 ). In an embodiment, longitudinal velocity (or forward speed) in knots can be converted into feet per second (fps) where signal  152  represents foot longitudinal velocity in fps (or foot forward speed). Signal  152  is inputted into advance ratio calculation block  154 , which outputs signal  156  that represents a rotor advance ratio. Fade in/out logic block  158  receives signal  156  and outputs a signal  160  that represents a corrective differential lateral cyclic command. 
         [0032]    Also, differential rolling moment target block  164  receives signal  162  as input and outputs a signal  166  that represents a desired differential lateral rolling moment. Further, differential rolling moment filter block  170  receives signal  168  and outputs a signal  172  that represents a reference value of actual differential rolling moment. Summation block  174  receives signals  166 ,  172  and outputs a signal  176  that represents an error value between reference signal  172  and signal  166 . Feedback controller block  178  receives the error value and outputs signal  180  that represents gain for determining a corrected differential rolling moment. Signals  160 ,  180  are multiplied in multiplier block  182  to output a multiplied signal  184 . Multiplied signal  184  is inputted into authority limiting block  186  to outputs a differential lateral cyclic command  190  based on limits of differential lateral cyclic in order to produce a desired differential rolling moment on rotors  12   a,    12   b  ( FIG. 1 ). 
         [0033]      FIG. 5  illustrates a schematic diagram of differential lateral model  110  for target lift offset as shown in  FIG. 3 . Signal  202  represents rotor advance ratio, signal  208  represents ganged collective mixing command, and signal  212  represents air density ratio. Signal  208  is applied to coefficient model  210  which is a linear model based on collective mixing command. Coefficient model  210  is a linear fit based on ganged collective mixing command and outputs signal  208  for coefficients that are linear with respect to ganged collective mixing command. In an example, output signal  208  comprises slope with advance ratio and bias. Linear model  204  receives signals  208  and  202  as inputs and outputs a signal  206  that is a quantity of corrective differential lateral cyclic command. Signal  206  is applied to divide block  212 , which outputs a signal  214  that is a normalized signal of differential corrective lateral cyclic commands based on air density ratio. 
         [0034]      FIG. 6  illustrates schematic detailed block diagram  220  for implementing control algorithm  72  by FCC  76  ( FIG. 2 ) according to an embodiment of the invention. As such,  FIG. 2  is also referenced in the schematic block diagram of  FIG. 6 . As shown, implementation of control algorithm  72 , in an embodiment, begins when FCC  76  receives and stores the helicopter&#39;s sensed parameters from sensors  78  such as, for example, a signal that represents an estimate of the magnitude of the helicopter&#39;s longitudinal velocity  222  and a signal that represents a percentage of an estimate of angular velocity  226  of rotor  12   a,    12   b  ( FIG. 1 ). Additionally, flight control commands that are generated based on pilot manipulation of controllers are applied by control algorithm  72  as a ganged collective mixing command  236  to model block  248  in order to determine the degrees of differential lateral cyclic command to apply at predefined speeds of the rotor  12   a,    12   b.    
         [0035]    Signal for angular velocity  226  (as a percentage) of rotors is inputted into a multiplier block  238  which outputs a signal that represents a linear velocity  240  of rotor (or rotor linear speed  240 ). The multiplier block  238  determines linear velocity of rotor  240  by multiplying a signal for angular velocity  226  according to the following: multiplied by a signal from limit block  224  in order to convert rotor angular velocity  226  to a value in the range of 0 to 1.15. In an embodiment, angular velocity  226  is converted to a value in the range of about 0.8 to about 1.15, multiplied by signal for a constant  228 , and multiplied by a signal that represents a rotor radius  230  of rotor  12   a,    12   b  in order to convert signal of angular velocity  226  to linear velocity  240  of rotor. In an embodiment, constant  228  has a value of about 0.010. Constant  232  represents divide by zero protection for rotor advance ratio block  246  and, in an embodiment, has a value of 0.010. Constant  232  and linear velocity  240  of rotor are inputted into maximum block  242 . Maximum block  242  determines a maximum value from constant  232  and linear velocity  240  to produce a signal that represents rotor linear velocity  244  for values greater than zero. Foot longitudinal velocity  222  (as an ‘X’ input) and rotor linear velocity  244  (as a ‘Y’ input) are inputted into rotor advance ratio block  246 . Advance ratio block  246  divides longitudinal velocity  222  by rotor linear velocity  244  and outputs a signal that represents a rotor advance ratio  256 . 
         [0036]    Block  234  represents an empirical model of how much corrective differential lateral cyclic command is required and includes a linear fit as a function of ganged collective mixing command  236 , slope  250  and bias  252 . Block  234  includes a model  248  having fixed gain blocks and parameter limits that receives a ganged collective mixing command  236  and outputs a signal representing slope  250  and bias  252 . Slope  250  represents a slope of the dependence on rotor advance ratio  256 . It is to be appreciated that at higher speeds of rotor, for example, above a predefined threshold value, control algorithm  72  implements a ganged collective mixing command  236  and does not utilize differential collective for determining corrective differential lateral cyclic command  288 . Further, multiplier block  258  receives rotor advance ratio  256  and slope  250  and outputs a signal  262  as a product of advance ratio  256  and slope  250 . Signal  262  and bias  252  are added together in summation block  266 , which outputs a summation signal  268 . Air density ratio  254  and constant  260  are inputted into a maximum block  264  to produce a signal  270  that represents a maximum value of air density ratio  254  and constant  260 . Air density ratio  254  is a ratio of the relative density of air in flight to the density of air at sea level. Constant  260  represents divide by zero protection for divide block  272  and, in an embodiment, has a value of 0.010. Divide block  272  divides summation signal  268  (as a ‘X’ input) by signal  270  (as a ‘Y’ input) and outputs a signal  276  that represents an empirical model of a quantity of corrective differential lateral cyclic command that is applied at predefined rotor speeds in order to control differential roll moments of the rotors  12   a,    12   b.    
         [0037]    Fade in/out logic block  274  includes a look-up table as a function of rotor advance ratio  256  and outputs a signal  278  to multiplier block  280 . Fade in/out logic block  274  utilizes a schedule of gain for rotor advance ratio  256  that is defined by a longitudinal velocity  222  of aircraft  10  and linear velocity  244  of rotor in order to determine whether to output a corrective differential lateral cyclic command to implement on rotors  12   a,    12   b.  The fade in/out logic block  274  determines whether to output a corrective differential lateral cyclic command based on a function of the rotor advance ratio  256  (which is a function of the rotor speed and forward speed). Multiplier block  280  receives signals  276 ,  278  and multiplies the respective signals  276 ,  278  in order to output a multiplied signal  282  to asymmetric limiter block  286 . Asymmetric limiter block  286  determines the amount of corrective differential lateral cyclic command that is applied based on its upper and lower limits (of differential lateral cyclic) as well as to preserve lateral cyclic for roll maneuvering. Asymmetric limiter block  286  provides a corrective differential lateral cyclic command  288  to rotors  12   a,    12   b  (See  FIG. 1 ) based on upper and lower limits in order to maintain a specified or targeted differential rolling moment for the rotors  12   a,    12   b.    
         [0038]    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.