Abstract:
A system and method for estimating rotor mixing commands for an aircraft includes receiving signals indicative of reference commands from one or more controllers; receiving signals indicative of airspeed and sideslip angle for the aircraft, the sideslip angle being indicative of a direction of flight for the aircraft; calculating a sine and cosine of the sideslip angle; determining gains for roll and pitch as a function of the airspeed, the determining including referencing a look-up table that indexes the gain constants with the airspeed; and determining the one or more rotor mixing commands from the determined gains, the one or more rotor mixing commands being applied synchronously to the rotors in the aircraft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional patent application Ser. No. 61/987,227, filed May 1, 2014, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The subject matter disclosed herein relates generally to the field of rotary-wing aircraft and, more particularly, to a system and method for determining estimated rotor mixing command signals in order to decouple the rolling or pitching responses during low-speed forward flight. 
       DESCRIPTION OF RELATED ART 
       [0003]    Many vehicles, including helicopters, use fly-by-wire (FBW) systems to control yaw, pitch and roll. In particular, for coaxial rotorcraft, yaw control is implemented through a differential collective blade pitch control (or differential collective) during low airspeed flight, which collectively pitches the rotor blades in one rotor with respect to the other coaxial rotor. However, during non-hovering flight (zero airspeed condition), the direction and magnitude of the air relative to the rotor induces a non-symmetric lift distribution and roll sensitivity as a function of collective on each rotor. In this condition, the advancing side of the rotor tends to create more lift than the retreating side when the collective pitch is changed. Since the advancing side of each rotor is on opposite sides and the differential collective induces twice the “undesirable” rolling moment when a differential collective yaw input is applied. 
         [0004]    For example, in a counter-clockwise rotor in forward flight (advancing blade on the starboard side): Adding positive collective pitch increases the angle of attack equally across the entire rotor. Due to dynamic pressure differences on the advancing versus retreating sides, the advancing (RIGHT) side sees greater increase in lift. Larger increase in lift on advancing side results in increase in left rolling moment. For a clockwise rotor in forward flight (advancing blades on the port side): adding positive collective pitch increases angle of attack on both sides equally. Due to dynamic pressure differences on the advancing versus retreating sides, the advancing (LEFT) side sees greater increase in lift. Larger increase in lift on advancing side results in increase in RIGHT rolling moment. 
         [0005]    Therefore, for a coaxial rotor applying positive GANG collective in forward flight: Adding positive collective pitch to counter-clockwise rotor increases lift on right (advancing) side relative to left (retreating) side. Adding positive collective pitch to clockwise rotor increases lift on left (advancing) side relative to right (retreating) side. For counter-clockwise rotor, larger increase in lift on advancing side results in increase of left rolling moment at the hub. For clockwise rotor, larger increase in lift on advancing side results in increase of right rolling moment at the hub. So, positive GANG collective increases lift on both rotors, biased to the advancing sides. Opposing roll moments balance out, with no net effect on flight path response. 
         [0006]    For a coaxial rotor applying positive differential collective in forward flight: Adding positive collective pitch to counter-clockwise rotor increases lift on right (advancing) side relative to left (retreating) side. Adding negative collective pitch to clockwise rotor decreases lift on left (advancing) side relative to right (retreating) side. For counter-clockwise rotor, larger increase in lift on advancing side results in increase of left rolling moment at the hub. For clockwise rotor, larger decrease in lift on advancing side results in decrease of right rolling moment at the hub. 
         [0007]    Improvements in decoupling these pitching or rolling moments or responses on the coaxial rotors would be well received in the art. 
       BRIEF SUMMARY 
       [0008]    According to one aspect of the invention, a method for estimating rotor mixing commands for an aircraft includes receiving signals indicative of reference commands from one or more controllers; receiving signals indicative of airspeed and sideslip angle for the aircraft, the sideslip angle being indicative of a direction of flight for the aircraft; calculating a sine and cosine of the sideslip angle; determining gains for roll and pitch as a function of the airspeed, the determining including referencing a look-up table that indexes the gain constants with the airspeed; and determining the one or more rotor mixing commands from the determined gains, the one or more rotor mixing commands being applied synchronously to the rotors in the aircraft. 
         [0009]    In addition to one or more of the features described above, or as an alternative, further embodiments could include receiving information indicative of an unmixed roll command signal, an unmixed pitch command signal, and a yaw command signal that produce a desired flight response for the aircraft. 
         [0010]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining a mixed pitch command as a function of a differential collective to ganged pitch mixing signal. 
         [0011]    In addition to one or more of the features described above, or as an alternative, further embodiments could include applying the differential collective to ganged pitch mixing signal for travel along a lateral axis of the aircraft. 
         [0012]    In addition to one or more of the features described above, or as an alternative, further embodiments could include summing the differential collective to ganged pitch mixing signal with an unmixed pitch command signal. 
         [0013]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the differential collective to ganged pitch mixing signal as a function of a yaw command signal, the sine of the sideslip angle, the airspeed, and the determines gain pitch for airspeed. 
         [0014]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining a mixed roll command as a function of a differential collective to ganged roll mixing signal. 
         [0015]    In addition to one or more of the features described above, or as an alternative, further embodiments could include applying the differential collective to ganged roll mixing signal for travel along a longitudinal axis of the aircraft. 
         [0016]    In addition to one or more of the features described above, or as an alternative, further embodiments could include summing the differential collective to ganged roll mixing signal with an unmixed roll command signal. 
         [0017]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the differential collective to ganged roll mixing signal as a function of a yaw command signal, the cosine of the sideslip angle, the airspeed, and the gain roll constant for airspeed. 
         [0018]    According to another aspect of the invention, a system for estimating mixing commands for an aircraft includes a processor and memory. The processor receives signals indicative of reference commands from one or more controllers, receives signals indicative of airspeed and sideslip angle for the aircraft and determines a sine and cosine of the sideslip angle. The processor determines gain constants for roll and pitch as a function of the airspeed and determines the one or more rotor mixing commands as a function of the determined gain constants. Also, the memory indexes gain constants for roll and pitch with the airspeed. 
         [0019]    In addition to one or more of the features described above, or as an alternative, further embodiments could include receiving information indicative of an unmixed roll command signal, an unmixed pitch command signal, and a yaw command signal that produce a desired flight response for the aircraft. 
         [0020]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the one or more rotor mixing commands from a mixed pitch command as a function of a differential collective to ganged pitch mixing signal. 
         [0021]    In addition to one or more of the features described above, or as an alternative, further embodiments could include applying the differential collective to ganged pitch mixing signal during travel along a lateral axis of the aircraft. 
         [0022]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the mixed pitch command by summing the differential collective to ganged pitch mixing signal with an unmixed pitch command signal. 
         [0023]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the differential collective to ganged pitch mixing signal as a function of a yaw command signal, the sine of the sideslip angle, the airspeed, and the determined gain pitch for airspeed. 
         [0024]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the one or more rotor mixing commands by determining a mixed roll command as a function of a differential collective to ganged roll mixing signal. 
         [0025]    In addition to one or more of the features described above, or as an alternative, further embodiments could include applying the differential collective to ganged roll mixing signal for travel along a longitudinal axis of the aircraft. 
         [0026]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the mixed roll command by summing the differential collective to ganged roll mixing signal with an unmixed roll command signal. 
         [0027]    In addition to one or more of the features described above, or as an alternative, further embodiments could include determining the differential collective to ganged roll mixing signal as a function of a yaw command signal, the cosine of the sideslip angle, the airspeed, and the determined gain roll for airspeed. 
         [0028]    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 
         [0029]    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: 
           [0030]      FIG. 1  is a perspective view of an exemplary rotary wing aircraft according to an embodiment of the invention; 
           [0031]      FIG. 2  is a schematic diagram of an exemplary computing system that is used with the rotary wing aircraft of  FIG. 1  according to an embodiment of the invention; and 
           [0032]      FIG. 3  illustrates a schematic block diagram for implementing the mixing algorithm according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Referring now to the drawings,  FIG. 1  illustrates a general perspective view of an exemplary vehicle in the form of a vertical takeoff and landing (VTOL) rotary-wing aircraft  100  according to an embodiment of the invention. As illustrated, the rotary-wing aircraft  100  includes a dual, counter-rotating rotor system  104 , which is attached to an airframe  102 . The rotor system  104  includes a rotor hub  106  with a plurality of blades  108  attached. The rotor hub  106  includes a rotating main rotor shaft  110  and a counter-rotating main rotor shaft  112  that each rotate about axis A. The rotor hub  106  is driven to rotate by one or more engines  114  through one or more gearboxes (not shown). The aircraft  100  also includes a flight control computer  202  (shown in  FIG. 2 ) that may interpret positions of collective and cyclic controllers and sensors  210  and receive direction and magnitude of wind relative to the plurality of blades  108  in order to implement a coaxial rotor low-speed mixing algorithm. The coaxial rotor low-speed algorithm produces, for example, a command for displacing one or more servos and linkages that are connected to the rotor hub  106  in order to drive the blades  108  on the rotor system  104  while taking into account the asymmetric moments on the plurality of rotor blades  108 . These commands correspond to forward and backward motion of the aircraft  100  relative to the wind-frame (i.e., airspeed), rotation of the aircraft  100  about axis X-X (i.e., roll), sideways rotation of the airframe  102  about axis Z-Z (i.e., yaw), and a rotation of the airframe  102  about axis Y-Y (i.e., pitch). Although a particular configuration of a rotary-wing aircraft  100  is illustrated and described in the disclosed embodiments, other configurations and/or machines, such as high speed compound rotary-wing aircraft with supplemental translational thrust systems, dual contra-rotating coaxial rotor system aircraft, tilt-rotors, tilt-wing aircraft, tandem rotor aircraft, and unmanned rotary wing aircraft with any of the previous configurations will also benefit from embodiments of the invention. 
         [0034]      FIG. 2  illustrates a schematic block diagram of a fly-by-wire (FBW) flight control system  200  (also referred to as FBW system  200 ) for the rotary-wing aircraft  100  according to an exemplary embodiment. As illustrated, the FBW system  200  implements a coaxial rotor low-speed mixing algorithm  212  (or low-speed mixing algorithm  212 ), which shapes the pilot&#39;s controller and displacement commands and produces a desired stability, response, and flight augmentation. In an embodiment, the FBW system  200  may determine an estimated mixed roll and pitch command in order to produce a desired proportional decoupling response in a proper orientation for pitch or roll moments induced on the plurality of rotors  108  ( FIG. 1 ). The FBW system  200  includes a computing system such as a flight control computer (FCC)  202 . The FCC  202  can receive reference commands from a collective controller  206  and a cyclic controller  208 , and sensed parameter signals from a plurality of sensors  210  including operating conditions such as lateral acceleration, attitude, and angular rate as well as magnitude and direction of wind speed relative to the rotor  104  in rotary-wing aircraft  100  in order to produce the desired stability response and flight augmentation. The collective controller  206  and the cyclic controller  208  may take various forms including sidearm controllers, a yaw pedal system or other such flight controllers. 
         [0035]    In an embodiment, the FCC  202  receives information such as, for example, a magnitude of the wind frame relative to the rotary-wing aircraft  100  from the sensors  210 , a direction of the wind relative to rotary-wing aircraft  100  from the sensors  210 , lateral acceleration, aircraft attitude, and aircraft angular rate, and interprets reference commands such as, for example, displacement positions of controllers  206 ,  208  based on reference commands in order to determine yaw and pitch command signals. In an embodiment, the FCC  202  receives information on airspeed for rotary-wing aircraft  100  while traveling during non-hover flight and a relative direction of the airspeed to the rotary-wing aircraft  100 . The FCC  202  inputs the received information into the low-speed mixing algorithm  212  in order to determine or calculate an estimated mixed pitch and roll command that forms part of the augmented flight control commands  220 . It is to be appreciated that mixing is used whenever differential collective is used for yaw control. These estimated mixed pitch and roll commands are provided to a mixing unit  214 , which communicates these commands to rotary-wing aircraft  100  for the displacement of servos on the rotor system  104  ( FIG. 1 ). 
         [0036]    Also shown in  FIG. 2 , the FCC  202  includes a memory  216 . The memory  216  stores the low-speed mixing algorithm  212  as executable instructions that is executed by a processor  218 . 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 low-speed mixing algorithm  212 . The processor  218  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  216  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. It is to be appreciated that the low-speed mixing algorithm  212  described below in  FIG. 3  may be implemented not only for aircraft travel along a longitudinal axis of rotary-wing aircraft  100  but also for aircraft travel along the lateral axis or for combinations of longitudinal and lateral travel. 
         [0037]      FIG. 3  illustrates a schematic block diagram for implementing the low-speed mixing algorithm  212  by FCC  202  ( FIG. 2 ) according to an embodiment of the invention and as such,  FIG. 2  is also being referenced in the description of low-speed mixing algorithm  212 . 
         [0038]    In an embodiment, implementation of the low-speed mixing algorithm  212  begins when the FCC  202  ( FIG. 2 ) receives and stores the aircraft&#39;s sensed parameters from sensors  210  ( FIG. 2 ) such as, for example, a signal  302  that represents an estimate of a magnitude of wind relative to the airframe  102  (i.e., airspeed incident on the airframe  102 ) and a signal  304  which represents an estimate of a direction of wind (i.e., sideslip angle on the airframe  102 ) relative to airframe  102 . The signals  302 ,  304  are representative of a direction of travel for the rotary-wing aircraft  100  ( FIG. 1 ). The signal  302  is applied to each of a multiplier block  308  on line  306 , a roll angle fixed gain block  310  on line  312 , a pitch angle fixed gain block  314  on line  316 , and a multiplier block  318  on line  320 . 
         [0039]    The fixed gain blocks  310 ,  314  include gain values, which are maintained in one or more lookup tables in memory  216  ( FIG. 2 ). Particularly, the gain blocks  310 ,  314  include one or more lookup tables which store gains for the respective signals  302 ,  304  representing the magnitude of wind relative to the rotor  104  and the direction of wind relative to the rotor  104 . These gains may be predetermined or derived from, in some non-limiting examples, simulated data or flight test data. 
         [0040]    The multiplier block  308  also receives a signal  322  on line  324  that is representative of a cosine of the signal  304  and multiplier block  318  receives a signal  326  on line  328  that is representative of a sine of the signal  304 . Additionally, flight control commands that are generated based on reference commands are applied to the multiplier blocks  308 ,  318  and summation blocks  344 ,  346 . Particularly, a signal representative of a roll command  330  based on reference commands is applied to summation block  344  on line  332 , a signal representative of a differential collective pitch (i.e., yaw) command  334  based on reference commands is applied to each of a multiplier block  308  on line  336  and multiplier block  318  on line  338 , and a signal representative of a pitch command  340  based on reference commands is applied to summation block  346  on line  342 . 
         [0041]    The multiplier block  308  multiplies its inputs of gain  310 , airspeed  302 , cosine signal  322  and yaw command  334  to produce a signal  348  that is representative of a differential collective to ganged roll mixing signal. The differential collective to ganged roll mixing signal is representative of a proportional decoupling response to the rolling moments that is applied for travel along a longitudinal axis of the rotary-wing aircraft  100  ( FIG. 1 ). The output of summation block  344  is signal  352  which is a sum of the roll command  330  and the decoupling signal  348 . The signal  352  represents a mixed roll command that is applied synchronously to the rotor system  104  ( FIG. 1 ) in order to decouple the roll moments that are induced in the rotary-wing aircraft  100  as described above. Similarly, the multiplier block  318  multiplies its inputs of gain  314 , airspeed  302 , sine signal  326  and yaw command  334  to produce a signal  350  that is representative of a differential collective to ganged pitch mixing signal. The differential collective to ganged pitch mixing signal is representative of a decoupling response to the pitching moments that is applied for travel along a lateral axis of the rotary-wing aircraft  100  ( FIG. 1 ). The output of summation block  346  is signal  354  which is a sum of the pitch command  340  and the decoupling signal  350 . The signal  354  represents a mixed pitch command that is applied synchronously to the rotor system  104  ( FIG. 1 ) in order to decouple the pitch moments that are induced in the rotary-wing aircraft  100  as described above. 
         [0042]    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.