Abstract:
A method for estimating airspeed of an aircraft includes receiving values indicative of operating conditions of the aircraft along an axis; estimating a tip path plane (TPP) angle along the axis from at least one of the operating conditions to create an estimated TPP angle; and determining an estimated airspeed as a function of the estimated TPP angle, the determining including referencing a look-up table that indexes the estimated TPP angle with the airspeed.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under N00019-06-C-0081 awarded by the Department of the Navy. The Government has certain rights in this invention. 
    
    
     FIELD OF INVENTION 
     The subject matter disclosed herein relates generally to the field of measurements in a rotary-wing aircraft and, more particularly, to a method and a system for determining an airspeed of a helicopter by utilizing a fly-by-wire system to estimate a tip path plane angle and resolving the steady state speed of the helicopter as a function of the known helicopter parameters. 
     DESCRIPTION OF RELATED ART 
     Many vehicles, including helicopters, use fly-by-wire (FBW) systems to control vehicle operation. Emerging FBW helicopters provide high levels of augmentation. These systems greatly reduce pilot workload and enhance safety. Part of the safety enhancements includes control inputs that allow pilots to aggressively maneuver within the airframe structural limits and not exceed these limits. With these systems, it is possible for the flight control system to provide a variety of tactile cues (such as power cues, level flight cues, control limit cues, etc.) to the pilot. Knowledge of rotorcraft airspeed is needed to facilitate optimal flight control operation for FBW systems. Typically, air speed is derived from an externally mounted sensor on the airframe. However, this sensor can fail due to mechanical defects or environmental conditions such as, for example, ice that may accumulate on the sensor. Improvements in providing an estimation of rotorcraft airspeed from information available within the FBW controls would be well received in the art. 
     BRIEF SUMMARY 
     According to one aspect of the invention, a method for estimating an airspeed of a rotary wing aircraft, includes receiving values indicative of operating conditions of the aircraft along an axis; estimating a tip path plane (TPP) angle along the axis from at least one of the operating conditions to create an estimated TPP angle; and determining an estimated airspeed as a function of the estimated TPP angle, the determining including referencing a look-up table that indexes the estimated TPP angle with the airspeed. 
     According to another aspect of the invention, a system for estimating an airspeed of an aircraft, includes a memory that indexes an estimated TPP angle with an estimated airspeed of the aircraft; a processor that receives values indicative of operating conditions of the aircraft along an axis and determines the estimated TPP angle along the axis from at least one of the operating conditions; where the processor determines an estimated airspeed as a function of the estimated TPP angle. 
     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 
       Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
         FIG. 1  is a perspective view of an exemplary rotary wing aircraft for use with embodiments of the invention; 
         FIG. 2  is a schematic diagram of an exemplary flight control system; and 
         FIG. 3  illustrates a schematic block diagram of an algorithm used for estimating airspeed of an aircraft according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of an airspeed estimating algorithm includes estimating a TPP angle as a function of a pitch attitude. The algorithm estimates the airspeed by utilizing a lookup table that has stored values of several TPP angles against an estimated aircraft airspeed. In an embodiment, the estimated airspeed may accommodate acceleration by correcting the estimated TPP angle as a function of the longitudinal acceleration prior to utilizing the lookup table for determining the estimated aircraft speed. 
     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  for use with estimating the airspeed of the rotary-wing aircraft  100  according to an embodiment of the invention. The rotary-wing aircraft  100  includes a main rotor assembly  102  and a tail rotor assembly  104 . Also, a tip path plane  106  is shown connecting the rotor blade tips as the rotor blades rotate during flight. The TPP  106  is horizontal to the longitudinal axis of the aircraft  100  as it hovers and its thrust vector is oriented along a vertical axis  108 . When the aircraft  100  travels forward, the thrust vector is rotated slightly in a forward direction, resulting in the TPP angle (α)  110 . The airspeed of the aircraft  100  may be estimated from the TPP angle  110  as a function of the parameters from a fly-by-wire flight control system  200  utilizing a known relationship between the airspeed of the aircraft and the TPP angle  110 . Although a particular helicopter configuration is illustrated and described in 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, and unmanned rotary wing aircraft with any of the previous configurations will also benefit from embodiments of the present invention. 
       FIG. 2  illustrates a schematic block diagram of a fly-by-wire flight control system  200  according to an exemplary embodiment. In an embodiment, Particularly, the system  200  is stored on a flight control computer system and estimates the TPP angle  110  ( FIG. 1 ) as a function of a displacement of the pilot&#39;s controllers  206 ,  208  in order to provide an estimate of the aircraft  100  airspeed from the TPP angle  110 . The system  200  includes a Primary Flight Control System (PFCS)  202  and an Automatic Flight Augmentation and Cuing System (FACS)  204 . 
     In an example of a flight control system  200 , the PFCS  202  and FACS  204  execute explicit control laws to provide both control and stability augmentation. The PFCS  202  is the flight critical portion of the flight control system, while the FACS  204  is the mission critical portion. The FACS  204  augments the performance of the PFCS  202 . In this control law architecture, pilot commands are shaped directly into desired aircraft responses. These desired commands are then passed through an aircraft model to obtain the control commands required to produce the desired response. The difference between the desired command and the aircraft response is also fed back to drive these errors towards zero, thus improving the model performance. 
     The PFCS  202  and FACS  204  each receive the force output command signals of a collective controller  206  on line  212 , a cyclic controller  208  on line  214 , and the aircraft&#39;s sensed parameter signals from sensors  210 , on lines  216 . 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. The pilot command signals on lines  212 ,  214  and the sensed parameter signals on lines  216  are shown consolidated within trunk lines  218  and  220  in the PFCS  202  and FACS  204 , respectively. 
     The PFCS  202  and FACS  204  may each contain separate control channel logic laws for controlling the yaw, pitch, roll and lift axes of the aircraft within the operational flight envelope. The logic is included in the PFCS  202  and FACS  204  control modules (schematically represented by blocks  222 - 228  for the PFCS  202  and blocks  230 - 236  for the FACS  204 ). The sensed parameter signals from aircraft sensors  210 , on lines  216 , provide the PFCS  202  and FACS  204  with the aircraft&#39;s angular rate and attitude response to the rotor command signals. In an embodiment, the FACS  204  utilizes an algorithm for estimating the tip path plane angle  110  and uses an airspeed model stored in memory for determining the airspeed of the aircraft based on a known relationship between angle  110  and airspeed, derived through flight test data or simulation. The FACS logic utilizes the sideslip envelope to limit the yaw rate available to the pilot for producing a desired response without exceeding the operational flight envelope. The PFCS logic provides rotor command signals and the FACS logic provides conditioning and/or trimming of the PFCS four axis logic functions based on the operational flight envelope. The PFCS and FACS logic modules interconnect through bus  238  to provide rotor command signals on output lines  240  to a mixing function  242 , which communicates commands on lines  252  for the displacement of servos  244  and linkages  246  to control the tip path plane  106  ( FIG. 1 ) of the main rotor  102 . A mixed command signal is also provided on line  250  to the tail rotor servos  248  which control the thrust of the tail rotor  104  through linkages  250 . 
       FIG. 3  illustrates a schematic block diagram of an airspeed estimation algorithm  302  implemented by the FACS  204  ( FIG. 2 ) of the fly-by-wire flight control system  200  which communicates with, in an exemplary embodiment, the control law system of the PFCS  202  ( FIG. 2 ) according to an embodiment of the invention. For ease of illustration and understanding, the functional block diagram in  FIG. 3  illustrates the airspeed estimation algorithm  302  stored in memory  306  on flight control computer system  300  and executed by a microprocessor  304  for implementing the algorithm  302  intended to provide an estimate of the airspeed from the TPP angle  110  ( FIG. 1 ) and as a function of aircraft operational parameters obtained from flight test data or simulated parameters during non-accelerated flight. The microprocessor  304  of computer system  302  can 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  306  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 data and control algorithms described below. It is to be appreciated that the airspeed estimation algorithm  302  described below for estimating the airspeed for travel along the longitudinal axis of aircraft  100  over a reference position but travel along the lateral axis may also be implemented. 
     In an embodiment, for a longitudinal axis direction of travel of the aircraft  100 , the microprocessor  304  initiates the airspeed estimation algorithm  302  by receiving and storing the aircraft&#39;s sensed parameter signals. In an embodiment, the aircraft&#39;s senses parameter signals include several aircraft operating parameters such as, in some non-limiting examples, lateral acceleration, aircraft attitude, aircraft&#39;s angular rate in order to estimate the aircrafts sideslip angle. The sensed parameter signals are received from appropriate sensors  210  in response to compensation in the flight attitude for, in an example, wind-induced perturbations from a steady-state horizontal flight. In one non-limiting example, the flight attitude may be compensated by displacing the controller  206  forward in order to change the longitudinal swash plate position and change the pitch angle θ to move the aircraft  100  forward. In one embodiment, the pitch angle θ is the angle between the longitudinal axis of the aircraft and a defined reference plane. The change in pitch angle θ causes a change in the longitudinal force along the longitudinal axis. In one example, displacing the controller  206  for moving the aircraft  100  forward causes the rotor to tip backwards in order to compensate for the prevailing wind, resulting in leveling the rotor angle. The microprocessor  304  uses the pitch angle or attitude θ and the displacement value  308  of the controller  206  for estimating the TPP angle  110  (α E ) according to equation (1) below:
 
α E   =K *(θ+( A*B   1   s )+ B );  (1)
         where:   α E =estimated TPP angle;   K, A, B=Constants derived from simulation data;   θ=Pitch angle; and   B 1 s=longitudinal main rotor swash plate position.       

     Further, the flight control computer  300  stores one or more lookup tables in memory  306  where operational conditions may be maintained for a particular airframe. particularly, the one or more tables defines estimates for the airspeed against several TPP angles  110  derived from, in some non-limiting examples, simulated data or flight test data. 
     In an embodiment, the algorithm  302  may correct the estimated TPP angle for accelerated flight along the longitudinal axis by utilizing equation (2) below:
 
α C =α E   −A   x   /G;   (2)
         where:   α C =the corrected TPP angle;   A x =longitudinal acceleration of the aircraft  100 ; and   G=Acceleration due to gravity of 9.80665 m/s 2  (or approx. 32.174 ft/s 2 ).       

     Further, one or more tables that defines estimates for the airspeed against the TPP angle  110  derived from, in some non-limiting examples, simulated data or flight test data may be utilized for estimating the airspeed. 
     Embodiments for estimating airspeed described herein may be used in conjunction with a pitot tube system for measuring airspeed. The airspeed estimating algorithm may be used to verify proper operation of a pitot tube system. Further, the airspeed estimating algorithm may be used to detect a common mode failure (e.g., icing) across multiple pitot tubes. 
     The technical effects and benefits of exemplary embodiments include an airspeed estimating algorithm from a TPP angle. In an embodiment, the system includes estimating the TPP angle as a function of the pitch attitude and utilizing a lookup table that defines stored values of several TPP angles against estimated aircraft airspeed. Also, the algorithm accommodates acceleration of the aircraft by correcting the estimated TPP angle as a function of the longitudinal acceleration. 
     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 embodiment 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.