Flight control laws for constant vector flat turns

An aircraft and method to control flat yawing turns of the aircraft while maintaining a constant vector across a ground surface. The aircraft includes a control system in data communication with control actuators, a lateral control architecture, a longitudinal control architecture, and an initialization command logic. The lateral control architecture controls the aircraft in the lateral direction, while the longitudinal control architecture controls the aircraft in the longitudinal direction. The initialization command logic automatically activates the lateral control architecture and the longitudinal control architecture to maintain a constant vector across the ground whenever a directional control input is made at low speed.

TECHNICAL FIELD

The present invention relates generally to flight control systems, and more particularly, to a flight control system having flight control laws which enable precise aircraft maneuvering relative to the ground.

DESCRIPTION OF THE PRIOR ART

Aircraft which can hover and fly at low speeds include rotorcraft, such as helicopters and tilt rotors, and jump jets, like the AV-8B Harrier and F-35B Lightning II. These aircraft can spend a large portion of their mission maneuvering relative to the ground. Sometimes, this maneuvering must be conducted in confined spaces around external hazards such as buildings, trees, towers, and power lines.

For traditional flight control systems, ground-referenced maneuvering (GRM) requires the pilot to make constant control inputs in multiple axes in order to counter disturbances caused by wind, as well as to remove the natural coupled response of the aircraft. The pilot workload during such maneuvers can become quite high since the pilot must sense un-commanded aircraft motions and then put in the appropriate control input to eliminate the disturbance. In a worst-case scenario, a pilot might be required to fly GRM in a degraded visual environment. With the lack of visual cues to detect off-axis motion, the pilot might accidentally fly into an external hazard while maneuvering in a confined space.

Traditional flight control law designs do not provide the pilot with an easy way to control aircraft crab angle during GRM. Crab angle is defined as the angle between the aircraft's heading and its actual ground path. With these prior designs, adjusting crab angle while maintaining ground track took considerable pilot concentration, since the pilot had to coordinate inputs to both the lateral and directional controllers.

Although pilots generally seek to minimize crab angle during GRM, some mission tasks may call for flat yawing turns while maintaining a constant vector across the ground. For example, on a steep approach, the pilot may need to fly with a crab angle so he or she can see the landing zone. Additionally, the pilot may want to quickly transition out of rearward or sideward flight while continuing along the same ground track. In a final example, the pilot may want to acquire and track a point on the ground without having to fly directly towards it. With prior flight control designs, such maneuvers required extraordinary pilot skill to coordinate the aircraft's motions in multiple control axes.

Although the foregoing developments represent great strides in the area of flight control laws, many shortcomings remain.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention will enable seamless and transient free GRM. More specifically this invention will enable a pilot to use the directional controller to command flat yawing turns at low groundspeeds, while maintaining a constant vector across the ground. The seamless integration of this design requires no manual cockpit switches to select a Constant Vector Flat Turn (CVFT) mode. As a result of this auto-moding logic, the control laws will automatically adjust pitch and roll attitude to keep the aircraft moving in the same direction at a constant speed whenever the pilot inputs a directional command at low speed.

The auto-moding logic of the present application enables seamless and transient free GRM without the need for manual cockpit switches. The control system utilizes relative groundspeed difference to automatically control pitch and roll attitudes so that the aircraft will maintain a constant vector during a low speed flat turn. The control system also allows the pilot to complete a CVFT with minimal workload since the ground vector will automatically be maintained by the control laws without the pilot having to use cockpit switches to change modes.

Referring now to the drawings,FIG. 1shows a representative flight envelope101with a plurality of control law modes designed to enable GRM. Flight envelope101comprises a region103depicting the CVFT region, wherein the CVFT region is preferably from 10 to 35 knots groundspeed in any direction relative to the aircraft's body axis. The lower bound of region103is set by the Hover Hold and Translational Rate Command (TRC) region105. The upper bound of region103is set by the aircraft's sideward and rearward flight airspeed limits.

FIG. 2is a schematic of an aircraft201utilizing a control system according to the preferred embodiment of the present invention.FIG. 2shows aircraft201in forward flight within region103. Directional inputs turn aircraft201in a complete 360 degree yaw movement R1, stopping every 90 degrees, and without changing the speed and flight heading of aircraft201, as represented with arrow D1. In the preferred embodiment, the control system is utilized with rotary aircraft, i.e., a helicopter; however, it should be appreciated that the control system is easily and readily adaptable with control systems of different types of aircraft, both manned and unmanned.

FIG. 2depicts aircraft201traveling between 10 to 35 knots in a forward direction. As is shown, aircraft201preferably turns in a yaw direction R1at approximately 90 degrees relative to direction D1. Aircraft201continues to turn in direction R1while maintaining a constant flight heading. It should be appreciated that the preferred control system is adapted to turn aircraft201at 90 degrees during each application; however, it should be appreciated that alternative embodiments could easily include a control system adapted to turn the aircraft at different angles, e.g., at 30 degrees in lieu of or in addition to 90 degrees. It should also be understood that although shown turning in a clockwise direction, the control system can also turn the aircraft in a counterclockwise movement.

FIG. 2provides an exemplary depiction of aircraft201turning 360 degrees while maintaining forward flight. Step1shows aircraft201traveling in a constant forward flight, as depicted with arrow D1, between 10 and 35 knots. Step2depicts application of the control system, namely, the pilot utilizes the control system to rotate aircraft201in the clockwise direction approximately 90 degrees, as indicated by arrow R1. Step2shows aircraft201traveling in forward flight while the fuselage faces 90 degrees relative to the directional movement. Steps3-5provide further illustration of the process being repeated. In particular, each time the control system is utilized, aircraft201rotates 90 degrees while maintaining a constant forward heading.

Referring now toFIG. 3in the drawings, measured flight data301of aircraft201is shown during a 360 degree CVFT. A plot303provides measured data representing the turning movement R1of aircraft201during the 360 degree turn. A plot305provides measured data representing the groundspeed of aircraft201during the 360 degree turn. Plot305shows aircraft201initially starting at 20 knots forward groundspeed during the entire 360 degree CVFT. Plot305shows that aircraft201holds a relatively steady groundspeed during the 360 degree CVFT. A plot307provides measured data representing the ground track of aircraft201during the 360 degree CVFT. The forward groundspeed plotted on a plot309essentially depicts a cosine curve during the turn, while the sideward groundspeed plotted on a plot311shows a sine curve.

Referring now toFIG. 4in the drawings, a schematic view of aircraft201is shown changing flight heading from sideward flight to forward flight. In the exemplary embodiment, the CVFT control system is utilized such that aircraft201changes heading from a forward groundspeed Vx of about 0 knots and a sideward groundspeed between 10-35 knots to a forward groundspeed between 10-35 knots and a sideward groundspeed about 0 knots. Step1ofFIG. 4shows aircraft401during hover, while a step2shows aircraft401traveling in a sideward groundspeed between 10-35 knots, as depicted with arrow D2. In step2, a right lateral control stick (not shown) is utilized to generate a left sideward heading. Thereafter, a right 90 degree pedal turn is applied to rotate aircraft201in a forward heading with a pedal203. In the preferred embodiment, pedal203is a pedal manipulated with the pilot's foot; however, it should be appreciated that other forms of devices, i.e, a hand switch could be utilized in lieu of or in addition to pedal203. For purposes of this invention, a lateral controller, longitudinal controller, and directional controller are characterized as pedal203or similarly suited devices. Step3depicts application of the CVFT control system, wherein aircraft201turns 90 degrees for changing the heading of aircraft201.

Referring now toFIG. 5in the drawings, an alternative application of the CVFT control system is shown. In the exemplary embodiment, the CVFT control system is utilized to turn aircraft201from a forward groundspeed Vx between 10-35 knots and a sideward groundspeed of about 0 knots to a sideward groundspeed between 10-35 knots and a forward groundspeed about 0 knots. Step1shows aircraft201during hover, while a step2shows aircraft201traveling in a forward heading having a groundspeed between 10-35 knots, as depicted with arrow D3. In step2, a forward longitudinal stick is utilized to generate forward flight. Thereafter, a left 90 degree pedal turn is applied to rotate aircraft201such that the forward flight of aircraft201changes to a sideward flight heading.

Those skilled in the art will understand that the methods for aircraft guidance disclosed in this invention can be applied to any combination of the following: (1) full authority fly-by-wire flight control systems, as well as partial authority mechanical systems; (2) traditional cockpit layouts with a center stick for longitudinal and lateral control, pedals for directional control, and a collective stick for vertical control, as well as more advanced designs which combine multiple control axes into a center or side stick controller; and, (3) any aircraft capable of GRM, including both rotorcraft and jump jets.

The key to enabling seamless and transient free GRM lies in the advanced control law architecture of the CVFT control system as shown inFIGS. 6 to 8.FIG. 6shows architecture601of the CVFT control system operably associated with one or more longitudinal control laws,FIG. 7shows architecture701of the CVFT control system operably associated with one or more lateral control laws, andFIG. 8shows architecture801of the CVFT control system operably associated with one or more directional control laws according to the preferred embodiment of the invention.

Referring now toFIG. 6in the drawings, architecture601includes one or more aircraft sensors603operably associated with the control laws to accomplish GRM. Aircraft sensors603can include: an inertial Navigation System (attitudes, attitude rates, and translational accelerations); a Global Positioning System (ground-referenced speeds and positions); an Air Data Computer (airspeed and barometric altitude); and, a Radar or Laser Altimeter (above ground level (AGL) altitude). An aircraft model can be obtained from aerodynamics data and a group of linear models can be developed based on its airspeed from aircraft sensors603. These linear models include both lateral and longitudinal equations of motion. Since the aircraft model matrices are large and contain coupling terms of lateral and longitudinal motions within the matrices, it is difficult to determine the best performance control gains for all at the same time. In order to overcome these issues, the linear model of aircraft performance is decoupled first. After the aircraft model is decoupled to lateral and longitudinal equations of motion, the effect of coupling terms between lateral and longitudinal motions can be reduced to a minimum, thus stabilizing the system.

In the preferred embodiment, architecture601preferably comprises of a longitudinal control law for forward speed, represented as block605“Long_SPD”; a longitudinal control law for pitch angle, represented as block607“Long_ATT”; and, a longitudinal control law for pitch rate, represented as block609“Long_RATE”. Architecture601is further provided with initialization logic611adapted for determining which loop is active in each axis based on flight conditions and pilot control inputs. Logic611will also re-initialize inactive loops in order to eliminate control jumps when switching between the loops to provide seamless and transient free mode changes.

Architecture601further includes a longitudinal command613generated in the control laws by referencing the pilot's cockpit control input in each axis. The input to the control laws is the difference between the controller's present position and the centered, no force position, which is also referred to as the “detent” position. The control commands can also be generated by a beep switch located in the cockpit to command small and precise changes in aircraft state. The control laws process these control inputs to generate the appropriate aircraft response commands. These commands are then sent out to the control law guidance blocks to maneuver the aircraft. The control law outputs are routed to an actuator615for each dynamic axis. For a conventional helicopter, the control laws send control signals to the following actuators: longitudinal axis—main rotor longitudinal swashplate angle; lateral axis—main rotor lateral swashplate angle; vertical axis—main rotor collective pitch; and, directional axis—tail rotor collective pitch.

Since pitch rate is the fastest longitudinal state, Long_RATE609is the inner loop of the longitudinal control laws. Next, the Long_ATT607loop feeds the Long_RATE control law609loop to control pitch attitude. Finally, the Long_SPD control law605loop feeds the Long_ATT607loop to control forward speed.

When flying with the longitudinal controller in detent outside of the Hover Hold/TRC region105, depicted inFIG. 1, the Long_SPD605loop will be active. At lower speeds, this loop will hold constant forward groundspeed, while at higher speeds, airspeed will be held. Once the pilot moves the longitudinal controller out of detent, the control laws can command either pitch attitude (Long_ATT607) or pitch rate (Long_RATE609).

Referring now toFIG. 7in the drawings, architecture701comprises one or more lateral control laws operably associated with sensors603, logic611, lateral commands702, and actuators615. The lateral control laws include: a lateral control of roll rate, represented as block703“Lat_RATE”; a lateral control of the roll attitude, represented as block705“Lat_ATT”; a lateral control of sideward groundspeed, represented as block707“Lat_SPD”; a lateral control of the crab angle, represented as block709“Lat_CRAB”; and, lateral control of heading, represented as block711“Lat_HDG”.

Similar to the longitudinal axis, Lat_RATE703is the inner loop of the lateral control laws and the Lat_ATT705loop feeds the Lat_RATE703loop to control roll attitude. The Lat_ATT705loop can be fed by one of three loops, Lat_SPD707, Lat_CRAB709, or Lat_HDG711.

The crab angle used in the Lat_CRAB709loop is computed in the control laws using the following equation:
η=tan−1(Vy/Vx)  (1)

where η is the crab angle, Vyis the sideward groundspeed with right positive, and Vxis the forward groundspeed. To avoid a singularity in Equation 1, Vxis limited to be above the Hover Hold/TRC region103.

When operating in the Ground-Coordinated Banked Turn (GCBT) envelope as shown by region107inFIG. 1, if both the lateral and directional controllers are in detent, lateral control law logic will hold crab angle through the Lat_CRAB709loop. If operating in the CVFT envelope, but not in the GCBT envelope, and the lateral and directional controllers are in detent, the control logic will hold sideward groundspeed constant with the Lat_SPD707loop. When operating at higher airspeeds with lateral and directional controllers in detent, the control logic will hold heading constant with the Lat_HDG711loop. When the pilot moves the lateral controller out of detent in any of these cases, the control laws can command either roll attitude (Lat_ATT705) or roll rate (Lat_RATE703).

Referring now toFIG. 8in the drawings, architecture801comprises one or more directional control laws operably associated with sensors603, logic611, commands613, and actuators615. The directional control laws include: directional control of yaw rate, represented as block803“Dir_RATE”; directional control of heading, represented as block805“Dir_HDG”; and, directional turn coordination, represented as block807“Dir_TC”

Since yaw rate is the fastest directional state, Dir_RATE803is the inner loop of the directional control laws. This loop is fed by the Dir_HDG805loop to control aircraft heading at lower speeds. Unlike traditional control law designs, this invention includes an additional loop, parallel to the Dir_HDG805loop, to feed the Dir_RATE803inner loop. The Dir_TC807loop is used to coordinate banked turns throughout the flight envelope.

In the GCBT envelope107shown inFIG. 1, the Dir_TC807loop will control crab angle during banked turns. With no directional input, the Dir_TC807loop will hold crab angle at zero. Any directional control inputs during a GCBT will result in a change in crab angle in the appropriate direction. Additionally, if the aircraft is in the GCBT envelope, but above the CVFT envelope, directional controller inputs will command changes in crab angle through the Dir_TC807loop even in non-turning flight. In this case, once the directional controller is returned to detent, heading hold will be re-engaged (Dir_HDG805loop) and the crab angle will be held though the Lat_CRAB709loop.

When in the BCBT envelope, the Dir_TC807loop will automatically adjust yaw rate based on actual bank angle, true airspeed, and lateral acceleration in order to keep the slip ball centered. Any directional controller inputs in the BCBT envelope will command a change in lateral acceleration, which will subsequently result in sideslip away from the pedal input. Pedal inputs will also result in a slight roll in the direction of the input to provide lateral stability.

In the absence of lateral or directional control inputs while operating in either the GCBT or CVFT envelopes, the directional axis will hold heading through the Dir_HDG805loop. If the pilot moves the directional controller out of detent in the CVFT envelope with both the lateral and longitudinal controllers in detent, the directional control laws will command a yaw rate through the Dir_RATE803loop. In this case, the control laws will maintain a constant ground vector by using the Long_SPD605and Lat_SPD707loops.

During the CVFT, when the directional controller is first moved out of detent, the control laws will capture the aircraft's current groundspeed in the earth axis coordinate system. The control laws keep track of the difference between the aircraft's actual groundspeed and the captured groundspeed. This relative groundspeed difference is converted from the earth axis to the aircraft's body axis using the following equations:
ΔVx=ΔVnorth·cos ψ+ΔVeast·sin ψ  (2)
ΔVy=ΔVeast·cos ψ−ΔVnorth·sin ψ  (3)

where ΔVxis the groundspeed difference in the body axis forward direction, ΔVyis the groundspeed difference in the body axis sideward direction with right positive, ΔVnorthis the groundspeed difference in the earth axis north direction, ΔVeastis the groundspeed difference in the earth axis east direction, and ψ is the aircraft heading. The values for ΔVxand ΔVyare then used in the Long_SPD605and Lat_SPD707blocks respectively to command the pitch and roll attitudes needed to minimize the relative groundspeed difference during the flat turn.

Referring now toFIG. 9in the drawings, control law logic901for the CVPT mode is shown. Control law logic901comprises one or more of latch903, latch905, and latch907adapted to control when a mode is turned on or off. If the reset conditions are met, then the mode will always be off. If the reset conditions are not met, then the mode will be latched on when the set conditions are met. The CVPT mode will be reset whenever lateral speed hold, depicted as block909“Lat_SPD_ON” or a longitudinal speed hold, depicted as block911“Long_SPD_ON” are not on and will be set when the directional controller is out of detent.

Both longitudinal speed hold latch905and lateral speed hold latch907will be reset when their respective controller is out of detent. Additionally, the lateral speed hold latch907will be reset when the banked turn (Bank_Turn_ON) or crab hold (Crab_ON) modes are on, or if forward speed exceeds the CVPT threshold (typically around 35 KGS). The longitudinal and lateral speed hold latches will be set when their respective acceleration falls below the acceleration threshold (typically around 2 ft/sec2).

Referring now toFIG. 10in the drawings, a control law flow chart1001for a CVFT is presented. When the directional controller is initially moved out of detent, the CVPT mode will be engaged. The longitudinal and lateral speed hold blocks (Long_SPD605and Lat_SPD707) will be reinitialized to feedback the relative groundspeed differences calculated in equations 2 and 3. Once the directional controller is returned to detent following the CVFT, directional control laws will hold heading by re-engaging the Dir_HDG805loop. The longitudinal and lateral axes will continue to hold a constant ground vector until the pilot commands a change by moving either the longitudinal or lateral controller out of detent. This design enables the pilot to command and hold any crab angle around the 360 degree circle, while the aircraft continues to move across the ground on a constant vector. As shown, when the pedal stops, if the crab angle equals 0 degrees, then aircraft201travels in pure forward flight; if the crab angle is 90 degrees, aircraft201travels in pure right side flight; if the crab angle is 180 degrees, then aircraft201travels in pure aft flight; and, if the crab angle is 270 degrees, then aircraft201travels in pure left sideward flight.

The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. It is therefore evident that the particular embodiments disclosed above may be altered or modified, and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the description. It is apparent that an invention with significant advantages has been described and illustrated. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof.