Control strategy and control distribution method of multi-tilt-rotor aircraft

Abstract

The invention discloses a control strategy and a control distribution method of a multi-tilt-rotor aircraft, and belongs to the field of helicopter flight mechanics. According to the control strategy, a control system is divided into three states of a helicopter mode, a transition mode and a fixed wing mode according to a nacelle tilt angle and a flight speed; the distribution relation of three control channels including pitching, rolling and yawing is defined in each mode; according to the multi-mode control strategy, stable transition can be achieved among three flight states, and the equal control sensitivity characteristic of each control channel is kept within the full flight envelope range. The method is suitable for the flight control of the tilt rotor aircraft with a plurality of control surfaces (such as rotor total pitch, nacelle tilt, fixed wing control surfaces and the like) in a helicopter mode, a transition mode and a fixed wing mode.

Description

Technical Field

[0001] This invention relates to the field of helicopter flight mechanics, specifically to a control strategy and control allocation method for equal control sensitivity of a multi-tilt rotor aircraft, which is particularly applicable to the flight control of tiltrotor aircraft with multiple control surfaces (such as rotor collective pitch, nacelle tilt, fixed-wing control surfaces, etc.) in helicopter mode, transition mode and fixed-wing mode. Background Technology

[0002] Tiltrotor aircraft achieve vertical takeoff and landing and high-speed cruise by changing the rotor tilt angle. While balancing the performance of helicopters and fixed-wing aircraft, their control systems exhibit significant redundancy and nonlinear characteristics, making control allocation particularly complex. Modern aircraft often employ multi-control surface layouts to enhance overall performance. For example, the B-2 strategic bomber is equipped with 10 control surfaces, and the B747 airliner has more than 50. Such systems are called overacted systems, meaning that the combination of control surface deflections is not unique when achieving the same control torque.

[0003] Common control allocation methods include generalized inverse allocation, chained allocation, direct allocation, and nonlinear programming. Generalized inverse allocation (such as pseudo-inverse allocation) aims to minimize control surface deflection and is computationally simple, but it struggles to simultaneously consider the actuator's physical limits and rate constraints. Chained allocation allocates control surfaces according to their priority, achieving some degree of division of labor, but under rate-constrained conditions, frequent switching between high and low priority surfaces can lead to control instability. Direct allocation relies on reachability set analysis of control torque, offering theoretical accuracy, but the computational burden increases dramatically with the command dimension. Nonlinear control allocation methods (such as schemes based on the Kriging surrogate model) can handle complex aerodynamic nonlinearities, but their heavy computational burden makes them unsuitable for real-time control.

[0004] In the research field of tiltrotor aircraft, the control assignment problem is particularly unique. The China Helicopter Design Institute has proposed a patent, "An Anti-saturation Control Assignment Method and System for a Tiltable Hexacopter UAV," which extends the concept to multi-rotor configurations. However, its core remains based on the traditional pseudo-inverse approach, obtaining a least-squares solution through a constant control assignment matrix. Because this method relies on linearization assumptions, it struggles to accurately describe the nonlinear and time-varying control performance characteristics under transient modes. Consequently, it cannot guarantee consistent control sensitivity across the entire flight envelope, limiting its application in complex multi-objective optimization tasks.

[0005] For multi-tilt rotor configurations, the increased number of rotors brings higher redundancy, but also introduces significant aerodynamic coupling effects. How to fully exploit the collaborative control potential of each rotor and control surface while ensuring consistent handling feel across the entire control envelope has become a core challenge in multi-tilt rotor control allocation research. Summary of the Invention

[0006] This invention addresses the problems existing in the aforementioned technologies by providing a method for achieving equal control sensitivity allocation across multiple flight modes (helicopter mode, transition mode, and fixed-wing mode). For different optimization objectives, it ensures that the attitude angle response (pitch, roll, yaw) generated by a unit control input of 1 cm within 1 second remains consistent across the entire flight envelope, eliminating control sensitivity differences caused by changes in flight state. It solves the problem that traditional pseudo-inverse methods and other linear allocation algorithms struggle to handle nonlinear control surface efficacy, dynamic saturation constraints, and multi-channel coupling issues. Furthermore, this invention optimizes control allocation through a multi-rotor, multi-control quantity control strategy, designing a control strategy that enhances the system's anti-saturation capability, improves control efficacy and response speed, and achieves a unified control strategy for multi-rotor cooperation.

[0007] This invention is implemented as follows:

[0008] To adapt to the aerodynamic characteristics and control coupling relationship of multi-tilt rotor aircraft in different flight modes, this invention proposes a mode-segmented, multi-channel control strategy. Based on the nacelle tilt angle and flight speed, this strategy divides the control system into three states: helicopter mode, transition mode, and fixed-wing mode, and defines the allocation relationship of the three control channels—pitch, roll, and yaw—in each mode.

[0009] A control strategy and control allocation method for a multi-tilt rotor aircraft, characterized in that the control strategy divides the control system into three states—helicopter mode, transition mode, and fixed-wing mode—based on the nacelle tilt angle and flight speed, and defines the allocation relationship of three control channels—pitch, roll, and yaw—in each mode.

[0010] In helicopter mode, the pitch channel is achieved through the combined action of differential collective pitch of the front and rear rotors, small-amplitude unidirectional tilt of the nacelle, and elevator; the roll channel is achieved by differential collective pitch of the left and right rotors and deflection of the ailerons; the yaw channel is achieved by combined control of counter-rotation of the left and right nacelles, deflection of the rudder, and diagonal differential collective pitch of the rotors to enhance directional control capability at low speeds.

[0011] In the transition mode, the pitch channel is mainly controlled by the differential collective pitch of the front and rear rotors and the elevator, the roll channel is mainly controlled by the differential collective pitch of the left and right rotors and the aileron deflection, and the yaw channel is achieved by the coordination of the differential collective pitch of the left and right rotors and the rudder.

[0012] When the aircraft enters fixed-wing mode, pitch, roll and yaw channels are independently completed by the elevator, aileron and rudder respectively, while the total distance difference between the left and right sides is retained as an auxiliary control quantity of the rudder to enhance yaw stability at high angles of attack or low-speed cruise.

[0013] The multi-mode control strategy can achieve a smooth transition between three flight states and maintain the same control sensitivity characteristics of each control channel throughout the entire flight envelope.

[0014] Furthermore, the control allocation method achieves unified control across multiple modes by constructing a nonlinear optimization allocation model under nominal joystick input conditions, aiming to achieve the same angle change in attitude response within 1 second.

[0015] The nonlinear dynamic model of the aircraft is expressed as:

[0016]

[0017] Among them, state variables Manipulation quantity This includes the control values ​​for rotor, control surfaces, and nacelle tilt angles; the nominal control stick inputs for the longitudinal, lateral, and directional channels are set as follows: Different control surface deflection amounts are mapped through the control effectiveness function. Map the joystick inputs to the actual control surface commands, where Regarding the rotor nacelle tilt angle and forward flight speed The nonlinear allocation matrix. The control allocation efficiency mapping function can be expressed as:

[0018] .

[0019] Furthermore, the aforementioned equal control sensitivity refers to the attitude angle (pitch angle, roll angle, yaw angle) response value of the aircraft within 1 second after applying a unit (1 cm) step input of control stick displacement; its mathematical expression is:

[0020]

[0021] The core requirement for control sensitivity is to maintain this sensitivity consistently throughout the entire flight envelope through control allocation, i.e., to meet the following conditions:

[0022]

[0023] Among them, the target sensitivity benchmark value It is not limited to fixed standards; it can be designed and set with reference to flight quality specifications or according to engineering application requirements.

[0024] Furthermore, a general multi-objective control allocation optimization problem is constructed to achieve optimal performance under different flight phases and mission requirements. The core of the multi-objective control allocation optimization problem is a configurable cost function, which can highlight different optimization objectives according to actual needs. The control allocation optimization problem for different optimization objectives is defined as follows:

[0025]

[0026] in, These are weighting coefficients used to adjust the weights of different optimization objective functions in the global cost function; For the first The optimal value of the objective function, and the constraints for different channels, can be expressed as:

[0027]

[0028] Furthermore, to more intuitively demonstrate the overall calculation approach and implementation steps of the equal manipulation sensitivity allocation method, the calculation flow of the equal manipulation sensitivity allocation method is as follows:

[0029] First, the flight state and control parameters are initialized, and nacelle tilt angle and forward velocity commands are generated sequentially. Dynamic simulation is performed, and the attitude angle response per unit stick input is calculated. Then, sensitivity constraints are checked to determine if they are met. If not, the control allocation is updated through iterative optimization while simultaneously satisfying convergence criteria, ultimately outputting the optimal allocation result. This process, centered on control sensitivity constraints, achieves multi-channel coordination through nonlinear control allocation and iterative optimization, thereby ensuring control consistency and response coordination across different flight modes.

[0030] The multi-tilt rotor aircraft in this invention is a tilt rotor aircraft with three or more rotors.

[0031] The advantages of this invention compared to the prior art are as follows:

[0032] 1. Consistent Control Sensitivity: This invention achieves consistent control sensitivity across the entire flight envelope by establishing a nonlinear control allocation model with equal control sensitivity constraints. This method can control the attitude response error within ±0.15° within 1 second after a 1 cm input to the control stick, thereby significantly improving the coordination accuracy and response consistency among multiple control surfaces, effectively enhancing flight quality and handling comfort.

[0033] 2. Effects of Multi-Tiltrotor Control Strategy: This invention designs a multi-mode systematic control strategy tailored to the spatial layout and aerodynamic coupling characteristics of multi-tilt rotor aircraft. This strategy rationally allocates the control surface combinations of pitch, roll, and yaw channels in helicopter, transition, and fixed-wing modes, enabling smooth transitions and coordinated responses between different flight modes. This strategy fully leverages the control redundancy and potential of the multi-rotor configuration, improving the controllability and stability of the aircraft across its entire flight envelope. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the calculation process in the control strategy and control allocation method for a multi-tilt rotor aircraft of the present invention;

[0035] Figure 2 This is a schematic diagram of pitch control in the control strategy and control allocation method for a multi-tilt rotor aircraft of the present invention;

[0036] Figure 3 This is a schematic diagram of roll control in the control strategy and control allocation method for a multi-tilt rotor aircraft of the present invention;

[0037] Figure 4 This is a schematic diagram of yaw control in the control strategy and control allocation method for a multi-tilt rotor aircraft of the present invention;

[0038] Figure 5 The three-axis step response during hovering in helicopter mode (nacelle angle 90°);

[0039] Figure 6 The three-axis step response at a forward speed of 35 m / s before transition mode (nacelle angle 60°);

[0040] Figure 7 The three-axis step response at a forward speed of 45 m / s before transition mode (nacelle angle 30°);

[0041] Figure 8 The three-axis step response at a forward speed of 60 m / s in aircraft mode (nacelle angle 0°). Detailed Implementation

[0042] To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the following examples provide a more detailed description of the invention. It should be noted that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention.

[0043] To adapt to the aerodynamic characteristics and control coupling relationship of multi-tilt rotor aircraft in different flight modes, this invention proposes a mode-segmented, multi-channel control strategy. Based on the nacelle tilt angle and flight speed, this strategy divides the control system into three states: helicopter mode, transition mode, and fixed-wing mode, and defines the allocation relationship of the three control channels—pitch, roll, and yaw—in each mode.

[0044] In helicopter mode, the pitch channel is achieved through the combined action of differential collective pitch of the front and rear rotors, small-amplitude unidirectional tilt of the nacelle, and elevator; the roll channel is accomplished by differential collective pitch of the left and right rotors and deflection of the ailerons; the yaw channel is achieved by combined control of counter-rotating tilt of the left and right nacelles, rudder deflection, and diagonal differential collective pitch of the rotors to enhance directional control capability at low speeds.

[0045] In the transition mode, the pitch channel is mainly controlled by the differential collective pitch of the front and rear rotors and the elevator, the roll channel is mainly controlled by the differential collective pitch of the left and right rotors and the aileron deflection, and the yaw channel is achieved by the coordination of the differential collective pitch of the left and right rotors and the rudder.

[0046] Once the aircraft enters fixed-wing mode, pitch, roll, and yaw are controlled independently by the elevator, ailerons, and rudder, respectively. Meanwhile, the total distance difference between the left and right sides is retained as an auxiliary control variable for the rudder to enhance yaw stability at high angles of attack or low-speed cruise.

[0047] This multi-mode control strategy can achieve a smooth transition between three flight states and maintain equal control sensitivity characteristics of each control channel throughout the entire flight envelope.

[0048] Building upon this foundation, to achieve continuous transitions and sensitivity consistency across different modes, this invention further proposes a nonlinear control allocation method based on equal control sensitivity constraints. This method constructs a nonlinear optimization allocation model by ensuring that the attitude response reaches the same angle change within 1 second under nominal joystick input conditions, thereby achieving unified control across multiple modes. This model features anti-saturation, constraint-based operation, and real-time optimization capabilities, effectively improving the aircraft's handling performance and control consistency in complex missions.

[0049] The nonlinear dynamic model of the aircraft is expressed as:

[0050]

[0051] Among them, state variables , Control inputs include those for rotor, control surfaces, and tilt angle. Let the nominal stick input be... By controlling the performance mapping function Map the joystick inputs to the actual control surface commands, where Regarding the rotor nacelle tilt angle and forward flight speed The nonlinear allocation matrix. The control allocation efficiency mapping function can be expressed as:

[0052]

[0053] The control sensitivity defined in this invention refers to the attitude angle (pitch angle, roll angle, yaw angle) response value of the aircraft within 1 second after applying a step input of a unit (1 cm) control stick displacement. Its mathematical expression is:

[0054]

[0055] The core requirement for control sensitivity is to maintain this sensitivity consistently throughout the entire flight envelope through control allocation, i.e., to meet the following conditions:

[0056]

[0057] Among them, the target sensitivity benchmark value It is not limited to fixed standards; it can be designed and set with reference to flight quality specifications or according to engineering application requirements.

[0058] To achieve optimal performance under different flight phases and mission requirements, this method constructs a general multi-objective control allocation optimization problem. The core of this problem is a configurable cost function that can highlight different optimization objectives according to actual needs. The control allocation optimization problem for different optimization objectives is defined as follows:

[0059]

[0060] in, These are weighting coefficients used to adjust the weights of different optimization objective functions in the global cost function; For the first The objective function is optimized to find the optimal value. The constraints for different channels can be expressed as:

[0061]

[0062] To more intuitively demonstrate the overall calculation approach and implementation steps of the equal manipulation sensitivity allocation method, Figure 1 The computational flow of this method is presented. This flow centers on control sensitivity constraints and achieves multi-channel coordination through nonlinear control allocation and iterative optimization, thereby ensuring control consistency and response coordination across different flight modes. Figure 1 The pitch channel control parameters and their operation modes of the research object of this invention are shown, including the elevator on the V-tail, as well as the collective pitch and nacelle tilt angle of each rotor; Figure 2The roll channel control amount and its operation mode of the research object of the present invention are shown, including the ailerons on the wing and the collective pitch of each rotor. Figure 3 The yaw channel control parameters and their operation methods of the research object of this invention are shown, including the rudder on the V-tail, as well as the collective pitch and nacelle tilt angle of each rotor.

[0063] The following specific data examples use a certain type of six-tilt-rotor eVTOL aircraft for verification. This aircraft has a total weight of 1950 kg, a wingspan of 11.5 m, and its nacelle tilt angle can be adjusted within the range of 0° to 90°. The main control components include six rotors with collective pitch and nacelle tilt control capabilities, as well as fixed-wing control surfaces such as elevators and rudders located on the ailerons of the wings and the V-tail.

[0064] The simulation was conducted based on a nonlinear dynamic model of the aircraft, with the rotor system described using the Pitt–Peters dynamic inflow model. To ensure the accuracy of the model, model verification was first performed by comparing the trim calculations and control responses with reference data to confirm the model's reliability.

[0065] The specific steps to achieve equal manipulation sensitivity control allocation are as follows:

[0066] 1. Define target values ​​for control sensitivity: Based on flight quality requirements or engineering needs, set target values ​​for attitude angle response within 1 second after a step input of a unit joystick displacement (1 cm). In this embodiment, the pitch angle change is set to 3°±0.1°, the roll angle change is set to 3°±0.15°, and the yaw angle change is set to 2.5°±0.1°.

[0067] 2. Select flight status points: Within the nacelle tilt angle-velocity flight envelope, select a series of status points with different forward flight speeds, covering helicopter mode (nacelle tilt angle 90°), transition mode (nacelle tilt angle 60°, 30°) and fixed-wing mode (nacelle tilt angle 0°).

[0068] 3. Define the control surface combinations for each control channel according to the flight mode:

[0069] Helicopter Mode: Pitch is controlled by differential collective pitch of the front and rear rotors, slight unidirectional tilt of the nacelles, and coordinated action of the elevator; Roll is controlled by differential collective pitch of the left and right rotors and ailerons; Yaw is controlled by counter-rotating tilt of the left and right nacelles, rudder, and differential collective pitch of the diagonal rotors. Transition Mode: Pitch is controlled by differential collective pitch of the front and rear rotors and the elevator; Roll is controlled by differential collective pitch of the left and right rotors and ailerons; Yaw is controlled by differential collective pitch of the left and right rotors and the rudder. Fixed-Wing Mode: Pitch, Roll, and Yaw are controlled independently by the elevator, ailerons, and rudder, respectively, supplemented by differential collective pitch of the left and right rotors for yaw assistance.

[0070] 4. Constructing a control allocation optimization problem: For each flight state point, solve for the control allocation coefficients so that the actual attitude angle response reaches the target value within 1 second when the nominal control stick input is 1cm. In the control allocation calculation, the optimization objective is to minimize the control surface deflection, and the allocation of fixed-wing control surfaces is given priority in the weight design to improve overall control efficiency and reduce rotor actuation load.

[0071] 5. Solving the optimization problem: The Particle Swarm Optimization (PSO) algorithm is used to solve for the control allocation coefficients at each state point.

[0072] 6. Verify the control allocation effect: Apply the obtained control allocation coefficients to the nonlinear model and perform a step response test. Simulation results are as follows: Figures 5-8 As shown, Figure 5 The three-axis step response during hovering in helicopter mode (nacelle angle 90°); Figure 6 The three-axis step response at a forward speed of 35 m / s before transition mode (nacelle angle 60°); Figure 7 The three-axis step response at a forward speed of 45 m / s before transition mode (nacelle angle 30°); Figure 8 The three-axis step response is given at a forward speed of 60 m / s in flight mode (nacelle angle 0°). The above indicates that the attitude angle response meets the target value under different flight modes, with errors within the allowable range. The system achieves good consistency in three-axis response across the entire speed range, meeting the design requirements for equal control sensitivity. This verifies the feasibility and effectiveness of the equal control sensitivity control allocation method on multi-tilt rotor aircraft, ensuring control consistency throughout the entire flight envelope.

[0073] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements can be made without departing from the principle of the present invention, and these improvements should also be considered within the scope of protection of the present invention.

Claims

1. A control strategy and control allocation method for a multi-tilt rotor aircraft, characterized in that, The control strategy is based on the nacelle tilt angle and flight speed, and divides the control system into three states: helicopter mode, transition mode and fixed-wing mode. In each mode, the allocation relationship of the three control channels of pitch, roll and yaw is defined respectively. In helicopter mode, the pitch channel is achieved through the combined action of differential collective pitch of the front and rear rotors, small-amplitude unidirectional tilt of the nacelle, and elevator; the roll channel is achieved by differential collective pitch of the left and right rotors and deflection of the ailerons; the yaw channel is achieved by combined control of counter-rotation of the left and right nacelles, deflection of the rudder, and diagonal differential collective pitch of the rotors to enhance directional control capability at low speeds. In the transition mode, the pitch channel is mainly controlled by the differential collective pitch of the front and rear rotors and the elevator, the roll channel is mainly controlled by the differential collective pitch of the left and right rotors and the aileron deflection, and the yaw channel is achieved by the coordination of the differential collective pitch of the left and right rotors and the rudder. Once the aircraft enters fixed-wing mode, pitch, roll, and yaw are controlled independently by the elevator, aileron, and rudder, respectively. At the same time, the total distance difference is retained as an auxiliary control variable for the rudder to enhance yaw stability at high angles of attack or low-speed cruise. The multi-mode control strategy can achieve a smooth transition between the three flight states and maintain the equal control sensitivity characteristics of each control channel throughout the entire flight envelope.

2. The control strategy and control allocation method for a multi-tilt rotor aircraft according to claim 1, characterized in that, The control allocation method described above aims to achieve the same angle change in attitude response within 1 second under nominal joystick input conditions, and constructs a nonlinear optimization allocation model to realize unified control under multiple modes. The nonlinear dynamic model of the aircraft is expressed as: Among them, state variables Manipulation quantity This includes the control values ​​for rotor, control surfaces, and nacelle tilt angles; the nominal control stick inputs for the longitudinal, lateral, and directional channels are set as follows: Different control surface deflection amounts are mapped through the control effectiveness function. Map the joystick inputs to the actual control surface commands, where Regarding the rotor nacelle tilt angle and forward flight speed The nonlinear allocation matrix; the control allocation efficiency mapping function can be expressed as: 。 3. The control strategy and control allocation method for a multi-tilt rotor aircraft according to claim 1, characterized in that, The so-called constant control sensitivity refers to the attitude angle (pitch angle, roll angle, yaw angle) response value of the aircraft within 1 second after applying a step input of a unit (1 cm) control stick displacement; its mathematical expression is: The core requirement for control sensitivity is to maintain this sensitivity consistently throughout the entire flight envelope through control allocation, i.e., to meet the following conditions: Among them, the target sensitivity benchmark value It is not limited to fixed standards; it can be designed and set with reference to flight quality specifications or according to engineering application requirements.

4. The control strategy and control allocation method for a multi-tilt rotor aircraft according to claim 1, characterized in that, A general multi-objective control allocation optimization problem is constructed to achieve optimal performance under different flight phases and mission requirements. The core of the multi-objective control allocation optimization problem is a configurable cost function, which can highlight different optimization objectives according to actual needs. The control allocation optimization problem for different optimization objectives is defined as follows: in, These are weighting coefficients used to adjust the weights of different optimization objective functions in the global cost function; For the first The optimal value of the objective function, and the constraints for different channels, can be expressed as: 。 5. The control strategy and control allocation method for a multi-tilt rotor aircraft according to claim 4, characterized in that, The calculation process for the equal manipulation sensitivity allocation method is as follows: First, the flight state and control inputs are initialized, and the nacelle tilt angle and forward speed commands are generated sequentially. Dynamic simulation is performed and the attitude angle response under unit stick input is calculated. Then, the sensitivity constraints are used to determine whether the requirements are met. If not, the control allocation is updated through iterative optimization while satisfying the convergence criterion. Finally, the optimal allocation result is output.

6. The control strategy and control allocation method for a multi-tilt rotor aircraft according to claim 1, characterized in that, The aforementioned multi-tilt rotor aircraft is a tiltrotor aircraft with three or more rotors.

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