Flight Mode Transitions Research Articles

Modeling and Transition Flight Control of Distributed Propulsion–Wing VTOL UAV with Induced Wing Configuration

The integration of propulsion and wing in distributed propulsion–wing UAVs (DPW UAVs) introduces significant propulsion-aerodynamic coupling, complicating dynamic modeling and flight control. This complexity is heightened by using induced wing surfaces for vertical takeoff and landing, requiring controllers to adapt to configuration changes and disturbances during transition flight. This paper develops a propulsion-aerodynamic coupling model for a medium-sized DPW UAV with induced wings (DPW-IW), enabling real-time aerodynamic performance calculations. Furthermore, a unified flight-control framework is proposed to avoid controller scheduling and switching during flight mode transitions. The proposed control framework employs the time-scale separation principle, divided into an outer loop and an inner loop. The outer loop uses a fuzzy controller to adjust allocation parameters, while the inner loop applies incremental nonlinear dynamic inversion (INDI) and control allocation (INCA) methods, providing robustness to nonlinear changes during flight transitions. Finally, simulations under various conditions demonstrate the controller’s effectiveness in ensuring smooth and robust transitions.

Time Optimal Altitude-Hold Flight Mode Transition Strategy for a Class of Ducted Fan Tail Sitter UAV

For special tail sitter configurations such as the ducted fan tail sitter unmanned aerial vehicle (UAV), the widely used trajectory planning methodology based on differential flatness might not be applicable due to complex aerodynamic coupling effects. As a result, the flight mode transition remains a challenging task. In this paper, we address the time optimal altitude-hold flight mode transition issue for a class of ducted fan tail sitter UAV. The foundation of the framework is the dynamic transition corridor in which the limitation of jerk is particularly considered, aiming to thoroughly reflect the dynamic feature of aggressive maneuvers. Based on this, we propose a time optimal strategy to generate feasible altitude-hold transition trajectories. Simultaneous, by fully utilizing the manifestation of time optimal altitude-hold flight behavior revealed by the transition corridor, we try to tackle the time optimal altitude-hold transition by means of a novel model-free control scheme. Comparative simulations show that both of the transition strategies achieve satisfactory performance on time optimal altitude-hold transition in the absence of disturbance, while the model-free control scheme exhibits better robustness under external disturbance.

Towards Flight Envelope Protection for the NASA Tiltwing eVTOL Flight Mode Transition Using Hamilton???Jacobi Reachability

Innovative electric vertical take-off and landing (eVTOL) aircraft designs and operational concepts, driven by advancements in battery and electric motor technologies, seek to achieve superior safety records with increased system redundancy. Validating safe flight operations within the prescribed flight envelope for passenger flights in densely populated urban environments remains a primary challenge. This paper establishes a framework for applying Hamilton???Jacobi reachability analysis to the full six-degree-of-freedom (6-DOF) dynamics of the NASA Tiltwing vehicle, verifying the flight envelope during the flight mode transition between near-hover and cruise flight, which prevents loss of control of the vehicle and ensures recoverability to safe trim conditions. This involves first verifying the nominal flight mode transition path as a series of trim points, defining the safe flight envelope using reachability, and decomposing the system dynamics into longitudinal and lateral subsystems. Our formulation guarantees the computed envelope’s robustness against modeling errors and uncertainties, and the usage of state decomposition significantly improves the tractability of the reachability computation. The result is validated through Monte Carlo 6-DOF nonlinear simulation of vehicle dynamics, demonstrating that the vehicle states within the flight envelope can successfully recover to trim states and continue the flight mode transition safely.

A Hybrid Controller Enhancing Transient Performance for an Aerial Manipulator Extracting a Wedged Object

Autonomous aerial manipulation requires the capability to handle inevitable dynamic changes during physical interaction. Previously, very few studies have addressed the stability and transient performance of the scenarios involving abrupt changes in dynamics. This paper proposes a hybrid controller enhancing transient performance for an aerial manipulator extracting an object wedged in a static structure. This task incurs a significant jump in the interaction force on the end-effector so that the analysis using the concept of hybrid dynamical systems is required. To demonstrate the dynamic characteristics of the object-extracting aerial manipulator, we derive the dynamic equations for two flight modes, i.e., free-flight and object-extracting, and the rule of state jumps. Also, we design control strategies which enhance the transient performance during flight mode transition. Then, the stability of the proposed control law is proven, and the overshoot reduction after the object extraction is analyzed. To show the improved performance, we conduct plug-pulling experiments with a quadrotor-based aerial manipulator using the proposed controller and two different existing controllers. The comparative results confirm that our controller enables the aerial manipulator to maintain its stability after the flight mode transition and shows the best transient performance in overshoot minimization among three controllers. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —The motivation for this article is the desire to prevent unexpected collisions between obstacles and an aerial manipulator after the vehicle extracts a wedged object from a static structure. To resolve this problem, we present a hybrid control method for such tasks while avoiding an excessive overshoot after the extraction. This method can be utilized in tasks involving abrupt changes in the dynamic model such as retrieving a device attached to a tall structure, reclaiming an object in disaster recovery, or pulling a plug out of a socket. Unlike existing methods, the proposed method simultaneously considers the stability <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">and</i> the initial overshoot right after pulling the object out of the structure, by employing a disturbance observer (DOB) in a hybrid control structure. It can be utilized for the aerial manipulator to avoid collision in a narrow space or to keep it in a safe operation envelope even though the task requires producing a relatively large pulling force.

Autopilot for Flight Mode Transistion in QuadPlanes

This paper presents the design of an autopilot for the flight mode transition in a hybrid quadrotor/fixed-wing aircraft known as QuadPlane. The autopilot includes an attitude stabilization subsystem, a control subsystem for takeoff and landing in quadrotor mode, and a control subsystem to regulate the longitudinal speed of the aircraft and determine the transition from quadrotor to fixed-wing and vice versa. During pure fixed-wing and quadrotor modes, the pilot has control authority on the QuadPlane. The proposed scheme is validated using the X-Plane flight simulator.

A corridor-based flight mode transition strategy for agile ducted-fan tail-sitter UAV: Altitude-hold transition

As an attractive transition approach, the altitude-hold transition is a special type of super-maneuvering and the vertical/horizontal flight mode transition that an agile aircraft conducts at fixed altitude. However, it is still challenging to implement an autonomous control of the altitude-hold transition while the existing optimal transition planning methods cannot avoid an evident altitude change during the transition process. This paper proposes a corridor-based flight mode transition strategy and presents a successful flight demonstration of the altitude-hold transition on a small ducted-fan tail-sitter unmanned aerial vehicle. In the proposed corridor-based methodology, we model and analyze the transition corridor, concentrate on the dynamic characteristics of the altitude-hold transition, and emphasize that a valid transition trajectory should be governed by its transition corridor. The identified transition corridor reveals that for a given velocity trajectory, the solution for the corresponding trajectories of pitch angle and thrust is unique. Based on this, the transition trajectory generation problem is addressed simply on the velocity-acceleration plane. Furthermore, a proper flight control scheme is devised to track the generated transition trajectories. Finally, the effectiveness of the proposed method is verified through practical flight tests, in which the altitude change is less than 1.1 m during the entire transition course.

Transition Analysis and Practical Flight Control for Ducted Fan Fixed-Wing Aerial Robot: Level Path Flight Mode Transition

For agile fixed-wing aerial robots, the vertical/horizontal flight mode transition is a fundamental requirement for high maneuverability. This letter focuses on a challenging implementation in which the aircraft conducts the transitional flight at fixed altitude along a straight path. Implemented on a small ducted fan fixed-wing, we propose a transition-corridor-based control strategy to conduct the flight transition neatly. The transition corridor is described as a region comprising all the admissible flight velocities, accelerations and attitudes, bounded by the capacity of both the propulsion and control system of the vehicle. By planning the transition trajectory in obedience with the identified transition corridor, as well as adopting a unified full envelope flight control scheme, the ducted fan fixed-wing successfully accomplishes the practical transitional flight. Experimental results are satisfactory in both flight mode transition and altitude stabilization.

Unified Accurate Attitude Control for Dual-Tiltrotor UAV with Cyclic Pitch Using Actuator Dynamics Compensated LADRC.

This paper proposes a unified attitude controller based on the modified linear active disturbance rejection control (LADRC) for a dual-tiltrotor unmanned aerial vehicle (UAV) with cyclic pitch to achieve accurate attitude control despite its nonlinear and time-varying characteristics during flight mode transitions. The proposed control algorithm has higher robustness against model mismatch compared with the model-based control algorithms. The modified LADRC utilizes the state feedbacks from the onboard sensors like IMU and Pitot tube instead of the mathematical model of the plane. It has less dependency on the accurate dynamics model of the dual-tiltrotor UAV, which can hardly be built. In contrast to the original LADRC, an actuator model is integrated into the modified LADRC to compensate for the non-negligible slow rotor flapping dynamics and servo dynamics. This modification eliminates the oscillation of the original LADRC when applied on the plant with slow-response actuators, such as propeller and rotors of the helicopter. In this way, the stability and performance of the controller are improved. The controller replaces the gain-scheduling or the control logic switching by a unified controller structure, which simplifies the design approach of the controller for different flight modes. The effectiveness of the modified LADRC and the performance of the unified attitude controller are demonstrated in both simulation and flight tests using a dual-tiltrotor UAV. The attitude control error is less than ±4° during the conversion flight. The control rising time in different flight modes is all about 0.5 s, despite the variations in the airspeed and tilt angle. The flight results show that the controller guarantees high control accuracy and uniform control quality in different flight modes.

Double Hybrid Tailsitter Unmanned Aerial Vehicle With Vertical Takeoff and Landing

We propose a new double-hybrid concept and architectural design of a tailsitter unmanned aerial vehicle with vertical takeoff and landing capability. Basically, it consists of a modified flying wing with a single combustion powertrain set and a multirotor with 2 powertrain sets with electric motors. With the electric propellers fixed at the leading wing edge, the tailsitter has two standard surfaces for elevation control and two vertical stabilizers that are used to give the necessary direction on vertical takeoff and landing. We have designed, built, and tested a prototype based on this new double-hybrid concept, which spends less energy on vertical taking off and landing and also on horizontal flight, thus maximizing flight endurance and travelled distance. We performed several experimental tests starting with the aircraft on the ground in vertical positioning. These tests include executing vertical takeoffs and landing, transitions from vertical to horizontal flight modes and transitions back (from horizontal to vertical), and hovering, which were carried out successfully. Transitions fourth and back from combustion to multirotor are inherent to some of those flight mode transitions, which have also been performed smoothly. We also performed tests (in bench) to estimate the maximum flight duration, which has demonstrated about 32 minutes of endurance. To this end, the proposed and currently built double-hybrid prototype has proven to be functional as an effective hybrid UAV system.

Robust flight mode transition control for tail-sitter formation flying with communication delays

In this article, the formation control problem is investigated for a team of unmanned tail-sitters subject to communication delays. A robust distributed and continuous formation control scheme is developed to achieve the desired aggressive time-varying formation flying in flight mode transitions between forward and vertical flights. The proposed control method consists of a translational controller and a rotational controller for each individual tail-sitter to govern the position and attitude motions, respectively. It is proven that the formation system stability can be guaranteed in the presence of communication delays and multiple uncertainties. Simulation results are presented for a team of tail-sitters to illustrate the advantages of the proposed formation flight control algorithm.

Robust Time-Varying Formation Control for Tail-Sitters in Flight Mode Transitions

This paper mainly addresses the formation control problem for a group of tail-sitters in transition flight between forward and vertical flight. A robust formation control method is proposed to achieve the aggressive time-varying formation subject to nonlinear dynamics and uncertainties. For each tail-sitter, the proposed control method results in a composite controller that includes a trajectory tracking controller and an attitude controller to achieve the translational and rotational motion control, respectively. It is proven that tracking errors of the proposed global closed-loop system can converge to a given neighborhood around the origin in a finite time. Finally, the simulation studies for multiple tail-sitters to accomplish the time-varying formation in transition flight are presented to show the effectiveness of the proposed control strategy.

A bio-inspired flight control strategy for a tail-sitter unmanned aerial vehicle

Wingbeat behavior and intermittent flight path are the two main characteristics of many birds. In this paper, to improve the efficiency of energy use and cruise range, a bio-inspired intermittent flight strategy with a whole flight envelope has applied to a tail-sitter aircraft. A total energy control system based transition control law has been proposed. The energy efficiency is investigated in terms of energy consumption per unit distance of different cruising modes, and the effectiveness and stability of proposed flight mode transition control law are verified by simulation. The mean mechanical power in flap-gliding flight is reduced compared with steady flight.

Adaptive attitude controller design for tail-sitter unmanned aerial vehicles

Tail-sitter unmanned aerial vehicles have two flight modes: they can fly long distances at high cruising speeds as fixed-wing aircrafts; or hover, take off, and land vertically as rotary-wing aircrafts. The tail-sitter dynamics involves serious nonlinearities and high uncertainties, especially in the two flight mode transitions. In this article, an adaptive control approach is proposed for a class of tail-sitter unmanned aerial vehicles to achieve the robustness properties. The control torque allocation problem is addressed based on the dynamic pressure in the transition flight. The proposed control method does not need to switch the coordinate system, the controller structure, or the controller parameters in different flight modes. It is proven that the attitude tracking errors can converge into a given neighborhood of the origin in finite time. Simulation results are presented to show the advantages of the proposed adaptive control method.

Using Radar Signatures to Classify Bird Flight Modes Between Flapping and Gliding

In this work, we find that radar signatures registered by wingbeats can work as a chronophotograph in radar version to record bird flight modes. When a bird flies, its wings and body compose a corner reflector, and the incident radiation upon either face of this wingbeat corner reflector impinges onto the other face and is reflected toward the illuminator, which enhances the bird signal intensity by 0–10 dB. When a bird flaps its wings, the corner works at a right angle, and the wingbeat corner reflector strongly modulates the radar signal; therefore, the contribution is strong and stably approaches 10 dB. During gliding, the wingbeat corner reflector fades away, the modulation effect significantly decreases, and the contribution approaches 0 dB. This difference can assist in classifying bird flight modes between flapping and gliding over a long sampling period. Both the Ku-band data from a duck in an anechoic chamber and the Ku-band radar data from a pigeon in an outside environment support the ability of this signature to classify gliding and flapping modes. We present flight mode transitions between flapping and gliding extracted from the radar echoes of small and large oncoming and outgoing birds.

Predictor-based model reference adaptive roll and yaw control of a quad-tiltrotor UAV

An attempt is made to apply modern control technology to the roll and yaw control of a rudderless quad-tiltrotor Unmanned Aerial Vehicle (UAV) in the latter part of the flight mode transition, where aerodynamic forces on the tiltrotor’s wings start to take effect. A predictor-based adaptive roll and yaw controller is designed to compensate for system uncertainties and parameter changes. A dynamics model of the tiltrotor is built. A Radial-Basis Function (RBF) neural network and offline adaptation method are used to reduce flight controller workload and cope with the nonlinearities in the controls. Simulations are conducted to verify the reference model response tracking and yaw-roll control decoupling ability of the adaptive controller, as well as the validity of the offline adaptation method. Flight tests are conducted to confirm the ability of the adaptive controller to track different roll and yaw reference model responses. The decoupling of roll and yaw controls is also tested in flight via coordinated turn maneuvers with different rotor tilt angles.

Robust Tracking Control for Tail-Sitters in Flight Mode Transitions

Tail-sitters can fly as rotary-wing or fixed-wing aircraft and the transition between the two flight modes is critical. This paper develops a robust nonlinear control method to achieve the trajectory tracking control in flight mode transitions. The coordinate system does not need to be switched neither the controller structures or parameters. Theoretical analysis and simulation studies are conducted to verify the effectiveness of the developed control approach.

Robust trajectory tracking control for unmanned tail‐sitters in aggressive flight mode transitions

SummaryIn this paper, the trajectory tracking control problem is investigated for a new typical tail‐sitter. A robust hierarchical control method is proposed to achieve aggressive flight mode transitions. The proposed control method results in a composite controller including a translational controller and a rotational controller to control the position and attitude respectively. Continuous aggressive flight mode transitions can be achieved without switching on the coordinate systems or the controller structures. It is proven that the tracking errors of the designed closed‐loop system can converge into a given neighborhood of the origin in a finite time. Simulation results show the effectiveness of the proposed control strategy.

Robust Fault-Tolerant Formation Control for Tail-Sitters in Aggressive Flight Mode Transitions

In this paper, the fault-tolerant time-varying formation control problem for a group of tail-sitters with multiple actuator faults and uncertainties is studied. A robust distributed fault-tolerant formation control strategy is developed to achieve aggressive time-varying formation flying in flight mode transitions. For each tail-sitter, the designed controller can be divided into an inner attitude controller and an outer position controller to govern the rotational and translational motions, respectively. The information of the actuator faults does not need to be identified online and the tracking errors of the global closed-loop control system can converge into a given neighborhood of the origin in a finite time. Simulation results are presented to show the effectiveness of the proposed control strategy.

Robust attitude control for tail‐sitter unmanned aerial vehicles in flight mode transitions

SummaryTail‐sitter unmanned aerial vehicles (UAVs) can flight as rotorcrafts as well as fixed‐wing aircrafts, but it is hard to control the flight mode transition. The vehicle dynamics involves serious parametric uncertainties, highly nonlinear dynamics, and is easy to be affected by external disturbances, especially during the mode transition. This paper presents a robust control method for a kind of tail‐sitter UAVs to achieve the flight mode transition. The robust controller is proposed based on the state‐feedback control scheme and the robust compensation method. The proposed control method does not need to switch the coordinate system, the controller structure, or the controller parameters during the mode transitions. Theoretical analysis is given to guarantee the robustness stability of the designed flight control system. Numerical simulation results are presented to show the advantages of the proposed control method compared with the state‐feedback control method and the sliding mode control approach.

Nonlinear robust control of tail-sitter aircrafts in flight mode transitions

In this paper, a nonlinear robust controller is proposed to deal with the flight mode transition control problem of tail-sitter aircrafts. During the mode transitions, the control problem is challenging due to the high nonlinearities and strong couplings. The tail-sitter aircraft model can be considered as a nominal part with uncertainties including nonlinear terms, parametric uncertainties, and external disturbances. The proposed controller consists of a nominal H∞ controller and a nonlinear disturbance observer. The nominal H∞ controller based on the nominal model is designed to achieve the desired trajectory tracking performance. The uncertainties are regarded as equivalent disturbances to restrain their influences by the nonlinear disturbance observer. Theoretical analysis and simulation results are given to show advantages of the proposed control method, compared with the standard H∞ control approach.