Direct Lift Control (DLC) is the capability to directly and intentionally influence lift on a fixed-wing aircraft by means of aerodynamic control devices, with minimum change of its angle of attack. Although several definitions exist, with various degrees of ambiguity, the combination of DLC and pitch attitude control has unambiguously proven to reduce pilot workload and improve flying comfort considerably. DLC has historically seen several applications on so-called inflight simulators and, recently, this capability has been rolled out over several aircraft types of the US Navy fleet, massively reducing pilot workload during carrier landings. On the civil front, only one aircraft type has been equipped with this capability, despite its very positive reception by flight crews and passengers. The intention of this paper is to revive interest in civil DLC applications, by reviewing in-depth its basic principles, characteristic features, benefits, and implementations so far. Several modern aircraft and disruptive wing configurations appear to be inherently capable of accommodating DLC functionality from a flight physical, systems, and software point of view. The proven benefits of DLC are likely to well outweigh the cost of the added functionality.

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Unforeseen failures during flight can lead to Loss of Control In-Flight, a significant cause of fatal aircraft accidents worldwide. Current offline synthesized flight control methods have limited capability to recover from failures, due to their limited adaptability. Incremental Approximate Dynamic Programming (iADP) control is a model-agnostic online adaptive control method, which integrates an online identified locally linearized incremental model, with a Reinforcement Learning (RL) based optimization technique to minimize an infinite horizon quadratic cost-to-go. A key challenge for adopting these self-learning flight control methods is validation through flight testing. This paper presents the iADP flight control law design for CS-25 class aircraft to achieve rate control. It outlines the controller evaluation strategy, controller integration, verification & validation procedures, and a discussion on flight test results. To the author’s understanding, this flight test marks the world’s first demonstration of an online RL based automatic flight control system for this aircraft category, demonstrating real-time learning and adaptation capabilities to aircraft configurations.@en

OnlineAdaptive Flight Control is interesting in the context of growing complexity of aircraft systems and their adaptability requirements to ensure safety. An Incremental Approximate Dynamic Programming (iADP) controller combines reinforcement learning methods, optimal control and online identified incremental model to achieve optimal adaptive control suitable for Nonlinear Time-Varying systems. The main contribution of this paper is twofold. Firstly, the iADP controller is designed to achieve automatic online rate control to track pilot commands via setpoints provided by the manual outer loop on Citation II Aircraft model. Secondly, to assess the controller performance in the presence of sensor dynamics and actuator dynamics, an analysis is carried out to identify causes of any performance degradation. The simulation results from iADP longitudinal control using full state feedback indicate that the discretization of sensor signals, sensor bias and transport delays did not have any significant effect on the controller performance or on the incremental model identification. However noisy signals and sensors delays are found to cause controller performance degradation. Appropriate filtering of signals resulted in better estimation of the incremental model subsequently improving the controller performance due to noisy signals. Control performance degradation due to sensor delays should be addressed in future before conducting flight tests on Citation II Aircraft.@en

This paper discusses the design, implementation and flight testing of an incremental Backstepping (IBS) based manual flight control law with angular accelerometer (AA) feedback. The main advantages of incremental control laws are that they only require a partial model of the system and are of low complexity. Incremental control laws for aircraft rotational motion, however, need angular acceleration measurements to compute the control increments. Previously, estimates based on angular rate measurements were used for this. The newly implemented AA feedback is expected to improve the performance of the controller by decreasing the sensor delay. The manual control laws command roll rate/angle, vertical load factor, and side slip angle and have been implemented on a Cessna Citation II aircraft, equipped with an experimental fly-by-wire system. The IBS based control law has an integrated integral control term and uses Pseudo Control Hedging to handle actuator saturations. The IBS based control law is shown to have highly satisfactory performance in flight. Test manoeuvres included standard roll and load factor commands and asymmetric thrust handling. Robustness to model mismatch has been compared in a nonlinear simulation for the controllers with and without AA feedback. In general, the AA feedback improved the tolerance to mismatch substantially.

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Traditionally, aircraft actuator control systems are commanded by means of control surface position or rate references from the flight control laws. This is natural for hydraulic actuators, but less so for electro-mechanical actuators (EMAs). EMAs apply forces or moments that are proportional to their current inputs. For this reason, it has been investigated if this principle can be used within an aircraft flight control system. This has several distinct advantages, including prevention of force fighting between parallel actuators, natural alleviation of structural and actuator loads in the face of atmospheric disturbances, and the possibility to restore reversible control behavior for irreversible systems. A complete design and a flight test campaign were performed using a Cessna Citation II with experimental fly-by-wire system. As a first step, current control loops were designed and tested for the electric aircraft servos. In a second step, an existing set of flight control laws based on incremental nonlinear dynamic inversion was adapted to command current instead of control surface angle commands. After implementation on the aircraft, the integrated system was intensively tested in flight. Direct comparisons with open-loop control were made. The tests were highly successful and very encouraging to further investigate this integrated approach for future applications.

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The Delft University laboratory aircraft Cessna Citation II has the capability to implement and test new auto pilot control laws. The aircraft is equipped as a Fly-By-Wire testbed, that enables the user to control the aircraft through an experimental computer and directly have control over the control surfaces. An extra control stick is mounted on the righthand side in the cockpit from where the aircraft can be controlled in a closed loop. The gains can be adjusted in-flight. An experimental display in the cockpit showed the setpoints for the rate and direct control in pitch and roll. The control laws that were tested are developed by the department of aircraft systems dynamics of the German Aerospace Center together with Delft University of Technology. The control laws are based on Incremental Nonlinear Dynamic Inversion (INDI). INDI uses synchronized measurements or estimations of (angular) accelerations and control surface deflections. This way it is not dependent on an airplane model, but it is able to automatically adapt flight control laws to changing dynamic behavior of the aircraft, even in case of major system failures or damage to the airframe. The INDI controller directly controls the current to the control surface actuators. This paper treats the execution of the test flights. @en

This paper presents the design and implementation of incremental backstepping (IBS) flight control laws for the attitude control and stabilization on a fixed-wing aircraft. The design consists of multiple functionalities such as command-filtered backstepping, angle of attack control and body attitude control, that are based around an incremental control inner loop that tracks the angular rates of the aircraft. The results include flight data of an integrated IBS design that validate simulation results of control laws shown previously in literature. The results show that it is possible to implement robust nonlinear flight control laws that are easy to tune and require only little knowledge about the system dynamics parameters.@en

This paper describes the design, implementation and flight testing of flight control laws based on Incremental Nonlinear Dynamic Inversion (INDI). The method compares com- manded and measured accelerations to compute increments on the current control de ec- tions. This results in highly robust control solutions with respect to model uncertainties as well as changes in aircraft dynamic characteristics of failure cases during flight. At the same time, the complexity of the algorithms is similar to classical ones. The key for practical implementation is in ensuring synchronization between angular acceleration and control de ection measurements or estimates. This paper describes the design and vali- dation of manual attitude control functions for a Cessna Citation II experimental aircraft, covering control structure design, application of INDI, design optimization, robustness analyses, software implementation, ground and flight testing. For comparison, also control laws based on classical Nonlinear Dynamic Inversion were implemented and flown. The flight tests were highly successful and marked the first successful demonstration of INDI on a CS-25 certified aircraft. The flight test results proved that INDI clearly outperforms NDI and provided valuable lessons-learnt for future applications.@en