Nonlinear Dynamic Inversion (NDI) control and its Incremental variant (INDI) provide a conceptually simple and modular control framework, making it an attractive technique for designing flight control laws. By coupling these control architectures with robust control synthesis procedures, the overall approach can systematically ensure compliance with certification-level robustness requirements. In this sense, the H _∞ Loop-Shaping Design Procedure (LSDP) is a strong contender as a robust control synthesis approach, as it provides controllers with a priori robust stability guarantees. Therefore, in this study, structured H _∞ synthesis based on the H _∞ LSDP is used to systematize the development of (I)NDI control laws. This has been made possible by the advent of non-smooth non-convex multi-objective H _∞ optimization with MATLAB ^® systune. Despite the inherently nonlinear nature of (I)NDI-based control laws, local stability and robustness can be assessed using established trim-and-linearize techniques, allowing LTI methodologies to address design trade-offs in alignment with well established practices. Consequently, a linear hybrid Incremental Dynamic Inversion (IDI) control architecture is proposed, combining linear model-based DI with sensor-based IDI to leverage their complementary robustness properties. Model-following requirements are included using a weighting filter, whose parameters are optimized together with the hybrid IDI controller via a co-design approach. The potential of the proposed methodology is assessed in a design case study focused on a digital pitch-rate controller for a simulation model of NASA’s X-29 experimental aircraft. Results demonstrate that the synthesis procedure allows to optimize hybrid IDI controllers with the robustness guarantees associated with the H _∞ Loop-Shaping setup while simultaneously allowing to meet performance requirements.

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.

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.

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.

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.