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.

The Flying V concept aircraft represents a notable candidate to reduce the carbon footprint of the aviation industry, with a potential 20% decrease in fuel use and a 17% higher lift-over-drag ratio. However, given the inherent design limitations concerning low control authority and pitch break-up tendencies, a well-designed control system is crucial for the aircraft’s safe operation. Thus, this study proposes a systematic design and tuning of a digital longitudinal flight control system that explicitly addresses robustness specifications a-priori. The flight dynamics simulation modeling is first detailed, followed by the outline and discussion of the design specifications. A C ^∗longitudinal control law is designed using a signal-based H _∞framework. Results indicate effective disturbance and noise rejection, stability under parametric uncertainties, Level 1 handling qualities, and satisfactory performance in the nonlinear model. These results validate the control law’s effectiveness, paving the way for future enhancements in gain-scheduled controllers for the Flying V.

Nonlinear Dynamic Inversion (NDI) has a long and successful history of research and development. The need for gain scheduling for nominal performance may be alleviated with the NDI method, which is accompanied by developmental benefits in terms of design modularity and transparency. However, the robustness of NDI-based control laws remains dependent on the nature of the open-loop plant. In this paper, a design and analysis framework based on quasi Linear Parameter-Varying (q-LPV) system theory is proposed that systematically considers this aspect across nonlinear operating regimes. The q-LPV model framework is presented in the context of robust hybrid incremental NDI control design, which incorporates inversion error compensation in addition to baseline model predictions. Based on a design case study for a simulated aeroservoelastic system, it is shown how systematic gain scheduling of the related inversion compensation design parameters can be performed with the proposed approach.

Flight control system design for the Flying-V has been an active research area. However, despite the strengths of H₈ control, this framework has not yet been considered for the system design. Therefore, this study details the synthesis of a longitudinal control law using the robust control signal-based H₈ framework. The trimming procedure used to obtain operating points and linearized flight dynamics is explained, followed by a description of the design requirements which are systematically converted into hard constraints for synthesis. A structured controller design is conducted and the resulting system is evaluated in terms of performance and robustness in linear and nonlinear settings. Results indicate effective disturbance and noise rejection, stability under parametric uncertainties, Level 1 handling qualities predictions, and adequate performance. The C^* control law effectiveness paves the way for future enhancements in gain-scheduled robust controllers for the Flying-V and for the extension to lateral-directional designs.

Incremental Nonlinear Dynamic Inversion (INDI) has received substantial interest in the recent years as a nonlinear flight control law design methodology that features inherent robustness against bare airframe aerodynamic variations. However, systematic studies into the robust design benefits of INDI-based control over the classical divide-and-conquer philosophy have been scarce. To bridge this gap, this paper compares the setup of hybrid INDI with a standard industry benchmark that is based on two-degree-of-freedom gain-scheduled proportional-integral-derivative control. This is done on an architectural basis and in terms of achievable robust stability and performance levels with respect to a common set of design requirements. To this end, a non-smooth, multi-objective H_∞-synthesis algorithm is used that incorporates mixed parametric and dynamic uncertainties in the design objective and constraints. It is shown that close similarities exist between hybrid INDI design and gain-scheduled PID control, which leads to virtually equivalent robustness and performance outcomes in both linear time-invariant and linear time-varying contexts. It is therefore concluded that the main benefit of the hybrid INDI does not lie in improved robustness properties per se, but in the opportunity to perform modular robust design in an implicit model-following context. Specifically, this implies that the areas of flying qualities, robustness, and nonlinear implementation are directly visible and accessible in the control law structure.

To reduce the impact of aviation on the environment, technological innovations, such as the Flying-V are required. The Flying-V is a proposed commercial flying wing, which uses the Airbus A350-900 as reference aircraft. In this work, a Flight Control system for the Flying-V is proposed with a longitudinal C ^* control law, and a Rate Control Attitude Hold roll control law. This Flight Control System also includes a Flight Envelope Protection law to prevent reaching angles of attack higher than 30 degrees, where the Flying-V becomes statically unstable. The FEP also prevents the Flying-V from reaching load factors above 2.5 and limits the roll angle. The control laws are tuned to be within level 1 handling qualities in the selected approach and cruise conditions, with the presence of sensor dynamics, and a digital control system. Robustness for aerodynamic uncertainties is also shown. Finally, it is shown that the FEP is able to prevent the angle of attack from becoming too large.

MegAWES is a reference design and simulation framework for ground-generation, fixed-wing airborne wind energy systems with a nominal power output of 3 MW. The winch size of MegAWES is based on a smaller system and needs to be scaled up because the current size leads to unrealistically fast dynamics, which require saturation. However, there is no available method to select an appropriate size for the winch. Additionally, while it has been hypothesized that the size of the winch has a significant effect on the dynamics of the overall system for ground-generation concepts, this effect has not been quantified. In this work, we first analyze the effects of the winch size on the system dynamics using a linearized model. Second, we present a method to find the upper bound for the size of the winch based on a selected maximum tether force overshoot during nominal operation. Third, we apply this method to find an upper bound for the winch size for the MegAWES reference design. Using the nonlinear MegAWES simulation framework, we validated this upper bound. At the upper bound, the system accurately tracked the reference tether force without overshoot and when exceeding our upper bound, the tether force response was oscillatory and overshot its ideal value.

Incremental nonlinear dynamic inversion (INDI) is a sensor-based control law design strategy that is based on the principles of feedback linearization. Contrary to its nonincremental counterpart (nonlinear dynamic inversion), this design method does not require a complete onboard model of the airframe dynamics and is therefore more robust against regular perturbations arising from aerodynamic variations. Therefore, INDI brings a natural design approach to desirable flying qualities. However, robustness against singular perturbations, which may arise due to transport delays, elastic airframe effects, or other types of badly modeled or unknown dynamics, is a known challenge for INDI-based control laws. Therefore, this paper addresses the question of robust stability and performance for INDI and its linear form (incremental dynamic inversion [IDI]) in the context of mixed regular and singular perturbations. This is done through analytical insights and by performing quantitative robustness assessments based on the structured singular value framework. Additionally, inversion loop augmentation solutions are investigated using robust synthesis techniques to improve the robustness characteristics of basic IDI designs.

The functional architecture of a flight control system (FCS) is driven by multiple objectives related to the aircraft’s operational mission and in-service performance targets. Control allocation (CA) is a common method to ensure adequate use of the available effector architecture once the number of control effectors linked to the FCS increases and the control problem becomes overdetermined. A primary CA design objective is to ensure that high-level motion control demands are met. However, the additional degrees of freedom offered by the control effector suite can also be exploited to perform secondary control tasks. In this light, this article focuses on the Incremental Control Allocation (INCA) framework in the context of in-flight optimization of arbitrary secondary flight control design objectives. An extension to the existing INCA concept is formulated that isolates this secondary control task from generating primary control demands. Moreover, an alternative, but closely related design method based on optimal control principles is proposed that extents the role of the control allocator to active control of the system dynamics. Design examples are demonstrated for least-squares minimization of total drag and control activity. These are analyzed in linear and nonlinear simulation scenarios based on an open-source General Dynamics F-16 simulation model.

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.