The Parametric Aircraft Noise Analysis Module - status overview and recent applications

The German Aerospace Center (DLR) is investigating aircraft noise prediction and noise reduction capabilities. The Parametric Aircraft Noise Analysis Module (PANAM) is a fast prediction tool by the DLR Institute of Aerodynamics and Flow Technology to address overall aircraft noise. It was initially developed to (1) enable comparative design studies with respect to overall aircraft ground noise and to (2) indentify promising low-noise technologies at early aircraft design stages. A brief survey of available and established fast noise prediction codes is provided in order to rank and classify PANAM among existing tools. PANAM predicts aircraft noise generated during arbitrary 3D approach and take-off flight procedures. Noise generation of an operating aircraft is determined by its design, the relative observer position, configuration settings, and operating condition along the flight path. Feasible noise analysis requires a detailed simulation of all these dominating effects. Major aircraft noise components are simulated with individual models and interactions are neglected. Each component is simulated with a separate semi-empirical and parametric noise source model. These models capture major physical effects and correlations yet allow for fast and accurate noise prediction. Sound propagation and convection effects are applied to the emitting noise source in order to transfer static emission into aircraft ground noise impact with respect to the actual flight operating conditions. Recent developments and process interfaces are presented and prediction results are compared with experimental data recorded during DLR flyover noise campaigns with an Airbus A319 (2006), a VFW-614 (2009), and a Boeing B737-700 (2010). Overall, dominating airframe and engine noise sources are adequately modeled and overall aircraft ground noise levels can sufficiently be predicted. The paper concludes with a brief overview on current code applications towards selected noise reduction technologies

Tilt-Wing Control Design for a Unified Control Concept

Urban Air Mobility (UAM) promises an economic and ecological solution for the growing mobility demand by utilizing Electric Vertical Take-off and Landing Vehicles (eVTOLs). Tilt-wing eVTOLs (e.g., Airbus A3 Vahana) appear to be the most promising ones because they offer an efficient wing-borne cruise flight while reducing the need for ground-based infrastructure at the cost of a complex control task. Tilt-wing vehicles increase the pilot's workload and introduce possible human and technical failures due to mechanical complexity. A unified control concept shall be able to handle the vehicle in every phase and provides a single clean and intuitive interface. This work develops a controller capable of decoupling the physical couplings of the flight dynamics. An integrated six-degree-of-freedom rigid body model in a compact mathematical representation is proposed, and flight control requirements are identified. An Incremental Nonlinear Dynamic Inversion (INDI) controller is designed which fulfills the requirements. Moreover, multiple command filters and outer-loop controllers are designed to handle different control modes and provide a proof-of-concept for a unified control scheme. Finally, the closed-loop system is evaluated by means of the control requirements and a generic UAM mission. The closed-loop system masters all parts of the mission and fulfills these requirements. The developed dynamic model and control system will be valuable for future tilt-wing eVTOL research, especially subsequent works on unified control systems

Flight Testing Total Energy Control Autopilot Functionalities for High Altitude Aircraft

In this paper the design and flight testing of a Total Energy Control System (TECS) autopilot for a High Altitude Long Endurance (HALE) aircraft is presented. Autopilot control for HALE aircraft is a well-fitting application for the TECS control strategy, as this enables energy-efficient, decoupled airspeed and flight path control with explicitly handling thrust limitation. To achieve a realistic validation of the controller before moving towards the integration on the HALE platform, the flight testing is carried out on a Cessna Citation passenger aircraft. It has been proven that the adjustments required to implement the control laws on the Cessna Citation passenger aircraft are minimal. This indicates that the Cessna Citation aircraft serves excellently as a hardware platform and can be utilized for the validation of flight control code integration and functionality. The results of the flight test are discussed, and insights gleaned for the future integration of TECS on the HALE aircraft are provided

Dynamic Modeling and Analysis of Tilt-Wing Electric Vertical Take-Off and Landing Vehicles

Electric vertical take-off and landing (eVTOL) aircraft enable new transport options in regional and urban air mobility. One promising but only little investigated and understood subcategory comprises tilt-wing eVTOLs. They offer high efficiency and long flight ranges but come with a trade-off in increased complexity. Consequently, a critical step towards market entry is the development of mature and safe hybrid pilot-autonomy control systems, including fault detection, identification, and recovery (FDIR) concepts. That requires a mid-fidelity dynamic model with sufficient accuracy, which is not yet available despite a long history of tilt-wing research. Without a representative model, no detailed analysis and identification of a trimmed transition trajectory could be performed. This, however, is a crucial step in the development of a control system. We approach the problem by applying and combining current modeling approaches. Furthermore, a trim analysis of different flight phases, including the transition, is conducted. The identified model lays the foundation for a representative and detailed development and investigation of future control designs, bringing tilt-wing eVTOLs closer to airworthiness

Dynamic Inversion-Based Control Concept for Transformational Tilt-Wing eVTOLs

Transformational electric vertical take-off and landing (eVTOL) vehicles, including tiltwings, have (re)gained popularity over the past decade owing to their advantages of efficient wing-borne cruise flight and reduced requirements on ground-based infrastructure. They promise a new mode of transportation for fast and versatile, short-to-medium-haul on-demand connections. However, they come at the cost of complex mechanics, flight dynamics, and aerodynamics. Among these factors, the different flight regimes and the transition between them make the control system design challenging. Ideally, a flight control system provides means for pilot interaction, autoflight functions, robustness to disturbances, and failure mitigation. The different flight regimes with distinct flight dynamics in a single vehicle motivate a holistic approach. So far, no control approach has prevailed, which raises the question of how to design a control concept that satisfies the above requirements for the full flight envelope. To solve this problem, we derive a generalized representation for transformational eVTOLs and propose a flight control approach for this system, consisting of a dynamic-inversion-based angular rate and velocity control law. Moreover, combining these control functions with optimization-based control allocation is motivated and presented. Finally, the concept is applied to a tandem tilt-wing configuration and analyzed. Results suggest the practicability of the proposed control approach

Transition Strategies for Tilt-Wing Aircraft

Despite being the critical flight phase for tilt-wings, the transition maneuver is still not well understood with respect to aerodynamics and flight dynamics. Flight and handling qualities in this regime are primarily defined by the size and shape of the transition corridor, which describes physical limitations and other boundaries. Typically, the corridor is obtained based on static trim analysis, which results in a conservative estimate that neglects the dynamic maneuverability of the aircraft. As a first fundamental step for the determination of the full transition corridor, this paper presents a comprehensive dynamic analysis of the longitudinal tilt-wing transition, considering various operational strategies for the maneuver. The analysis is based on an optimal control approach where the dynamic control problem is transcribed into a nonlinear programming (NLP) problem. The developed framework successfully explores different regions of the transition corridor by adapting constraints and objective functions, and thereby lays the foundation for future determination of transition boundaries. The optimized trajectories show that the phenomena of transition folds found in static analyses are damped within dynamic investigations. Furthermore, the re-transition maneuver proves to be more challenging from a flight physics perspective because of high effective angles of attack, and requires upward motion to avoid flow separation

Decentralized Control of Ailerons for Gust Load Alleviation

This paper describes the design of a so-called decentralized gust load alleviation system for an aircraft with high aspect ratio wings. The system uses control loops at each individual aileron actuator that aim to reduce local structural dynamics at each respective point of installation in the wing structure. To this end, each actuator is equipped with a remote electronic unit that incorporates an inertial measurement unit and a processing unit that allows for implementation of a fast, local control loop that combines control surface positioning with local structural damping. At an aircraft level, this renders a decentralized control structure. Although the number of design degrees of freedom is lower than for a centralized system, over-all system complexity is considerably reduced. The presented design shows that remarkable improvement of structural damping and reduction of wing root bending moments can be achieved

Flight Testing Air Data Sensor Failure Handling with Hybrid Nonlinear Dynamic Inversion

Loss of one or more air data signals to the flight control laws typically results in activation of backup modes. These reduce functionality and work solely with inertial sensors. Since the failure situation is already a significant stress for the flight crew, this reduction of supporting control functionality comes at an awkward moment. This work shows how a combination of classical and so-called sensory nonlinear dynamic inversion can be used to maintain more or less the same handling characteristics in case of partial or even complete loss of air data. Under nominal conditions, hybrid NDI uses a complementary filter to combine NDI and sensory NDI. In case of failure, it is possible to degrade to a control law based purely on sensory NDI, which is predominantly inertial sensor dependent. This paper describes the application of the proposed modification to a CS-25 class aircraft, as well as the validation of its intended features in flight tests

Aeroservoelastic Modeling and Robust Control for Gust Load Alleviation of an Experimental Wing

Active gust load alleviation is a promising technique to reduce the structural weight of flexible transport aircraft wings with high aspect ratio, ensuring lower fuel consumption and environmental impact. The challenge is to develop a controller that is not only performant, but also robust, since the aircraft's environment cannot fully be captured within a computational model. Robust control is well suited to address this challenge by including uncertainties within the controller design. This paper describes using µ-synthesis to develop a gust load alleviation controller for a flexible wing, to be tested in a wind tunnel experiment. The wing features both sweep and high aspect ratio. Five trailing edge control surfaces are installed for active load alleviation. Distributed acceleration sensors provide measurements for feedback control. The paper details the entire controller development, starting with the creation of an aeroservoelastic plant, using the Loewner framework as a novel addition to model the gust disturbance. After controller synthesis, the designed controller is validated in a nonlinear simulation, constituting a virtual representation of the wind tunnel experiment