This report provides a historical survey and assessment of the state of the art in the modeling and application of active control to aircraft encountering atmospheric disturbances in flight. Particular emphasis is placed on applications of active control technologies that enable weight reduction in aircraft by mitigating the effects of atmospheric disturbances. Based on what has been learned to date, recommendations are made for addressing gust alleviation on as the trend for more structurally efficient aircraft yields both lighter and more flexible aircraft. These lighter more flexible aircraft face two significant challenges reduced separation between rigid body and flexible modes, and increased sensitivity to gust encounters due to increased wing loading and improved lift to drag ratios. The primary audience of this paper is engineering professionals new to the area of gust load alleviation and interested in tackling the multifaceted challenges that lie ahead for lighter-weight aircraft.

https://ntrs.nasa.gov/api/citations/20120013450/downloads/20120013450.pdf

Survey of Applications of Active Control Technology for Gust Alleviation and New Challenges for Lighter-Weight Aircraft

Model Predictive Control for Gust Load Alleviation

A high-performance airborne UV Rayleigh lidar system was developed within the European project DELICAT. With its forward-pointing architecture, it aims at demonstrating a novel detection scheme for clear air turbulence (CAT) for an aeronautics safety application. Due to its occurrence in clear and clean air at high altitudes (aviation cruise flight level), this type of turbulence evades microwave radar techniques and in most cases coherent Doppler lidar techniques. The present lidar detection technique relies on air density fluctuation measurement and is thus independent of backscatter from hydrometeors and aerosol particles. The subtle air density fluctuations caused by the turbulent air flow demand exceptionally high stability of the setup and in particular of the detection system. This paper describes an airborne test system for the purpose of demonstrating this technology and turbulence detection method: a high-power UV Rayleigh lidar system is installed on a research aircraft in a forward-looking configuration for use in cruise flight altitudes. Flight test measurements demonstrate this unique lidar system being able to resolve air density fluctuations occurring in light-to-moderate CAT at 5 km or moderate CAT at 10 km distance. A scaling of the determined stability and noise characteristics shows that such performance is adequate for an application in commercial air transport.

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Airborne forward-pointing UV Rayleigh lidar for remote clear air turbulence detection: system design and performance.

This study develops an active control technology to reduce the incremental dynamic loads of a large four-engine transport airplane flying through a gust field The mathematical model of the proposed gust-alleviation system features composite structural motions (for example, rigid-body motions, elastic vibrations, and deflections of control surfaces) and unsteady aerodynamic forces induced by the structural motions and the gusts A clear outline of the procedure is first provided to determine the aeroservoelastic equation of the system Then, an adaptive feedforward controller that uses the preview information of the gust sensed by an onboard alpha probe is designed to operate the ailerons symmetrically to alleviate the wing-root bending moment induced by the gust The rigid-body motions due to travelling gusts are also compensated for using symmetrical deflections of the elevators To solve the problems of weight drift and weight bias that are commonly encountered in adaptive control, the circular leaky l

Gust Load Alleviation on a Large Transport Airplane

This paper presents a combined feedback and feedforward active load alleviation system and its associated design and tuning methodology The feedback part is strongly structured and its robust performance across the flight envelope is ensured by the use of a multi-model and multi-objective controller design approach The feedforward function is based on a Doppler LIDAR sensor and the processing of the measurements as well as their physical interpretation combines various ideas from the system identification, the signal processing, and the control design domains The proposed solution remains very easy to use and tune, thanks to a limited number of parameters that can easily be interpreted physically and that exhibit only very little and very predictable couplings The performance and behavior of the active load alleviation functions is shown extensively based on a representative flexible long range aircraft model (based on the Airbus XRF1 configuration)

/pdf/combined-feedback-and-lidar-based-feedforward-active-load-3uz7ffib43.pdf

Combined Feedback and LIDAR-Based Feedforward Active Load Alleviation

Design, development, installation, and flight test of a novel airborne forward-looking lidar sensor are presented, aiming at accurately and remotely measuring the airspeed in front of the aircraft for automatic control of turbulence in-flight phenomena. The development is driven by the stringent requirements of the capability of measuring airspeed at cruise altitude, in clear air, and at all weather environments with the required precision. The sensor was installed in an A340-300 testbed for flight testing, inside a dedicated fairing in a forward-looking configuration. The flight-test measurements proved that the direct-detection Rayleigh/Mie lidar is capable of detecting the wind speed under flight-test conditions up to 39,000 ft in clear air, rain, dense clouds, and ice rain. A measurement range of 50 m is achieved for the ultraviolet lidar at a 60 Hz update rate and at a standard deviation of line-of-sight speed as low as 1.0―1.5 m/s. Those parameters, as required for forward-looking automatic control sensors, are a unique basis for the usage of such a remote sensor for automatic turbulence control.

Guillermo Jenaro Rabadan

Papers

Airborne Lidar for Automatic Feedforward Control of Turbulent In-Flight Phenomena