COAST - A Simulation and Control Framework to Support Multidisciplinary Optimization and Aircraft Design with CPACS

This paper describes COAST (CPACS-oriented aircraft simulation tool), a fixed-wing aircraft simulation framework tailored to an XML-based open source common exchange format for multidisciplinary aircraft design, called CPACS (Common Parametric Aircraft Configuration Schema). COAST enables designers to utilize flight simulation on desktop computers or even on full motion simulators in early stages of aircraft design, which facilitates the assessment of flight characteristics and handling qualities as well as early flight control design. The core model of COAST is presented along with its interface to the data exchange format CPACS, which is implemented via import functions based on XML parsers. Due to the necessity of generically working autopilot functionalities, a nonlinear flight control concept based on the idea of the nonlinear model following control (NMFC) methodology is proposed. In order to handle novel aircraft configurations with a redundant set of control effectors, an optimization-based control allocation module is integrated. The process of integrating the model into the German Aerospace Center's full motion flight simulator AVES (Air Vehicle Simulator) in Braunschweig is discussed

Parameter Analysis of a Doppler Lidar Sensor For Gust Detection and Load Alleviation

This paper analyzes the performance of Doppler lidar sensors for gust detection and load alleviation purposes w.r.t. selected parameters of the lidar system and the wind reconstruction algorithm. The presented sensitivity studies are the first part of a larger investigation focused on identifying ideal lidar sensor configurations for gust load alleviation. This first study consists of two parts: in the first part, the effect of the measurement geometry is investigated by varying the scan angle, the number of measurements per laser pulse, and the rotational speed of the sensor's line of sight. In the second part, the effect of wind reconstruction parameters is investigated by varying the power aperture product of the lidar and the smoothing parameters of the wind reconstruction process

Aerodynamic Design of a Reusable Booster Stage Flight Experiment

The DLR Project ReFEx is a flight experiment to gain experience for future HTVL reusable rocket stages. The focus of this paper lies on the aerodynamic properties and difficulties during the reentry flight. The stable flight envelope, regions with little and without aerodynamic stability and the analysis of various forms of stable and unstable motion are covered. The test flight is scheduled for 2022

Flight Control Challenges of the Winged Reusable Launch Vehicle ReFEx

The German Aerospace Center (DLR) is currently studying different technologies for Reusable Launch Vehicles (RLVs) in order to evaluate and compare their benefits. The project CALLISTO (Cooperative Action Leading to Launcher Innovation in Stage Toss-back Operations) investigates a VTVL (Vertical Takeoff, Vertical Landing) concept. In the DLR project ReFEx (Reusability Flight Experiment), in the context of which this paper is presented, a winged VTHL (Vertical Takeoff, Horizontal Landing) concept is investigated in order to develop the key technologies for future winged RLV applications, culminating in a flight experiment to demonstrate the capability of controlled autonomous return flight from supersonic to subsonic speeds. The ReFEx re-entry vehicle has a length of 2.7 m, approximately 1.1 m wingspan and its mass is expected to be around 400 kg. In order to perform controlled autonomous re-entry, the Guidance subsystem - as part of the Guidance, Navigation and Control (GNC) system - generates and adapts a target trajectory during the mission, depending on the current position and energy state of the vehicle. Using the commands provided by the Guidance and the navigation information generated by a Hybrid Navigation System (HNS), the Flight Control System (FCS) stabilizes the vehicle and ensures that it follows the target trajectory as closely as possible, even in the presence of disturbances such as wind gusts. In this paper, first the challenging stability and control characteristics of the vehicle are examined, both at supersonic and subsonic speeds, which pose certain requirements on the FCS and the flight envelope of the vehicle. Second, a flight control architecture is proposed, and its suitability to the considered problem is shown via stability analyses and time-domain simulations

Stability analysis and flight control design of the winged reusable launch vehicle ReFEx

The German Aerospace Center (DLR) is currently studying different technologies for reusable launch vehicles (RLVs) to evaluate and compare their benefits. The project CALLISTO (Cooperative Action Leading to Launcher Innovation in Stage Toss-back Operations) investigates a VTVL (vertical takeoff, vertical landing) concept. In the DLR project ReFEx (reusability flight experiment), in the context of which this paper is presented, a winged VTHL (vertical takeoff, horizontal landing) concept is investigated to develop the key technologies for future winged RLV applications, culminating in a flight experiment to demonstrate the capability of controlled autonomous return flight from supersonic to subsonic speeds. In this paper, analysis of stability and controllability is used on a three-dimensional envelope of points to derive a suitable flight corridor for the re-entry. Second, a controller concept based on inversion of the rotational equations of motion is derived. The validity of the presented controller concept is shown on a preliminary level via comparison of open-loop and closed-loop dynamics at two representative flight points and a time simulation which includes a segment of the planned mission

Frequency-domain performance characterization of lidar-based gust detection systems for load alleviation

The application of Doppler-lidar wind sensors for feedforward gust load alleviation has been an emerging research topic in the recent years. It offers the potential to reduce structural loads on the aircraft, thereby improving safety of operations, passenger comfort, and allowing for lighter aircraft design. This paper discusses a frequency domain metric for evaluation and characterization of lidar sensors to be used to gust load alleviation. The presented method can also be used to derive a transfer function of a lidar sensor

Method to Account for Estimator-Induced Previewed Information Losses - Application to Synthesis of Lidar-Based Gust Load Alleviation Functions

Preview control using wind estimates derived from Doppler wind lidar measurements is a promising technique for designing active gust load alleviation functions. Due to high noise levels in the lidar measurements, the associated estimator must use some type of smoothing to obtain a reasonable estimate, which entails a loss of some information. Taking this loss into account during control synthesis should help ease the tuning procedure and improve the performance and robustness of the resulting controller. This paper proposes a method to consistently design a linear filter which closely approximates the behavior of the wind estimator and which can be integrated into a linear robust control framework. Its characteristics are shown to closely match the real estimator over a range of types of turbulence using a standard set of system parameters, and in a control synthesis example, it demonstrates a significant improvement in load alleviation performance