Method for designing multi-input system identification signals using a compact time-frequency representation

A flight test campaign for system identification is a costly and time-consuming task. Models derived from wind tunnel experiments and CFD calculations must be validated and/or updated with flight data to match the real aircraft stability and control characteristics. Classical maneuvers for system identification are mostly one-surface-at-a-time inputs and need to be performed several times at each flight condition. Various methods for defining very rich multi-axis maneuvers, for instance based on multisine/sum of sines signals, already exist. A new design method based on the wavelet transform allowing the definition of multi-axis inputs in the time-frequency domain has been developed. The compact representation chosen allows the user to define fairly complex maneuvers with very few parameters. This method is demonstrated using simulated flight test data from a high-quality Airbus A320 dynamic model. System identification is then performed with this data, and the results show that aerodynamic parameters can still be accurately estimated from these fairly simple multi-axis maneuvers

Evaluation of the controllability of a remotely piloted high-altitude platform in atmospheric disturbances based on pilot-in-the-loop simulations

In the context of the project HAP, the German Aerospace Center (DLR) is currently developing a solar-powered high-altitude platform that is supposed to be stationed in the stratosphere for 30 days. The development process includes the design of the aircraft, its manufacturing and a flight test campaign. Furthermore, a high-altitude demonstration flight is planned. While the high-altitude flight will be performed using a flight control and management system, during take-off and landing and at the beginning of the low-altitude flight test campaign, the aircraft will be remotely piloted. The aircraft has a wing span of 27 m and operates at extremely low airspeeds, being in the magnitude of around 10 m/s equivalent airspeed, and is therefore profoundly susceptible to atmospheric disturbances. This is particularly critical at low altitudes, where the airspeed is lowest. Hence, both time and location for take-off, landing or low-altitude flight test campaigns need to be selected thoroughly to reduce the risk of a loss of aircraft. In this regard, the knowledge about the operational limits of the aircraft with respect to atmospheric conditions is crucial. The less these limits are known, the more conservative the decision about whether to perform a flight on a certain day or not tends to be. On the contrary, if these limits have been adequately investigated, the amount of days and locations that are assessed as suitable for performing a flight might increase. This paper deals with a pilot-in-the-loop simulation campaign that is conducted to assess the controllability of the high-altitude platform in atmospheric disturbances. Within this campaign, the pilots are requested to perform practical tasks like maintaining track or altitude, flying a teardrop turn or performing a landing while the aircraft is subject to different atmospheric disturbances including constant wind, wind shear, continuous turbulence, and discrete gusts of different magnitudes. This paper describes the desktop simulator used for the campaign, outlines the entity of investigated test points and presents the assessment method used to evaluate the criticality of the respective disturbances. Finally, a set of restrictions on the acceptable wind conditions for the high-altitude platform are found. The underlying limits comprise a constant wind speed of 3.0 m/s in any direction, except during landing, maximum wind shear of 0.5 m/s^2 and gusts with peak speeds of 1.5 to 2.0 m/s, depending on the direction

Impact of ATC speed instructions on fuel consumption and noise exposure: an assessment of real operations in Zurich

Approach operations at busy airports are louder and less fuel efficient than they technically could be. Carrying out a safe approach under fluctuating wind and weather conditions while following individual air traffic control (ATC) instructions imposes a significant workload on the flight crew, especially with the limited systems support and information availability on the flight deck today. The SESAR exploratory research project DYNCAT (Dynamic Configuration Adjustment in the TMA) aims to highlight the impact of current approach operations on the environment based on all relevant data sources (on-board operational data, ATC commands, noise measurement data, surrounding traffic and weather information) allowed to evaluate individual approach operations in their full context, exemplarily for the Airbus A320 at Zurich airport. The evaluations performed are a unique opportunity to analyse the impact of ATC instructions on fuel consumption and noise exposure during real operations. The results showed that speed instructions, especially when given early during the transition phase, lead to high fuel consumption, due to the long-time flight in low speed levels correlated to earlier usage of Flaps configurations. A mean difference of almost 50 kg in fuel burned is observed when comparing the flights with speed restrictions to those without. On the other hand, speed restrictions provide a well-defined airspeed guidance for the pilots during transition and final approach, which leads to lower usage of speed brakes and also to lower speed levels for the landing gear extension. The less speed restrictions were given to the pilots by ATC, the higher was the usage of speed brakes during the final approach, and also the landing gear deployment was performed at higher speed levels, which ceteris paribus contributes significantly to the noise exposure and noise footprint. Based on these evaluations, requirements for a novel flight management system (FMS) function, which is set to be developed in the next steps of the project, were derived to support pilots and controllers through extended information exchange, thus increasing predictability of the lateral and vertical flight profiles for both sides. A central component is a novel airborne energy management assistance system including a configuration management functionality, to be implemented through an extension of the FMS capabilities

Generation of a Dataset for System Identification with VIRTTAC-Castor

This paper presents a new workbench for VIRTTAC-Castor, called GenerateDatasetForSysID, which enables users to generate a dataset that is suited for system identification purposes. This paper describes the motivations for this workbench and the main capabilities provided. The dataset provided by simply running this workbench with no modification is already well suited for identifying a flight dynamics model of VIRTTAC-Castor. However, the authors consider that system identification is not reduced to the application of parameter estimation algorithms to an exisiting set of experimental data. System identification specialists should be able to define the test points needed to model their system (which in the case of aircraft usually consist of maneuvers, flight points, flight conditions) and therefore the authors encourage the users of this workbench to customize the pre-defined test points or at least to question whether the pre-defined ones are suited for their needs. The aim of this paper is to inform the community of the release of this new workbench and presents its capabilities and foreseen uses

Multi-Axis Maneuver Simulation for System Identification of Flexible Transport Aircraft by High-Fidelity Methods

The development, testing and production of new aircraft are associated with considerable temporal and financial risks due to the product and manufacturing complexity. In order to accelerate the introduction of innovative technologies for more economical, more environmentally friendly and safer air transport vehicles and to better control the technological risks involved, DLR's guiding concept The Virtual Product aims at virtualizing the design, development and manufacturing processes. The availability of high fidelity simulation tools with the capability to model multidisciplinary phenomena is a key factor in this context. High demands are put on the numerical methods and solvers as the tools must provide reliable results in a robust manner for the entire flight envelope where nonlinearities are prominent in most of the disciplines. In this paper we present an approach for the high-fidelity multi-axis maneuver simulation, the Virtual Flight Test, based on multidisciplinary analysis of the free-flying elastic aircraft with CFD in the time domain. The goal is to generate virtual flight test data of high accuracy to substitute time and cost consuming real flight test campaigns to identify the flight mechanic and aeroelastic characteristics of the aircraft. Wavelet-type control surface inputs with carefully selected frequency ranges and amplitudes are used to excite the aircraft. A medium-range jet transport serves as the test case for which the approaches as well as the results of two identification maneuvers are presented and discussed

A Process for Time Domain Identification of a Flexible Aircraft Model from CFD/CSM Simulations

The present work describes a process to estimate parameters of a complete flexible aircraft model from virtual flight test (VFT) data using the output error method (OEM) in time domain. The VFT data are generated with a multidisciplinary, numerical, high-fidelity tool which couples CFD aerodynamics, flight mechanics, and structural dynamics in space and time. The challenge to identify all relevant parameters was met by a dedicated iteration process accounting for different structural modes. One single model identification at a specific flight point was performed and shows the successful applicability of the process. The applicability of the process to other aircraft types and the expansion to other flight points is pointed out and also subject to further work

Evaluation of the Controllability of a High-Altitude Platform in Atmospheric Disturbances Based on Pilot-in-the-Loop Simulations

In the context of the project HAP, the German Aerospace Center (DLR) is currently developing a solar-powered high-altitude platform that is supposed to be stationed in the stratosphere for 30 days. The development process includes the design of the aircraft, its manufacturing and a flight test campaign. Furthermore, a high-altitude demonstration flight is planned. The aircraft operates at extremely low airspeeds, being in the magnitude of around 10 m/s equivalent airspeed, and is therefore profoundly susceptible to atmospheric disturbances. This is particularly critical at low altitudes, where the airspeed is lowest. Hence, both time and location for take-off, landing or low-altitude flight test campaigns need to be selected thoroughly in order to reduce the risk of a loss of aircraft. In this regard, the knowledge about the operational limits of the aircraft with respect to atmospheric conditions is crucial. The less these limits are known, the more conservative the decision about whether to perform a flight on a certain day or not tends to be. On the contrary, if these limits have been adequately investigated, the amount of days and locations that are assessed as suitable for performing a flight might increase. This paper deals with a pilot-in-the-loop simulation campaign that is conducted in order to assess the controllability of the high-altitude platform in atmospheric disturbances. Within this campaign, the pilots are requested to perform practical tasks like maintaining track or altitude, flying a teardrop turn or perform a landing while the aircraft is subject to different atmospheric disturbances including constant wind, wind shear, continuous turbulence, and discrete gusts of different magnitudes. This paper describes the desktop simulator used for the campaign, outlines the entity of investigated test points and presents the assessment method used to evaluate the criticality of the respective disturbances. Finally, a set of restrictions on the acceptable wind conditions for the high-altitude platform are found. The underlying limits comprise a constant wind speed of 3.0 m/s in any direction, except during landing, maximum wind shear of 0.5 m/s^2 and gusts with peak speeds of 1.5 m/s to 2.0 m/s, depending on the direction

Impact of ATC Speed Instructions on Fuel Consumption and Noise Exposure: An Assessment of Real Operations in Zurich

Approach operations at busy airports are louder and less fuel efficient than they technically could be. Carrying out a safe approach under fluctuating wind and weather conditions while following individual air traffic control (ATC) instructions imposes a significant workload on the flight crew, especially with the limited systems support and information availability on the flight deck today. The SESAR exploratory research project DYNCAT (Dynamic Configuration Adjustment in the TMA) aims to highlight the impact of current approach operations on the environment based on all relevant data sources (on-board operational data, ATC commands, noise measurement data, surrounding traffic and weather information) allowed to evaluate individual approach operations in their full context, exemplarily for the Airbus A320 at Zurich airport. The evaluations performed are a unique opportunity to analyse the impact of ATC instructions on fuel consumption and noise exposure during real operations. The results showed that speed instructions, especially when given early during the transition phase, lead to high fuel consumption, due to the long-time flight in low speed levels correlated to earlier usage of Flaps configurations. A mean difference of almost 50 kg in fuel burned is observed when comparing the flights with speed restrictions to those without. On the other hand, speed restrictions provide a well-defined airspeed guidance for the pilots during transition and final approach, which leads to lower usage of speed brakes and also to lower speed levels for the landing gear extension. The less speed restrictions were given to the pilots by ATC, the higher was the usage of speed brakes during the final approach, and also the landing gear deployment was performed at higher speed levels, which ceteris paribus contributes significantly to the noise exposure and noise footprint. Based on these evaluations, requirements for a novel flight management system (FMS) function, which is set to be developed in the next steps of the project, were derived to support pilots and controllers through extended information exchange, thus increasing predictability of the lateral and vertical flight profiles for both sides. A central component is a novel airborne energy management assistance system including a configuration management functionality, to be implemented through an extension of the FMS capabilities

VicToria HAP3: Virtual Flight Testing

A decisive part of the design and certification process of a transport aircraft is the identification of its flight mechanic stability and control characteristics. This task is typically complied with a time and cost consuming flight test campaign with carefully selected control surface inputs. Given the accuracy of state of the art multidisciplinary analysis methods including aerodynamics, flight dynamics, and structural dynamics, the substitution of the real flight test by numerical simulation is appropriate and promises time and cost savings. In order to obtain highly accurate simulation results only CFD aerodynamic methods solving the URANS equations seem adequate for the calculation of the aerodynamic forces. In this work we present an approach for the high-fidelity multidisciplinary maneuver simulation based on CFD. The overall goal of this effort is to generate virtual flight test data of high accuracy to be used in subsequent system identification processes which estimate – amongst others – the stability and control characteristics of the aircraft [1]. In order to earn trust in such highly sophisticated and complex simulation tools, a careful validation process is necessary. Unfortunately, flight test data of jet transport aircraft are rare, and corresponding simulation models (CAD, FEM) of high quality must be available for the numerical analysis. Within the DLR-project VicToria, a number of flight tests have been conducted with the DLR research aircraft ATRA, an Airbus A320 configuration. The tests comprise steady and unsteady maneuvers at various combinations of speeds, Mach numbers, altitudes and loadings, such as bank-to-bank, 1-3-2-1-1 inputs, doublets, and maneuvers with high normal acceleration (up to 2g). Selected maneuvers of this unique and valuable set of experimental flight test data are used for the validation of the multidisciplinary process chain. In the full paper, the approaches, the simulation models, as well as the results of two longitudinal and two lateral maneuvers are presented and discusse

Flight mechanical analysis of a solar-powered high-altitude platform

The German Aerospace Center (DLR) is currently developing an unmanned experimental solar-powered fixed-wing high-altitude platform designed to be stationed in the stratosphere for several days and to carry payload for earth observation missions. This paper deals with a flight mechanical analysis of the aircraft within the preliminary design phase. For this purpose, it briefly describes all disciplines involved in the preliminary design and gives an insight into their methods used. Subsequently, it presents an assessment of the aircraft in terms of stability and control characteristics. Doing so, it first deals with a dynamic stability investigation using a non-linear 6-degrees-of-freedom flight dynamic model with a simple quasi-stationary approach to account for flexibility, in which the aerodynamic derivatives are given for different airspeed-dependent flight shapes. The investigations show that the aircraft is naturally stable over the complete flight envelope. It does not have a typical short period mode. Instead, the corresponding mode involves altitude and airspeed changes to a large extent. At low airspeeds, the Dutch roll and spiral modes couple and form two non-classical modes. Second, it presents a control surface design evaluation process for the aircraft based on a flight mechanical requirement. This requirement addresses the necessary control authority to counteract the aircraft's responses due to gust encounters to not exceed afore-defined limits and to prevent the aircraft from entering a flight condition that it cannot be recovered from