Fig 2: In-flight simulation principle of ATTASIn-Flight Simulation
ATTAS was designed to serve as an in-flight simulator in order to present unconstrained motion and visual cues for the pilot under real flight tasks. In-flight simulation technique allows to fly a 'virtual aircraft' in the real world. This unique feature, i.e., configuring the dynamic characteristics by an on-board programmed model aircraft which can be evaluated by the pilot under real flight conditions, provides high quality results. Especially flying qualities, flight control law research, and database establishment for handling qualities guideline development depend heavily on realistic experiments and high fidelity data.In-flight simulation is based on specific aircraft control and functional system capabilities such as a fly-by-wire flight control system with configurable control functions and actuation of all control surfaces via the on-board computer system. The overall principle to provide the pilot with realistic motion and visual cues is shown in Fig. 2. It is seen that the pilot commands are fed to a computed aircraft model, the 'virtual aircraft'. The host aircraft ATTAS is forced by a model following controller to follow exactly the model response by deflecting all control surfaces in a proper way. The specially developed direct lift control system gives ATTAS a 5-degrees-of-freedom simulation capability.
Fig 3: Aircraft modificationsAircraft Modifications
An overview of the technical modifications incorporated in ATTAS is illustrated in Fig. 3. Besides the conventional elevator, rudder, and aileron control surfaces, the engines, six direct lift control flaps, landing flaps, and stabilizer are under fly-by-wire (FBW) control.
Fig 5: Model Following Control SystemModel Following Control System
The key element for in-flight simulation is the model following control system, Fig. 5. In ATTAS that is a software package in which both, an explicit model (the virtual aircraft) and a controller comprising a feedforward path with the inverse dynamics of ATTAS and a feedback path, is calculated under real time to provide precise model following response. The so-called nonlinear in-flight simulation package allows in-flight simulation over a wide range of the flight envelope.
Fig 6: Hermes spaceplane in-flight simulationSimulation Fidelity
Dynamic simulation fidelity is primarily limited by control power and actuator dynamics. Therefore, high bandwidth electro-hydraulic actuators were specifically developed for ATTAS to get adequate simulation fidelity in a frequency range typical for transport aircraft. Specific simulation criteria in time and frequency domain were developed. The range of in-flight simulation capability is exemplified in Fig. 6 by comparing model and ATTAS response for a Hermes spaceplane model. An excellent simulation fidelity was obtained. Up to 100 virtual aircraft models can be programmed and selected during flight tests ('dial-a-model').
Since 1986, ATTAS was involved in a large number of research programs with different objectives. The applications comprise typical in-flight simulation experiments for handling qualities investigations, flight control law investigations, autopilot functions, air traffic management functions, aerodynamic airfoil investigations, avionic system demonstrations, a new cockpit display technologies demonstration, and finally FBW-flight control research applications. The usage of ATTAS with roughly 100 to 150 flight test hours per year has accumulated to about 1600 h.
Fig 7: Computer system installationSimulation Capability Enhancement
To fulfil user requirements for future experiments, the ATTAS simulation capability will be improved. This program comprises three main items: first, the flight envelope expansion for final approach and landing in the fly-by-wire/light flight control mode, second, the performance improvement of the user dedicated on-board computer system, and, third, increased efficiency in integrating user defined software functions and hardware.At present, ATTAS fly-by-wire/light operations in the simulation mode are limited to 500 ft minimum flight altitude due to flight safety reasons. In order to ensure safe operation below that altitude limit, the authority of all electro-hydraulic actuators is reduced such that the safety pilot is able to recover the airplane even in critical failure events. The authority limiting functions can be switched on and off. In addition, the disconnecting system reliability was improved and the fly-by-wire/light system was expanded to a full dual redundant system with failure detection and system passivation capabilities, Fig. 7. To improve the experimental computer performance and to speed up the experiment integration process, a new freely programmable on-board experimental computer system, named EXEC, was designed. It is based on a high performance VME-Bus computer providing the necessary computing power for future applications as well as high speed data interfacing to connect user supplied computer systems for real-time applications.
The experiment development process itself was improved by a new approach using Matlab/ Simulink tools to design the experimental functions while target code generation for the on-board system EXEC is fully automated. This approach reduces development and validation effort and ensures that the program will run on ATTAS during flight tests as intended.
The system is in the certification process and the demonstration flights are scheduled to take place in the first half of 1999. The dual redundant flight control system is under operation since the beginning of 1998 while the new experiment computer system EXEC combined with the improved implementation process of user functions will become operational in the second half of 1999.
Flight Test Operation
ATTAS flight test operation is conducted by a joint team of personnel from the Institute of Flight Systems, the Institute of Guidance and Control, and the Flight Operation Department. DLR is authorized by the German aviation certification authority (LBA) to modify the aircraft and to install test equipment. The scientific experimenter is assisted in software implementation, flight testing, data acquisition, and data preprocessing. Before the flight test, experiment and experimenter's software must be verified in the ATTAS ground based simulator which provides a full representation of all ATTAS functions and data using to a certain extent aircraft-identical hardware (hardware-in-the-loop simulation). This approach gives full potential in preparing flight tests and provides the basis for efficient flight test conduction.