Robert Luckner

Modeling and Simulation of Wake Vortex Encounters: State-of-the-Art and Challenges

Wake vortices are one of the major constraints on separations between aircraft pairs, as wake vortex encounters (WVEs) can be harmful to other aircraft during any phase of flight. The vortex flow field disturbs the encountering aircraft. This affects pilot’s workload, comfort and safety of passengers and crewmembers and it has an impact on structural loads. Pilot workload refers to the effort for compensation of perturbations in aircraft’s attitude and flight path. The level of tolerable disturbances and acceptable aircraft response depends on the flight phase. Modeling and simulation of WVEs is an important prerequisite for investigations of this phenomenon in fast-time or in piloted real-time flight simulations. This paper describes how the effects of wake-vortex-induced velocities on aircraft flight dynamics can be modeled and how such models are applied. The model requirements for different applications are discussed e.g. for WVE simulations in real-time, piloted simulator tests or for flight simulator training (real-time models), for risk assessments with fast-time Monte Carlo simulations in the rule making process (fast-time models). An overview on the state-of-the-art of WVE modeling is given, i.e. quality, constraints and maturity of models are discussed and directions for future research are given.

Simulation of Aircraft Encounters with Perturbed Vortices Considering Unsteady Aerodynamic Effects

Today, wake-turbulence-related aircraft separations are a limiting factor for the capacity of airports worldwide. A detailed knowledge about the severity of potential wake encounters is necessary for a safe revision of the current International Civil Aviation Organization separation requirements. Vortex encounter simulation is an important part in building an understanding how the encountering aircraft is adversely affected by the vortex-induced disturbances. Whereas most research has focused on aircraft encounters with a pair of straight vortices, this work focuses on the simulation of encounters with perturbed vortices as they are observed under various atmospheric conditions. In addition, a method to account for unsteady aerodynamic effects during the computation of vortex-induced forces and moments is presented. Those effects are becoming especially important for vortex-induced disturbances of a higher-frequency range as they may appear during encounters with perturbed vortices. This combination of mo...

Experimental Validation of a Flight Simulation Model for Slightly Flexible Aircraft

The paper presents a methodology for modeling the coupled flight dynamics with aeroelasticity in the time domain. The equations of motion are based on linearized mean-axes formulation. Unsteady, incremental aerodynamics due to elastic deformations are modeled in the time domain, applying the strip theory and a indicial function. The methodology is demonstrated with a prototype of the utility aircraft Stemme S15. Comparisons between the simulation and the flight-test data regarding the added aeroelastic dynamics have indicated that the model is suited for the flight control law and aircraft design, and that care must be taken in modeling the control surface lift slope for low-aspect-ratio lifting surfaces.

Modeling Wake Vortex Roll-Up and Vortex-Induced Forces and Moments for Tight Formation Flight

Migratory birds that fly in tight formation use the upwash of the leading birds to save energy. Aircraft can use the same principle to save fuel. To date, commercial aircraft do not fly in tight formation because of unresolved technical and operational issues. However, increasing fuel cost and environmental constraints are drivers to investigate tight formation flight and the potential fuel savings. For such investigations a good knowledge of the vortexinduced velocities and the effect on the encountering aircraft is fundamental. In research on safe separation distances the vortex far field (more than 40 wingspans behind the generating aircraft) is modeled with a pair of straight vortices. This paper addresses a method to model the wake vortex roll-up that occurs in the near field (up to 15 wingspans behind the generating aircraft) using discrete vortex filaments. The motion of these filaments due to the mutual velocity induction is simulated numerically, followed by the calculation of the resulting induced velocities in a second step. To find an appropriate set of simulation parameters, the computed velocities are compared with those induced by a validated wake vortex velocity model. An already existing wake vortex encounter simulation software is improved to model the wake vortex roll-up. First simulation results are presented.

nxControl Instead of Pitch-and-power : A Concept for Enhanced Manual Flight Control

AbstractA command nsystem for manual control of the longitudinal load factor in flight path direction of an aircraft is designed that completes existing flight control command systems (e.g. with sidesticks that command normal load factor). The system is called nxControl. It aims to assist pilots during manual flight by reducing the workload for monitoring flight parameters as well as for controlling thrust and airbrakes. Important for the nxControl concept is the direct flight mechanical relation between longitudinal load factor and changes of the total aircraft energy. This paper presents the system concept and a prototype realization. The nxControl system consists of the control law that combines the actuation commands for engines and airbrakes, a new input device for the longitudinal load factor command and augmented display elements informing pilots about aircraft energy states to assure situation awareness. In order to investigate the feasibility of the concept as well as to evaluate consequences on human performance, a flight simulator study with airline pilots was conducted. nThe nxControl prototype was used by the pilots as expected. Changes in instrument scanning behaviour and thrust lever usage confirmed this. After just a short familiarization and practice, the pilots were able to perform standard flight tasks with nxControl without exceeding given tolerance limits. So, the results provide first evidence for the feasibility of the concept.

The Total Energy Control Concept for a Motor Glider

In this article the Total Energy Control System (TECS) that was introduced by Lambregts to control the vertical flight path and the velocity of an aircraft by using the total energy and the energy distribution between the flight path and the acceleration, will be taken up, modified, extended and tested on a motor glider. The TECS concept has been extended by using the airbrakes as additional control elements to manipulate the total energy. For motor gliders and utility aircraft with a high glide ratio this increases the sink performance and the range of possible missions, like steep approaches. Further modifications are done to improve the height accuracy during normal operation and during flare manoeuvre and to improve the control response reaching its saturations. A height protection is introduced to make a safe flight near to the ground possible. The usage and generation of required sensor signals from existing sensor data is introduced. Examples of flight test results are given.

Modeling and Design Considerations for Robust Control of Aircraft in Tight Cruise Formation

This paper addresses controller design for a follower aircraft (wingman) flying in tight cruise formation with a preceding aircraft (leader). The envisioned scenario is that of two similar commercial medium / large size freight or passenger aircraft in cruise flight. A VFW614 aircraft flying in the wake of another aircraft of the same type serves as example. A control design procedure for uncertain systems is used to obtain lateral and longitudinal controllers, yielding control loops with guaranteed robustness properties because wake vortex influence on the wingman is considered at modeling and design time in the uncertain system framework. In combination with appropriate references the resulting controllers are suitable for the three phases of formation flight: forming up, maintaining formation and leaving formation. Simulation results are presented.

Bridging the tool gap for model-based design from flight control function design in Simulink to software design in SCADE

Development processes of software for safety critical, complex aircraft systems, for example flight control systems, are very demanding. In the context of an aircraft certification, strict process guidelines and objectives from the Radio Technical Commission for Aeronautics according to the Development Assurance Level of the Software have to be complied to. Efficiency and compliance are important goals when defining processes from these requirements. Our approach to gain efficiency is, to reuse models from function design for code generation, and to use qualified tools. In control engineering, MATLAB, Simulink and Stateflow are widely used tools to build such models. The code generator Embedded Coder of the MathWorks tool chain is not available as qualifiable tool. Hence, it is common in the avionics domain to use SCADE Suite and its qualifiable code generator KCG for software design. To enable reuse of Simulink/Stateflow models for code generation with KCG it is necessary to translate them to Scade. In the project CERTT-FBW231 a feasibility study of the automatic translation from Simulink/Stateflow to Scade using the SCADE Suite Gateway for Simulink was performed. An existing control law model of an automatic flight control system was used as example. Due to its size and functionality it is a demanding and suitable example. To enable the translation of Simulink/Stateflow models, modeling guidelines were derived in the project MCAS2. We motivate our approach and show how it can be used effectively by automatic guideline checking and model repair. Our tool chain, the translation and configuration management processes are presented after briefly introducing the example model. A selection of guidelines and repair algorithms are shown, that helped to increase the translatability of our example.

A utility aircraft for remote sensing missions with a high-precision automatic flight control system

Light civil utility aircraft are well suited for remote sensing missions and to serve as an airborne platform for efficient reconnaissance, surveillance, exploration, and measurement tasks in a growing market. Missions where the pilot has to operate a payload require an automatic flight control system (AFCS) to support the pilot. In missions that are extremely difficult, long or dangerous, the AFCS may even replace the pilot. Such an AFCS has to follow precisely predefined trajectories and should compensate disturbances due to atmospheric turbulence. For un-manned aircraft operations it has to have full authority, it has to be highly reliable, and it must be able to take off and land automatically. The development and certification of such an AFCS at competitive cost is a major challenge. This paper gives an overview on the STEMME S15 utility aircraft, for which an AFCS has been developed. It describes the design objectives, the selected flight control architecture, the system, and the flight test equipment as well as flight test results that demonstrate the high precision of flight path control.

Simulation Methods for Aircraft Encounters with Deformed Wake Vortices

Simulation of aircraft wake-vortex encounters is regularly applied in the research toward revised aircraft separation minima. Most encounter flight simulation studies have used a pair of straight counter-rotating vortices. However, after being shed by the generating aircraft, the wake vortices begin to develop a growing deformation due to the long-wave Crow instability, especially under low-turbulent atmospheric conditions when vortices decay slowly. In this study, two methods for simulation of aircraft encounters with such perturbed wake vortices are described and compared. The first method uses a vortex simulation model, which is based on analytical models of the deformation that consider the large-scale vortex shapes. The second method applies vortex-velocity fields from high-fidelity large-eddy simulations. It offers the highest possible level of realism and is used as the reference. The resulting aircraft responses induced by wavy vortices and vortex rings are reproduced with good quality by the vort...