Matthias Heller

Lateral-Directional Coupling and Unsteady Aerodynamic Effects of Hypersonic Vehicles

Results from a joint research program of the Institutes of Fluid Dynamics and of Flight Mechanics and Flight Control of the Technical University of Munich on a two-stage space transportation system are presented. The unsteady aerodynamics resulting from an orbital stage at stage separation e ow conditions at Mach number 6.8 are e rst discussed. Unsteady e owe elds associated with yaw and roll oscillations of the orbital stage at a certain distanceabovethecarrierstageareinvestigated.Thecalculationsarebased ona e nitevolumemethodforreal-time solutions of the unsteady Euler equations. Theresultsfocus on pressuredistributions and aerodynamic coefe cients providing an essentialmeansforcarrying outstabilityand control investigations.Inthesecond part, e ightmechanics stability and control problems are addressed considering key issues of lateral ‐directional dynamics. Inherent vehicle characteristics can show specie c stability and e ying quality dee ciencies in hypersonic e ight, concerning a partially unstable dutch roll mode with high roll ‐yaw coupling and weak roll damping. There is also a coupling of the roll and spiral poles for certain cone gurations, yielding a slow, eventually unstable oscillation called lateral phugoid. Reasons and effects for the stability dee ciencies are discussed, including conditions of existence for the lateral phugoid.

Configuration assessment and preliminary control law design for a novel diamond-shaped UAV

The development of a novel aircraft configuration is typically accompanied by high technical risks. In order to ensure the success of such a development, a risk reduction phase must be considered in the design cycle. The paper at hand gives insight into the activities related to flight dynamics and control that have been performed during the risk reduction phase of a novel diamond-shaped UAV, the so-called SAGITTA Demonstrator. In particular, the main design drivers for the configurations Flight Control System are provided and details on the inherent lateral dynamics and worst case stabilization capability are presented. Furthermore, a preliminary controller structure that is suitable to meet the requirements and an appropriate gain design method are proposed. The resulting preliminary closed-loop system is analyzed in frequency domain (e.g. by means of Nichols charts) and time domain (i.e. gust response). By the activities, summarized as Controllability Study, main requirements are validated and significant development risk reduction is achieved.

Development of control laws for the simulation of a new transport aircraft

Abstract This article presents the development of fly-by-wire control laws used for the high-fidelity simulation of a new transport category aircraft. Beyond structural and gain design aspects for normal operations, envelope protections and limitations as well as mode transition issues are also addressed. As the control system is to be adjusted to changing aircraft datasets at different levels of fidelity, particular emphasis has been put on a high level of automation in gain design and system assessment routines. For lateral dynamics, eigenstructure assignment is used as the design methodology whereas pole placement is used for the pitch axis.

Development of a Lateral-Directional Flight Control System for a New Transport Aircraft

ठThe paper presents the development of a Flight Control System for the lateral dynamics of a transport aircraft to be used for simulation analys is. For a manufacturer-independent evaluation of the handling characteristics of a future transport aircraft, it is inevitable to replicate its flight control system as close as possible. While some data on the basic design characteristics of the flight control system of the real aircraft could be obtained, almo st no information was available concerning its internal implementation. Thus, own control laws have been implemented. As both the fidelity of the flight simulation model as well as the underlying d ata are changing quite rapidly, robust routines for automated gain and filter computation have been developed, accounting for handling qualities, stability and robustness requirements which can be easily stated in definition files. Much attention i s drawn to the high level of automation in gain desig n and the flexibility in specifying the characteristics of the closed-loop system as those features on the one hand allow the system to be use d during all stages of the project and on the other h and provide the capability to analyze the impact of different design requirements on the overall charac teristics of the system. For achieving appropriate dynamics characteristics, eigenstructure assignment is used. The system is augmented by limitations as well as compensations.

Nonlinear Non-cascaded Reference Model Architecture for Flight Control Design

A nonlinear reference model architecture motivated by dynamic inversion based flight control is introduced. As a novel feature, only one integrated reference model is used to provide reference commands, for longitudinal axis: the flight path angle, vertical load factor (or angle of attack), and pitch rate, while admitting flight path rate command as input; for lateral axis, bank angle and roll rate; for directional axis, lateral load factor and yaw rate. Flight dynamics, actuator dynamics with rate and position limits, and envelope protections can also be incorporated in a straight forward way in the reference model. One advantage of this non-cascaded reference model is that at least the attitude of the reference response can be restored and flown at an early stage of the flight control system design cycle. The second feature is that the reference model is parameterized, allowing the opportunity of updating the knowledge of aircraft dynamics (e.g. damaged) and flying qualities design. With these two aspects, the physical consistency in terms of the reference commands among different channels and reference commands reasonable with respect to true aircraft dynamics can be assured. Although designed for General Aviation aircraft, the framework can be generalized for other aircraft considering only rigid body dynamics.

Hybrid Control System for a Future Small Aircraft

A Fly-by-Wire like Flight Control System for small general aviation aircraft designed for excellent flying qualities and pilot assistance with particular emphasis on acceptable development and especially certification efforts (cost-benefit ratio) is considered. The manifold benefits of advanced Active Fly-by-Wire Flight Control Systems within modern transport aircraft featuring a wide range of functionalities and applications are undisputable. They represent an effective means of reducing pilots workload significantly, monitoring pilot’s inputs, providing warnings and protections and hence, increasing the passenger, crew and aircraft safety (carefree handling). However, the general aviation aircraft sector is still missing such Active Flight Control Systems because of their tremendous (development, system/hardware and certification) cost which can easily exceed the actual airframe cost of the small aircraft several times. Affordable Fly-by-Wire technology for small aircraft can be achieved by using a Hybrid Control System which combines additional (limited) electrical actuated control surface deflections commanded by the Flight Control Computer together with retaining a (full-authority) permanent mechanical direct link between pilot inceptor and the control surface via a mechanical mixing unit. This approach offers the opportunity of utilizing the benefits of state of the art Fly-by-Wire control technology simultaneously enhanced by the reliability of conventional control systems and thus, reducing certification effort and cost dramatically. The Hybrid Fly-by-Wire like control concept discussed is integral part of a joint Technology Research project concerning upcoming future small aircraft representing a cooperation of an Austrian aircraft manufacturer and Technische Universitat Munchen.

Enhancement of the Nonlinear OLOP-PIO-Criterion Regarding Phase-Compensated Rate Limiters

The unique dynamic coupling effects between airframe, flight control system, and the pilot of modern, highly or super-augmented aircraft introduce new stability and handling qualities problems which do not occur on conventional airplanes. In particular, phenomena leading to a destabilization of the closed-loop system consisting of the airframe, the control and stability augmentation system, and the pilot, triggered by an undesired and unexpected interaction between pilot and augmented aircraft dynamics are known to be very dangerous for the aircraft and crew. They are commonly referred to as Pilot Involved or Pilot-In-the-Loop Oscillations. This paper focuses on the enhancement of the so called Open-Loop-Onset-Point-criterion, originally developed to predict PIO-susceptibility due to nonlinear effects such as position and non-phase-compensated rate limiting. The criterion is modified to also cover phase-compensated rate limiters, which are now commonly found in modern flight control systems.

A New Flight Test Technique for Pilot Model Identification

For today’s highly augmented fighter aircraft, the aircraft dynamics are specifically tailored to provide Level 1 handling qualities over a wide regime of the service flight envelope. This requires a profound understanding of the human pilot to assure that stability margins of the airframe plus controller dynamics are sufficient to accommodate the additional pilot dynamics introduced into the system during closed loop tasks. Whereas the mathematical formulations of the airframe and controller dynamics are reasonably exact, the human pilot remains to be the most unpredictable element in the Pilot Vehicle System. In the past decades various pilot models have been developed in conjunction with analytical handling qualities and Pilot Involved Oscillations prediction criteria, mainly focusing on air-to-air tracking tasks. This paper focuses on the development of a novel flight test technique, which allows the identification of the pilot dynamics during air-to-surface aiming tasks. During an extensive flight test campaign, data was gathered and processed, using state of the art systemidentification techniques to derive a mathematical model of the human pilot during air-tosurface tracking tasks. Flight test and model-based data are compared with each other to support the validity of the developed pilot models.

Advanced Uncertainty Modeling and Robustness Analysis for the Basic Flight Control System of a Modern Jet Trainer

[Abstract] Due to the continuously increasing design requirements on modern aircraft the robustness properties of their flight control systems against multiple model uncertainty becomes a more and more important issue for flight control system certification and flight clearance, particulary in the case of an unstabel aerodynamic basic design. Most classical approches (like stability margins in the Nichols Chart) only provide incomplete information about the qualitative and quantitative degree of robustness. Therefore, an innovative approach for robustness analysis of the primary control laws of a modern jet trainer will be presented here which uses the structured singular value (SSV) and a physically motivated parametric uncertainty model. The methodology is applied to both the longitudinal and lateral basic controller and shows a direct and exact way from the definition of parametric uncertainties to an uncertain system model suitable for -analysis. Particularly, the introduction of a trim point uncertainty emphasizes the davanced character of this analysis method by incorporating the dependence of the control loop parameters (plant and controller gains) on the current trim point into the uncertainty model. Consequently the analysis of the controller can be accomplished for entire regions of the flight envelope in a single robustness test. Therefore, the proposed approach demonstrates the possibility of substituting the conventional proceedures for proving controller robustness by a more efficient methodology.

Testing and performance enhancement of a model-based designed ground controller for a diamond-shaped unmanned air vehicle (UAV)

At the Institute of Flight System Dynamics of the Technische Universität München (TUM), a digital flight control system for a fixed wing Unmanned Air Vehicle (UAV) featuring a novel diamond-shaped configuration is designed, implemented and tested up to its first flight. The capabilities of the UAV comprises a fully automated flight, including ground control for centerline tracking and runway alignment during automatic take-off and landing. This paper presents the testing and performance enhancement of a model-based designed ground controller for the diamond-shaped UAV. The focus is set on the challenges of testing the ground controller within the framework of an existing flight software and ground control station.