| 1. |
- Persson, Björn, 1986-
(author)
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Assessment of Aircraft Radar Cross-Section for Detection Analysis
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Hiding from and surprising an opponent are tactics that have been used in warfare throughout history. They were features that aircraft originally possessed when they were first used in military operations. However, development of military technology is an endless struggle between advances in technology and counter technology. During World War II this struggle led to the development of a new technology called radar, which was designed to detect sea vessels and aircraft at a distance and deny them the element of surprise. This laid the foundation for modern air defenses and simultaneously created a need for aircraft to penetrate such defenses. Central to the tactics and technological development that followed from the deployment of radar on the modern battlefield is the radar cross-section (RCS) of aircraft, which dictates the range at which aircraft can be detected by radar. In this thesis some aspects of the RCS of aircraft in radar detection are investigated. A combination of experimental measurement of aircraft and digital model development of the RCS of aircraft has been used.From flight experiments, the uncertainty in aspect angle to a threat sensor, due to aircraft dynamics, is quantified for various aircraft. In addition, the RCS fluctuation behavior of a military jet trainer is investigated by dynamic in-flight measurement. The monostatic and bistatic RCS of an F-117 are modeled and findings show that spline interpolation provides superior accuracy when interpolating the RCS data. Smooth and conservative RCS models are suggested and a new RCS sampling scheme is presented. A model based on experimental data is suggested for determining the range of aspect angles that an aircraft is likely to orient towards a threat sensor, and experimental RCS data is compared to the classical Swerling radar target models.Possible consequences for military operations and the design of military systems are discussed and considerations for modeling the interaction between air defenses and aircraft penetrating those defenses are given. This thesis should be of interest to military actors and the defense industry, since the analyses of the ability to detect aircraft using radar are important for military operations and their planning.
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| 2. |
- Jacobsen, Marianne, 1980-
(author)
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On improving efficiency of flight using optimization
- 2009
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Doctoral thesis (other academic/artistic)abstract
- In this thesis, optimization is used to improve the performance of aircraft. The focus is on operating current generation aircraft more efficiently rather than designing new aircraft. Drag minimization and aircraft trajectory optimization is used to increase efficiency. Optimization methods are implemented and evaluated on different problem formulations. The first part of the thesis presents a drag minimization strategy using multiple control surfaces distributed across the span of an elastic wing. Aeroelasticity is exploited to reduce drag for a wide range of flight conditions. A method to minimize drag during a long distance flight is developed and tested in a wind tunnel environment. The method is based on continuously changing the control surface deflections to obtain a more beneficial load distribution from a drag point of view for the current flight condition. In a second study, the method is extended to include the angle of attack as a variable together with the control surface deflections in the drag minimization algorithm. Extensive wind tunnel testing demonstrates the possibility to reduce drag significantly with the presented method for a wide range of flight conditions. The second topic in the thesis is optimizing the aircraft trajectory. The emissions from the aircraft engine are modeled as smooth functions suitable for optimization using published certification data. These emissions are combined in different environmental indices and used as objective functions in the aircraft trajectory optimization problem. The optimization problem is formulated by discretizing the trajectory in time. The resulting large scale nonlinear optimization problem is solved using a sequential quadratic programming method. The trajectory optimization problem is first studied using a model of the Boeing 737 and the results show that the optimal trajectory depends significantly on the definition of the environmental objective function. A method to treat restricted airspace is also presented and evaluated using a model of the Swedish Air Force trainer SK60. The results show that the method for imposing airspace constraints on the flight path works well, especially when the initial point for the optimization is feasible.
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| 3. |
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| 4. |
- Jacobsen, Marianne, 1980-
(author)
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Real Time Drag Minimization
- 2006
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Licentiate thesis (other academic/artistic)abstract
- This thesis focuses on the use of multiple redundant control surfaces to increase performance during flight. There is no clear-cut definition of performance. It may differ between applications, but here, the amount of drag for a given flight condition is used. The work is concentrated on minimizing drag with the use of measurements instead of numerical simulations. Measured data contains noise and there are problems with repeatability and hysteresis. These difficulties are considered and a method for drag minimization during flight is presented. In the first study the drag minimization algorithm is discussed. Focus is put on describing the implemented method and the treatment of constraints to the optimization problem. The constraints include keeping the lift constant as well as having bounds on the control surface deflections. In the second work, a more complex wind tunnel model is used to validate the drag optimization algorithm. Drag reduction for different flight conditions is studied, as well as the impact of the number of control surfaces. Different layouts of the control surfaces are also tested. The results show that the constraints are satisfied and that the drag is reduced substantially.
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| 5. |
- Jansson, Natascha, 1987-
(author)
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Analysis of Dynamic Flight Loads
- 2012
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Licentiate thesis (other academic/artistic)abstract
- This thesis deals with the determination of loads on an aircraft struc- ture during flight. The focus is on flight conditions where the loads are significantly time-dependent. Analysis of flight loads is primarily motivated to ensure that structural failure is avoided. The ability to ac- curately determine the resulting structural loads which can occur during operation allows for a reduction of the safety margins in the structural design. Consequently it is then possible to decrease the aircraft struc- tural weight. The demand for safe and fuel efficient aircraft creates a desire for efficient and accurate methods for determining the structural loads. The first paper of this thesis discusses the use of control laws for robust atmospheric turbulence load alleviation in the time domain. A numerical aircraft model including structural elasticity and unsteady aerodynamic effects is used. A limited set of longitudinal flight mechanic degrees of freedom are considered and two methods for structural load analysis are compared for evaluation of the wing root bending moment. In the second paper a method to perform time domain simulation of both motion of center of gravity and elastic deformation is described. The intention with the development of this simulation method is to enable efficient analysis of dynamic flight loads. A third study is finally included, where steady and unsteady pressure measurements have been carried out during wind tunnel testing. The motivation for performing these experiments is that knowledge about the aerodynamic force distribution which affect an aircraft structure is needed to correctly determine the structural loads.
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| 6. |
- Lokatt, Mikaela
(author)
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On a Viscous-Inviscid Interaction Model for Aeronautical Applications
- 2016
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Licentiate thesis (other academic/artistic)abstract
- The work presented in this thesis concerns the development of a viscous-inviscid interaction model for prediction of viscous flow properties in aeronautical applications. The inviscid model is based on an existing potential flow model and the thesis thus focuses on the development of a viscous model based on the three-dimensional integral boundary layer equations. The model is to be applicable to complex geometries with unstructured meshes and this requirement, in combination with the fairly complex character of the integral boundary layer equations, sets high standards for the discretization. In order to arrive at a stable and well-conditioned scheme a number of topics related to the formulation and discretization of the integral model are analyzed and discussed. These include the challenge of finding a discretization which ensures flow conservation on curved surfaces and provides a stable and well-conditioned discretization for mixed-hyperbolic systems of conservation laws. Different coupling strategies for the viscous and inviscid models are discussed and analyzed and so are the singularities in the integral boundary layer equations in separated flow regions. The predictions of the resulting viscous-inviscid coupling scheme are validated by comparison to experimental measurements as well as to predictions from other numerical models. The currently developed coupled model is found to provide reasonably accurate predictions of viscous flow properties in laminar as well as turbulent flow regions while being stable and convergent in separated flow regions.
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