Aerodynamics, Signature- and Structural Technology
Flight mechanics covers the whole field of aerial vehicle dynamics, including control theory and control technology applicable to aerial vehicles, as well as methods for analysing the role of the pilot in manned aircraft.
An important part of this field involves analytical or numerical methods for determining whether a given control system makes an aerial vehicle manageable in a way that is safe. These activities also apply to the control of autonomous aerial vehicles and are closely linked to both aerodynamics and structural technology via the aeroelastic phenomena mentioned above.
FOI conducts applied and research-driven activities in aerodynamics, focusing on manned and unmanned military aircraft, including weapons and missiles. The objective is to enhance the capability for evaluating aircraft design concepts, with regard to their fundamental flight performance, stability, weapons integration, weapons separation, and propulsion integration, including the design of air intakes, and so on, to produce aero-data for simulation models and noise analyses.
The research is directed towards simulations of non-steady flow phenomena using CFD (Computational Fluid Dynamics) methods, as well as flow control, aerodynamic design, and shape optimisation.
CFD is a branch of fluid mechanics in which numerical methods are used to analyse flow problems. The principal foundations on which CFD rests are the Navier-Stokes equations. These consist of a system of partial differential equations, which primarily describe the time- and space-dependent velocity field in a fluid. The equations are non-linear and must be solved numerically. FOI is developing its own CFD software, the code M-Edge, to be able to solve the compressible Navier-Stokes equations on computer networks, using a so-called finite volume method.
The aerodynamic studies are normally based on the assumption that the geometry of the aircraft remains unchanged during flight. However, the aircraft’s component materials are not rigid, but elastic, so that the loads acting on the aircraft during flight cause significant deformation of the various structural components, such as the wings. This in turn leads to new loads on the structure and then new deformations, and so on, until equilibrium is established, or severe vibrations occur, in the form of flutter. In working with aeroelasticity, models and computational codes are developed to cope with these problems, which become increasingly important as aircraft become more flexible. The problem of deformation of control surfaces such as rudders in turn creates a connection to the research on flight mechanics and control systems.
Signature reduction technology
FOI conducts multi-disciplinary analyses of stealth vessels and detailed analyses for use in the design of subsystems with low signature properties. Radar and IR propagation characteristics of materials and objects are predetermined and analysed so that signatures can be minimised without affecting other performance-critical properties, for example aerodynamic or structural characteristics.
FOI is responsible for the two national signature codes, SAFIR, for IR calculations, and EMCAV, for radar signature calculations of cavities (typically air intakes). These tools are being developed in collaboration with various companies in the SAAB Group and with Volvo Aero, which are both users of the codes
Structural and materials technology
Structural mechanics is a relatively mature discipline, which has produced numerous commercial computer codes, although not all equally powerful, based on the finite element method. FOI has developed a computer code, called Stripe that is specially designed as a tool for performing reliable large-scale analysis of damaged metal and composite structures. It is used for virtual testing, certification, and accreditation. Stripe is an hp-adaptive code that is especially adapted for large computer systems of up to a thousand or more computers (CPUs). It is based, furthermore, on a mathematical formulation that allows the code to compute with exact correctness under given boundary conditions and structural loads. Another of its properties is that the shape of the finite element can be highly distorted, but without compromising the final result; also, the number of necessary elements can be less than in conventional finite element codes.
FOI’s tool-box also includes other codes that it has developed; these are based on fracture mechanics, and are useful for predicting the growth of cracks as a function of cyclic loading. The codes have been specially adapted for damage tolerance analysis.