Jonathan Cooper

Identification of a Nonlinear Wing Structure Using an Extended Modal Model

The nonlinear resonant decay method identifies a nonlinear dynamic system using a model based in linear modal space comprising the underlying linear system and a small number of additional terms that represent the nonlinear behavior. In this work, the method is applied to an aircraftlike wing/store/pylon experimental structure that consists of a rectangular wing with two stores suspended beneath it by means of nonlinear pylons with a nominally hardening characteristic in the store rotation degree of freedom. The nonlinear resonant decay method is applied to the system using multishaker excitation. The resulting identified mathematical model features five modes, two of which are strongly nonlinear, one is mildly nonlinear, and two are completely linear. The restoring force surfaces obtained from the mathematical model are in close agreement with those measured from the system. This experimental application of the nonlinear resonant decay method indicates that the method could be suitable for the identification of nonlinear models of aircraft in ground vibration testing.

Active Aeroelastic Control Using the Receptance Method

The control and manipulation of dynamic instabilities, such as flutter, is termed as aeroelastic control and is extremely important in designing next generation flexible and maneuverable aircrafts. One of the goals of an aeroelastic control is to extend stable fight conditions for a large range of aerodynamic flow conditions. The associated control problem deals in adjusting and assigning the eigenvalues (which determine the natural frequencies and damping ratios) of the aeroelastic system for achieving the desired closed-loop behavior by active or passive means. In this paper, an active aeroelastic control problem, associated with the wing model, is formulated and eigenvalue assignment to achieve flutter free flight envelope is developed by using a new control methodology known as the Receptance Method. This method is entirely based upon transfer functions, typically obtained from a standard modal test by using actuators and sensors. This method has several advantages over traditional aeroelastic control approach which leads to state-space formulations. For example, it does not require the estimation of structural matrices (i.e. mass, stiffness and damping) as well as rational function approximations of aeroelastic influence coefficient matrices. The control gains are obtained without the knowledge of system matrices and purely from the receptance matrices. The feasibility study of this approach for aeroelastic control is considered here using several simple numerical aeroelastic systems. The control gains for eigenvalue assignments are obtained from receptance matrices and the performance of the controller is compared with those obtained by the state-space approach. Numerical examples associated with the eigenvalue assignment problems to adjust the natural frequency as well as damping ratio and to extend the flutter envelope by active means are presented. The actuator dynamics of the control surface and its effect on receptance based control is also studied. We envision that this new approach will facilitate an alternative method to address aeroelastic control problems and eventually will provide a practical solution for implementing active aeroelastic control.Copyright

Optimization of a Scaled Sensorcraft Model with Passive Gust Alleviation

High altitude long endurance UAVs such as sensorcraft are extremely flexible and consequently are susceptible to excessive gust loads; therefore it is desirable to incorporate some form of gust load alleviation system into the air vehicle design. If successful, this could result in a significant weight reduction. This paper describes part of an EOARD / AFOSR and ESF supported research project aiming to design, manufacture and test a wind tunnel model of a sensorcraft structure incorporating a passive gust alleviation device. Various elements of the project are described, including initial simulated validation of the device, design and test of a concept prototype and aeroelastic scaling of the sensorcraft wind tunnel model. Conclusions are made as to the effectiveness and feasibility of incorporating the gust alleviation device on an aeroelastically scaled wind tunnel model.

Optimisation of Composite Structures for Aeroelastic Applications using Evolutionary Algorithms

This paper investigates the possibility of utilising the directional properties of composites for passive gust load alleviation. A composite flat plate representing a typical commercial aircraft wing planform is constructed using Finite Elements. The composite layup is optimised to reduced maximum gust loads for a discrete ”1-cosine” vertical gust specified under cruise conditions but with consideration of the mass, flutter and static aeroelastic displacement. Genetic Algorithms and Particle Swarm Optimisation are used to optimise the structure via changes in the orientation and thickness of the layers in the composite layup. It was found that compared to a metallic wing it was possible to reduce the gust loading and mass, however, there were difficulties in maintaining the same flutter speed.

Force Appropriation for Flight Flutter Testing using Smart Devices

A novel approach involving the use of the force appro priation testing for flight flutter testing using PZT patches is described. It is shown how the traditional flight flutter test procedure can be improved through the use of multi -excitation force appropriation resulting in superior estimates of frequenci es, damping ratios and mode shapes. This improved identification procedure enables better estimates of critical flutter boundaries to be achieved compared to traditional phase separation methods. The approach is demonstrated upon simulated rectangular an d swept wings. A procedure for determining the optimal positioning of the PZT patches is also described.