Methodology for Modelling the Dynamics of Flexible, High-

Aspect-Ratio Aircraft in the Time Domain for ......


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Pages: 231
Language: English
Publication date: 30/11/-0001
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Dissertation Luft- und Raumfahrttechnik
In this thesis, a methodology for modelling the dynamics of flexible aircraft in the time domain for aeroservoelastic investigations is developed. The methodology consists in extending the rigid-body equations of motion with the aeroelastic dynamics and is applicable to slightly flexible, high-aspectratio aircraft in incompressible regime. The necessary aeroelastic database can be generated knowing the aircraft geometry, the lifting surfaces aerodynamic properties and the desired elastic modes from a ground vibration test or a finite element model.
The modelling of the flexible aircraft dynamics is based on the linearised mean axes constraints, i.e. without considering the inertial coupling between the rigid-body and the elastic degrees of freedom. The structural dynamics is linearly represented in terms of the aircraft in-vacuum orthogonal elastic modes and the principle of superposition is applied, resulting in the linear second order differential equations in the modal coordinates. To determine the incremental aerodynamics due to elastic deformations, an unsteady strip theory formulation in the time domain is used, considering the Jones exponential representation of the Wagner function and the resulting stripwise aerodynamic lag states. Spanwise correction to account for three-dimensional effects at the wing tip based on the quasi-steady circulation distribution was applied. The effect of the wind is similarly accounted for using an exponential representation of the K¨ussner function. The final system of equations of flexible aircraft is formally the system of the rigid body equations of motion, added by the equations of the structural dynamics and the aerodynamic lag states.
The methodology is demonstrated using the STEMME S15 prototype, an experimental aircraft, for which a control system enabling a fully autonomous flight is being developed in the project LAPAZ. Aeroelastic stability analysis of the S15 prototype was compared to calculations using ZAERO. Comparisons of damping and frequency show a good agreement and the flutter speed is calculated with the actual methodology within a 5% error in relation to ZAERO. Aeroservoelastic invertigations are performed for the S15 prototype regarding the influence of the flexibility in the flight control system depending on the sensor positioning.
For the validation of the methodology, a flight test campaign with the S15 prototype was planned and accomplished. It was demonstrated that simulation results using this methodology have an excellent agreement with the flight test data, justifying the use of this methodology in aeroservoelastic investigations for this class of aircraft. Moreover, the manoeuvres specified in the flight test campaign, prepared and conducted within this work, have proven to be very efficient in the validation of flexible aircraft simulation models.
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