11:00
Very flexible aircraft 2
Chair: Andrea Castrichini
11:00
30 mins
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Flight-dynamics aeroelastic coupling in flexible and very flexible wings
Dor Naftaly, Daniella E. Raveh
Abstract: The paper presents a study of the coupled flight-dynamics and aeroelasticity of flexible
vehicles, with focus on the impact of hard maneuvers on the aircraft’s stability. The paper
presents an original derivation of the nonlinear coupled equations of motion, their subsequent linearization, and a modal representation of the linearized equations of motion. The inertial coupling terms are examined to elucidate their magnitude and influence on the system’s stability. A stability analysis of the coupled flight-dynamics-aeroelastic system for a flying wing UAV configuration is presented, highlighting the impact of maneuvering on the system’s flutter onset speed and characteristics.
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11:30
30 mins
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Linearized CFD-and CSM based flutter process for very flexible aircraft
Bernd Stickan, Reik Thormann, Michael Wrightson, Paolo Mastracci, Thomas Wilson
Abstract: New aircraft developments might include higher aspect ratio wings compared to the current commercial transport aircraft. Their aspect ratio is currently around nine, whereas new studies, like truss-braced wings, envisage an aspect ratio between 15 and 19. To reduce the mass penalty for such designs, these wings will usually become more flexible. The modal mass and stiffness matrix as well as the mode shapes themselves become dependent on the static aeroelastic equilibrium state and therefore, the challenges for flutter stability analysis are increased compared to a purely linear analysis.
The application case for this paper is an aircraft with aspect ratio 17 and a wing deformation of up to 11% relative to half-wing-span. It is designed for transonic Mach numbers. Figure 1 shows the differences of flutter results when considering a jig shape or a flight-shape structural model. The aerodynamics for both cases have been computed in flight shape.
Updated Flutter Process
The challenge for the p-k-based, linear flutter process is that matrices for mass, stiffness and aerodynamics can vary during each velocity step.
The two main influence parameters on the wing deformation are the lift coefficient and the dynamic pressure, which are spanning up the interpolation space I. For each sample point in I, a static CFD-CSM solution, the structural eigenmodes in the deformed shape and the CFD-LFD aerodynamics are computed. Afterwards, for a certain production mass cases the eigenmode analysis must be repeated for the same sample points in I and the aerodynamics must be approximated accordingly from the LFD database. Afterwards, a mode-mapping between the structural sample point mode matrices must be performed to gain structural and aerodynamic matrices which allow direct interpolation. During the p-k solver run these matrices are interpolated in each velocity step.
Finally, the paper will compare differences of the jig-based approach and the approach based on interpolation between different CSM deformation conditions.
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12:00
30 mins
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Flight dynamic-aeroelastic response of highly flexible aircraft with distributed propellers
Alberto Gallego Pozo, Rauno Cavallaro
Abstract: The increased concern about climate change has driven an international effort to develop zero-emission air transport. Within the emergent clean electric aircraft market, Distributed Electric Propulsion (DEP) is a promising concept. The freedom in the disposition of the propellers allows higher aerodynamic efficiency and enables new capabilities in vehicle control [1].
However, distributing rotary devices along the aircraft can promote traditional aeroelastic instabilities or result in unconventional instabilities like whirl flutter. Furthermore, propellers enhance couplings between rigid flight dynamics and aeroelastic effects. Moreover, maximizing the lift-over-drag ratio leads to high-aspect-ratio wings, which undergo large deformations and exhibit geometrically nonlinear behaviour. This also reduces the typical frequencies of elastic modes, enlarging the flight dynamics-aeroelasticity coupling. To achieve actual implementation of DEP technologies in the market, it is critical to develop methods to evaluate these effects since the early design stages.
Efforts have been made to propose purely aeroelastic frameworks capable of analyzing the interaction between propellers and wings, considering classical wing flutter and whirl flutter [2], [3]. They consist of rigid propellers flexibly attached to a wing undergoing out-of-plane bending and torsion. The wing aerodynamics is modelled with a DLM performed on the rigid wing and propeller aerodynamic with Blade Element Theory, with no interaction between them. None of these methods consider the free motion of the aircraft and they are not valid for very flexible wings.
There is the need for the development of frameworks capable of analyzing the coupled non-linear flight dynamic-aeroelastic response of free-flying aircraft with distributed propellers. This proceeding aims to present a method capable of this, while still being computationally efficient.
The aircraft structure is modelled with a non-linear beam formulation and a lumped mass model. The unsteady aerodynamics are modelled with a Vortex Particle Method, coupled with an enhanced Doublet-Lattice Method for the wing and Blade Element Theory for the propeller. Geometrical non-linearities are considered both in the structure and aerodynamics. Propeller slipstream effects are taken into account.
Finally, a synthetic test aircraft is analyzed to showcase the capabilities of this framework and demonstrate the importance of considering the coupling of non-linear flight dynamics and aeroelasticity in the design of highly flexible Distributed Electric Propulsion (DEP) aircraft.
[1] H. D. Kim, A. T. Perry, and P. J. Ansell, “A Review of Distributed Electric Propulsion Concepts for Air Vehicle Technology,” AIAA, no. 2018–4998, Jul. 2018, doi: 10.2514/6.2018-4998.
[2] N. Böhnisch, C. Braun, S. Koschel, and P. Marzocca, “Whirl flutter for distributed propulsion systems on a flexible wing,” AIAA SCITECH 2022 Forum, Jan. 2022, doi: 10.2514/6.2022-1755.
[3] N. Böhnisch, C. Braun, S. Koschel, P. Marzocca, V. Muscarello, P. Marzocca, “Dynamic Aeroelasticity of wings with distributed propulsion system featuring a large tip propeller”, IFASD 2022 Forum, Jun 2022
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