PrandtlPlane Joined Wing: Body Freedom Flutter, Limit Cycle Oscillation And Freeplay Studies

Dynamic aeroelastic behavior of a joined-wing PrandtlPlane configuration is
investigated herein. The baseline model is obtained from a configuration previously
designed by partner universities through several multidisciplinary optimizations
and ad-hoc analyses, including detailed studies on the layout of
control architecture. An equivalent structural model has then been adopted to
qualitatively retain similar aeroelastic properties.
Flutter and post-flutter regimes, including limit cycle oscillations (LCOs) are
studied. A detailed analysis of the energy transfer between fluid and structure
is carried out; the areas in which energy is extracted from the fluid are identified
to gain insights on the mechanism leading to the aeroelastic instability. Starting
from an existing design of control surfaces on the baseline configuration, freeplay
is also considered and its effects on the aeroelastic stability properties of the
joined-wing system are investigated for the first time.
Both cantilever and free flying configurations are analyzed. Fuselage inertial
effects are modeled and the aeroelastic properties are studied considering plunging
and pitching rigid body modes. For this configuration a positive interaction
between elastic and rigid body modes yields a flutter-free design (within the
range of considered airspeeds).
To understand the sensitivity of the system and gain insight, fuselage mass
and moment of inertia are selectively varied. For a fixed pitching moment of
inertia, larger fuselage mass favors body freedom flutter. When the moment of
inertia is varied, a change of critical properties is observed. For smaller values
the pitching mode becomes unstable, and coalescence is observed between pitching
and the first elastic mode. Increasing pitching inertia, the above criticality
is postponed; meanwhile, the second elastic mode becomes unstable at progressively
lower speeds. For larger inertial values “cantilever” flutter properties,
having coalescence of first and second elastic modes, are recovered.