  Frequency domain Unsteady Lifting-Line Theory (ULLT) provides a means by
which the aerodynamics of oscillating wings may be studied at low computational
cost without neglecting the interacting effects of aspect ratio and oscillation
frequency. Renewed interest in the method has drawn attention to several
uncertainties however. Firstly, to what extent is ULLT practically useful for
rectangular wings, despite theoretical limitations? And secondly, to what
extent is a complicated wake model needed in the outer solution for good
accuracy? This paper aims to answer these questions by presenting a complete
ULLT based on the work of Sclavounos, along with a novel ULLT that considers
only the streamwise vorticity and a Prandtl-like pseudosteady ULLT. These are
compared to high fidelity Euler CFD for cases of rectangular wings at multiple
aspect ratios and oscillation frequencies. The results of this work establish
ULLT as a low computational cost model capable of accounting for interacting
finite-wing and oscillation frequency effects, and identify the aspect ratio
and frequency regimes where the three ULLTs are most accurate. This research
paves the way towards the construction of time-domain or numerical ULLTs which
may be augmented to account for non-linearities such as flow separation.
