The filtered lifting line theory is an analytical approach to solving
the equations of flow subjected to body forces with a Gaussian
distribution, such as used in the actuator line model. In the original
formulation 1, the changes in chord length along the blade were assumed
to be small. This assumption can lead to errors in the induced
velocities predicted by the theory compared to full solutions of the
equations. In this work, we revisit the original derivation and provide
a more general formulation, that can account for significant changes in
chord along the blade. The revised formulation allows for applications
to wings with significant changes in chord along the span, such as wind
turbine blades.