Unsteady Aerodynamics from the Doublet Lattice Method
Aircraft flutter is a destructive phenomenon which requires special attention in
the design process. The elements of flutter are structural dynamics and unsteady
aerodynamics. Of these, it is generally recognized that unsteady aerodynamics
are the more difficult to model and the least reliable. In 1935, Theodorsen was
the first to develop a practical unsteady incompressible aerodynamic formula!
for a flutter analysis of a two dimensional airfoil. It was fifty years ago that
Smilg and Wasserman of the Aircraft Laboratory of the Wright Air Development
Center wrote their landmark report on flutter clearance using the K-method and
strip theory. Of course such methods can be addressed with manual calculations.
Compressibility is normally associated with the flutter of high speed aircraft. It
is impractical to solve the compressible unsteady aerodynamic equations by
hand. The doublet lattice method” was developed slong with improvements in
digital computer technology. Hopefully, the doublet lattice method represents
the most rudimentary unsteady aerodynamic technique in practice where
subsonic compressible flow is a consideration. With the introduction of today’s
supercomputers, non-linear aerodynamics are now heing addressed, in spite of
the high cost. It is because of the high cost a. y:chaical complications
associated with non-linear Computational Fluid Dynamics (CFD) that the
doublet lattice method is still used almost exclusively for the subsonic flutter
clearance of flight vehicles being designed today. It is difficult to imagine the
day when non-linear CFD will replace the doublet lattice method in the
preliminary design environment.
The quote above is from the reference document #1 by Max Blair.
Blair wrote this report to help understand the flow equations and basic algorithms of
the doublet lattice method.
Upon reading Blair’s document, many folks would have liked to test the equations that have been encoded
in the C programming language, but have not wanted to enter them manuallly.
Witold Waldman has prepared the instructions from the reference document and the necessary support
calculations so that students learning from Blair's paper will not have to type them in.