Unsteady Aerodynamics

If the motion of the profile or the wing in a free stream changes by time, so do the acting aerodynamic coefficients. When the changes in the motion are fast enough, the aerodynamic response of the body will have a phase lag. For faster changes in the motion, the inertia of the displaced air will contribute as the apparent mass term. If the apparent mass term is negligible, this type of analysis is called the quasi-unsteady aerodynamics.

1.1.5 Compressible Aerodynamics

When the free stream speeds become high enough, the compressibility of the air starts to change the aerodynamic characteristics of the profile. After exceeding the speed of sound, the compressibility effects changes the pressure distribution so drastically that the center pressure for a thin airfoil moves from quarter chord to midchord.

1.1.6 Vortex Aerodynamics

A vortex immersed in a free stream experiences a force proportional to density, vortex strength and the free stream speed. If the airfoil or the wing in a free stream is modeled with a continuous vortex sheet, the total aerodynamic force acting can be evaluated as the integral effect of the vortex sheet. In rotary aerodynamics, the
returning effect of the wake vorticity on the neighboring blade can also be mod­eled with vortex aerodynamics. At high angles of attack, at the sharp leading edge of highly swept wings the leading edge vortex generation causes such suction that it generates extra lift. Further angle of attack increase causes asymmetric gener­ation of leading edge vortices which in turn causes wing rock. The sign of the leading edge vortices of unswept oscillating wings, on the other hand, determines whether power or propulsive force generation, depending on the frequency and the center of the pitch. For these reasons, the vortex aerodynamics is essential for analyzing, especially the unsteady aerodynamic phenomenon.