EFFECTS OF OSCILLATORY PLUNGING MOTION ON ROTARY WING BLADES

ABSTRACT
The effect of blade oscillation on resulting air loads has promoted new interest in
oscillatory blade aerodynamics. Rotary wings aerodynamics is considerably more
complex than fixed wing aerodynamics. As an airfoil performs an oscillation, vortices
are shed into the medium with a circulation strength equal in magnitude to the increase
in circulation about the airfoil but opposite in direction. These disturbances are stored in
the fluid because the shed vorticity convicts down stream at a local flow field velocity.
The wake of the rotor consists of helical vortex sheet below the rotor b1ades. The wake
remains near the rotor and therefore passes close to the following blades.
The fundamental closed form solution of Theodorsen [1], and Lowey [2], provide the
basis for theoretical work in this area. The closed form theory shows rapid changes in
the lift deficiency function with changes in reduced frequency, wake spacing and
frequency ratio. In the past, emphasis in the study of unsteady aerodynamics has
tended to focus on flutter instability and the effect of unsteady aerodynamics on
generating lift and torsional loads. The classic reference on this subject is that by
Garrick [3]. While Garrick's work shows that an airfoil oscillating in pitch will typically
produce drag in the lower reduced frequency range, k, it is found that the presence of
another layer of shed vorticity of the proper phase can reduce the drag on plunging
airfoils and depending upon wake spacing, reduced frequency and phase, may even
enhance the propulsive force acting on the plunging airfoils.