Experimental Investigation of the Aerodynamics and Aeroelasticity of Flapping, Plunging and Pitching Wings

This thesis concerns the experimental analysis of the aerodynamics and
aeroelasticity of oscillating wings. It is divided into two sections, an inves-
tigation of wings undergoing imposed root flapping and pitching motion,
such as seen in avian flight and an experimental investigation of the self-
excited stall flutter oscillations of a wing undergoing plunging and pitching
motion.
The objective of the flapping wing investigation is to improve the under-
standing of the unsteady aerodynamics of flapping flight. The work con-
sists of designing and building a flapping and pitching bird-like mechanical
model and testing it in a low speed wind tunnel. The model’s size is sim-
ilar to that of a goose. The wind tunnel tests involved the measurement
of forces, power, kinematics and local flow velocities using Particle Im-
age Velocimetry (PIV). The mechanical model was tested at four different
frequencies and three airspeeds. In addition, three different wing profiles
were tested. The kinematic modes included pure flapping and combined
flapping and pitching. It was shown that pure flapping and pitch-lagging
combined motion can lead to interesting aerodynamic phenomena, such
as flow separation and dynamic stall. Dynamic stall can cause significant
increases in instantaneous lift production, although it also generates a lot
of drag. On the contrary, pitch leading combined motion can generate a
net propulsive force over the entire cycle.
In the stall flutter study, a 6 degree of freedom NACA 0018 wing was sus-
pended by springs in the wind tunnel. The wing was then exposed to an
airflow at a number of airspeeds and wind-off angles of attack. The wing
was instrumented with pressure sensors and accelerometers. In addition,
PIV flow measurements were carried out on the wing’s upper surface. The
work demonstrates that stall flutter is the result of a combination of dy-
namic stall with the elastic properties of the system. The dynamic stall
mechanism was studied using the PIV measurements and was found to be
different to the classical stall mechanisms that are reported in the litera-
ture. The sensitivity of the stall flutter phenomenon to parameters such
as airspeed and wind-off angle of attack is demonstrated by means of a
two-parameter bifurcation diagram.