Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 474

Two types of compliance mechanisms can be distinguished: lumbed and
continuous compliance. The disadvantage of lumped compliance - where the
deformability is concentrated in discrete points - is that stress concentrations occur
around the fl exible points, whereas with distributed compliance the strain is
distributed over the whole mechanism.
A second form of adaptive wings aims at fl ight control trough wing twist. Gern
et al . [31] mentions that wing twist can be employed to reduce roll reversal due to
fl ap actuation. Wing twist requires a large amount of distributed actuators because
the entire wing needs to be deformed to get a signifi cant change in angle of attack.
Stanewski [32] also mentions the suitability of 'smart' materials for this task
because of their high power density and the possibility of distributed actuation,
which is needed when deforming a wing though gradual torsional deformation,
either on the wing itself, like Krakers [33] suggests, or by a added torsion tube,
like Jardine et al . [34] has implemented.
Shape change of an aerofoil is also used for reconfi guration. In this case the
aerofoil is deformed to set an optimal shape for different fl ight modes, e.g. from
sub- to transonic fl ight or from a landing to cruising modes and vice versa. Recon-
fi guration concepts [35, 36] are often similar to other adaptive aerofoil concepts,
but the needed actuation speeds are much lower. Therefore it opens up new pos-
sibilities for low frequent control concepts and adaptive materials such as SMAs
and shape memory polymers (SMPs) [34, 37-39].
A fi nal application of adaptive materials in aircrafts is vibration control. On the
subject of 'smart' materials for vibration reduction in general numerous publica-
tions exist, but here the focus will be on vibration reduction of aerodynamically
excited panels and wings. Vibration suppression then usually involves increasing
aeroelastic damping. With increasing size, fl utter might also become an issue for
wind turbine blades [40] because the fi rst bending and torsion mode move closer
to each other, so it is an interesting topic for the wind turbine community as well.
Guo et al . [41] reports increased stability of a panel by activating embedded SMA
wires. Wu et al . [42] achieves vibration suppression by adhering PZT patches to a
panel of a F15 fi ghter that suffers from aeroelastic excitation. These patches are
connected to a shunt circuit which is tuned for maximum energy dissipation at spe-
cifi c frequencies. Hopkins et al . [43] reduces the vibration due to twin tail buffeting
by adhering PZT patches to the tails and controlling these using feedback control
based on accelerometers and strain gages. An increase in aeroelastic stability cannot
only be attained by actively or passively increasing the structural damping, but also
by increasing the aerodynamic damping, e.g. though control of a fl ap [ 44 , 45 ].
A program that encompassed many of these concepts is the DARPA Smart Wing
program. In the fi rst phase of this program wing twist and SMA-activated trailing
edge fl aps were implemented on a scaled wing of a fi ghter aircraft [46]. Wing twist
was induced by a SMA torque tube [34, 47] and the trailing edge fl aps were
designed as fl exible glass-epoxy plates, covered with aluminum honeycomb and
room temperature vulcanized (RTV) sheets to give the trailing edge its shape and
smoothness respectively. In the second phase fl aps, driven by piezoelectric motors,
were implemented on a unmanned aerial vehicle (UAV) [48]. This concept is