Implementation of the ‘smart’ rotor concept - Wind Power Generation and Wind Turbine Design - page 475

allowed for a high rate, continuously deforming trailing edge in both cordwise and
spanwise direction. As a base structure the same construction as the trailing edge
fl aps, but aramid instead of aluminum honeycomb were used. The piezoelectric
motors were chosen because of their high power-to-weight-ratio. Interesting con-
clusions from the program were that active surfaces can indeed be employed but
that the actuation bandwidth of SMA material is very low and that the technology
readiness level [49] was around fi ve [25]. This means that it is at the level of com-
ponent and/or breadboard validation in a relevant environment. This point is also
addressed by Boller [50]: adaptive structures have shown great potential over the
last 20 years, but only little real structural implementations have been achieved.
The overview of all these concepts of 'smart' wings leads to some refl ections.
First of all, a distinguish between different amounts of required deformation can
be made: vibration control requires the littlest deformation of the structure since it
means controlling the stiffness of a structure through stressing, or actively coun-
teracting the vibration with a force. For wing twist medium strains are required
because the resulting twist results from the accumulated strain along the blade's
span. For integrated control surfaces relatively large strains are needed. This is
also illustrated by the fact that many studies focus on the use of servo or servo-like
actuators and that often silicone or latex skins are employed to allow for large
strains in the skin, e.g., [48]. Classical concepts mentioned by Campanile [21]
often employ surfaces that slide over each other.
Another distinction can be made in the different speeds required for actuation.
Active vibration control requires very high actuation frequencies, whereas recon-
fi guration is quasi-static. Concepts for fl ight control require medium actuation
speeds which are similar to the current control surfaces. Thirdly, for all adaptive
concepts, but especially with wing twist, a trade-off must me made between on
one hand the possible weight reduction due to the integration of several functions
in one structure and on the other the added actuator mass. A fi nal consideration is
the readiness of the technology.

2.2 Smart helicopter rotor blades

A great deal of research on adaptive (aero)elastic structures has been conducted
in the fi eld of helicopter rotors. The research has been into different features
such as torsion tubes, active twist control, trailing edge fl aps, etc. Straub presents
a good overview of early concepts [51]. In the following they are grouped and
discussed.

2.2.1 Trailing edge fl aps
Although helicopter rotors are much smaller than wind turbine blades and operate at
much higher rotational speeds, they still pose an interesting benchmark as adaptive
structures primarily because the structure under consideration is also a rotor. Sec-
ondly, because the intended effect, obtaining vibration reduction through load con-
trol, is usually the same. And fi nally because the fl ap defl ections that are aimed at are
roughly the same [9, 52], viz. several decrees for a fl ap size of ∼ 10% of the cord.