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Innovative Blade Trailing Edge Flap Design Concept using Flexible Torsion Bar and Worm Drive

In this paper, a simple but effective trailing edge flap system was proposed. This preliminary concept uses a more practical and stable actuation system which consists of a motor-driven worm gear drive and flexible torsion bar. The flexible torsion bar is designed to be easily twisted while keeping bending rigidity as a sup-port and the worm gear drive not only provides a high torque to overcome aero-dynamic forces on the flap area and the torsional rigidity of the support bar, but also acts as a brake to avoid instability due to the high torsional flexibility of sup-port bar. A preliminary level design study was performed to show the applicability of the new trailing edge flap system for wind turbine rotor blade or helicopter blade.


Introduction
Rotor blade vibrations and noise are generated during all operating conditions, primarily due to unsteady aerodynamic loads. The reduction of such vibratory loads are quite important, so that a much research has been performed to develop various passive and active methods and mechanisms for achieving this goal [1][2][3]. Also, the application of active materials for reductions of vibration and noise in rotor blade has been the focus of numerous studies in recent years [4,5]. In the aerodynamic point of view, the outboard region is subject to the highest values of dynamic pressure and consequently offers the greatest potential for the generation of rotor blade control air loads with minimal actuation effort (i.e. minimal deformation of the blade), as well as having the largest effect on blade loads due to a large leverage. Straub et al. [6] researched trailing edge flaps with various mechanisms and actuators, and Bernhard and Chopra [7] investigated an active blade tip rotor with piezoelectric actuation using a bending-torsion actuator. However, most of actuation system use additional amplification mechanism such as linkage system to generate large deflection of control surface for effective load control.
Ha and Dancila [8] proposed and analyzed a star-shaped composite cross section which optimizes the extensiontwist response. Composite star beams are ideal solution for the frictionless tension-torsion hinge bar support of the rotor blade tip against centrifugal forces. A design level study on active wind turbine blade tips with a flexible torsion bar and piezoelectric actuation was done by Dancila et al. [9]. The high axial and bending stiffness and strength of the composite bar provide effective and frictionless support against axial (centrifugal) loads and transverse (lift and dead weight) loads, while the low torsional stiffness allows an effective actuation by the coiled piezoelectric actuator. However, the suggested coiled actuator requires further research for realistic manufacturing and also current bender type actuators do not provide enough power to generate control surface deformation in the rotor blade and also do not 102 provide force to constrain the support bar to avoid torsional instability. Another wind blade tip control mechanism concept was also proposed by Xie et al. [10]. It used a servo motor and worm-gear reducer to control folding of a wind turbine blade. Experiments with a small blade showed the advantages in reducing rotation torque and thrust through the implementation of a worm gear drive.
There were also several studies in finding what angle of attack of an equivalent rigid section will produce the same lift as a flapped section with flap deflection angle. Fehlner worked on the design of control surfaces for hydrodynamic applications and found that slope effectiveness factor was about 3.13 corresponding to 15% flap chord [11]. Madsen, Barlas, and Andersen developed morphing trailing edge flap system for wind turbine and demonstrated the functionality and aerodynamic performance of the flap concept. From the research, they found that 3 degree flap deflection gives the same lift change as 1 degree pitch of the whole blade system [5].
In the present work, a more practical and stable actuation system, the motor-driven worm drive, is suggested to provide trailing edge flap motion and to avoid instability due to torsional flexibility of star shaped composite beam using intrinsic self-locking principle of single worm drive. A preliminary design level study was performed to show the applicability of the new trailing edge flap system for wind turbine blades.

Characteristic of Flexible Trailing Edge Flap System
The characteristics of the flexible torsion bar and worm gear drive, which comprise the new trailing edge flap system are briefly explained analytically. Figure 1 shows possible redistributions of a circular cross sectional material to star configuration section and a representative flexible torsion bar with three arcs. From Equations 1 to 3, Figures 2 and 3 are obtained visually, which show that the transition improves the performance [8]. That is, it is possible to achieve a beneficial stiffness decreases in torsional stiffness and an increase in bending stiffness while keeping the axial stiffness unchanged.

Flexible Torsion Bar
Where, EA is axial stiffness, EI is bending stiffness, N is the arm number of star shape, is the ratio of wall thickness to radius of star shape cross section, ∅ is a nondimensional parameter, and GJ is torsional stiffness. Also, subscript s and circular represent star shaped cross section and circular solid cross section, respectively.

Figure 3. Variation of bending stiffness ration
Also, from Figures 2 and 3, it is shown that the torsional rigidity can be reduced to less than 7% while the bending stiffness can be increased more than seven times and the axial stiffness is preserved. Figure 4 shows typical worm gears consisting of worm as the driving part and wheel gear as the driven part. It has been mainly used to applications requiring a large gear reduction, that is, a high torque with for this application, a maximum 90° motion transfer at the wheel gear end. Another interesting part is that the worm can easily turn the wheel gear, but the wheel gear cannot turn the worm reversely. This is a useful feature to the currently proposed trailing edge flap system because it keeps the flexible torsion bar from turning excessively to cause torsional instability. Kel : elastic torsional stiffness;

Worm Gear Drive
KF : apparent torsional stiffness due to axial load; KNL : nonlinear term due to trapeze effect; s : solid cross section type; Considering the tip torque, T, at the wheel gear from worm gear drive motor, the moment equilibrium equation at the tip is given by: Where, Pi is a power (Watt) given from the worm gear drive motor specification, is a rotational speed. Therefore, the flap angle is expressed in Equation 4 with integration by parts rule in terms of input power from the worm gear drive and the torsion stiffness from the flexible torsion bar.
Where, t is operation time by the worm gear drive motor. Figure 5 shows the example of a flexible trailing edge flap system application to rotor blade such as wind turbine blade or helicopter blade. For simplicity, assume that the flap hinge position is located aft to the aerodynamic center axis. In the presence of air loads, a positive nose-up rotation of the trailing edge flap device is generated by a positive aerodynamic hinge moment, which tends to amplify the flap deflection.

Conclusion
A conceptual flexible trailing edge flap system was proposed in this work. This preliminary concept uses a more practical and stable actuation system which consists of motor-driven worm gear drive as an input power device and flexible torsion bar as a support bar. The flexible torsion bar showed a beneficial decrease in torsional stiffness, while increasing the bending stiffness, all at the same axial stiffness as a massive bar for comparison. It was also shown that the worm gear drive not only provided a high torque to overcome aerodynamic force on the flap area and the torsional rigidity of support bar, but also plays as a brake to avoid instability due to the high torsional flexibility of the support bar. A preliminary level design study was performed to show the equilibrium condition analytically and the applicability of the new trailing edge flap system for wind turbine blades with regard to the aerodynamic force.