Actuation Requirements of a Warp Induced Variable Twist Rotor Blade

Abstract

This paper examines a warp induced twist concept to obtain quasi-static large amplitude twist changes of helicopter or tiltrotor blades for performance benefits over diverse operating conditions. The concept presented in this paper has a cylindrical spar with rotating ribs, with the ribs attached to the skin which is slit along the trailing-edge. Warping the skin then produces twisting of the blade section, while bending loads are still transferred to the main spar. Warp actuation is implemented by rotating a threaded rod passing through alternating unthreaded and threaded blocks connected to the interior of the upper skin near the trailing edge. During warp actuation the torsion stiffness is reduced and the blade twists easily, but in “power off” condition, the threaded rod effectively converts it to a closed section with correspondingly higher torsion stiffness. A prototype based on this concept was built and tested for both the warp-twist relationship and the actuation torque requirements for producing the twist of the blade. The prototype has a NACA 0012 airfoil profile, a 10.75 in chord and a 42 in span, and was capable of variations of up to ±18 deg of tip twist. The finite element model developed in the paper correlated very well with the experimental measurements made on the prototype. The validated finite element model is further used to conduct parametric studies to understand the effect of the beam span length, skin thickness, rib spacing and actuation block spacing variation on the system characteristics.

Design, fabrication, and benchtop testing of a helicopter rotor blade section with warp-induced spanwise camber variation

“…The study in this article uses the reverse concept whereby directly applying a warping force along the beam span produces spanwise camber variation. The work builds on previous efforts where warp-twist coupling was utilized to vary wing twist in fixed-wing applications (Vos et al, 2008(Vos et al, , 2010, to helicopter rotor blade morphing (Mistry, 2008;Mistry et al, 2011;Thomas et al, 2008;Van Weddingen et al, 2010), and most recently to wind-turbine blade twist morphing (Lachenal et al, 2013). In particular, it represents a continuation and development of the authors' work in Mistry (2008) and Mistry et al (2011).…”

“…While the rotor blade design in Mistry (2008) and Mistry et al (2011) was shown to be capable of producing large twist variation, it also had a couple of major limitations. The first was the significant reduction in blade bending and torsion stiffness associated with the circular spar and rotating ribs supporting the shell skin.…”

“…The work builds on previous efforts where warp-twist coupling was utilized to vary wing twist in fixed-wing applications (Vos et al, 2008(Vos et al, , 2010, to helicopter rotor blade morphing (Mistry, 2008;Mistry et al, 2011;Thomas et al, 2008;Van Weddingen et al, 2010), and most recently to wind-turbine blade twist morphing (Lachenal et al, 2013). In particular, it represents a continuation and development of the authors' work in Mistry (2008) and Mistry et al (2011). In these references, the authors use an airfoil-shaped blade skin shell, which is slit along the trailing edge and affixed to ribs in a manner which allowed for the free sliding of the skin in spanwise direction, while restricting motion in the other two directions.…”

“…A concept developed by Mistry (2008) and Mistry et al (2011) was based on spanwise warping of the skin of the rotor blade, slit along the trailing edge, to produce twist. The skin was supported by ribs that rotated about a circular spar, and spanwise warping of the upper skin relative to the lower skin was achieved by rotation of a threaded rod running through alternately placed threaded and non-threaded housings attached to the upper and lower skins along the span.…”

Concept for morphing airfoil with zero torsional stiffness

“…Creating a discontinuity in the skin, and thereby creating an open walled cross-section, reduces the torsional stiffness significantly. The actuated screw thread is able to resist torsion while enabling controlled twisting of the blade [25,26].…”