This article 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 presented concept has a cylindrical spar with rotating ribs which are attached to the blade skin that is slit along the trailing edge. Warping the skin then produces twisting of the blade section. Warp actuation is implemented by rotating a threaded rod assembly attached to the interior of the upper and lower skin near the trailing edge. Due to the slit at the trailing edge, the blade is soft in torsion during actuation, but is effectively a closed section due to the threaded rod assembly when in power off state. A prototype (NACA 0012, 10.75 in chord, 42 in span) 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 (up to ±18°). The finite element model developed in this article correlated very well with the experimental measurements made on the prototype. The validated finite element model is further used to conduct a study to understand the effect of various structural parameters on the system behavior.

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.

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