This paper examines rotor power reductions achievable through a combination of radius and RPM variation. The study is based on a utility helicopter similar to the UH-60A and considers +17% to −16% variation in radius and ±11% variation in RPM about the baseline, over a range of airspeed, gross weight, and altitude. Results show that decreasing RPM alone effectively reduced power at cruise velocities in low-and-light conditions, but the power reductions diminished at increasing altitude and/or gross weight, and in low-speed flight. Increasing radius alone, on the other hand, had greatest effectiveness in power reduction in high-and-heavy operating conditions and at lower flight speeds. When radius and RPM variation is used in combination, minimum RPM is always favored, along with radius increases at increasing altitude and gross weight, and in low-speed operation. At low-to-moderate gross weight, the significant power reductions seen in cruise and at low altitude with RPM variation alone are obtained even at higher altitude, and over the airspeed range, using radius and RPM variation in combination. In high-and-heavy conditions, the combination of RPM reduction and radius increase yields very large power reductions of over 20% and up to 30% over the baseline. Power reduction in low-and-light conditions comes almost entirely from profile power reduction due to RPM decrease. In cruise and high-speed flight, the profile power reductions progressively give way to induced power reductions at increasing gross weight and altitude. At low speeds, reduction in induced power due to increased radius and decreased disk loading dominates.

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.

No 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.

This article focuses on the design, fabrication, testing, modeling, and validation of a spanwise variable camber section of a helicopter rotor blade. The lower surface skin was slit aft of the trailing edge spar and subjected to a spanwise warping actuation input. A kinematic linkage facilitates a corresponding chordwise motion of the skin along the span length (from zero at the no-camber end to maximum at the maximum camber end). Input warping actuation of 0.18 in length produced an 18°camber variation over a 45 in span section of a modified CH-46 blade. Finite element model predictions of the active camber section showed good agreement with benchtop test data for both the output camber for a given warping actuation input and the corresponding actuation force required. Finite element results suggest that some geometry distortions in the active camber section could be experienced due to the presence of centrifugal and aerodynamic loads, but these distortions could be alleviated with the introduction of a shear-flexible (but stiff through the thickness) core.