Mission-based optimal morphing parameters for rotors with combined chord and twist morphing

Background: The fixed geometry rotor blades in today’s helicopters do not give the best performance throughout the duration of any mission. However, low-speed and high-speed flights have different geometrical requirements for the shape of the most efficient rotor blades. With advancements in morphing technologies, these can be applied to change the shape of the blades in different flight regimes. Methods: Two different helicopter rotor morphing concepts – namely, the linearly variable chord extension and the torque-tube based twist - under the framework of the European project SABRE were investigated for their optimal geometric parameters using a Particle Swarm Optimization (PSO) algorithm. Since the morphing parameters were dependent on the mission profile, three different missions representing typical helicopter applications were chosen. The optimization problem was posed both as single objective (power) and as multi-objective (power, tip elastic torsion and vibratory hub load). Based on the insights drawn from these investigations, a rotor was set up including both morphing concepts in a single blade. Results: The rotor with combined chord and twist morphing was shown to have better performance than the baseline blade, while keeping the penalty on the elastic torsion and vibration of the rotor to a minimum. Conclusions: Chord and twist are both important parameters determining the efficiency of a rotor blade. Since they have non-overlapping requirements, combining the two morphing concepts into a single blade can yield higher performance than the individual ones.

Linearly variable chord-extension morphing for helicopter rotor blades

A new morphing concept called linearly variable chord extension was studied for its effectiveness in improving the efficiency of a helicopter rotor. Apart from chord extension itself, an additional feature which is deflection of the extended part of the chord resulting in an effective camber and additional twist to the airfoil, is also studied for its effect on rotor efficiency improvement. Trim analyses were carried out for various chord-extended rotors for hover as well as various forward flight velocities using DLR’s in-house comprehensive analysis code S4. Chord extension of up to 100% and chord-extension–deflection of up to 15° were considered. Results show that the linearly variable chord-extension concept is effective in reducing power requirement in both hover and forward flight. Deflection of the extended chord also helps reduce power requirement in hover, especially at higher blade loadings. However, the root torsional moments and hence, the pitch-link loads are seen to increase substantially for the morphed rotors.

Extendable Chord Rotors for Helicopter Envelope Expansion and Performance Improvement

This study explores the benefits of rotor chord extension in stall-dominant conditions. Simulations are based on a UH-60A Blackhawk helicopter with an effective chord increase of 20% realized by extending a trailing-edge plate (TEP) through a slit in the trailing edge between 63% and 83% blade span. Since TEP extension changes the baseline SC-1094R8 airfoil profile, two-dimensional aerodynamic coefficients of the modified profile from Navier–Stokes computational fluid dynamics calculations are used, coupled with 12 × 12 dynamic inflow and the Leishman–Beddoes dynamic stall model in the Rotorcraft Comprehensive Analysis System. While a fixed 20% larger chord produces comparable advantages to TEP extension in stall-dominant conditions, the rotor power requirements increase by up to nearly 4% for low gross weight, low-altitude operations, a penalty easily avoided with TEP retracted. From the simulations in the study, reductions of up to nearly 18% in rotor power requirements were observed with TEP for operation at high gross weight and altitude. Furthermore, increases of around 18 kt in maximum speed, 1500 lb in maximum gross weight capability, and 1800 ftin maximum altitude were observed. TEP extension generally reduces maximum angles of attack on the retreating side and weakens stall. Lift generally increases over the annulus where the TEP is present but reduces over the outer rim because the nose-down pitching moments produce larger nose-down elastic tip twist. With TEP extension, the offloading of the outer rim reduces drag, rotor torque, and power.

Helicopter Rotor Blade Chord Extension Morphing Using a Centrifugally Actuated von-Mises Truss

Previous studies have shown that chord extension morphing over a spanwise section of helicopter rotor blades can reduce main rotor power requirement in stall-dominant flight conditions while at the same time being able to increase the maximum gross weight, altitude, and flight speed capability of the aircraft. This study examines a centrifugally driven, fully passive chord morphing mechanism for helicopter rotor blades. It is based on a von-Mises truss situated aft of the leading-edge spar, connected to a rigid extension plate which deploys through a slit in the trailing-edge. When the rotor RPM increases beyond a critical value the chordwise component of centrifugal (CF) force on the von-Mises truss and plate assembly results in the deployment of the plate beyond the slit in the trailing edge, effectively increasing chord length. On reducing the RPM, a retraction spring pulls the plate back within the confines of the blade. This study presents the design process, iterations and the final design solution for a configuration that undergoes 20% chord extension. A prototype was fabricated and tested on the bench-top as well as on a rotor test stand at rotational speeds simulating 70% full-scale CF loads. The test results demonstrate that the concept works. However, effects such as friction lead to higher force (or RPM) requirements for deployment than predicted by simulation, and are present during retraction as well. The effects are more pronounced in the high CF field in the rotor test.