STAR: Smart Twisting Active Rotor
Smart rotor blades featuring flaps, trailing edge extensions, morphing tips, active twist, etc. are currently studied to provide better performance over conventional designs. In the UK, the trailing edge flap has been extensively studied as part of projects like REACT and RTVP of Leonardo Helicopters. In Europe, the Clean Sky 2 project also considered main rotor blades with flaps of the Gurney type. In the US, model scale rotors with several trailing edge flaps have also been studied aiming at noise and vibration reduction.
The STAR project (Smart Twist Active Rotor) is an international effort to study active twist blades. A conventional BO-105 planform is used and fitted with piezoelectric Macro Fibre Composite (MFC) actuators, which can change the twist in a static way (hover) and dynamically (forward flight) at multiples of the rotor rotational frequency. Glasgow University is working with the STAR team to deliver high fidelity aeroelastic modelling.
The University of Glasgow leads the pre-test prediction effort of the high-speed rotor flight condition in the STAR project. It was found that a 2/rev active twist can improve the rotor efficiency and reduce the hub vibrations. With a 0/rev static increase of blade twist, the vibration worsened. Results contributed to the high-load dynamic stall condition, predict a significant improvement of rotor efficiency and vibration with 2/rev active twist over the passive rotor. Increased static blade twist worsened the rotor performance and vibration.
The experimental rotor blades have completed whirl tower and bench testing in 2023 and will be tested extensively at the DNW-LLF (German-Dutch Wind tunnels - Large Low-speed Facility).
Fig. 1: (a) STAR blade photograph in the whirl tower of DLR. The piezoelectric actuator patches cured onto the blade skin. (Kalow., S., et. al. (2017) Experimental investigation and validation of structural properties of a new design for active twist rotor blades. In: 43ʳᵈ European Rotorcraft Forum, Milan, Italy.) (b) Q-Criterion and Vorticity magnitude of the hovering rotor simulation in HMB3.
Fig. 2: STAR blade aeroelastic simulation results of HMB3 in highly-unsteady forward flight at maximum lift and advance ratio µ=0.3. Primary and secondary blade tip vortex (BVI) interactions can be observed in the unsteady pressure-coefficient contours and are best visible around Ψ=0°.
Aero-servo-elastic modelling
The STAR blades are designed to maximise the actuator effectiveness, with a very soft structure. Aeroelastic modelling is therefore absolutely necessary for good simulation results. Most project partners are applying a 1D-beam model approach using cross-sectional properties.
A 2D/3D-combined FEM structural model of the STAR has been created at UofG and implemented in HMB3 to achieve a novel high-fidelity aeroservoelastic simulation of the piezoelectric, actuated rotor. The on-blade actuators are modelled using a thermal analogy method (TAM). Instead of applying a torsion moment on a 1D-beam, the piezo materials are embedded and have a thermal expansion coefficient, which is equivalent to the piezoelectric coefficient. This allows to accurately predict the blade response and stiffening when voltage is applied to the blade. A large effort of the PhD project was spent on this topic, as the geometry and structural properties of the blade resulted in a difficult problem for the structural solver, which needed to be overcome.
The structural model with the thermal analogy is directly included in the aero-servo-elastic coupling loop of steady hover cases in HMB3. After a set number of CFD time-steps, either the structural displacement, the pitch trim or both are automatically updated. The actuator voltage can also be adjusted. Additionally to the aeroelastic modal method, where structural amplitudes are computed in HMB, coupling structural simulation in NASTRAN is supported in unsteady flow. This uses a loosely coupled approach, similar to delta-airloads. The feature will be demonstrated using the harmonic balance method for time-discretisation and transient analysis of MSC NASTRAN.
Fig. 3: Structural FEM model of the STAR blade tip region, with materials of the: (a) outer shell, (b) inner volumes indicated by colour.
Fig. 4: Thermal Analogy Method applied to the cantilevered STAR blade, from -500V to +1500V. The uneven distribution of strain through the different materials can be observed in the colour contours.
Aeroelastic Hover Validation
In addition to the STAR, where experimental data will only be available after the duration of the PhD, other blades were simulated. The aeroelastic hover methodology has been validated on the very similar HVAB (Hover Validation and Acoustic Baseline) rotor. The HVAB, part of the AIAA HPW (Hover Prediction Workshop) is the highest fidelity hover benchmark case in available literature. It was tested in 2023 at the NASA Ames NFAC facility. The fully turbulent k-ω SST turbulence model by Menter was used in HMB3 to predict the hover figure of merit curve to within 1 count of the experiment with tripped flow. The integrated loads of thrust and moment coefficient matched accurately with the pressure-measurements. The blade deformations were mostly matched to the experiment. This is because the available structural properties have a larger error margin, and similar differences were seen in other prediction works.
Fig. 5: (a) HVAB figure of merit prediction of rigid and elastic blades, at nominal and increased twist compared to the experiment. (b) Sectional moment coefficient. (c) Aeroelastic flap deflection. (d) Aeroelastic blade torsion. (Norman, T. R., et. al. (2023) Fundamental test of a hovering rotor: Comprehensive measurements for CFD validation. In: Vertical Flight Society 79th Annual Forum and Technology Display, West Palm Beach, FL, USA.)
Benefits of Active Twist
From the literature review and the current numerical study, active twisting blades promise some improvements in efficiency, especially in high-speed flight. In hover, a four degree increase in blade twist resulted in 2-4 counts figure of merit improvement, depending on the base twist level and geometry.
In the tested forward flight conditions, the Lift-to-drag equivalent ratio was increased by up to 1%, which covers the additional energy cost of the system. However, large hub vibration reductions could be observed, by better distributing the lift over the full blade revolution. Up to 22% reduction in overall vibration index was found. The system can also be tailored in phasing and amplitude, to specifically eliminate some hub vibration contributions at the cost of slighly increased vibration in some other metrics. With the current trend towards higher twisted rotor blades, which were shown to drastically increase vibration in high-speed flight, this system could offset the disadvantages of high-twisted blades, while retaining all benefits. Using 2/rev active twist, the dynamic stall on the retreating blade could be decreased at maximum rotor lift, aiding the rolling moment trim.
Fig. 6: High-speed flight simulation results of the STAR with nominal twist, increased twist, and 2/rev 210° active twist. (a) Lift-to-drag, (b) Vibration Intrusion Index, (c) Vibration Index breakdown.
Fig. 7: Comparison of sectional moment coefficient in a maximum thrust, high-speed condition to showcase dynamic stall. (a) nominal twist, (b) increased twist, (c) 2/rev 0° active twist. The active twist significantly reduces retreating blade stall, improving rolling moment trim and load carrying capacity.
Fig. 8: Vorticity magnitude slices of the retreating side blade in the maximum lift, high speed case. The blade vortex interaction effects on the boundary layer are shown. The reversed flow region inboard is highlighted.
Publications
In Journals
In Conference Proceedings
This project at the University of Glasgow is funded by the Defence, Science and Technology Laboratory [dstl] of the UK, under contract number DSTLX10000129255.
Project Sharepoint (authorized users access only)
Partner Institutions
DNW - German-Dutch Wind Tunnels
JAXA - Japan Aerospace Exploration Agency
KARI - Korea Aerospace Institute
NASA - National Aeronautics and Space Administration
ONERA - The French Aerospace Lab
Contact
George Barakos, Professor & Supervisor
Rinaldo Steininger, PhD Student