Andreas P. F. Bernhard

Analysis of a bending-torsion coupled actuator for a smart rotor with active blade tips

Active rotorblade tips offer an alternative approach to the challenge of main rotor active vibration control. The tips are pitched with respect to the main blade via a piezo-driven bending-torsion coupled actuator beam that runs down the length of the blade. A Vlasov based, specialized one-dimensional finite beam element is developed to model the rotating actuator beam and is validated with the free-vibration and static forced response of 4:1 and 2:1 aspect ratio, bending-torsion coupled, active and passive plates. A one-eighth scale, reduced tip-speed rotor model (tip Mach 0.26), incorporating the bending-torsion actuator beam, has been previously hover tested (open loop). In these tests, blade tip deflections of the order of 2° (half peak-to-peak) were achieved at 2, 3, 4, 5/rev with corresponding dynamic vertical blade root shear variations of the order of 10-20% of the nominal blade lift at 8° collective (CT/σ = 0.07). The test results are used to validate a coupled actuator and elastic rotorblade model. The correlation of the predicted active blade-tip pitch deflections and the experimental data is within 20%. The predicted values for the active vertical root shears are within the same margin for 4° and 6° collective.

Hover Testing of Active Rotor Blade-Tips Using a Piezo-Induced Bending-Torsion Coupled Beam

This paper presents the development of an active on-blade vibration-reduction system using smart active blade tips (SABT), that are driven by a piezo-induced, bending-torsion coupled actuator. The actuator beam has a graphite substructure with surface bonded piezoceramic elements. A spanwise variation in both the bending-torsion coupling and the piezo element phasing is used to generate a pure tip twist. A small scale rotor, with 10% span active tips, was tested on the hover stand, at a reduced tip speed of Mach 0.25. At a mean thrust loading (CT/σ) of 0.07, and for an activation of 100 Vrms, SABT deflection amplitudes from 1.8 deg at 2/rev to 2.25 deg at 4/rev were achieved (half peak-to-peak). The rotor normal force measurements show a distinct coupling of the activation with the first and second flap frequencies of the rotor. The corresponding dynamic thrust, generated by a single active tip, relative to the steady thrust, ranges from 4.5% at 2/rev to 8.3% at 5/rev. For a 1/rev excitation, the single active tip generates a dynamic lift amplitude of 15% of the steady rotor thrust. The same actuator beam was used to test a rotor with controllable twist blades. The active twist blades used the same main blade section as the rotor blades with the active tips, and hence were not optimized for active twist configuration. Nonetheless, in hover, at a mean thrust loading (CT/σ) of 0.07, and with an activation of 100 Vrms, dynamic tip-twist amplitudes of 0.4 deg at 4/rev and 0.5 deg (at 5/rev) were achieved (half peak-to-peak).

Development of a smart moving blade tip activated by a piezo-induced bending-torsion coupled beam

This paper presents an active moving blade tip for a smart rotor with torsional actuation via a piezo-induced bending-torsion coupled composite beam. A novel spanwise variation in the beam layup and piezoceramic element phasing is used to maximize the twist response, while minimizing the bending response. The composite beam is located lengthwise within the blade with surface bonded piezoceramic elements excited to induce spanwise and chordwise bending of the beam. The bending results in an induced twist via the structural coupling and this twist deflects the moving tip. A proof-of-concept actuator beam was developed and tested. Tip twist with essentially zero tip bending deformation was demonstrated. Two operational 1/8th scale model rotor blades were manufactured for hover testing. In non-rotating calibration tests, at a 60 Hz actuator excitation, a peak to peak moving tip deflection of 4 deg was achieved.

Hover Test of Mach-Scale Active Twist Rotor Using Piezo-Bending-Torsion Actuators

The active twist rotor investigated in this research is a derivative of the previously developed smart-activeblade-tip (SABT) rotor. On the SABT rotor, the blade tips are independently pitched, with respect to the main blade. A novel piezo-induced bending ‐torsion coupled actuator beam, located spanwise in the hollow midcell of the main rotor blade, is used to actuate the blade tip. When the blade tip is locked to the main blade, the actuator beam twists the entire blade. A Mach scale rotor with a 1.542 m diameter was hover tested, open loop, to evaluate the control authority for vibration reduction. A nonrotating tip-twist amplitude of 0.78 deg was achieved (below resonance, 150 V rms ). Analysis indicates that no signie cant twist actuation degradation is expected at full rotor speed. In 2000-rpm hover (tip Mach 0.47 ), at 8-deg collective, and for a single blade actuation of 150 V rms at 1, 2, and 3 per revolution, respectively, the measured oscillatory thrust coefe cients are 1 :4 £ 10 3 , 0:55 £ 10 3 , and 0:7 £ 10 3 . The corresponding e nite element model estimated blade twist amplitudes are 0.8, 1.0, and 1.9 deg. Good correlation of the predicted and measured rotor thrust was achieved up to 3 per revolution. The hover test demonstrates the potential of the active twist rotor system using an internal actuation beam and warrants further research for a dedicated next-generation model-scale design and full-scale feasibility study.

Hover testing of an active rotor blade tip and structural analysis of the actuator beam

This paper presents the continued development of an active on-blade vibration-reduction system using smart active blade tips (SABT), that are driven by a piezo-induced bending-torsion coupled actuator. A Vlasov based, specialized one-dimensional finite bar-element is developed to model the (nonrotating) actuator beam and is validated with the free-vibration and static forced response of 8:1 and 2:1 aspect ratio bending-torsion coupled plates. The FEM over-predicts the actuator torsional frequency by 4%, and accurately captures the actuator dynamics up to 60Hz. In hover, at thrust coefficient of 0.0035, and for an activation of 100Vrms the SABT deflection ranges from l.Sdeg at 2/rev to 2.25deg at 4/rev. The results show a distinct coupling of the activation with the first and second flap frequencies of the rotor. The corresponding dynamic thrust, generated by a single active tip, relative to the steady thrust, ranges from 4.5% at 2/rev to 8.3% at 5/rev.

Development of Mach Scale Rotors with Tailored Composite Coupling for Vibration Reduction

This paper presents the design, structural and aeroelastic analyses, fabrication, and structural tests of Mach-scale articulated composite rotors with tailored flap-bending/torsion couplings for vibration reduction. The rotor design was nominally based on the UH-60 Black Hawk rotor. A new fabrication process was developed to manufacture Mach-scale composite tailored blade. Five sets of Mach-scale composite tailored rotors were successfully fabricated in-house, including a baseline rotor without coupling, two rotors with uniform spanwise flap-bending/torsion couplings, and two rotors with spanwise segmented flap-bending/torsion couplings. Bench-top and nonrotating dynamic tests were performed to verify the structural analysis of these composite tailored blades. The measured results correlated well with predictions. Comprehensive aeroelastic analysis (using University of Maryland Advanced Rotorcraft Code) of these Mach-scale composite tailored rotors indicated that blade elastic flap-bending/ torsion couplings can significantly impact rotor vibration characteristics. It was shown that spanwise segmented flap-bending/torsion couplings can provide larger rotor vibration reduction than uniform spanwise couplings.

DESIGN AND HOVER TEST OF LOW VIBRATION MACH SCALE ROTOR WITH TWISTED COMPOSITE TAILORED BLADES

Several different Mach scale model rotors with composite tailored blades were designed for reduction of vibratory hub loads. Comprehensive aeroelastic analysis shows that a non-uniform spanwise segmented composite flap-bending/torsion coupling distribution can lead to reduced vibratory hub loads in forward flight. Two Mach scale rotor sets with pre-twisted composite tailored blades (baseline and optimum coupling) were fabricated. Static bench-top tests and dynamic tests were carried out to validate the structural analysis of these composite blades. The rotors were tested up to the nominal rotor speed of 2300rpm (with tip Mach number 0.65) on the hover stand. Predicted rotor thrust matched the test data in hover.