The tensile fatigue behavior of a [0/ ± 45]s carbon/epoxy laminate with and without tightly consolidated epoxy-impregnated carbon nanotube (CNT) interlayers is evaluated. The interlayers contain either imperfectly aligned CNT yarns or aligned CNT films. Optical edge micrographs and measurements of loss factor, quasi-static elastic modulus, and tensile strength are compared before and after 106 cycles of fatigue loading at 3,000 or 4,000 με. X-ray radiographs were also recorded after 106 cycles. Though specimens with CNT interlayers had virgin tensile strengths similar to specimens without CNTs (baseline), the CNT interlayer specimens had less resistance to fatigue damage than the baseline. All the experimental methods employed could detect damage, though the direct imaging methods (edge microscopy and X-ray radiography) missed some early damage mechanisms, while quasi-static elastic modulus and especially loss factor measured below roughly 50

Carbon fiber reinforced epoxy composite stiffened panels are increasingly being used for structural components in large transport rotorcraft. However, problems are arising with high levels of vibration and interior noise due to the increased stiffness-to-density ratio of composites. The current investigation explores the potential of reducing vibrations in carbon/epoxy stiffened panels with the integration of acoustic black holes (ABH), namely features that incorporate a power law thickness taper. The proposed approach involves designing a taper into the thickness of the blade stiffeners as well as the thin plate. Integration of ABHs into the fuselage structure has the potential to reduce broadband vibrations. Multiple parametric studies with either an ABH integrated into the blade stiffener or a grid of ABHs integrated into the plate were conducted, and the tradeoffs between vibration amplitudes, panel mass, and compressive buckling load were examined. Carbon/epoxy panels were fabricated using vacuum-bag-oven processing with out-of-autoclave prepreg and verified to be of good quality. The integrated velocity response, a proxy for the radiated noise from a panel, and compressive buckling were simulated using finite elements. Comparisons were made to experimentally measured data from modal testing and compression buckling testing. Experimental results indicated that when an ABH is integrated into the blade stiffener and 15 ABHs are integrated into the plate in a grid configuration, the panel mass was unchanged, the integrated velocity response decreased by 2.82 dB, and the buckling load increased by 2.9% compared to a baseline non-tapered design.

Vibration damping in aerospace structures can decrease the likelihood of failures and instabilities and improve passenger comfort. This paper introduces the novel idea of damping vibration using the electric proprotors on aircraft without compromising flight control. The equations of motion of a cantilevered beam with a propeller at the tip driven by an electric motor are obtained using Hamilton’s principle, solved analytically in the frequency domain, and approximately in the time domain. Feeding back the beam tip angular rate to the motor torque is shown to asymptotically stabilize all transverse beam vibration modes. The overall vibration control consists of the inner rate feedback damping loop with an outer rotor speed control loop. Experimental frequency response and step response validate the models and show that the closed-loop damping in the first mode is three times higher than open loop with less than 1% rotor speed change for a 3% initial tip displacement. Theoretical results give good agreement with experiments. Parametric studies based on a 12 kg quadcopter indicate that if the rotor inertia is sufficiently large the proposed control can provide 8% damping with minimal impact on rotor speed.

Over 4 decades of research works on the nutating, now pericyclic, mechanical transmission have studied its capability to achieve high power density, low noise, and amplified single-stage reduction ratios of up to 100:1. These analytical efforts have culminated into the fabrication of a 50 HP and 32:1 reduction ratio pericyclic transmission prototype. This work introduces the prototype with highlights of the assembly and alignment procedures validated by static testing evaluation. Then, discussion of the dynamic test stand integration, instrumentation, and lubrication components lay out the framework of the high-speed testing plan. Power transmission data validated the pericyclic reduction ratio model. Accelerometer data demonstrated the transmission's capability to operate at low vibration, with peak amplitudes of 1.2 and 2.5 inches per second on the pericyclic gear train and output shaft respectively. Acoustic emission data captured the first 5 harmonics of the shaft speed as well as gear mesh frequencies. The thermal profile showed the shaft bearings remained below 180°F throughout testing, implying safe operating conditions. Finally, strain and vibration data showed the pericyclic gear train maintained load sharing throughout the entire operating envelope, further validating static testing and assembly procedures. The results of the experiments demonstrate the technological readiness of pericyclic mechanical transmission.

Electric Vertical Takeoff Landing (eVTOL) aircraft feature heavy electric motors, battery packs, and rigid fixed-pitch rotors supported on flexible arms. Under substantial time-varying aerodynamic loads associated with variable rotor speeds and, with low intrinsic damping, such lightweight arms respond in bending and torsion at relatively high levels. In this paper, two methods of reducing vibration response in the operating frequency range are explored, one based on damping, the other on stiffness. A tailored particle impact damper system was evaluated experimentally to address near-periodic vibration over a range of frequencies. A forced torsional response test showed consistent 50% vibration reduction, with a 5% mass penalty. To stiffen the system, a cross-braced strut approach linked two arms such that the natural frequencies of their torsion modes would be increased beyond the rotor operating frequency range. A finite element model was developed and validated for a representative eVTOL configuration. Validation was conducted using a scale model aluminum beam set. The addition of a cross-braced strut efficiently stiffened the system, increasing its natural frequency by almost 120%, thus greatly reducing resonant torsional vibration within the operating range. Both approaches to vibration reduction for variable-speed eVTOL aircraft merit continued consideration and research.

Hybrid-electric propulsion could provide numerous benefits for full-size rotorcraft, including reduced peak turbine power demand, reduced transmission system weight and complexity, and reduced operating costs. Variable speed electric motors, furthermore, could be configured to enable continuously variable rotor speed. Achieving these benefits requires accounting for coupling between the hybrid-electric drivetrain and vehicle performance within a large, unexplored design space. This paper presents a framework for simultaneous optimization of vehicle and electrified powertrain conceptual design using Geometric Programming (GP) methods. Four hybrid-electric powertrain architectures are evaluated relative to a baseline non-electrified powertrain for single main rotor, compound coaxial-rotor, and tiltrotor configurations. For designs with an upper limit on turbine power, electrification increases the maximum cruise speed for the compound coaxial-rotor configuration. Variation of the rotor speed by 15% allows the vehicle to carry 8% more fuel, relative to the non-electrified baseline, and 1,246 lb of battery. Operating the rotor at optimal speeds across the mission results in increased off-design mission performance, most notably a 43% increase in transport radius relative to a baseline powertrain. The results demonstrate the utility of the design optimization framework for exploration of novel hybrid-electric concepts as well as the challenges associated with incorporating electrical components into the drivetrain.

Rotorcraft experience significant vibrations due to periodic aerodynamic forces and moments on the rotor blades and wings. Rotor torque damping is a novel vibration damping method which uses small torque perturbations from the main electric motor to reduce vibrations. The large inertial and aerodynamic rotor loading and relatively high frequency torque perturbations mean that the rotor speed changes are small, so the rotor thrust and flight control performance are not significantly affected. This paper investigates the application of electric motor torque control for damping structural vibrations of an aircraft. The structural dynamics of the aircraft are represented using a finite element model of a quad tiltrotor eVTOL. Using collocated angular rate feedback on all four rotors provides more than 10% damping in controllable modes. The RMS value of flap-wise angular rate can be reduced by 91% with less than 1.2 RPM rotor speed change in response to a 20% vertical step gust in airplane mode. For N/rev disturbance cancellation, an optimal controller is designed assuming known disturbance location and frequency for active vibration control (AVC). The transfer matrix of a single cantilevered wing is calculated and used to feed forward harmonic rotor torques. The wing undergoes aerodynamic disruptions at N/rev and the harmonic controller reduces N/rev shear force and bending moment at the root by 20% and 58% with less than 1 RPM rotor speed change, respectively.

In this paper, we consider the design optimization of electrically-augmented gearbox concepts for different electrified aircraft propulsion applications. Two systems are considered: one with two independent electric machines coupled through a separate drive trains to the low- and high-pressure spools of a turbofan engine, the other the Versatile Electrically Augmented Turbine Engine (VEATE) gearbox, including an additional electric machine and coupling of the two shafts through a variable-speed epicyclic gear train. Design of the systems is formulated as a numerical optimization, with different sets of input parameters for different applications of the system, including electrical power boost, power extraction, and turbine electrified energy management (TEEM). The results shows the masses of the designs considered range from 0.97%-3.1% of total engine mass, depending on the application of interest. The results also show the electrical components account for more than half the total system mass, which is most sensitive to efficiency and power density of these components, as well as the maximum power and torque requirements of the system.

Adding multiwalled carbon nanotubes (CNTs) to polymer composites such carbon fiber reinforced epoxies (c/ep) has the potential to add beneficial damping in vibration prone structures, such as rotorcraft blades, because of slip at the CNT/epoxy interface and between the multiple walls of the CNTs themselves. Recent research has demonstrated remarkable damping improvement by incorporating CNT-rich interlayers into c/ep laminates, and there is current interest in flight testing this damping approach in rotorcraft blades. In order to avoid the time and cost of developing and flight-certifying an entirely new blade material with CNT interlayers, it has been proposed to bond a CNT-rich patch onto the surface of an existing blade. Thus, this investigation compares and analyzes damping in three types of cantilevered [±45/0]s c/ep beams: without any CNTs; with CNT interlayers; and with CNT layers co-bonded to the surface. The CNT layers were used in 64 g/m2 pairs, covering 20% of the length of the beam near the clamped end, and comprising about 1 vol% of the cantilevered portion of the beam. Damping ratio, , was measured during free vibration in a vacuum chamber. The damping ratio increased from 0.65% in a plain c/ep beam to 0.93 (+43%) and 1.24% (+91%) by adding CNT interlayers and externally bonded CNTs, respectively. Dynamic finite element (FE) analysis of the beams confirmed these results and was further used to parametrically investigate the damping attainable with CNT surface films covering higher lengths of the beam or stacked to higher thicknesses at the root of the beam. The parametric investigation showed that increasing the length of the CNT surface film is the most efficient use of CNTs and that a damping ratio 277% greater than that of the beam without CNTs could be realized. In conclusion, externally bonded CNT layers offer a promising avenue for the rapid, cost-effective flight testing of CNT-augmented damping in composite rotorcraft blades.

This paper introduces additively manufactured plate stack isolators capable of broadband vibration isolation over multiple frequency ranges. The isolator consists of axisymmetric stepped plates stacked in series and connected using compliant hinges. The monolithic design eliminates friction, leading to deeper stop bands and a potentially larger cycle life. A Kirchhoff–Love plate model predicts the frequency response. Additively manufactured polymer plate network isolators show that multiple band gaps can be achieved by model based tuning of plate geometries. A sensitivity study shows that additive manufacturing tolerance errors affect the width and center frequency of the band gap. Finally, the experimentally validated model predicts that metal isolators manufactured with Laser Powder-Bed fusion are capable of achieving low frequency band gaps and other design metrics of commercially available vibration isolators.

Performance of ball bearings in the motion converter subassembly of an internally driven, single-speed, torque-split, twin configuration pericyclic transmission prototype is evaluated to extend the analytical knowledge base on this innovative transmission concept. A dynamic model of the transmission is developed with high-fidelity models of the installed rolling element bearings to determine their reactions. Attention is focused on the pair of ball bearings supporting the motion converter subassembly which are subjected to a complex combination of loads, including radial and axial forces, moments, carrier motion, and possibly internal preload. Then the influence of internal axial clearance and preload on the behavior of the rolling elements is analyzed with a fully dynamic ball bearing model. Provisions to consider the carrier motion and a robust integration algorithm for component orientations are presented. Finally, a microstructure-based fatigue life simulation of the critical bearing component is performed to demonstrate the effect of clearance/preload on bearing reliability.

This paper modified the Dynamic Antiresonant Vibration Isolator (DAVI) with a flexible lever, reducing weight and/or isolation frequency. An analytical model predicts that a compliant lever design can provide the same isolation frequency as that of a rigid DAVI but with half the auxiliary mass. The analytical model also shows that for the same auxiliary mass, compliant levers can provide lower isolation frequencies compared to DAVIs with rigid levers. The proposed isolator includes monolithic compliant features that are realizable with additive manufacturing, which reduces friction and wearing losses compared to traditional isolators. Two fabricated and tested devices validate the model and demonstrate the improved performance of a flexible design.

The objective of the current investigation is to evaluate the dynamic mechanical properties of a hybrid carbon fiber reinforced epoxy composite laminate containing CNT-rich interlayers, where the CNTs were prepared with ten different surface treatments and two different degrees of alignment. The surface treatments, including surfactants and oxidation, alter the CNT/epoxy stress transfer characteristics. Testing was done in uniaxial tension at different temperatures, loading frequencies and in dry and wet conditions. In the case of imperfectly aligned CNT yarn interlayers, the best surfactant, which was Triton X-100, increased the loss modulus by 182% over the baseline (no CNTs) and increased the storage modulus by 2.2%. Oxidation of the CNTs increased the loss modulus by 219% and the storage modulus by 2.3%, versus the baseline. Highly aligned CNT film provides higher loss modulus (+44%) and storage modulus (+8.4%) than imperfectly aligned CNT yarns, for the case of no surface treatment. Alignment of the CNTs also reduces the sensitivity of dynamic properties to temperature change and moisture content. CNT interlayers reduce the frequency dependency of the dynamic properties. Improvements in damping with surface treatments are hypothesized to be due to improved epoxy impregnation of the condensed CNTs.

Vibration damping in rotorcraft structures can reduce failures and instabilities and improve the ride comfort for passengers. This paper introduces the novel idea of damping vibration using electric proprotors on eVTOL aircraft without compromising the rotors ability to provide thrust. Feeding back the beam tip angular rate to the motor voltage is shown to stabilize all transverse beam vibration modes. The experimental results show that the closed loop damping in the first mode is three times higher than open loop. The torque bandwidth of the electric motor exceeds 100 Hz so the damping performance on the first mode (5.6 Hz) is very good. Damping on the second mode, however, is not improved due to the 40 Hz bandwidth of the angular rate sensor. The rotor speed frequency response rolls off at 20 dB/dec, indicating smaller vibration induced rotor speed variations at high frequency. Experimental step response results match the frequency domain damping predictions and show only 0.8% rotor speed variation for a 3% initial tip displacement.

The ability to understand speech in complex environments depends on the brain’s ability to preserve the precise timing characteristics of the speech signal. Age-related declines in temporal processing may contribute to the older adult’s experience of communication difficulty in challenging listening conditions. This study’s purpose was to evaluate the effects of rate discrimination training on auditory temporal processing. A double-blind, randomized control design assigned 77 young normal-hearing, older normal-hearing, and older hearing-impaired listeners to one of two treatment groups: experimental (rate discrimination for 100- and 300-Hz pulse trains) and active control (tone detection in noise). All listeners were evaluated during pre- and post-training sessions using perceptual rate discrimination of 100-, 200-, 300-, and 400-Hz band-limited pulse trains and auditory steady-state responses (ASSRs) to the same stimuli. Training generalization was evaluated using several temporal processing measures and sentence recognition tests that included time-compressed and reverberant speech stimuli. Results demonstrated a session × training group interaction for perceptual and ASSR testing to the trained frequencies (100 and 300 Hz), driven by greater improvements in the training group than in the active control group. Further, post-test rate discrimination of the older listeners reached levels that were equivalent to those of the younger listeners at pre-test. Generalization was observed in significant improvement in rate discrimination of untrained frequencies (200 and 400 Hz) and in correlations between performance changes in rate discrimination and sentence recognition of reverberant speech. Further, non-auditory inhibition/attention performance predicted training-related improvement in rate discrimination. Overall, the results demonstrate the potential for auditory training to partially restore temporal processing in older listeners and highlight the role of cognitive function in these gains.

Variable rotor speed technology implemented in a helicopter can improve the flight performance, reduce the required power, and increase the flight speed. However, variable rotor speed changes the frequencies of rotor vibratory loads and may produce helicopter fuselage resonance under the excitation of the rotor vibratory loads. Active vibration control (AVC) has been effectively used in vibration reduction of helicopter fuselages. However, the frequency domain control algorithms that are currently used have poor adaptability in controlling vibration with variable frequencies (i.e., during time varying rotor speeds). In order to effectively improve control convergence, adaptability, and effectiveness, the normalized adaptive hybrid control algorithms containing both the normalized adaptive harmonic control algorithm and the normalized frequency tracking algorithm have been presented in this paper. Simulations of AVC with variable frequencies on a dynamically similar frame structure of a helicopter fuselage driven by piezoelectric stack actuators installed on the gearbox support struts show that the normalized adaptive hybrid control algorithms can accurately track the changes in rotor load frequencies and can be effectively used in the AVC of a helicopter with variable rotor speed.

The focus of this work is to integrate component-level design analyses developed for different machine elements of a twin pericyclic drive into a comprehensive design decisions framework. The integrated system loads, bearing loads, and tooth contact analysis procedure is used for designing a prototype for minimum weight within the constraints posed by assembly, component life, and system efficiency. Simultaneous sizing of the gears, bearings, and shafts was performed for given input power, speed, and reduction ratio. The effect of inertial loads due to nutational gear motion is significant on support bearing loads, and the gear bodies are designed to minimize these loads. It was demonstrated that a torque density greater than 50 Nm/kg can be achieved for a low Technology readiness level (TRL) pericyclic transmission prototype design. The test article is designed to operate at a 50-HP, 5000 RPM input with a speed reduction ratio of 32:1 and system efficiency greater than 93%.

This paper introduces additively manufactured beam networks that isolate vibration with multiple broad bandgaps. The isolators consist of symmetric stacks of beams with tip masses connected in series via compliant hinges near nodal points. An analytical model is derived to predict the vibration behavior of a beam stack. An isolator with four beams is designed to provide a band gap of 32 Hz with a center frequency at 72 Hz. A Polylactic acid polymer prototype is 3D printed and the frequency response of input force to transmitted force is experimentally measured showing a large 41 Hz band gap with a center frequency also around 72 Hz. The experimentally validated model is used to design a 10 beam isolator with a low frequency band gap from 10 to 86 Hz, and a high frequency band gap from 202 to 543 Hz.

Carbon nanotubes (CNTs) embedded in carbon/epoxy (c/ep) composites offer a lightweight, stiff solution with high damping for structural components. To manufacture CNT/c/ep hybrid composites with high concentrations of CNTs, a CNT yarn interlayer concept is used. Through a mechanism known as stick-slip, the interface between CNTs and c/ep laminates dissipate energy during dynamic cycling. It is predicted that altering the interfacial bond strength can enhance dynamic properties: loss factor, loss modulus, and storage modulus. In this research, the effect of surface treatments on CNT yarns prior to inserting the CNTs into the laminate was explored to determine if the dynamic properties of the composite were enhanced. It was determined that 2,3-dibromo-1,4-butanediol (23D14B), Triton X-100 (TX) and a solution of sulfuric and nitric (S/N Acid) were the best treatments of 12 tested based on the increased dynamic properties. For a composite with 5 vol% CNT yarn interlayers, 23D14B, TX, and S/N Acid increased Loss Factor by 230%, 130%, and 160%, respectively. Elevated temperature, moisture-saturated/elevated temperature, and cyclic loading frequency were investigated on 5vol. % CNT interlayers to determine their effects on dynamic properties of the CNT hybrid composites.

An acoustic black hole (ABH) plate is a lightweight and high loss panel structure for effective reduction of vibration and radiated sound. It is understood that the high loss local ABH modes can be designed at desired frequencies by changing the size of the ABH cell(s). The ABH cell diameter (size) and minimum thickness play dominant roles in the performance of the ABH effect. In addition, attaching tuning masses at the center of the ABH cells has been shown to alter the local ABH modes with the result of improved low-frequency performance. In this work, the transmission loss (TL) of an embedded multi-scale ABH plate was investigated. The embedded large and small ABH cells were particularly designed to cut-on below and above the critical frequency of the plate, respectively. The results were compared with a uniform plate and an embedded single-scale ABH plate. Discrete tuning masses were attached at the ABH cells' center to manipulate the ABH cut-on modes to increase the TL further. The results show that the damped multi-scale ABH plate achieved a 10 dB TL increase, flattened the TL curve, and nearly eliminated the plate coincidence dip. Manipulating the high loss ABH modes by adding tuning masses (20 g each) demonstrated a 2 dB increase at low frequencies within the mass-law range. Although damping material was applied, adding some mass, an overall weight advantage was still attained compared to the uniform plate. The damped multi-scale ABH plate is 7% lighter than the uniform plate.