Browse Topic: Electric motors
Emerging technologies in the field of electrified propulsion systems offer a promising solution to reduce the dependence on fossil fuels and improve efficiency. However, the design of high-power density electric machines introduces new challenges, including limited passive cooling potential and the issue of the weight of electric motors. To address these challenges, this paper considers analysis and design methods for high torque-to-weight ratio axial flux motors. A magnetic equivalent circuit model coupled with a lumped parameter thermal network is developed for design space exploration and optimization. This inexpensive analytical model predicts the performance of a single-stator dual-rotor axial flux motor based on geometry, loading condition, and slot and pole pair combination. To enable comparisons against real-world data, the optimization study was demonstrated using the hover mission requirements from the Research Aircraft for eVTOL Enabling techNologies (RAVEN) vehicle to minimize the mass of the motor. In tandem with the analytical model, a higher-fidelity finite element model was also developed, and good agreement between predicted power and efficiency was demonstrated across a range of axial flux motor designs. The lightest weight design that satisfied the hover mission requirements was the 12 pole pair 27 slot (12PP 27S) configuration with a fixed weight of 9.28 kg. The analytic model undersized the output power of the electric motor by approximately 9% across a range of slot and pole pair combinations.
Vertical Take-Off and Landing (VTOL) aircraft introduce complex monitoring challenges due to distributed propulsion, lightweight structures, and variable operating conditions. This paper presents advanced Frequency and Orders domain techniques that repurpose existing flight control, propulsion, and structural sensor data to enhance observability without additional instrumentation. By transforming vibration, acoustic, and electrical signals into frequency and order domains, the approach enables detection of harmonics, resonance, and fault signatures tied to rotor dynamics, supporting adaptive control and predictive maintenance. Beyond rotor systems, these techniques are equally effective for monitoring electric motor health, gearbox wear, bearing degradation, and structural coupling effects in composite airframes. They also provide insight into power electronics and thermal management systems by identifying spectral anomalies linked to electrical imbalance or cooling inefficiencies. Aggregated fleet data strengthens prognostic capabilities, enabling early detection of systemic issues and trend analysis. Applications include mitigating ground resonance and modal instabilities, as well as improving reliability of propulsion and structural subsystems. Integration into avionics emphasizes computational efficiency, scalability, and compliance with standards such as DO-160 [1], DO-178 [2], ARP4761 [3] and ARP4764 [4]. Simulation and bench testing confirm feasibility, demonstrating potential to enhance safety, reliability, and lifecycle cost for next-generation urban air mobility platforms.
This paper, explores the design and sizing of a planetary gear-based electronic continuously variable transmission (ECVT) for implementation of a parallel gas-electric hybrid helicopter propulsion system. The ECVT consists of a differential planetary gear transmission (PGT) and an electric motor/generator (MG) unit. The ECVT enables power-flow between engine, motor and helicopter main rotor. The parallel arrangement enables the main rotor speed to varied continuously based on the MG speed while the engine speed can remain constant. The performance benefits enabled by the main rotor speed variation capability are offset by the added weight penalties introduced by the ECVT system. By considering factors such a as gear tooth bending and contact stress, bearing loads, required motor torque, planetary gear kinematics and pitch-line velocity constraints, this paper conducts a minimum mass design study for several PGT / ECVT arrangements. Here, three different single stage PGT/ECVT arrangements are compared along with an improved two stage ECVT. The three single stage ECVT configurations can be summarized as; I) Sun-Engine / Carrier-Motor / Ring-Out, II) Sun-Engine / Ring-Motor / Carrier-Out, and III) Carrier-Engine / Sun-Motor / Ring-Out. Of these three types, it was found that type III was significantly lighter in weight compared with types I and II since type III would have the highest relative motor speed. When sized for a 3000 Hp engine-side power input at 6000 rpm, the minimum mass design for type III was on the order of 100 lbs compared to 400 lbs and 700 lbs respectively for types I and II. Despite the seemingly obvious advantage of design type III, it's drawback is that it is effectively a speed increasing stage with respect to the engine. To address this, a two-stage ECVT with compound planetary arrangement of Type III and II was designed which achieved an overall minimum weight of 219 lbs at the 3000 Hp level while providing 1:0.351 gear reduction form engine to output. The analysis tools developed and sizing results flowing from this study will provide a baseline for evaluating performance benefits and weight penalties introduced by parallel hybrid drive-systems for rotorcraft applications.
Electrification could improve full-size rotorcraft performance by reducing peak turbine power demand, reducing transmission system weight and complexity, and reducing operating costs. Integrating electric machines with mechanical powertrains requires careful consideration of the system-level weight and efficiency impacts. This paper presents an optimization framework for evaluating parallel hybrid powertrain configurations using Geometric Programming (GP). Both retrofit and clean-sheet vehicle designs are considered. The results show that high-speed electric motors integrated into a parallel hybrid configuration using batteries can reduce the sized gas turbine power, enabling more efficient engine operation at lower power levels. For retrofit designs, with a fixed vehicle gross weight, adding batteries and motors reduces usable fuel, decreasing mission capability. Clean-sheet designs offer additional flexibility to re-size the vehicle and rotor, resulting in energy savings for an equivalent design mission.
Electric vertical take-off and landing vehicles are proposed as a viable solution for urban air mobility due to their potential for reducing carbon emissions, noise, and operational costs. However, the shift towards electrified aircraft introduces new thermal management issues due to the excess heat generated by electric motors and power electronics. This heat is challenging to dissipate during the mission, resulting in transient motor temperatures, especially during high-power mission segments. In addition, electrified aircraft also encounter design challenges associated with the fixed weight of electric motors and batteries. To address these challenges, this work presents a multifidelity framework for performing shape optimization of an electric motor subject to performance, geometric, and thermal transient constraints. A preliminary sizing of the electric motor is performed using a low fidelity Fourier series model. Next, the sizing is refined by utilizing a coupled electromagnetic-thermal finite element model of the the motor physics. To demonstrate the framework, a numerical optimization is performed for a hover hold mission with a relevant scale eVTOL vehicle, reducing the structural mass of the motor by 2.7kg while ensuring power and thermal constraints are satisfied.
A propeller driven rotor uses small electric motors and propellers attached to the rotor blade to spin the main rotor. Recent propeller driven rotor hover test campaigns suffered propeller failures at relatively low main rotor rotational speeds. The dynamics of spinning a fast propeller at the end of a spinning main rotor blade were the suspected cause of the propeller blade failure. An experiment using the 10 ft diameter vacuum chamber was designed to isolate and measure the propeller flapping motion of an articulated propeller blade from inertial loads. A Coriolis coupling exists between the propeller and the main rotor, resulting in large 20° sinusoidal propeller flapping motions. The vacuum chamber experiment also demonstrated that for the propeller/rotor speed ranges tested, increasing the propeller or the main rotor speed resulted in larger propeller flapping motion. An analytical model was developed to study the coupled propeller flapping motion due to the main rotor rotation. The predicted solutions for the analytical model capture the vacuum chamber experimental results well. Key insights were obtained by analytically solving the propeller flapping equation of motion assuming periodic coefficients (β0, β1c, β1s). This analytical solution method showed that the β1s term was associated with the Coriolis coupling between the main rotor and the propeller, whereas the β0, β1c terms result from the location of the propeller forward of the rotor 1/4 chord location.
This paper presents handling qualities (HQs) research findings for electrical Vertical Take-off and Landing vehicles. Testing in the Vertical Motion Simulator (VMS) investigated handling qualities of vehicle configurations having a degraded powertrain. Powertrain components, including batteries and electric motors, can degrade as the vehicle is flown. This paper investigates the impact of low battery charge and high motor temperature degradations on the pilot's ability to execute precise maneuvers. Pilot comments and ratings that were collected from four rotorcraft test pilots in VMS testing are used to quantify the effects that powertrain degradations had on the HQs of the vehicle.
WHY DO WE NEED SIMULATIONS? This paper is intended to provide a broad presentation of the simulation techniques focusing on transmission testing touching a bit on power train testing. Often, we do not have the engine or vehicle to run live proving ground tests on the transmission. By simulating the vehicle and engine, we reduce the overall development time of a new transmission design. For HEV transmissions, the battery may not be available. However, the customer may want to run durability tests on the HEV motor and/or the electronic control module for the HEV motor. What-if scenarios that were created using software simulators can be verified on the test stand using the real transmission. NVH applications may prefer to use an electric motor for engine simulation to reduce the engine noise level in the test cell so transmission noise is more easily discernable.
The Joint Tactical Aerial Resupply Vehicle (JTARV) project is supporting the expansion of the Army's unmanned aerial reconnaissance capability by working to obtain high-quality test data that is scarce for group 2 and group 3 UAVs to validate physics-based models. This paper will evaluate the system identification results from transient testing at a wind tunnel speed of 29.2 kts (15 m/s) of the commercial off the shelf T-motor '28x9.2' rotor which is used on the TRV-80 platform, a group 3 UAV. Rotor angles of attack of -90° to 0°, which represent climb to edgewise flight respectively, were tested in the wind tunnel. Chirp Pulse-Width Modulation (PWM) sweep inputs that varied the ΔPWM amplitude by 50, 100, and 150 (approximately 3%, 6%, and 10% of the steady state PWM) were analyzed to derive an electric motor model that can be used in a physics-based simultion. Additionally, it was found that the identified coefficients of thrust and torque were higher in transient condtions versus steady for most angle of attack test condtions. Lastly, the closed loop wind tunnel system, thrust due to ΔPWM amplitude sweep, was compared to the closed loop system of the TRV-80. A similar motor lag frequency was identified in both systems which shows the relevance of the electric motor model and rotor performance data derived from wind tunnel data preseneted herein.
The advent of electric propulsion is revolutionizing the paradigm of rotorcraft design, leading to new electric Vertical Take-Off and Landing (eVTOL) aircraft. Direct drive topologies are common within these new designs, and some designers have chosen to utilize this mechanism for Primary Flight Control (PFC), effectively utilizing the aircraft engines as PFC actuators to control the speed of the rotors. This decision integrates the propulsion and flight control systems, and intrinsically couples the aircraft sizing and control. Four separate tools were exercised throughout this study to conduct a conceptual design exploration of eVTOL aircraft handling qualities. The main tasks for these tools were: 1) aircraft sizing and performance analysis, including the calculation of trim; 2) flight dynamics modeling and analysis; 3) handling qualities-centric control law optimization; and 4) electric motor sizing. Sizing of an RPM-controlled Hexacopter concept explored the dependency of aircraft size to fundamental design parameters: 1) disk loading and 2) blade loading coefficient. Increasing the design disk loading resulted in designs with high agility, but at the cost of significant growth in the design gross weight. Finally, Categories II and III pilot-induced-oscillation (PIO) are known potentially-catastrophic handling qualities deficiencies of fly-by-wire flight control systems such as those expected of eVTOL aircraft. Consideration of PIO predictive metrics in conceptual design control synthesis led to increased PIO robustness.
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.
Manual analysis of the aerodynamic behavior of small unmanned aircraft is a lengthy and repetitive task. This paper shows the current state of an automated flight analysis tool that calculates the aerodynamic coefficients of small unmanned aerial vehicles. It covers the tool's workflow, shows the current quality of the data processing, and lessons learned. It compares the results of different aircraft types, including standard electric motor and glider aircraft, and shows specifically designed and tested flight patterns. The results of the tool are compared to manually computed data from the glider using a low-wind flight before sunrise. Finally, the impact of thermals on the measured data is presented. An outlook will show the remaining limitations and possible future additions to the tool.
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.
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.
This white paper discusses the application of carbon fiber roving for rotor magnet retention in high-performance Brushless DC (BLDC) motors, focusing on sectors like Advanced Air Mobility and motorsports. Highlighting the benefits of carbon fiber's tensile strength, thermal characteristics, and electrical resistivity, it compares thermoset and thermoplastic matrices, analyzing their trade-offs. It delves into manufacturing methods, particularly the advantages of in-situ winding of Hexcel® HexTow® IM7 12k carbon fiber directly onto rotors, versus pre-wound sleeves, emphasizing controlled processes for even stress distribution and preventing failure. Key design factors such as operating speed, temperature, and air gap dimensions are considered to optimize carbon fiber's application. Windings' expertise in fabricating high-tolerance carbon fiber wound rotors is showcased, highlighting its potential to enhance motor power output and offering collaboration for innovative retention solutions in BLDC motors.
This paper investigates the role of the aerodynamic torque on propeller whirl flutter stability. The generalized force due to the torque is first computed and subsequently included in the equations of motion of a rigid propeller-pylon system. Preliminary evaluations indicate that the torque modifies the real part of the backward and forward modes, providing a stabilizing effect on powered propellers. Analyses are conducted on a 3-bladed propeller driven by an electric motor. Stability predictions are obtained with a simple analytical model and validated by multibody simulations coupled with a mid-fidelity aerodynamic solver, based on a vortex particle method. Furthermore, a simple control law acting on the propeller's collective pitch and rotational speed is presented. The control variables are modified to increase the whirl flutter stability margins, without altering the trim conditions of the aircraft. Results demonstrate the effectiveness of the proposed control strategy, although propeller efficiency is reduced. In a failure scenario, the control law can be exploited to compensate for a weak mounting stiffness, allowing the aircraft to land safely.
In the emerging market of Advanced Air Mobility (AAM), aerospace companies have been designing and prototyping electric and hybrid vehicles to revolutionize travel. These vehicles must have low noise and particulate emissions while also having enough propulsive efficiency to complete the mission. This paper presents the relationship between noise and propulsive efficiency as related to any aircraft equipped with an electric motor and a variable pitch rotor/propeller. The combination of the electric motor with the variable pitch propeller/rotor allows for a decoupled rotational speed and torque generation, meaning that the electric motor can generate the same amount of torque while operating at different rotational speeds. This feature allows the rotor/propeller to hold constant thrust at different combinations of rotational speeds and torque, by adjusting the collective pitch of the blades. Therefore, for a rotor at constant thrust, the minimum noise (from loading and thickness contributions) and minimum power operating points in terms of rotor RPM and collective blade pitch, are not the same thus leading to the fact that it takes increased energy to decrease noise. A MATLAB code is developed to investigate the power and noise relationship by employing several functions to integrate XFOIL and Blade Element Momentum Theory for the rotor performance calculations and WOPWOP for thickness and loading noise analysis.
AAM concepts use multiple distributed electric motors driving propellers and rotors to augment or directly generate lift and propulsive forces. Several current concepts incorporate separate drive systems for providing vertical lift, for takeoff and landing, and propulsive thrust for wing-borne cruising flight. Measurement of loads and performance on these rotating systems is very important in both the design and development stage, as well as for certification use and ultimately supporting HUMS monitoring. However, providing instrumentation in the rotating frame and extracting their associated measurements is often problematical, as it requires some means for both power and signals to bridge the rotating interface between the blade of the rotor/propeller and the fixed frame (fuselage) system. This paper describes work conducted to leverage prior CDI development of a novel optical telemetry/instrumentation system to create a prototype unit that can support ground and flight tests, allowing for multiple installations on the many rotors that constitute current AAM configurations. The resulting hardware was designed to expand the capabilities developed previously in types and rates of data collected, on-board processing, and user configuration options, supporting NASA and commercial organizations in their testing activities.
A blade-tip-propeller driven rotor consists of small electric motors and propellers attached to the rotor blade tip to spin the main rotor. This study address a propeller driven shortcoming that was identified in previous research: a high required power to spin the main rotor. To investigate this, a series of wind tunnel and hover stand test campaigns were conducted to experimentally characterize the 6 ft diameter propeller driven rotor performance. A streamlined tip nacelle was designed to house the blade tip motor, and featured an embedded load cell to measure the tip propeller's aerodynamic forces and moments. A propeller aerodynamic model was developed from propeller hover tests and then validated through wind tunnel testing of the propeller in axial flow. Next, the interactional aerodynamics between the stationary rotor blade and tip mounted propeller were investigated through wind tunnel testing. These tests were performed between 0 and 12 degrees wing angle of attack, and at airspeeds of 15 and 20 m/s. Finally, the shaft driven isolated rotor, the shaft driven rotor with tip nacelle, and the propeller driven rotor were tested in hover, and their figures of merit and power loading were compared.
Electrical vertical takeoff and landing (eVTOL) vehicles for urban air mobility (UAM) are garnering increased attention from both the automotive and aerospace industries, with use cases ranging from individual transportation, public service, cargo delivery, and more. Distributed electric propulsion systems are their main technical feature; they determine vehicle size and propulsion efficiency and provide distributed thrust to achieve attitude control. Considering the intended role of eVTOL vehicles, ducted-fan systems are ideal choice for the propulsor, as the duct provides a physical barrier between the rotating blades and the human, especially during the take-off and landing phases. Key Technology Challenges of Electric Ducted Fan Propulsion Systems for eVTOL introduces the main bottlenecks and key enablers of ducted-fan propulsion systems for eVTOL applications. Based on the introduction and discussion of these important issues, this report will help eVTOL engineers understand the key technical issues and inspire them to develop the ideal solutions that will enable eVTOL vehicle deployment for UAM operations. Click here to access the full SAE EDGETM Research Report portfolio.
This paper describes the development of a semi-empirical sizing algorithm for the outrunner brushless DC (BLDC) motors used in electric unmanned aerial systems (UAS) with a maximum gross takeoff weight of 25 kg (55 lb.). Given an operating torque and a desired geometric aspect ratio (stator diameter/length), the algorithm can predict the mass, geometry, and figure of merit (km) of a motor rated for the applied torque. Given an operating voltage and operating speed, the algorithm can predict the optimal torque constant (kt ), speed constant (kv), and winding resistance (R). To develop, tune, and validate the algorithm, complementary theoretical and experimental methods were used to overcome two key barriers: (1) motor performance theory was used to generate non-existent performance data, and (2) empirical trends in motor design were leveraged to connect independent theoretical torque, volume, and mass models. The resulting mass and figure of merit predictions are within ±20% of actual values for motors ranging in mass from 25 - 500 g (1-18 oz.) and varying geometries. Moreover, the derived electrical constants can be fed into equivalent circuit performance models to validate the output of the sizing algorithm and drive optimization feedback in a broader UAS conceptual design tool. The validated algorithm also revealed that motor size (and therefore mass) grows with torque, not power, which is a critical distinction for vehicle designers in the VTOL industry who are increasingly transitioning to electric powertrains.
This paper presents an object-oriented, equation-based framework for multi-engineering modeling of a quadrotor UAV, which includes the rigid body dynamics, simplified aerodynamics, gyroscopic effects, electrical power system and battery losses, and DC motor dynamics. An open-source drone modeling library is introduced by explaining the mathematical models and multi-domain components used to model the drone. Animation and visualization techniques for the drone using CAD models are also introduced and explained. The proposed drone model is simulated under different flight scenarios using motor and power system models with different levels of detail, aiming to provide better means for design and understanding, of multi-engineering aspects of UAVs. This model provides a foundation for future UAV open-source model development, electrified power propulsion design, visualization and interaction, and system identification.
Modern system identification techniques were used to identify a linear model based on a nonlinear simulation of a concept Urban Air Mobility quadcopter, and compared to a perturbation-based model. These models were used to develop feedback controllers for both variable-pitch and variable-RPM variants of the quadcopter, with the handling qualities requirements determining current requirements for the electric motors. To have sufficient stability margins and bandwidth, the motor time constant for the variable-RPM system must be no greater than 0.122s. Both variable- RPM and variable-pitch systems were limited by the yaw axis, which relies on differential motor torque for control. The introduction of rotor cant alleviated this problem for the variable-pitch vehicle, allowing a 47% reduction in motor weight, relative to the uncanted variable-pitch system.
The paper discusses the application of the Array Controlled Turn-less Structures (ACTS) motor for VTOL application. The motor enhances the three main competing characteristics of electric motors; namely specific power, efficiency and reliability. The motor arrays an ensemble of elemental turn-less motors which include turn-less elements each with their dedicated inverters which are operated in synchronism. The resulting small pole size enhances the power density, the enhanced conductor packing enhances the efficiency, and the massive parallelism enhance the reliability. Vertical takeoff requires much higher thrust compared to wing assisted takeoff. With limited on-board power, this higher thrust is presently provided by in ordinary larger propulsion disk area which reduces the craft aerodynamics, and the cruising Lift-to-Drag (L/D) ratio and accordingly the flight efficiency and range. The high specific power of the ACTS motor allows for a different scenario and thus craft architecture. By substantially increasing the takeoff power which is now possible with the higher specific power of the ACTS motor, the propulsion disk area can be substantially reduced, with the resulting greater streamlined, high L/D craft, and thus longer range. The paper discusses key aspect of the motor and inverter architecture and technology. Furthermore, it discusses its application to a high L/D VTOL and prospective performance.
In this work the use of a Nonlinear Dynamic Inversion flight control system is investigated for use on electric-VTOL aircraft. This included studying the use of an airspeed scheduled switching system that would switch the aircraft’s control architecture from a low speed helicopter control system to a high speed airplane control system. Additionally, a novel thrust control allocation scheme is presented. This new scheme combines the variable collective pitch and variable rotor speed allocation schemes into a unified, complimentary filtering based, control allocation scheme. This new scheme is compared against the original constituent schemes on the basis of time simulations, stability margins and handling qualities. The successful operation of the control architecture switching system, was demonstrated via time domain simulations. It was also found that the combined control allocation scheme did not have better performance than the variable collective pitch scheme. However, the combined scheme did offer some improved performance over the variable rotor speed scheme. Especially when electric motor torque limits are enforced. The combined control allocation scheme was able to perform maneuvers that the variable rotor speed scheme could not.
A phenomenological simulation for a variable-voltage hybrid-electric powertrain was developed and compared with test data acquired on a 4 hp powertrain to understand the fundamental characteristics of such a system. The powertrain was modeled component by component, and compared with over 500 experimental data points, from the engine alone to the engine generator, to the engine-generator with four distributed propulsors. The principal conclusion of the predictive simulation and the experimental data was that generator voltage is a key parameter that needs careful control relative to rotor speed. For any operating state -- defined by rotor torque and RPM -- the generator voltage should be minimized to minimize engine specific fuel consumption. In general the system is influenced more by the engine generator than electric motors. Hence greater rotor torque and lower rotor RPM is desired. It was found that steady state performance can be confidently predicted with the engine model, if the thermal efficiency is calibrated with engine data. The overall understanding gained from this work is that the optimal operation of hybrid-electric powertrains in VTOL is closely coupled with controls and rotor aeromechanics as well as engine gas dynamics and thermodynamics, but can be captured with relatively simple phenomenological models.
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