Browse Topic: Vehicle performance
This study investigates the performance and vibration characteristics of representative lift rotors for a notional lift+cruise electric vertical takeoff and landing (eVTOL) configuration. As new eVTOL concepts continue to be developed and others progress towards FAA certification, it is crucial to understand the performance and vibratory considerations associated with different lift rotor design choices, including the number of blades and the method of thrust control (e.g., variable blade pitch/fixed RPM vs. fixed blade pitch/variable RPM). The NASA Revolutionary Vertical Lift Technology (RVLT) Lift+Cruise configuration was chosen as the baseline vehicle for this analysis (Ref 1). The investigation includes the evaluation of multiple lift rotors with 2-, 3-, and 4-bladed configurations as well as variable- vs. fixed-pitch designs. Both isolated rotor and full vehicle simulations were assessed to demonstrate some of the design variables applicable to the full vehicle performance and vibratory content. Industry-standard rotorcraft comprehensive analysis software, the Rotorcraft Comprehensive Analysis System (RCAS) (Ref 3), was used to evaluate and compare each configuration for performance and vibratory loads at key points in the rotors and in the fuselage
The work performed for the Adaptive Resilient Engineered Structures (ARES) program sponsored by the U.S. Army constitutes a trade study and resulting proposal for a structural demonstrator platform. The trade study was conducted using the Quality Function Deployment (QFD) process and a subsequent Artificial Intelligence (AI) exercise to find clusters of technologies for structural efficiency and resilience from Boeing's internal research activities. From a selection of approximately 150 technologies at different TRLs, Boeing subject matter experts (SMEs) for structural technologies identified several characteristics that could potentially determine the development of ARES structural demonstrator. Through the QFD process, the list of technologies was down selected about 50 unique technologies for consideration. The next stage of the QFD process entailed in identifying 37 different attributes or criteria long which each of these technologies would be assessed. They were grouped under two different categories: vehicle performance criteria and program performance criteria. Importance scores were provided by the SMEs independently and then a statistical approach for AI was used to distill them to 9 significant ones (labeled as 'Pillars') and a further distillation to 3 significant features (labeled as 'Super Metrics'). Clustering algorithms were then employed to group the set of technologies that could provide the resiliency targets sought for the demonstrator platform. The clusters were compared a hypothetical ideal platform to determine suitability and finally, 12 technologies merited attention toward the stated goals of the demonstrator platform.
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.
The influence of ground, wall, and corner boundaries on multirotor vehicle performance was investigated through a series of controlled flight tests. Changes in rotor inflow profiles were represented by near-field rotor pressure measurements captured by a custom Kiel probe wake rake. Ground effect was characterized by reduced thrust and power requirements, primarily driven by the vehicle fuselage, which induced regions of reduced pressure and increased flow unsteadiness around the airframe. Operating near a wall boundary was found to restrict airflow into the portion of the rotor disk closest to the wall, leading to increased power requirements to maintain hover and a consequent reduction in performance. While vehicle orientation had minimal impact on overall rotor performance, it did influence local rotor inflow behavior near the wall, depending on the relative position of the interaction region formed with adjacent rotors. As the vehicle descends from the isolated wall effect into corner effect, created by the intersection of the wall and ground, an exchange between the dominating ground and wall effects is observed, with corresponding ground heights and wall distances identified.
Rotors and propellers in edgewise flight typically encounter reverse-flow on the retreating blade, especially when operating at low rotational speeds and high speed flight. This phenomenon is well known and has been observed in rotorcraft and vertical take-off and landing (VTOL) applications, with impacts on vehicle performance and aerodynamic loads. Reverse flow is characterized by flow incident to the trailing edge of an airfoil with an angle of attack (AoA) of around 180°. Aerodynamic coefficients for reverse flow conditions are difficult to find in literature, and wind tunnel measurements often focus on the normal operating range of airfoils. This study investigates the fundamental aerodynamic characteristics of airfoils in reverse flow using high fidelity computational fluid dynamics, and analyzes the impact of using accurate aerodynamic coefficients on comprehensive rotorcraft analysis. Although the effect on flight performance is well understood, for applications on lift rotors of eVTOL configurations, reverse flow phenomena are expected to have a significant impact on rotor loads, especially during flight transition from VTOL to fixed-wing modes.
Aerodynamic interactions impact multirotor vehicle performance throughout its entire flight envelope and change with vehicle orientation, attitude, and forward flight speed. This paper presents efforts in incorporating these interaction effects into a reduced-order numerical quadrotor model informed by experimental flight test data. The interaction model employed system identification tools to compensate for discrepancies between actual rotor performance data and a Blade Element Theory (BET) based baseline model. Incorporation of the interaction model derived from system identification techniques improved the accuracy of model predicted rotor performance. The interaction model also provided insight into interaction effects predominantly influencing rotor performance for multiple flight conditions. The results demonstrate the utility of system identification techniques for accurate multirotor modeling capabilities.
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.
An aeromechanics analysis of a Mach-scaled rotor with lift compounding was conducted to understand the impact of various wing configurations on performance and loads. An assessment of the single retreating side wing and dual wing configurations was conducted for advance ratios up to μ = 0.7, two wing incidence angles (4° and 8°), and three rotor shaft angles (-4°, 0°, and 4°). Aircraft performance, control angles, blade structural loads, hub vibratory loads, and aerodynamic interactions between the rotor and wing were evaluated using the University of Maryland Advanced Rotorcraft Code (UMARC). Additionally, UMARC coupled rotor-wing analysis was validated with wind tunnel data of a lift and thrust compounded rotor. The study shows that the single wing configuration is beneficial for peak vehicle performance (L/D), though the dual wing configuration minimizes blade loads. The single wing configuration observed a 7% greater wing L/D than the dual wing configuration for the same 8° wing incidence angle at μ = 0.5 and αs = 0°, however, the dual wing configuration yielded a 20% lower steady flap bending moment. The study showed that the wing wake has a negative effect on the rotor performance especially at rearward rotor shaft angles; this is overcome with the efficiency gains from lift offset present in a single wing configuration but absent in the dual wing configuration. Hence, the single wing configuration attains the highest performance while the dual wing configuration minimizes structural loads. A lift compounded rotor is limited at high advance ratios since it is almost entirely offloaded, resulting in increased total lift to drag ratio at the cost of large blade structural loads.
ABSTRACT
LaunchPoint Electric Propulsion Solutions is developing mission-optimized electric propulsion systems. Unlike traditional aircraft development where designs are often driven by available propulsion systems, the relative simplicity of electric propulsion opens up the possibility that bespoke propulsion components may be developed and optimized for a particular vehicle configuration and mission. Electric propulsion is new to many aerospace designers and there is not yet a good body of knowledge about the performance of electric propulsion components. LaunchPoint aims to fill this gap by developing user-friendly physics-based electric propulsion models for multi-disciplinary optimization in vehicle/mission designs. To date, the existing vehicle configuration studies have largely used curve fits of existing electric propulsion components that do not accurately capture all of the relationships between motor and power electronics size, mass, efficiency, voltage, torque, and rpm and how those different motor parameters affect the overall vehicle performance. In a step toward revealing these relationships, this paper presents the design results from the coupling of detailed parametric motor and drive models with a simple eVTOL vehicle model.
The Sikorsky Boeing SB>1 DEFIANT is a technology demonstrator aircraft that is being built under the Joint Multi-Role Technology Demonstrator (JMR TD) program to address the next generation performance requirements of the Future Vertical Lift program which is led by the US Army. The design and development of the main rotor gearbox (MRGB) on the SB>1 DEFIANT Technology Demonstrator incorporated a number of lower TRL technologies and innovative design solutions to meet challenging technical requirements imposed by a generational leap in vehicle performance. The SB>1 DEFIANT main rotor drive system (MRDS) configuration is summarized with an account of advanced technologies incorporated and ground testing performed to successfully incorporate low TRL technologies in a flight vehicle that breaks performance barriers.
The Advanced AH-64 research and development program has evaluated approaches to improve the speed, payload, and range characteristics of the current AH-64E Guardian attack helicopter. Through several multi-disciplinary design studies, a cost effective manner to expand vehicle performance was determined to evolve the configuration to a lift and thrust compounded vehicle. Wind tunnel testing was performed to reduce the risk of the new configuration, inform design studies, and provide a relevant correlation database for advanced design tools. The test campaign utilized powered and unpowered models at 16.0%, 24.5%, and 30.0% scale to address high risk areas of the compounded design. The carefully planned sequence of tests addressed high speed, lightly loaded main rotor performance/loads/control/motions, main rotor and wing interactions in hover and forward flight, drag reduction achievable within program constraints, and rotor/wing/fuselage and propulsor interactions. Data from the testing has been applied to configuration trade studies guiding the development of the Advanced AH-64.
This article demonstrates application of a probabilistic analysis facility called AURA to rotary-wing control and handling-qualities problems. It is based on a Generalized Polynomial Chaos approach to compute and reason over probabilistic quantities. The AURA software has interfaces to both SIMULINK and MATLAB and bindings to other programming languages. It can model arbitrary random distributions and has the potential to fully represent the effects of the uncertainty with a single simulation run. Modeling and analyzing rotorcraft flight dynamics, handling-qualities, performance, structural loads, and design requires sophisticated tools to capture the complex physical phenomena germane to their operation. Present and future rotorcraft operational requirements demand the vehicles perform in new and increasingly austere environments. In the design and maintenance of such complex systems, there are associated uncertainties from either internal processes or environmental factors that affect the performance and stability of the system. Traditionally, uncertainties are treated as Gaussian random variables and their impact assessed using Monte Carlo techniques. This approach has proven successful but requires numerous simulation runs and approximations regarding the true distribution of the uncertainty, and there is no guarantee that Monte Carlo techniques provide comprehensive coverage of the uncertainty space. AURA is designed to help bridge these gaps in an environment that treats arbitrary random sources in an intuitive manner. This article briefly outlines the approach and demonstrates applications to rotorcraft control and handling-qualities analysis.
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