Browse Topic: Scale models
This paper discusses the development of a quantitatively-accurate non-linear hybrid flight dynamics model of a hover-capable Air-Launched Tailsitter Unmanned Aerial System (ALUAS) in order to 1) understand its dynamics during complicated maneuvers, and 2) provide a high-fidelity framework to develop novel control laws. Wind tunnel tests were conducted on a 1:1 scale model of the full aircraft to measure the airloads, which were used in the simulation as a lookup table. Flight tests of the ALUAS were performed in hover, transition, and cruise to collect a large amount of unique state measurements by providing large excitations to induce highly transient motion. The flight dynamics predictions using Rotorcraft Comprehensive Analysis System (RCAS) software were then compared with experimental flight test data. To correct any discrepancies in the RCAS physics-based predictions, a correction was learned from the experimental measurements, making use of the large amount of collected flight test data. Using a neural network to learn this correction, the end result was a quantitatively accurate neural network assisted flight dynamics model. The accuracy of current simulations in complex flight states successfully demonstrates the applicability of the proposed methodology for correcting the dynamics model of novel out-of-the-box aircraft configurations.
Hybrid additive manufacturing (AM) and subtractive manufacturing (SM) processes utilize the combination of AM (e.g., LPBF and DED) and SM (e.g., milling and turning operations) to produce the final part. Due to the poor surface roughness resulting from the uneven melting of powders in AM, the subtractive process is a necessary finishing operation to improve the surface roughness of the AM part. The hybrid AM/SM technology combines the benefits of AM and SM processes to create complex geometry while introducing good surface finish and compressive stress to prevent crack initiation. However, the relationship between large process parameter space and the residual stress/distortion in the part is not well understood, which impedes the adoption of hybrid AM/SM to minimize the residual stress in the final product. To expedite the process optimization, we establish a pipeline for the sequential modeling of additive manufacturing (AM) and subtractive manufacturing (SM) processes. Key accomplishments achieved under this study include (1) development of thermal abstraction technique for the AM process to speed up the macroscale level heat transfer analysis based on the manufacturing factors including scanning vector, laser power, dwelling time, etc.; (2) development of the sequentially coupled thermal-mechanical model to predict the residual stress and distortion after AM process by passing the temperature history obtained from heat transfer analysis to the mechanical analysis at each time point; (3) validation of the thermal-mechanical model for AM using thin-wall structure from literature and cantilever beam structure from UNT’s experiments data; (4) conduction of the parametric study on the chamber temperature and part design in the AM process to demonstrate how the temperature gradient and supporting structure affect the residual stress and distortion; (5) exploration of macro and micro scale models to predict the bulk and surface residual stress after cutting; (6) applying the developed modeling framework to tailoring the hybrid AM/SM process. To support model verification and demonstration, we print cantilever beam structure with different supporting structure designs and cutting strategies to study how these factors affect the final part residual stress and distortion. The data collected in the printing and cutting process is used to examine the applicability of the developed simulation tool.
This paper presents an overview of the comprehensive aerodynamic framework developed at ERC for the analysis and simulation of electric vertical takeoff and landing (eVTOL) aircraft. Addressing the challenges inherent to distributed propulsion architectures and the complex transition between hover and forward flight, the methodology integrates multi-fidelity simulation tools ranging from analytical models and low-fidelity simulation to fully-resolved transient CFD. The framework addresses all phases of aircraft design and validation, and includes dedicated insight into aeroacoustics, aeroelasticity, and interactional aerodynamics problems. A modular approach is adopted, where individual phenomena are first studied in isolation before being synthesized into an aircraft model. Experimental validation through wind tunnel testing, full-scale static thrust test stand measurements, and scaled model flight tests is essential to ensuring model accuracy and validity. The paper concludes with an outlook to further enhance data generation, simulation efficiency, and fidelity in future eVTOL development programs.
Aeroelastic stability prediction is critical to the successful design, development and flight testing of rotorcraft. As configurations reach higher speeds, new challenges in high Mach number unsteady aerodynamic modeling need to be addressed, especially for higher frequency aeroelastic modes with significant coupling. In this paper, Linear Unsteady aerodynamics and Leishman-Beddoes attached flow models are applied and compared to 2D CFD (airfoil) and 3D CFD/CSD (rotor) analysis for operating conditions of interest. The Leishman-Beddoes model demonstrates improved agreement with CFD data. In the 2D assessment, RCAS is used to model a representative airfoil undergoing prescribed pitch and heave oscillations. CFD results are presented to compare each model (Linear Unsteady and Leishman-Beddoes). In the 3D assessment, a full rotor CFD/CSD test case is evaluated for aeroelastic stability and compared to RCAS standalone analysis. The RCAS rotor structural model is coupled with the HELIOS CFD code and a swashplate cyclic pitch input is used to excite a lightly-damped rotor mode. The transient response based on RCASHELIOS is compared to the standalone RCAS internal aerodynamic result for both Linear Unsteady and Leishman- Beddoes unsteady aerodynamics. This study demonstrates that the Leishman-Beddoes model can produce similar stability results to the computationally expensive coupled CFD/CSD approach of RCAS-HELIOS, but at a lower computational cost, even for critical high-speed conditions.
Current paper summarizes a correlation study of two flow solvers (CREATETE-AV Helios and Simcenter STAR-CCM+), routinely used at Sikorsky, with multiple model-scale wind-tunnel tests. The Helios modeling approach was aiming for a high-fidelity accurate simulation, whereas the STAR-CCM+ modeling approach was aiming for a fast turn-around time with reasonable solution accuracy with a relatively coarse mesh and simplifications. The two solvers generally agreed well with the test data within reasonable accuracy and captured the airloads and flowfield trends. The calculations presented herein show the impact of the turbulence model on component loads, the aerodynamic interactions among components, and the effect of transition modeling on rotor performance. The Reynolds-Averaged Navier-Stokes CFD model generally delayed separation and resulted in lower drag. By modeling the airframe supporting structure in CFD simulations, an improvement on correlation for inflow on the propeller plane was shown. Additionally, improvements in the rotor system L/De correlation were realized by including a turbulence-transition model, which reduces the rotor drag.
This paper explores a significant step forward, regarding the further detailed understanding of the Fenestron®. Since its patent in 1968 – for the Gazelle helicopter –, the shrouded tail rotor has been resized, inclined, modulated, etc. and has thus been continuously enhanced on different rotorcraft. Half a century after its invention, Airbus is once again exploring in more detail the magic of the Fenestron®, with the objective of optimizing it even further, for future helicopter applications. To grasp and observe properly some specific phenomena, a model (scaled to one third) capable of both unprecedented functions and modularities, was developed. The present paper will describe in detail the novel model and the related challenges and solutions. This model is capable of high rotor speed and dynamic pitch inputs, delivering power levels high enough to reach stall effects, while allowing the measurement of propulsive efficiency and to differentiate rotor vs fairing thrust. Furthermore, the model had to provide aerodynamic-shape modularity, both in the shroud, on the covers and on the tail gearbox supports. The first test campaign performed on this model allowed us to define design drivers and aerodynamic preferences. And since one idea often leads to another, another challenge is being addressed on this scaled mock-up: Reduction of the tail rotor noise level.
Rotorcrafts frequently operate in environments with severe atmospheric turbulence, for instance transferring people offshore to and from oil rigs as well as operating from and around ships. The presence of high turbulence can deteriorate performance, stability, and controllability of the rotorcraft. Additionally, such challenging conditions also generate loads that both airframe and rotor components must withstand. Following this, it is crucial to consider the impact of these operational atmospheric conditions during rotorcrafts design and development. In this context, numerical models are a fundamental tool to provide an easier and quicker way to explore the operative envelopes of the helicopter compared to performing experimental activities. This paper presents a rotor loads correlation activity between an experimental test designed and carried out by Leonardo Helicopters in which an AW189 helicopter was placed in the wake of a C-27J Spartan aircraft and a multibody structural model built using MSC Adams®. Furthermore, an atmospheric turbulence model is proposed and compared with the wind experimental records with the purpose of defining an analytical tool for the estimation of the wind time histories. The main purpose of the work is to detail the extensive correlation activity and highlight the modeling key ingredients to consider for an accurate load prediction in a turbulent wind environment. The correlation focuses on the effect of the wind during ground operations, considering the rotor at rest or running up/shutting down.
The CH-53K® King Stallion™ is the most advanced heavy lift helicopter developed by Sikorsky, a Lockheed Martin Company, to address the requirements of the United States Marine Corps. The aircraft was designed to support missions with a maximum design gross weight of 88,000 lbs and can carry external loads up to 36,000 lb. Performance flight tests for the CH-53K® have been completed as part of its System Design and Development (SDD) phase. Tethered hover and level forward flight performance measurements have been acquired that are used as a basis for Naval Air Training and Operating Procedures Standardization (NATOPS) flight manual performance charts. They were also used in the Key Performance Parameter (KPP) verification analysis, demonstrating that the CH-53K® exceeds its KPP for mission effectiveness. In addition to overview descriptions of the performance flight test program, the test results are herein compared with predictions from aircraft performance modeling tools that were largely based on earlier comprehensive model scale hover stand and wind tunnel tests. In some cases, the flight test results dictated that adjustments be made to the aircraft performance prediction tools so they consistently represent the overall vehicle flight performance characteristics. Recent computational fluid dynamics (CFD) simulations were employed to assist with the overall understanding of the complex aerodynamic flow field in both hover and forward flight. This includes modeling simulations using CREATE-AV™ Helios full-aircraft tool for rotor performance and interactional effects and the Simcenter STAR-CCM+ tool for isolated fuselage drag and fuselage variations. The insights gained from the CFD analysis, such as parasitic drag contributions, main rotor and tail rotor interactions, and other aerodynamics interactions, were used to make appropriate input adjustments to the aircraft performance modeling tools for enhanced correlation to the flight-test results.
A two-phase wind tunnel test was conducted to evaluate aerodynamic performance on a 1/5th scale model of the Sikorsky/Boeing X2™ technology representative aircraft for Future Vertical Lift (FVL). The test program provided valuable aerodynamic data for two important elements of the design: the faired coaxial hub system and the main inlet flow leading to the engine interface. Studies from previous X2™ technology aircraft have shown that hubs, pylons and sail fairings have strong interactions, and if well integrated can lead to low drag aircraft designs. Rotorcraft main inlets generally have aggressive turns; therefore, this inlet design was investigated for distortion and total pressure loss. Accuracy of modeling these aerodynamic interactions using Computational Fluid Dynamics (CFD) and other forms of computational aerodynamic assessment requires supporting empirical testing for validation. The two wind tunnel facilities used in Phase 1 and 2 offered different and unique advantages compared to each other, which allowed the Sikorsky/Boeing team to generate a large, complementary aerodynamic and propulsion database for comprehension as well as simulation validation.
Multirotor UAS spanning Groups 3 and 4 have received increased attention as candidates for tactical resupply missions due to their VTOL capability and payload capacity. The objective of this work is to better understand how the parameters of multicopter UAS flight dynamics models scale with size in support of expanding the Army's unmanned aerial reconnaissance capability. A family of coaxial multirotor UAS spanning Groups 2 and 3 have been flight tested to gather data for flight dynamics modeling and validation. These UAS consist of the TRV-80, TRV-150, and the subscale Eagle platform. A series of test points including static stability, trim shot, frequency sweeps, doublets, and maximum climb rate maneuvers were collected. Wind data was simultaneously collected using a 3-axis ultrasonic anemometer to characterize wind conditions and characteristics during testing. Flight data were collected in varying payload configurations ranging from 0-120 pounds and at flight conditions ranging from hover to 50 knots. Point state space models were identified from frequency sweeps and verified using doublet time histories. Point models of the TRV-150 were combined with trim data and experimentally measured moments of inertia to develop a stitched simulation model. The stitched model was used to extrapolate for the flight tested off-nominal payload configurations for analysis. The unstable roll and pitch hover eigenvalues between the vehicles within the TRV family and smaller Group 1-2 UAS were analyzed and compared to Froude-scaled results to establish trends based on vehicle aspect ratio.
Researchers at the National Aeronautics and Space Administration (NASA) Langley Research Center (LaRC) have conducted a series of structural component and seat level tests to improve finite element model (FEM) characterization of a representative vertical take-off and landing (eVTOL) test article developed by NASA. A full-scale dynamic test was conducted on the representative eVTOL test article in November of 2022. The test article represented a high wing, six passenger eVTOL design concept and is referred to as the lift plus cruise (LPC) test article. The full-scale test identified limitations in the analytical models used to predict aircraft structural response, in particular the composite material models did not effectively capture brittle failure of the structure which were measured during dynamic loading. To better understand the mechanism behind the composite material failure mechanisms observed and to improve the FEM, intact sample specimens of the composite airframe structure were recovered from the test article post-test and used in material characterization testing. In addition, the seat configurations used in the LPC test article were further studied using isolated seat and anthropomorphic test device (ATD) drop tower testing. Dynamic compression tests and three-point bend tests, conducted at varied impact speeds, were performed on the recovered frame section specimens. Additional testing was conducted to characterize the material properties of the forming foam, which remained in the frames after fabrication. These tests were used to improve characterization of the damage and failure parameters of the composite material model used in the FE model of the LPC test article. Seat level tests were conducted on the seats used in the LPC test article using acceleration pulses inclusive of current general aviation and rotorcraft certification load levels as well as conditions representative of those measured at the seat base during the LPC test. The structural material models and seat environment models of the LPC test article FEM were calibrated using the generated component test data. The updates made to these models were then integrated into the LPC FEM and simulated in the full-scale test condition. Results demonstrated the effectiveness of component testing to improve predictive capability of composite aerospace structural models within the crash and dynamic loading environments. Demonstration of the LPC FEM response across an accumulation of coupon, component, seat environment, and full-scale test levels provides confidence in the predictive capability of this model for future use in the study of occupant safety within eVTOL relevant crash environments.
ABSTRACT A proof of concept test to measure the unsteady boundary layer transition locations on the lower surface of a Machscaled rotor in forward flight was performed during the Summer of 2017 in the NASA Langley 14- by 22-Foot Subsonic Tunnel. The transition locations were measured using high-speed infrared thermography with a rotating mirror assembly that could be remotely actuated to acquire data at several rotor azimuths. Data were acquired for eight unique rotor flight conditions for a range of advance ratios (μ=0:10 : 0:38), thrust coefficients (CT/α =0:04 : 0:12) and rotor shaft angles (αs = -6 deg : 0 deg). This paper presents the transition locations as a function of azimuth and radius for an advance ratio of, μ, of 0.30, and thrust coefficent, CT/α, of 0.08. At this condition, the lower surface is fully laminar on the retreating side and mostly turbulent on the advancing side except near the tip. The tip airfoils were greater than 60 percent laminar on the lower surface advancing side. Capturing the location of natural transition on a rotating blade in forward flight represents a new advancement toward understanding the boundary layer state and its important contribution to rotor aerodynamics. Documentation of the boundary layer transition location during testing is critical to understanding scaling model to full-scale performance data, validation of newly developed turbulence models, and the design of the next generation of high performance rotor blades.
This article presents aeroelastic analysis of the ERATO blade with double-swept design and an homogenised structure, using both computationally intensive and rapid aerodynamics solvers coupled with a projection-based reduced-order model (ROM) for the structure. The study focuses on investigating the impact of blade flexibility on aerodynamic performance during hover flight, and comparing with experimental data. In terms of modelling, the ROM allows for efficient computation of structural displacements, while capturing the non-linear physics and the complex structural response induced by the double-swept configuration. The aerodynamic analysis incorporates different solvers including among others Computational Fluid Dynamics (CFD) with elsA, Vortex Particle Method (VPM) and Blade Element Momentum Theory (BEMT). This multi-solver approach is employed to assess the capability of fast aerodynamic methods to reproduce the desired flow, coupling properties and flight performance. The coupling methods employed such as field transfers and mesh deformation are rather classical but were adapted. First, results highlight the need of aeroelastic coupled analysis to model the ERATO blade in hover. Second, the VPM is proven to be a valid method for reproducing the studied flight condition offering a good compromise between the computational cost and accuracy. Third, the proposed VPM/ROM coupling is validated with respect to the reference CFD/ROM. Finally, this article contributes to evaluate a wide range of methodologies and to propose an alternative method for predicting the rotorcraft blades performance and for assessing the effect of the aeroelastic coupling on such performance. Future work is needed to characterize in more details the aeroelastic behaviour of the ERATO blades with a realistic structural model for hover and in forward-flight, for which the flow is intrinsically unsteady.
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.
Computational Fluid Dynamics (CFD) analyses are compared with 1/9th model-scale wind-tunnel test measurements for the RAIDER X® Competitive Prototype (CP). A multitude of comparisons with experiment are made, including measured airframe aerodynamic forces and moments, surface pressures, propulsor performance and propulsor-airframe interactions, surface flow visualization, and flow field velocimetry. The CFD tools STAR-CCM+ and CREATE-AVTM Helios were both utilized to simulate the test conditions. The CFD analyses, both using the Spalart-Allmaras turbulence model, yielded results which showed good trending with the experimental data. Overall, the CFD tools demonstrated their ability to accurately analyze the behavior of flow over a complex geometry at a variety of orientations. At the same time, a few areas of improvement were identified, such as in regimes of flow separation and complex airflow interactions (such as the hubs' wake impacting the tail). In these areas, trends are often captured, but further investigation and refinements in the CFD analyses are warranted to improve correlations with the experimental data.
This paper investigates optimal wing arrangements for electric Vertical Take-Off and Landing (eVTOL) aircraft, leveraging on their design flexibility with electric propulsion system. The study employs a multidisciplinary approach with the objective of integrating aerodynamic analysis, static and dynamic stability assessments, and pilot feedback to evaluate various wing configurations. Analytical techniques were adopted to evaluate aerodynamic performance and static stability, while experimental flight testing on scale models was conducted to validate these findings. Additionally, the Cooper-Harper rating system was introduced to capture pilot perceptions of aircraft handling qualities. Results inform eVTOL designers on wing arrangements that offer enhanced aerodynamic efficiency, stability, and handling qualities, ultimately expanding the operational scope and applications of eVTOL aircraft. The study concludes the versatility of the high aspect ratio conventional wing on eVTOL aircraft, providing excellent overall performance in the various aspects. This research aims to provide a comprehensive understanding on how different wing configurations influence range and handling qualities in eVTOL aircraft.
The constant, undisturbed rotor hub rotational speed is a commonly applied boundary condition and simplification in computational analyses of helicopter rotors. Revoking this simplification and considering rotor-drivetrain interactions in the hub's rotational degree of freedom can - but doesn't necessarily - improve the predictions of structural blade loads, especially in the lead-lag direction. To estimate the drivetrain's potential to influence the lead-lag loads, this paper proposes the systematic evaluation of the modified collective lead-lag modes. These eigenmodes, as well as the resulting modification of lead-lag loads in the aeromechanic simulation, are presented and compared for the rotordrivetrain configurations of the Eurocopter Bo105 and the Sikorsky UH-60A. The study focuses on understanding the drivetrain's influence rather than on making high fidelity predictions. In the Bo105 case, the drivetrain impact on the lead-lag moments is significantly more pronounced than for the UH-60A. The discussion of this difference includes the assessment of the blade passage frequency magnitudes (4/rev for a number of nb = 4 rotor blades) and their sensitivity to the modified collective second lead-lag eigenfrequency, which in turn is changed by a varying drivetrain stiffness. While a very stiff drivetrain causes an eigenfrequency of 4/rev with high-magnitude resonance for the Bo105, the UH-60A configuration maintains an eigenfrequency above 4/rev for the whole range of applied stiffness values. For rotor-drivetrain systems, especially near resonance, the nb/rev magnitudes of the lead-lag loads are very sensitive to changes in the drivetrain properties as well as changes in the nb/rev excitations. The accurate modeling of these nb/rev excitations, e. g. by airloads, is essential to capture changes in the nb/rev dynamic response that are caused by the drivetrain. Therefore, including the drivetrain in the structural model may only be useful if the fidelity of other models is increased simultaneously.
ABSTRACT T-tail configurations are a promising approach to increase vertical tail efficiency, reduce fuselage download and hub load cycle amplitudes in low speed transition. However, the horizontal tail can be subject to rotor wake impingement in cruise flight which might lead to high dynamic loads and structural fatigue. The involved aerodynamics are in addition highly complex and hence difficult to be predicted by simulation. In this work a simulation approach for empennage structural loads and vibration prediction is established based on free-wake analysis and modal fuselage approximation, focusing on the expectedly most dominant aerodynamic interaction effects at the T-tail. The results are compared to flight test data to evaluate the approach, and sensitivities of the framework are assessed. The results indicate that the motion of the horizontal tail is characterized only by a few modeshapes, predominantly driven by rotor wake influence, rather than rotor loads via the structural load path. At the same time, high sensitivities are associated with these particular modes and are evaluated in this work to identify the driving mechanisms of T-tail vibrations of the investigated configuration. Discrepancies in the structural model are identified against bang test data. Taking these discrepancies into account, the simulation approach yields reasonable results for T-tail vibrations and loads in comparison to flight test data. In the front part of the fuselage, flight test data is significantly underpredicted as expected and attributed to the employed simplifications in the main rotor blade model.
High fidelity code-to-code comparisons have been made between the University of Glasgow HMB3 code and the HPCMP CREATE™-AV Helios code under The Technical Cooperation Program collaboration project, Next Generation Rotor Blade Design. The comparisons are made for two model-scale rotors - Langley baseline (LBL) rotor and Pressure Sensitive Paint (PSP) rotor. Hover and forward flight performance results are compared against test data. For the LBL rotor, hover performance is in a good agreement between the test data and HMB3 results over a full range of CT. However, the comparison between the HMB3 and Helios results at a CT of 0.0084 shows the difference in Figure of Merit (FM) by approximately 2 counts (2.2-3.2%). In forward flight, the HMB3 and Helios performance results overpredict the test data at the low advance ratios but improve the predictions at the high advance ratios. At an advance ratio of 0.31, the code-to-code comparison indicated that the Helios torque was lower by 2.8-3.1% compared with the HMB3 torque. For the PSP rotor, the FM trend computed using the Helios-OVERFLOW code agrees well with the fully-turbulent test data. The Helios-OVERFLOW result shows that the fuselage interaction effect is favorable for rotor performance. For a code-to-code comparison, the HMB3 FM result shows about 3.0 counts (4.0%) lower than the HeliosOVERFLOW result at a CT/σ of 0.091. In forward flight, the HMB3 rotor result agrees well with the test data whereas the Helios rotor result underpredicts by 2-3 counts in CP/σ (3.5-5.7%) at the high advance ratios although it was significantly improved at the low advance ratio. Overall, the code-to-code comparisons are successfully conducted for both the LBL and PSP rotors.
A time-parallel algorithm is developed for large-scale three-dimensional rotor dynamic analysis. A modified harmonic balance method with a scalable skyline solver forms the kernel of this algorithm. The algorithm is equipped with a solution procedure suitable for large-scale structures that have lightly damped modes near-resonance. The algorithm is integrated in X3D, implemented on a hybrid- shared and distributed memory architecture, and demonstrated on a three-dimensional structural model of a UH-60A-like fully articulated rotor. Flight test data from UH-60A Airloads Program transition flight C8513 are used for validation. The key conclusion is that the new solver converges to the time-integration solution more than 75 times faster, and achieves a performance of greater than 1 teraFLOPS. The significance of this conclusion is that the principal barrier of computational time for trim solution using high-fidelity three-dimensional structures can be overcome with the scalable harmonic balance method demonstrated in this paper.
Two- and three-dimensional models representative of a helicopter rotor blade element during forward flight have been implemented. The rotor blade element is considered in pitching oscillation motion with a non-uniform translation to take into account the speed variation in forward flight. Two stalled flight conditions of the 7A rotor have been selected in wind tunnel test data. These flight conditions have been investigated in a previous study and the aerodynamic behavior of the rotor blades in realistic rotor environment is known, including stall mechanisms. The capability of simplified models to reproduce the aerodynamic behavior of the blade element has been validated for a first case. Then, the influence of the blade-vortex interaction on stall onset has been investigated since the previous work on full articulated-rotor configurations does not allow to conclude on the role of the blade-vortex interaction on stall onset. The simplified models allow to isolate the influence of a vortex passing close to the blade element on aerodynamic loads. This work shows that a dynamic stall event is triggered in both cases while a vortex is passing close to the blade element. This clearly highlights that blade-vortex interaction can trigger stall in realistic rotor environment.
Experimental measurements of the unsteady flow fields generated by a scale model rotor, hub, and fuselage, plus the unsteady loads generated on a horizontal stabilizer, have been used as the basis for comparison to two computational fluid dynamics (CFD) simulations. The STAR-CCM+ commercial solver and CREATETM-AV HELIOS using the KCFD and SAMCART solver were applied to a series of seven test cases. The configurations were fuselage and hub with blades-on and blades-off for velocity fields, as well as the stabilizer in two locations for unsteady normal forces. The quantities examined included time averaged rotor, hub, fuselage, and tail forces and moments, time averaged, unsteady, and periodic velocities, and stabilizer forces. Overall for the forces and velocities, both codes did well for the time averages, and captured the trends and qualitative features of the unsteady quantities. Cases driven by a strong tip vortex – stabilizer interaction were modelled well, the key issue being rotor tip path plane trim. Cases driven by combined wakes from the hub, fuselage, and forward pointing blades were more challenging, and the codes often under-predicted the unsteady amplitudes or differed in the distribution of frequencies. Since this was accompanied by higher than measured Reynolds stresses, one cure may be resolved smaller scales in the solutions, increasing the order, or improving the turbulence modelling to better preserve the unsteady flow structures. However, application for industrial design still requires computational efficiency.
Icing of the fuselage and blades may occur when the helicopter is flying in the icing area. If ice accretion occurs in the ADS(Air Data System) of the fuselage, normal speed and altitude information are lost, making it difficult to flight. When windshield icing occurs, the view of pilot is limited and flight is difficult. Also, the ice accretion of the blades deforms the outer shape of the blades (Ref. 1) and makes the dynamic characteristics unstable due to an abnormal weight increase, resulting in deterioration of performance, deterioration of maneuverability, and structural instability. To avoid this, an anti-icing or de-icing system is required. Therefore, if the aircraft is not fitted with a proper anti-icing system, it is not possible to operate under icing conditions. However, it is difficult to design a proper anti-icing system considering the position of anti-icing protection area and icing phenomenon due to limitation of electric power, weight, thermal damage temperature limit, shape and so on. It is essential of understanding of anti-icing mechanism for selection and design of appropriate anti-ice system considering configuration (impingement limit, collection efficiency), material (thermal fatigue limit, heating source) and icing amount (collection efficiency, stagnation point). This paper introduces the preparation procedure (analysis and simulation, design, scaled model test) of icing certification test and the artificial/natural icing flight test method for proving helicopter icing through KUH case, and describes the characteristics of each system for the de/anti-icing system design of the helicopter, the computational analysis method, comparison with the test result, and the improvement method of the de/anti-icing system.
BCFD (Boeing CFD) computations of airframe (or fuselage) drag of an AH-64 helicopter are presented and compared with high-quality wind tunnel test data. These computations use a steady state implementation of the solver for different components of the airframe. The BCFD solver has also been tailored for subsequent drag reduction applications. Drag predictions were made for each component of the airframe including fuselage canopy, EFAB, wing stores, main rotor hub, etc. and their overall contributions to airframe drag for the primary mission configuration to help arrive at low drag design drivers. The CFD predictions have been validated extensively against the 16 percent model scale test data obtained in the University of Washington Wind tunnel to establish the accuracy as well as viability of CFD as a design tool to reduce the airframe drag associated with a complex geometry such as Apache. Further improvements of drag prediction are made using EPIC (Edge Primitive Insertion Collapse) adaptive grid tool.
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