Browse Topic: Stall
This paper presents the investigation of experimental data belonging to main rotor loads during Never-Exceed-Speed demonstration of T625 Gökbey helicopter. Load data from the critical flight conditions in the VVNNNN envelope including cold-weather testing are collected. Maximum advancing tip Mach number demonstration, power-on and power-off flight conditions are investigated in terms of pitch link loads and blade loads. Blade loads including flapwise and chordwise bending moments, torsional moments and pitch link loads are examined to assess any divergence due to compressibility effects and the onset of stall. Load trends that are correlated with the tip Mach number are isolated from the effect of increasing dynamic pressure. Compressibility effects are observed to be the most dominant factor on the blade torsional moment and pitch link loads in advancing blade. The retreating blade stall phenomenon is apparent cases with a high advance ratio and mainly leads to dynamic stall cycles on the retreating blade, resulting in torsional moments and pitch link loads. Experimental results are compared with blade-resolved Unsteady Reynolds-Averaged Navier-Stokes simulations.
The Sikorsky BLACK HAWK® is the primary medium lift helicopter for the U.S. Army performing a wide range of missions that encompass Air Assault, MEDEVAC, CSAR, Command and Control, and VIP transport. The Multimission UH-60M is one of the latest in the BLACK HAWK helicopter product family, more capable, more survivable, more maintainable, more powerful, and more effective than its predecessors. In previous efforts, a high-fidelity CFDCSD based full-aircraft trim and maneuvering simulation methodology was developed and applied to model both coaxial aircraft and single main/tail rotor configurations (Refs. 1-4). The CFD solver is based on the CREATE™-AV HELIOS toolset (Ref. 5) and the CSD solver is based on Rotorcraft Comprehensive Analysis System (RCAS) (Ref. 6). The current paper further enhances the previously developed 6-DOF CFD-CSD full-aircraft trim methodology to robustly handle the trim solution for the single main/tail rotor configurations. The enhanced methodology was applied to rotor control loads and aircraft performance calculations at both steady level flight and aggressive Angle-of-Bank (AOB) turn maneuver conditions, including comparisons with available flight test data. The simulation results demonstrate a consistent correlation with flight test, including at stall conditions. The high-fidelity full-aircraft CFDCSD simulation approach provides an efficient, accurate and robust methodology for aircraft performance, control loads and stall boundary predictions to support advanced design at early stage.
This paper explores novel airfoils for rotorcraft applications using a gradient-free, multi-objective genetic algorithm with 2D URANS simulations. The study considers dynamic kinematics at a Reynolds number of 5×105 and a mean Mach number of 0.35. Two optimization scenarios are analyzed: 1) pre-stall kinematics (0° ≤α ≤10°) and 2) dynamic stall kinematics (0° ≤ α ≤ 20°). The paper compares two objective functions: f1, based on the cycle averaged lift, and ˜ f1, which modifies f1 by penalizing hysteresis in the lift coefficient. The effects of uniform vs. fluctuating freestream velocity and reduced frequency on optimal airfoils are also discussed. The proposed optimization approach has resulted in novel airfoil shapes that are characterized by a drooped nose, with a convex surface on the aft upper surface similar to a reflex camber in pre-stall kinematics and less unsteadiness in the air loads for the optimized airfoils under the dynamic stall kinematics.
A wind tunnel investigation to characterise the aerodynamic performance and aeroelastic response of a tiltrotor blade set operating in propeller mode is presented. A custom blade set was instrumented with fully bridged axial strain gauges to monitor the flap bending and torsional strain at several radial locations. Propeller thrust and torque measurements were acquired using a custom six component Rotating Shaft Balance. Measurements of blade tip deflection were obtained via stereoscopic Digital Image Correlation. Testing was performed at a range of rotational frequencies, blade pitch angles and advance ratios to assess the blade aerodynamic performance and aeroelastic response in both attached and stalled operating conditions. Strain measurements were shown to identify stall and blade eigenmode frequencies, where flap bending bridges show a more reliable capture of stalled flow than torsional bridges. Furthermore, blade tip deflection measurements were shown to reduce with increased advance ratio and excessive values of advance ratio resulted in negative thrusting at the blade tip.
Dynamic stall is an undesirable flow phenomenon that could occur on rotor blades of helicopters in forward flight due to azimuthal changes in local angle of attack resulting from blade motion, blade deformation and blade-vortex interactions. It is characterized by leading-edge vortex (LEV), or dynamic-stall vortex (DSV) shedding and significantly affects rotor performance and longevity. Therefore, the capability to predict dynamic stall, especially using rapid low-order approaches, is beneficial for vehicle design and flight-dynamics simulation. Recent work has resulted in the development of a theoretical parameter called leading-edge section parameter (LESP), which provides a measure of the suction force acting on the leading edge. It has been shown that the occurrence of dynamic stall on airfoils and finite wings corresponds to the time in an unsteady motion when the instantaneous LESP crosses a predetermined critical value. The current work shows that the critical LESP value, determined from relatively inexpensive 2D computational fluid dynamics (CFD) on an airfoil undergoing pitch and surge motions, can be used to predict the onset of dynamic stall on the section of a rotor blade in forward flight.
Dynamic stall continues to be a limiting factor for rotorcraft performance in forward flight. The complex flow physics, resulting from blade kinematics, aeroelastic deformations, and blade-vortex interactions, makes this problem challenging. The availability of results from recent high-fidelity coupled computational aerodynamics-structural dynamics simulations provides an opportunity to gain new insights into the physics of dynamic stall on rotor blades in realistic operating conditions. Recent research efforts have also resulted in the identification of a leading-edge suction parameter (LESP), whose critical value has been shown to correlate with the flow events leading to dynamic stall. Critical LESP is largely independent of motion parameters, and is dependent mostly on the airfoil shape, Reynolds number, and Mach number. In this work, LESP variation along the blades of a UH-60A rotor in forward flight is extracted from high-fidelity computational results. The objective is to explore the correlation between criticality of LESP and the onset of dynamic stall in a complex rotor flow. The results show a high correlation between LESP behavior and the signatures for the different occurrences of dynamic stall on the rotor blades. This excellent correlation provides the impetus for further application of the LESP concept to rotor aerodynamics.
This paper focuses on an experimental investigation of rotor loads during dynamic stall on a rotating pitching blade. In particular, the effect of rotor control parameters—rotor speed, collective pitch, and cyclic pitch—on the structural load dynamics of a rotor blade are analyzed in hover. The rotor platform used is the Mach-scaled, two-bladed Munich Experimental Rotor Investigation Testbed (MERIT) rotor at the Technical University of Munich (TUM). The dynamic stall cases selected vary in collective and cyclic pitch angles: 14°±6°, 14°±10°, and 20°±6°. Static and dynamic stall data are measured at three different rotor speeds: 900, 1200, and 1500 RPM with the highest corresponding tip Mach and Reynolds numbers of Matip = 0.41 and Retip = 1.2•106. Increasing pitch and rotor speed shows a considerable positive trend in the load overshoot, and hysteresis of the blade root moments of most cases. Cycle-to-cycle variations with bifurcation occur in some load graphs of light dynamic stall cases and are not restricted to reattachment regions.
This paper outlines the investigation into the effect of static stall onset in hover on the deformation of rotor blades, comparing the behaviour of a stiff blade featuring a NACA0012 aerofoil, rectangular planform and no taper, and a hingeless blade attachment; with a more flexible blade featuring a NACA23012 aerofoil, twist and taper, and a leadlag hinge. The Munich Experimental Rotor Investigation Testbed (MERIT) at the Technical University of Munich (TUM) was operated in a two-blade configuration at a variety of rotational speeds and collective pitch angles, paired with a stereooptic high speed photogrammetry system. The post-processing methodology used to extract flap and torsional deformations despite the presence of a hinge is outlined, and it was shown that the hinge affected the onset of flow separation and subsequent deformations. A comprehensive set of experimental deformation data for a repeatable setup has been generated and published.
ABSTRACT Reverse flow is the source of several unsteady effects that complicate load predictions for high advance ratio rotorcraft. As a way of improving load predictions, an ongoing series of experiments has been aimed at gaining a physical understanding of the unsteady aerodynamics of the reverse flow region. The current work contributes phase-averaged, three-component velocity field measurements collected on a rotor at high advance ratios. Stereoscopic particle image velocitmetry (PIV) was performed on a Mach-scale rotor across three advance ratios (0:6 ≤ μ ≤ 0:8), three radial stations (0:3 ≤ r/R ≤ 0:6), and one collective (θ0 = 10°). The present analysis focuses on how the reverse flow dynamic stall vortex, which results from flow separation about the sharp geometric trailing edge of a rotor blade in reverse flow, evolves over time in three dimensions. For a constant advance ratio, the size of the reverse flow dynamic stall vortex increases with decreasing radial station, creating a gradient in vorticity along the blade span. Tip-to-root radial flow, the amount of which increases for inboard radial stations, was also associated with the development of the reverse flow dynamic stall vortex. The current work postulates that due to an increase in radial flow, inboard radial stations are subject to a greater radial transport of vorticity than outboard sections and, in turn, produce less dimensional vorticity than outboard sections at the same convective time. Coupled with previous experimental models of reverse flow, the current work represents an important step in identifying the dominant unsteady characteristics of the reverse flow region and is designed to inform a low order model of rotors at high advance ratio.
ABSTRACT In this work, a genetic algorithm was implemented to perform an airfoil shape optimization with constraints applied to the airfoil cross-sectional area and pitching-moment coefficient. Constraints are enforced through the use of an augmented Lagrange penalty function. The design variables are formed through a class shape transformation approach with orthogonal, polynomial basis modes. The use of an orthogonal basis provides decreased levels of multicollinearity in higher-order design spaces, while still maintaining the completeness of lower-order spaces. The optimization methodology is demonstrated on the tip airfoil of a UH-60A baseline rotor. The design trade-offs of a new tip airfoil are investigated where the optimized tip section shows improvements in forward-flight performance in exchange for a small reduction in the rotor's stall margin.
A towing tank investigation of a single rotor blade operating at hovering and high advance ratio conditions is presented. A custom blade was manufactured and instrumented with fully bridged axial strain gauges to monitor the flap bending strain at three radial locations. Measurements of rotor thrust and torque were obtained to characterise the rotor aerodynamic environment for advance ratios ranging from 0.4 to 1.00 and to identify the presence of stalled and reverse flow. Strain measurements obtained at three locations across the blade span show minima and maxima at approximately the same azimuthal location as the load data. Moreover, the strain distribution shows a growth in strain magnitude with increasing advance ratio. Spectra of strain shows a dominant 1/rev signal and for the ∅ = 25° collective, non-harmonic frequencies are observed due to aperiodic vortex shedding from the presence of stalled flow.
The flow behavior of the two-blade MERIT rotor in hover, focusing on both pre-stall and stall regimes, is investigated through a comprehensive numerical-experimental approach. The study leverages unsteady RANS simulations to compute rotor thrust and power polars and validates them against experimental measurements. Valuable insights are provided into the capabilities of unsteady RANS methods and modern turbulence models for predicting rotor performance across these critical operating conditions. Furthermore, the numerical model incorporates blade deformations by implementing the experimentally measured flap and torsion displacements. A more realistic depiction of the rotor's aerodynamics is provided accounting for the structural deformations of the blades under aerodynamic loads. Highfidelity simulations closely predict the experiments in pre-stall conditions while discrepancies are present when the flow exhibits extended stalled regions. Blade deformations demonstrated to have only a minor effect on thrust and power polars due to the high stiffness of the system.
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This paper introduces a methodology for an optimization-based trajectory planner for the autonomous transition of a quadrotor biplane tailsitter (QRBP) between the flight modes of hover to forward flight and forward flight to hover. The trajectory planner uses a simplified first principles dynamic model of the QRBP in the formulation of a optimization problem for trajectory planning. Additional constraints on the trajectory are imposed based on physical limitations, such as available power, stall limits, among others. The cost function for the optimization problem is chosen to be the time-of-transition. The solution of this problem generates time-optimal state and input trajectories for transition. To validate the algorithm, the trajectories are tested on a flight dynamics simulation of a QRBP to demonstrate feasibility and tracking performance with an inner-loop PID feedback controller; and compared against trajectories generated from a heuristic approach. The results of the simulated tracking performance indicate the proposed trajectory planner is capable of generating feasible transition trajectories for the previously specified flight modes.
Dynamic stall has been studied for more than fifty years; in the last decade significant advances have been accomplished in the understanding, prediction, modeling and control of dynamic stall on rotors. In September 2019, an Army Research Office-funded workshop was held at the Georgia Institute of Technology to evaluate the state of the art and future directions in the understanding and control of dynamic stall found on rotors, specifically for vertical lift vehicles. Approximately forty attendees drawn from top experts in the field to graduate students convened to discuss experimental, computational, theoretical, and control research in the field over a two-day period. This paper provides a summary of the findings from this workshop, including a synopsis of best practices for experiments and first-principles-based computational prediction of rotor dynamic stall. Experimental data sets are discussed, as well the direction of research for empirical (non-first-principles) modeling and control of dynamic stall.
The effects of key design parameters of tilting distributed ducted fans are investigated through steady-state CFD simulations to assess the benefits of using variable geometry ducts in urban air mobility applications. The analysis is made on three adjacent ducted fans mounted at the trailing edge of a semi-span wing. The fans are represented by body forces calculated using the blade element theory. The duct expansion ratio, the duct thickness and the fan design expansion ratio are varied along with the fan speed, the crosswind speed in hover and the airspeed in forward flight. For each combination of the parameters, the hover Figure of Merit and crosswind stall speed as well as the forward flight lift coefficient, thrust coefficient and propulsive efficiency are evaluated. From these results, variable geometry ducted fans are benchmarked against fixed geometry ducted fans using a simplified 1 hour mission with 10% of hover time. It is found that a ducted fan equipped with a Krueger flap and a variable expansion ratio diffuser consumes nearly 11% less energy to complete the mission.
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.
Rotor blade aerodynamics are significantly influenced by dynamic stall. This study is part of a larger effort to delay the adverse effects of dynamic stall on rotor blade aerodynamics through airfoil shape optimization. This paper summarizes the experimental results of this effort. Time-accurate surface pressure measurements along with lift and moment coefficients were determined experimentally for the baseline and optimized airfoil geometries at relevant conditions. Particle Image Velocimetry was also utilized to shed light on the state of the flow on the two airfoils at select conditions. The sensitivity of dynamic stall under different flow conditions was studied for both airfoils. The result indicate significant improvements in the behavior of the optimized airfoil geometry. Approved for public release: distribution unlimited
A state-space, semi-empirical dynamic stall model is presented for use in helicopter flight simulators and onboard flight computers. Dynamic stall is a limiting flight condition for helicopters at high advance ratios and during high-g maneuvers, and to represent it requires the development of reduced-order models that compute loads in real-time. This work validates the stall model of Ref. [1] against wind tunnel tests of NACA 0012 wing sections in unsteady freestream and yawed flow. An improvement of the optimization technique in Ref. [2] is presented to greatly reduce time required to identify parameters.
A comparison study between modeling approaches of a quadrotor biplane tailsitter aircraft is conducted. A blade element theory model with dynamic inflow is used to validate a reduced order model that incorperates a simple interference model for trajectory planning and dynamic simulation. With an appropriate interference model, the predicted power requirement through transition from hover to forward flight drops by 30-45% as the interference velocity reduces the effective angle of attack for the wing. A trajectory generation scheme is developed, which shows the importance of accurate stall modeling for the transition maneuver. Without interference modeling all transition trajectories are expected to violate the installed motor power limit or pass through an excessively stalled wing state (> 60%). The interference model dynamics are used to design a trajectory that avoids stall of the aircraft by adding a vertical climb element to the transition maneuver. A transition controller is linear dynamic inversion transition controller is described for inner loop stability over the entire flight regime.
Dynamic stall is an important source of vibrations on a rotor at high advance ratios. Dynamic stall loads are induced by periodic flow separation and reattachment. In this study, the flow separation is modeled as the shedding of concentrated vorticity, known as a vortex particle, from the leading edge of the airfoil. The rotor wake is obtained from the generation of vortex particles over the rotor blade using the Viscous Vortex Particle Method. Blade loads are calculated using a reduced order model obtained from CFD, and dynamic stall loads are calculated using the ONERA dynamic stall model. Results are presented for isolated and coaxial rotors at advance ratios of μ = 0.3 and μ = 0.4. The results indicate that the separated wake modifies the vibratory hub loads by 15 percent-30 percent for an isolated rotor at μ = 0.3. The vibratory hub loads for the coaxial rotor are modified by 10%-65% at μ = 0.4. The separated wake modifies the angle of attack distribution on the rotor and hence influences the loads. The upper and lower rotor tip path planes are tilted such that the blade and wake interaction increases on the retreating side of the upper rotor, and decreases on the advancing side.
Dynamic stall is a highly complex phenomenon characterized by unsteady massive separated flow. It limits the flight envelope of helicopters by generating vibrations and large dynamic loads which can lead to fatigue and structural failure of blades. Dynamic stall involves several mechanisms which make the numerical prediction of stall difficult and the understanding of the phenomenon still incomplete. A loose coupling methodology between a Computational Fluid Dynamics and a Comprehensive Analysis codes is used to simulate the problem. Three stalled flight conditions have been selected in the wind tunnel 7A rotor test data to investigate the RPM effect on the dynamic stall onset and the related mechanisms. The lower the RPM, the more severe the stall is. A double stall has been observed on the lowest RPM case. The coupled simulations are in satisfactory agreement with experiment and are used to identify the mechanisms leading to stall. Simulations indicate that the blade-vortex interaction is an important factor in triggering the different stall events in these configurations.
Pitching airfoil measurements are known to exhibit significant scatter at near- and post-stall angles of attack. Applying data-driven algorithms revealed the presence of bimodal distribution within the data scatter, suggesting that the statistical mean and standard deviation often used to represent cycle-to-cycle variations are incorrect. Considering the historical significance of dynamic stall measurements, a thorough assessment was undertaken to ascertain that the observed furcation in the data is not a result of facility or post processing error. Once confirmed, cluster-averages, associated variances, and group probability were identified as the best alternative to represent groups in the data. Several existing clustering techniques were tested, however, their shortcomings led to the development of two new data-driven algorithms. A uniqueness that is common to both of the new algorithms is that the clustering process is based on the flow phenomena that contribute the most energy to the overall flow variations. By operating in the optimal basis that maximizes the variance in the measurements, separation of clusters became efficient. When applied to several test cases, the clusters revealed the causes for such grouping, such as the variations in the separation location, occurrence of LE/TE stall, presence/absence of a dynamic stall vortex (or vortices), reattachment angle, etc. In all the cases, the physical processes and their effects were obscured by the phase-average curves. Further analyses to study the effects of Mach number, reduced frequency, mean angle and amplitude of oscillation revealed trends in the group probability. Aerodynamic damping, peak values of pitching moment and lift were substantially different between the clusters, as well as with the phase-average. Considering future semi-empirical models that need to account for cycle switching from one group to another, Markov process (and chain) was studied.
ABSTRACT Aeroelastic stability of stiff-in-plane hingeless rotors is investigated using the comprehensive analysis RCAS. Aeroelastic stability analysis of stiff-in-plane rotors in hover is compared to experimental measurements that shows an overall fair to good agreement for various rotor parameters. The analysis reveals that blade lead-lag damping decreases sharply and the blades become aeroelastically unstable when the blades stall. Stiff-in-plane rotor aeroelastic stability analysis in forward flight is compared to a previous numerical study. Then, using the rotor models as a baseline, a parametric study is performed for various rotor parameters including aerodynamic models, rotor speed, rotor thrust, lead-lag frequency, precone, contol system flexibility, and tip sweep. The parametric study covers lead-lag frequencies of stiff-in-plane rotors from 1.1 /rev to 1.4 /rev with a flap and a torsional frequencies of 1.15 /rev and 3.0 /rev. The parametric study shows that blade lead-lag mode of the stiff-in-plane hingeless rotors tends to become aeroelastically unstable at high advance ratios. Blade lead-lag damping of the stiff-in-plane rotors decrease as the lead-lag frequencies increase from 1.1 /rev to 1.4 /rev. Aeroelastic stability boundaries (advance ratio) increase as the rotor speed increases for a fixed rotor lift. Aeroelastic stability boundaries increase as the rotor lift decreases for a fixed rotor speed. Aerodynamic stall demonstrates significant effects on aeroelastic stability of stiff-in-plane hingeless rotors.
ABSTRACT An experiment was conducted on a two-dimensional SC1094R8 airfoil model with the intention of investigating the cycle-to-cycle variations observed in dynamic stall. Unsteady surface pressure measurements were recorded at numerous points along the airfoil surface at specific pitching conditions that displayed more than one preferred reattachment process. Sets of individual cycles that represented the dominant reattachment processes were identified and compared to the phase-average, showing significant differences in pressure distributions and aerodynamic loads. Proper Orthogonal Decomposition was then employed to the unsteady pressure distributions with the intent of isolating the influence of specific modes on the cycle-to-cycle variations. The results indicate that the importance of the modes changes depending on the particular reattachment process observed. Approved for public release: distribution unlimited. Review completed by the AMRDEC Public Affairs Office (PR3 3750, 10 April 2018).
ABSTRACT This work presents a conceptual variable diameter tiltrotor sized in the NDARC conceptual design code. Both the maximum allowable diameter change and the weight increment due to morphing were varied with the intent of investigating the system level benefits (improved weight, fuel use, etc.) associated with a variable diameter rotor system. Generally, for a 0% increment in rotor weight due to morphing, diameter morphing reduced vehicle gross and component weights, as well as mission fuel weight. As the rotor weight penalty for morphing was increased, the entire system saw a corresponding increase in weight. For these increased rotor weight conditions, diameter morphing was again able to reduce total system weight back to, or lower than, the baseline design. At very high levels of morphing however, (large reductions in rotor radius for cruise) weight was seen to drastically increase. This was traced to the rotor approaching stalled conditions during cruise due to increased blade loading. A conceptual approach for estimating cruise blade loading for a variable diameter or variable speed tiltrotor was then presented. In general, this work shows that even for fairly substantial increases in rotor weight, variable diameter morphing can have a system wide benefit in the form of reduced gross weight and reduced fuel weight for the given design mission.
ABSTRACT A new active flow control strategy by placing a synthetic jet actuator (SJA) and a trailing-edge flap (TEF) has been proposed, and its control effects on mitigation of large negative pitching moments and drag caused by rotor dynamic stall are numerically investigated by CFD method. A moving-embedded grid method and an unsteady Reynolds averaged Navier–Stokes (URANS) solver are established for predicting the complex flowfields of rotor and airfoil. Calculated results of VR-12 and SC1095 airfoils indicate that TEF and SJ can suppress the formation of dynamic stall vortex and postpone flow separation over rotor airfoil, resulting in much lower Cdmax and Cmmax comparing to the baseline state, and aerodynamic characteristics of airfoil could be further improved by the new control method comparing to individual control method. Furthermore, parametric analyses on dynamic stall control of airfoil by the combinational method are conducted, and it indicates that aerodynamic characteristics of an oscillatory rotor airfoil could be significantly improved when non-dimensional frequency (k*) of TEF is about 1.0, and larger angular amplitude help to increase the lift coefficient. At last, simulations are conducted for the flow control of rotor airfoil by the combinational method, and the numerical results indicate that the TEF can better improve the lift characteristics of rotor airfoil, and synthetic jet further increases the aerodynamic characteristics of rotor airfoil.
ABSTRACT Helicopter reaches its flight domain limit at high-thrust forward flight or maneuver with high load factors because of dynamic stall. This phenomenon is due to complex unsteady three-dimensional flow separation mechanisms that occur on the retreating blade. These flow separations can be of different natures depending on the flight conditions. This paper proposes to investigate numerically two significantly different cases of dynamic stall on a helicopter rotor in forward flight. The results show that the numerical simulation can capture the variations of section pitching moment associated with dynamic stall, for each of these two flight cases. A deep analysis of the numerical results allows to identify distinct flow separation regions appearing on the rotor disk. Similarities and differences are highlighted between the dynamic stall characteristics of these two rotor flight conditions.
ABSTRACT A rotor test facility was developed to investigate dynamic stall under optimized boundary conditions compared to conventional hover chambers. It features a defined axial inflow, reduction of ground effect and recirculation of the rotor wake and good optical access to apply non-intrusive measuring techniques. The rotor consists of two blades with an aspect ratio of 6:8 and a tip radius of 0:65m. It was operated at a chord based Reynolds number of 350,000 and a Mach number of 0:21, both at 75% radius. The flow and blade deformation were analyzed by means of unsteady blade pressure transducers, particle image velocimetry and tip deflection measurements covering the whole azimuth and different radii. Three measurement approaches to detect flow separation: tufts, differential infrared thermography and surface pressure analysis showed comparable results and flow separation over one half of the cycle. A radius-dependent separation point in time and an altering of the dynamic stall behavior near the tip were observed. The recirculation area remained close to the blade's surface with radial jet flow increasing towards the tip. A maximum increase of the effective angle of attack of 1:4° was observed due to blade deformation. For a fully attached flow case the laminar-turbulent boundary layer transition differed along the radius and moved over 70% of the blade's chord.
ABSTRACT A whirl and wind tunnel test was conducted to measure performance and stall vibration benefits associated with a novel Retreating Side Blowing (RSB) concept, Miniature Trailing edge Effectors (MiTEs), and Lateral Lift Offset (LLO) trim. A new 28 ft diameter rotor mounted to an articulated S-76R rotor hub was flown at the National Full Scale Aerodynamics Complex (NFAC). Technical objectives were slightly improved performance and significantly reduced retreating blade stall vibration at high blade loading-advance ratio combinations. The blades were well instrumented with five fully strain gauged spanwise locations as well as external, internal, and blowing slot pressure and temperature sensors. Retreating blade stall conditions were reached between 0.24 to 0.61 advance ratios. The RSB pumping power and slot profile drag effects reduced L/De at mid blade loadings but leading edge blowing regained the performance loss at high blade loadings. The MiTEs showed a slight L/De improvement at high blade loadings. Lateral lift offset showed up to 35% L/De improvements as blade loading was increased. The RSB concept slightly reduced retreating side torsional vibration. The RSB dynamic lift and moment improvements were not as beneficial as expected due to the dynamic Mach number on the retreating side. The MiTEs reduced retreating blade stall vibration by 20% to 40%. Test data revealed rotor blade peak torsional vibration was not improved above advance ratio 0.32 by any concept. This is because peak torsional blade loads were not generated by retreating side stall. They were generated roughly between 20 deg and 200 deg azimuth positions. Therefore even if the retreating side technologies worked perfectly the rotor stall boundary would not have improved. Subsequent review of UH-60A slowed rotor and AH-56A flight test data show the same phenomena.
ABSTRACT A novel passive flow control concept - based on the local modification of an airfoil's surface - is proposed and examined via CFD for the mitigation of the negative effects of dynamic stall, i.e. for the reduction of peak negative pitching moment while not deteriorating significantly the original lift and drag characteristics. 2D CFD simulations of a NACA 0012 airfoil exposed to a freestream of Mach 0.3 and Re = 3.76×10⁶ and undergoing a 15°±10° pitch oscillation with a reduced frequency of 0.101 were conducted. The baseline airfoil simulations were carefully verified and validated, showing excellent agreement with wind tunnel data. Twenty-six different local geometry modifications were proposed and examined, all functioning as a trapped-vortex generator. The surface modifications were examined on both the upper and lower surfaces. In case of the upper surface modifications, the best geometries could reduce the peak negative pitching moment by as much as 37-63%, while sacrificing only 2-10% of peak lift and reducing drag by 14-38%. On the other hand, the lower surface modifications demonstrated the ability to increase lift by 4-16% with only minor penalty in pitching moment and drag.
ABSTRACT In order to research the three-dimensional effects on the dynamic stall of rotor blade, the unsteady flowfields of finite-wing and rotor are simulated under dynamic stall conditions respectively. The unsteady RANS equations coupling with third -order Roe-MUSCL spatial discretizat ion scheme are chosen as the governing equations to predict the three dimensional flowfields of finite-wing and rotor, and the Spalart -Allmaras turbulence model is employed to calculate the viscidity of unsteady flowfields. From the simulat ion results of finite-wing, it is illustrated that the aerodynamic loads of wing would be reduced due to the effect of wing-tip vortex. As a result, the dynamic stall vortex of wing would be weakened near the wing-tip. Therefore, the peaks of lift coefficient, drag coefficient, and pitching moment coefficient are decreased near wing-tip. The spanwise flow on finite-wing would cause accumulat ion of dynamic stall vortex. As a result, the dynamic stall is restricted near the wing-root, and aggravated near wing-tip. By simulating the flowfield of a rotor in forward flight, it is illustrated that the centrifugal force would lead to generate spanwise flow. As a result, the dynamic stall characteristics of rotor would be restricted due to the Coriolis force. What is more, the decrease of angle of attack of rotor blade induced by downwash flow would inhibit the dynamic stall characteristics further.
ABSTRACT This work presents results of an experimental investigation into synchronized active flow control of a Sikorsky SSC-A09 airfoil undergoing periodic pitching motion in an unsteady free stream using leading edge blowing. The airfoil was evaluated at reduced pitching frequencies up to k=0.05 at steady Mach numbers of 0.2 and 0.4, and at k=0.025 with phase-locked pitch and Mach oscillations at Mach 0.4±0.07 at Reynolds numbers from 1.5 to 3 million. A spanwise row of vortex generator jets (VGJs) located at 10% chord is fed by an oscillating valve that is phase-locked to the pitch oscillation of the airfoil. The oscillating valve can be set to produce a peak jet mass flux ratio (Cq) of 0.0022 or 0.0028 with a background Cq of half this value over the remainder of the period. The phase and duration of the peak Cq were varied to optimize the flow control benefits to both CL and CM hysteresis loops and reduce negative damping. Peak performance was observed with actuation initiated just after lift stall and continuing for 11% of the pitch cycle. Blowing beyond 11% resulted in no perceptible benefit. Compared to steady blowing flow control, the 11% synchronized control case delivers comparable (or better) performance with less than 50% of the massflow. The degree of stall control is a function of reduced frequency, mass flux ratio, and Mach number.
ABSTRACT A variable stiffness composite optimization framework for wind turbine rotor blades is presented. The framework consists of a multi-fidelity approach for wind turbine rotor analysis, where both structural and aerodynamic constraints are considered during the optimization. The potential of twist coupled blades to regulate the power on stall controlled wind turbines is investigated by exploiting the characteristic of unbalanced laminates to induce twist coupling. A complete stiffness variation along the blade span is considered during the optimization, while using the cost of energy as the objective function. Results show that unbalanced laminates provide a greater capabilitiy (compared to balanced laminates) to reduce the cost of energy of stall controlled wind turbines by exploiting extention-twist and bend-twist coupling of composite blades.
ABSTRACT The Peters-Modarres semi-empirical dynamic stall model is extended to the simulation of pitching moment and drag. The new model is correlated with experimental data. In particular, wind tunnel and water tunnel tests of harmonically pitching Boeing VR-12 and VR-7 airfoils are used for validation, including Mach numbers of 0.2, 0.3, and 0.4, and reduced frequencies of 0.02 - 0.25. Secondary peaks in the pitching moment stall data are modeled with an additional secondary stall model: a simple second-order equation driven by a rectangular pulse, requiring no additional states in simulation. As a state-space model, the extended model is in a form useful for design of controls and flutter analyses. An optimization routine is used to determine the empirical parameters of the stall model by comparison of the computed results with each experimental case, producing a set of 12 stall parameters and 2 static corrections. The results show good agreement for each case, and are an improvement over previous efforts by Ahaus to correlate pitching moment and drag data.
ABSTRACT Rotor blade aerodynamics are significantly influenced by dynamic stall. The objective of this study is to use a combined experimental/ computational approach toward better understanding of the dynamic stall phenomenon on the SC1094R8 airfoil. This study is part of a larger effort to alleviate the adverse effects of dynamic stall on rotor blade aerodynamics through airfoil shape optimization. In order to accomplish this goal, time-accurate surface pressure measurements along with lift and moment coefficients are gathered experimentally. Boundary layer tripping is performed to ensure behavior similar to that of the airfoil at larger Reynolds numbers. Sensitivity of dynamic stall under different flow conditions is also studied. Numerical simulations have been performed for similar flow conditions and compared with the experimental results. The result have aided in clarifying the sensitivity of dynamic stall as well as providing encouraging indications of the ability of the simulations to capture the important features associated with dynamic stall.
ABSTRACT An aeromechanics analysis was conducted of a large-winged, single main rotor, compound helicopter modified from the AH-56 Cheyenne, in cruise and high-speed flight (250 knots) at sea level and high altitude (20,000 ft.) conditions. Performance and representative loads were evaluated with the comprehensive code RCAS to show the effect of compound configuration decisions. Suitability of the analysis for high advance ratio predictions was demonstrated through comparison to the UH-60A slowed rotor test data, and validation of compound performance prediction was shown with AH-56 Cheyenne test data. An assessment of the role of compound configuration, collective setting, wing pitch, rotor speed, altitude and trim control strategy on performance and loads was made. The study shows how reducing collective, for a constant wing pitch, is beneficial for peak L=De and reducing loads. Increasing wing pitch, at a constant collective, improves peak L=De, but can reduce L=De at high airspeeds and limit maximum airspeed. A 30% reduction in rotor speed achieved significant performance increases, while larger reductions limited high airspeed and high altitude trim due to stall. High altitude flight improves L=De at all airspeeds if stall on the prop rotor can be prevented. Ailerons and horizontal stabilizer trim control does not adversely affect L=De, but free rotor cyclics to trim for reduced flap bending loads. Interference effects from the rotor on the wing reduced peak L=De, but the effect is marginal above 170 knots. The final compound configuration achieved a peak L=De = 7.6 around 140 knots and L=De > 5.0 at the maximum airspeed considered of 250 knots. The static control derivatives for the compound showed less authority than a conventional helicopter, however increasing control authority is shown across the flight envelope.
ABSTRACT Retreating Side Blowing (RSB) is a concept to blow air through the blade to suppress dynamic stall on the retreating side of the rotor, and enhance a vehicle's flight envelope at high speed and high loading forward flight conditions. Passive RSB utilizes a rotating blade as a centrifugal pump to drive flow from the inlet at the root to the outboard region. Current numerical studies examined the effectiveness of RSB in conjunction with validation against wind-tunnel measured data at high advance ratio conditions. The impact of varying freestream velocity on the performance of a blown pitching airfoil was also examined using two-dimensional airfoil calculations. The variation and timing of the freestream velocity significantly decreased the stall suppression benefit of a blown airfoil versus a fixed freestream. The validation of three-dimensional rotor simulations showed good correlation with measured data in predictions of integrated performance, section loading, and duct flow properties. Both measured data and numerical simulation showed no significant performance benefit from RSB in the tested flight conditions. Detailed examination indicated the RSB was able to reduce retreating side separation, but pumping torque, geometric modification due to slots, and increased retreating side horizontal force overwhelmed the retreating side torque benefit. Additional numerical explorative studies showed that powered-blowing could achieve significant performance improvement from pure aerodynamic perspective. However, the additional blowing power cancelled out most of benefit. An inlet valve model for simulation of scheduled blowing was also studied, but meaningful benefit was not observed. The retreating side blowing was able to reduce aerodynamic sources of retreating side vibration torsional load. However, the peak blade torsional load emerged on the advancing side at high advance ratio conditions.
ABSTRACT Time-dependent Navier-Stokes simulations have been carried out for a flexible UH-60A rotor in forward flight, where the rotor wake interacts with the rotor blades. These flow conditions involved blade vortex interaction and dynamic stall, two common conditions that occur as modern helicopter designs strive to achieve greater flight speeds and payload capacity. These numerical simulations utilized high-order spatial accuracy and delayed detached eddy simulation. Emphasis was placed on understanding how improved rotor wake resolution affects the prediction of the normal force, pitching moment, and chord force of the rotor. Adaptive mesh refinement was used to highly resolve the turbulent rotor wake in a computationally efficient manner. Moreover, blade vortex interaction was found to trigger dynamic stall. Time-dependent flow visualization was utilized to provide an improved understanding of the numerical and physical mechanisms involved with three-dimensional dynamic stall.
ABSTRACT This paper provides a fundamental understanding of the unsteady aerodynamic phenomena on a cycloidal rotor blade operating at ultra-low Reynolds numbers (Re∼18,000) by utilizing a combination of experimental (force and flowfield measurements) and computational (CFD) studies. For the first time ever, the instantaneous blade fluid dynamic forces on a rotating cyclorotor blade were measured, which, along with PIV-based flowfield measurements revealed the key fluid dynamic mechanisms acting on the blade. A 2D CFD analysis of the cycloidal rotor was developed and systematically validated using both force and flowfield measurements. Studies were performed with both static and dynamic blade pitching. Direct comparison of the static and dynamic pitch experimental results helped isolate the unsteady phenomena (such as dynamic stall, unsteady virtual camber, etc.) from the steady effects. The dynamic blade force coefficients were almost double the static ones clearly indicating the role of unsteady mechanisms on force production on cyclorotor blades. For the dynamic case, the blade lift monotonically increased even up to ±45° pitch amplitude due to dynamic stall phenomenon; however, as expected, for the static case, the flow separated from the leading edge after around 15° with large laminar separation bubble (LSB) and eventually completely separating at higher pitch angles. For both static and dynamic pitching cases, there was significant asymmetry in the lift and drag coefficients between positive and negative pitch angles due to the flow curvature effects (virtual camber). CFD flow solution and PIV measured flowfield correlated well and both showed the formation and shedding of strong dynamic stall or leading edge vortices, especially at higher pitch amplitudes, which is the reason for the stall delay and force enhancement. Also, the dynamic stall process during the upper half of the trajectory was significantly different from the lower half even with symmetric blade pitch kinematics because of the reversal of dynamic virtual camber from the upper to the lower half. Even at such low Reynolds numbers the pressure forces, as opposed to viscous forces, were found to be dominant on the cyclorotor blade. The power required for rotation (rather than pitching power) was the domineering component of the total blade power for the dynamic pitching case.
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