Browse Topic: Stall
A research team developed a smart strake system that dynamically adapts to flight conditions, showing a promising drag reduction in the wind tunnel with respect to passive strakes. This approach has the potential to save airlines hundreds of kilograms of fuel per flight. University of Washington Department of Aeronautics & Astronautics (A&A), Seattle, WA For decades, aircraft have carried a fundamental compromise between their engines and wing flow interactions by using strakes. These are small fins attached at the sides of engine nacelles that generate helpful vortices during takeoff and landing that boost lift and avoid stall, but create unwanted drag during cruise flight. Now, seven William E. Boeing Department of Aeronautics & Astronautics (A&A) undergraduates have advanced a solution that improves this trade-off, achieving up to 33 percent drag reduction, on the limited tested conditions, during cruise while maintaining critical safety benefits at high angles of attack. The team
ABSTRACT 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.
ABSTRACT 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
ABSTRACT 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
ABSTRACT 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
ABSTRACT 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
ABSTRACT 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 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.
ABSTRACT 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
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ABSTRACT Experimental investigations of three-dimensional dynamic stall on a four-bladed Mach-scaled semi-elastic rotor with an innovative double-swept rotor blade planform are presented. The study focuses on the coupling between the aeroelastic behavior of the blade and the underlying aerodynamics. Blade bending moment and flap displacement measurements were conducted using strain gauges and optical tracking of blade tip markers. The aerodynamic behavior was characterized by means of unsteady surface pressure measurements using unsteady pressure-sensitive paint (iPSP) across the outer 65 % of the blade span and fast response pressure transducers at discrete locations. Different cyclicpitch settings were investigated at a rotation frequency of frotor = 23.6 Hz, that corresponds to blade tip Mach and Reynolds numbers of Mtip = 0.282 - 0.285 and Retip = 5.84 - 5.95 x 105. The findings reveal a detailed insight into the non-linear behavior in the flap movement during downstroke. iPSP and
ABSTRACT The unsteady laminar separation and subsequent dynamic stall vortex (DSV) formation is investigated on a NACA 0012 airfoil section subject to a constant pitch rate motion using Delayed Detached Eddy Simulations (DDES) in NASA's OVERFLOW 2.3 solver. This study focuses on the complex flow features during the initial DSV formation and analyzes the distinct mechanisms from which the vortex is formed. It is shown that DDES accurately predicts the bursting of a laminar separation bubble (LSB), which triggers the onset of a DSV. In parallel to studying the feasibilty of DDES in terms of capturing distinct flow features compared to Large Eddy Simulation (LES) results, a turbulence model study is also carried out, analyzing the influence of stateof-the-art turbulent and transition models on the DSV formation and subsequent stall onset. These include the fully turbulent Spalart Allmaras (SA) turbulence model and three different transition models: SA Coder Amplification Factor Transport
ABSTRACT This work presents the state-of-the-art of the validated buffet loads determination methodology developed by Leonardo Helicopters, with particular attention to the aerodynamic modelling and the related structural loads calculation. The modern CFD, validated by WT data, is the first major topic addressed in the paper: simulations are performed with ANSYS Fluent to characterize the oscillatory pressure distributions generated by the wing stall. The structural loads calculation is the next part of this process, with a focus on the interface with the aerodynamic input and on the necessary statistical analyses to predict safe loads. Buffet is considered a random phenomenon, which is robust and repeatable only in terms of statistic quantities, and, to estimate the maximum likely values, a probabilistic approach is used to calculate a reasonable safety factor. The last topic is the analysis of actual flight data in compatible high angle of attack conditions in order to substantiate
ABSTRACT Hover trim and dynamic analyses were performed on a UAM-scale quadcopter with both variable rotor speed and variable collective blade pitch. The bare-airframe dynamics were first considered at three different hover trim points, where power consumption is increased to improve authority. The control and stability derivatives were examined at each trim point and an increase in base RPM caused increased authority for pitch inputs (and decreased authority for RPM inputs) in thrust-dominated axes. Explicit model following control laws wre then optimized using CONDUIT to meet ADS-33E-PRF handling qualities specifications. Design margin optimization was then performed on each axis. Heave and yaw responses of the linearized system were examined for the three trim points with either RPM or pitch control. It was found that pitch-control outperformed RPM-control in heave, while the opposite was true for yaw. Hybrid control mixing was considered using a complementary filter, so that it
ABSTRACT This paper discusses the benefit of the multi-ducted angled rotor (M-DAR) distributed electric propulsion system in a V/STOL aircraft called the M-Star. The M-DAR propulsion system consists of an array of fixed pitched ducted fans. Traditional fan-in-wing designs can suffer from pitch up effects, stall, instability, and momentum drag when transitioning to forward flight as the oncoming air vector increases. In such planar orientations the air must accelerate rapidly into the ducted fan to prevent blade instability. The M-DAR solves these challenges by lowering the angle at which air enters the fans. A 45-degree M-DAR system enables a higher transition speed than 90 degree ducted fans since the air does not need to turn the additional 45-degrees into the ducts. The M-DAR system is comprised of an array of fans which increase the total disk area while the front plate area remains fixed. The fixed fans also eliminate the weight and complexity of actuators-essentially reducing the
ABSTRACT A model of a coaxial helicopter with a rigid rotor system is modified to have dual side-mounted propellers. The aircraft is simulated in trim at different flight speeds to investigate the potential benefits of the dual propellers with regard to fault tolerance and yaw control authority. At low speed, the dual propellers impact on the main rotor system actuator failure ranges is analyzed, demonstrating an increase in the allowable trim range from 25-30% in the nominal platform to 50-60% with the dual propeller configuration. Relaxation of the stall constraint for the dual propellers expands the upper rotor maximum actuator limits to the maximum geometric limit, minimum actuator limits for the lower rotor are constrained by the upper rotor stall limits. Upper rotor minimum and lower rotor maximum actuator limits are determined by tip clearance restrictions. At mid speed, the dual propellers improve the yaw control power in the 50-100 kt range and allow for a Level 1 aggressive
ABSTRACT A preliminary investigation of impact of piloting and flight control strategies on maneuver noise is conducted on a generic eVTOL configuration undergoing a 50 knot level-turn maneuver. The piloting strategy involved control of aircraft pitch to change split between rotor lift and wing lift, while the control strategy involved comparing a rotor thrust control with fixed pitch rotors operating with variable rotation rate and a rotor thrust control strategy with variable pitch rotors operating at constant angular velocity. With the rotors operating in the low tip-Mach number flow regime, it was revealed that broadband noise due to airfoil self-noise dominates the noise levels overwhelmingly. The turbulent boundary layer trailing edge noise contributed the most, with blade stall found to result in significant addition to noise levels (nearly 10 dBA). Deterministic noise was found to be sensitive to rotor thrust control strategies, with control biases offering an additional layer
Passengers would always like to reach their destinations with minimum commute time. Generating a higher thrust is a necessity. This implies that the turbomachinery associated with the power plant has to rotate faster and with higher efficiencies. However, high rotational speeds, mainly in the transonic regime, often lead to boundary layer separation, shocks, compressor stall, and surge. The current investigation is an attempt to reduce the abovementioned phenomena. It involves the performance study of a smoothened controlled diffusion airfoil (CDA) blade that has been optimized by “Multi-Objective Genetic Algorithm” (MOGA) by altering maximum camber location and stagger angle. Inlet pressure is varied from 15 kPa to 30 kPa and the angle of attack ranging from 40.4° to 56.4°. C48-S16-BS1 is validated and considered as the baseline profile, and all other blades are collated to this. It is observed that shifting the location of the maximum camber close to the leading edge and increasing
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When an aircraft veers upwards too much, the decrease in lift and increase in drag may cause the vehicle to suddenly plummet. Known as a stall, this phenomenon has prompted many drone manufacturers to err on the side of extreme caution when they plan their vehicles’ autonomous flight movements. For vertical takeoff and landing (VTOL) tail-sitter drones, most manufacturers program the aircraft so that the vehicle body turns very slowly whenever it transitions from hover to forward flight and vice versa.
ABSTRACT 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
ABSTRACT 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
ABSTRACT 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
ABSTRACT 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
Dual mass flywheel (DMF) is an excellent solution to improve the noise, vibration and harshness (NVH) characteristic of any vehicle by isolating the driveline from the engine torsional vibrations. For the same reason, DMFs are widely used in high power-density diesel and gasoline engines. However, the real-world usage conditions pose a lot of challenges to the structural robustness of the DMF. In the present work, a new methodology is developed to evaluate the robustness of a DMF fitted in a compact sports utility vehicle (SUV) with rear-wheel drive architecture. The abuse conditions (mis-gear, sudden braking, etc) in the real-world usage could lead to a sudden engine stall leading to an abnormally high angular deceleration of the driveline components. The higher rate of deceleration coupled with the higher rotational moment of inertia of the systems might end up in introducing a significantly high impact torque on the DMF. Hence, prolonged usage of the vehicle in abuse conditions
The present work is focussed on the real-world challenges of a dual mass flywheel (DMF) equipped vehicle in the Indian market. DMFs are widely used to isolate the drivetrain from the high torsional vibrations induced by the engine. While DMFs can significantly improve noise, vibration and harshness (NVH) characteristics of a vehicle, there are multiple challenges experienced in real-world operating conditions when compared with the single mass flywheel (SMF). The present work explains the challenges of using a DMF in a high power-density diesel powertrain for a multi-purpose vehicle (MPV) application in the Indian market. Measurements on the flat-road operating conditions revealed that the DMF vehicle is very sensitive for launch behaviour and requires a higher clutch modulation. Vibration measurements at the driver’s seat confirm that the SMF vehicle could be launched more comfortably at the engine idle speed of 850 RPM. However, the DMF vehicle needs a "launch assist" of an
Since the torque converter and fluid coupling are commonly used components of automatic transmissions in industry, the SAE appointed a committee to standardize terminology, test procedure, data recording, design symbols, and so forth, in this field. The following committee recommendations will facilitate a clear understanding for engineering discussions, comparisons, and the preparation of technical papers. The recommended usages represent the predominant practice or the acceptable practice. Where agreement is not complete, alternates have been included for clarification. EXAMPLE: Two systems of blade angle designations are described. Consequently, when a blade angle is specified, the system should be designated. This SAE Recommended Practice deals only with the physical parts and dimensions and does not attempt to standardize the design considerations, such as the actual fluid flow angle resulting from the physical blade shape.
This paper presents a coupled numerical and experimental study of an unconventional wing profile such as cp-180-050-gn (Cambered plate C = 18% T = 5% R = 0.78). This wing profile deals with low speeds. It is not currently used on any aircraft model. Otherwise, it presents interesting performances that can be exploited for the design of low-speed STOL or VTOL aircraft by mean of the very high lift that it can generate and can fit with different uses such as VAWT, cyclorotors drones, which are designed explicitly for low-speed operations. After a preliminary CFD assessment of the wing a complete experimental characterisation also at high angles of attack has been performed. The excellent agreement between CFD and experiments has allowed producing a complete analysis of the behaviour of the wing profile both before and after stall conditions. This study has the objective of analysing the viability of such an unconventional wing in traditional or over-stalling conditions. A complete
Cars in several motor sports series, such as Formula 1, make use of multi-element front wings to provide downforce. These wings also provide onset flows to other surfaces that generate downforce. These elements are highly loaded to maximise their performance and are generally operating close to stall. Rubber debris, often known as marbles, created from the high slip experienced by the soft compound tyres can become lodged in the multiple elements of a front wing. This will lead to a reduction in the effectiveness of the wing over the course of a race. This work will study the effect of such debris, both experimentally and numerically, on an inverted double element wing in ground effect at representative Reynolds numbers. The wing was mounted at two different ride heights above a fixed false-floor in the Loughborough University wind tunnel and the effect of debris blockage modelled by closing sections of the gap between elements with tape. The reduction in downforce compared to the
The scope of this SAE Aerospace Information Report (AIR) is to present a guide for the determination of probable power output and the effect on the aircraft system that will be experienced when operating three-phase motors with one phase open. Unfortunately, the above subject cannot be resolved by specific rules. Modern aircraft or missile electrical systems are composed of a wide variety of electrical and electronic components. These components react differently under identical impetus due to the latitude of their design. This latitude of design must be allowed wherever possible to the accessory designer due to the various specification requirements. Therefore, it cannot be over-emphasized that the effect on the airplane or missile system, as well as motor operation, of three-phase motors on two-phase power must be thoroughly investigated.
This recommendation establishes objectives for high performance control motors to be used with aeronautical and associated equipment in protective enclosures or completely within the shell of the aircraft so that they are subjected only to the internal climatic conditions of heat, cold, shock, vibration, altitude, and humidity. Control motors larger than size #23 are not covered in this document.
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