Helicopter In Forward Flight Research Articles

Numerical study of airfoil used for helicopter rotor tip to investigate the enhancement in the aerodynamic characteristics using active flow control at forward flight conditions

Helicopter forward flight is the most important state of flight due to time spent in this mode during the complete helicopter trip. In this study, a main rotor blade tip airfoil was simulated to obtain the effect of applying active flow control on the tip part of the rotor blade. A blowing excitation jet is applied to the leading edge of the airfoil just before the separation point with small momentum and high frequency to enhance the near wall boundary layer characteristic. This jet is used with different amplitude, frequency and excitation location from a leading edge to study the effect of this parameter. A numerical model is used to solve two different angles of attack and all results are compared with NACA report that used the same helicopter hub and airfoil. Results show good agreement with the experimental results and the chosen excitation parameter gives good enhancement to the performance of the airfoil used in the tip part of the helicopter blade in this mode of flight. Performance represented in lift and drag and lift to drag ratio shows improvement, reaching about 10% increase in lift and 30% decrease in drag, which means more than 50% change in lift to drag ratio. Furthermore, this study gives a good estimation of the best excitation location that is required for real application on the helicopter blade to improve of forward flight performance.

Helicopter performance enhancement by alleviating retreating blade stall using active flow control

In this study, a model was created numerically to study the effect of active flow control on helicopter blade to improve the retreating blade stall. Active flow control is applied to helicopter blades to improve retreating blade stall along with the overall helicopter performance. The novel aspect of this study is using active control in a helicopter with a complex aerodynamic environment and using it in all significant flight modes. This has the advantage of practical application since active control can be used constantly without having to turn it off. The flow control is established by using blowing jets at the leading edge of the airfoil that is used at the tip part of the main rotor blade. 2D and 3D computational fluid dynamics (CFD) Turbulent-unsteady models are developed to investigate the effect of applying blowing jets on different helicopter modes of flight (hover and forward flight). Different excitation parameters and turbulence models are compared to get the maximum possible enhancement of the aerodynamic characteristic of the flow both in 2D and 3D. The results are validated against a NASA helicopter main rotor hub with known geometry and available performance parameters. Both models illustrated a good agreement with the published benchmark problem. The results revealed that the active control blowing jets can enhance the stall characteristics and aerodynamic performance of helicopter blades. The method delays the flow separation and increase the blade lift by about 10% in addition to a 40% increase in the lift to drag ratio. Thus, we can conclude that the present active control technique can enhance the helicopter in forward flight by delaying the retreating blade stall. This research offers insightful information on the usage of blowing jets for rotor flow control.

Analysis of Aerodynamic Characteristics of Different Tail Configurations of Compound High-speed Helicopters

When a compound high-speed helicopter is in forward flight, there are serious aerodynamic disturbances among the fuselage components, among which the tail configuration has a greater influence on its aerodynamic performance. To study the influence of different tail configurations on aerodynamic performance, the flow field of a compound high-speed helicopter in forward flight was numerically simulated based on the commercial software XFlow. Firstly, the ROBIN standard helicopter fuselage model was calculated to verify the scientific validity of the method. Then, the aerodynamic data of a high-speed helicopter was analyzed for different tail configurations in forward flight with different angles of attack and yaw. The results show that the V-tail configuration has the lowest lift and resistance coefficients and the best longitudinal manoeuvrability and stall characteristics; the conventional tail configuration has the highest lift-to-resistance ratio and the fastest wake decay; the lateral stability of the V-tail and conventional tail configurations is weaker than that of the H-tail and fixed tail configurations; and the presence of the vertical tail in the tail increases the pressure value around the wing surface.

The Aerodynamic Interaction Effects between the Rotor and Fuselage on the Drag Performance of a Civil Helicopter in Forward Flight

The aerodynamic interaction between the rotor and fuselage is an important consideration in relation to the drag performance prediction of helicopters. In this paper, based on dynamic patched-grid technology coupled with a three-dimensional N–S equation solver, a numerical study of the aerodynamic interaction effects between the rotor and fuselage on the aerodynamic performance and flow field of the fuselage of a light civil helicopter is carried out under the forward flight condition. The deterioration of the drag performance and the evolution of the flow field structure caused by the rotation of the blades are illustrated. Due to the aerodynamic interaction, the drag of the fuselage is increased by 124.1% while the cycle of drag fluctuation is observed in one full revolution of the rotor with a change in amplitude of ±20%. The comparative results between the rotor-off and rotor-on cases show that the aerodynamic interaction effects between the rotor and fuselage on the drag performance are mainly reflected in the influence of the backward-developing rotor wake on the flow field of the rear part of the fuselage. An additional afterbody vortices system is induced, leading to a large reverse pressure gradient on the rear surface of the helicopter, which contributes to an increase in the pressure drag.

Numerical Analysis of Glauert Inflow Formula for Single-Rotor Helicopter in Steady-Level Flight below Stall-Flutter Limit

This article addresses the numerical computation problem of induced inflow ratio based on the helicopter momentum theory in forward flight. The Glauert inflow formula (equation) is a nonlinear equation usually solved by the Newton–Raphson method in a relatively small number of iterations. However, many high-order convergence multipoint iterative methods have been developed over the last decade. The study examines several selected methods in terms of finding ones that provide a solution in only one iteration with acceptable accuracy. Furthermore, the influence of initial guesses on the accuracy of the obtained solutions has been investigated. In this regard, the practical range of parameters of the Glauert inflow equation for helicopters in forward flight is roughly determined by simplified modeling of a power and stall-flutter limitation. For these purposes, a basic low-fidelity longitudinal trim model of a single-rotor helicopter in steady-level flight is modified and numerically solved by a symbolic transformation of a system of 20+ nonlinear equations into a single nonlinear equation.

Interactional Aerodynamic Analysis of an Asymmetric Lift-Offset Compound Helicopter in Forward Flight

Recently, an asymmetric lift-offset compound helicopter has been conceptualized at the University of Maryland with the objective of improving the overall performance of a medium-lift utility helicopter. The investigated form of lift-compounding incorporates an additional stubbed wing attached to the fuselage on the retreating side. This design alleviates rotor lift requirements and generates a roll moment that enables increased thrust potential on the advancing side in high-speed forward flight. In this study, a numerical model was developed based on the corresponding experimental test case. Threedimensional unsteady Reynolds-averaged Navier–Stokes equations were solved on overset grids with computational fluid dynamics–computational structural dynamics (CFD–CSD) coupling using the in-house CPU–GPU heterogeneous Mercury CFD framework. Simulations were performed at high-speed, high-thrust operating conditions and showed satisfactory agreement with the experimental measurements in terms of the cyclic control angles, rotor thrust, and torque values. CFD results indicated that for an advance ratio of 0.5 with a collective pitch of 10.6°, a vehicle lift-to-equivalent-drag ratio improvement of 47% was attainable using 11% wing-lift offset. The CFD-computed flow fields provide insights into the origin of a reverse flow entry vortex that was observed in particle image velocimetry data, and they characterize the wing–rotor interactional aerodynamics.

Helicopter Rotor Ice Shedding and Trajectory Analyses in Forward Flight

Ice accretion on a helicopter’s aerodynamic surfaces during flight can lead to incidents and fatal accidents. An iced helicopter blade results in a decrease in stall angle, an increase in required torque, as well as potential damage from shed ice. While the high centrifugal forces on a rotor may serve as a natural deicing mechanism, they can also lead to uneven ice shedding, causing rotor imbalances or hitting other blades, the fuselage, or the tail rotor. Computational methods are important for predicting in-flight ice formation and are a vital tool in the design of ice protection systems. The objective of this paper is to present a computationally inexpensive methodology to determine for a helicopter in forward flight, following a period of ice accretion, rotor ice crack locations, shedding times, and potential impact zones.

Analytic Model of Proprotor Forces and Moments at High Incidence

The paper presents an analytical model for estimation of proprotor aerodynamic loads at elevated incidence angles. Previous theories have concentrated on either small incidence angle for aircraft stability analysis or edge-wise flow for helicopter forward flight. This development attempted an engineering method that covers the full incidence angle range from 0 to π/2. Blade element theory was applied to known proprotor geometry, and off-axis loads including normal force and in-plane moment were obtained in closed form based on thrust and torque in axial condition. The model was found to be sufficiently accurate over a broader flight conditions compared to classical models, and computationally more efficient than numerical methods. Hence it could be easily used as a preliminary design and analysis tool for future convertible aircraft proprotors. The paper further discusses a dedicated wind tunnel campaign on proprotor off-axis load measurement. Experimental data from the test campaign was considered in model validation. The results suggested that the model was capable to accurately estimate proprotor performance in nominal flight regimes.

Unsteady boundary-layer transition measurements and computations on a rotating blade under cyclic pitch conditions

The presented work tackles the lack of experimental investigations of unsteady laminar-turbulent boundary-layer transition on rotor blades at cyclic pitch actuation, which are important for accurate performance predictions of helicopters in forward flight. Unsteady transition positions were measured on the blade suction side of a four-bladed subscale rotor by means of non-intrusive differential infrared thermography (DIT). Experiments were conducted at different rotation rates corresponding to Mach and Reynolds numbers at 75% rotor radius of up to {M}_{75} = 0.21 and {mathrm {Re}}_{75} = 3.3 times 10^5 and with varying cyclic blade pitch settings. The setup allowed transition to be measured across the outer 54% of the rotor radius. For comparison, transition was also measured using conventional infrared thermography for steady cases with collective pitch settings only. The study is complemented by numerical simulations including boundary-layer transition modeling based on semi-empirical criteria. DIT results reveal the upstream and downstream motion of boundary-layer transition during upstroke and downstroke, a reasonable comparison to experimental results obtained using the already established sigma c_p method, and noticeable agreement with numerical simulations. The result is the first systematic study of unsteady boundary-layer transition on a rotor suction side by means of DIT including a comparison to numerical computations.

Immersogeometric analysis of compressible flows with application to aerodynamic simulation of rotorcraft

We present an immersogeometric analysis (IMGA) approach for the simulation of compressible flows around complex geometries. In this method, compressible flow simulations are performed directly on various boundary representations (B-reps) of mechanical designs, circumventing the labor-intensive and time-consuming cleanup of complex geometric models. A new formulation for the weak imposition of essential boundary conditions in the context of non-body-fitted meshes is proposed. The formulation employs the non-symmetric Nitsche method, which yields good performance especially when the penalty parameters are difficult to estimate. We test the proposed immersogeometric formulation on benchmark problems for a wide range of Reynolds and Mach numbers, showing its robustness and accuracy. Finally, the methodology is applied to the simulation of a UH-60 Black Hawk helicopter in forward flight, illustrating the ability of the proposed approach to support the design of real-world engineering systems through high-fidelity aerodynamic analysis.

Numerical Investigation of Deep Dynamic Stall for a Helicopter Blade Section

For helicopters in forward flight at high advance ratios, because of the cyclic variation of the angle of attack, the flow over the retreating blade can stall. This phenomenon, which is known as dynamic stall, limits the maximum airspeed of the helicopter. Physical understanding of the highly complex unsteady flow can serve as a catalyst for the development of novel flow control strategies that counter the detrimental effects of dynamic stall. For the present implicit large-eddy simulations, the complex flow over a rotor blade section with a Sikorsky SCC-A09 airfoil is decomposed into a pitching, surging, and yawing flow (dynamic crossflow). The heaving/plunging motion resulting from the unsteady blade bending under load is disregarded. Instead, the centrifugal and Coriolis accelerations are modeled. Three different combinations of the elementary flows are considered: 1) pitching and surging (baseline); 2) pitching, surging, and rotational accelerations; and 3) pitching, surging, and yawing. Simulations for an outboard blade section at a chord-based Reynolds number and a reduced frequency exhibit all relevant stages of dynamic stall reported in the literature: The bursting of a laminar leading-edge bubble is followed by the emergence of a strong dynamic stall vortex, which is eventually shed. When the harmonic yawing motion is added, the laminar bubble bursts earlier and the dynamic stall vortex is shed sooner. The addition of the rotational accelerations reduces the coherence of the dynamic stall vortex and, as a result, diminishes the cycle-to-cycle variations of the lift coefficient.

Amplified Pilot Head Vibration and the Effects of Vibration Mitigation on Neck Muscle Strain.

Rotary wing pilot neck strain is increasing in prevalence due to the combined effects of head supported mass (e.g., Night Vision Goggles, head mounted displays) and whole-body vibration. This study examined the physiological responses of pilots during exposure to whole-body vibration (WBV) representative of the National Research Council's Bell 412 helicopter in forward flight. WBV levels were measured and evaluated using the ISO-2631-1-1997 WBV standards. Twelve pilots (aged 20-59 yr, 7 of the 12 with 20+ years flight experience) underwent six 15-min vibration trials on a human rated shaker platform. Participants were exposed to three vibration levels (-25%, normal, and +25% amplitude; Levels 1-3, respectively) while seated on an Original Equipment Manufacturer (OEM) or vibration mitigating (MIT) cushion. Upper back and neck electromyography (EMG) and acceleration were continuously recorded. Normalized EMG amplitude was higher using the OEM compared to the MIT during Level 2 (0.18 vs. -0.27) and Level 3 (0.24 vs. -0.14) for the anterior neck muscles. Health weighted vibration amplitude at the head (Mean of 3 levels: OEM = 1.19 and MIT = 1.11 m · s-2) was larger than the vibration amplitude at the seat (Mean of 3 levels: OEM = 0.77 and MIT = 0.70 m · s-2). The amplification of head vibration relative to the seat, and the significant effects of vibration level, as well as the vibration mitigation cushion, on neck EMG amplitude support the need for revisions to the ISO-2631-1 standard to account for the head and neck response to whole-body vibration.Wright Beatty HE, Law AJ, Thomas JR, Wickramasinghe V. Amplified pilot head vibration and the effects of vibration mitigation on neck muscle strain. Aerosp Med Hum Perform. 2018; 89(6):510-519.

Airborne Acquisition of Blade Tip Displacements and Vortices on a Coaxial Helicopter

A series of comprehensive flight tests investigated the flight dynamics and explored the flight envelope of a newly developed ultralight, coaxial, counter-rotating helicopter from the company edm aerotec. The present paper focuses on optical measurement techniques that were applied to study the minimum blade tip clearance and convection of blade tip vortices around the dual-rotor system. A calibrated single-camera system was used to quantify the movement and distances of the blade tips at a critical rotor azimuth for multiple steady and unsteady flight maneuvers. The measurements allowed critical flight conditions to be identified and provided flight data for the validation of comprehensive rotor codes. In an additional flight test, an airborne background-oriented schlieren system was operated aboard a second helicopter to visualize the blade tip vortices of the ultralight helicopter in forward flight. The background-oriented schlieren measurements resulted in the first visualization of blade–vortex and vortex–structure interaction effects on a full-scale coaxial helicopter. The application of optical measurement systems in the present flight tests gives valuable insight into the aerodynamics and blade dynamics of a free-flying counter-rotating helicopter and demonstrates how these methods can effectively support the development process of new rotorcraft.

Flight dynamic simulation of helicopter forward flight through microburst wind field

A nonlinear dynamic model of a helicopter flying through a variable microburst wind field is developed. Equivalent wind field models are established based on the Soesman and gust models to analyze quantitatively the abrupt change when a helicopter enters and leaves the wind field. The comparison of the simulation results with and without the equivalent model shows that the equivalent model has good accuracy for flight dynamic simulation of a helicopter under atmospheric disturbance. Real-time simulation of helicopter forward flight through the microburst wind field is also conducted at different positions and heights to analyze the influence of wind at the side direction and the downward flow on helicopter flight performance. The results indicate that wind at the side direction enables the helicopter to carry out complicated lateral movements with different yaw characteristics. Furthermore, with the increase in flight height, the downward flow leads to faster rate of descent and stronger vertical overload, both of which will affect the flight safety of the helicopter.

Perpendicular blade–vortex-interaction over an oscillating airfoil in light dynamic stall

An experimental and numerical study was performed to investigate the effects of perpendicular blade vortex interactions on the aerodynamic performance of an oscillating airfoil. The selected test cases studied the aerodynamic interaction of a stream-wise vortex impacting on a NACA 23012 airfoil oscillating in light dynamic stall regime, representing a typical condition of the retreating blade of a helicopter in forward flight. The analysis of particle image velocimetry surveys and time-accurate simulation results enabled to point out the different effects due to the blade pitching motion on the interacting flow field. Thus, numerical results enabled to achieve a detailed insight about the aerodynamic loads acting on the oscillating airfoil in the interacting cases. In particular, the comparison with the clean airfoil case shows that a severe loss of performance is produced by the interaction of the vortex during the airfoil downstroke motion, as the vortex impact triggers the local stall of the blade section.

Inflow Prediction and First Principles Modeling of a Coaxial Rotor Unmanned Aerial Vehicle in Forward Flight

When the speed of a coaxial rotor helicopter in forward flight increases, the wake skew angle of the rotor increases and consequently the position of the vena contracta of the upper rotor with respect to the lower rotor changes. Considering ambient air and the effect of the upper rotor, this study proposes a nonuniform inflow model for the lower rotor of a coaxial rotor helicopter in forward flight. The total required power of the coaxial rotor system was compared against Dingeldein’s experimental data, and the results of the proposed model were well matched. A plant model was also developed from first principles for flight simulation, unknown parameter estimation and control analysis. The coaxial rotor helicopter used for this study was manufactured for surveillance and reconnaissance and does not have any stabilizer bar. Therefore, a feedback controller was included during flight test and parameter estimation to overcome unstable situations. Predicted responses of parameter estimation and validation show good agreement with experimental data. Therefore, the methodology described in this paper can be used to develop numerical plant model, study non-uniform inflow model, conduct performance analysis and parameter estimation of coaxial rotor as well as other rotorcrafts in forward flight.

Stereo particle image velocimetry measurements of perpendicular blade–vortex interaction over an oscillating airfoil

The aerodynamic interaction of a stream-wise vortex impacting on a NACA 23012 oscillating airfoil was investigated using stereo particle image velocimetry. The experimental rig enabled the study of the aerodynamic effects due to the blade pitching motion in the interaction with the vortex. The experimental study focused on the light dynamic stall regime, which represents a typical condition of the retreating blade of a helicopter in forward flight. Particle image velocimetry was applied to a measurement volume close to the airfoil upper surface in order to obtain the three-dimensional interacting flow field. In particular, the experimental results show that during the airfoil downstroke motion, the vortex impact triggers the stall of the local blade section, indicating that detrimental effects on the blade performance can be introduced by perpendicular vortex interactions.

관절형 허브 로터를 이용한 전진비행조건에서의 플래핑 운동에 대한 실험적 연구

본 논문에서는 헬리콥터 전진비행 조건에서 플래핑모션에 대하여 축소 로터 실험을 통하여 실험결과와 이론적인 예측결과를 비교하였다. 축소로터 성능 실험은 충남대학교 아음속 풍동에서 수행하였으며 1.8 x 1.8m의 개방형 시험부를 사용하였다. 전진비행조건에서의 실험 결과에 의하면 축소로터의 전진비행조건에서의 추력 결과를 고정한 조건에서 동력계수는 차이가 있는 것을 확인 할 수 있었다. 또한, 공력 성능 측정 결과와 이론적인 예측결과의 비교를 통하여 헬리콥터의 플랩핑 각도의 범위에 대하여 비교하여 보았다. Coning 각도, 횡방향과 종방향에 대한 플래핑 각도에 대해서는 실험결과와 예측결과의 유사함을 확인하였다. In this paper, wind tunnel test and analytical prediction are compared for result of flapping motion in helicopter forward flight condition. Tests were performed at low speed wind tunnel at Chungnam National University, test section of wind tunnel has 1.8 by 1.8 meter open-jet test section area. According to the results of measured data for aerodynamic performance of model rotor in forward flight. It has to observed the difference of analytical and measured results of power coefficient for fixed thrust coefficient. And calculated and measured data of helicopter rotor flapping angles in forward flight are compared for a model rotor in a wind tunnel. A test was conducted to verify the measured data of coning and lateral/longitudinal flapping angle with predicted values.

The Aerodynamics Investigation of Vortex Trap on Helicopter Blade

During helicopter forward flight, the retreating blade revolves at high angle of attack compared to advancing blade in order to balance the lift and also to stabilise the helicopter. However, due to the aerodynamics limitations of the retreating blade at forward flight, stall may occur at high angle of attack compared with the advancing blade. This phenomenon is dangerous for pilot when controlling and balancing the helicopter while flying against strong wind. This paper investigates the capabilities of introducing multiple vortex traps on the upper surface of the helicopter airfoil in order to delay the stall angle of retreating helicopter blade. Blade Element Theory (BET) was applied to scrutinize the lift force along the helicopter blade. Computational Fluid Dynamic (CFD) analyses using the Shear-Stress Transport (SST) turbulence model was carried out to investigate the effect of groove on delaying the stall and to predict the separation of flow over the airfoil. Based on the CFD analyses, the optimization of the groove was done by analyzing the numbers and locations of the grooves. Finally, the results from both BET and the CFD analyses were utilised to obtain the lift force achieved by the vortex trap. The study showed that the presence of multiple vortex traps has successfully increased the lift coefficient and most importantly, delaying the stall angle.

Rotorcraft simulations: a challenge for CFD

This paper gives an overview of CFD techniques developed and used at ONERA for rotorcraft applications. First, the complex multidisciplinary environment around helicopters, in which aerodynamics, flight dynamics, aeroelasticity and aeroacoustics strongly interact, is highlighted. Rotorcraft simulations are thus performed by comprehensive codes capable of dealing with the whole system efficiently, using integrated simplified models for each discipline, e.g. the aerodynamics. However, fast aerodynamic models cannot accurately represent the full complexity of rotorcraft aerodynamics, in particular as far as nonlinear phenomena are concerned, contrary to CFD. Nevertheless, helicopter problems are particularly demanding for numerical methods, requiring efficient simulation of unsteady flows with shock waves, massive flow separation, concentrated vortex structures and deforming bodies with large amplitude relative motion, while allowing fine description and analysis of local flow phenomena impacting the vehicle behaviour. Helicopter trim in the CFD solution is obtained by iterative coupling with comprehensive analysis, so that the global multidisciplinary simulation can be achieved with an advanced aerodynamic model. The approaches taken by ONERA for the comprehensive code and the CFD solvers are outlined in the paper. Examples of applications typical of rotorcraft problems are given to illustrate current possibilities and difficulties. They include an isolated rotor in hover, the dynamic stall of an oscillating wing, an isolated rotor in descent flight with Blade-Vortex Interactions, the dynamic-aerodynamic coupling of a rotor in high-speed forward flight and the simulation of a complete helicopter in forward flight. Finally, expected and needed developments are reviewed in order to make CFD a more efficient tool in the design office of helicopter manufacturers.