Hover Performance Research Articles

Aeroelastic analysis of a helicopter rotor in hover

The paper deals with the numerical analysis of a helicopter rotor in hover conditions, including aeroelastic effects. In existing literature the effects of an aircraft structural deformation on airloads distribution and performance are fairly well described but mainly for airplanes. The purpose of the presented work is to indicate the possible influence of blades elasticity on helicopter rotor loads and performance in hover. Additionally, it aims to validate a new computational method based on a set of experimental results for a UH-60A helicopter rotor. The method used combines a high fidelity Navier-Stokes aerodynamic model coupled with a low-order beam structural model. This approach made it possible to take into consideration blade deformations in simulations and revealed the high influence of aeroelastic effects on rotor loads and performance at hover conditions. To authenticate the results, the presented method was validated with experimental data from two different flight test programs, three small-scale wind tunnel tests and one full-scale wind tunnel test. They were conducted for the UH-60A helicopter rotor by different organizations. The comparison of results revealed that the created computational method has high accuracy.

Systematic design methodology for development and flight testing of a variable pitch quadrotor biplane VTOL UAV for payload delivery

This paper discusses the conceptual design and proof-of-concept flight demonstration of a novel variable pitch quadrotor biplane Unmanned Aerial Vehicle concept for payload delivery. The proposed design combines vertical take-off and landing (VTOL), precise hover capabilities of a quadrotor helicopter and high range, endurance and high forward cruise speed characteristics of a fixed wing aircraft. The proposed methodology is described by taking an example of a VTOL Unmanned Aerial Vehicle (UAV) for a mission requirement of carrying and delivering 6 kg payload to a destination at 16 km from the point of origin. First, the design of proprotors is carried out using a physics based modified Blade Element Momentum Theory (BEMT) analysis, which is validated using experimental data generated for the purpose. Proprotors have conflicting requirement for optimal hover and forward flight performance and a compromise solution has to be chosen. Next, the biplane wings are designed using simple lifting line theory. The airframe design is followed by power plant selection and transmission design. Finally, weight estimation is carried out to complete the design process. The proprotor design with 24° preset angle and −24∘ twist is found to be suitable for the current mission profile, based on 70% weightage to forward flight and 30% weightage to hovering flight conditions. A reduction in operating RPM of the proprotors from 3200 during hover to 2000 during forward flight results in 27% increase in proprotor efficiency. The VTOL UAV design proposed in this paper is also predicted to consume 64% less power in airplane mode compared to the quadrotor mode, thereby establishing the benefit of this hybrid concept. A proof-of-concept scaled prototype is fabricated using commercial-off-the-shelf parts and a Proportional Integral Derivative (PID) controller is developed and implemented on open source PixHawk autopilot board to enable stable hovering flight and attitude tracking during hover flight tests.

Biomechanics of hover performance in Neotropical hummingbirds versus bats.

Hummingbirds and nectar bats are the only vertebrates that are specialized for hovering in front of flowers to forage nectar. How their aerodynamic performance compares is, however, unclear. To hover, hummingbirds consistently generate about a quarter of the vertical aerodynamic force required to support their body weight during the upstroke. In contrast, generalist birds in slow hovering flight generate little upstroke weight support. We report that nectar bats also generate elevated weight support during the upstroke compared to generalist bats. Comparing 20 Neotropical species, we show how nectarivorous birds and bats converged on this ability by inverting their respective feathered and membrane wings more than species with other diets. However, while hummingbirds converged on an efficient horizontal wingbeat to mostly generate lift, bats rely on lift and drag during the downstroke to fully support their body weight. Furthermore, whereas the ability of nectar bats to aerodynamically support their body weight during the upstroke is elevated, it is much smaller than that of hummingbirds. Bats compensate by generating more aerodynamic weight support during their extended downstroke. Although, in principle, it requires more aerodynamic power to hover using this method, bats have adapted by evolving much larger wings for their body weight. Therefore, the net aerodynamic induced power required to hover is similar among hummingbirds and bats per unit body mass. This mechanistic insight into how feathered wings and membrane wings ultimately require similar aerodynamic power to hover may inform analogous design trade-offs in aerial robots.

Effects of hawkmoth-like flexibility on the aerodynamic performance of flapping wings with different shapes and aspect ratios

The effect of hawkmoth-like flexibility on the aerodynamic hovering performance of wings at a Reynolds number of 400 has been assessed by conducting fluid structure interaction simulations incorporating a finite difference based immersed boundary method coupled with a finite-element based structure solver. The stiffness distribution of a hawkmoth forewing was mapped onto three wing shapes (r¯1 = 0.43, 0.53, and 0.63) defined by the radius of the first moment of wing area each with aspect ratios, AR = 1.5, 2.96, 4.5, and 6.0 using elliptic mesh generation, the Jacobi method for iterations, and the concept of the barycentric coordinate system. The results show that there is a dominant chordwise deformation at AR = 1.5, and the wings also deform in the spanwise direction and their tips deviate from the horizontal stroke plane as AR increases. At AR = 1.5, 2.96, and 4.5, flexibility increases the mean lift (up to 39%, 18%, and 17.6%, respectively) for all wing shapes. At AR = 6.0, the r1¯ = 0.53 and 0.63 flexible wings give lesser lift than the rigid equivalents because of negative lift or small positive lift during the early stroke as the vortical structures remain on the bottom surface. This is attributed to the rapid pitch-down rotation, lesser stroke angular velocity than the rigid wing, and upward motion of the wingtip, away from the horizontal stroke plane. From the design perspective, the anisotropic flexible wings (except r1¯ = 0.53 and 0.63 with AR = 6.0) can be used in micro aerial vehicles for high lift requirements, such as for a high payload. Results here show that in nature, the hawkmoth wings with r1¯ and AR of 0.43-0.44 and 2.73-2.92, respectively, appear to have a combination of the shape, AR, and flexibility that optimizes power economy.

멀티콥터의 추진 시스템 실험 결과를 통한 제자리 비행 성능 예측 및 검증

본 논문은 최근에 각광 받고 있는 드론(멀티콥터)과 관련하여 비행체의 비행 성능, 특히 비행 가능 시간에 대한 예측과 실험 데이터를 통한 검증 등을 목표로 하여, 제자리 비행에 대한 연구 결과를 제시 하고 있다. 연구 방법을 드론 시스템을 구성하고 있는 여러 부품들을 기능별로 분류하여 부품 수준에서의 제원과 기능에 대한 정리 및 수학적 수식화를 통하여 단위 부품의 성능을 분석 및 실험 데이터를 확보하고, 이들 단위 부품 데이터의 조합을 통하여 드론 시스템의 제자리비행 성능을 예측하는 연구 결과를 보여 주고 있다. 또한 5kg급 쿼드콥터를 이용하여 제자리 비행에 대한 분석을 통하여 비행시간에 따른 전압 변화를 예측, 검증 하였으며 해당 방법을 통해 제자리 비행시간을 예측하였다.

Role of Blade Flexibility on Cycloidal Rotor Hover Performance

In this paper, the aeromechanics of a cycloidal rotor (or cyclorotor) is investigated to understand the effect of blade flexibility on the performance of the rotor in hover. Toward this, experiments are conducted on cyclorotors using moderately and highly flexible blades. These studies show that, with increasing blade flexibility, the rotor thrust decreases; whereas the power consumption increases, leading to a significant drop in power loading (thrust/power). To investigate these phenomena, a coupled aeroelastic model of cyclorotor is developed by coupling an unsteady aerodynamics model with a beam-based structural model and is systematically validated with experimental results. The aerodynamic model contains rigorous modeling of various physics behind the operation of the cyclorotor, such as the nonlinear dynamic virtual camber, the effect of near and shed wakes, and the leading-edge vortex. In comparison with a second-order nonlinear structural model, inclusion of a geometrically exact structural model proves to be essential in accurate prediction of rotor deflections and performance. Based on a systematic analysis performed using the validated model, it is observed that the large torsional deflections of the blades are a key reason for the drop in thrust. The torsional deflections are mainly produced by centrifugal force and nonlinear moments due to bending curvatures.

Analysis of Classical and Alternate Hexacopter Configurations with Single Rotor Failure

This study examines hexacopter performance in hover and forward flight with single rotor failure. A classical configuration (adjacent rotors spinning in opposite directions) and an alternate configuration are considered. The simulation model uses calculates rotor forces and moments by using blade element theory coupled with a finite-state dynamic inflow model. For the alternate hexacopter, if one of four rotors (out of six) fails, trim is possible in hover and forward flight, and the aircraft remains fully controllable. Recovery from the failure of one of the other two rotors is impossible. For the classical hexacopter, if any of the forward-facing rotors fail, the aircraft can be trimmed in forward flight and is fully controllable. If an aft rotor fails, trim is not possible but the aircraft could be turned around to orient the failed rotor forward. Although the classical hexacopter can be trimmed in hover by turning off the rotor diametrically opposite to the failed rotor, the aircraft is not independently controllable about all axes. In the event of a rotor failure, the classical hexacopter could cruise and land, but it cannot maintain a sustained hover. Power penalties of up to 23% are observed in the event of failure due to increased drag on the operational rotors.

Effects of flexibility on the hovering performance of flapping wings with different shapes and aspect ratios

The effects of isotropic homogeneous flexibility on the aerodynamic performance of flapping wings with different shapes and aspect ratios in hover at a Reynolds number of 400 have been studied numerically with a 3-D Navier–Stokes solver coupled with a structure solver. Three wing shapes, defined by the radius of the first moment of wing area, r1¯ (=0.43, 0.53 and 0.63), with four aspect ratios, AR (=1.5, 2.96, 4.5 and 6.0) are considered. We used a set of moderately flexible wings with an effective stiffness of 14 and 2.31 (for the mass ratio, m∗=4.0 and 0.66 respectively) and a set of more flexible wings with an effective stiffness of 6.12 and 1.01 (for m∗=4.0 and 0.66 respectively). The wings have a limited spanwise twist and a dominant chordwise flexibility because the leading edge is modeled as rigid. The results show that although the prescribed kinematics is advanced pitch rotation, it becomes symmetric or delayed pitch rotation depending on the value of r1¯, the degree of flexibility and the mass ratio. This change in pitch angle kinematics causes variations in the time histories of lift and power with flexibility including the timings and magnitudes of lift and power peaks. Flexible wings with high AR such as 4.5 and 6.0 produce less lift than rigid wings for both mass ratios because of lower pitch angles during the mid-stroke, but they are more efficient in terms of power economy; for example, 11% less lift but 33% higher power economy at AR = 6.0, r¯1=0.63 and m∗=0.66. At m∗=4.0, the low r1¯ and high AR wings maximize PE for a given lift. However, at m∗=0.66, there is a limited range of lift for which low r1¯ and high AR wings are efficient, as r1¯=0.63 wing at higher AR (=6.0) consumes lesser aerodynamic power than r1¯=0.43 and 0.53 wings by flapping at a lower pitch angle; therefore, the PE of low r1¯ (=0.43 and 0.53) may drop below r1¯=0.63 wing for a given lift.

Switching PD-based sliding mode control for hovering of a tilting-thruster underwater robot.

This paper presents a switching PD-based sliding mode control (PD-SMC) method for the 6-degree-of-freedom (DOF) hovering motion of the underwater robot with tilting thrusters. Four thrusters of robot can be tilted simultaneously in the horizontal and vertical directions, and the 6-DOF motion is achieved by switching between two thruster configurations. Therefore, the tilting speed of thruster becomes the most essential parameter to determine the stability of hovering motion. Even though the previous PD control ensures stable hovering motion within a certain ranges of tilting speed, a PD-SMC is suggested in this paper by combining PD control with sliding mode control in order to achieve acceptable hovering performance even at the much lower tilting speeds. Also, the sign function in the sliding mode control is replaced by a sigmoid function to reduce undesired chattering. Simulations show that while PD control is effective only for tilting duration of 600 ms, the PD-based sliding mode control can guarantee the stable hovering motion of underwater robot even for the tilting duration of up to 1500 ms. Extensive experimental results confirm the hovering performance of the proposed PD-SMC method is much superior to that of PD method for much larger tilting durations.

드론용 동축 프로펠러 시험장치 개발 및 제자리비행 성능특성에 대한 실험적 연구

본 논문에서는 저 레이놀즈 동축 프로펠러의 제자리 비행 특성연구를 위한 시험장치 개발과정과 실험결과를 기술하였다. 상/하부 프로펠러간의 간격 변화에 따른 제자리 비행 성능측정이 가능하도록 시험장치를 설계하였다. 그리고 상/하부 프로펠러 각각의 추력, 토크, 회전수, 진동, 전류량을 측정할 수 있도록 장치를 구성하였다. 센서의 신호 수집을 위하여 자료획득시스템을 구축하고 LabVIEW 기반의 소프트웨어에서 모터의 제어와 신호 수집을 하였다. 단일 프로펠러의 지상회전 실험을 통한 성능 특성 데이터를 확보한 후, 동축 프로펠러 사이의 간격과 상/하부 직경의 차이에 따른 성능 특성과 효율향상을 위한 실험을 진행하였다. 성능 실험분석 결과 동축 프로펠러 사이 간격이 로터 직경의 20~30%일 때 효율이 높은 것으로 확인되었으며, 그 이상일 경우는 큰 차이가 없었다. 상부 프로펠러의 직경이 하부 프로펠러의 직경보다 작을 경우가 다른 직경 조합에 비해 가장 높은 효율을 보였다.

Locomotion of a bioinspired flyer powered by one pair of pitching foils

We numerically investigate the flight dynamics and aerodynamics of a two-dimensional model for the jellyfishlike ornithopter recently devised by Ristroph and Childress [L. Ristroph and S. Childress, J. R. Soc. Interface 11, 20130992 (2014)]. This simplified model is composed of two rigid thin foils which are forced to pitch in antiphase fashion. The Navier-Stokes equations for the fluid and the dynamics equations for the flyer are solved together in the simulations. We first consider the constrained-flying condition where the flyer model is only allowed to move in the vertical direction. The influences of the control parameters on the hovering performance are studied. With the variations in parameter values, three different locomotion states, i.e., ascending, descending, and approximate hovering, are identified. The wake structures corresponding to these three locomotion states are explored. It is found that the approximate hovering state cannot persist due to the occurrence of wake symmetry breaking after long-time simulation. We then consider the free-flying condition where the motions in three degrees of freedom are allowed. We study the postural stability of a flyer, with its center of gravity located at the geometric center. The responses of the flyer at different locomotion states to physical and numerical perturbations are examined. Our results show that the ascending state is recoverable after the perturbation. The descending state is irrecoverable after the perturbation and a mixed fluttering and tumbling motion which resembles that of a falling card emerges. The approximate hovering state is also irrecoverable and it eventually transits to the ascending state after the perturbation. The research sheds light on the lift-producing mechanism and stability of the flyer and the results are helpful in guiding the design and optimization of the jellyfishlike flying machine.

Aerodynamic Performance of the Three-Dimensional Lumped Flexibility Bionic Hovering Wing

Understanding the effect of flexibility on the aerodynamic characteristics of the wing is one of the most important considerations for successfully designing a flapping wing micro aero vehicle (FMAV). This paper aims at providing a systematic synthesis on the flexibility effects on the hovering performance of the bionic wing based on the numerical analysis approach. We construct a novel three-dimensional bionic wing, which has lumped flexibility at the root, and develop an iterative coupling program to simulate the interaction between the flexible wing and fluid. The effects of flexibility on the hovering performance of the three-dimensional flapping wing are investigated, and the results indicate that the best performance of the wing is achieved when the wing flaps at resonance and has the density close to the natural insect wing. The feasibility of using lumped flexibility wings driven by a simple harmonic flapping for designing efficient FMAV is also concluded in this study.

From Aeroacoustics Basic Research to a Modern Low-Noise Rotor Blade

In 2015, Airbus Helicopters unveiled the secrecy around its Dauphin successor and presented the all new H160 helicopter. A special feature that immediately attracted attention was its unusual and revolutionary fore-aft swept main rotor blade. This design, which aims to significantly reduce the blade-vortex interaction noise signature and reduce high-speed forward flight power requirements, was first investigated in the 1990s. At this time, DLR and ONERA formed a joint team to acoustically optimize a rectangular reference rotor blade. Based on state-of-the-art comprehensive rotor codes and a 50/50 work share, the ERATO rotor blade design was developed, patented worldwide and tested on both a rotor test rig and in the wind tunnel. Airbus Helicopters (then: Eurocopter) adopted the design, optimized hover and forward flight high lift performance and prepared it for serial production, as the Blue EdgeTM rotor blade on the Airbus Helicopters H160 helicopter.

Comparison of Computational Fluid Dynamics Hover Predictions on the S-76 Rotor

The S-76 rotor is used as a baseline case to assess the isolated rotor hover predictions of different computational fluid dynamics solvers. These predictions are compared to the available test data as well as to one another. Both grid and parametric studies of the options available within each code are included. The grid studies look into not only blade grid density but also Cartesian and unstructured far-field grid computations and the use of adaptive mesh refinement. A study examining the impact of the hub on the performance predictions is also included. The results show that, in addition to blade-tip grid refinement, leading-edge and trailing-edge grid refinement are important to compute the hover performance. The dual-mesh methodology is shown to preserve the wake for a longer distance when compared to the fully unstructured methodology. This has some impact on the final wake structure. The presence of the hub is found to have a small impact on the final integrated performance parameters. However, there is a greater impact on the convergence rate of these performance parameters as well as the magnitude of the 4/rev time history characteristics. The final integrated performance results obtained from each solver using the “best” set of grid and input parameters are found to be comparable to one another.

Application of Vortex Methods to Coaxial Rotor Wake and Load Calculations in Hover

The coaxial rotor has distinct advantages in hover performance and forward flight compared to a conventional isolated rotor. The viscous vortex particle approach based on the Lagrangian formulation of the incompressible Navier–Stokes equations is a grid-free method suitable for capturing the vortex wake interaction over long distances. It is applied to evaluate performance, wake evolution, and loads in hover for a coaxial rotor system. A two-dimensional analysis is used to evaluate the effects of blade pitch angle and vertical separation distance. It is found that the interaction is significant only for distances of less than four to six chord lengths between the airfoils. In the coaxial rotor analysis, the performance is sensitive to the viscous drag coefficient of the airfoil. The rational function approximation of unsteady airfoil loads was used to reduce computational effort and improve accuracy of the calculations. The hover performance was predicted well with both the methods. Oscillations in thrust occurred primarily at blade passage azimuths. The rational function approximation calculations indicated a reduction in the magnitude of the thrust oscillations compared to the vortex panel method.

Linear vs Nonlinear MPC for Trajectory Tracking Applied to Rotary Wing Micro Aerial Vehicles

Precise trajectory tracking is a crucial property for Micro Air Vehicles (MAVs) to operate in cluttered environment or under disturbances. In this paper we present a detailed comparison between two state-of-the-art model-based control techniques for MAV trajectory tracking. A classical Linear Model Predictive Controller (LMPC) is presented and compared against a more advanced Nonlinear Model Predictive Controller (NMPC) that considers the full system model. In a careful analysis we show the advantages and disadvantages of the two implementations in terms of speed and tracking performance. This is achieved by evaluating hovering performance, step response, and aggressive trajectory tracking under nominal conditions and under external wind disturbances.

Computational Investigation of a Full-Scale Proprotor Hover Performance and Flow Transition

The full-scale XV-15 proprotor hover performance and flow transition phenomena are numerically investigated using an unsteady Reynolds-averaged Navier–Stokes flow solver. The rotor figure of merit is predicted over a range of blade collective angles, and computed skin frictions on the rotor surface are compared with the wind-tunnel experimental data. A grid resolution study is performed to investigate the impact of the mesh resolution on hover predictions using a baseline mesh and a refined mesh. The Langtry–Menter local correlation-based transition model is used to predict the skin friction and transition onset associated with the XV-15 rotor under the framework of both the Spalart–Allmaras one-equation turbulence model and Menter’s shear-stress transport two-equation model. Computational results show satisfactory predictions for the XV-15 hover performance and transition phenomena and offer an improved understanding of complicated rotor flow physics for the XV-15 proprotor.

Accurate Predictions of Rotor Hover Performance at Low and High Disc Loadings

This paper presents evidence on the ability of modern computational fluid dynamics methods to accurately predict hover performance of rotors with modest computer resources. The paper uses two well-studied blades, the S-76 main rotor blade and the XV-15 tiltrotor blade. The results are compared with experiments and show that the performance is well predicted. In addition, the employed computational fluid dynamics method was able to capture the effects of the tip Mach number, tip shape, blade aeroelasticity, and flow transition on the performance of the blade as well as on the wake structure and the rotor acoustics.

Modified Hybrid Navier–Stokes/Free-Wake Method for Hovering Rotor Analysis

The analysis of rotors in hover has always been a significant challenge. The presence of strong vortices underneath the rotor requires a large amount of grid points to properly capture the vorticity, and many time steps are required to reach a time-independent state. To bring the simulation cost to a more practical state for industry, a modified hybrid Navier–Stokes/free-wake method has been introduced, which successfully reduced grid count and simulation time by one order of magnitude compared to full Navier–Stokes simulations. In this hybrid method, Navier–Stokes and free-wake solvers are loosely coupled for fast convergence in hover. The S-76 model-scale blades with three tip designs were studied to assess current approaches. The proposed approaches showed reasonable-to-good correlation with measured figure of merit, thrust, and torque data. The effects of tip-design and tip-speed variations were captured well. The physical mechanisms of tip-design impact on hover performance were also studied.

Multi-Fidelity Coupled Trim Analysis of a Flapping-Wing Micro Air Vehicle Flight

This study provides a methodology to analyze the performance of a millimeter-scale flapping-wing micro air vehicle in hover and steady forward flight. Using a flight dynamic model of the insect, which comes embedded with simplified quasi-steady wing aerodynamics and is coupled to high-fidelity computational fluid dynamics analysis, trim solutions are obtained in realistic time frames. This procedure is analogous to rotorcraft periodic coupling for trim. Six iterative coupling cycles are sufficient to obtain convergence in the wing loads and trim variables for the cases investigated. This multifidelity approach, where many quasi-steady calculations are combined with a judicious number of computational fluid dynamics simulations, may be used in parametric sweeps and design studies to improve hover and cruise performance. The trim variables chosen in this study are the parameters used to describe the wing kinematics (stroke amplitude , stroke flapping bias , and stroke plane tilt ). Flight conditions investigated are hover and three forward flight speeds. At hover, the wing kinematics parameters produce linearly independent force and moment components in the longitudinal plane. In forward flight, cross couplings manifest between the individual wing kinematics parameters and the longitudinal forces and moments. Thus, the one-to-one correlation between forces and individual controls that exists at hover is lost in forward flight.