Tiltrotor Blades Research Articles

Comparative Study of Soft In-Plane and Stiff In-Plane Tiltrotor Blade Aerodynamics in Conversion Flight, Using CFD-CSD Coupling Approach

Tiltrotors permit aircrafts to operate vertically with lift, yet convert to ordinary forward flight with thrust. The challenge is to design a tiltrotor blade yielding maximum lift and thrust that converts smoothly without losing integrity or efficiency. The two types of blades, soft in-plane and stiff in-plane—the designation depending on the value of the blade’s natural lag frequency—exhibit different structural responses under the same flight conditions, differently affecting the aerodynamics of the blades, especially in the complex aerodynamic environment of conversion flight where the aerodynamic differences are significant. This phase of flight is not deeply researched, nor is the analytical coupling method much used. To study the influence of blade type on aerodynamics during conversion, models suitable for the conversion flight simulation are established for the application of coupled computational fluid dynamics and computational structural dynamics (CFD-CSD) methods. Each method is implemented with well-accepted techniques (the Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations, the Reverse Overset Assembly Technique (ROAT), and the Timoshenko beam model. To improve the solving efficiency, a loose coupling strategy is used in constructing the two-way coupled model. The XV-15 tiltrotor is used for verification. The aeroelastic simulation of soft in-plane and stiff in-plane blades in conversion flight indicates an impactful role on the modal shapes, with a significant difference in the third flap modal shapes for the XV-15 rotor. However, the effect on aerodynamic performance is relatively small. In the first half of the flight conversion, the thrust of stiff in-plane blades is larger than that of soft in-plane blades, but in the last half, the influence of structural characteristics on aerodynamic performance is negligible and the thrust of the blades tends to be equal.

Fabrication, Testing, and 3D Comprehensive Analysis of Swept-Tip Tiltrotor Blades

This paper covers the design, fabrication, testing, and modeling of a family of Froude-scale tiltrotor blades. They are designed with the objective of gaining a fundamental understanding of the impact of a swept tip on tiltrotor whirl flutter. The goal of this paper is to describe the development of the blades needed for this purpose. The rotor is three bladed with a diameter of 4.75 ft. The blades have a VR-7 profile, chord of 3.15 inches, and linear twist of –37° per span. The swept-tip blades have a sweep of 20° starting at 80% R. The blade properties are loosely based on the XV-15 design. A CATIA and Cubit-based high-fidelity three-dimensional (3D) finite element model is developed. It accurately represents the fabricated blade and is analyzed with X3D. Experiments in a vacuum chamber were carried out to demonstrate the structural integrity of the blades. Measured frequencies and strains were validated with X3D predictions proving the fidelity of the 3D model. Thus, even though the wind tunnel facilities were closed due to COVID-19, hover and forward flight calculations for the blade stress could be performed using the high-fidelity 3D structural model. The results prove the blades have sufficient structural integrity and stress margins to allow for wind tunnel testing.

Role of Transition Modeling in Rotor Hover Predictions

The Langtry–Menter transition model is used to predict the aerodynamic performance of three realistic helicopter and tiltrotor blades in hover: the XV-15 proprotor, the joint-service vertical takeoff/landing experimental rotor, and the S-76 conventional scaled rotor. The role of transition modeling in rotor hover predictions is investigated based on comparative studies using the full turbulence model and the transition model. The subsequent predicted transition phenomena are then validated with the experimental data. The focus of this study is to explore three major issues pertinent to rotor hover predictions: 1) the effect of transition modeling on the rotor aerodynamic prediction, 2) rotor prediction at high thrusts or collective angles, and 3) rotor flow characteristics and their influence on aerodynamic performance. Numerical computations indicate different impacts of the transition modeling on predicting the aerodynamic performance for conventional rotors and proprotors.

Tiltrotor CFD Part II - aerodynamic optimisation of tiltrotor blades

ABSTRACTThis paper presents aerodynamic optimisation of tiltrotor blades with high-fidelity computational fluid dynamics. The employed optimisation framework is based on a quasi-Newton method, and the required high-fidelity flow gradients were computed using a discrete adjoint solver. Single-point optimisations were first performed to highlight the contrasting requirements of the helicopter and aeroplane flight regimes. It is then shown how a trade-off blade design can be obtained using a multi-point optimisation strategy. The parametrisation of the blade shape allowed the twist and chord distributions to be modified and a swept tip to be introduced. The work shows how these main blade shape parameters influence the optimal performance of the tiltrotor in helicopter and aeroplane modes, and how an optimised blade shape can increase the overall tiltrotor performance. Moreover, in all the presented cases, the accuracy of the adjoint gradients resulted in a small number of flow evaluations for finding the optimal solution, thus indicating gradient-based optimisation as a viable tool for modern tiltrotor design.

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.

단면의 차원축소/복원해석과 에너지 해방률 계산을 위한 복합재 블레이드 해석 라이브러리 개발에 대한 연구

In this paper, numerical results of sectional analysis, stress recovery and energy release rate were compared with the results of VABS, 3-D FEM through the blade analysis library. The result of stress recovery analysis for one-dimensional model including the stiffness matrix is compared with stress results of three-dimensional FEM. We discuss the configuration of the blade analysis library and compare verifications of numerical analysis results of VABS. Blade analysis library through dimensional reduction and stress recovery is intended to be utilized in conjunction with pre- and post-processing of the analysis program of the composite blade, high-altitude uav`s wing, wind blades and tilt rotor blade.

복합재 로터 블레이드의 차원축소와 복원해석을 위한 블레이드 해석 라이브러리 개발

본 논문에서는 블레이드 해석 라이브러리를 통해 단면해석 및 응력복원 해석 결과를 VABS의 결과와 비교하였다. 강성 행렬로 연결된 1차원 보 모델을 가상일의 원리에 의해 계산된 에너지를 복원 이론에 기반한 차원 복원해석 과정을 통해 계산된 응력과 3차원 해석모델과 비교하였다. 블레이드 해석 라이브러리의 구성, 입력 및 출력 형태, 차원 축소와 복원을 통한 로터 블레이드의 구조해석 프로시져를 기술하고, 블레이드 해석 라이브러리의 검증은 고형모델, 박 벽 모델의 단면해석, 3차원 복합재료 유한요소 모델로 구성하여 VABS를 통하여 검증하였다. 차원 축소와 복원해석을 통하여 블레이드 해석 라이브러리는 복합재료 블레이드의 전후처리 프로그램와 연계되어 고고도 무인기, 로터 블레이드, 풍력 블레이드 및 틸트로터 블레이드의 모델링에 활용될 수 있을 것이다. In this paper, numerical results of sectional analysis and stress recovery were compared with the results of VABS through the blade analysis library. The results of recovery analysis for one-dimensional model including the stiffness matrix is compared with the calculated three-dimensional stress results of three-dimensionial FEM based on the principle of virtual work. We discuss the configuration of the blade analysis library and compare verifications of numerical analysis results of VABS. Blade analysis library through dimensional reduction and stress recovery is intended to be utilized in conjunction with pre- and post-processing of the analysis program of the composite blade, high-altitude uav's wing, wind blades and tilt rotor blade.

Aerodynamic blade design with multi-objective optimization for a tiltrotor aircraft

Purpose – The purpose of this paper is to present the aerodynamic blade design of a tiltwing aircraft with a multi-objective optimization procedure. The aerodynamic design of tiltrotor blades is a very challenging task in the project of this type of aircraft. Design/methodology/approach – Tiltrotor blades have to give good performance both in helicopter and aeroplane modes. According to the design parameters (the chords, the twists and the airfoils along the blade), as the optimization objectives are different from one operating condition to another, the blade is the result of a multi-objective constrained optimization based on a controlled elitist genetic algorithm founded on the NSGA-II algorithm. The optimization process uses a BEMT solver to compute rotor performance. To avoid negative effects due to compressibility losses in aeroplane mode, the blade shape has been refined following the normal Mach number criterion. Findings – It has been found that the optimized rotor blade gives good performance bo...

Actuation Requirements of a Warp-Induced Variable Twist Rotor Blade

This article examines a warp-induced twist concept to obtain quasi-static large amplitude twist changes of helicopter or tiltrotor blades for performance benefits over diverse operating conditions. The presented concept has a cylindrical spar with rotating ribs which are attached to the blade skin that is slit along the trailing edge. Warping the skin then produces twisting of the blade section. Warp actuation is implemented by rotating a threaded rod assembly attached to the interior of the upper and lower skin near the trailing edge. Due to the slit at the trailing edge, the blade is soft in torsion during actuation, but is effectively a closed section due to the threaded rod assembly when in power off state. A prototype (NACA 0012, 10.75 in chord, 42 in span) based on this concept was built and tested for both the warp-twist relationship and the actuation torque requirements for producing the twist of the blade (up to ±18°). The finite element model developed in this article correlated very well with the experimental measurements made on the prototype. The validated finite element model is further used to conduct a study to understand the effect of various structural parameters on the system behavior.

Optimal design of a variable-twist proprotor incorporating shape memory alloy hybrid composites

The tiltrotor blades, or proprotor, act as a rotor in the helicopter mode and a propeller in the airplane mode. The helicopter mode generally requires relatively a low built-in twist angle, whereas in the airplane mode, a high built-in twist is desired. Meeting these rather conflicting requirements make the tiltrotor design a challenging task. This paper explores an optimal design of a variable-twist proprotor that changes the built-in twist in an adaptive manner by using the shape memory alloy hybrid composite (SMAHC). The optimum design problem attempts to find the cross-section internal layout that maximizes the twist actuation of the variable-twist proprotor while satisfying a series of design constraints. An optimum design framework is constructed in the current work by combining various analysis and design tools, such as an active composite cross-sectional analysis, a nonlinear flexible multibody dynamics analysis, a 3-D strain analysis, and a gradient-based optimizer. The MATLAB is used to integrate and synthesize the individual tools. A static tip twist is chosen as an objective function that should be maximized for the best performance. The optimum results exhibit that the twist actuation of the variable-twist proprotor can be maximized while satisfying all the prescribed design constraints.

Design and analysis of variable-twist tiltrotor blades using shape memory alloy hybridcomposites

The tiltrotor blade, or proprotor, acts as a rotor in the helicopter mode and as a propellerin the airplane mode. For a better performance, the proprotor should have different built-intwist distributions along the blade span, suitable for each operational mode. This paperproposes a new variable-twist proprotor concept that can adjust the built-in twistdistribution for given flight modes. For a variable-twist control, the present proprotoradopts shape memory alloy hybrid composites (SMAHC) containing shape memory alloy(SMA) wires embedded in the composite matrix. The proprotor of the KoreaAerospace Research Institute (KARI) Smart Unmanned Aerial Vehicle (SUAV),which is based on the tiltrotor concept, is used as a baseline proprotor model.The cross-sectional properties of the variable-twist proprotor are designed tomaintain the cross-sectional properties of the original proprotor as closely aspossible. However, the torsion stiffness is significantly reduced to accommodatethe variable-twist control. A nonlinear flexible multibody dynamic analysis isemployed to investigate the dynamic characteristics of the proprotor such as naturalfrequency and damping in the whirl flutter mode, the blade structural loads ina transition flight and the rotor performance in hover. The numerical resultsshow that the present proprotor is designed to have a strong similarity to thebaseline proprotor in dynamic and load characteristics. It is demonstrated thatthe present proprotor concept could be used to improve the hover performanceadaptively when the variable-twist control using the SMAHC is applied appropriately.

Modeling and Experimental Characterization of SMA Torsional Actuators

This article presents the material modeling and experimental characterization of SMA rod and tube actuators undergoing pure torsional deformations. The investigation of the torsional characteristics of SMAs is carried out in order to gain a fundamental understanding of the behavior of SMA torsional actuators. The proposed application is to alter the twist distribution of a tiltrotor blade from hover to forward flight modes, providing improved flight efficiency in both modes of flight. To describe the behavior of the actuator, a torsional model involving the extension of the one-dimensional (uniaxial) formulation of SMA phenomenology is presented. As an example, Brinson's model is chosen as the representative uniaxial model; however, the approach used here to extend the uniaxial model into the torsional domain is applicable to nearly any uniaxial SMA model. The parameters for the uniaxial model are derived from extensional testing of an SMA rod or tube, and are then used to predict the torsional characteristics of the same material. A major advantage of this method compared to prior art in torsional modeling is that the simplicity of deriving and implementing the parameters from uniaxial testing is carried over into the torsional domain. This simplicity derives from the fact that the measured properties (forces and displacements) in tensile loading are more readily suited to determining the model parameters than the measured properties in torsional loading which have been used by previous researchers (angles and torques which are, in fact, integrated quantities from strains and stresses). This article also presents a comprehensive experimental investigation and model correlation with both tension and torsional behavior of SMA rods and tubes. The model is compared with experimental data including torsional actuation tests against torsional springs. It is shown that the theoretical model demonstrates good agreement with the experimental data over a wide range of thermomechanical conditions. In addition, experimental phenomena associated with the torsional behavior of the SMA actuator such as the effects of heat treatment, twist rate, and loading pattern are examined.

Embedded Laser Doppler Velocimetry Boundary-Layer Investigation on a Tilt-Rotor Blade

A fundamental study is made of a rotating boundary layer (BL) in the inboard region of a tilt-rotor blade, using an embedded laser Doppler velocimetry probe inside a rotating blade. Velocity measurements have been performed to get insight into the BL behavior around a blade section close to the rotor hub.,BL transition and separation phenomena are investigated on the basis of both chordwise and spanwise velocity profiles measured for different collective pitch angles on a two-bladed hovering rotor. The velocity. experimental data bank appears to be in good agreement with theoretical velocity profiles in attached How conditions (laminar, transitional, or turbulent). Moreover, validation of a transitional criterion based on the second thickness of energy delta'(3) of the BL is extended to the rotational flow configuration. Usually such a configuration classically exhibits a separation of the BL for high collective pitch angles. However, in the particular case of a high twisted tilt-rotor blade operating in, hover, centrifugal effects are shown to be responsible for an important delay in the BL separation process.

Demonstration of an elastically coupled twist control concept for tilt rotor blade application

The purpose of this Note is to present results from an analytic/experimental study that investigated the potential for passively changing blade twist through the use of extension-twist coupling. A set of composite model rotor blades was manufactured from existing blade molds for a low-twist metal helicopter rotor blade, with a view toward establishing a preliminary proof concept for extension-twist-coupled rotor blades. Data were obtained in hover for both a ballasted and unballasted blade configuration in sea-level atmospheric conditions. Test data were compared with results obtained from a geometrically nonlinear analysis of a detailed finite element model of the rotor blade developed in MSC/NASTRAN.

Navier-Stokes calculations of inboard stall delay due to rotation

Momentum blade element methods, which have historically been used to predict rotor blade thrust, underpredict the power that is required for highly twisted rotors at high thrust levels. During a test of the V-22 tilt rotor blade conducted at the NASA Ames Research Center Outdoor Aerodynamic Research Facility (OARF), measurements of the thrust as a function of collective angle showed that the thrust levels achieved were higher than anticipated and drag levels at the high thrust levels were much lower than predicted. Because the maximum thrusting capability of the rotor in hover usually determines the maximum pay load of the vehicle, it is important to be able to accurately predict high thrust performance characteristics. A three-dimensional Navier-Stokes method was used to investigate the flow phenomenon that causes the increase in lift coefficients and reduction in drag coefficients obtained near the stall of a rotating blade. Comparison of the computational results with the experimental data indicates that this method can be used to predict thrust levels. The Navier-Stokes results indicate that flow separation near the hub is curtailed for a rotating blade compared with the momentum blade element predictions.

Behavior of Thin-Walled Beams Made of Advanced Composite Materials and Incorporating Non-Classical Effects

Several results concerning the refined theory of thin-walled beams of arbitrary closed cross-section incorporating non-classical effects are presented. These effects are related both with the exotic properties characterizing the advanced composite material structures and the non-uniform torsional model. A special case of the general equations is used to study several problems of cantilevered thin-walled beams and to assess the influence of the incorporated effects. The results presented in this paper could be useful towards a more rational design of aeronautical or aerospace constructions, as well as of helicopter or tilt rotor blades constructed of advanced composite materials.