Sang Joon Shin

VIBRATORY LOADS REDUCTION TESTING OF THE NASA/ARMY/MIT ACTIVE TWIST ROTOR

Recent studies have indicated that controlled strain-induced blade twisting can be attained using piezoelectric active fiber composite technology, and that such advancement may provide a mechanism for reduced rotorcraft vibrations and increased rotor performance. In order to validate these findings experimentally, a cooperative effort between the NASA Langley Research Center, the Army Research Laboratory, and the MIT Active Materials and Structures Laboratory has been developed. As a result of this collaboration a four-bladed, aeroelastically-scaled, active-twist model rotor has been designed and fabricated for testing in the heavy gas test medium of the NASA Langley Transonic Dynamics Tunnel. Initial wind tunnel testing has been conducted to assess the impact of active blade twist on both fixed- and rotating-system vibratory loads in forward flight. The active twist control was found to have a pronounced effect on all system loads and was shown to generally offer reductions in fixedsystem loads of 60% to 95%, depending upon flight condition, with 1.1o to 1.4o of dynamic blade twist observed. A summary of the systems developed and the vibratory loads reduction results obtained are presented in this paper.

On the modeling of integrally actuated helicopter blades

This paper presents an asymptotical formulation for preliminary design of multi-cell composite helicopter rotor blades with integral anisotropic active plies. It represents the first attempt in the literature to asymptotically analyze such active structure. The analysis is broken down in two parts: a linear two-dimensional analysis over the cross-section, and a geometrically non-linear (beam) analysis along the blade span. The cross-sectional analysis revises and extends a closed form solution for thin-walled, multi-cell beams based on the variational-asymptotical method, accounting for the presence of active fiber composites distributed along the cross-section of the blade. The formulation provides expressions for the asymptotically correct cross-sectional stiffness constants in closed form, facilitating design-trend studies. These stiffness constants are then used in a beam finite element discretization of the blade reference line. This is an extension of the exact intrinsic equations for the one-dimensional analysis of rotating beams considering small strains and finite rotations, and now taking account of the presence of distributed actuators. Subject to external loads, active ply induced strains, and specific boundary conditions, the one-dimensional (beam) problem can be solved for displacements, rotations, and strains of the reference line. Analytical and numerical studies are presented to compare the proposed theory against the previously established analytical models. Discrepancies are found for general blade cross-section and discussed herein in details, especially for the piezoelectric actuation components. Direct results of the present formulation are also compared with experimental data.

Dynamic response of active twist rotor blades

Dynamic characteristics of active twist rotor (ATR) blades are investigated analytically and experimentally in this paper. The ATR system is intended for vibration and potentially for noise reductions in helicopters through individual blade control. An aeroelastic model is developed to identify frequency response characteristics of the ATR blade with integral, generally anisotropic, strain actuators embedded in its composite construction. An ATR prototype blade was designed and manufactured to experimentally study the vibration reduction capabilities of such systems. Several bench and hover tests were conducted and those results are presented and discussed here. Selected results on sensitivity of the ATR system to collective setting (i.e. blade loading), blade rpm (i.e. centrifugal force and blade station velocity), and media density (i.e. altitude) are presented. They indicated that the twist actuation authority of the ATR blade is independent of the collective setting up to approximately 10P, and dependent on rotational speed and altitude near the torsional resonance frequency due to its dependency on the aerodynamic damping. The proposed model captures very well the physics and sensitivities to selected test parameters of the ATR system. The numerical result of the blade torsional loads show an average error of 20% in magnitude and virtually no difference in phase for the blade frequency response. Overall, the active blade model is in very good agreement with the experiments and can be used to analyze and design future active helicopter blade systems.

On the twist performance of a multiple-cell active helicopter blade

This paper discusses how torsional stiffness is related with twist actuation in an integrally twist actuated rotor blade. It shows that increase in torsional stiffness does not necessarily reduce the twist actuation. Improvements in twist actuation for a realistic airfoil-shaped beam can be achieved while controlling the blade torsional stiffness within aeroelastic requirements. These design trends can be studied once an appropriate anisotropic beam analytical formulation with integral anisotropic piezoelectric actuators is developed. The new analytical model revises and extends the closed form solution for thin-walled multi-cell beams based on the variational-asymptotical method and introduces the distributed anisotropic piezoelectric actuator effects along the cross section of the blade. The model is applied to specifically investigate the relationship between twist actuation performance and torsional stiffness of a realistic active blade. In this study, both analytical expressions and numerical examples are examined. The results show that the torsional stiffness can be increased by up to 20% with an increase in twist actuation of approximately 5% from their corresponding baseline values. Moreover, the results also indicate that a single-cell model is insufficient to address qualitatively and quantitatively the complex interaction that exists among the wall members of a two-cell airfoil cross section.

Design and Manufacturing of a Model-scale Active Twist Rotor Prototype Blade

The design and manufacturing of an active twist rotor blade for vibration reduction in helicopters are presented. The rotor blade is integrally twisted by direct strain actuation through embedded piezoelectric fiber composite actuators distributed along the span of the blade. Highlights of the analysis formulation used to design this type of active blade are presented. The requirements for the prototype blade, along with the final design results are also presented. Detailed aspects of its manufacturing are described. Experimental structural characteristics of the prototype blade compare well with design goals, and bench actuation tests characterize its basic actuation performance. The design and manufacturing processes permit the realization of an active blade that satisfies a given set of design requirements. This is used to later develop a fully active rotor blade system.

System Identification Technique for Active Helicopter Rotors

System identification methodology is developed for a linear time-periodic (LTP) system and applied to an experimental setup of an integrally twist-actuated helicopter rotor blade. Identification is conducted for a controller design, which alleviates vibratory loads induced in forward flight. Since a rotor in forward flight is a time-periodic system due to the aerodynamic environment varying once per rotation, the adopted methodology requires determination of the multicomponent harmonic transfer functions. A simplified identification formula is also derived for a linear time-invariant (LTI) system, such as a rotor system in hover. The latter approach gives another estimate of the primary component among the harmonic transfer functions. The identification experiment is conducted at NASA Langley Transonic Dynamics Tunnel. The magnitude of the higher-order harmonic transfer functions is observed to be small in the frequency range of interest when compared with that of the primary component. This indicates that the present active rotor system may be regarded as a LTI system under the level flight conditions considered. Results obtained in system identification are interpreted in terms of the closed-loop controller design.

Forward flight response of the active twist rotor for helicopter vibration reduction

Dynamic characteristics of active twist rotor (ATR) blades during forward flight is investigated analytically in this paper. An aeroelastic model for an active rotor system is developed to identify dynamic characteristics of ATR blades with integral strain actuators embedded in their composite construction. More specifically, a time domain integration scheme for the geometrically exact formulation of passive beams is extended with the active materials constitutive relations for the forward flight analysis of the ATR system. In parallel, forward flight wind-tunnel tests are conducted at NASA Langley to collect control sensitivity functions experimentally using dynamically-scaled four-active-bladed rotor system. Preliminary results from the present analytical model are presented and compare well with experimental observations. The theoretical model will be used as a design and evaluation tool for closed-loop controller for twist actuation of the ATR system.

Computational material characterization of active fiber composite

Active fiber composite (AFC) is composed of many different materials–piezoelectric fiber, polymer matrix, kapton mold, and kapton electrode and it is usually embedded in the glass fiber composites. In addition, there is an active/inactive region in the fiber. Therefore, it is ideal to adopt a full microscopic model and directly analyze the model without any simplifying assumptions. In this work, all the constituents of AFC are modeled and simulated directly in microscopic scale level. Material properties and actuation performances are characterized and compared with the previous experimental measurements. Some material constants which are difficult to be experimentally determined but needed for three-dimensional (3-D) finite element (FE) simulations can be obtained by this approach. Effects of mesh density are examined and local stresses are observed in detail. To solve a large scale problem, parallel computing technology is introduced.

Helicopter Rotor Load Prediction Using a Geometrically Exact Beam with Multicomponent Model

In this paper, an accurate structural dynamic analysis was developed for a helicopter rotor system including rotor control components, which was coupled to various aerodynamic and wake models in order to predict an aeroelastic response and the loads acting on the rotor. Its blade analysis was based on an intrinsic formulation of moving beams implemented in the time domain. The rotor control system was modeled as a combination of rigid and elastic components. A multicomponent analysis was then developed by coupling the beam finite element model with the rotor control system model to obtain a complete rotor-blade/control-system aeroelastic analysis. The rotor blade analysis was in good agreement and validated by comparing with DYMORE. Numerical results were obtained for a four-bladed, small-scale, articulated rotor rotating in vacuum and in a wind tunnel to simulate forward-flight conditions and its aerodynamic effects. The complete rotor-blade/control-system model was loosely coupled with various inflow and wake models in order to simulate both hover and forward-flight conditions. The resulting rotor blade response and pitch link loads are in good agreement with those predicted by CAMRAD II. The present analysis features both model compactness and robustness in its solution procedure while capturing the sophisticated behavior of individual rotor components. The analysis is expected to be part of a framework useful in the preliminary design phase for helicopters.

Advanced Analysis on Tiltrotor Aircraft Flutter Stability, Including Unsteady Aerodynamics

The whirl-flutter-instability phenomenon imposes a serious limit on the forward speed in tiltrotor aircraft. In this paper, an advanced analysis is formulated to predict an aeroelastic stability for a gimballed three-bladed rotor with flexible wing based on three different types of aerodynamic model. Among them, the one with the full unsteady aerodynamics is most sophisticated, because it may represent more realistic operating conditions. A nine-degree-of-freedom model is newly developed to predict the complete tiltrotor aircraft. Numerical results are obtained in both time and frequency domains. A generalized eigenvalue is used to estimate whirl-flutter stability in the frequency domain, and the Runge-Kutta method is used in the time domain. Control system flexibility is further included in the present analysis to give the capability for a more accurate stability prediction. Results from such an improved analysis are validated with the other existing predictions and show good agreement. The present model with unsteady aerodynamics will be extended to further consider an aerodynamic interaction between the rotor and wing.