The objective of this investigation is to illustrate the steps involved in developing a multibody dynamics analytical model to simulate the aeroelastic stability and blade loading of a soft-inplane tiltrotor wind tunnel model and to correlate those simulations with experimental data. Development of soft-inplane tiltrotor technology is beneficial for providing viable lightweight hub design options for future heavy lift transport rotorcraft application. Experimental verification of such advanced configurations using either subscale models in wind tunnels or full-scale flight testing is becoming prohibitively expensive. Advanced modeling and simulation of complex tiltrotor hub configurations using multibody dynamics analyses offers an alternative to such expensive experimental verifications. Comprehensive rotorcraft-oriented multibody analyses enable the modeling and simulation of rotor hub systems to a level of detail that allows the complex kinematics and nonlinear effects associated with rotor hub control systems and drive train free play to be considered. The influence of these and other nonlinear effects on the aeromechanical behavior of a tiltrotor model is examined in this study.

An investigation into the effects of aerodynamic and aeroelastic scaling parameters on model scale helicopter rotors has been conducted in the NASA Langley Transonic Dynamics Tunnel. The effect of varying Reynolds number, blade Lock number, and structural elasticity on rotor performance has been studied and the performance results are discussed herein for two different rotor blade sets at two rotor advance ratios. One set of rotor blades were rigid and the other set of blades were dynamically scaled to be representative of a main rotor design for a utility class helicopter. The investigation was conducted in forward flight at rotor advance ratios of 0.15 and 0.35. Additionally, the rotors were tested over a range of nominal test medium densities from 0.00382 slugs! It 3 to 0.009 slugs! It . This range of densities permits the acquisition of data for several Reynolds and Lock number combinations.

The objective of this investigation is to illustrate a modular approach in the development and validation of sophisticated rotorcraft analytical models within the framework of multibody dynamics simulations. This approach is demonstrated with the development of a stiff-inplane tiltrotor wind-tunnel model using two multibody dynamics analyses. The two multibody dynamics codes used are: DYMORE and MBDyn, which are both capable of supporting comprehensive, multibody-based rotorcraft modeling and dynamic simulation. The multibody dynamics models developed in this effort include the gimballed hub, rotor blades, pitch links, swashplate, conversion actuators which are attached to the pylon, and the elastic wing. The natural frequencies, mode shapes, and stability characteristics of key sub-component structures and the kinematic couplings of rotor system are correlated with the predictions of the two analyses. Furthermore, experimental data obtained in ground vibration tests and wind-tunnel tests is extensively used to validate the analytical models. The validated models are then used to predict the tiltrotor whirl-flutter stability boundary, which shows good agreement with the experimental measurement.

This paper focuses on the development of multibody numerical models to predict the dynamic response, aeroelastic stability, and blade loading of a soft-inplane tiltrotor wind-tunnel model. Comprehensive rotorcraft-based multibody analyses enable modeling of the rotor system to a high level of detail such that complex mechanics and nonlinear effects associated with control system geometry, and joint deadband may be considered. The influence of these and other nonlinear effects on the aeromechanical behavior of the tiltrotor model is examined. To assess the reliability of the multibody approach to rotorcraft analysis, the results obtained using two different rotorcraft-based multibody solvers are analyzed and compared. A parametric study of some design parameters that may influence the aeromechanics of the soft-inplane rotor system is also included in this investigation.

This paper presents the development of two multibody dynami cs odels to predict the whirl-flutter stability of a stiff-inplane tiltrotor wi nd-tunnel model and correlates the predictions with experimental data. Comprehensive, multibody-b ased dynamics analyses of rotorcraft enable modeling and simulation of the rotor system at a high l evel of detail so that complex mechanics and nonlinear effects associated with control sy stem geometry and various types of hinges and joints are able to be considered. A parametric stu dy of key design variables, such as control system stiffness, pitch-flap coupling, and aerodyn amic compressibility was completed as part of this investigation, which may help to determine su itable design criteria for future stiff-inplane tiltrotor systems. Analytical results indi cate consistent capabilities of multibody dynamics analyses in predicting the stiff-inplane tiltrot or whirl-flutter stability and show generally good agreement with the experimental results. Both a nalytical and experimental results show the destabilizing effects of pitch-flap coupling and ae rodynamic compressibility on the whirl-flutter stability of a stiff-inplane tiltrotor where as both results also indicate that a reduction in control system stiffness has little effect on whirlflutter stability.

A new four-bladed, semi-articulated, soft-inplane rotor system, designed as a candidate for future heavy-lift rotorcraft, was tested at model scale on the Wing and Rotor Aeroelastic Testing System (WRATS), a 1/5-size aeroelastic wind-tunnel model based on the V-22. The experimental investigation included a hover test with the model in helicopter mode subject to ground resonance conditions, and a forward flight test with the model in airplane mode subject to whirl-flutter conditions. An active control system designed to augment system damping was also tested as part of this investigation. Results of this study indicate that the new four-bladed, soft-inplane rotor system in hover has adequate damping characteristics and is stable throughout its rotor-speed envelope. However, in airplane mode it produces very low damping in the key wing beam-bending mode, and has a low whirl-flutter stability boundary with respect to airspeed. The active control system was successful in augmenting the damping of the fundamental system modes, and was found to be robust with respect to changes in rotor-speed and airspeed. Finally, conversion-mode dynamic loads were measured on the rotor and these were found to be significantly lower for the new soft-inplane hub than for the previous baseline stiff-inplane hub.

The sensitivity of blade tracking in hover to variations in root pitch was examined for two rotor configurations. Tests were conducted using a four-bladed articulated rotor mounted on the NASA-Army aeroelastic rotor experimental system (ARES). Two rotor configurations were tested: one consisting of a blade set with flexible fiberglass spars and one with stiffer (by a factor of five in flapwise and torsional stiffnesses) aluminum spars. Both blade sets were identical in planform and airfoil distribution and were untwisted. The two configurations were ballasted to the same Lock number so that a direct comparison of the tracking sensitivity to a gross change in blade stiffness could be made. Experimental results show no large differences between the two sets of blades in the sensitivity of the blade tracking to root pitch adjustments. However, a measurable reduction in in-track coning of the fiberglass spar blades with respect to the aluminum blades is noted at higher rotor thrust conditions.

A preliminary study of a soft-inplane gimballed tiltrotor model subject to ground resonance conditions in hover has been completed. Parametric variations of the rotor collective pitch and blade root damping, and their associated effects on the aeromechanical stability of both the isolated rotor system and the coupled wing/pylon/rotor system were examined. Results showed aeromechanical behavior which is significantly different from that associated with classical soft-inplane helicopter rotor systems. The unstable mode of the coupled aeromechanical system was a fixed system wing mode while in classical ground resonance the rotor lag mode becomes unstable. Damping of the isolated rotor lag mode was determined to be extremely sensitive to the collective pitch setting, as was damping and the associated stability of the critical wing mode for the coupled aeromechanical system.

The results of a joint NASA/Army/Bell Helicopter Textron wind-tunnel test to assess the potential of Generalized Predictive Control (GPC) for actively controlling the swashplate of tiltrotor aircraft to enhance aeroelastic stability in the airplane mode of flight are presented. GPC is an adaptive time-domain predictive control method that uses a linear difference equation to describe the input-output relationship of the system and to design the controller. The test was conducted in the Langley Transonic Dynamics Tunnel using an unpowered 1/5-scale semispan aeroelastic model of the V-22 that was modified to incorporate a GPC-based multi-input multi-output control algorithm to individually control each of the three swashplate actuators. Wing responses were used for feedback. The GPC-based control system was highly effective in increasing the stability of the critical wing mode for all of the conditions tested, without measurable degradation of the damping in the other modes. The algorithm was also robust with respect to its performance in adjusting to rapid changes in both the rotor speed and the tunnel airspeed.

A wind-tunnel investigation of whirl-flutter stability boundaries has been conducted on a 1/5-size semispan tiltrotor model known as the Wing and Rotor Aeroelastic Test System (WRATS) in the NASA-Langley Transonic Dynamics Tunnel as part of a joint NASA/Army/Bell Helicopter Textron, Inc (BHTI) research program. The model was developed by BHTI as part of the JVX (V-22) development program in the 1980s and was modified to incorporate a hydraulically actuated swasliplate control system for use in active controls research. The modifications have changed the model's pylon mass properties sufficiently to warrant testing to re-establish its baseline stability boundaries. A parametric investigation of the effect of rotor design variables on stability was also conducted. Experimental baseline stability boundaries in air are presented with comparisons to results from parametric variations of rotor pitch-flap coupling and control system stiffness. Increasing the rotor pitch-flap coupling (53 more negative) has a destabilizing effect on stability, while a reduction in control system stiffness has little effect on whirl-flutter stability. The results from tests conducted in R-134a heavy gas indicate that matching full-scale blade Mach number has a destabilizing effect on whirl-flutter. This finding demonstrates that stability boundaries obtained from tests at reduced Mach number in air are unconservative.

A team of researchers from the Army Research Laboratory, NASA Langley Research Center (LaRC), and Bell Helicopter-Textron, Inc. have completed hover-cell and wind-tunnel testing of a 1/5-size aeroelastically-scaled tiltrotor model using a new active control system for stability augmentation. The active system is based on a generalized predictive control (GPC) algorithm originally developed at NASA LaRC in 1997 for unknown disturbance rejection. Results of these investigations show that GPC combined with an active swashplate can significantly augment the damping and stability of tiltrotors in both hover and high-speed flight.

Soft-inplane rotor systems an significantly reduce the inplane rotor loads generated during the maneuvers of large tiltrotor, thereby reducing the strength requirements and the associated structural weight of the hub. Soft-inplane rotor systems, however, are subject to instabilities associated with ground resonance, and for tiltrotors this instability has increased complexity as compared to a conventional helicopter. Researchers at Langley Research Center and Bell Helicopter-Textron, Inc. has completed an initial study of a soft-inplane gimballed tiltrotor model subject to ground resonance conditions in hover. Parametric variations of the rotor collective pitch and blade root damping, and their associated effects on the model stability were examined. Also considered in the study was the effectiveness of an active swashplate and a generalized predictive control (GPC) algorithm for stability augmentation of the ground resonance conditions. Results of this study show that the ground resonance behavior of a gimballed soft-inplane tiltrotor can be significantly different from that of a classical soft-inplane helicopter rotor. The GPC-based active swashplate was successfully implemented, and served to significantly augment damping of the critical modes to an acceptable value.

An investigation into the effects of aerodynamic and aeroelastic scaling parameters on model scale helicopter rotors has been conducted in the NASA Langley Transonic Dynamics Tunnel. The effect of varying Reynolds number, blade Lock number, and structural elasticity on rotor performance has been studied and the performance results are discussed herin for two different rotor blade sets at two rotor advance rations. One set of rotor blades were rigid and the other set of blades were dynamically scaled to be representative of a main rotor design for a utility class helicopter. The investigation was conducted in forward flight at rotor advance ratios of 0.15 and 0.35. Additionally, the rotors were tested over a range of nominal test medium densities from 0.00382 slugs/ft\super{3} to 0.009 slugs/ft\super{3}. This reange of densities permits the acquisition of data for several Reynolds and Lock nuymber combinations.

Data for rotors using unconventional airfoils are of interest to permit an evaluation of this technology’s capability to meet the U.S. Army’s need for increased helicopter mission effectiveness and improved safety and survivability. Thus, an experimental investigation was conducted in the NASA Langley Transonic Dynamics Tunnel to evaluate the effect of using slotted airfoils in the rotor blade tip region (85%-100% radius) on rotor aerodynamic performance and loads. Four rotor configurations were tested in forward flight at advance ratios from 0.15 to 0.45 and in hover inground effect. The hover tip Mach number was 0.627, which is representative of a design point of 4000-ft geometric altitude and a temperature of 95° F. The baseline rotor configuration had a conventional single-element airfoil in the tip region. A second rotor configuration had a forward-slotted airfoil with a −6° slat, a third configuration had a forward-slotted airfoil with a −10° slat, and a fourth configuration had an aft-slotted airfoil with a 3° flap (trailing edge down). The results of this investigation indicate that the −6° slat configuration offers some performance and loads benefits over the other three configurations at the higher rotor lift coefficients at each advance ratio.

Abstract : This report provides data obtained during a wind-tunnel test conducted to investigate parametrically the effect of blade nonstructural mass on helicopter fixed-system vibratory loads. The data were obtained with aeroelastically scaled model rotor blades that allowed for the addition of concentrated nonstructural masses at multiple locations along the blade radius. Testing was conducted for advance ratios ranging from 0.10 to 0.35 for 10 blade-mass configurations. Three thrust levels were obtained at representative full-scale shaft angles for each blade-mass configuration. This report provides the fixed-system forces and moments measured during testing. The comprehensive database obtained is well-suited for use in correlation and development of advanced rotorcraft analyses.

This report provides data obtained during a wind-tunnel test conducted to investigate parametrically the effect of blade nonstructural mass on helicopter fixed-system vibratory loads. The data were obtained with aeroelastically scaled model rotor blades that allowed for the addition of concentrated nonstructural masses at multiple locations along the blade radius. Testing was conducted for advance ratios ranging from 0.10 to 0.35 for 10 blade-mass configurations. Three thrust levels were obtained at representative full-scale shaft angles for each blade-mass configuration. This report provides the fixed-system forces and moments measured during testing. The comprehensive database obtained is well-suited for use in correlation and development of advanced rotorcraft analysis. Aeroelasticity

An investigation was conducted in the Langley Transonic Dynamics Tunnel to obtain data to permit evaluation of paddle-type tip technology for possible use in future U.S. advanced rotor designs. Data were obtained for both a baseline main-rotor blade and a main-rotor blade with a paddle-type tip. The baseline and paddle-type tip blades were compared with regard to rotor performance, oscillatory pitch-link loads, and 4-per-rev vertical fixed-system loads. Data were obtained in hover and forward flight over a nominal range of advance ratios from 0.15 to 0.425. Results indicate that the paddle-type tip offers no performance improvements in either hover or forward flight. Pitch-link oscillatory loads for the paddle-type tip are higher than for the baseline blade, whereas 4-per-rev vertical fixed-system loads are generally lower.