Higher Harmonic Pitch Control Research Articles

Study on Blade Property Measurement and Its Influence on Air/Structural Loads

In this study, the structural properties of Second Higher-Harmonic Aeroacoustic Rotor Test blades are determined using state-of-the-art test techniques. The measurement includes bending and torsion stiffnesses, section geometric offsets, and mass and inertia properties. Several Second Higher-Harmonic Aeroacoustic Rotor Test blades, including the original instrumented blade used for the wind-tunnel test campaign in 2001, as stated by Yu et al. (The HART-II Test: Rotor Wakes and Aeroacoustics with Higher-Harmonic Pitch Control (HHC) Inputs—The Joint German/French/Dutch/US Project, Proceedings of the 58th American Helicopter Society (AHS) Annual Forum, American Helicopter Soc., Montreal, June 2002) are used for the activity. A finite element-based cross-section analysis combined with an x-ray computer tomography technique is employed for the cases where no mechanical measurement is available or attempted. The resulting structural properties are correlated against the earlier estimated values, which have extensively been used in the literature for the validation of the Second Higher-Harmonic Aeroacoustic Rotor Test rotor. A substantial deviation is observed between the present measurement and the earlier property result. The comprehensive rotor dynamics analysis is performed to quantify the impact of the measured blade properties on the aeromechanics behavior of the rotor. The location of the center of gravity is demonstrated to be the most influential factor affecting the deviation and the sensitivity of the aeroelastic response of the rotor.

Prediction of the aeromechanics for HART II rotor in descending flight using mixed variational geometrically exact beam analysis

This paper presents validation of the helicopter rotor aeromechanics in descending flight. In its structural analysis, mixed variational geometrically exact beam model was used to capture nonlinear behavior of the rotor blade precisely. The present beam model was expressed by a mixed varational formulation in terms of the displacements, rotations, inertial forces, moments, and linear/angular momenta. Then displacements, inertial forces, and momenta can be directly extracted and solved simultaneously. And the finite-state dynamic inflow aerodynamics combined with blade element theory was used to predict induced inflow. The present results are compared by the wind tunnel test data for the higher-harmonic aeroacoustics rotor test (HART) II rotor with and without higher harmonic pitch control in descending flight. In the numerical results, trim control angles, section normal force, structural deflections, and structural loads were compared with those of the measurement. Predicted trim control angles and structural deflections show a good agreement with the measurement. But predicted flap bending moments show certain discrepancy with the measurement.

Comprehensive multibody dynamics analysis for rotor aeromechanics predictions in descending flight

AbstractThis paper studies the rotor aeromechanics in descending flight using a nonlinear flexible multibody dynamic analysis code, DYMORE. A freewake model is included in DYMORE to improve the rotor wake modelling. The wind-tunnel test data of the Higher-harmonic Aeroacoustics Rotor Test (HART) II rotor, with and without higher harmonic pitch control (HHC), and the flight test data of the full-scale utility helicopter rotor in descent are used for the aeromechanics correlation at an advance ratio of 0·15. The blade-vortex interaction (BVI) airloads are reasonably predicted for both the HART II and utility helicopter rotors, although some BVI peaks are missed on the advancing sides for both the rotors. The flap deflections and elastic torsion deformations at the blade tip are fairly correlated against the measured data of the HART II rotor. The correlation of blade structural moments for both HART II and utility helicopter rotors are not as good as the lift predictions; however, a reasonable prediction is obtained for the utility helicopter rotor.

Correlation of helicopter rotor aeroelastic response with HART‐II wind tunnel test data

PurposeThis paper aims to validate a comprehensive aeroelastic analysis for a helicopter rotor with the higher harmonic control aeroacoustic rotor test (HART‐II) wind tunnel test data.Design/methodology/approachAeroelastic analysis of helicopter rotor with elastic blades based on finite element method in space and time and capable of considering higher harmonic control inputs is carried out. Moderate deflection and coriolis nonlinearities are included in the analysis. The rotor aerodynamics are represented using free wake and unsteady aerodynamic models.FindingsGood correlation between analysis and HART‐II wind tunnel test data is obtained for blade natural frequencies across a range of rotating speeds. The basic physics of the blade mode shapes are also well captured. In particular, the fundamental flap, lag and torsion modes compare very well. The blade response compares well with HART‐II result and other high‐fidelity aeroelastic code predictions for flap and torsion mode. For the lead‐lag response, the present analysis prediction is somewhat better than other aeroelastic analyses.Research limitations/implicationsPredicted blade response trend with higher harmonic pitch control agreed well with the wind tunnel test data, but usually contained a constant offset in the mean values of lead‐lag and elastic torsion response. Improvements in the modeling of the aerodynamic environment around the rotor can help reduce this gap between the experimental and numerical results.Practical implicationsCorrelation of predicted aeroelastic response with wind tunnel test data is a vital step towards validating any helicopter aeroelastic analysis. Such efforts lend confidence in using the numerical analysis to understand the actual physical behavior of the helicopter system. Also, validated numerical analyses can take the place of time‐consuming and expensive wind tunnel tests during the initial stage of the design process.Originality/valueWhile the basic physics appears to be well captured by the aeroelastic analysis, there is need for improvement in the aerodynamic modeling which appears to be the source of the gap between numerical predictions and HART‐II wind tunnel experiments.

Analysis of SPR measurements from HART II

The wind tunnel test of HART II (Higher Harmonic Control Aeroacoustic Rotor Test), performed in October 2001 in the Large Low-Speed Facility (LLF) of the German-Dutch Wind-tunnel (DNW), is part of an international cooperative program by the German DLR, French ONERA, DNW, NASA Langley and the US Army Aeroflightdynamics Directorate (AFDD). The main objective of the program is the investigation of rotor wake and its influence on rotor blade–vortex interaction (BVI) noise with and without higher harmonic pitch control (HHC). For blade position and deflection measurements the Stereo Pattern Recognition (SPR) technique was used for the first time. This technique is based on a 3-dimensional reconstruction of visible marker locations by using stereo camera images. An evaluation of these images leads to the spatial position of markers which are attached to each of the four blades and to the bottom of the fuselage and thus to the blade motion parameters in flap, lead-lag and torsion. In this paper the different analysis methods and post-processing of SPR data and the final results for all four rotor blades are presented and the advantages, drawbacks and the potential of this technology are shown.