Hyeonsoo Yeo

Rotor Airloads Prediction Using Loose Aerodynamic Structural Coupling

A computational fluid dynamics (CFD) code and rotorcraft computational structural dynamics (CSD) code are coupled to calculate helicopter rotor airloads across a range of flight conditions. An iterative loose (weak) coupling methodology is used to couple the CFD and CSD codes on a per revolution, periodic basis. The CFD code uses a high fidelity, Navier‐Stokes, overset grid methodology with first principles-based wake capturing. Modifications are made to the CFD code for the aeroelastic analysis. For a UH-60A Blackhawk helicopter, three challenging level flight conditions are computed: 1) high speed, μ = 0.37, with advancing blade negative lift, 2) low speed, μ = 0.15, with blade‐vortex interaction, and 3) high thrust with dynamic stall, μ = 0.24. Results are compared with UH-60A Airloads Program flight test data. For all cases the loose coupling methodology is shown to be stable, convergent, and robust with full coupling of normal force, pitching moment, and chord force. In comparison with flight test data, normal force and pitching moment phase and magnitude are in good agreement. The shapes of the airloads curves are well captured. Overall, the results are a noteworthy improvement over lifting line aerodynamics used in rotorcraft comprehensive codes.

Assessment of comprehensive analysis calculation of airloads on helicopter rotors

Blade section normal force and pitching moment were investigated for six rotors operating at transition and high speeds: H-34 in flight and wind tunnel, SA 330 (research Puma), SA 349/2, UH-60A full-scale, and BO-105 model (Higher-Harmonic Acoustics Rotor Test I). The measured data from flight and wind-tunnel tests were compared with calculations obtained using the comprehensive analysis CAMRAD II. The calculations were made using two free-wake models: rolled up and multiple trailer with consolidation models. At transition speed, there is fair to good agreement for the blade section normal force between the test data and analysis for the H-34, research Puma, and SA 349/2 with the rolled-up wake. The calculated airloads differ significantly from the measurements for the UH-60A and BO-105. Better correlation is obtained for the UH-60A and BO-105 by using the multiple trailer with consolidation wake model. In the high-speed condition, the analysis shows generally good agreement with the research Puma flight data in both magnitude and phase. However, poor agreement is obtained for the other rotors examined. The analysis shows that the aerodynamic tip design (chord length and quarter-chord location) of the research Puma has an important influence on the phase correlation.

Optimum Design of a Compound Helicopter

A design and aeromechanics investigation was conducted for a 100,000-lb compound helicopter with a single main rotor, which is to cruise at 250 knots at 4000 ft/95 deg F condition. Performance, stability, and control analyses were conducted with the comprehensive rotorcraft analysis CAMRAD II. Wind tunnel test measurements of the performance of the H-34 and UH-1D rotors at high advance ratio were compared with calculations to assess the accuracy of the analysis for the design of a high speed helicopter. In general, good correlation was obtained when an increase of drag coefficients in the reverse flow region was implemented. An assessment of various design parameters (disk loading, blade loading, wing loading) on the performance of the compound helicopter was conducted. Lower wing loading (larger wing area) and higher blade loading (smaller blade chord) increased aircraft lift-to-drag ratio. However, disk loading has a small influence on aircraft lift-to-drag ratio. A rotor parametric study showed that most of the benefit of slowing the rotor occurred at the initial 20 to 30% reduction of the advancing blade tip Mach number. No stability issues were observed with the current design. Control derivatives did not change significantly with speed, but the did exhibit significant coupling.

Performance and Design Investigation of Heavy Lift Tilt-Rotor with Aerodynamic Interference Effects

Abstract : The aerodynamic interference effects on tiltrotor performance in cruise are investigated using comprehensive calculations, to better understand the physics and to quantify the effects on the aircraft design. Performance calculations were conducted for 146,600-lb conventional and quad tiltrotors, which are to cruise at 300 knots at 4000 ft/95 deg F condition. A parametric study was conducted to understand the effects of design parameters on the performance of the aircraft. Aerodynamic interference improves the aircraft lift-to-drag ratio of the baseline conventional tiltrotor. However, interference degrades the aircraft performance of the baseline quad tiltrotor, due mostly to the unfavorable effects from the front wing to the rear wing. A reduction of rotor tip speed increased the aircraft lift-to-drag ratio the most among the design parameters investigated.

Prediction of Rotor Structural Loads with Comprehensive Analysis

Blade flap and chord bending and torsion moments are investigated for five rotors operating at transition and high speed: H-34 in flight and wind tunnel, SA 330 (research Puma), SA 349/2, UH-60A full-scale, and BO-105 model (HART-I). The measured data from flight and wind tunnel tests are compared with calculations obtained using the comprehensive analysis CAMRAD II. The calculations were made using two free wake models: rolled-up and multiple-trailer with consolidation models. At transition speed, there is fair to good agreement for the flap and chord bending moments between the test data and analysis for the H-34, research Puma, and SA 349/2. Torsion moment correlation, in general, is fair to good for all the rotors investigated. Better flap bending and torsion moment correlation is obtained for the UH-60A and BO-105 rotors by using the multiple-trailer with consolidation wake model. In the high-speed condition, the analysis shows generally better correlation in magnitude than in phase for the flap bending and torsion moments. However, a significant underprediction of chord bending moment is observed for the research Puma and UH-60A. The poor correlation of the chord bending moment for the UH-60A appears to be caused by both the airloads model (at all radial locations) and the lag damper model (mostly at inboard locations).

Aeromechanics Analysis of a Heavy Lift Slowed-Rotor Compound Helicopter

A heavy lift slowed-rotor tandem compound helicopter was designed as a part of the NASA heavy lift rotorcraft systems investigation. The vehicle is required to carry 120 passengers over a range of 1200 nautical miles and cruise at 350 knots at an altitude of 30,000 feet. The basic size of the helicopter was determined by the United States Army Aeroflightdynamics Directorates design code RotorCraft. Then performance, loads, and stability analyses were conducted with the Comprehensive Analytical Model of Rotorcraft Aerodynamics and Design II. Blade structural design (blade inertial and structural properties) was carried out using the loading condition from the Comprehensive Analytical Model of Rotorcraft Aerodynamics and Design II. A rotor parametric study was conducted to investigate the effects of the twist, collective, tip speed, and taper on aircraft performance. Designs were also developed for alternate missions to explore the influence of the design condition on performance.

Investigation of Rotor Blade Structural Dynamics and Modeling Based on Measured Airloads

The work presented herein treats measured airloads from the UH-60A Airloads Program as prescribed external loads to calculate the resulting structural loads and motions of a rotor blade. Without the need to perform any aerodynamic computations, the coupled aeroelastic response problem is reduced to one involving only structural dynamics. The results, computed by RCAS and CAMRAD II, are compared against measured results and against each other for three representative test points. The results from the two codes mostly validate each other. Seven more test points, with responses computed by RCAS, to form thrust and airspeed sweeps are evaluated to better understand key issues. One such issue is an inability to consistently predict pushrod loads and torsion moments well, and this is found to be amplified at the two test points with the highest thrust coefficient. For these two test points, harmonic analysis reveals that the issue is due to excessive amounts of 5/rev response that stem from high levels of 5/rev pitching moment excitation. Another issue that concerns all test points is that the phase of the 1/rev blade flapping motion is not predicted well, which reflects the high sensitivity of this quantity that is developed due to having a first flap frequency of approximately 1/rev. Results also show that current force-velocity relationships, used in describing the lead-lag damper, are not satisfactory to consistently yield accurate inboard chordwise bending moment predictions. Overall, the investigation here, conducted with numerous test points, further confirms the methodology of prescribing measured airloads for assessing the structural dynamics capability of a computational tool.

Investigation of Rotor Performance and Loads of a UH-60A Individual Blade Control System

Wind tunnel measurements of performance, loads, and vibration of a full-scale UH-60A Black Hawk main rotor with an individual blade control (IBC) system are compared with calculations obtained using the comprehensive helicopter analysis CAMRAD II and a coupled CAMRAD II/OVERFLOW 2 analysis. Measured data show a 5.1% rotor power reduction (8.6% rotor lift to effective-drag ratio increase) using 2/rev IBC actuation with 2.0 ◦ amplitude at μ = 0.4. At the optimum IBC phase for rotor performance, IBC actuator force (pitch link force) decreased, and neither flap nor chord bending moments changed significantly. CAMRAD II predicts the rotor power variations with the IBC phase reasonably well at μ = 0.35. However, the correlation degrades at μ = 0.4. Coupled CAMRAD II/OVERFLOW 2 shows excellent correlation with the measured rotor power variations with the IBC phase at both μ = 0.35 and μ = 0.4. Maximum reduction of IBC actuator force is better predicted with CAMRAD II, but general trends are better captured with the coupled analysis. The correlation of vibratory hub loads is generally poor by both methods, although the coupled analysis somewhat captures general trends.

Investigation of UH-60A Rotor Performance and Loads at High Advance Ratios

Wind tunnel measurements of the performance, airloads, and structural loads of a full-scale UH-60A Black Hawk main rotor operating at high advance ratios (up to 1.0) are compared with calculations obtained using the comprehensive rotorcraft analysis Comprehensive Analytical Model of Rotorcraft Aerodynamics and Dynamics II to understand physics and quantify this comprehensive code’s accuracy and reliability in the prediction of rotor performance and loads at high advance ratios. Detailed comparisons are made on rotor thrust, control angles, power, and section loads to illustrate and understand unique aeromechanics phenomena in this regime. The analysis correctly predicts the thrust reversal with collective at high advance ratios. Rotor induced plus profile power is also reasonably well predicted with proper modeling of the shank. Airloads and structural loads correlation is fair. A significant underprediction of 2-per-revolution structural loads is observed.

Coupled Rotor/Fuselage Vibration Analysis for Teetering Rotor and Test Data Comparison

Acomprehensivevibrationanalysisofacoupledrotor/fuselagesystemforatwo-bladedteeteringrotorusinge nite elementmethodsinspaceandtimeisdevelopedthatincorporatesconsistentrotor/fuselagestructural,aerodynamic, and inertial couplings and a modern free-wake model. Coupled nonlinear periodic blade and fuselage equations are transformed to the modal space and solved simultaneously. The elastic line airframe model of the AH-1G helicopter is integrated into the elastic rotor e nite element model. Analytical predictions of rotor control angles, blade loads, hub forces, and vibration are compared with AH-1G operation load survey test data. The blade loads predicted by the present analysis show generally fair agreement with the e ight test data. Calculated 2 and 4 per revolution vertical vibration levels at the pilot seat show fair correlation with the e ight test, but the predicted 2 per revolution lateral vibration level is higher than the measurement, particularly at high advance ratios. Modeling of pylon e exibility is essential in the two-bladed teetering rotor vibration analysis. Ree ned aerodynamics such as free wake and unsteady aerodynamics have an important role in the prediction of vibration. Nomenclature eg = blade center-of-gravity offset from the elastic axis lu = undersling m = blade mass per unit length pfe = temporal fuselage elastic modal displacement vector pfr = temporal fuselage rigid modal displacement vector R = blade radius u;v;w = blade elastic displacements in the axial, lag, and e ap directions x = longitudinal coordinate P x f ; P yf ; P f = fuselage translational velocities