Wayne Johnson

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.

Navier-Stokes and Comprehensive Analysis Performance Predictions of the NREL Phase VI Experiment

A vortex lattice code, CAMRAD II, and a Reynolds- Averaged Navier-Stoke code, OVERFLOW-D2, were used to predict the aerodynamic performance of a two- bladed horizontal axis wind turbine. All computations were compared with experimental data that was collected at the NASA Ames Research Center 80- by 120-Foot Wind Tunnel. Computations were performed for both axial as well as yawed operating conditions. Various stall delay models and dynamics stall models were used by the CAMRAD II code. Comparisons between the experimental data and computed aerodynamic loads show that the OVERFLOW-D2 code can accurately predict the power and spanwise loading of a wind turbine rotor.

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).

Overview of the Helios Version 2.0 Computational Platform for Rotorcraft Simulations

This article summarizes the capabilities and development of the Helios version 2.0, or Shasta, software for rotary wing simulations. Specific capabilities enabled by Shasta include off-body adaptive mesh refinement and the ability to handle multiple interacting rotorcraft components such as the fuselage, rotors, flaps and stores. In addition, a new run-mode to handle maneuvering flight has been added. Fundamental changes of the Helios interfaces have been introduced to streamline the integration of these capabilities. Various modifications have also been carried out in the underlying modules for near-body solution, off-body solution, domain connectivity, rotor fluid structure interface and comprehensive analysis to accommodate these interfaces and to enhance operational robustness and efficiency. Results are presented to demonstrate the mesh adaptation features of the software for the NACA0015 wing, TRAM rotor in hover and the UH-60A in forward flight.

Assessment of aerodynamic and dynamic models in a comprehensive analysis for rotorcraft

The history, status, and lessons of a comprehensive analysis for rotorcraft are reviewed. The development, features, and capabilities of the analysis are summarized, including the aerodynamic and dynamic models that were used. Examples of correlation of the computational results with experimental data are given, extensions of the analysis for research in several topics of helicopter technology are discussed, and the experiences of outside users are summarized. Finally, the required capabilities and approach for the next comprehensive analysis are described.

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.

Performance Analysis of the Slowed-Rotor Compound Helicopter Configuration

Abstract : The calculated performance of a slowed-rotor compound aircraft, particularly at high flight speeds, is examined. Correlation of calculated and measured performance is presented for a NASA Langley high advance ratio test and the McDonnell XV-1 demonstrator to establish the capability to model rotors in such flight conditions. The predicted performance of a slowed-rotor vehicle model based on the CarterCopter Technology Demonstrator is examined in detail. An isolated rotor model and a model of a rotor and wing are considered. Three tip speeds and a range of collective pitch settings are investigated. A tip Mach number of 0.2 and zero collective pitch are found to be the optimum condition to minimize rotor drag. Performance is examined for both sea level and cruise altitude conditions.