A computational fluid dynamics (CFD) model is coupled with a computational structural dynamics (CSD) model to improve prediction of helicopter rotor vibratory loads in high-speed flight. The two key problems of articulated rotor aeromechanics in high-speed flight-advancing blade lift phase, and underprediction of pitch link load-are satisfactorily resolved for the UH-60A rotor. The physics of aerodynamics and structural dynamics is first isolated from the coupled aeroelastic problem. The structural and aerodynamic models are validated separately using the UH-60A Airloads Program data. The key improvement provided by CFD over a lifting-line aerodynamic model is explained. The fundamental mechanisms behind rotor vibration at high speed are identified as: 1) large elastic twist deformations and 2) inboard wake interaction. The large twist deformations are driven by transonic pitching moments at the outboard stations. CFD captures 3-dimensional unsteady pitching moments at the outboard stations accurately. CFD/CSD coupling improves elastic twist deformations via accurate pitching moments and captures the vibratory lift harmonics correctly. At the outboard stations (86.5% radius out), the vibratory lift is dominated by elastic twist. At the inboard stations (67.5% and 77.5% radius), a refined wake model is necessary in addition to accurate twist. The peak-to-peak pitch link load and lower harmonic waveform are accurately captured. Discrepancies for higher harmonic torsion loads remain unresolved even with measured airloads. The predicted flap-bending moments show a phase shift of about 10 deg over the entire rotor azimuth. This error stems from 1, 2, and 3/rev lift. The 1/rev lift is unaffected by CFD/CSD coupling. The 2 and 3/rev lift are significantly improved but do not fully resolve the 2 and 3/rev bending moment error. IntroductionT HE objective of this paper is to improve the prediction of rotor vibratory loads by replacing the lifting-line aerodynamic model of a comprehensive rotor analysis with computational fluid dynamics (CFD). The focus is on high-speed level flight of the UH-60A Blackhawk (155 kn, μ = 0.368). The state of the art in helicopter vibration prediction in high-speed flight is far from satisfactory 1 even though both vibratory airloads and structural response show consistent patterns for a large number of helicopters. 2,3 Prediction accuracy of vibratory blade loads is less than 50%. Measurements from the UH-60A Air Loads program 4 open the opportunity to trace back the sources of prediction deficiencies to discrepancies in airload calculation.Bousman in 1999 (Ref. 5) identified two key discrepancies in articulated rotor aeromechanics: 1) prediction of negative lift phase on the advancing side in high-speed flight and 2) underprediction of pitch link load (by 50%). The error in pitch link load stems from errors in pitching moment predictions. Figure 1 shows state-of-the-art lift and pitching moment predictions from lifting-line comprehensive analyses CAMRAD/JA and 2GCHAS (Ref. 6). The drop in th...

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This paper reviews the state of the art in helicopter rotor loads prediction with the emergence of computational fluid dynamics (CFD) and computational structural dynamics (CSD) coupling. The focus is on steady level flight, where most of the current CFD/CSD analyses are being applied. The application of CFD to rotorcraft problems has evolved, over the period 1990-2005, as a viable means to improve the aerodynamic modeling used in rotorcraft comprehensive analyses (CA). It has the potential to meet the ultimate objective of a coupled rotor-fuselage analysis that can predict loads and vibration accurately at all critical flight conditions without semi-empirical inputs. The paper begins by identifying three critical level flight conditions. These are useful to isolate the key aerodynamic and structural dynamic mechanisms. It is followed by a review of the capabilities and limitations of current CFD analyses in representing the key aerodynamic mechanisms: threedimensional airloads and wake, dynamic stall, unsteady transonic effects near the tip, and fuselage aerodynamic effects. The structural dynamic methods are then briefly reviewed. Finally, the recent rotorcraft CFD/CSD coupling methods are described and evaluated on the basis of their loads prediction capability. The emphasis is on the fundamental aeroelastic mechanisms which determine the critical rotor loads, and with which the accuracy and efficiency of all CFD/CSD coupled analyses should be assessed. Nomenclature

The design of a Martian autonomous rotary wing vehicle (MARV) is described. MARV is a 50-kg gross takeoff mass, coaxial helicopter designed for Mars exploration. Powered by a fuel cell system, it carries a payload of 10.8 kg over a range of 25 km with an endurance of 39 min including hover capability for 1 min. MARV is designed in response to the Request For Proposal from NASA/Sikorsky for the Year 2000 American Helicopter Society student design competition. The design covers aerodynamic and structural design of rotor blades, vehicle power plant, fuselage and landing gear, control system, transmission, and vehicle lander communications. A detailed mechanism for autonomous deployment of the vehicle from the lander is also described. This preliminary design study indicates that controlled vertical ight on Mars is feasible with existing technology.

This paper describes and analyzes the measurements from a full-scale, slowed revolutions per minute (rpm), UH-60A rotor tested at the National Full-Scale Aerodynamics Complex 40-by 80-ft wind tunnel up to an advance ratio of 1.0. A comprehensive set of measurements that includes performance, blade loads, hub loads, and pressures/airloads makes this data set unique. The measurements reveal new and rich aeromechanical phenomena that are unique to this exotic regime. These include reverse chord dynamic stall, retreating side impulse in torsion load, large inboard-outboard elastic twist differential, diminishing rotor forces and yet a dramatic buildup of blade loads, and high blade loads and yet benign levels of vibratory hub loads. The objective of this research is the fundamental understanding of these unique aeromechanical phenomena. The intent is to provide useful knowledge for the design of high-speed, high-efficiency, slowed rpm rotors of the future and a database for validation of advanced analyses. Nomenclature