CFD/CSD Prediction of Rotor Vibratory Loads in High-Speed Flight

Abstract

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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