This study predicts, analyzes, and isolates the mechanisms of main rotor airloads, structural loads, and swashplate servo loads in a severe unsteady maneuver. The objective is, to develop a comprehensive transient rotor analysis for predicting maneuver loads. The main rotor structural loads encountered during unsteady maneuvers are important to size different critical components of the rotor system, particularly for advanced combat helicopters. These include the blade structural loads, control/pitch-link loads, and swashplate servo loads. Accurate and consistent prediction of maneuver loads is necessary to reduce the risks and costs associated with use of prior flight test data as a basis for design. The mechanism of rotor loads in different level flight regimes is well understood – transonic shock in high speed flight, inter-twinning of blade tip vortices below the rotor disk at low speed transonic flight, and two dynamic stall cycles on retreating blade during high altitude dynamic stall flight. All these physical phenomena can occur simultaneously during a maneuver.
The goal is to understand the key mechanisms involved in maneuver and model them accurately. To achieve this, the aerodynamics and structural dynamics of UH-60A rotor in unsteady maneuvering flight is studied separately. For identification of prediction deficiencies in each, first, the measured lift, drag, pitching moment and damper force from the UH-60A Flight Test Program for UTTAS pull-up maneuver (C11029: 2.16g pull-up maneuver) are used to obtain an accurate set of deformations. A multibody finite element blade model, developed for this purpose, is used to perform measured airloads analysis. Next, the resultant blade deformations are used to predict the airloads using lifting-line and RANS CFD aerodynamic models. Both lifting-line as well as CFD analyses predict all three stall cycles with prescribed deformations. From the airloads predicted using prescribed deformations, it is established that the advancing blade transonic stall, observed from revolution 12 onwards, is a twist stall triggered by 5/rev elastic twist deformation resulting in shock induced flow separation. The 5/rev elastic twist is triggered by the two retreating blade stalls from previous revolution, which are separated by 1/5th rev. The accurate prediction of both stall cycles on retreating blade holds the key to prediction of advancing blade stall. In analysis, advancing blade stall is triggered by a correct combination of control angles and 5/rev elastic twist. Some discrepancies are observed in higher harmonics of predicted torsion moment, which are not resolved by using measured airloads.
The structural model and the aerodynamic models are coupled together to predict blade loads for the maneuver. The structural model is refined to include a three degrees of freedom swashplate model to calculate servo loads and to study the effect of swashplate dynamics on rotor loads. Lifting-line coupled analysis, though of low fidelity, is ideally suited to isolate the effects of free wake and dynamic stall. It is concluded that the UTTAS maneuver is almost entirely dominated by stall with little or no wake induced effect on blade loads, even though the wake cuts through the disk twice during the maneuver. At the peak of the maneuver, almost 75% of the operating envelope of a typical airfoil lies beyond stall. The peak-to-peak structural loads prediction from the lifting-line analysis show an under-prediction of 10%–20% in flap and chord bending moments and 50% in torsion loads. The errors stem from the prediction of 4 and 5/rev stall loads. Swashplate dynamics appears to have a significant impact on the servo loads - unlike in level flight – with more than 50% variation in peak loads.
The coupled analysis using CFD/CSD tight coupling shows considerable improvements in the predicted results by using a CFD model over a traditional liftingline approach. In particular, the coupled CFD/CSD simulation is able to correctly predict the magnitude and phasing of the two dynamic stall cycles on the retreating side of the rotor disk during the maneuver. Further it shows significant improvement in the predicted peak-to-peak structural loads. The advancing blade stall is not predicted by either of the analyses. CFD/CSD analysis is not able to predict the advancing blade stall due to less satisfactory prediction of retreating blade dynamics stall cycles which are sensitive to the grid refinements and turbulence modeling.