Prediction, Analysis, and Validation of Main Rotor Blade Loads in a Prescribed Pull-Up Maneuver

Abstract

This paper predicts, analyzes, and validates main rotor airloads, structural loads, and swashplate servo loads in a prescribed high-g pull-up maneuver. A multibody finite element structural model is coupled to a transient lifting-line aerodynamic model. The structural model includes a swashplate model to calculate servo loads. The lifting-line model combines airfoil tables, Weissinger-L near wake, time marching free wake, and a semi-empirical dynamic stall model. The maneuver is the Army/NASA UH60A Airloads Program Flight Counter 11029. The primary objective of this paper is to isolate the eects of structural dynamics, free wake, dynamic stall, and pitch control angles, to determine the key loads mechanisms in this flight. The structural loads are first calculated using airloads measured in flight. The measured airloads are then replaced with a lifting-line coupled analysis ‐ that is ideally suited to isolate the eects of free wake and dynamic stall. It is concluded that the maneuver is almost entirely dominated by stall with little or no wake induced eect 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 mechanism of dynamic stall, in the analysis, consists of multiple cycles within a wide disk area. 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.