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

Abstract

This paper predicts and analyzes 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 with a transient lifting-line aerodynamic model. The structural model includes a swashplate model to calculate servo loads. The lifting-line model combines airfoil tables, a Weissinger-L near-wake time-marching free wake, and a semiempirical dynamic stall model. The maneuver data were taken from the Army/NASA UH-60A Airloads Program Flight Counter 11029. The primary objective of this paper is to isolate the effects of structural dynamics, free wake, dynamic stall, and pitch control angles in order 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, which is ideally suited to isolate the effects of free wake and dynamic stall. It is concluded that the 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 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 shows an underprediction of 10-20% in flap and chord bending moments and 50% in torsion loads. The errors stem from the prediction of fourand five-revolution stall loads. Swashplate dynamics appear to have a significant impact on the servo loads (unlike in level flight), with a more than 50% variation in peak loads.

A Novel Inverse Simulation Method of Helicopter Maneuvering Flight

“…To validate our novel inverse simulation method, a case study of a helicopter flying a dynamic pull-up maneuver (in the longitudinal/vertical channel) was conducted. This maneuver was based on the Utility Tactical Transport Aerial System (UTTAS) maneuver of the original UH−60A design specification [30]. It was an instantaneous down-to-up turning maneuver in the vertical plane, and was designated by the Counter 11029 flight from the UH−60A flight test database, which is a terrain-avoidance maneuver in the longitudinal/vertical channel, and is initiated from a high-speed steady flight by pitchingup the helicopter for a rapid gain in altitude [31].…”

Air and Structural Loads Analysis of a 5-Ton Class Rotorcraft in a Pull-Up Maneuver Using CFD/CSD Coupled Approach

“…In developing new helicopters, detailed load analyses around the extreme corner points of the flight envelope are needed to ensure the safe operation of the vehicle. For helicopters, the most critical loadings and severe vibration levels are generated during the transient flights such as the high-g pull-up maneuver [2][3][4].…”

Prediction and Fundamental Understanding of Stall Loads in UH-60A Pull-Up Maneuver

“…5,6), and wake-capturing methodologies (Refs. 7,8) to analyze this maneuver. The lifting line analyses in general show unsatisfactory correlation with measured flight test data, especially in the magnitude of pitching moment drop-off during dynamic stall events, and the blade structural loads.…”