Dynamic analysis of bearingless main rotor (BMR) system with hub flexbeam damage

Abstract

Dynamic analyses of a bearingless main rotor helicopter system with damaged flexbeam configurations were conducted using a comprehensive helicopter aeroelastic code based on finite elementlblade element theory. The bearingless main rotor (BMR) system, including flexbeams, torque tubes, and main rotor blades, was modeled as a number of elastic beam finite elements, wherein each beam element undergoes flap bending, lag bending, elastic twist, and axial deflections. Aerodynamic forces on rotor blades are calculated using quasi-steady aerodynamic theory with a linear inflow model. Flexbeam ballistic damage was simulated by changes in the span-wise distribution of the mass, bending and torsional stiffness of a flexbeam element. Results are first calculated for a soft in-plane five-bladed bearingless rotor helicopter with a undamaged (baseline) configuration. Results then are calculated for this helicopter with ballistically damaged configurations. The effects of this damage on rotor and helicopter system performances are assessed in terms of blade modal shapes and frequencies, rotor system aeroelastic response and load variations. Ballistic damage to the hub flexbeam can affect the dynamic behavior of the bearingless rotor system. I N T R O D U C T I O N The bearingless rotor is rapidly gaining acceptance as a design configuration for the next genera*Presented at the 36th AIAA/ASME/AHS/ASCE/ASC Structures, Structural Dynamics and Materials (SDM) Conference, New Orleans, LA, April 1995. r~erospace Engineer, Air Systems Branch, Member AIAA. This paper is declared a work of the U S . Government and is not subject to copyright protection in the United States. tion helicopter; it offers design simplicity (fewer parts), weight reduction, better maintenance, and more control power and maneuverability. A bearingless rotor is a special case of a hingeless rotor, in which the pitch bearing as well as the flap and lag hinges are eliminated (See Figure 1). Pitch control from the pitch link to the main blade is transmitted via a torsionally stiff torque tube. This in turn twists a torsionally soft flexbeam, which functions effectively as a pitch bearing. The flexbeam is, in fact, a major component of the bearingless rotor design; it carries the centrifugal load, and allows for the blade flap, lead-lag, and twist motions. There is, however, a lack of vulnerability information for flight-critical flexbeam element attributable to ballistic damage mechanisms. In addition, the effects of flexbeam ballistic damage on the rotor and helicopter system's performance are not adequately understood. The objective of the present research is to analytically investigate rotor and aircraft performance effects caused by flexbeam damage. The present investigation was performed using the University Maryland Advanced Rotorcraft Code (UMARC) [I]. The bearingless main rotor (BMR) system, including flexbeams, torque tubes, and main rotor blades, was modeled as a number of elastic beam finite elements, wherein each beam element undergoes flap bending, lag bending, elastic twist, and axial deflections. Flexbeam ballistic damage was simulated as changes in the span-wise distribution of the mass, bending and torsional stiffness of flexbeam element. Results are first calculated for an advanced fivebladed BMR helicopter with a baseline (undamaged) configuration. Results then are calculated for this helicopter with representative levels of ballistic damage to the hub flexbeam. The effects of this damage on this helicopter's performance are determined in terms of blade modal shapes and frequencies, rotor system dynamic response and loads. In the context of the U S . Army Research Laboratory's (ARL) process structure for analyzing combat system vulnerability, this study and its associated engineering-based methods address the mapping from Level 2, the Target Component Damage State, to Level 3, the Target Capability State (i.e., 0 2 , ~ mapping); other levels include 1, Threat/Target Initial Conditions and 4, Target Combat Utility. See Reference 2 for more details. A significant feature of this process is that at each level (or space), distinct, measurable information is available defining the threat-target encounter and vulnerability/lethality outcome. Here, for example, physical and structural factors defining hub flexbeam damage (Level 2) are mapped via engineering methods into parameters that define the rotor and helicopter system's functional capability (Level 3); all the defining terms are explicit and measurable through experimentation. Application of these and other engineering analysis tools to the vulnerability/lethality process structure will occur largely through implementation in the degraded states vulnerability methodology for level 02,3 mapping now being developed for aircraft targets at ARL.