Session: 02-01-01: Dynamic Loads, Response, Vibration and Alleviation of Aerospace Structures - I

Paper Number: 110224

110224 - Vibration Reduction of a Lift-Offset Co-Axial Rotorcraft System Using Individual Blade Control Approach 

High level of vibration and large noise emission with poor aerodynamic characteristics are prime concerns in rotorcraft aeromechanics fields. The problems may become aggravated with higher flight speeds as the edgewise rotor experiences greater differences in incoming air velocities between the advancing and retreating sides of the rotor. Many innovative ideas have been devised to overcome the shortcomings and the speed barriers of the helicopter flights. One of the prominent concepts is to apply the lift-offset (LO) mechanism with which the lift generation capacity of the advancing side is maximized without interfering with the aerodynamic complexities in the retreating side. The roll moment equilibrium is satisfied with the lift generated by a pair of coaxial, counterrotating rotors that operate with equal and opposite roll moments. The XH-59A is the first aircraft that has demonstrated the LO concept successfully, achieving the maximum forward speed of 160 knots in helicopter mode and 236 knots in level cruise mode when installed with auxiliary propulsion. However, the test program was terminated due to the occurrence of severe vibrations and high fuel consumptions in flight.

To analyze the problem closely, precise modeling of the main rotor and the fuselage system taking into account their interactional behavior is crucial for reliable predictions of the rotorcraft vibration, especially at the cockpit location. The rotor-body coupled vibration analysis can be performed using two different approaches: one-way coupling and two-way coupling. The former considers the main rotor and the fuselage system separately: the isolated rotor computes the vibratory hub loads and then apply them to excite the fuselage (from the hub) for estimating the vibration at the desired stations of the fuselage. The process is unilateral (i.e., one way) with no interactions in between. On the other hand, in the latter method, they interact with each other, in which the vibratory loads produced by the rotor are transferred to the fuselage, which in turn changes the motion of the hub by the dynamic response of the fuselage so that the changed hub motions modify the hub loads. These interactions are repeated until the converged response is reached. Despite the complexities in the modeling and larger computational costs needed for the converged solutions, the latter one is deserved to apply for more accurate evaluation of rotor-body coupled rotorcraft vibration analysis.

The objectives of the present study are threefold: 1) assessing the accuracy of the rotor-body coupled vibration predictions obtained between the one-way and two-way coupled methods in reference to XH-59A flight test data in high-speed cruise condition (200 knots); 2) predicting the blade and hub vibratory loads of the rotor with varying LO’s, and the vibration (acceleration) at the pilot seat of the vehicle; and 3) reducing the vibration at the cockpit (pilot seat) through applying the individual blade control scheme. To facilitate the rotor-body coupled analysis, one-dimensional finite element stick model including the ground vibration test model is constructed based on the available information of XH-59A fuselage modes (e.g., airframe symmetric mode) found in the literature while exploiting the existing data set for conventional helicopters such as BO-105 and AH-1G. The present results obtained for the blade and hub vibratory loads, acceleration on the pilot seat, and forward flight performance characteristic showed excellent agreements with the test data of XH-59A in high-speed flight, especially with the two-way coupled approach. The final presentation covers a comprehensive correlation of the present predictions with the flight test data and other analysis results. In addition, significantly reduced vibration responses enabled due to the individual blade pitch control strategies will be demonstrated for more practical application of the lift-offset rotorcraft combined with the IBC control concept.

Presenting Author: Seong Hyun Hong Konkuk University

Presenting Author Biography: Graduate research assistant in Konkuk University.
Research interests: Rotor-body coupling analysis , Vibration reduction of helicopter rotor system, Comprehensive analysis of rotorcraft

Authors:

Seong Hyun Hong Konkuk University
Dong Kyun Kim Konkuk University
Sung Nam Jung Konkuk Unviersity