Conceptualization, Modeling, and Characterization of a Cf Driven Multi-State Lead-lag Bypass Damper

Open Access
Author:
Marr, Conor Matthew
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 23, 2012
Committee Members:
  • Edward Smith, Committee Chair
  • George A Lesieutre, Dissertation Advisor
  • Cengiz Camci, Committee Member
  • Christopher Rahn, Special Member
Keywords:
  • Damper
  • Fluidlastic
  • Hydraulic
  • CFD
  • Modeling
  • Multi-State
  • Bypass
  • Passive Damping
  • Lead-Lag Damper
  • Ground Resonance
Abstract:
Conventional lead-lag dampers are necessary to prevent ground and air resonance in most helicopter designs. Though high damping is needed to eliminate these instabilities only over a small region of the helicopter’s operating conditions, the conventional damper continues to produce high damping in all conditions, leading to high root end loads and reducing component life. A simple passive or semi-passive multi-state bypass damper, with predictable behavior validated by experimental data, could help to solve the problem of high root loads associated with the use of conventional lead-lag dampers. To meet this need, a passive multi-state lead-lag damper was designed to reduce damper forces by approximately 50% when high damping is not required. This variable damping was achieved via a set of bypass channels that are opened or closed via the changing centrifugal force associated with the rotor speed. A first generation prototype was fabricated with an internal sliding spring/mass system to open or close the bypass channels with varying rotor speed. A semi-empirical analytical model was created to predict the damper behavior. To get a more accurate prediction of the damper behavior, a detailed computational fluid dynamics (CFD) model was developed using the commercial program FLUENT®. This model was made to capture the dynamic, time varying flow through the piston orifices, around the piston head, and through the bypass channels. The prototype damper was bench tested with the bypass channels open and closed to validate the multi-state behavior of the design over a range of frequencies and dynamic displacements. The damper was then tested in the rotating environment on a spinning test stand to demonstrate the centrifugally activated multi-state behavior. The spin tests proved that the bypass channels could be successfully opened via increasing centrifugal force. At low rotor speeds, corresponding to the regions in which ground resonance could occur, the damper produced high damping. As the rotor speed increased beyond 140 RPM, damper forces decreased, providing lower damping as the rotor speed approached nominal RPM. Comparison with the experimental bench top test data revealed the semi-empirical analytical model to be very inaccurate, with average errors in peak force prediction around 60%. Conversely, the CFD model accurately predicts damper forces in the closed configuration with an average error in peak force prediction of approximately 7% and an average error in loss stiffness prediction of less than 4.4%. The CFD model over predicts damper forces in the open configuration, with an average peak force error approaching 100%. Prediction errors of the loss stiffness for the open case are close to 150%. The final stage of the research involved utilizing the knowledge gained from the other models and experimental data to create a CFD augmented analytical model. This model combined the accuracy of the CFD model, while retaining the low computational requirements of the semi-empirical analytical model, making this new model ideal for incorporation into rotorcraft prediction codes. Comparison of the revised analytical model with the CFD model and experimental data show errors in peak force prediction under 13% for most cases and errors in the loss stiffness of fewer than 13% for most cases. Prediction of the open bypass channel configuration is less accurate, with an average error in peak force of 24% and an average error in loss stiffness of 37%. All of these predictions show a marked improvement over the pure semi-empirical analytical model. The centrifugal force driven bypass multi-state lead-lag damper concept was proven effective through a series of experimental tests and modeling efforts. The first generation prototype demonstrated high damping at low rotor speeds followed by reductions in damping up to 50% once the rotor speed reached 250 RPM. The effectiveness of the CFD augmented analytical model was also demonstrated. Expansion of the CFD correction factors to cover a broader range of flow feature sizes and geometries could be investigated to improve the model.