Fluidic Flexible Matrix Composite Vibration Treatments for Helicopter Airframes and Rotor Blades

Open Access
Krott, Matthew Joseph
Graduate Program:
Mechanical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
August 31, 2018
Committee Members:
  • Christopher Rahn, Dissertation Advisor
  • Edward Smith, Committee Chair
  • Bo Cheng, Committee Member
  • Jose Palacios, Committee Member
  • Charles E Bakis, Outside Member
  • Christopher Rahn, Committee Chair
  • Edward C. Smith, Dissertation Advisor
  • F2MC
  • fluidic flexible matrix composite
  • helicopter
  • rotorcraft
  • structural dynamics
  • vibration absorber
  • damper
  • rotor blade
  • vibration control
Vibrations caused by periodic and unsteady loading in rotorcraft must be minimized to maintain acceptable fatigue life in structural parts and ride quality for passengers and crew. Rotorcraft vibrations are typically addressed through some combination of passive and active solutions that focus on reducing steady-state vibrations at the n/rev frequency, where n is the number of rotor blades. Currently existing passive treatments are often heavy, bulky variations of the classical tuned vibration absorber. Active treatments either attempt to isolate the cabin from hub vibratory loads or reduce cabin vibrations using a set of actuators, but they are more difficult to implement because they require a power supply and controller. This research covers the modeling, design, and experimental verification of fluidic flexible matrix composite (F2MC) vibration treatments for two main rotorcraft applications: airframe vibration control and rotor blade damping. The main advantages to using F2MC tubes over conventional hydraulic devices with pistons are their high strain-induced pumping capability and high force output per unit pressure. A laboratory-scale rotorcraft tailboom was used as a testbed for demonstrating new F2MC vibration absorber concepts. The tailboom is modeled using Euler-Bernoulli beam finite elements and coupled to a model of the F2MC tubes and fluidic circuit. Based on the combined structural and fluid system model, an F2MC damped vibration absorber was designed and built using four F2MC tubes placed near the corners of the rectangular tailboom. Experimental results showed reduction of both lateral and torsional vibrations in a 26.7 Hz coupled tailboom vibration mode by up to 80%. Three fluidic circuits were tested for performance and model verification. This single-mode F2MC vibration absorber was then modified so that two tailboom vibration modes can be treated by the same device. A lateral absorber frequency was tuned by selecting lengths of short branch segments connecting the left and right F2MC tube pairs. Then, a vertical absorber frequency was tuned by selecting the appropriate length of tubing to connect the top and bottom F2MC tube pairs. The tuned multi-mode vibration absorber reduced vibration by 63% in the vertical mode and 65% in the lateral mode, whereas a comparable absorber designed to only treat the vertical mode reduced vibration by 68% in the vertical mode but only 42% in the lateral mode. The weight penalty from modifying the circuit to treat both modes was only 2% of the original absorber weight. New F2MC devices are proposed to augment the damping of both articulated and hingeless rotor blades. The proposed device for articulated blades dissipates energy by using an F2MC tube to pump fluid through an orifice. In contrast, the proposed device for hingeless blades uses an F2MC tube as part of a damped vibration absorber with a tuned inertia track. Models are derived for both conceptsto assess the feasibility of these dampers for representative articulated, stiff-inplane, and soft-inplane rotor blades. Parametric studies are conducted to understand how fluidic circuit design variables impact damper performance. For the articulated blade damper, increasing orifice resistance increases the damping ratio of the blade-damper system at the cost of increased F2MC tube oscillatory pressures. Increasing the accumulator capacitance reduces the F2MC damper stiffness and also increases the achievable damping, although the benefits diminish as the accumulator becomes larger. A stiff-inplane hingeless blade is modeled with beam finite elements, and F2MC damped absorbers are tuned for the first chordwise bending blade mode. Eigenvalue analysis predicts an increase in the first chordwise blade mode damping ratio from a baseline of 0.02 to a range of 0.059-0.066 with the F2MC damped absorber. Using a large accumulator in the absorber fluidic circuit improves the absorber effectiveness and reduces the inertance required for tuning the fluidic circuit to a specific frequency. Benchtop tests were conducted on a 9.7-foot diameter rotor integrated with a prototype articulated blade F2MC damper. Springs were attached to the blade to simulate centrifugal stiffness, and both frequency-domain and time-domain data were collected to assess damper performance. Model predictions of blade displacement and F2MC tube pressure were verified by experiments. Blade damping ratios in frequency-domain benchtop tests increase from a range of 0.054-0.064 with the orifice fully closed to a range of 0.300-0.335 with the orifice tuned to maximize damping. In time-domain benchtop tests, measured damping ratios increase from 0.062-0.090 with the orifice fully closed to a range of 0.298-0.404 with the orifice tuned. The benchtop tests and model verification are a key first step in developing functional F2MC damper technology for rotor blade applications.