STABILITY ANALYSIS AND EXPERIMENTAL TESTING OF FLUIDIC PITCH LINKS IN HELICOPTERS WITH ARTICULATED ROTORS
Open Access
- Author:
- Treacy, Shawn Michael
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 27, 2017
- Committee Members:
- Christopher Rahn, Dissertation Advisor/Co-Advisor
Christopher Rahn, Committee Chair/Co-Chair
Edward Smith, Committee Member
Reuben H Kraft, Committee Member
Joseph Paul Cusumano, Outside Member - Keywords:
- fluidic pitch link
FPL
stability
articulated
helicopter
experiment
test
rotor - Abstract:
- Vibration levels in helicopters impact many aspects of flight including performance, comfort, and reliability. Three types of vibration control exist to dissipate these effects: passive control, semi-active control, and active control. Historically, rigid pitch links have been used on rotorcraft, however, researchers have been exploring alternatives to conventional rigid pitch links as a means to reduce vibrations. One viable passive vibration reduction device is the fluidic pitch link. Replacing rigid pitch links on rotorcraft with fluidic pitch links provides a method for vibratory control. At high frequencies, the pitch link impedance can be tuned to change the blade pitching response to higher harmonic loads. Although all have not been demonstrated simultaneously, fluidic pitch links have been shown to reduce rotor power and all six hub forces and moments. While reduced vibrations have been observed upon substitution of rigid pitch links for fluidic pitch links, the impact of fluidic pitch links on stability has not yet been examined. As such, this investigation explores the aeroelastic stability of a helicopter with fluidic pitch links. Each rotor is articulated and is modeled by rigid pitch and rigid flap degrees of freedom. Quasi-steady aerodynamics are used for the lift and moment terms in the aeroelastic model. The control system stiffness is modeled as an axial spring. The fluidic pitch links have two degrees of freedom: axial displacement of the piston, which is directly related to pitch, and volume of fluid entering the inertia track. A positive impact on aeroelastic stability from several fluidic pitch link designs is demonstrated for hover. The positive stability margins found for the fluidic pitch links in hover are marginally affected by the periodic terms that appear in forward flight. The fluidic pitch links are shown to help stabilize the pitch mode and enable use of substantially larger aft center of gravity offsets in rotor design. To test the fluidic pitch link for model verification purposes, the model for a double pumper fluidic pitch link was derived. This is a more viable and popular choice than the single pumper design used previously due to greater safety and reduced risk of cavitation. A prototype fluidic pitch link was designed, built, and tested at LORD Corporation. Displacement, load, and pressure were recorded during testing. Frequency and time response results were compared between simulation and experiment to validate the model. Three different fluid circuits were used, and the model accurately predicted performance for each of them with the exception of inaccuracy at low frequency due, in part, to the frequency dependence of the elastomer. An additional fourth circuit was tested that included a needle valve. The model did not accurately predict results across the entire range of valve positions, but the model was able to accurately match the dynamic stiffness amplitude using empirical parameters from a parameter study.