SEMI-ACTIVE CONTROL OF HELICOPTER VIBRATION USING CONTROLLABLE STIFFNESS AND DAMPING DEVICES

Open Access
Author:
Anusonti-Inthra, Phuriwat
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 11, 2002
Committee Members:
  • Edward C Smith, Committee Member
  • George A Lesieutre, Committee Member
  • Farhan Gandhi, Committee Chair
  • Christopher Rahn, Committee Member
Keywords:
  • controllable damper
  • controllable stiffness
  • semi-active control
  • helicopter vibration reduction
Abstract:
Semi-active concepts for helicopter vibration reduction are developed and evaluated in this dissertation. Semi-active devices, controllable stiffness devices or controllable orifice dampers, are introduced; (i) in the blade root region (rotor-based concept) and (ii) between the rotor and the fuselage as semi-active isolators (in the non-rotating frame). Corresponding semi-active controllers for helicopter vibration reduction are also developed. The effectiveness of the rotor-based semi-active vibration reduction concept (using stiffness and damping variation) is demonstrated for a 4-bladed hingeless rotor helicopter in moderate- to high-speed forward flight. The rotor blade is modeled as an elastic beam, which undergoes elastic flap-bending, lag-bending, and torsional deformations, and is discretized using finite element analysis. Aerodynamic loads on the blade are determined using blade element theory, with rotor inflow calculated using linear inflow or free wake analysis. The stiffness variation is introduced by modulating the stiffness of the root element or a discrete controllable stiffness device that connects the rotor hub and the blade. The damping variation is achieved by adjusting the damping coefficient of a controllable orifice damper, introduced in the blade root region. Optimal multi-cyclic stiffness/damping variation inputs that can produce simultaneous reduction in all components of hub vibrations are determined through an optimal semi-active control scheme using gradient and non-gradient based optimizations. A sensitivity study shows that the stiffness variation of root element can reduce hub vibrations when proper amplitude and phase are used. Furthermore, the optimal semi-active control scheme can determine the combination of stiffness variations that produce significant vibration reduction in all components of vibratory hub loads simultaneously. It is demonstrated that desired cyclic variations in properties of the blade root region can be practically achieved using discrete controllable stiffness devices and controllable dampers, especially in the flap and lag directions. These discrete controllable devices can produce 35-50% reduction in a composite vibration index representing all components of vibratory hub loads. No detrimental increases are observed in the lower harmonics of blade loads and blade response (which contribute to the dynamic stresses) and controllable device internal loads, when the optimal stiffness and damping variations are introduced. The effectiveness of optimal stiffness and damping variations in reducing hub vibration is retained over a range of cruise speeds and for variations in fundamental rotor properties. The effectiveness of the semi-active isolator is demonstrated for a simplified single degree of freedom system representing the semi-active isolation system. The rotor, represented by a lumped mass under harmonic force excitation, is supported by a spring and a parallel damper on the fuselage (assumed to have infinite mass). Properties of the spring or damper can then be controlled to reduce transmission of the force into the fuselage or the support structure. This semi-active isolation concept can produce additional 30% vibration reduction beyond the level achieved by a passive isolator. Different control schemes (i.e. open-loop, closed-loop, and closed-loop adaptive schemes) are developed and evaluated to control transmission of vibratory loads to the support structure (fuselage), and it is seen that a closed-loop adaptive controller is required to retain vibration reduction effectiveness when there is a change in operating condition.