Laplace-domain approximation to the transfer functions of a rotor blade in forward flight

A continuous frequency domain method with roots on the classic Hill's determinant analysis is presented to approximate the time-varying characteristics of a linear periodic system. The method is particularly useful to derive a time-invariant equivalent form of the time-varying aeroelastic problem of a rotor blade in forward flight. The proposed technique allows methodology usually employed in fixed wing aircraft to obtain closed-loop control laws be extended to rotary wings. The method is first validated solving Mathieu's equation. Next, the two-degree-of-freedom (flap bending and torsion) problem of rotating beam subject to unsteady and incompressible aerodynamics in forward flight is solved in the laplace domain. As a demonstration of the proposed method, the transfer functions in the 's' plane between a sudden and uniformly distributed input pressure perturbation applied along the beam and the output response of the two elastic degrees of freedom considered are obtained at a set of local sections

Modeling an adaptive impedance boundary condition device for helicopter individual blade control

In the present investigation, the helicopter blade is modeled as a rotating beam with two degrees of freedom, namely the elastic flatwise bending and torsion. A mathematical model in the frequency domain is developed, incorporating the unsteady aerodynamic loads associated with helicopters in forward flight. The effects of dynamic adaptation of the rot boundary conditions on the beam aeroelastic response are studied. The results suggest that it is possible to control the local dynamic response at particular sections of the beam by varying the frequency and relative phase of the control signal

Individual blade control with the smart spring - A closed-loop independent harmonic control approach

In steady-state forward flight the cyclic variation of the aerodynamic loads acting on the blade generates forces and moments that are predominantly transmitted to the fuselage at the Nb/rev harmonic of the rotor frequency, where Nb is the number of rotor blades. The Smart Spring is a semi-active device that allows actively modulating the blade pitch link axial stiffness throughout the indirect action of a piezoelectric actuator. It performs dynamic parametric excitation of the rotor system and introduces Individual Blade Control with the objective of reducing these harmonic cyclic loads transmitted to the fuselage. Previous experimental studies demonstrated that the transmissibility reduction of some harmonics could be greater than 90% for a given combination of the Smart Spring parameters. In this paper, the capability of the Smart Spring to act in the closed-loop control configuration is analytically explored for the first time. The control action results in the realization of an independent harmonic control of the rotor blade system, an improvement in the current state of the art of the technology. The fundamentals for the Smart Spring closed-loop independent harmonic control concept are discussed

Active and passive control of the aeroelastic response of helicopter rotors using smart materials to tailor the blade root flexibility

An individual blade controller designed to attenuate the aeroelastic response of helicopter rotors in forward flight by tailoring the blade root attachment conditions is developed. A feasibility analysis indicates that the open-loop controller which incorporates both passive and active design techniques is energy efficient and may be realized by available adaptive structures

Whirl-flutter investigation on an advanced turboprop configuration

The whirl-flutter problem of an advanced turboprop configuration with two pusher propellers positioned at the aircraft fuselage cone is analyzed. Coupling between the two propellers and the flexible backup structure— pylons and aft fuselage cone—is allowed. Very interesting results lead to the conclusion that, for typical stiffness ratios between the backup structure and the engine suspension system, a special type of flutter involving mainly the backup structure may be dominant over traditional propeller-nacelle whirl flutter. This type of flutter is solely due to the propeller whirl and may be critical either in some modern configurations of aircrafts (as propfans) or in new conceptions of power plants installations employing additional vibration insulators at the pylon-fuselage attachments

Aeroelastic analysis of a helicopter rotor blade with active impedance control at the root

In the present investigation, the helicopter blade is modelled as a rotating beam with two degrees of freedom, namely the elastic flatwise bending and torsion. A mathematical model in the frequency domain is developed, incorporating the incompressible unsteady aerodynamic loads associated with helicopters in forward flight. The effects of dynamic adaptation of the root boundary conditions on the beam aeroelastic response are studied. The results suggest that it is possible to control the local dynamic response at particular sections of the blade by varying the frequency and relative phase of the control signal

A comparative study on different techniques to control rotary wing vibration using smart structures

This investigation discusses some proposed techniques to attenuate rotary wing vibration using smart materials. Advantages and disadvantages of these solutions are analysed in greater detail. Numerical results from feasibility studies are summarised to present the accomplishments already achieved and the goals to be pursued in the near future to develop the next generation of the so-called 'smart rotors'

Nonlinear dynamic response of an accelerating composite rotor blade using perturbations

The general nonlinear intrinsic differential equations of a composite beam are solved in order to obtain the elastodynamic response of an accelerating rotating hingeless composite beam. The solution utilizes the results of the linear variational asymptotic method applied to cross-sectional analysis. The integration algorithm implements the finite difference method in order to solve the transient form of the nonlinear intrinsic differential equations. The motion is analyzed since the beam starts rotating from rest, until it reaches the steady state condition. It is shown that the transient solution of the nonlinear dynamic formulation of the accelerating rotating beam converges to the steady state solution obtained by an alternative integration algorithm based on the shooting method. The effects of imposing perturbations on the steady state solution have also been analyzed and the results are shown to be compatible with those of the accelerating beam. Finally, the response of a nonlinear composite beam with embedded anisotropic piezocomposite actuators is illustrated. The effect of activating actuators at various directions on the steady state forces and moments generated in a rotating beam has been analyzed. These results can be used in controlling the nonlinear elastodynamic response of adaptive rotating beams

Review of active rotor control research in Canada

The current status of Canadian research on rotor-based actively controlled technologies for helicopters is reviewed in this paper. First, worldwide research in this field is overviewed to put Canadian research into context. Then, the unique hybrid control concept of Carleton University is described, along with its key element, the "stiffness control" concept. Next, the smart hybrid active rotor control system (SHARCS) project's history and organization is presented, which aims to demonstrate the hybrid control concept in a wind tunnel test campaign. To support the activities of SHARCS, unique computational tools, novel experimental facilities and new know-how had to be developed in Canada, among them the state-of-the-art Carleton Whirl Tower facility or the ability to design and manufacture aeroelastically scaled helicopter rotors for wind tunnel testing. In the second half of the paper, details are provided on the current status of development on the three subsystems of SHARCS, i.e. that of the actively controlled tip, the actively controlled flap and the unique stiffness-control device, the active pitch link

Comparison of the variational asymptotic beam sectional analysis methods applied to composite beams

In the analysis of isotropic and prismatic beams, the usual approach is to assume that the stress field is uni-axial (Bernoulli hypothesis). Such an assumption is not valid in the case of composite beams and if applied, results in large errors. The analysis becomes even further complex if the beam performs large deflections (even though at small strain) and the equations become geometrically nonlinear. One successful solution method in such cases is the so-called Variational-Asymptotic Method (VAM). This method starts from the elastic energy functional and has the common advantage of asymptotic methods of being mathematically well-grounded with no ad hoc assumptions. This method splits the three dimensional (3-D) geometrically nonlinear elasticity analysis for beam problems into a nonlinear 1-D analysis along the beam that utilizes the results of a linear 2-D analysis used to determine the cross-sectional stiffness matrices. The linear 2-D cross-sectional analysis is performed by the Variational Asymptotic Beam Sectional Analysis program (VABS). At present, there are two versions of VABS: the Georgia Tech version (GT/VABS release 2.1), released and maintained by Professors Yu and Hodges, and the UM/VABS release 1.30, released and maintained by Professor Cesnik at the University of Michigan. Both of these codes are widely used. The main aim of this paper is to provide a comparative study of the capabilities and the results obtainable by using the two existing VABS programs. The programs are used to solve a series of problems including solid section, open, and multi-cell thin-walled structures made of isotropic or composite materials and in search of classic, Timoshenko and Vlasov stiffness matrices (as well as mass matrices). The results obtained for a few cases reveal that these two programs provide about identical mass matrices and identical diagonal elements of stiffness matrices. However, the off-diagonal (coupling) elements in the stiffness matrices are not, in general, close enough to each other. Therefore, one may conclude that even though the outcomes are about identical in the mass matrix calculation, these two VABS programs do not, in general, provide close enough stiffness matrices