Peretz P. Friedmann

Vibration reduction in rotorcraft using active control - A comparison of various approaches

This paper presents a concise review of the state of the art for vibration reduction in rotorcraft using active controls. The principal approaches to vibration reduction in helicopters described in the paper are 1) higher harmonic control, 2) individual blade control, 3) vibration reduction using an actively controlled flap located on the blade, and 4) active control of structural response. The special attributes of the coupled rotor/flexible fuselage vibration reduction problem are also briefly discussed to emphasize that vibration reduction at the hub is not equivalent to acceleration reduction at specific fuselage locations. Based on the comparison of the various approaches, it appears that the actively controlled flap has remarkable potential for vibration reduction.

Renaissance of Aeroelasticity and Its Future

The primary objective of this paper is to demonstrate that the field of aeroelasticity continues to play a critical role in the design of modern aerospace vehicles, and several important problems are still far from being well understood. Furthermore, the emergence of new technologies, such as the use of adaptive materials (sometimes denoted as smart structures technology), providing new actuator and sensor capabilities, has invigorated aeroelasticity, and generated a host of new and challenging research topics that can have a major impact on the design of a new generation of aerospace vehicles.

Aeroelastic and Aerothermoelastic Analysis in Hypersonic Flow: Past, Present, and Future

H YPERSONIC flight began in February 1949 when a WAC Corporal rocket was ignited from a U.S.-captured V-2 rocket [1]. In the six decades since this milestone, there have been significant investments in the development of hypersonic vehicle technologies. The NASA X-15 rocket plane in the early 1960s represents early research toward this goal [2,3]. After a lull in activity, the modern era of hypersonic research started in the mid-1980s with the National Aerospace Plane (NASP) program [4], aimed at developing a single-stage-to-orbit reusable launch vehicle (RLV) that used conventional runways. However, it was canceled due mainly to design requirements that exceeded the state of the art [1,5]. A more recent RLV project, the VentureStar program, failed during structural tests, again for lack of the required technology [5]. Despite these unsuccessful programs, the continued need for a low-cost RLV, as well as the desire of the U.S. Air Force (USAF) for unmanned hypersonic vehicles, has reinvigorated hypersonic flight research. An emergence of recent and current research programs [6] demonstrate this renewed interest. Consider, for example, the NASA Hyper-X experimental vehicle program [7], the University of Queensland HyShot program [8], the NASA Fundamental Aeronautics Hypersonics Project [9], the joint U.S. Defense Advanced Research Projects Administration (DARPA)/USAF Force Application andLaunch fromContinentalUnited States (FALCON) program [10], the X-51 Single Engine Demonstrator [11,12], the joint USAF Research Laboratory (AFRL)/Australian Defence Science and Technology Organisation Hypersonic International Flight Research Experimentation project [13], and ongoing basic hypersonic research at the AFRL (e.g., [14–20]). The conditions encountered in hypersonic flows, combined with the need to design hypersonic vehicles, have motivated research in the areas of hypersonic aeroelasticity and aerothermoelasticity. It is evident from Fig. 1 that hypersonic vehicle configurations will consist of long, slender lifting body designs. In general, the body, surface panels, and aerodynamic control surfaces are flexible due to minimum-weight restrictions. Furthermore, as shown in Fig. 2, these

Helicopter vibration reduction using structural optimization with aeroelastic/multidisciplinary constraints - A survey

This paper presents a survey of the state-of-the-art in the field of structural optimization when applied to vibration reduction of helicopters in forward flight with aeroelastic and multidisciplinary constraints. It emphasizes the application of the modern approach where the optimization is formulated as a mathematical programming problem, the objective function consists of the vibration levels at the hub, and behavior constraints are imposed on the blade frequencies and aeroelastic stability margins, as well as on a number of additional ingredients that can have a significant effect on the overall performance and flight mechanics of the helicopter. It is shown that the integrated multidisciplinary optimization of rotorcraft offers the potential for substantial improvements, which can be achieved by careful preliminary design and analysis without requiring additional hardware such as rotor vibration absorbers or isolation systems.

Reduced-Order Nonlinear Unsteady Aerodynamic Modeling Using a Surrogate-Based Recurrence Framework

A reduced-order nonlinear unsteady aerodynamic modeling approach suitable for analyzing pitching/plunging airfoils subject to fixed or time-varying freestream Mach numbers is described. The reduced-order model uses kriging surrogates to account for flow nonlinearities and recurrence solutions to account for time-history effects associated with unsteadiness. The resulting surrogate-based recurrence framework generates time-domain predictionsofunsteadylift,moment,anddragthataccuratelyapproximate computational fluiddynamicssolutions, but at a fraction of the computational cost. Results corresponding to transonic conditions demonstrate that the surrogate-based recurrence framework can mimic computational fluid dynamics predictions of unsteady aerodynamic responses when flow nonlinearities are present. For an unsteady aerodynamic modeling problem considered in this study, an accurate reduced-order model was generated by the surrogate-based recurrence framework approach with significantly fewer computational fluid dynamics evaluations compared to results reported in the literature for a similar problem in which a proper-orthogonal-decomposition-based approach was applied. Furthermore, the results show that the surrogate-based approach can accurately model time-varying freestream Mach number effects and is therefore applicable to rotary-wing applications in addition to fixed-wing applications.

Aeroelastic and Aerothermoelastic Behavior in Hypersonic Flow

The testing of aeroelastically and aerothermoelastically scaled wind-tunnel models in hypersonic flow is not feasible; thus, computational aeroelasticity and aerothermoelasticity are essential to the development of hypersonic vehicles. Several fundamental issues in this area are examined by performing a systematic computational study of the hypersonic aeroelastic and aerothermoelastic behavior of a three-dimensional configuration. Specifically, the flutter boundary of a low-aspect-ratio wing, representative of a fin or control surface on a hypersonic vehicle, is studied over a range of altitudes using third-order piston theory and Euler and Navier-Stokes aerodynamics. The sensitivity of the computational-fluid-dynamics-based aeroelastic analysis to grid resolution and parameters governing temporal accuracy are considered. In general, good agreement at moderate-to-high altitudes was observed for the three aerodynamic models. However, the wing flutters at unrealistic Mach numbers in the absence of aerodynamic heating. Therefore, because aerodynamic heating is an inherent feature of hypersonic flight and the aeroelastic behavior of a vehicle is sensitive to structural variations caused by heating, an aerothermoelastic methodology is developed that incorporates the heat transfer between the fluid and structure based on computational-fluid-dynamics-generated aerodynamic heating. The aerothermoelastic solution procedure is then applied to the low-aspect-ratio wing operating on a representative hypersonic trajectory. In the latter study, the sensitivity of the flutter margin to perturbations in trajectory angle of attack and Mach number is considered. Significant reductions in the flutter boundary of the heated wing are observed. The wing is also found to be susceptible to thermal buckling.

Higher-Harmonic-Control Algorithm for Helicopter Vibration Reduction Revisited

The higher-harmonic-control (HHC) algorithm is examined from a control theory perspective. A brief review of the history and variants of HHC is given, followed by a careful development of the algorithm. An analytic convergence and robustness analysis is then performed. Online identification with the adaptive variant of the algorithm is also addressed. A new version of the algorithm, relaxed HHC, is introduced and shown to have beneficial robustness properties. Some numerical results comparing these variants of the HHC algorithm applied to helicopter vibration reduction are also presented. The results presented unify and extend previous work on the higher-harmonic-control algorithm.

Approximate Modeling of Unsteady Aerodynamics for Hypersonic Aeroelasticity

DOI: 10.2514/1.C000190 Various approximations to unsteady aerodynamics are examined for the aeroelastic analysis of a thin doublewedge airfoil in hypersonic flow. Flutter boundaries are obtained using classical hypersonic unsteady aerodynamic theories: piston theory, Van Dyke’s second-order theory, Newtonian impact theory, and unsteady shock-expansion theory. The theories are evaluated by comparing the flutter boundaries with those predicted using computational fluid dynamics solutions to the unsteady Navier–Stokes equations. Inaddition, several alternative approaches to the classical approximations are also evaluated: two different viscous approximations based on effective shapes and combined approximate computational approaches that use steady-state computational-fluid-dynamics-based surrogatemodelsinconjunction withpistontheory.Theresultsindicatethat,with theexceptionof first-order piston theory and Newtonian impact theory, the approximate theories yield predictions between 3 and 17% of normalized root-mean-square error and between 7 and 40% of normalized maximum error of the unsteady Navier–Stokes predictions. Furthermore, the demonstrated accuracy of the combined steady-state computational fluid dynamics and piston theory approaches suggest that important nonlinearities in hypersonic flow are primarily due to steadystate effects. This implies that steady-state flow analysis may be an alternative to time-accurate Navier–Stokes solutions for capturing complex flow effects.

OPTIMUM DESIGN OF ROTOR BLADES FOR VIBRATION REDUCTION IN FORWARD FLIGHT.

Modern structural optimization techniques are applied to vibration reduction of helicopter rotor blades in forward flight. The objective function minimized consists of the oscillatory vertical hub shears or the hub rolling moments at one particular advance ratio. The behavior constraints are the frequency placements of the blade and the requirement that aeroelastic stability margins, in hover, remain unaffected by the optimization process. The aeroelastic stability and response analysis is based on a fully coupled flap-lag-torsional analysis of the blade. Numerical results are presented for some typical soft-in-plane hingeless rotor configurations indicating a 15-40 percent reduction in vibration levels, as well as a blade which is 20 percent lighter than the initial design. These results imply that structural optimization techniques can yield substantial practical benefits in the design process of rotor systems.

Vibration analysis of composite turbopropellers using a nonlinear beam-type finite-element approach

An analytical model for determining the free vibration characteristics of advanced composite turbopropellers (prop-fans) is presented. The blade is modeled using a number of straight beam-type finite elements, where the elastic axis of each element is a piecewise straight representation of the curved line of shear centers of the swept blade. The finite-element model is obtained from Hamiltons principle with allowances for: generally anisotropic material behavior, arbitrary cross-sectional properties, large pretwist angles, out-of-plane cross-section warping, and geometrically nonlinear behavior based upon moderate deflection theory. The natural frequencies and mode shapes of the rotating blade are calculated assuming linear perturbations about the nonlinear static equilibrium position of the blade. This model is sufficiently general to analyze other advanced composite aerospace structures. Numerical results are presented to illustrate the versatility of the method by applying it to 1) a conventional propeller (TRW-Hartzell 101/16) and 2) a highly swept and pretwisted isotropic turbopropeller (NASA SR-3). Excellent agreement with experimental test results is obtained for the lower modes of both the conventional propeller and the advanced turbopropeller.