Showing papers in "Journal of The American Helicopter Society in 2005"

Generalized Timoshenko Theory of the Variational Asymptotic Beam Sectional Analysis

Abstract: The generalized Timoshenko theory for composite beams embedded in the computer program VABS has the same structure as Timoshenko’s original theory for isotropic beams without the restrictive assumptions of the original theory. An overview of this theory is presented to show its general and rigorous framework. Certain theoretical details missing from previous developments are supplied, such as the proof of a kinematical identity and the expression of the recovery theory in terms of sectional stress resultants. It has been demonstrated that the VABS generalized Timoshenko theory reproduces the elasticity solution for the flexure problem of isotropic prisms. Numerical results are presented in support of the long-term validation effort, focusing especially on calculation of sectional stiffnesses (including shear correction factors) and shear center location, making use of the VABS model for composite beam analysis (including buckling and vibration), and recovering three-dimensional field variables over the section. The accuracy of the VABS generalized Timoshenko theory is demonstrated, and some of its practical advantages over three-dimensional finite element analysis are exhibited. Presented at the 44th Structures, Structural Dynamics and Materials Conference, Norfolk, Virginia, April 7 – 10, 2003 Assistant Professor, Department of Mechanical and Aerospace Engineering, Utah State University, Logan, Utah 84322-4130. Formerly, Post Doctoral Fellow, School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, Georgia. Email: wenbin.yu@usu.edu. Professor, School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, Georgia, 303320150. Email: dewey.hodges@ae.gatech.edu. Fellow, AHS.

CFD Simulations of Tiltrotor Configurations in Hover

Abstract: Navier-Stokes computational fluid dynamics calculations are presented for isolated, half-span, and full-span V-22 tiltrotor hover configurations. These computational results extend the validity of CFD hover methodology beyond conventional rotorcraft applications to tiltrotor configurations. Computed steady-state, isolated rotor performance agrees well with experimental measurements, showing little sensitivity to grid resolution. However, blade-vortex interaction flowfield details are sensitive to numerical dissipation and are more difficult to model accurately. Time-dependent, dynamic, half- and full-span installed configurations show sensitivities in performance to the tiltrotor fountain flow. As such, the full-span configuration exhibits higher rotor performance and lower airframe download than the half-span configuration. Half-span rotor installation trends match available half-span data, and airframe downloads are reasonably well predicted. Overall, the CFD solutions provide a wealth of flowfield details that can be used to analyze and improve tiltrotor aerodynamic performance.

Wind tunnel testing of a helicopter fuselage and rotor in a ship airwake

Abstract: The demanding task of landing a maritime helicopter on a ship at sea is constrained by an operational envelope that places limits on wind direction and speed. An operational envelope is normally developed by first-of-class flight trials, but flight testing is an expensive means of qualifying a helicopter for shipborne operations. This paper describes the development of an experimentally-based simulation method which is intended to complement flight testing and mitigate its cost. The basis of the methodology lies with establishing correlations of unsteady aerodynamic fuselage loads (measured in a wind tunnel) with pilot workload (obtained by flight test), assessed for cases where airwake turbulence is chiefly responsible for the workload. With these correlations, contours of unsteady fuselage loading can assist with the definition of the operational envelope. An experiment was conducted to measure the unsteady aerodynamic fuselage loads in a wind tunnel. In this investigation the fuselage of a Sea King helicopter was immersed in both the downwash of a spinning main rotor and the airwake of a Canadian Patrol Frigate. Measurements of unsteady side force, yawing moment and drag force were made over a combination of wind directions, speeds, and hover positions. The results indicate that a spinning main rotor generating appropriate levels of thrust is a necessary feature of the wind-tunnel simulation. Specifically, in comparison to the rotorless case unsteady loading at low hover over the flight deck was found to increase to the levels of unsteadiness that exist at high hover, and the variation of unsteady loading with wind speed is changed by the interaction of the ship airwake and rotor downwash. Generally the unsteady loading increases with the additional influence of main rotor downwash compared to the baseline, rotorless case. The wind tunnel data, particularly side Assistant Research Officer Senior Research Officer Presented at the 29th European Rotorcraft Forum, Friedrichshafen, Germany, September 16-18, 2003. Copyright c 2003 by the National Research Council Canada. force and drag, are then shown to correlate well with flight-test derived operational limits. The correlation of unsteady yawing moment with the operational limit is less straight forward.

Optimization of Critical Trajectories for Rotorcraft Vehicles

Abstract: This paper focuses on the development of computational procedures for the trajectory management of maneuver- ing rotorcraft vehicles. The ight mechanics models are based on classic blade element theory, and are applicable to both helicopters and tilt-rotor aircrafts. The computation of an optimal maneuver is viewed here as an optimal control problem, where the solution minimizes an appropriate cost function subjected to constraints that translate the ight envelope limita- tions of the aircraft and all the necessary safety and operational requirements. A nite element based transcription process is used for discretizing the problem, obtaining a nite dimensional parameter optimization problem. Ad hoc procedures are proposed for ensuring yable and realistic computed control time histories, that are compatible with the hidden (unmod- elled) actuator dynamics. The methodology is used for developing and studying various models of the take-off of helicopters and tilt-rotors in the one-engine failure case under Category-A certication requirements.

Water Impact Test and Simulation of a Composite Energy Absorbing Fuselage Section

Abstract: In March 2002, a 25-ft/s vertical drop test of a composite fuselage section was conducted onto water. The purpose of the test was to obtain experimental data characterizing the structural response of the fuselage section during water impact for comparison with two previous drop tests that were performed onto a rigid surface and soft soil. For the drop test, the fuselage section was configured with ten 100-lb. lead masses, five per side, that were attached to seat rails mounted to the floor. The fuselage section was raised to a height of 10-ft. and dropped vertically into a 15-ft. diameter pool filled to a depth of 3.5-ft. with water. Approximately 70 channels of data were collected during the drop test at a 10-kHz sampling rate. The test data were used to validate crash simulations of the water impact that were developed using the nonlinear, explicit transient dynamic codes, MSC.Dytran and LS-DYNA. The fuselage structure was modeled using shell and solid elements with a Lagrangian mesh, and the water was modeled with both Eulerian and Lagrangian techniques. The fluid-structure interactions were executed using the fast general coupling in MSC.Dytran and the Arbitrary Lagrange-Euler (ALE) coupling in LS-DYNA. Additionally, the smooth particle hydrodynamics (SPH) meshless Lagrangian technique was used in LS-DYNA to represent the fluid. The simulation results were correlated with the test data to validate the modeling approach. Additional simulation studies were performed to determine how changes in mesh density, mesh uniformity, fluid viscosity, and failure strain influence the test-analysis correlation.

Flow Visualizations and Extended Thrust Time Histories of Rotor Vortex Wakes in Descent

Abstract: : An experimental study is performed on a three-bladed rotor model in two water tanks. The blade pitch, rotational velocity, descent angle, and descent speed are all varied in order to simulate a wide range of rotorcraft operating states, focusing on descent cases where the rotor is operating in or near vortex ring state--an area in which there is currently very little available data. Flow visualization is done by injecting air bubbles and fluorescent dye tangentially from the blade tips to mark the vortex core, showing the development of both short-wave ("sinuous") and long-wave ("leapfrogging") instabilities on the helical vortices in the wake. Strain gages are used to record transient loads, allowing a correlation between the rotor thrust performance and the development of the vortex wake. Reynolds numbers are of order 105 and test runs are performed for extended periods--up to 500 rotor revolutions-- demonstrating the repeatability of the patterns of thrust variation. The data indicate that as the instabilities develop, adjacent vortices merge and form thick vortex rings, particularly during descent. Periodic shedding of these rings from the wake associated with vortex ring state is observed, resulting in peak-to-peak thrust fluctuations of up to 95% of the mean and occurring at regular intervals of 20-50 rotor revolutions, depending on flow parameters.

Modeling Multi-Layer Matrix Cracking in Thin Walled Composite Rotor Blades

Abstract: Helicopter rotor blades are made of fiber-reinforced composite materials that are prone to matrix cracking. Matrix cracking precedes more serious damage mechanisms such as delamination and fiber breakage and is therefore a useful indicator of structural health. In the present study, the effect of matrix cracking on composite blade stiffness and deflections is investigated. A stiff inplane rotor blade with a rectangular box and two-cell airfoil section with [0/ +/-45/90], family of laminates is considered. It is observed that the stiffness decreases rapidly in initial phase of matrix cracking and then becomes saturated. Study of the behavior of composite rotor blade from matrix cracking in single, two and complete lamina group show a bending stiffness loss of 6-12 percent and a torsion stiffness loss of 25-30 percent at the point where matrix cracking saturates, and more severe forms of damage such as debonding/delamination and fiber breakage begin.

State-space representation of vortex wakes using the method of lines

Abstract: The method of lines (MOL) is applied to the equations of helicopter rotor vortex wakes, and converts the governing partial differential equations into a system of ordinary differential equations (ODE). These ODE can then be coupled to other ODE modeling helicopter dynamics, for time-marching simulations or to extract linearized models. The MOL is applied to a simplified set of wake equations that has an analytical solution. Because these simplified equations neglect key wake physics, the study is only a first step toward applying MOL to realistic models. Therefore, the conclusions only apply to the simplified problem considered. The results show that the MOL is a suitable method to formulate vortex wake models in state-space form. The solutions are accurate and numerically stable. Refining the space discretization increases the stiffness of the ODE, but explicit solvers can still be used. Computational efficiency increases when the accuracies of space and time discretizations are matched, that is, the local error tolerance set for the time solver is matched with the order of accuracy of the space discretization. Formulas of several orders were used in the space discretization. In all cases, the explicit ODE solver used (DE/STEP) was much more computationally efficient than another, implicit ODE solver (DASSL). Linearized state-space wake models can be easily obtained. The MOL could also provide a systematic methodology to extract state-space models from computational fluid dynamics formulations, and therefore to increase the accuracy of helicopter simulation models.

Effects of V-22 Blade Modifications on Whirl Flutter and Loads

Abstract: A CAMRAD II model of the V-22 Osprey tiltrotor was constructed for the purpose of analyzing the effects of blade design changes on whirl flutter. The model incorporated a dual load-path grip/yoke assembly, a swashplate coupled to the transmission case, and a drive train. A multiple-trailer free wake was used for loads calculations. The effects of rotor design changes on whirl-mode stability were calculated for swept blades and offset tip masses. A rotor with swept tips and inboard tuning masses was examined in detail to reveal the mechanisms by which these design changes affect stability and loads. Certain combinations of design features greatly increased whirl-mode stability, with (at worst) moderate increases to loads.

A subjective test of modulated blade spacing for helicopter main rotors

Abstract: Analytically, uneven (modulated) spacing of main rotor blades was found to reduce helicopter noise. A study was performed to see if these reductions transferred to improvements in subjective response. Using a predictive computer code, sounds produced by six main rotor configurations: 4 blades evenly spaced, 5 blades evenly spaced and four configurations with 5 blades with modulated spacing of varying amounts, were predicted. These predictions were converted to audible sounds corresponding to the level flyover, takeoff and approach flight conditions. Subjects who heard the simulations were asked to assess the overflight sounds in terms of noisiness on a scale of 0 to 10. In general the evenly spaced configurations were found less noisy than the modulated spacings, possibly because the uneven spacings produced a perceptible pulsating sound due to the very low fundamental frequency.