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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

Handling Qualities Considerations in Control Allocation for Multicopters

Due to multicopters’ role in advanced air mobility, there has been an increasing interest in handling qualities. This paper investigates the effect of control allocation of multicopters on handling qualities. Typically, multicopter aircraft are trimmed with equal speed on each rotor. The force generated by each rotor can be modeled, where thrust and torque of the rotor are related to the square of the rotor speed. The dynamic equations indicate that trimming pairs of motors at different RPM results in an increase of control effectiveness in the associated axis, improving bandwidth and disturbance rejection. A tradeoff occurs where power usage is no longer optimized, but handling qualities are improved in that control axis. A blade element model for multicopter, RMAC, was used to simulate how this off-nominal control allocation would affect the dynamics of a multicopter. System identification flight tests were used to validate the trends seen in RMAC. Control systems were optimized to provide improved disturbance rejection bandwidth in the axis associated with increased rotational speed. Unmanned mission task elements were performed in flight test, showing improved tracking in the lateral axis by using off-nominal mixing to increase trim speed on the rolling motors.

Handling Qualities↗
NASA NTRSDate not supplied

Medium-Fidelity CFD Modeling of Multicopter Wakes for Airborne Sensor Measurements

A steady-state multicopter simulation capability leveraging automated Cartesian grid generation and a blade element source term rotor model was used to investigate multi-rotor aerodynamics in and out of ground effect. Simulations demonstrated that this model is able to predict reasonably accurate thrust in ground effect for both single and multiple propeller cases. The method yielded accurate lift and drag predictions for a commercial quadcopter when compared to recent wind tunnel test data. This aircraft was then simulated in a variety of flight conditions, including both hover and edgewise forward flight, to determine if generalized guidelines for airborne sensor placement could be developed. Velocity perturbation magnitudes showed large regions of affected air upstream of the vehicle at low speeds, which contracted as the vehicle’s speed was increased. Placing the sensors more than one characteristic length ahead of or above the aircraft reduces errors from rotor-induced flow at higher speeds. For low-speed forward flight, the entrainment of flow into the propeller wakes introduces significant flow turning above the aircraft, suggesting that a forward location, with more moderate perturbations, could be advantageous for these conditions.

ARMD↗
NASA NTRSDate not supplied

Towards Low-Noise Design of a Proprotor with the Vortex Lattice Method and Gradient-Based Optimization

This work aims to combine an aerodynamic model based on the unsteady vortex lattice method with an acoustic model provided by Farassat's formulation 1A to perform gradient-based optimizations of a proprotor with aerodynamic and acoustic constraints. The resulting combination of tools is applied to the problem of designing a single proprotor operating at a cruise condition, with and without an acoustic constraint and wing placed downstream of the proprotor rotation plane. Results are compared to a baseline design studied previously, and to similar optimizations performed with a simpler blade element momentum theory aerodynamic model. Each optimization case achieved feasibility and made significant improvements in the objective function, but the optimality criterion was not satisfied. Overall the designs the optimizer found roughly comported with our previous experience with similar problems, with some discrepancies that are discussed.

Aerodynamics↗
NASA NTRSDate not supplied

Towards Low-Noise Design of a Proprotor with the Vortex Lattice Method and Gradient-Based Optimization

This work aims to combine an aerodynamic model based on the unsteady vortex lattice method with an acoustic model provided by Farassat's formulation 1A to perform gradient-based optimizations of a proprotor with aerodynamic and acoustic constraints. The resulting combination of tools is applied to the problem of designing a single proprotor operating at a cruise condition, with and without an acoustic constraint and wing placed downstream of the proprotor rotation plane. Results are compared to a baseline design studied previously, and to similar optimizations performed with a simpler blade element momentum theory aerodynamic model. Each optimization case achieved feasibility and made significant improvements in the objective function, but the optimality criterion was not satisfied. Overall the designs the optimizer found roughly comported with our previous experience with similar problems, with some discrepancies that are discussed.

Aerodynamics↗
NASA NTRSDate not supplied

Blade and Takeoff Trajectory Optimization of a Propeller-Driven Electric Aircraft with Acoustic Constraints

In this work, a multi-disciplinary toolchain is described and used to optimize the takeoff trajectory of a electrified general aviation aircraft, subjected to acoustic constraints. The Dymos multi-disciplinary optimal control library is used to optimize the trajectory, with propeller aerodynamic and acoustic models provided by blade element momentum theory (CCBlade.jl) and acoustic analogy (AcousticAnalogies.jl) codes, respectively. Each model is implemented in the OpenMDAO framework, with all derivatives calculated either analytically or via automatic differentiation tools. The toolchain is applied to a hypothetical electrified form of the Cirrus SR20 and compared to the conventional piston-driven form.

Aerodynamics↗
NASA NTRSDate not supplied

Blade and Takeoff Trajectory Optimization of a Propeller-Driven Electric Aircraft with Acoustic Constraints

In this work, a multi-disciplinary toolchain is described and used to optimize the takeoff trajectory of a electrified general aviation aircraft, subjected to acoustic constraints. The Dymos multi-disciplinary optimal control library is used to optimize the trajectory, with propeller aerodynamic and acoustic models provided by blade element momentum theory (CCBlade.jl) and acoustic analogy (AcousticAnalogies.jl) codes, respectively. Each model is implemented in the OpenMDAO framework, with all derivatives calculated either analytically or via automatic differentiation tools. The toolchain is applied to a hypothetical electrified form of the Cirrus SR20 and compared to the conventional piston-driven form.

gradient-based optimization↗
NASA NTRS1982

Aeroelastic Stability of Rotor Blades Using Finite Element Analysis

The flutter stability of flap bending, lead-lag bending, and torsion of helicopter rotor blades in hover is investigated using a finite element formulation based on Hamilton's principle. The blade is divided into a number of finite elements. Quasi-steady strip theory is used to evaluate the aerodynamic loads. The nonlinear equations of motion are solved for steady-state blade deflections through an iterative procedure. The equations of motion are linearized assuming blade motion to be a small perturbation about the steady deflected shape. The normal mode method based on the coupled rotating natural modes is used to reduce the number of equations in the flutter analysis. First the formulation is applied to single-load-path blades (articulated and hingeless blades). Numerical results show very good agreement with existing results obtained using the modal approach. The second part of the application concerns multiple-load-path blades, i.e. bearingless blades. Numerical results are presented for several analytical models of the bearingless blade. Results are also obtained using an equivalent beam approach wherein a bearingless blade is modelled as a single beam with equivalent properties. Results show the equivalent beam model.

Chopra, I.↗
NASA NTRS1981

Dynamic stability of a rotor blade using finite element analysis

The aeroelastic stability of flap bending, lead-lag bending, and torsion of a helicopter rotor blade in hover is examined using a finite element formulation based on the principle of virtual work. Quasi-steady two-dimensional airfoil theory is used to obtain the aerodynamic loads. The rotor blade is discretized into beam elements, each with ten modal degrees of freedom. The resulting nonlinear equations of motion are solved for steady-state blade deflections through an iterative procedure. The flutter solution is calculated assuming blade motion to be a small perturbation about the steady solution. The normal mode method based on the coupled rotating natural modes about the steady deflections is used to reduce the number of equations in the flutter eigenanalysis. Results are presented for hingeless and articulated rotor blade configurations.

Sivaneri, N. T.↗
NASA NTRS1987

Aeroelastic modeling of swept tip rotor blades using finite elements

A special finite element for the modeling of a swept tip rotor blade is derived. The swept tip blade undergoes moderate deflections in flap, lag and torsion. The nonlinear, partial differential equations of motion are discretized using a Galerkin finite element method. Tip sweep introduces flap-torsion and lag-axial couplings, and may lead to aeroelastic instabilities associated with frequency coalescence. When frequency coalescence does not occur, sweep is usually stabilizing. A comparison of the approximate model of a swept tip blade, using a blade with straight elastic axis and offsets of aerodynamic centers of gravity, with the exact swept tip model developed in this paper indicates that the approximate model can produce inaccurate results for the case of hingeless rotor blades.

Celi, R.↗
NASA NTRS1994

Advance finite element modeling of rotor blade aeroelasticity

An advanced beam finite element has been developed for modeling rotor blade dynamics and aeroelasticity. This element is part of the Element Library of the Second Generation Comprehensive Helicopter Analysis System (2GCHAS). The element allows modeling of arbitrary rotor systems, including bearingless rotors. It accounts for moderately large elastic deflections, anisotropic properties, large frame motion for maneuver simulation, and allows for variable order shape functions. The effects of gravity, mechanically applied and aerodynamic loads are included. All kinematic quantities required to compute airloads are provided. In this paper, the fundamental assumptions and derivation of the element matrices are presented. Numerical results are shown to verify the formulation and illustrate several features of the element.

Straub, F. K.↗
NASA NTRS1982

Finite element analysis for bearingless rotor blade aeroelasticity

A conventional articulated rotor blade has mechanical flap and lag hinges, a lag damper, and a pitch bearing. In connection with an interest in designs of greater mechanical simplicity and increased maintainability, hingeless and bearingless rotors have been developed. A hingeless blade lacks the hinges and is cantilevered at the hub. It does have a pitch bearing for pitch control. A bearingless design eliminates the hinges and the pitch bearing as well. In the present investigation of bearingless rotor blade characteristics, finite element analysis has been successfully applied to determine the solutions of the nonlinear trim equations and the linearized flutter equations of multiple-load-path blades. The employed formulation is based on Hamilton's principle. The spatial dependence of the equations of motion is discretized by dividing the flexbeams, the torque tube, and the outboard into a number of elements.

Sivaneri, N. T.↗
NASA NTRS1993

Finite state aeroelastic model for use in rotor design optimization

In this article, a rotor aeroelastic model based on a newly developed finite state dynamic wake, coupled with blade finite element analysis, is described. The analysis is intended for application in rotor blade design optimization. A coupled simultaneous system of differential equations combining blade structural dynamics and aerodynamics is established in a formulation well-suited for design sensitivity computation. Each blade is assumed to be an elastic beam undergoing flap bending, lead-lag bending, elastic twist, and axial deflections. Aerodynamic loads are computed from unsteady blade element theory where the rotor three-dimensional unsteady wake is described by a generalized dynamic wake model. Correlation of results obtained from the analysis with flight test data is provided to assess model accuracy.

He, Chengjian↗
NASA NTRS1987

Thermal finite-element analysis of space shuttle main engine turbine blade

Finite-element, transient heat transfer analyses were performed for the first-stage blades of the space shuttle main engine (SSME) high-pressure fuel turbopump. The analyses were based on test engine data provided by Rocketdyne. Heat transfer coefficients were predicted by performing a boundary-layer analysis at steady-state conditions with the STAN5 boundary-layer code. Two different peak-temperature overshoots were evaluated for the startup transient. Cutoff transient conditions were also analyzed. A reduced gas temperature profile based on actual thermocouple data was also considered. Transient heat transfer analyses were conducted with the MARC finite-element computer code.

Abdul-Aziz, Ali↗
NASA NTRS1987

Finite element analysis of flexible, rotating blades

A reference guide that can be used when using the finite element method to approximate the static and dynamic behavior of flexible, rotating blades is given. Important parameters such as twist, sweep, camber, co-planar shell elements, centrifugal loads, and inertia properties are studied. Comparisons are made between NASTRAN elements through published benchmark tests. The main purpose is to summarize blade modeling strategies and to document capabilities and limitations (for flexible, rotating blades) of various NASTRAN elements.

Mcgee, Oliver G.↗
NASA NTRS1983

A refined finite element for vibration analysis of twisted blades based on beam theory

A finite element method of discretizing beam segments of pretwisted rotating blades is presented. Employing the matrix displacement method, stiffness and mass properties are developed from basic mechanics of a pretwisted beam theory. By introducing the proper displacement functions, the effect of rotor blade rotational motion on the stiffness matrix is obtained systematically from the kinetic energy expression. Comparing with other beam elements the derivation of this element is more fundamental. This allows one to apply the same approach to more complicated problems including nonlinear effects or complex dynamic motions. Illustrative examples are given comparing numerical results with available data and other numerical solutions from rotating and nonrotating force fields. These examples show that accurate prediction of vibration frequencies for pretwisted blades can be obtained by employing a quite modest number of degrees of freedom.

Sisto, F.↗
NASA NTRSDate not supplied

On the Representation of Through-The-Thickness Reinforcements in Finite Element Analysis of Stitched, Blade Stiffened Panels

Modern aircraft employ laminated composites for their tailorable in-plane properties, high specific strengths, and high specific stiffnesses. However, laminated composites exhibit relatively poor interlaminar properties without through-the-thickness reinforcements. Quantifying the necessary amount of through-the-thickness reinforcements is necessary to reduce cost and meet damage tolerance certification requirements. In this study, a discrete superposed cohesive element (DSCE) approach is applied to represent the mixed-mode delamination behavior of stitched stiffened panels subjected to seven-point bending. This approach is compared to a one-dimensional embedded spring element (ESE) method. The DSCE approach uses two superposed bilinear traction-separation laws to obtain a representative load-displacement response determined from interlaminar tensile and shear tests. Additionally, several stitch configurations (unstitched, stitched, and overstitched) are evaluated in terms of their load-displacement response and crack-arrestment capability. Results indicate that the DSCE and ESE approaches show good agreement with respect to the predicted load-displacement response, but the ESE method tends to overpredict the crack growth behavior by approximately 13%. Stitches were not observed to fail during skin-stringer separation. Using an overstitched laminate with stitches near the flange edge provides the greatest crack-arrestment capability. Furthermore, the skin retains 92% of its stiffness after skin-stringer separation occurs.

composites↗
NASA NTRSDate not supplied

On the Representation of Through-The-Thickness Reinforcements in Finite Element Analysis of Stitched, Blade Stiffened Panels

Modern aircraft employ laminated composites for their tailorable in-plane properties, high specific strengths, and high specific stiffnesses. However, laminated composites exhibit relatively poor interlaminar properties without through-the-thickness reinforcements. Quantifying the necessary amount of through-the-thickness reinforcements is necessary to reduce cost and meet damage tolerance certification requirements. In this study, a discrete superposed cohesive element (DSCE) approach is applied to represent the mixed-mode delamination behavior of stitched stiffened panels subjected to seven-point bending. This approach is compared to a one-dimensional embedded spring element (ESE) method. The DSCE approach uses two superposed bilinear traction-separation laws to obtain a representative load-displacement response determined from interlaminar tensile and shear tests. Additionally, several stitch configurations (unstitched, stitched, and overstitched) are evaluated in terms of their load-displacement response and crack-arrestment capability. Results indicate that the DSCE and ESE approaches show good agreement with respect to the predicted load-displacement response, but the ESE method tends to overpredict the crack growth behavior by approximately 13%. Stitches were not observed to fail during skin-stringer separation. Using an overstitched laminate with stitches near the flange edge provides the greatest crack-arrestment capability. Furthermore, the skin retains 92% of its stiffness after skin-stringer separation occurs.

composites↗
NASA NTRS1991

The design/analysis of flows through turbomachinery - A viscous/inviscid approach

A new design/analysis system for the flows through turbomachinery is currently being developed for studying turbomachinery problems with an axisymmetric viscous/inviscid 'average-passage' throughflow code. The advantage of this approach, compared to streamline curvature codes, is that the solutions obtained simulate some of the unsteadiness, compressibility and viscous effects of a multistage turbomachine. The design/analysis system consists of three elemental parts, the axisymmetric block grid generator, the blade surface element code, and the axisymmetric flow code. Each element of the system will be discussed and the flow solutions for three axisymmetric geometries will be shown compared to experimental data where available. The computations are shown to be in very good agreement with test data for SR7 spinner body and transonic boattail geometry obtained in the wind tunnels at NASA Lewis Research Center. The VIADAC Rotor 67 Fan results were compared to PARC2D calculated results and shown to be in very good agreement.

Miller, D. P.↗