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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Optimization of helicopter rotor blade design for minimum vibration

The optimization approach discussed is part of an ongoing effort to develop a general automated procedure for rotor blade design. This procedure can be used to determine the necessary geometric, structural, and material properties of a rotor system to achieve desired objectives relating to vibration, stress, and aerodynamic performance. The approach used for helicopter vibration is emphasized. Based on analytical studies performed at the United Technologies Research Center (UTRC), a simplified vibration analysis was developed to be used in conjunction with a forced response analysis in the optimization process. This simplified analysis improves the efficiency of the design process significantly. Results of applying this approach to the design of an existing rotor blade model are presented.

Davis, M. W.↗
NASA NTRS1972

Examination of the collision force method for analyzing the responses of simple containment/deflection structures to impact by one engine rotor blade fragment

An approximate collision analysis, termed the collision-force method, was developed for studying impact-interaction of an engine rotor blade fragment with an initially circular containment ring. This collision analysis utilizes basic mass, material property, geometry, and pre-impact velocity information for the fragment, together with any one of three postulated patterns of blade deformation behavior: (1) the elastic straight blade model, (2) the elastic-plastic straight shortening blade model, and (3) the elastic-plastic curling blade model. The collision-induced forces are used to predict the resulting motions of both the blade fragment and the containment ring. Containment ring transient responses are predicted by a finite element computer code which accommodates the large deformation, elastic-plastic planar deformation behavior of simple structures such as beams and/or rings. The effects of varying the values of certain parameters in each blade-behavior model were studied. Comparisons of predictions with experimental data indicate that of the three postulated blade-behavior models, the elastic-plastic curling blade model appears to be the most plausible and satisfactory for predicting the impact-induced motions of a ductile engine rotor blade and a containment ring against which the blade impacts.

Zirin, R. M.↗
NASA NTRS2001

Wind Tunnel Evaluation of a Model Helicopter Main-Rotor Blade With Slotted Airfoils at the Tip

Data for rotors using unconventional airfoils are of interest to permit an evaluation of this technology's capability to meet the U.S. Army's need for increased helicopter mission effectiveness and improved safety and survivability. Thus, an experimental investigation was conducted in the Langley Transonic Dynamics Tunnel (TDT) to evaluate the effect of using slotted airfoils in the rotor blade tip region (85 to 100 percent radius) on rotor aerodynamic performance and loads. Four rotor configurations were tested in forward flight at advance ratios from 0.15 to 0.45 and in hover in-ground effect. The hover tip Mach number was 0.627, which is representative of a design point of 4000-ft geometric altitude and a temperature of 95 F. The baseline rotor configuration had a conventional single-element airfoil in the tip region. A second rotor configuration had a forward-slotted airfoil with a -6 deg slat, a third configuration had a forward-slotted airfoil with a -10 slat, and a fourth configuration had an aft-slotted airfoil with a 3 deg flap (trailing edge down). The results of this investigation indicate that the -6 deg slat configuration offers some performance and loads benefits over the other three configurations.

Noonan, Kevin W.↗
NASA NTRS1979

Computation of subsonic and transonic flow about lifting rotor blades

An inviscid, nonconservative, three-dimensional potential flow code has been developed for computing the quasi-steady flow about an isolated lifting rotor blade. Calculations from the code were compared with chordwise pressure measurements obtained in a wind tunnel on a nonlifting rotor at transonic tip speeds at advance ratios from 0.40 to 0.55. The overall agreement between theoretical calculations and experiment was good. To illustrate the early capability of the program, the flow about a hypothetical lifting rotor blade having twist, airfoil thickness taper, and a 20 deg sweptback tip was analyzed at azimuthal positions of 60, 90, and 120 deg for an advance ratio of 0.342. A typical run on a CDC 7600 computer required about 5 min for one rotor position at transonic tip speeds.

Arieli, R.↗
NASA NTRS1976

The nature of flow distortions caused by rotor blade wakes

The distortion caused by wakes of rotor blades is one of the least understood phenomena in turbomachinery. An attempt is made to predict the decay characteristics of the defect in axial and tangential velocity components, and the decay of the maximum radial velocity inside the rotor wake using the momentum integral analysis. The predictions agree well with the experimental data taken at the exit of an axial flow fan operating at zero incidence. The measurements are carried out with a three sensor hot wire probe. The wake width is found to grow linearly with distance downstream and the defect in tangential velocity inside the rotor wake decays fastest of the three components. An empirical expression is also provided for the axial velocity profile of a rotor wake.

Lakshminarayana, B.↗
NASA NTRS1972

Random gust response statistics for coupled torsion-flapping rotor blade vibrations.

An analysis of coupled torsion-flapping rotor blade vibrations in response to atmospheric turbulence revealed that at high rotor advance ratios anticipated for future high speed pure or convertible rotorcraft both flapping and torsional vibrations can be severe. While appropriate feedback systems can alleviate flapping, they have little effect on torsion. Dynamic stability margins have also no substantial influence on dynamic torsion loads. The only effective means found to alleviate turbulence caused torsional vibrations and loads at high advance ratio was a substantial torsional stiffness margin with respect to local static torsional divergence of the retreating blade.

Gaonkar, G. H.↗
NASA NTRS1979

Characteristics of lightly loaded fan rotor blade wakes

Low subsonic and incompressible wake flow downstream of lightly loaded rotor was studied. Measurements of mean velocity, turbulence intensity, Reynolds stress, and static variations across the rotor wake at various axial and radial locations were investigated. Wakes were measured at various rotor blade incidences to discern the effect of blade loading on the rotor wake. Mean velocity and turbulence measurements were carried out with a triaxial hot wire probe both rotating with the rotor and stationary behind the rotor. Results indicate that increased loading slows the decay rates of axial and tangential mean velocity defects and radial velocities in the wake. The presence of large radial velocities in the rotor wake indicate the extent of the interactions between one radius and another. Appreciable static pressure variations across the rotor wake were found in the near wake region. Similarity in the profile shape was found for the axial and tangential components of the mean velocity and in the outer layer for axial, tangential, and radial turbulence intensities.

Reynolds, B.↗
NASA NTRS1986

Flap-lag-torsional dynamics of helicopter rotor blades in forward flight

A perturbation/numerical methodology to analyze the flap-lead/lag motion of a centrally hinged spring restrained rotor blade that is valid for both hover and for forward flight was developed. The derivation of the nonlinear differential equations of motion and the analysis of the stability of the steady state response of the blade were conducted entirely in a Symbolics 3670 Machine using MACSYMA to perform all the lengthy symbolic manipulations. It also includes generation of the fortran codes and plots of the results. The Floquet theory was also applied to the differential equations of motion in order to compare results with those obtained from the perturbation analysis. The results obtained from the perturbation methodology and from Floquet theory were found to be very close to each other, which demonstrates the usefullness of the perturbation methodology. Another problem under study consisted in the analysis of the influence of higher order terms in the response and stability of a flexible rotor blade in forward flight using Computerized Symbolic Manipulation and a perturbation technique to bypass the Floquet theory. The derivation of the partial differential equations of motion is presented.

Crespodasilva, M. R. M.↗
NASA NTRSDate not supplied

Transition measurements of upper and lower rotor blade surfaces in forward flight with thermography

Thermography allows for imaging the location of laminar-to-turbulent boundary layer transition on surfaces in air flows. The determination of BL transition of rotor blades was done in 2017 on the lower surface of a rotor blade in forward flight in the NASA Langley 14- by 22-Foot Subsonic Tunnel Subsonic Wind Tunnel. This article reports on the follow-on experiment in the same facility and the same blades where thermography was performed on both upper and lower surface synchronously, with the upper surface mapped throughout the full azimuthal range. Blade-tracking rotating mirrors were used for each thermal camera and were synchronized for simultaneous image acquisition. Furthermore, the Differential Infrared Thermography (DIT) technique depicts the unsteady nature of the BL transition in forward flight. Results are presented and discussed. These data are the first ever recorded of both surfaces of a rotor in forward flight.

Boundary layer transition↗
NASA NTRS1977

Design, fabrication, and test of a composite material wind turbine rotor blade

The aerodynamic design, structural design, fabrication, and structural testing is described for a 60 foot long filament wound, fiberglass/epoxy resin matrix wind turbine rotor blade for a 125 foot diameter, 100 kW wind energy conversion system. One blade was fabricated which met all aerodynamic shape requirements and was structurally capable of operating under all specified design conditions. The feasibility of filament winding large rotor blades was demonstrated.

Griffee, D. G., Jr.↗
NASA NTRS1977

An analysis of the rotor blade stresses of the three stage compressor of the Ames Research Center 11- by 11-foot transonic wind tunnel

The static and dynamic rotor blade stresses of the three stage compressor were measured. Data are presented in terms of total blade stress for the complete operational range of compressor speeds and tunnel total pressures. Modal frequencies and variations with tunnel conditions were measured. Phase angles and coherences between various gage combinations are also presented. Recommendations for improvements are given for future rotor blade experimental investigations.

Dods, J. B., Jr.↗
NASA NTRS1974

Unsteady Subsonic and Transonic Potential Flow over Helicopter Rotor Blades

Differential equations and boundary conditions for a rotor blade in forward flight, with subsonic or transonic tip Mach number, are derived. A variety of limiting flow regimes determined by different limits involving blade thickness ratio, aspect ratio, advance ratio and maximum tip Mach number is discussed. The transonic problem is discussed in some detail, and in particular the conditions that make this problem quasi-steady or essentially unsteady are determined. Asymptotic forms of equations and boundary conditions that are valid in an appropriately scaled region of the tip and an azimuthal sector on the advancing side are derived. The equations are then put in a form that is valid from the blade tip inboard through the strip theory region.

Isom, M. P.↗
NASA NTRS1976

Nonlinear Curvature Expressions for Combined Flapwise Bending, Chordwise Bending, Torsion and Extension of Twisted Rotor Blades

The nonlinear curvature expressions for a twisted rotor blade or a beam undergoing transverse bending in two planes, torsion, and extension were developed. The curvature expressions were obtained using simple geometric considerations. The expressions were first developed in a general manner using the geometrical nonlinear theory of elasticity. These general nonlinear expressions were then systematically reduced to four levels of approximation by imposing various simplifying assumptions, and in each of these levels the second degree nonlinear expressions were given. The assumptions were carefully stated and their implications with respect to the nonlinear theory of elasticity as applied to beams were pointed out. The transformation matrices between the deformed and undeformed blade-fixed coordinates, which were needed in the development of the curvature expressions, were also given for three of the levels of approximation. The present curvature expressions and transformation matrices were compared with corresponding expressions existing in the literature.

Kvaternik, R. G.↗
NASA NTRS1987

Analysis, design and elastic tailoring of composite rotor blades

The development of structural models for composite rotor blades is summarized. The models are intended for use in design analysis for the purpose of exploring the potential of elastic tailoring. The research was performed at the Center for Rotary Wing Aircraft Technology.

Rehfield, Lawrence W.↗
NASA NTRS1989

Three-dimensional viscous drag prediction for rotor blades

The state-of-the-art in rotor blade drag prediction involves the use of two-dimensional airfoil tables to calculate the drag force on the blade. One of the most serious problems with the current methods is that they cannot be used for airfoils that have yet to be tested. Most of the drag prediction methods also do not take the Reynolds number or the rotational effects of the blade into account, raising doubts about the accuracy of the results. These problems are addressed with the development of an analytical method which includes the shape of airfoil, the effects of Reynolds number, and the rotational motion of the blade.

Chen, Ching S.↗
NASA NTRS1993

A new sensitivity analysis for structural optimization of composite rotor blades

This paper presents a detailed mathematical derivation of the sensitivity derivatives for the structural dynamic, aeroelastic stability and response characteristics of a rotor blade in hover and forward flight. The formulation is denoted by the term semianalytical approach, because certain derivatives have to be evaluated by a finite difference scheme. Using the present formulation, sensitivity derivatives for the structural dynamic and aeroelastic stability characteristics, were evaluated for both isotropic and composite rotor blades. Based on the results, useful conclusions are obtained regarding the relative merits of the semi-analytical approach, for calculating sensitivity derivatives, when compared to a pure finite difference approach.

Venkatesan, C.↗