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NASA NTRS1981

An extension of the local momentum theory to a distorted wake model of a hovering rotor

The local momentum theory is based on the instantaneous balance between the fluid momentum and the blade elemental lift at a local station in the rotor rotational plane. Therefore, the theory has the capability of evaluating time wise variations of air loading and induced velocity distributions along a helicopter blade span. Unlike a complex vortex theory, this theory was developed to analyze the instantaneous induced velocity distribution effectively. The boundaries of this theory and a computer program using this theory are discussed. A concept introduced into the theory is the effect of the rotor wake contraction in hovering flight. A comparison of this extended local momentum theory with a prescribed wake vortex theory is also presented. The results indicate that the extended local momentum theory has the capability of achieving a level of accuracy similar to that of the prescribed wake vortex theory over wide range variations of rotor geometrical parameters. It is also shown that the analytical results obtained using either theory are in reasonable agreement with experimental data.

Kawachi, K.↗

NASA NTRS1988

Wake model for helicopter rotors in high speed flight

Two alternative approaches are developed to calculate blade-vortex interaction airloads on helicopter rotors: second order lifting-line theory, and a lifting surface theory correction. The common approach of using a larger vortex core radius to account for lifting-surface effects is quantified. The second order lifting-line theory also improves the modeling of yawed flow and swept tips. Calculated results are compared with wind tunnel measurements of lateral flapping, and with flight test measurements of blade section lift on SA349/2 and H-34 helicopter rotors. The tip vortex core radius required for good correlation with the flight test data is about 20 percent chord, which is within the range of measured viscous core sizes for helicopter rotors.

Johnson, Wayne R.↗

NASA NTRS1989

Comparison of measured induced velocities with results from a closed-form finite state wake model in forward flight

An unsteady, three-dimensional induced-flow model is applied to compute the induced-flow distribution of a rotor in forward flight, and numerical results are compared against LDV measurements for both time-averaged and unsteady induced flow at the disk. The former flow shows good agreement with measured data, except just behind the pylon at the lowest advanced ratio and near the blade tips for rectangular blades at high advance ratios. The method performs as well or better than other codes that have been applied to these data, takes less computing time, and is better suited to aeroelastic analysis. Results with only four harmonics and 15 state variables converge to all fundamental characteristics of the time-averaged flow.

Peters, David A.↗

NASA NTRS1992

Extension and validation of an unsteady wake model for rotors

A new three-dimensional, finite-state induced-flow model is extended to treat nonlinearities associated with the mass flow induced through the rotor plane. This new theory is then applied to the correlation of a recent set of unsteady, hover laser Doppler velocimetry inflow measurements conducted in the Aeroelastic Rotor Test Chamber at Georgia Institute of Technology. Although the model is intended primarily as a representation of unsteady aerodynamics for aeroelasticity applications, the results show that it has an excellent capability in predicting the inflow distribution in hover except near the root and tip. In addition, the computed unsteady spanwise lift distribution of a rotor is compared with that from an unsteady vortex lattice method for pitch oscillations at various frequencies. The new model is shown to be capable of prediction of unsteady loads typical of aeroelastic response.

Su, AY↗

NASA NTRS2016

A Coupled Probabilistic Wake Vortex and Aircraft Response Prediction Model

Wake vortex spacing standards along with weather and runway occupancy time, restrict terminal area throughput and impose major constraints on the overall capacity and efficiency of the National Airspace System (NAS). For more than two decades, the National Aeronautics and Space Administration (NASA) has been conducting research on characterizing wake vortex behavior in order to develop fast-time wake transport and decay prediction models. It is expected that the models can be used in the systems level design of advanced air traffic management (ATM) concepts that safely increase the capacity of the NAS. It is also envisioned that at a later stage of maturity, these models could potentially be used operationally, in groundbased spacing and scheduling systems as well as on the flight deck.

Gloudemans, Thijs↗

NASA NTRS1988

Development and validation of an advanced low-order panel method

A low-order potential-flow panel code, PMARC, for modeling complex three-dimensional geometries, is currently being developed at NASA Ames Research Center. The PMARC code was derived from a code named VSAERO that was developed for Ames Research Center by Analytical Methods, Inc. In addition to modeling potential flow over three-dimensional geometries, the present version of PMARC includes several advanced features such as an internal flow model, a simple jet wake model, and a time-stepping wake model. Data management within the code was optimized by the use of adjustable size arrays for rapidly changing the size capability of the code, reorganization of the output file and adopting a new plot file format. Preliminary versions of a geometry preprocessor and a geometry/aerodynamic data postprocessor are also available for use with PMARC. Several test cases are discussed to highlight the capabilities of the internal flow model, the jet wake model, and the time-stepping wake model.

Ashby, Dale L.↗

NASA NTRS2014

Review of Idealized Aircraft Wake Vortex Models

Properties of three aircraft wake vortex models, Lamb-Oseen, Burnham-Hallock, and Proctor are reviewed. These idealized models are often used to initialize the aircraft wake vortex pair in large eddy simulations and in wake encounter hazard models, as well as to define matched filters for processing lidar observations of aircraft wake vortices. Basic parameters for each vortex model, such as peak tangential velocity and circulation strength as a function of vortex core radius size, are examined. The models are also compared using different vortex characterizations, such as the vorticity magnitude. Results of Euler and large eddy simulations are presented. The application of vortex models in the postprocessing of lidar observations is discussed.

Ahmad, Nashat N.↗

NASA NTRS1989

The influence of the rotor wake on rotorcraft stability and control

The effect of the time-averaged rotor wake flowfield on the aerodynamic behavior of the tail rotor and fixed tail surfaces is discussed. The flowfield at the location of these surfaces is predicted by two wake models, a simplified flat wake model and an accurate free wake model. Both models are shown to give similar predictions of the flowfield in the vicinity of the empennage that are generally in agreement with experiment. The contributions of these aerodynamic interactions to the helicopter stability derivatives are described and control responses using different wake models are compared with flight test.

Curtiss, H. C., Jr.↗

NASA NTRS2015

Multi-Model Ensemble Wake Vortex Prediction

Several multi-model ensemble methods are investigated for predicting wake vortex transport and decay. This study is a joint effort between National Aeronautics and Space Administration and Deutsches Zentrum fuer Luft- und Raumfahrt to develop a multi-model ensemble capability using their wake models. An overview of different multi-model ensemble methods and their feasibility for wake applications is presented. The methods include Reliability Ensemble Averaging, Bayesian Model Averaging, and Monte Carlo Simulations. The methodologies are evaluated using data from wake vortex field experiments.

Koerner, Stephan↗

NASA NTRS2015

Multi-Model Ensemble Wake Vortex Prediction

Koerner, Stephan↗

NASA NTRS1989

Evaluation of VSAERO in prediction of aerodynamic characteristics of helicopter hub fairings

A low-order panel code, VSAERO, was used to predict the aerodynamic characteristics of helicopter hub fairings. Since the simulation of this kind of bluff body by VSAERO was not documented before, the VSAERO solutions were correlated with experimental data to establish their validity. The validation process revealed that simulation of the aerodynamic environment around a hub fairing was sensitive to several modeling parameters. Some of these parameters are body and wake panels arrangement, streamwise and spanwise separation location, and the most prominent one-the wake modeling. Three wake models were used: regular wake, separated wake, and jet model. The regular wake is a wake with negligible thickness (thin wake). It is represented by a single vortex sheet. The separated wake and the jet model in the present application are wakes with finite thickness (thick wake). They consist of a vortex sheet enclosing a region of low-energy flow. The results obtained with the reqular wake were marginally acceptable for sharp-edged hub fairings. For all other cases under consideration, the jet model results correlated slightly better. The separated wake, which seemed to be the most appropriate model, caused the solution to diverge. While the regular wake was straight-forward to apply in simulations, the jet model was not. It requires the user to provide information about the doublet strength gradient on wake panels by guessing the efflux velocities at the wake shedding location. In summary, VSAERO neither predicts accurately the aerodynamic characteristics of helicopter hub fairings nor was cost effective.

Louie, Alexander↗

NASA NTRSDate not supplied

A Physics-Based Spatial Wake Interactional Model of Fixed-Wing Aircraft and Rotorcraft for Urban Air Mobility

This paper presents a study of a physics-based spatial wake interactional model for urban air mobility. A modeling method for spatial wake interaction of fixed-wing aircraft and rotorcraft is presented. The method validates an existing wake vortex model with flight data from the existing literature. The model is then adopted for both fixed-wing aircraft and rotorcraft. A lift circulation model is developed for rotorcraft to account for unsteady aerodynamics and the compressibility effect. This lift circulation model provides more realistic estimate of lift circulation for rotorcraft for used in wake vortex modeling. A vehicle-building wake interaction is studied using the method of image. The effects of wake vortex encounters on aerodynamic loads, stability, and control are examined. A flight control simulation is performed to demonstrate recovery during a simulated wake vortex encounter.

Wake Vortex Encounter↗

NASA NTRS2019

Correlation of Full-Scale Isolated Proprotor Performance and Loads

A full-scale isolated proprotor test was recently conducted in the USAF National Full-Scale Aerodynamics Complex (NFAC) at NASA Ames Research Center. The test article was a 3-bladed research rotor derived from the right-hand rotor of the AW609. For this test, the NASA Tiltrotor Test Rig (TTR) and rotor were installed in the 40- by 80-foot test section. This paper presents correlations between data and predictions of rotor performance and blade moments using the newly acquired test data and the comprehensive analysis CAMRAD II. The operational conditions covered in this analytical study are: hover (actually, low speed vertical climb), cruise (airplane mode), conversion, and helicopter mode. Mean and 1/2 peak-to-peak quantities (hpp) are correlated; time-history correlation for the helicopter condition is also included. The correlation is reasonable to good. Also, the hover calculations turned out to be useful in providing reality checks on the test hardware such as: a) the functioning of the blade strain gages and b) calibration of the measurement of the collective pitch hardware. The time-history correlation shows that, compared to the rolled-up wake model, the multiple-trailer wake model improves the correlation slightly; the longitudinal cyclic correlation is reasonable but the lateral cyclic correlation is not good, and the collective is predicted well by the rolled-up wake model; the flap moment correlation is reasonable; the pitch link load and lag moment are underpredicted; and the torsion moment correlation is poor and needs further study.

Kottapalli, Sesi↗

NASA NTRS2008

Documentation for Three Wake Vortex Model Data Sets from Simulation of Flight 587 Wake Vortex Encounter Accident Case

This document contains a general description for data sets of a wake vortex system in a turbulent environment. The turbulence and thermal stratification of the environment are representative of the conditions on November 12, 2001 near John F. Kennedy International Airport. The simulation assumes no ambient winds. The full three dimensional simulation of the wake vortex system from a Boeing 747 predicts vortex circulation levels at 80% of their initial value at the time of the proposed vortex encounter. The linked vortex oval orientation showed no twisting, and the oval elevations at the widest point were about 20 meters higher than where the vortex pair joined. Fred Proctor of NASA?s Langley Research Center presented the results from this work at the NTSB public hearing that started 29 October 2002. This document contains a description of each data set including: variables, coordinate system, data format, and sample plots. Also included are instructions on how to read the data.

Switzer, George F.↗

NASA NTRS2014

NASA AVOSS Fast-Time Wake Prediction Models: User's Guide

The National Aeronautics and Space Administration (NASA) is developing and testing fast-time wake transport and decay models to safely enhance the capacity of the National Airspace System (NAS). The fast-time wake models are empirical algorithms used for real-time predictions of wake transport and decay based on aircraft parameters and ambient weather conditions. The aircraft dependent parameters include the initial vortex descent velocity and the vortex pair separation distance. The atmospheric initial conditions include vertical profiles of temperature or potential temperature, eddy dissipation rate, and crosswind. The current distribution includes the latest versions of the APA (3.4) and the TDP (2.1) models. This User's Guide provides detailed information on the model inputs, file formats, and the model output. An example of a model run and a brief description of the Memphis 1995 Wake Vortex Dataset is also provided.

Ahmad, Nash'at N.↗

NASA NTRS1987

A new methodology for free wake analysis using curved vortex elements

A method using curved vortex elements was developed for helicopter rotor free wake calculations. The Basic Curve Vortex Element (BCVE) is derived from the approximate Biot-Savart integration for a parabolic arc filament. When used in conjunction with a scheme to fit the elements along a vortex filament contour, this method has a significant advantage in overall accuracy and efficiency when compared to the traditional straight-line element approach. A theoretical and numerical analysis shows that free wake flows involving close interactions between filaments should utilize curved vortex elements in order to guarantee a consistent level of accuracy. The curved element method was implemented into a forward flight free wake analysis, featuring an adaptive far wake model that utilizes free wake information to extend the vortex filaments beyond the free wake regions. The curved vortex element free wake, coupled with this far wake model, exhibited rapid convergence, even in regions where the free wake and far wake turns are interlaced. Sample calculations are presented for tip vortex motion at various advance ratios for single and multiple blade rotors. Cross-flow plots reveal that the overall downstream wake flow resembles a trailing vortex pair. A preliminary assessment shows that the rotor downwash field is insensitive to element size, even for relatively large curved elements.

Bliss, Donald B.↗

NASA NTRS1999

Coherent Pulsed Lidar Sensing of Wake Vortex Position and Strength, Winds and Turbulence in the Terminal Area

NASA Langley Research Center (LaRC) has field tested a 2.0 gm, 100 Hertz, pulsed coherent lidar to detect and characterize wake vortices and to measure atmospheric winds and turbulence. The quantification of aircraft wake-vortex hazards is being addressed by the Wake Vortex Lidar (WVL) Project as part of Aircraft Vortex Spacing System (AVOSS), which is under the Reduced Spacing Operations Element of the Terminal Area Productivity (TAP) Program. These hazards currently set the minimum, fixed separation distance between two aircraft and affect the number of takeoff and landing operations on a single runway under Instrument Meteorological Conditions (IMC). The AVOSS concept seeks to safely reduce aircraft separation distances, when weather conditions permit, to increase the operational capacity of major airports. The current NASA wake-vortex research efforts focus on developing and validating wake vortex encounter models, wake decay and advection models, and wake sensing technologies. These technologies will be incorporated into an automated AVOSS that can properly select safe separation distances for different weather conditions, based on the aircraft pair and predicted/measured vortex behavior. The sensor subsystem efforts focus on developing and validating wake sensing technologies. The lidar system has been field-tested to provide real-time wake vortex trajectory and strength data to AVOSS for wake prediction verification. Wake vortices, atmospheric winds, and turbulence products have been generated from processing the lidar data collected during deployments to Norfolk (ORF), John F. Kennedy (JFK), and Dallas/Fort Worth (DFW) International Airports.

Brockman, Philip↗

NASA NTRS2011

Enhanced Correlation of SMART Active Flap Rotor Loads

This is a follow-on study to a 2010 correlation effort. Measured data from the SMART rotor test in the NASA Ames 40- by 80- Foot Wind Tunnel are compared with CAMRAD II calculations. As background, during the wind tunnel test, unexpectedly high inboard loads were encountered, and it was hypothesized at that time that due to changes in the flexbeams over the years, the flexbeam properties used in the analysis needed updating. Boeing Mesa, recently updated these properties. This correlation study uses the updated flexbeam properties. Compared to earlier studies, the following two enhancements are implemented: i) the inboard loads (pitchcase and flexbeam loads) correlation is included for the first time (reliable prediction of the inboard loads is a prerequisite for any future anticipated flight-testing); ii) the number of blade modes is increased to better capture the flap dynamics and the pitchcase-flexbeam dynamics. Also, aerodynamically, both the rolled-up wake model and the more complex, multiple trailer wake model are used, with the latter slightly improving the blade chordwise moment correlation. This sensitivity to the wake model indicates that CFD is needed. Three high-speed experimental cases, one uncontrolled free flap case and two commanded flap cases, are considered. The two commanded flap cases include a 2o flap deflection at 5P case and a 0o flap deflection case. For the free flap case, selected modifications to the HH-06 section flap airfoil pitching moment table are implemented. For the commanded 2o flap case, the experimental flap variation is approximately matched by increasing the analytical flap hinge stiffness. This increased flap hinge stiffness is retained for the commanded 0o flap case also, which is treated as a free flap case, but with larger flap hinge stiffness. The change in the mid-span and outboard loads correlation due to the updating of the flexbeam properties is not significant. Increasing the number of blade modes results in an effective, commanded flap hinge stiffness of 4X baseline, not 3X as reported earlier. The inboard loads correlation is reasonable, but needs further study. Overall, the free flap case correlation is reasonable, thus confirming the basic correctness of the current semi-empirical modifications; the correlation for the commanded 2o flap at 5P case and the 0o flap case is also reasonable.

Kottapalli, Sesi↗