The National Aeronautics and Space Administration (NASA) is conducting research to improve airport capacity by reducing the separation distance between aircraft. The limiting factor in reducing separation distances and improving airport capacity is the wake vortex hazard. The ability to accurately model wake vortices and predict the outcome of a vortex encounter is critical in developing a system to safely improve airport capacity. This is the focus of the wake vortex research being done at NASA Langley Research Center (LaRC). This paper will concentrate on two topics. The first topic is the control system developed for the Boeing 737 freeflight model in support of vortex encounter tests to be conducted in the 30- by 60- foot tunnel at NASA Langley Research Center later this year. The second topic discussed is the limited degree of freedom (DOF) trajectory generation study that is being conducted to determine the relative severity of a multitude of paths through a wake vortex

Wilson, Jeffrey M.

English

NASA Technical Reports Server

Wake Vortex Encounter
Research
Jeffrey M. Wilson
The University of Kansas
Langley Aerospace Researchers Summer Scholar
Robert A. Stuever
NASA Langley Research Center
Hampton, VA
Research and Technology Group
Flight Dynamics and Control Division
Vehicle Dynamics Branch
817
https://ntrs.nasa.gov/search.jsp?R=19970003129 2020-06-18T00:12:25+00:00Z
Abstract
The National Aeronautics and Space Administration (NASA) is conducting research to
improve airport capacity by reducing the separation distance between aircraft. The limiting
factor in reducing separation distances and improving airport capacity is the wake vortex hazard.
The ability to accurately model wake vortices and predict the outcome of a vortex encounter is
critical in developing a system to safely improve airport capacity. This is the focus of the wake
vortex research being done at NASA Langley Research Center (LaRC). This paper will
concentrate on two topics. The first topic is the control system developed for the Boeing 737
freeflight model in support of vortex encounter tests to be conducted in the 30- by 60- foot tunnel
at NASA Langley Research Center later this year. The second topic discussed is the limited
degree of freedom (DOF) trajectory generation study that is being conducted to determine the
relative severity of a multitude of paths through a wake vortex.
Introduction
While the number of aircraft in the world fleet is projected to more than double by the
year 2015, many of the world's major airports are already capacity limited.(1) The National
Aeronautics and Space Administration (NASA) is currently conducting research to enable safe
improvements in the capacity of the nation's air transportation system. One part of the NASA
research program is the Terminal Area Productivity (TAP) Program. This program is responsible
for conducting the necessary research to support the Federal Aviation Administration (FAA) and
the aviation industry in safely achieving fair weather airport capacity in instrument
meteorological conditions. The TAP Program is subdivided into four elements: Air Traffic
Management, Aircraft-Air Traffic Control Systems Integration, Low-Visibility Landing, and
Reduced Spacing Operations.
The Reduced Spacing Operations element is responsible for conducting research on
reducing the in-trail spacing distance between airplanes. Improvements in landing frequency by
reducing in-wall spacing distances have the potential to increase system capacity by 10-15%.(2)
The limiting factor in reducing the spacing distances is the wake turbulence phenomena. The
wake of an aircraft is characterized by two counter-rotating vortices that are spaced at a distance
of slightly less than the aircraft span. Another aircraft encountering one of these vortices could
either be mildly perturbed or catastrophically upset. The degree of upset is dependent on a
number of factors including relative sizes of the aircraft.
The current aim of the wake vortex research program at NASA Langley Research Center
(LaRC) is to create a methodology that will allow researchers to accurately model wake vortices
and characterize the hazard posed by them. The purpose of this hazard characterization research
is to relate the potential vortex hazard to sensor observable quantities. A sensor based system
designated as Aircraft Vortex Spacing System (AVOSS) will then be developed to predict,
detect, track, and quantify wake vortices.(3) Hazard analyses will be applied to determine if an
aircraft could safely penetrate a vortex that has been detected in the approach corridor. This
report will discuss the development of a freeflight model control system and the trajectory
generation study conducted at LaRC as part of the wake vortex research program.
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Freeflight Model Control System
As part of the wake vortex research program, tests will be conducted to determine the
dynamic effects of flying a Boeing 737 within the wake of another aircraft. In one test, a scale
model of the Boeing 737 aircraft will be flown behind a simulated second aircraft within the test
section of the 30- by 60- foot tunnel at LaRC. Control laws were developed for the Boeing 737
freeflight model based on the control system for the Boeing 737-100 at LaRC. The control laws
represent the Boeing 737 for eight candidate approach flight conditions in which the flap settings
and airspeed vary. Aerodynamic data for each of the eight flight conditions were obtained for
the state-space matrices from a Boeing 737 simulation database. Actuator models, maximum
control surface deflections, and yaw damper models were obtained from Boeing documentation.
A SIMULINK file was then created to analyze the dynamics of the full scale aircraft. The
control laws for the full scale aircraft were scaled down to the model size using Froude number
scaling as explained by Wolowicz, et al.(4) The model control system was then analyzed to
ensure dynamic similarity existed between the model and the actual aircraft.
Trajectory Generation Study
The purposes of the trajectory generation study were twofold. The first objective was to
determine the validity of using simple and approximate planform and mass characteristics to
predict the severity of vortex encounters. The primary advantage of using such relatively simple
techniques is the ease of which the study could be applied as a means of hazard prediction for the
entire commercial fleet. The second objective of the study was to obtain a database about the
severity of a vortex-induced upset for a multitude of paths through a wake for several follower-
generator aircraft pairs.
For this study, a MATLAB simulation was developed based on simplified math models
of aircraft. Generator aircraft were assumed to have elliptic spanwise lift distributions. Using
the known characteristics of vortices shed from aircraft with ellipitic spanwise lift distributions,
the wake vortices of the generator aircraft were then modeled by an accepted vortex velocity-
distribution. Strip theory was used to calculate the aerodynamic forces and moments induced on
the follower aircraft by the vortices. Explicit solutions for the induced lift and rolling moment
coefficients were obtained from Tatnall by integration of the strip theory equations.(5) Using the
induced lift and rolling moment coefficients, Bowles determined the equations of motion of the
follower aircraft in the wake of the generator aircraft.(6) With these equations, the simulation
could be altered to allow the follower aircraft either one degree of freedom (DOF), roll, or three
DOF, roll and translation along the Y and Z axes.
To simulate the decay of actual vortices with time in the atmosphere, the wake vortices
in the vortex encounter simulation were decayed linearly with non-dimensional time. This
vortex decay model is based on worst case situation of tower fly-by data in research conducted
by Greene. (7)
In the simulation runs completed for this report, the Boeing 737 was used as the follower
aircraft. In previous vortex encounter simulations, the follower-generator span ratio was
determined to be a key parameter in predicting the severity of a vortex encounter. Accordingly,
generator aircraft were selected to represent a range of follower to generator span ratios. The
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selected ratios were 0.5 (Boeing 747 as the generator aircraft), 1.0 (Boeing 737), and 2.0
(Learjet 60). An additional aircraft pair with a span ratio of 0.7 was analyzed. This span ratio
represents the Lockheed C-130 as the generator aircraft and was selected since the C-130 is a
candidate vehicle for flight experiments.
Each simulation run consisted of 99 passes through the generator wake in which each
pass differed in the lateral and vertical approach angles. The lateral approach angles selected
were -20" to 20" in 5" increments. The vertical approach angles used were -10 ° to 10" in 2"
increments. The Boeing 737 began each pass two generator spans away from the location of
maximum induced rolling moment in the wake and aimed for this location. The pass ended
when the follower aircraft was once again two generator spans away from the location of
maximum rolling moment. Simulation runs will be completed for cases in which the follower
aircraft will have no roll control power and full roll control power.
After the data is collected, a sensitivity analysis will be conducted for a nominal
approach condition. The objective of the analysis is to evaluate the relative importance of
certain parameters in determining the outcome of a vortex encounter. This information will be
useful in hazard prediction.
Trajectory Generation Study Results
The trajectory generation study was not completed at the time this report was written.
Sample 1 and 3 DOF results of a Boeing 737 encountering the wake of a Boeing 747 are
available and will be discussed presently. These results are for the follower aircraft with no roll
control inputs.
Figure 1 is a series of plots which contain the results of a 1 DOF penetration of the
location of maximum rolling moment. The penetration occurs at the FAA specified separation
distance of 5 nautical miles for this aircraft combination. The encounter shown in Figure 1
occurs when the Boeing 737 is on a 4" glideslope in the approach flight condition. The plot in
the upper fight comer indicates the follower aircraft reaches a maximum bank angle of -243".
The plot on the middle fight shows that the maximum induced roll rate is -58 "/see. These results
are unrealistic but are explained by the fact that since the penetration occurs a such a slow rate,
the aircraft is allowed to remain inside the vortex for a considerable period of time. Furthermore,
the follower aircraft is constrained to only roll. As will be seen in Figure 2, if the aircraft would
have had additional degrees of freedom, the magnitudes of the vortex induced perturbations
would have been smaller.
Figure 2 contains the 3 DOF results for the same initial trajectory as in Figure 1. As can
be seen in the plot in the upper left comer, the follower aircraft is removed from the intended
path and is driven over 200 feet off course. It is apparent from the plot in the lower left that the
aircrazt is also accelerated downward. If this dangerous situation had occurred, the follower
aircraft could have potentially crashed. At the very least, the aircraft would have been forced out
of the approach corridor and would have had to execute a 'go-around'. It should also be noted
that the bank angle and roll rate perturbations are smaller that the 1 DOF values but nonetheless
result in unacceptable passenger comfort levels.
Once the trajectory generation study is completed, the data will be analyzed to determine
a criteria for the estimating the potential of a wake vortex upset on a given trajectory. A worst
case trajectory for a vortex encounter will then be determined.
820
Acknowledgments
I would like to thank the personnel at NASA Langley Research Center who have assisted with
the research conducted in this paper. I would like to especially thank Robert A. Stuever for his
guidance and assistance not only with this project but also with my professional development.
References
1. Mecham, Michael; and McKenna, James T.: "Cost, Not Size, To Drive Success of
Superjumbo". Aviation Week & Space Technology, November 21, 1994, pp. 45-46.
2. Scott, William B.: "Technology, Economics, Fuel, Public-Private Cooperation".
Aviation Week & Space Technology, June 7, 1993, pp. 69-76.
3. Vicroy, Dan; et al: "NASA Wake Vortex Research". ICAS-94-6.2.2, September 1994.
4. Wolowicz, Chester H.; Bowman, James S., Jr.; and Gilbert, William P.: "Similitude
Requirements and Scaling Relationships as Applied to Model Testing". NASA TP 1435, 1979.
5. Tatnall, Chris R.: Unpublished Notes, 1995.
6. Bowles, Roland: Unpublished Notes, 1995.
7. Greene, George C.: "An Approximate Model of Vortex Decay in the Atmosphere".
Journal of Aircraft, Vol. 23, No. 7, July 1986, pp. 566-573.
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