American Institute of Aeronautics and Astronautics
092407
1
Initial Results from the Variable Intensity Sonic Boom
Propagation Database
Edward A. Haering, Jr.,
Larry J. Cliatt, II,
†
and Thomas J. Bunce
‡
NASA Dryden Flight Research Center, Edwards, California, 93523-0273
Thomas B. Gabrielson,
§
Victor W. Sparrow,
**
and Lance L. Locey
††
Pennsylvania State University, University Park, Pennsylvania, 16802
An extensive sonic boom propagation database with low- to normal-intensity booms
(overpressures of 0.08 lbf/ft
2
to 2.20 lbf/ft
2
) was collected for propagation code validation,
and initial results and flight research techniques are presented. Several arrays of
microphones were used, including a 10 m tall tower to measure shock wave directionality
and the effect of height above ground on acoustic level. A sailplane was employed to measure
sonic booms above and within the atmospheric turbulent boundary layer, and the sailplane
was positioned to intercept the shock waves between the supersonic airplane and the ground
sensors. Sailplane and ground-level sonic boom recordings were used to generate
atmospheric turbulence filter functions showing excellent agreement with ground
measurements. The sonic boom prediction software PCBoom4 was employed as a preflight
planning tool using preflight weather data. The measured data of shock wave directionality,
arrival time, and overpressure gave excellent agreement with the PCBoom4-calculated
results using the measured aircraft and atmospheric data as inputs. C-weighted acoustic
levels generally decreased with increasing height above the ground. A-weighted and
perceived levels usually were at a minimum for a height where the elevated–microphone
pressure–rise time history was the straightest, which is a result of incident and ground-
reflected shock waves interacting.
Nomenclature
ASEL = A-weighted sound exposure level
BADS = boom amplitude and direction sensor
BASS = boom amplitude and shape sensor
CSEL = C-weighted sound exposure level
DGPS = differential global positioning system
FTE = flight-test engineer (of the sailplane)
GPS = global positioning system
HUD = head-up display
INS = inertial navigation system
IRIG = Inter-Range Instrumentation Group
IT = information technology
LCASB = Loudness Code for Asymmetric Sonic Booms
NASA = National Aeronautics and Space Administration
PC = personal computer
PL = Stevens Mark VII perceived level
RQDS = Research Quick Data System
Aerospace Engineer, Research Aerodynamics, Mail Stop D-2228, AIAA nonmember.
†
Aerospace Engineer, Research Aerodynamics, Mail Stop D-2228, AIAA member.
‡
Aerospace Engineer, Research Aerodynamics, Mail Stop D-2228, AIAA nonmember.
§
Senior Scientist, Applied Research Lab, 218 Applied Science Building, AIAA nonmember.
**
Associate Professor, Graduate Program in Acoustics, 201 Applied Science Building. AIAA Senior member.
††
Research Assistant, Graduate Program in Acoustics, 201 Applied Science Building, AIAA nonmember.
American Institute of Aeronautics and Astronautics
092407
2
SODAR = SOnic Detection and Ranging
VIBES = Variable Intensity Boom Effect on Structures
UTC = Universal Time Coordinated
Z = Zulu (North Atlantic Treaty Organization phonetic for UTC)
az
B
= shock wave propagation azimuth angle at the BADS toward the source, deg from true north
az
s
= shock wave propagation azimuth angle at the sailplane toward the source, deg from true north
az
t
= shock wave propagation azimuth angle at the tower toward the source, deg from true north
dB
b
= uncorrected acoustic level of the sonic boom, dB re 20
µ
Pa
dB
c
= corrected acoustic level, dB re 20
µ
Pa
dB
n
= uncorrected acoustic level of noise 1 s before the sonic boom, dB re 20
µ
Pa
el
B
= shock wave propagation elevation angle at the BADS toward the source, deg above horizontal
el
s
= shock wave propagation elevation angle at the sailplane toward the source, deg above horizontal
el
t
= shock wave propagation elevation angle at the tower toward the source, deg above horizontal
N
#
,
E
#
,
D
#
= north, east, and downward locations of microphones, m or ft
R
air
= specific gas constant of air
T = temperature
t = time, s after midnight UTC
tac
s
= time the shock wave left the F-18 airplane for the sailplane, from PCBoom4, s after midnight UTC
tac
t
= time the shock wave left the F-18 airplane for the tower, from PCBoom4, s after midnight UTC
tg
s
= time the bow shock wave hit the sailplane, from PCBoom4, s after midnight UTC
tg
t
= time the bow shock wave hit the tower, from PCBoom4, s after midnight UTC
t
b
ow
= time the bow shock is measured, s after midnight UTC
t
L
23
= measured IRIG-B time on the sailplane, s after midnight UTC
t
so
= stationary observer time, s after midnight UTC
V
N
,
V
E
,
V
D
= shock wavefront ground-relative velocities in the north, east, and downward directions, ft/s
V
i
nv
= inverse of shock wavefront velocity vector, s/ft
v
ns
, v
es
, v
ds
= DGPS velocities of the sailplane in the north, east, and downward directions
v
nw
,v
ew
,v
dw
= wavefront airmass-relative velocities at the sailplane in the north, east, and downward directions
w
n
, w
e
= wind components at the array from the north and east directions, ft/s
w
ns
, w
es
= wind components at the sailplane from the north and east directions, ft/s
x, y = DGPS sailplane position east and north of the origin, ft
p = sonic boom overpressure, lbf/ft
2
= ratio of specific heats for air
0
,
0
= origin longitude and latitude of -118° east and 35° north
s
,
s
= longitude and latitude of the sailplane, deg
s
= angle of wavefront propagation from the F-18 airplane toward the sailplane, deg
t
= angle of wavefront propagation from the F-18 airplane toward the tower, deg
µ
= Mach angle, deg
I. Introduction
series of flights generating low- to conventional-intensity sonic booms were flown in July 2007 to investigate
their effect on a house of modern construction, and to provide a database for the validation of sonic boom
propagation codes. This project is known as House VIBES (Variable Intensity Boom Effect on Structures), part of
the National Aeronautics and Space Administration (NASA) Fundamental Aeronautics’, Aeronautics Research
Mission Directorate’s, Supersonics Research Program. The United States Air Force Test Pilot School at Edwards
Air Force Base (Edwards, California, USA), the Gulfstream Corporation (Savannah, Georgia, USA), and
Pennsylvania State University (University Park, Pennsylvania, USA) also were participants in the project. Seven
flights were flown with NASA Dryden Flight Research Center (Edwards, California, USA) F-18 airplanes
(McDonnell Douglas, now the Boeing Company, Chicago, Illinois, USA) generating 31 low-intensity and 12
normal-intensity N-waves. An extra flight was flown to validate the airdata calibration of the airplanes. These sonic
booms at the house ranged in overpressure from 0.08 lbf/ft
2
to 2.20 lbf/ft
2
, and had risetimes of approximately 50 ms
to 0.7 ms. These risetimes were determined from 0 to 100 percent of maximum overpressure on a ground-level
microphone.
A
American Institute of Aeronautics and Astronautics
092407
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This house structural test is a follow-on test to one performed on an older house the previous year.
1
The data
from this newer house are not part of this paper, but these supersonic flights gave the opportunity to also measure
other sonic boom propagation phenomena, outlined below. Bütikofer and Thomann
2
reported that A-weighted sound
levels were lower at 1.2 m than at 10.0 m heights far to the side of a jet engine aircraft, where the received noise is
dominated by low frequencies. Since the low-intensity sonic booms for these flights have low propagation elevation
angles it was of interest to see if sonic boom acoustic level varied with height over 10 m, since most residences are
of this scale. A 10 m tall tower with 10 microphones along its height and four ground-level microphones around its
base was erected in an empty field a few hundred feet from the house. These tower data can be used to determine the
effects of height on shock wave incidence and reflection angles, reflection factor, and attenuation or amplification.
Ground impedance data were taken at the 10 m tall tower consisting of white noise and swept-sine recordings in
order to determine ground impedance as a function of frequency, but this analysis is not yet complete.
Advances in computational techniques have made it possible to design supersonic aircraft having something
other than the normal-intensity N-wave sonic boom.
3
The sonic booms generated by these new aircraft are referred
to as “low-booms” because they in theory will be quieter and less annoying than normal-intensity sonic booms. Such
“low-boom” designs could lead to commercial supersonic flight in the near future. Before such aircraft can be built,
engineers must come up with a way to model the effects of the atmosphere. They need to understand how the
atmosphere will alter the “low-boom” waveform. One way to do this is by modeling the atmosphere as a finite-
impulse-response filter.
A sailplane was instrumented with microphones, and flown above the planetary turbulent boundary layer to
intercept the shock wave on its way to the tower. The finite-impulse-response filter is estimated from the sailplane
measurement and the ground measurement. The resulting filter then is a representation of the atmosphere at those
two points in space and time. The filter can be used as a design tool to investigate the variability of proposed “low-
boom” waveforms, with the intention of producing an acceptably quiet supersonic aircraft.
The airborne and ground systems used in this project will be presented, along with the maneuvers, preflight
planning, and analysis techniques used. Representative sonic boom flight results will be shown along with
comparisons to the sonic boom prediction code PCBoom4.
4
The Appendix contains details of the system hardware
and flight research techniques used, as well as tabulated data from all the supersonic passes.
II. System Descriptions
The two main aircraft of this project will now be described, including the instrumentation used. A variety of
sensors at and near the ground as well as their recording systems will be described. Atmospheric data were also
gathered from various sensors, and these are also described.
A. The F-18B Sonic Boom Generating Airplane
The NASA F-18 airplanes, tail numbers 852 (a two-seat trainer B model), Fig. 1, and 850 (a single-seat A
model), are fighter airplanes built by McDonnell Aircraft (now the Boeing Company, Chicago, Illinois). Details of
the instrumentation used on these F-18 airplanes are given in the Appendix.
American Institute of Aeronautics and Astronautics
092407
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Figure 1. The instrumented NASA F-18B airplane, tail number 852, used for most of the sonic boom flights.
B. The 10 m Microphone Tower
A microphone tower approximately 130 m from the house held an array of ten microphones at 0, 1.2, 2, 3, 4, 5,
6, 7, 8, and 10 m above the ground. Four microphones on groundboards were positioned at 5 m east and 10 m east,
and at 5 m south and 10 m south of the tower, as shown in Fig. 2 through Fig. 4. The 0 m tower microphone was
also on a groundboard. Some data loss occurred due to conflicts between data recording software and mandated
information technology (IT) security software. Details of the tower construction, microphones, recording equipment,
and ground impedance measurements are given in the Appendix.
American Institute of Aeronautics and Astronautics
092407
5
Figure 2. The 10 m tall tower with microphones at 2 m to 10 m; the four guy-wires can be seen connected at
the apex of the tower.
Figure 3. The 10 m tall tower with microphones at 1.2 m and 0 m; 2 ft by 2 ft by 0.75 inch groundboards were
used under the ground-level microphones.
