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AeroRocket
Wind Tunnel Testing
Subsonic and Supersonic
Wind Tunnel Testing
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Copyright © 1999-2015 John Cipolla/AeroRocket
SUBSONIC
WIND TUNNEL TESTING
The
AeroRocket subsonic wind tunnel
is a suction
system powered by a two speed 1/3 horsepower fan.
The test section is 7 inches wide x 10 inches high x 16
inches long. The
basic subsonic wind tunnel design is by
Donald D. Baals of NASA and was fabricated and
redesigned by John Cipolla. A research quality pitot
tube is used to measure the difference between static pressure
and dynamic pressure in the wind tunnel. An analog velocity meter
is used to convert the resulting pressure differential between
the static and dynamic pressure to determine test section flow
velocity in feet per minute. Recently, the insertion of aluminum honeycomb
material before
and after the test section resulted in a significant increase in
measurement accuracy for drag and lift coefficients by
decreasing flow turbulence by several orders of magnitude.
VALIDITY OF USING SMALL MODELS TO
REPRESENT FULL SIZE VEHICLES
When testing small wind tunnel models, AeroRocket uses
aerodynamic scaling laws to convert data from inexpensive
wind tunnel models into design information for very expensive large scale
prototypes. In practice the aerospace industry giants do not
build an expensive multi-million dollar airplane and then
attempt to see if the aircraft has enough lift to fly and is stable enough
to avoid crashing. Instead, they measure lift and center of
pressure on
small models and use scaling laws to predict lift and
stability of the full scale prototype. Wind tunnel models have
been tested in small wind tunnels to determine the aerodynamics of every aircraft from the Wright Brothers first airplane to the
NASA Space Shuttle. Therefore, it is insulting for an
individual or corporation to request that AeroRocket test for
example a full size
motor cycle in the AeroRocket subsonic wind tunnel
while knowing the test section is about as big as a breadbox and
then in the same telephone request insult the AeroRocket
subsonic wind tunnel for its small size. For their ignorance
these individuals and corporations can take their wind tunnel testing business
elsewhere.
An example of testing a complex aerodynamic prototype in the
AeroRocket subsonic wind tunnel is the Schafer Corporation
V-Ship, second model from the left illustrated above. While
the V-Ship model is only 3.5" long
the AeroRocket subsonic wind tunnel accurately modeled the
laminar aerodynamics of the 60 foot long airship when operating
at 70,000 feet. Therefore,
models of very large designs may be successfully tested in the AeroRocket wind tunnel at relatively
high Reynolds number as long as the flow is assured to be either
laminar or turbulent depending on the actual characteristics of
the full-scale flow. Small models that represent large designs are
routinely tested in the AeroRocket wind tunnel because aerodynamic shapes including
plates, spheres and cylinders exhibit relatively constant drag
coefficient (CD) over a wide range of Reynolds number as long as
the flow is turbulent or made turbulent using an attached trip wire. In
the illustration below, please notice that a 2-D plate normal to
the flow has a relatively constant CD for Reynolds
number ranging from approximately 1,000 to nearly seven million.
Similarly, cylinders have relatively constant CD for Reynolds
number ranging from approximately 1,000 to nearly one
million. The
phenomena of constant CD over a wide range of
Reynolds number is also valid for 3-D flow and is caused by the
transition to "forebody" turbulent flow at Reynolds number
1,000 and NOT any effect the base-region turbulent boundary layer
has above the critical Reynolds number for reducing base
drag. This
principal can be extended to many other complex designs where
shape and Reynolds number must be maintained for valid wind
tunnel results based on flow similarity. Please read Fluid Dynamic Drag by S.F.
Hoerner, pages 3-7 to 3-15 for more explanation.
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Drag (CD) verses Reynolds number (Re) for 2-D flow over common
aerodynamic shapes. |
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Basic Features
 Velocity:
35 to 80 ft/sec with a test item installed
Reynolds Number: Laminar
and turbulent flow with trip wires
Test Section:
7 inches wide X 10 inches high X 16 inches long
Drag
(CD) and lift (CL) using 2-component force balance system
Unique wind tunnel model fabrication
skills
X-30 Wind Tunnel Model |
Subsonic Wind Tunnel |
HL-20 Wind Tunnel Model |
Flow Visualization
Smoke Flow
Visualization
Filament Probe Flow Visualization
Schlieren Photography for
Supersonic Flow
(presently
under development)
Support Systems
CD &
CL using rear mounted sting (0 to 40 degrees AOA)
CD & CL using vertical
mounted Sting (0,
5, 7.5, 10, 12.5, 15 and 17.5 degrees AOA)
Low turbulence due to very fine honeycomb flow straightener
before and after test section
Center of pressure location
measurement
(XCp)
SUPERSONIC WIND TUNNEL TESTING
(M = 0.5 to M = 3)
Back
 A
new supersonic blow-down wind tunnel is available for testing
aerodynamic shapes from M = 0.5 to M = 3. The AeroRocket supersonic blow-down wind tunnel is
the result of an urgent need to replace the previous shock
tube wind tunnel with a more robust and cost effective system to
measure
projectile drag coefficient (Cd). The shock tube proved to be
extremely expensive to operate while producing results that
lasted only a few milliseconds making Cd measurement
extremely difficult. The new AeroRocket blow-down wind tunnel
achieves nearly constant Mach 3 flow for approximately 4
seconds, then quickly transitions to nearly constant Mach 1.37
flow for another 4 seconds during the blow-down process. As a
result standard
and inexpensive off-the-shelf data acquisition devices can be
used to measure long duration drag forces for determining Cd
verses Mach number. The supersonic blow-down wind tunnel has
been calibrated using the supersonic Narrow Wedge
relationship for Cd verses Mach number from Fluid Dynamic Drag by
S.F.Hoerner. The converging-diverging nozzle featured in the new
supersonic blow-down wind tunnel was designed using
Nozzle 3.7 and classical
analyses to use the concept of a normal shock
diffuser to increase
pressure ratio efficiency. The normal shock is slightly upstream
of the divergent duct and completely enclosed by the exhaust
nozzle.
This system is operational for supersonic
wind tunnel testing. AeroRocket's expertise in the fabrication
of miniature wind tunnel models makes possible the measurement
of transonic and supersonic Cd for designs ranging from
relatively simple high power rockets to the
HTV-3X. However,
shape limitations apply because of the small size of the wind
tunnel models required to fit in the test section.
Mars Entry Capsule Example
 The
following case illustrates the measurement of drag coefficient (Cd) at Mach 2.94 for
a shape similar to the Mars Phoenix entry capsule. The Cd
determined by the AeroRocket supersonic blow-down wind
tunnel test is Cd =
1.390. To account for sting base drag
interference the following equation from Fluid Dynamic Drag is
used to correct for sting base drag effects. Cd_base =
K*1.43/Mn^2 where K equals 0.7. Base drag is determined by this
equation to be 0.116. Total Cd for the entry capsule is
Cd = 1.506 as tested in the AeroRocket supersonic blow-down
wind tunnel. Drag coefficient of a conical capsule of the type
tested here has been tested before and may be found in NACA
TN D-1085 (1963). The value for drag coefficient for the
capsule tested in the NACA report is Cd = 1.49. These
results may also be found in Fluid Dynamic Drag on page 18-19 in
Figure 33.
Next Generation 1"
Diameter Supersonic Wind Tunnel
A new
supersonic blow-down wind tunnel is available for testing
aerodynamic shapes.The new 1" inside-diameter supersonic blow-down wind tunnel, having a test section blockage factor
less than 3%, now joins the successful 1/2"
supersonic wind tunnel. Tests conducted using the
next generation wind tunnel indicate drag coefficient (Cd) for the
HTV-3X is 0.1016 at Mach 2.64.
Next
generation 1" diameter supersonic blow-down wind tunnel with air
supply.
HTV-3X mounted in the
1" diameter
supersonic blow down wind tunnel.
Determining Mach Number
in the
1/2" and 1" Supersonic Blow-Down Wind Tunnels
Total Pressure (0 to 30 psig) and Static Pressure (0 to 30
in-Hg) gages
used to determine Mach Number in the Supersonic Wind Tunnel.
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1/2"
Diameter Wind Tunnel: Mach number
verses time is measured during the blow down process
using a pitot-static pressure probe for measuring total
pressure (Po) and static pressure (Ps) of a compressible
fluid (air). Click here
to view a QuickTime movie of a 6.0 second segment
of a wind tunnel test using the 1/2 inch diameter AeroRocket
supersonic
blow down wind tunnel. The image below displays Mach
number verses time using total and static pressure
results from pressure testing. In this test nearly
constant Mach 2.0 flow is maintained for approximately
6.0 seconds. |
Click
image to view 1/2: diameter wind tunnel movie
Requires
QuickTime from Apple Computer
|
1"
Diameter Wind Tunnel: Mach number
verses time is measured during the blow down process
using a pitot-static pressure probe for measuring total
pressure (Po) and static pressure (Ps) of a compressible
fluid (air). Click here
to view a QuickTime movie of a 2.5 second segment
of a wind tunnel test using the one inch diameter AeroRocket
supersonic
blow down wind tunnel. The image below displays Mach
number verses time using total and static pressure
results from pressure testing. In this test nearly
constant Mach 1.6 flow is maintained for approximately
2.5 seconds. |
Click
image to view 1" diameter wind tunnel movie
Requires
QuickTime from Apple Computer
|
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SUBSONIC
WIND TUNNEL TECHNICAL
DETAILS |
Wind Tunnel
Test Features
BACK
Wind tunnel measurements at a maximum wind speed of 80 ft/sec
(54.5 mph).
Drag (CD) and lift (CL) at 0,
5, 7.5, 10, 12.5, 15 and 17.5 degrees AOA using vertical-sting or rear-sting mounted
models for angles of attack ranging from 0.0 degrees to 40
degrees AOA.
Center of Pressure location (XCp)
measurement.
Flow visualization using probe-mounted
yarn filaments or smoke that define areas of reverse flow and vortical
motion corresponding to lift.
Experiments photographically
documented.
Turbulent flow measurement using
trip-wire if required.
Report summarizing the results.
Drag (CD) and Lift (CL) Coefficients BACK
Figure-1 illustrates the force balance
system used to determine rocket drag (CD) and lift (CL) of a
wind tunnel model mounted on a vertical-sting. Displacement in
the axial (drag) and vertical (lift) directions are measured
using the two load cells labeled DRAG and LIFT
respectively and then converted to drag and lift forces in Newtons
using the Vernier CBL-2 computerized data acquisition system.
The force balance system pictured in Figure-1 is designed to
separate the aerodynamic forces and the associated displacements
in the axial and vertical directions when the weight on the force-balance
plate causes the model to be freely suspended. Models tested
in the AeroRocket wind tunnel may be mounted on a vertical-sting
as in Figure-2 or mounted on a rear-sting as in Figure-3. Either
mounting configuration may be selected depending on the objectives
of the wind tunnel test.
Center
of Pressure (XCp) Location Measurement BACK
Center of pressure location measurements
are performed using a special XCp-Caliper that secures the model
in the wind tunnel test section using two opposing low friction
points. Figure-4 illustrates a ring-fin model rocket being tested
in the AeroRocket wind tunnel for the determination of center
of pressure location. The ring-fin model in this configuration
is stable because the support point is ahead of the actual center
of pressure. The actual center of pressure location (XCp) is
determined by moving the sting support location rearward until
the model becomes unstable and "noses over" to one
side or the other when the wind tunnel is operating. Figure-5
further illustrates how the ring-fin rocket model is secured
in place during center of pressure location testing. Please notice
the pitot tube used to measure the difference between static
pressure and dynamic pressure for determining flow velocity in
the wind tunnel. An analog velocity meter is used to convert
the resulting pressure differential to test section flow velocity
in feet per minute (fpm).
Figure-4,
Center of Pressure (XCp) Location Measurement
Figure-5,
XCp Measurement Instrumentation |
Filament Flow
Field Visualization BACK
A single-strand filament of low mass
yarn on a long slender probe illustrates the flow on and around
an object. Regions of reverse flow behind blunt bodies become
visible. Please refer to Figure-6, Figure-7 and Figure-8 to see
how the base flow of the Sprint ABM is investigated using a yarn
tuft on a probe. In addition, regions where the flow rotates
indicate stream wise vorticity and therefore lift and circulation.
Please refer to Figure-9, Figure-10 and Figure-11 to see how
the fin-tip vortical flow pattern of the X-43 at an angle of
attack of 15 degrees is investigated using a yarn tuft on a probe.
Please note the three photographs of the X-43 are of the yarn
filament as it rotates in the clockwise direction as viewed from
the front of the wind tunnel model.
FILAMENT FLOW VISUALIZATION
Figure-6,
Sprint ABM Base Flow on Rear-Sting Mount
Figure-7,
Sprint ABM Base Flow Circulation on Rear-Sting Mount
Figure-8,
Sprint ABM Base Flow Circulation on Vertical-Sting Mount
Figure-9,
X-43 Fin-Tip Vortical Circulation (1)
Figure-10,
X-43 Fin-Tip Vortical Circulation (2)
Figure-11,
X-43 Fin-Tip Vortical Circulation (3) |
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Smoke
Flow
Field Visualization
BACK
A smoke generator and blower are
used to test models outside the AeroRocket wind tunnel. Larger
models may be accommodated because the smoke visualization tests are
conducted without the constraints of the relatively small dimensions
of the AeroRocket wind tunnel's test section. Figure-12 and
Figure-13 display the vortex flow pattern from the leading edge of a
60 degree triangular wing.
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SMOKE FLOW VISUALIZATION
Figure-12, 60 degree Wing Tip Vortex Flow
Figure-13, Filmstrip of Smoke Visualization Testing
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PAST WORKS
Figure-14, AeroRocket
Wind Tunnel with X-43 mounted on vertical sting
Figure-15, Wind
tunnel models from past and present work
Figure-16, X-43
mounted on vertical sting
Figure-17, X-20 tested to measure lift slope, CNa
Figure-18, X-20 CL verses angle
of attack, a
Figure-19, X-20 tested to determine pitch center of pressure (Xcp)
BACK |
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