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External Balance
View of external balance beneath test
section.
The external balance on the first floor of
the building below the test section is a Dynametrics
Incorporated, six component, pyramidal, virtual center,
mechanical balance which resolves the aerodynamic forces
acting on the test model into three orthogonal forces and
their associated moments. These components are measured
along and about a system of wind oriented axes having their
origin at the balance resolving center, which corresponds to
the geometric center of the test section. In addition, the
balance also supports the model and positions its pitch and
yaw attitudes. Force, moment and attitude measurements are
transmitted to the control console via optical encoders that
transmit digital signals directly to the digital data
acquisition and processing system.
Many different mounting systems are used to
mount models to the external balance. Information concerning
these mounting systems can be found under
Model Mounting. Load resolution,
attitude ranges, and accuracy for the external balance are
available in the Specifications table.
Balance beam used to measure
one of the six load components.
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Internal Balance
Six component strain gauge internal balances
are frequently used for measuring force and moment loads.
Due to the wide range of sizes and load capacities of
internal balances, they are normally supplied by the
customer with either the customer or the wind tunnel
providing the required signal conditioning equipment and
balance calibration. However, a 1.75 in. diameter NASA 711-A
balance and two 1.25 in. diameter Task Corporation internal
balances, the Mark X and Mark XIII are available for
customer use. More specific details are available by
clicking on the information of interest:
Sting-mounted internal balance to be placed
inside model.
Internal balance attached to a sting mount in
the test section.
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Auxiliary Air System
Chicago pneumatic TCB-4 compressors.
An auxiliary compressed air system provides
the ability to test turboprop, jet powered models, and
special nozzle testing, as well as a source for other
applications requiring high pressure air. Two four-stage
Chicago Pneumatic TCB-4 compressors that were formerly
installed in a NASA rocket vehicle test facility are now
located in a building adjacent to the wind tunnel. These
units are powered by two 150 horsepower electric motor
drives and are rated to provide 250 standard cubic feet per
minute air flow at 3,500 psi. Air is pumped through
intercoolers, filters, and regulators to an A. D. Smith high
pressure storage tank, which has an internal volume of 822
cubic feet and operating at pressures up to 2,300 psi.
Air is piped from the compressor-tank system
into the wind tunnel balance room at pressures up to 750
psig. Air flows are accurately controlled and measured using
a Daniel flow meter system. A control panel and digital
readouts facilitate control room adjustments to the test
section during testing, as digital inputs provide rapid data
acquisition and processing. A high pressure Chromalox heater
using 80 KW electrical service and employing solid state
controls, allows heating of air to models on a closed-loop
basis for temperatures up to 750° F. These conditions are
also set and monitored from the control room.
A bridge system of opposing tanks and
flexible hoses is used to bring air through the external
balance into the model without drag or thrust tare effects.
Final filtering of the air before entering the model is
provided by 10 microns of stainless steel filters.
Typical wind tunnel run schedules can
maintain airflow rates up to 2.0 pounds per second. At
higher rates, some allowances for pumping may be required in
addition to normal periods of pumping during model changes.
This clean, heated air is ideal for driving air turbine
units of turboprop models or for producing jet thrust
simulations. It can also be employed in research modes for
thrust vectoring, aerodynamic camber changing, and for
boundary layer control.
Thruster on missile using compressed air in
test section.
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Data Acquisition
System
A stand alone data acquisition and analysis
system is used at the wind tunnel. This system includes a
network of IBM compatible personal computers and a
Hewlett-Packard data acquisition system with a standard
IEEE-488 communication interface. The HP system acquires all
digital and analog data and sends it to the PC network for
processing. The network allows the customer to have access
to their own local processing unit(s), while maintaining
access to system-wide files and peripheral drives and remote
access via the internet. Data is regularly reduced to
coefficient form for engineering applications within one or
two minutes after completion of a standard test run. Plots
of report quality and digital tabular data are immediately
available.
This combination of equipment and appropriate
software provides a capability for the acquisition and
analysis of force and moment data from either internal or
external balance systems or a combination thereof. A number
of programs are available to incorporate all of the
customary corrections applied to aircraft model testing
including tare, alignment, and interference data.
Software also exists for pressure tests
involving several hundred pressure measurements. In addition
to obtaining raw data, pressures are readily computed into
coefficient form for each use and reference. The
presentation of data in coefficient form resolved to the
appropriate axis/axes is immediately available. In some
cases, customers will have special requirements concerning
data collection or reduction; software can usually be
tailored to meet these individual needs.
Data Reduction
The data reduction process reduces data to
coefficient form and applies corrections to remove unwanted
effects due to test conditions and techniques. Data are
first reduced to coefficient form by dividing by dynamic
pressure and a characteristic area for forces and area and
length for moments. Data are collected with all moments
resolved about the center of the test section, known as the
balance center. The moments can be transferred to a model
center, a point usually specified by the Principal
Investigator. The reduced data can be readily transferred to
two other axis systems - stability axis and body axis. Force
and moment data can then be presented as data tables:
Wind axis, Balance center
Wind axis, Model center
Stability axis, Model center
Body axis, Model center
Corrections
Before data can be presented in final form, a
number of corrections must be made. These include
corrections for weight tare, buoyancy, blockage, strut tare,
interference, alignment and wall presence.
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Weight Tare Corrections
Weight tare corrections simply remove the effects of
model weight and its distribution from the data. A
wind-off run is made to determine weight effects and
they are subtracted from the raw data before any
corrections are made.
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Buoyancy Corrections
Buoyancy corrections account for the thrust effects
caused by a static pressure gradient within the test
section.
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Blockage Corrections
When correcting data for blockage, two blockage effects
on dynamic pressures must be taken into account. The
first of these is due to the effect of the solid model
and support system, and the second is due to the model
wake which effectively adds to the blockage volume.
These volumes reduce the effective cross-sectional test
section area around the model. Reducing the
cross-sectional area around the model increases local
flow velocities and changes the pressure distribution in
the vicinity of the model.
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Strut Tare and Interference
Corrections
Strut tare and interference corrections are an
approximate means of eliminating the effects of the
model support struts. Strut tares are forces and moments
resulting from direct exposure of the support system to
the flow while interference is a result of the
disturbance of the pressure fields on the model caused
by the presence of the support system.
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Alignment Correction
The alignment correction provides for a means of
accounting for any flow inclination or curvature of flow
in the test section. In addition, the alignment
correction is used to insure that the lift and drag
directions on the balance system are mutually
perpendicular.
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Wall Correction
Wall corrections account for an alteration to normal
flow patterns caused by the confinement of the flow by
the walls of the test section. The drag coefficient,CD,
pitching moment coefficient, Cpm, and
angle of attack are all corrected for wall effects.
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Electric Drive
System
The motor generator set.
The electrical drive system is particularly
well-suited to propeller or rotor research. Both sting and
strut mounts can be accommodated. The electric drive system
is capable of powering three phase AC motors as large as 75
horsepower. A motor generator set consisting of a 150
horsepower DC motor and a variable frequency AC generator
provide a potential from 0 to 600 volts AC and a maximum
amperage of 180 amps AC. The generator output frequency is
variable from 0 to 450 hertz. Model drive motors are
available for propeller or rotor system testing.
A solid state control system allows full
range operation of the motor generator set from the control
room, and is able to maintain a given generator output
voltage/frequency ratio over the operating frequency or
speed range of the generator. The voltage/frequency ratio is
adjustable from 0.8 E/F to 1.2 E/F. The remote control
system uses feedback to maintain speeds.
Rotor using the electric drive system.
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Flow Visualization
Several methods of flow visualization are
employed ranging from simple yarn tufts to fluorescent oil
dyes. Flow visualization using tufts
include many small tufts attached to the model or a tuft
wand employing one long strand of yarn.
Smoke wands are available for examining local flow
regions. Oil flows using sublimating techniques (often a
mixture of tempera paint and
kerosene) allow viewing of surface flow conditions.
Fluorescent oils and
ultraviolet lights have been used successfully for studying
laminar flow transition and other surface phenomena.
Tufts
Two models with tufts during a run.
Smoke
Flow visualization using a smoke wand.
Tempera and
Kerosene
Tempera paint on a model after sublimation.
Fluorescent Oil
Fluorescent oil on a model without
ultraviolet light.
Fluorescent oil on a model with ultraviolet
light.
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High Attitude Robotic Sting
(HARS)
Click here for more
information on HARS
HARS is a sting mount that
allows changes in the
yaw, pitch, and roll angles for the
model during actual testing.
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Pressure Measurement
Electronic pressure instrumentation
systems are available to measure up to 240 pressures
simultaneously. Several types of pressure data can be
obtained through the use of probes mounted to the
traversing mechanism.
Wake rakes are also available to survey the wake behind a
model. A variety of small boundary layer rakes are available
for surveying floor or wall boundary layer conditions.
A seven hole probe mounted to the traversing
mechanism.
The wake rake behind a propeller.
Traversing
Mechanism
The traversing mechanism is a motorized arm
which is able to move a probe into various locations within
the tunnel. The traversing mechanism arm is located near the
exit of the test section and can be remotely positioned
vertically and laterally. Several types of probes can be
mounted to the traversing mechanism including a pitot-static
probe, a seven-hole probe, or a hot wire anemometer. Probes
can be positioned behind a model to determine the wake
properties or the drag through integration. For the testing
of earth-based structures the traversing mechanism is
employed in establishing the correct flow gradient. A hot
wire anemometer is often used to determine the turbulence
intensity within the tunnel.
Pitot tubes being mounted on the traversing
mechanism
behind a model before a run.
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Tare and Interference (Image System)
Dummy struts attached to model for tare and
interference runs.
Fairings and dummy struts are available to
all tare and interference measurements. The image
system simplifies assessment of mounting system effect so
that they can be subtracted from the test data.
Because there is no turntable in the ceiling, the image
system for the three strut mounting system cannot implement
changes in yaw angle.
Inverted model using a single strut mount
with dummy strut attached for tare and interference runs.
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Wind Gradient Tailoring
For wind engineering tests, the test section
wind gradient can be tailored to produce velocity and
turbulence profiles approximating the geotropic wind. Tests
involving offshore structures often use a boundary layer
fence to fix a certain velocity profile. The traversing
mechanism is used to determine the velocity profiles using a
pitot-static probe.
A boundary layer fence setup is placed
upstream of the model.
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