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110229

Wing Twist Measurements Transonic Facility

A. W. Burner, R. A. Wahls, and W. K. Goad Langley Research Center, Hampton, Virginia

February

1996

National Aeronautics and Space Administration Langley Research Center Hampton, Virginia 23681-0001

at the National

WING

TWIST

MEASUREMENTS

NATIONAL

TRANSONIC

AT THE

FACILITY

A.W. Burner R.A. Wahls W. K. Goad ABSTRACT A technique

for measuring

described.

of two dimensional The wing

currently

upon

for selection

transformation

Examples

run-to-run

of wing twist

The advantages

of this particular

in cryogenic

third dimensional

coordinate. and wind-off

wind-on

and limitations

mode

is

of the technique

are discussed.

repeatability

nitrogen

Facility

determination

between technique

and test-to-test

Transonic

photogrammetric

with a fixed (and known)

from a conformal

to illustrate

in use at the National

a single camera

in the plane of rotation.

as well as the rationale are presented

is based

coordinates

twist is found

2-D coordinates

mode.

wing twist

The technique

Examples

of the technique

in air

are also presented.

INTRODUCTION Model

deformation

wings

and control

design results

geometry can cause differences between the acquired and expected wind tunnel if the expected results are based upon rigid body assumptions. Differences can also

occur body

between

may be defined surfaces)

acquired

assumptions.

process. years

under

as the change aerodynamic

wind tunnel

These

The measurement

and was identified

data

differences of model as especially

as early as the mid 1970's.

More

measuring

distributions

surface

the importance

pressure

of model

loads,

because

model

has moved

attitude

of requirements and changed

and computational

measurements wind-on

of different

types

photogrammetric incorporation required. options tunnels.

used to measure

video

systems

In some

limited

to the requirement

wind tunnel

(NTF)

for has renewed

aerodynamic

between

which

the

tests (ref.

model

systems system

low contrast

deformation.

because

measurement video

to a number Video

not suitable

of the user and limited systems imagers

imaging,

in low cost

techniques

are generally

targets,

1) 2-dimensional

are used in

low cost electronic

photogrammetric

for custom

to be

imagers

with improvements

including

but these

continues

electronic

of relatively

data acquisition

view ports,

deformation

coupled

of video

problems,

are available,

into a wind tunnel

all contribute

market,

the application

of measurement

In addition,

model

ago, but today

The rapid development

have enabled

rigid

for over 20 Facility

(PSP)

under images

upon

shape.

by the consumer

computing

of interest

paints

in the

design

in the potential

sensitive

and deformation

based

Transonic

interest

to ratio wind-offand

as was the case 20 years

driven

widespread with pressure

technique

of film cameras.

for the National

the

This change

the aircraft

has thus been

desirable

recently,

(particularly

predictions

and degrade

deformation

photogrammetry largely

of a model

load in a wind tunnel.

can lengthen

The fundamental place

in shape

for

interaction illumination for many wind

have been

replacedwith instead

1-dimensional

of passive

active

sources

installation

linear arrays

rub-on or painted

and active infrared

targets.

This combination

more readily enables automation

of active

targets

is acceptable.

made

by Northern

the Optotrak

® system

determining

3-dimensional

locations

sources

are used as targets

of 1-dimensional

of the measurement,

A good Digital

example

arrays

provided

and

the

of this latter type of system

Inc. of Canada.

This system

at up to 600 Hz (for a limited number

is

is capable

of

of markers).

Well-controlled laboratory static calibration tests at Northern Digital have shown that the rms error in angle of attack (AOA) measurements can approach several arcseconds over a range of 180 ° (ref. 2). One disadvantage emitting

diodes

(LED's)

then be activated

in sequence

with the Optotrak

® measurement

which may make

it difficult

MODEL Transonic

pressurized

wind

configuration.

tunnel

(ref.

The wind

140 ° F, normally

provide

length of 9.75 wide range effects

inches.

at constant Reynolds

dynamic

pressure

pressure

environment

The constraints

development,

testing

for a

number effects

effects

imposed Even

number

Reynolds

aeroelastic

number

of temperatures

for the facility.

1984, instrumentation

loads

encountered

for an instrumentation

was identified

early.

Several

modulated

at

at constant

by operation

in a high

have had a significant though

the facility

improvement,

film cameras

techniques

or image

dissector

selected

for initial tests at the NTF

cameras

at the NASA

thought

that there

Langley

would

to measure

model

were considered moir6 contouring

model

has been

and

geometry

cameras. Foot

tunnel

Transonic

Pressure

implementation

Two parallel

successful problems

photogrammetric

approach tunnel

including using was

tests with film

(ref. 6).

It was also

with photogrammetry

efforts

a

heterodyne

techniques

Tunnel

a

at test conditions

(ref. 4), scanning

The photogrammetric

due to earlier

deformations,

for the measurement

(ref. 5), and photogrammetric

largely

Eight

be fewer

techniques.

in the NTF can cause

system

techniques

laser technique,

holographic

than with the other

2

number.

range for the

Reynolds

model

and Math

to

inside the

1.0, based on a chord

are possible:

pressure;

as a coolant,

insulation

and cold test gas can

at Mach full-scale

up to T =

are still underway.

requirement

interferometry,

number;

development

high aerodynamic

microwave

and dynamic

mode

total pressure

of pressure

afford

has a slotted-wall

temperature

using liquid nitrogen

of investigations

over such a wide range

since August

optimization

either

and Reynolds

continuous-flow

-250 ° F. Thermal

of 120,000,000

and Math

on instrumentation

operational

Since

number

number

closed-circuit,

The design

characteristics

Three types

Math

BACKGROUND

The combination

number

These

of aircraft.

constant

impact

Reynolds

problem sensors

FACILITY

mode,

consumption.

15 psia to 130 psia.

a maximum

Another

large size of the image

in an elevated

range down to about energy

must

markers

The 8.2 x 8.2 x 25-fl long test section

using air, and in a cryogenic

shell minimizes

NTF is from

TRANSONIC

is a fan-driven,

3).

light

These

for use in some wind tunnels.

tunnel can operate

obtain a test temperature pressure

is the relatively

DEFORMATION

Facility

is that infrared

real time maker discrimination.

system

to adapt

® system

in the model as markers.

to provide

NATIONAL

The National

of the Optotrak

must be placed

were initiated

for

the NTF.

ITT was selected

to develop

the Stereo

Electro-Optical

Tracking

System

(SETS) for the NTF (ref. 7). This rather large developmental effort to provide an automated model deformation measurement system for the NTF was based upon image dissector cameras which at the time were state of the art. A second, smaller, in-house effort concentrated

on film photogrammetry

tunnel experience.

However,

in order to more rapidly

in just a few years, the image

gain initial lab and

dissector

technology

obsolete due to the rapid advancement in solid state image sensors. made to abandon the SETS and concentrate on the in-house effort.

became

The decision was then For initial tests at the

NTF film cameras were replaced with high resolution, 875 scan line, video tube cameras because of inaccessibility of cameras which must be housed within the cryogenic, high pressure

plenum.

Additional

distortions

introduced

photogrammetric measurement were investigated distortions were evaluated (ref. 8). Initial model video

photogrammetric

technique

in both air and cryogenic

the NTF became

operational

Solid state video vibration-induced

cameras were chosen electronic distortion

severely

degraded

addition, eliminate

into the

modes

were made soon after

(ref. 9).

the video

for subsequent testing at the NTF since tunnel associated with the camera tube construction

data at low temperature

and high pressure

(ref. 10).

In

a personal computer (PC) controlled image acquisition system was employed the manual measurement of targets associated with the previous tube camera

technique. establish

by video tube cameras

and techniques to correct for these deformation tests with the tube-based

Camera

calibration,

the uncertainty

data reduction

of this technique

procedures,

(denoted

and laboratory

the Video

Model

to

tests to

Deformation

or

VMD system) were reported in reference 10 where the accuracy for measuring wing deflection was shown to be about 0.005 inch rms under best case wind-off ambient conditions

over a 26.5 inch semi-span

measurement conditions,

errors

as large

test wing.

It was also determined

as 0.2 ° might be experienced

even with suitable

least-squares

fits to a large

under number

In order to improve the accuracy of the VMD system, laboratory on the calibration and characterization of industrial and scientific cameras

(refs.

techniques

11 and 12).

and cameras,

These

while

studies

helpful

indicated

under

ambient

that state wind-off

that wing twist

best case wind-off of targets. studies were conducted solid state area array

of the art calibration conditions,

were

limiting factors for VMD system performance during wind-on test conditions The harsh environmental conditions at the NTF, which can severely degrade thought

to be the limiting

factors

since temperature-induced

contractions

not

at the NTF. imagery, are

from ambient

to

cryogenic conditions, coupled with the high refractive index in the test section, can lead to bias errors in the photogrammetric measurement which are much larger than the precision error. errors

Attempts to develop suitable correction techniques to reduce or eliminate these bias were unsuccessful. The technique currently used at the NTF to measure wing twist,

however, rationale

has the potential for development

to partially

compensate

of this technique

follows.

for some

of the bias errors.

The

RATIONALE In discussions people

about

involved

aerodynamic

model

FOR

WING

deformation

in aerodynamic

measurement

testing,

load is considered

TWIST

TECHNIQUE requirements

the determination

to be the primary

among a number

of the induced

concern,

of

wing twist under

with wing deflection

(bending)

being of secondary importance. Laboratory investigations, simple error analyses, and preliminary wind tunnel tests at the NTF of wind-on and wind-off conditions indicated flow-induced

wing twist

was decided

to investigate

technique

measurement

in order to provide

The resolution

acceptable

of photogrammetric

the field-of-view.

Thus

uncertainty

ways to increase

was larger than desired.

the resolution

wing twist

generally

to increase

Therefore

it

of the photogrammetric

uncertainties.

measurements

it is possible

that

resolution

is inversely

proportional

to

at the expense

of limited

field-

of-view by using longer focal length lenses to zoom in on the outboard portion of the wing near the tip. However, once this is done the fuselage is no longer in the field of view to serve

as a reference

deflection.

in order

Thus,

semispan

without

locations

to remove

fuselage

will contain

bending.

If wing

a second

camera will be required

bending

Photogrammetric

deflection

is desired,

coordinates

component

data, deflection

this sting deflection calculated

values

are based upon

to the 2-D image

from the wing

measurements

component

to view the fuselage

data reductions

the 3-D object

the sting deflection

at various

as well as the wing

for sting deflection

must be used

in order to measure

sting deflection.

the collinearity

equations

plane coordinates.

which relate

For high accuracy

static

measurements the data reduction procedure of choice in photogrammetry is bundle adjustment with self calibration (ref. 13). In this procedure a single camera is moved number

of locations

of high contrast the test field.

(usually

targets,

8 or more) at which photographs

properly

The 3-dimensional

parameters

associated

adjustment

with the 2-dimensional

unknowns,

and photogrammetric

suitable

for wind tunnel port locations

limit the practicality calibration

disks,

of the targets

along

are computed

image

use because

of multi-camera

applications.

can still be used for pre-test data reductions installation

before

determined

by a space

plane coordinates dimensional

resection

In multi-camera simultaneously

intersection,

photogrammetric optimize

nature

widely

method. of targets

of tests.

or simply

solution

restricted

high contrast

targets

Even so, the bundle method camera

calibrations angles

resection,

For

cameras

of the camera

image

locations

equations

triangulation,

of wing

are

fief.

given

13).

sometimes

twist it is often difficult

for two or more cameras

are

are

the 2-dimensional

of the collinearity

by multi-camera

with self

(ref. 12).

is inappropriate,

is not

intersection.

measurements

the lighting

this method

in the field of view with known

are then found

for the

In addition,

distributed

and pointing

In space

throughout

least-squares

however,

where bundle adjustment

least-squares

coordinates

to as space

laboratory

and the locations

of a number

used as input to a nonlinear

to utilize

distributed

initial guesses

Generally,

to a

of a large number

with calibration

by a nonlinear

plane coordinates,

of the dynamic

and the inability

calibrated

referred

retroreflective

scale as inputs.

photogrammetric

Three

illuminated coordinates

with the camera

viewing

are taken

or

to

the wind tunnel

lighting

restrictions

surface typical photogrammetric point

and the difficult

task of illuminating

for the NTF, at various AOA measurements, computation

is required

along

with precise

targets

on a mirror-like

wing

setpoints. In addition, for multi-camera ofcentroids from each image for each data

time synchronization

in order

to match

corresponding

fields from the cameras. Target identification and matching between the cameras is also required. However, if a single camera photogrammetric technique can be used, which is possible lessened

whenever one of the three target coordinates is known, then these problems considerably. A single camera technique may also be more amenable to

automation.

The major

of the coordinates

limitation

be known.

is known.

than one camera

is required

A single camera

photogrammetric

in the spanwise

in the flow direction coordinates effect

are recorded coordinate

solution

direction

a one second

means

are computed.

Once X, Z coordinates conditions

dimensional

data recording

estimates

transformations of rotation of rotation. A major change

optical

instrumentation

range.

In addition

in temperature

introduce Since

on a pure rotation

been fully successful,

in the wing twist

difference

in wing

pitch angle between

angle at wind-on

and wind-offtest

plane

are a good

dynamic

yaw, since over linear even

angle on the wing

desirable

center

from

of a conformal

location. properties.

of rotation

two-

Least-squares

of the least-squares

The change along

with

reduction.

The

For example, will yield the angle

due simply to the offset

such as window

and non-uniform bias errors

which

to account

to use wind-offpolars

computation.

image

and linear motion.

problems

wall targets

it was decided

reference pitch

measurement

at using test section

fields

from the center

at the NTF has been the wide operational

to operational

can lead to uniform

photogrammetric

attempts

a displaced

in Z coordinates

problem

the

10 video

in X and Z are determined

the angular

about

To reduce

by means

has several

If the Y

exists.

at each semispan

as a byproduct

also separates

only the X coordinate

are very nearly

in pitch

is

the Y

Y to vary,

without

are determined

displacements

since motion

vary with pitch.

plane coordinates

equations

Z coordinates

transformation

on/y, not the change

temperature

conditions

of)(,

sweeps

from which

coordinates

the differences

for each variable

transformation

on

then more

pitch sweeps

solution causes

interval,

object-image

and the corresponding

of precision

conformal

plane

pitch

direction

which

The mean image image

to wind-on

of the conformal

in pitch angle

will then depend

does not change;

in the vertical

at the NTF,

are computed,

transformation

computation

that one

are unknown,

wind-off

then a single camera

typical

to the expected

during

During

of each target

are known

yaw motion

small intervals of Y the collinearity though they are globally nonlinear.

wind-off

is possible

at least for wind-of

over

approximation

solution

In case all coordinates

and the Z coordinate

of all targets

of dynamic

is the requirement

to find a solution.

to the pitch plane,

coordinate

solution

The error in a single camera

how well the third coordinate

confined

of a single camera

are

Wind-on wind-off points

frosting,

contractions.

These

are very difficult

and wind-on

conditions.

had not

condition

wing twist is then computed

may not be equal,

tend to

to quantify.

for these bias errors at each tunnel

the large

as

as the

Since the model

it is necessary

to

subtract out the difference as determined by the primary model pitch angle measurement system, to obtain flow-induced wing twist. Since the wind-on and wind-off data are computed from images at nearly the same temperature and pressure, the effects of these variables are lessened. If the same model pitch angle is used for the wind-on and wind-off images, then the images from each will be at nearly the same image plane location neglecting sting bending. Thus any potential optical and sensor distortion will be comparable and tend to cancel. This error cancellation is only approximated at wind-on test conditions due to sting deflection and wind-on flow structure such as shock waves or boundary layers which can cause refractive index variations. EXPERIMENTAL

PROCEDURE

AND DATA REDUCTION

The optical technique used to determine wing twist data presented herein is based upon the recording and analysis of digitized video images. A video signal from a standard RS170 solid state camera with 752 horizontal by 240 vertical pixels per field is routed to a frame grabber controlled by a 386-33 MI-lz PC which records ten video fields in one second into the frame grabber memory. Ten fields are recorded in order to reduce the effects of dynamic yaw as discussed later. Only ten fields are presently recorded, instead of the desired 60 fields in one second, in order to reduce data storage time and volume. The contents of the grabber memory are stored on a hard disc with the current point number as file name. Once the hard disc is full, currently 144 data points, the digital images are transferred to an optical disc for archiving. The charge-coupled device (CCD) video camera used for wing twist measurements at the NTF has an adjustable field integration time in order to reduce the effects of dynamics on image recording. An 11.5 to 69 mm focal length remote zoom lens is currently used for imaging. Considerations when calibrating zoom lenses for wind tunnel use are discussed in reference 14. The CCD camera is mounted in a protective housing in the test section sidewall. The camera looks over the fuselage at one of the wings of the model (fig. 1). The camera is rotated 90 ° so that the horizontal X axis is vertical on the image plane in order to provide additional viewing flexibility. In addition, the 90 ° rotation more nearly matches the number of pixels vertically and horizontally across a target image since perspective causes the images to be longer in the X (streamwise) direction. The protective housing is equipped with insulation and sheath heaters to maintain camera temperature. The housing is pressure rated to greater than 9 atm. In order to prevent frost, air heated by an inline heater flows to a purge ring with a number of holes to direct the heated air over the inside surface of the one inch thick fused silica window viewport. A purge air vent to atmosphere maintains the camera housing pressure at approximately 1 atm. Circular targets, with an approximate diameter of 5 mm, are applied to the wing surface (fig. 2) with a Sharpie ®marking pen. it should be pointed out that such targets have not been acceptable for all test conditions as discussed later. A template is used to place targets in several rows in the streamwise direction. For the data reported here the target

rows

were located

Initial Xand

at normalized

Y coordinates

reference

locations

drawings

of the wing.

Data

recording

is taken.

with a script

grabber

menu

driven

code

was written

language. control

versions,

and configure the trial code

NASA

the data recording

Langley

on high-contrast

Transonic

In order

Tunnel

during

The initial

data

grabber

frame

acquisition

several

are used to

grabber

images

both wind-off

during

the

in the C programming

first saving the digital

targets

Dynamics

trigger as

and control

to reduce

for the frame

as well to access

without

upon

configure

for initial developments.

4.5.

functions

tap and other

acquisition

of code have been written

C language

can be computed

file for automatic

and 0.922.

from cross-sectional

files to automatically

was convenient

0.778,

from pressure

are estimated

in QuickBASIC

time, trial versions

In these

that centroids using

program

equal to 0.635,

are determined

The Z coordinates

The use of script

data reduction and reduction

locations

of the targets

on the wing.

is programmed

each data point frame

semispan

recent

video

memory

so

as DOS

files.

Tests

and wind-on

tests

at the

semispan

tests were

encouraging. The initial pre-test

calibration

procedure

camera

necessary

for conversion

parameters

coordinates. through

Techniques

for determining

14. The need for extensive

calibration

using the model

temperature

and pressure

The pointing determined

angles

run.

wind-off

points

these parameters

camera

and rotation

determines

calibration

image

are discussed

is lessened

those

plane

in references

somewhat

11

at the tunnel

by on-line total

test conditions. of the camera

set of targets

into a single reference

run temperature

technique

pitch angle for wind-offreference

and location

A known

optical

from pixels to corrected

in the tunnel

at the start of the test by photogrammetric

reference rotation

for the video

angle.

for resection target

over the range

resection

a range

of the center

a wind-off

expected

are

a wind-off

by merging

on knowledge

step requires

of angles

system

during

are established

field based

The final calibration

and pressure

coordinate

during

pitch

of

of

sweep

at

the wind-on

testing. Image

plane coordinates

inverting

The background pixels which from

is found

is slightly

the ten recorded

the photogrammetric coordinates equations

are determined

the gray scale and subtracting

by gray level center-of-mass the background

from the maximum

larger

than the target

gray scale on the border images

fields are then transformed principal

are determined

calculation

plus a few additional

(ref.

11).

and corrected

from a single camera

solution

scales

of a window

The mean (including

point xp, Yv) to give units of length.

after

gray

correction

The X and Z

of the following

of

pixei coordinates

collinearity

for

x = - c [m. (x- Xo)+ ml_(r- to) + m_ (z- zOJ [m31

(X-

X¢)

t- m32 (Y - ]'Pc) + m33

(Z - Z¢)]

(1) y = - c [m21 (X- Xc) + mz_ (Y[m31 (XX¢) + m32 (Y-

Yc) + mz_ (Z - Zc)] Y_)

+ m33

(Z - Zc)]

where x andy are the corrected image plane coordinates, c is the principal distance (or camera constant) which will be slightly larger than the focal length, X, Y, and Z are the object space center,

coordinates

of the target,

and the m terms

are elements

mt]

K"

= cos _cos

Xc, Y¢, and Z_ are the coordinates of the following

rotation

of the perspective

matrix

m_z = sin co sin _ cos t¢ + cos 09 sin ?¢ m_a = -cos cosin _cos

to+ sin co sin I¢

m21 = cos _ sin tc

(2)

m22 =- sin co sin _ sin x + cos co cos x mz_ = cos co sin _ sin ?¢+ sin ca cos ?¢ m31

=

sin

m32 = -sin co cos m33

= COS CO COS #

The pointing

angles

respectively,

are defined

of the camera, as positive

co, 4, and _', which

positive end of their axes. The X and Z coordinates equations (1) for a single camera solution are given X = Xc + (Y-

rotate

about

if they are counterclockwise

the X, Y, and Z axes

when viewed

determined below

fi'om the

from the collinearity

Yc) (Q206 - aj o.O / (a,_ a3 - al f16)

(3)

z = zc - (x- xo) a,/a3 - (r- to) a2/a_

(4)

where

al

= x m31

+ c ml!

a2

= x m32

+ c ml2

as = x mz._ + c

(5)

m13

a4 = y m_t + c m2_ a5 = y m32 + c m_ a6 = y m33 + c m23

Expression

(4) above

that the horizontal the a3 term is nearly

is suitable

for use at the NTF where

X axis is vertical

on the image

plane.

the camera When

zero so that al, a2, and a3 in expression

a4, as, and a6 respectively.

is rotated

the camera

(4) should

90 ° so

is not rotated,

be replaced

with

Thewind-ondataat eachsemispan locationis thenmatchedwith a wind-off pointwith similarmodelpitchangleto determineangleanddisplacement andcorrespondingstandard deviations,usingthe following leastsquaresconformaltransformation X' = X cos 0- Z sin 0 + Tx (6) Z' =Xsin/9+ where

Zcos

Tx and Tz are the translation

coordinates

of the wind-off

for the conformal freedom.

terms

reference

transformation,

one degree

where

in the X and Z directions

point.

If a nonlinear

at least two targets

If a linear least-squares

angle directly,

O+ Tz

technique

least-squares

are required

is used to solve

a = cos 0 and b = sin 0, a minimum

of freedom

since scale is implicit

and X' and Z' are the method

is used

for one degree

for a and b rather of three

in the a and b terms

targets without

of

than the

is needed

for

the constraint

that a 2 plus b 2 be equal to one. In the linear least-squares solution, the standard deviation of the rotation angle is not computed directly, but is instead computed from the standard deviations of the a and b coefficients. However, given three or more targets, nearly identical

results

have been

obtained

with both nonlinear

and linear-least

squares

conformal

transformations. The model reference wing

pitch angle, point,

measured

ao_-, is added

angle at a particular

Ot_st, is then found

with an onboard

to the angle

semispan

by subtracting

0 found

location. the model

accelerometer above

of the wind-off

in (6) to yield the streamwise

The wing twist pitch

package,

due to aerodynamic

angle of the wind-on

point,

load,

ao,, or

(7)

Oiwist= 0 + Otoff- ao. WING Examples

TWIST

of repeated

uncertainty

EXAMPLES

measurements

is expressed

AND

ERROR

of wing twist

as the sum of systematic,

CONSIDERATIONS

are presented or bias error,

below.

The total

and precision,

or

repeatability error. Bias errors are generally very difficult to determine under flow conditions, whereas repeatability can be computed. In addition to laboratory and wind tunnel wind-off determinations of error, run-to-run and test-to-test repeatabilities can be used

to gauge

the adequacy

The uncertainty are unresolved. corresponds

of wing twist measurements

requirements It has been

for measurement of wing twist caused by aerodynamic loads suggested that the desired uncertainty for wing twist which

to an uncertainty

of 0.01 ° for the model

not 0.01%

In other

have

about

the same magnitude

pitch

angle.

Wing

twist

angles

which

words,

measurement

with flow.

an uncertainty effect

on drag

error can occur

are used in equations

of the order

pitch

angle is of the order

of 0.05 ° in wing

measurements

(5) to determine

twist is thought

as 0.01 ° uncertainty

due to errors in the camera

(2) through

of 0.05 °,

position

to

in model

and pointing

the X and Z coordinates.

Pre-test

calibration

lens distortion that errors

errors

or frame

can also contribute grabber

The Y coordinate, well-behaved

assumed

for ambient

measurements under

to be known

contributes

images

image coordinates error

angle,

However,

the expected

error.

solution,

reduces

this error

to the onboard

in the pixel coordinates

that the variations

are typically

of several

pixels.

ceiling

camera.

Note

Wing

transformation

bending

causes

the g coordinate

error in the computation tip deflection

of X and Z. Assuming

bending

would

inboard

of the tip would

experience

be approximately

swept

wings

the wing

rapidly.

locations

are not too far separated,

in bending

under

load.

wing.

to decrease

which

bending

at the tip.

example

would

causes

which

experience

a wing due to wing

The shit_ in Y for targets station

will

Note

that it is this

wing

twist for

produces

in the streamwise

a wing twist

at the tip separated

a bias

dependence, in Yvalue

and shifts in Y value.

fore and aft targets

For two targets

out in the

at the same semispan

fore and ait targets

For instance,

tip of the previous

30 ° swept-back

in both bending

between

cancel

0.

a 2nd order

Targets

by plots

of time which show

equal to 580 mm, the change

0.5 mm for targets

decrease

only slight differences

small difference

angle

of wing targets

of 20 mm, and semispan

also that any

of a pixel even for total

Thus as long as the image

(6) used to determine

in Y

to determine

This is verified

as a function

a fraction

conditions

is as large as :1:3

This variation

in Y by averaging.

of the targets

angle sensor

over one second

the errors in X and Z will also be similar and will tend to partially conformal

and

which typically

during wind-on

Lateral model motion 10 images

equal to within

is constant

inertial

in Y will be nearly the same for all wing targets.

of the variation

to the

in ao_iS much

technique,

Y is not constant

Recording

error

(7)

This is verified by independent

bending.

from a test section

incorrect

ao., will contribute

as well as by the single camera

and wing

to the precision

excursions

sweeps.

if, for instance,

Also note from equation

for the single camera

wind-offpitch

conditions.

mm based on video

are used.

generally

of 0.03 ° or less when compared

wind-offambient

remaining

ao_; and wind-on

8t.z,t, although

due to model yaw dynamics

mean

angle,

in the test section

has an rms error

twist error

corrections

in wind-offreference

error in the wing twist angle, smaller than the error in ao..

to wing

affinity

direction

of almost

at

-2 ° for a

by 50 mm the difference

in

bending would be 1.7 mm out of a total bending of 20 mm with a corresponding difference in the shift of the Yvalue for the two targets of 0.06 mm. A shift in Yvalue of 0.06 mm will cause typical

a difference

object

in angle

in image

distances

caused

scale between

at the NTF

(-1.8

by this small difference

m).

the fore and aft targets For the geometry

in scale will be negligible

of only 1.00003

for

used at the NTF the error compared

to other error

sources.

Wind-off: flow,

Data

compute

apparent

temperature runs differed

possible

error for the video

in Table 1. Two wind-offruns wing

twist,

which ideally

for both runs was nearly considerably

22 °. The mean

10

illustrating

are presented

error

wing twist technique on adjacent

be zero without

deviation

without

days were used to flow.

the same (-105 ° F), but the pressure

(20 and 94 psia).

and standard

should

taken

The for these two

The AOA range for both runs was from 0 ° to of the error for the 13 data points

are

presented

for the three normalized

semispan

stations

(Y/b2)

equal to 0.635,

0.778,

and

0.922. Only three wing targets were used at Y/b/2 = 0.922, whereas four wing targets were used at 0.778 and 0.635. The wing twist error for Y/b/2 = 0.922 is presented in figure 3. The "error bars" in the figure represent the computed standard deviation from the conformal least squares adjustment for each data point and should not be confused with the standard wind-off

deviations

denoted

run used as reference

by cr presented

in determining

wing

in Table 1. Note twist would

that normally

the

be taken within an hour at

the same total temperature and pressure as the wind-on run. Thus the data in Table 1 may be taken to be a conservative estimate of possible wind-off error at non-cryogenic conditions.

Table 1. Error

to-run

0.635

0.778

0.922

mean

-0.001

-0.018

-0.019

o-

0.018

0.026

0.016

in degrees

from the previous Run-to-run

Y/b/2

when

measuring

wing twist without

repeatability: number,

a wind-off

run

day as reference. The repeatabilities

on the same day of a low aspect-ratio

for Mach

flow using

M, of 0.3 and dynamic

of the video research

pressure,

wing twist

model

during

technique

from run-

air runs are presented

Q, of 153 psfin

Table

2 and M = 0.9

and Q = 965 psfin Table 3. The results for 4 runs with 30 data points per run are shown in Table 2. Results for 4 runs with 23 data points per run are shown in Table 3. Wing twist,

O_,t, was computed

equation repeat

(7).

set of four data

AOA sensor accelerometer

at normalized

The mean and maximum points

semispan

stations

of the computed

are denoted

0.635,

sample

0.778,

and 0.922

standard

deviation

as o',,_on and crm_ in the tables.

The

with of each

arcsector

(ARCSEC) is much less affected by test dynamics than the onboard so that o-m,_, for the ARCSEC variable may be taken as an indicator

of

model pitch angle variability for repeat points. These two tables show that the mean standard deviation in 0,_,,, for repeat points was less that 0.02 ° in air mode. In general standard

deviation

of the wing twist,

0, since any real variations are subtracted

out when

the onboard

angle

repeatability

versus

worse

repeatability

the other presented symbol

O_,_t, is less than the standard

in angle-of-attack is computed.

Otwist

of attack

for aon or ao//will

aon are presented at higher

two semispan in figures size) represent

settings

in figures

aon, especially

stations

behaved

6 and 7 where

between

However,

repeat

to the 0,,,,_, value. Mach

number

The corresponding

bars (which

plus and minus one standard

present

are plotted

deviation

in 0

and variability

4 and 5 for Y/b/2 = 0.922

at the higher

of the angle,

points

note that any error

be added

similarly.

the error

deviation

the

in

Plots of the which

and Q. wing

if greater

show Data

twist

for

plots are

than the

of the four repeats

at each

_on-

11

Y/b/2

--

0.635

0.778

...........A_cs_ aato,., a0_,_ ....A0

a0_,_

0.922 A0

a0_,_,t

aO

cr,,,_,

0.010

0.011

0.008

0.009

0.007

0.012

0.006

0.018

or,,,,=

0.015

0.019

0.019

0.018

0.024

0.029

0.017

0.018

Table 2.

Run-to-run

repeatability

in degrees

for four repeat

air runs at M = 0.3 and Q =

153 psf. Y/b/2

=

0.635

0.778

0.922

.......................... _c_c........4._., ........... Aq._,:::. ............... ..4..q. ........... ..a..q._:_ ............... A.q. ........ .....4..q._,.,.:. ............. ..4..q. .......... o',_o,

0.006

0.011

0.016

0.016

0.013

0.015

0.014

0.016

o-,,_

0.012

0.015

0.029

0.032

0.025

0.033

0.037

0.051

Table 3. Run-to-run 965 ps£ Upright

repeatability

and inverted

angularity

by comparison

on the underside

Inverted

reference

due to the inversion.

could

an error check

the wing twist,

between

12

A wind-off

inverted for change

0.3 and Q = 153 psfare

linear interpolation

(extrapolation

in angle signs presented

nearest

0.778

0.922

mean

-0.045

-0.018

-0.013

o"

0.035

0.023

0.032

and standard deviation,

or, in degrees

for the differences

Y/b/2

0.635

0.778

0.922

mean

-0.010

-0.002

0.038

0.030

0.051

0.042

deviation,

o;, in degrees

air runs at M = 0.9 and Q = 965 psfat

semispan

for the differences various

semispan

in

in Table 5. coincide was used

data points.

pitch range of-3 ° to 5 ° were used in the computations

0.635

and standard

twist

run was used as

for end points)

of the pitch angle between

air runs at M = 0.3 and Q = 153 psf at various

5. Mean

and inverted

with flow.

Y/'b/'2

cr

Table

When

to upright runs at the

0.9 and Q = 965 psf are presented

twist values at the midpoint

Mean

and inverted

wing twist.

pitch angle at which the data were taken did not necessarily

Nine data points over a model Tables 4 and 5.

Table 4.

be made to compare

Data from runs made atM=

upright and inverted,

to determine

flow

air runs targets were placed

to determine

making proper allowance

Table 4 and data from runs made atM= Since the model

For two inverted

to determine

180 ° these targets were then in the field of view of the wing

to provide

to compute

air runs at M = 0.9 and Q =

runs are conducted

wing normally viewed

so that wing twist measurements

same conditions

for four repeat

model

to upright runs.

of the opposite

the model was inverted camera

runs:

in degrees

between locations,

between locations,

for

upright Y/b/2.

upright Y/b/'2.

Test-to-testrepeatability: Comparisonsof repeatrunsfrom two five months are presented

in Table 6. Linear interpolation

model pitch angle setpoint standard

deviation,

Mach number, comparisons

varied from

are presented

different

symbols.

pressure.

18 to 26.

are presented

pressures

number and total pressure

M

Q

example

of wing twist, in figure

0.635

,

mean

in

The mean and

of semispan

a plot comparing

12 for Y/b/2 = 0.922. as computed

The error

from the least squares

0.922

.... .a ................ .mean. ....... .a ...........

mean

0.022 -0.013 0.030

a

......... .........

534

-0.004

0.059

0.013

0.071

0.011

0.049

0.3

805

0.022

0.093

0.017

0.109

0.026

0.087

0.9

967

0.001

0.027

-0.016

0.026

-0.005

0.047

in degrees

by

air runs made

dynamic pressure.

0.778

repeatability

location

The Mach

0.6

Table 6. Test-to-test

location,

11. Data from the two tests are represented

is presented

....... .......

for differences

Wing twist data for these runs at a semispan

were varied to give the desired

=

by over

above.

as a function

bars represent plus and minus one standard deviation conformal transformation (6). Y/b/2

separated

The number of data points used for these

in figures 8 through

As an additional

dynamic

to account

the tests was used as described

or, of the differences

and dynamic

of 0.922

at different

between

tests

during air mode.

Units for dynamic

pressure

Q are psf. Wing twist measurements at cryogenic conditions have been limited by frosting on the inside surface of the window of the camera protective housing. In the past, the amount window

frosting

gradually

increased

as the tunnel

causing a degradation in video imagery. run-to-run and test-to-test repeatability Preliminary figures

psf.

of 0.922.

data for two runs under

psf.

For figure

The error

computed

14 the Mach

conformal

are expected

operation

number

increase

number

efforts

are expected

viewing the quality

to include a detailed

predicted

and measured

wing twist,

system

for on-line

data recording

and reduction

grabber

data reduction

board nearly

with dual TMS320C40 simultaneous

the

in

to image

pressure

the standard (6).

conditions of video

Recent

was

was

1795

deviation

improvements

and flexibility data

pressure

during

under

at low temperatures.

WORK

between frame

are presented

was 0.6 and the dynamic

transformation

FUTURE Future

conditions

14 also represent

to improve

and should

cryogenic

was 0.9 and the dynamic

13 through

in the least squares

of

of time

runs at -152 ° F and -250 ° F at a semispan

13 the Mach

bars in figures

a facility upgrade cryogenic

For figure

cold for long periods

Additional wing twist data are needed before under cryogenic conditions can be evaluated.

13 and 14. The data are for nitrogen

location 2670

wing twist

remained

uncertainty

analysis,

and the development to provide

digital capture.

a nearly

signal processors In addition,

comparisons

of a measurement automated is expected innovations

system.

A

to enable are sought

13

to obtainhighcontrast,durablewing targetswhich requirements

at the NTF.

microinches,

resulting

also contain

additional

successfully

automate

needed

which

white

circles

applied

produced

some

uncertainty

about

would

of the targets

a measurable

These

are neither

prefer

on aerodynamic

adverse

effect.

In order

targets

should

contrast.

applied

to are

be fiat-

The nor durable.

the targets

performance;

Targets

may

targets

high contrast

not to apply

l0

surface

high contrast

or the opposite

pen black targets

finish

of a wall or ceiling.

finish requirements.

of the facility

the effects

do not indicate

of the wing

at the NTF

background

the surface

at the NTF can approach images

by reflections

the surface

customers

Thus

measurement

on a fiat-black

Sharpie ® marking

In addition, to date

artifacts

finish of models

like" surface.

the wing twist

do not exceed

solid-filled

currently

The surface

in a "mirror

do not exceed

due to

however,

results

by a chemical

etching

technique would be durable, but of low contrast. Gun bluing could also produce durable targets on at least some of the materials used for models at the NTF, but would still produce

low contrast

rusting

process".

targets

Ideas

and have the additional

for a suitable

target

problem

application

of being

method

a "controlled

at the NTF

are solicited.

SUMMARY The history National camera

of the development

Transonic

Facility

photogrammetric

been presented. given.

It has been speculated technique

at Mach

better

than 0.02 ° . Upright

were

numbers

and equations

of wing twist

for data with error

for wing

and inverted

pressures

runs agreed

than 0.03 ° were also noted.

at tunnel

along

total temperatures

of-152

twist

has

have been

considerations

were

to 0.01 ° model

1-sigma

error

in the

repeatabilities

up to 965 psfwere

Wing

single

reduction

twist equivalent

to within

for the

of wing twist

to be less than 0.03 °. Run-to-run

up to 0.9 and dynamic

of better

presented

for the current

of 0.05 °. The wind-offnon-cryogenic

was shown

mode

capability

on the measurement

that the uncertainty

may be of the order

repeatabilities

procedure

of the measurement

measurement

The rationale

with emphasis

The experimental

Examples

measurement

deformation

has been presented. technique

given.

pitch angle

of a model

in air

shown

to be

0.05 ° . Test-to-test measurement

° F and -250 ° F. Future

examples efforts

include

the use of frame grabbers with onboad digital signal processors and the development of high contrast targets suitable for cryogenic operation which do not exceed the surface finish requirements the measurement.

necessary

at the NTF.

These

efforts

should

aid in the automation

of

REFERENCES 1. Smith, D. G. and Crowder,

J. P.:

The Northern

Digital

OPTOTRAK

Measurement of Model Deflections. Presented at the 71 st Meeting Tunnel Association, Burbank, CA, April 3-4, 1989. 2.

Sherk,

T. and Crouch,

Attack Measurement. Canada, N213V2.

14

D. G.:

Available

Using

the OPTOTRAK®/2010

from Northern

Digital

® for Wind-On

of the Supersonic

System

Inc. 403 Albert

for Angle St., Waterloo,

of

3. Fuller,D. E.

and Williams,

Facility.

AIAA

86-0748-cp.

Beach,

FL, March

M. S.: AIAA

Testing

14th Aerodynamic

Techniques

with the National Testing

Transonic

Conference

West Palm

5-7, 1986.

4. Harding, K. and Harris, J.: Evaluation UDR-TR-81-111, U. of Dayton Research 5. Hildebrand,

Experience

B. P. and Doty,

Applicable

Within

of Moire technique for Wind Tunnel Metrology. Institute, Dayton, OH, Oct 1981.

J. L: A Study of Model the National

Transonic

Deformation

Facility.

Measurement

NASA

CR-165853,

Feb.

1982. 6. Brooks, the Wind

J. D. and Beamish, Tunnel

at Transonic

J. K.:

Measurement

Speeds

Using

of Model

Aeroelastic

Stereophotogrammetry.

Deformations

NASA

in

TP 1010,

Oct. 1977. 7. Hertel, R. J.: Stereo-Electro-Otical Deformation at the National Transonic 8. Burner, Video

A. W., Snow,

Cameras.

published

in proceedings A. W.,

Snow,

a Cryogenic

Wind

Tunnel

May 622.

1985; published

10. Burner, Model

A. W.,

Deformation

published

W. L., and Goad,

presented

9. Burner,

at Annual pp.

W. L., and Goad,

published

of ASPRS,

W. K.:

Using Photogrammetry.

in Instrumentation

Snow,

for the Measurement of Model CR-159146, Oct 1979.

W. K.: Close-Range

meeting

Photogrammetry

Washington,

Mar.

W. L., Goad, presented

'87 RECORD,

Photogrammetry

Deformation Industry

W. K., and Childers, at 12th ICIASF,

IEEE,

Meets

in SPIE Proceedings

Model

with

1985;

pp.

Measurements

at

San Diego,

CA,

31st ISA Symposium,

in the Aerospace

11. Burner, A. W., Snow, W. L., Shortis, Calibration and Characterization of Video Close-Range

System NASA

62-77.

System.

in ICIASF

Tracker Facility.

- vol 31 ISA pp.

B. A.: A Digital

Williamsburg,

615-

Video

VA, June

1987;

210-220.

M. R., and Goad, W. K.: Laboratory Cameras. presented at ISPRS Symposium:

Machine

Vision,

Zurich,

Switzerland,

Sept.

1990;

1395 pp. 664-671.

12. Shortis, M_ R., Burner, A. W., Snow, W. L., Goad, W. K.: Calibration tests of industrial and scientific CCD cameras. Invited paper presented at First Australian Photogrammetry 13. Karara, Photogrammetry

Conference, H. M., ed.:

Sydney,

Non-Topographic

and Remote

14. Burner,

A. W.:

Proceedings

Vol. 2598,

Nov.

Zoom

Sensing,

Lens

7-9, Paper

6, 11 pages,

Photogrammetry. 2nd edition,

Calibration

American

Society

for

1989.

for Wind

pp. 19 - 33, Videometrics

1991.

Tunnel

Measurements.

IV, Philadelphia,

SPIE

PA, Oct. 22-26,

1995.

15

f / f_./¢ d

f f

Figure

1. Wing twist camera

location

at the NTF.

0

Figure 2. Wing

16

twist camera view.

Flow axis is vertical.

2t

.1

Y/b/2

= 0.922

.05

{

c

_

-.05

-5

I

I

I

I

I

I

0

5

10

:15

20

25

ALPHA,

Figure 3. Error previous deviation

in measuring

day as reference.

wing twist

The "error

from the conformal

without

runs

flow using

a wind-offrun

bars" in the figure represent

least squares

.04

deg

adjustment

24,

30,

32,

from the

the computed

standard

for each data point.

34

.03 C_ (D _D

.02 03 0 0 0

tt)

.01 O0

0 000000

0

-5

0 0

,

,%e,%

0

5

dynamic

4. One sigma repeatability pressure

= 153 ps£

0

0

, 15

:10 ALPHA,

Figure

0

Oo

corresponding

0

I

I

20

25

deg

to Table

2 for air runs at Mach

= 0.3 and

Y/b/2 = 0.922.

17

runs

.O4

25,

2g,

.O3 0J "O 0

0

.O2

0

CO I.--4

0

F-

0

0

0

0

0

8

o

.01

O0

o

0

0

0

0 O 0 I

]

I

I

I

I

0

5

10

15

20

25

0

-5

ALPHA.

deg

Figure 5. One sigma repeatability corresponding to Table 3 for air runs at Mach = 0.9 and dynamic pressure = 965 psf. Y/b/2 = 0.922.

• 15

24,

30,

32,

34

Y/b/2

:

0.922

_0 D o

0

O0

o 0

):

c-

-.15

]:

o

-.3 -5

i 0

j 5

i t0 ALPHA,

i _5

t 20

i 25

deg

Figure 6. Mean wing twist corresponding to Table 2 for air runs at Mach = 0.3 and dynamic pressure ffi 153 psf. The standard deviations of the scatter are plotted as error bars if greater than the symbol size. Y/b/2 = 0.922.

18

_

29,

25,

31,

33

Y/b/2

= 0.922

O

.5

O 0

Q) "O

0 0

0

.d

0

0

(/3 .,-4 "a: 0

-.5 O3 C.,-4

000000

°

-1

-I

000000

.5

i 0

-4

I 4 ALPHA,

Figure dynamic

7. Mean pressure

bars if greater

wing twist corresponding = 965 psf

than the symbol

• 15

57-7

I 12

deg

to Table 2 for air runs at Mach

The standard size.

i 8

deviations

of the scatter

= 0.9 and

are plotted

as error

Y/b/2 = 0.922.

g

50-12

Y/b/2

= 0.922

cn 02

xJ

0

.5 O3 .,--_

c_

C .,-4

-.15

-.3

-5

I

I

I

I

I

I

0

5

10

15

20

25

ALPHA,

Figure 8. Wing twist corresponding pressure = 154 psf Y/b/2 = 0.922.

deg

to Table 6 for an air run at M = 0.3 and dynamic

19

57 -8

.5

& 60-14

Y/b/2

= 0.922

Q

o% 0

°°_

-,-4

--.5

_"

C

-_

i 0

-5

E 5

i t0

I

15

ALPHA,

25

deg

to Table 6 for an air run at M = 0.6 and dynamic

Figure 9. Wing twist corresponding pressure = 534 psf. Y/b/2 = 0.922.

•5

I

20

O 57-13

g 60-11

Y/b/2

= 0.922

D []

0

Q.Z

% o%

.j • r-i

--

.

3

)= .iJ

i-

-_ 5 -5

I 0

_ 5

J _0

ALPHA.

Figure pressure

20

10.

Wing twist corresponding

= 804 psi'. Y/b/2

-- 0.922.

i 15

I

I

20

25

deg

to Table 6 for an air run at M = 0.3 and dynamic

1

57-9

6 60-_9

Y/b/2

= 0.922

D .5

rl

E_ GJ "C3

[]

0 tD 4.J

-,5

t2_ t.r.