December 2005 

NASA/TM-2005-213790 

Aeroelastic Deformation: Adaptation of Wind 

Tunnel Measurement Concepts to Full-Scale 

Vehicle Flight Testing

Alpheus W. Burner 

Langley Research Center, Hampton, Virginia 

William A. Lokos 

Dryden Flight Research Center, Edwards, California 

Danny A. Barrows 

Langley Research Center, Hampton, Virginia 
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December 2005 

NASA/ TM-2005-213790 

Aeroelastic Deformation: Adaptation of Wind 

Tunnel Measurement Concepts to Full-Scale 

Vehicle Flight Testing

Alpheus W. Burner 

Langley Research Center, Hampton, Virginia 

William A. Lokos 

Dryden Flight Research Center, Edwards, California 

Danny A. Barrows 

Langley Research Center, Hampton, Virginia 
Available from: 

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1

Abstract 

The adaptation of a proven wind tunnel test technique, known as 

Videogrammetry, to flight testing of full-scale vehicles is 

presented. A description is presented of the technique used at 

NASA’s Dryden Flight Research Center for the measurement of 

the change in wing twist and deflection of an F/A-18 research 

aircraft as a function of both time and aerodynamic load. 

Requirements for in-flight measurements are compared and 

contrasted with those for wind tunnel testing. The methodology 

for the flight-testing technique and differences compared to wind 

tunnel testing are given. Measurement and operational 

comparisons to an older in-flight system known as the Flight 

Deflection Measurement System (FDMS) are presented. 

Introduction 

Photogrammetry and image processing techniques (videogrammetry) have been used for the 

measurement of wind tunnel aeroelastic deformations at five major NASA ground-based facilities 

at Langley Research Center and Ames Research Center [1]. Wind tunnel applications ranging 

from subsonic to supersonic include many types of models such as those used in the former 

Advanced Subsonic Technology (AST) and High Speed Research (HSR) programs. This 

technique, developed to meet the needs of production wind tunnel testing with minimal impact to 

productivity, is known as the videogrammetric model deformation (or VMD) technique. The 

technique uses single-camera, single-view photogrammetry coupled with image processing. The 

term single-view is used to distinguish between techniques employing a single-camera, multiview configuration, during which a single camera is moved about a static object to record 

multiple images that can then be used in a simultaneous solution to determine spatial coordinates. 

The use of such a single-camera, multi-view configuration is the technique of choice in the 

photogrammetric community [2]. The single-camera, single-view technique utilized for this 

effort however, while not normally chosen for photogrammetric applications, has several 

advantages for wind tunnel testing (and for flight testing). These advantages include: (1) only one 

camera is needed, reducing space and viewport requirements in the wind tunnel or on the aircraft; 

(2) the base separation that would be required for multiple cameras (requiring additional 

mounting locations) is not a factor; and (3) the constrained coordinate that enables the singleview solution can usually be measured with more accuracy during pre-test or pre-flight than is 

possible with multi-camera photogrammetry during wind tunnel or flight testing. 

In-flight measurements of aeroelastic deformations in the past have typically been costly, 

non-routine, and required significant setup and aircraft adaptations. Special light emitting diodes 

(LEDs) are generally required that must be installed on the aerodynamic surfaces of interest for a 

limited amount of testing [3-7] with large, specialized, high-cost, and older measurement systems, 

such as the electro-optical Flight Deflection Measurement System (FDMS) [3] or the 

commercially available Optotrak® measurement system from Northern Digital Corporation [7]. 

In addition, some aerodynamic surfaces, such as ailerons and leading edge slats, are very difficult 

to measure with the older systems due to the requirement for LEDs, wiring, and special mounts 

that protrude above the aerodynamic surface. A technique that does not require LED targets, is 

relatively inexpensive, requires less aircraft adaptations, and is more flexible to measurement 

needs is highly desirable for flight testing. Videogrammetry with existing video cameras or 

digital still cameras offers promise to meet these measurement requirements. This paper 

describes efforts toward adaptation of a videogrammetric method for aeroelastic deformation 
2

measurements in-flight that does not require LEDs, is low cost, and may eventually be more 

robust, accurate, and adaptable to measurement needs than current in-flight measurement 

systems. Initial efforts investigating the use of videogrammetry for flight testing are presented in 

[8]. 

Comparing Wind Tunnel and Flight Testing 

There are several notable differences between measuring wing and control surface 

deformation of a wind tunnel model compared with measuring wing and controls surface 

deformation on an aircraft in flight. For instance, the in-flight data camera would likely be 

mounted on the fuselage of the aircraft, thus the view presented to the data camera would be 

essentially the same throughout testing (except of course for any deformation or scale changes 

due to varying refractive index at different altitudes). In contrast, for wind tunnel measurements 

the camera must typically be mounted on the test section wall. Figure 1 compares the differences 

in camera geometry for wind tunnel measurements with a camera mounted on the test section 

wall and in-flight measurements with an onboard camera (which is shown as mounted on the 

fuselage). 

 During wind tunnel testing, the pitch range of the model must remain in the field-of-view so 

that the amount of the image plane occupied by the wing at any given pitch angle is reduced, 

which correspondingly reduces the accuracy of the measurements. A given target may also move 

significantly on the image plane due to pitch-sweep testing during wind tunnel testing, unlike the 

case for flight testing. In addition, the typical image plane motion during wind tunnel testing (due 

to the excursions in model pitch angle during a run) makes it more critical to determine and 

properly account for optical lens distortion than for flight testing, where the motion on the image 

plane is much smaller and confined to that produced by differential dynamics or the deformation 

itself. Unlike flight testing, wind tunnel testing requires the measurement of large angles because 

the local angles on the wing follow the model pitch angle which can change considerably relative 

to the fixed camera on the test section wall. Thus, it is necessary during wind tunnel testing to 

calibrate the optical measurement system in terms of model pitch angle with an onboard precision 

accelerometer under static test conditions in order to improve the measurement accuracy. Such 

additional calibrations may not be necessary for flight testing where the angular change relative to 

the camera is confined to that due to the aeroelastic deformation itself. 

 Wind tunnel measurements are further complicated by the need to accommodate and account 

for rigid body motion of the model due to sting bending induced by the overall aerodynamic 

loading on the model. The measurement of wing deformation during flight testing is closer to a 

differential measurement, can make better use of the available image area, and is affected less by 

rigid body motion because the camera is on the body (fuselage). Dynamics of the model can 

further exacerbate potential problems during wind tunnel testing. Dynamics may be less of an 

issue for in-flight testing in one respect because the camera moves with the fuselage, thus any 

dynamics common to the fuselage and wing would tend to cancel, unlike for wind tunnel testing. 

 Differences also occur with targeting. For most wind tunnel tests, retro-reflective tape targets 

are used to provide very high-contrast targets to greatly increase the chances of successful 

automated image processing. (A notable exception to this is the National Transonic Facility at 

Langley Research Center, where a special polished paint technique has been developed to reduce 

the potentially harmful aerodynamic effect of the thicker and rougher surface-finish tape targets.) 

An example of a high-contrast image from a wind tunnel test using the polished paint technique is 
3

shown in figure 2. A typical video image of the left wing of an F/A-18 is shown in figure 3 for 

comparison. Images with retro-reflective tape targets are even higher contrast, such that often 

only the targets themselves are seen. 

 Another difference is that the wind tunnel model is typically viewed at an angle to the wing 

of about 30°, whereas for aircraft in flight, the data camera located on the fuselage would likely 

have a view angle to the wing closer to 10° or less. Because the light return from retro-reflective 

material drops significantly as the angle of incidence moves away from normal, the retro-effect 

for aircraft with a shallow angle of incidence would be reduced significantly. One way around 

this would be to place the targets on raised tabs to orient the tape targets toward the camera. 

However, the potential aerodynamic disturbance would then be equivalent to that now 

encountered with the protruding FDMS targets, which is undesirable. Another aspect of retroreflective material that complicates their use for deformation measurements on aircraft in flight is 

the requirement for a light source near the camera. The space required for such a light source, 

along with its additional wiring, mounting, survivability, and operational concerns, limit the use 

of retro-reflective tape targets for in-flight testing. The use of natural features or passive targets 

that do not require additional wiring or special mounting is highly desirable for in-flight testing. 

 One major difference between wind tunnel and flight testing is the very nature of the tests 

themselves. It is common in wind tunnel testing to require that data be available at least within a 

few minutes after the completion of a run series. Thus, near real-time data processing and rapid 

final reduction are necessary for an effective and useful model deformation measurement system 

for a production wind tunnel. Flight testing, on the other hand, normally consists of an hour or so 

flight where, during the flight, there is usually no pressing need for near real-time deformation 

data. The requirements for data processing and final reduction for in-flight measurements of 

wing deformation might be better expressed as rapid, next-day delivery of reduced data. Also, 

the very nature of the unknowns in flight testing make it desirable to record images for post-flight 

analyses rather than the procedure of capturing images, performing image processing, recording 

the results of image processing, and overwriting the images, as is commonly done during more 

well-defined wind tunnel testing. The recording of images for post-test processing is especially 

important during early phases of technique development. In fact, during the early stages of our 

development of a wind tunnel model deformation measurement system, images were likewise 

recorded for post-run or post-test analyses in order to develop a databank of troublesome images 

for technique development and enhancement. 

 Finally, flight-hardened hardware is a major concern for any in-flight test technique. Flight 

hardware must be able to endure a variable and hostile environment and still continue to function. 

Ambient temperature can vary over a range of up to 150 °C or more. Low air pressure at higher 

altitudes may cause electrical circuits to arc. Structural vibrations can sometimes cause electrical 

or mechanical components to failure. Moisture, as well as jet fuel, can potentially harm flight 

instrumentation. In addition, flight systems must be able to run unattended. 

F/A-18 Research Aircraft (Active Aeroelastic Wing) 

 The test bed subject of this study is the NASA Active Aeroelastic Wing (AAW) F/A-18 

aircraft undergoing flight testing at the NASA Dryden Flight Research Center. The AAW effort 

is a joint program of the Air Force Research Laboratory (AFRL), NASA, and The Boeing 

Company. The program [9-10] started in 1996 with a goal to develop flight-validated, active 

aeroelastic wing technology design guidance suitable for use on future aircraft design programs. 
4

 The traditional roll control paradigm requires a torsionally stiff wing box structure. Rolling 

moment is expected to be produced exclusively through anti-symmetric control surface loads. 

Using this control approach, higher performance aircraft designs struggle to provide enough wing 

stiffness to produce the desired roll rates. At high free stream dynamic pressure flight conditions 

the wing box elastically twists enough to generate an opposing rolling moment of its own. The 

higher the dynamic pressure, the higher is the required stiffness. The structural design penalty for 

this added stiffness is either greater wing mold-line thickness or increased structural weight, or 

both. The goal of the AAW research program is to provide design guidance to allow lighter and 

thinner wing designs by using the wing box itself as a primary control surface intentionally driven 

by the wing control surfaces. This coupling of the aerodynamic loads with the structural 

elasticity can now be a design benefit under the AAW roll control paradigm. 

 To support this aeroelastic flight research the AAW aircraft, shown in figures 4 and 5, has 

added research flight control computers in addition to the standard computers so that alternative 

flight control laws may be tested. The wing structure has been modified to provide more 

torsional flexibility. This involved modifying the auxiliary (aft) wing box upper and lower cover 

panels. The drive mechanisms for the leading edge control surfaces were modified to allow 

independent actuation of the inboard and outboard leading edge control surfaces. Some control 

surface drive rates and ranges were also modified. New flight control laws were developed to 

exploit the desired aeroelastic behavior with the more capable control surface drive system. The 

research flight control laws produce roll control without the use of differential horizontal tail 

deflection. 

 Considerable instrumentation has been added to the aircraft including strain gages, 

accelerometers, surface air pressure sensors, control surface position sensors, and in-flight 

deflection measurement instrumentation. The AAW F/A-18 project envisioned the application of 

in-flight structural deflection measurement from its earliest formation. Provisions were designed 

in the test aircraft to accommodate the Flight Deflection Measurement System (FDMS). This 

included a dorsally mounted receiver pod with window, shown in figure 6, sixteen infrared LED 

targets on the upper left-hand wing surface with lead wires to a target driver box, a control unit 

box, power supplies, data telemetry interface circuit, and a cockpit-mounted on/off switch. Image 

data for this research paper were acquired from a flight-qualified video camera mounted just aft 

of the FDMS Pod, which looks out over the left hand wing. A planform of the aircraft showing 

the locations of the video camera, FDMS receivers, and FDMS targets (indicated with small dots) 

is shown in figure 7. A digital still camera is located between the FDMS receivers and video 

camera. As the FDMS installation was being designed, additional provisions were built in to 

support a developmental deflection measurement system. These provisions include additional 

space in the FDMS receiver pod and a longer window. Additionally, a pod heater/cooler was 

provided. These features make the AAW aircraft an ideal test-bed for the flight testing of new 

deflection measurement approaches. 

Data Reduction Procedures 

Overview 

 One of the long-term goals of this effort is to develop a test technique able to provide nextday, continuous, time-resolved aeroelastic results from takeoff to landing [8]. It is envisioned 

that an image sequence of the entire flight will provide the raw data from which time-resolved 
5

deflection can be extracted. As mentioned earlier, a variation of the videogrammetric model 

deformation measurement technique used in wind tunnel testing offers promise for such in-flight 

measurements. Central to the extraction of meaningful image data that can be transformed to 

deflection is the applicability of automated image processing that is robust and requires little user 

intervention. In order to apply single-camera, single-view photogrammetry, which is the basis of 

the measurement, the pointing angles and location of the data camera must be determined in the 

aircraft coordinate system. This is accomplished with a well-known photogrammetric procedure 

known as space resection with assumed or known values of several camera parameters. Once all 

camera parameters and the pointing angles and location are known, two of three coordinates (with 

the third coordinate known) can then be computed from a single-camera view. To determine 

deflection, the vertical coordinates at a reference condition are subtracted from the vertical 

coordinates at the flight condition of interest. Angles are determined from the slope of computed 

coordinates in chordwise planes at the various spanwise locations where targets or suitable image 

patches (natural features) exist. Local angles at various span stations at the reference condition 

are subtracted from the flight condition of interest to yield the change in angle (induced twist) due 

to aeroelastic loading. An alternative approach to single-camera, single-view photogrammetry 

(requiring resection) is the determination of scale factors for each target (or image patch) 

location. This is essentially the approach used for the FDMS and, depending on the availability 

of targets with known 3D coordinates, may be desirable for the videogrammetric technique in 

some situations. 

 Two recording approaches are being investigated. The first approach for image recording 

uses a standard resolution flight-hardened video camera for recording. This camera, normally 

used for surveillance of the left wing, provided the image sequences for the results reported here. 

These video data sets expedite test technique development with real-world flight-test data for 

evaluation and offer a minimum impact data recording. The second approach uses a highresolution (3 Megapixel) digital still camera for selected data samples. The digital still camera 

had to be mounted upside down in order to fit into the camera pod. The pilot triggers the digital 

still camera from the control stick to acquire a short sequence of images. The camera aperture is 

fixed at a mid-range f-number to provide reasonable depth of field. The camera exposure time is 

automatically adjusted internally by the camera to provide reasonable contrast images, even with 

varying wing illumination. A USB interface cable running from the camera pod to the cockpit 

enables the images to be downloaded on the ground (while clearing the memory card) without 

having to open the camera pod (which can be very time-consuming). Results from the digital still 

camera will be evaluated and compared to results from the over-the-wing video camera and the 

FDMS during the 2nd phase of flight testing which began in December 2004. 

Determining Pointing Angles and Location of Data Camera 

 The first task in the data reduction procedure is to determine the location and pointing angles 

of the data camera in the aircraft coordinate system. This can be accomplished 

photogrammetrically with a process known as space resection, or simply resection. In resection, 

2D image plane coordinates are mathematically compared with their known 3D object space 

coordinates (which defines the coordinate system). Nonlinear least squares is used to extract 

estimates of the Euler angles ω, φ, κ (defining the pointing angles of the data camera) and the 

location of the effective perspective center of the camera Xc, Yc, Zc as well as precision estimates 

of these computed quantities. The basic equations used for this computation are referred to as the 

collinearity equations [1]. The collinearity equations (1) are the most fundamental and important 

data reduction equations in photogrammetry. They express the ideal imaging relationship that the 

object point, perspective center, and image point should be on a straight line. In equations (1) 

below, the image coordinates x, y have been corrected for optical lens distortion [11] (or the 
6

distortion is negligible). Unlike wind tunnel testing, the image motion of targets (or a given 

image patch) is small compared with wind tunnel testing; therefore, lens distortion has much less 

effect on the results and is ignored here. The photogrammetric principal point, which can be 

thought of as the point of symmetry for imaging, is represented by xp, yp, the principal distance is 

represented by c, and the object space location of each target is represented by X, Y, Z. The 

principal distance is the perpendicular image distance from the rear perspective center to the 

image plane and would ideally be equal to the effective focal length if the lens were focused on 

infinity. If the lens is focused on a near object (as for the examples presented here) the principal 

distance will be a little longer than the effective focal length and can be estimated from the wellknown Gaussian object-image relationship among object distance, image distance, and effective 

focal length. 

( ) ( ) ( )

( )( )( )⎥

⎦

⎤ ⎢

⎣

⎡

− + − + −

− + − + − = −

c c c

c c c p m X X m Y Y m Z Z

m X X m Y Y m Z Z

x x c

31 32 33

11 12 13

 (1) 

( ) ( ) ( )

( )( )( )⎥

⎦

⎤ ⎢

⎣

⎡

− + − + −

− + − + − = −

c c c

c c c p m X X m Y Y m Z Z

m X X m Y Y m Z Z

y y c

31 32 33

21 22 23

 The Euler angles ω, φ, κ, which orient the image plane to the coordinate system of interest 

about the X, Y, Z axes, respectively, are used to compute the nine elements of the rotation matrix 

given by equation (2). 

 For the efforts presented here, the following assumptions were made. The principal point, xp,

yp, was approximated as a fixed location at the center of the digital image extracted from the 

DVD recordings at a resolution of 704 horizontal pixels by 480 vertical pixels. In pixel space, the 

coordinates for xp, yp were taken to be 352, 240, which during the conversion from pixel space to 

millimeters on the image plane corresponded to a value of xp, yp = (0, 0). The principal distance 

was estimated to be 16.08 mm based on an assumed focus toward the outboard portion of the 

wing and an effective focal length of 16 mm. The results are only weakly dependent on the 

actual value of the principal distance used due to the differential nature of the computations to 

establish the difference in Z (or in angle) from a given measurement condition to another. It is 

common for wind tunnel testing to perform a lens calibration to determine principal distance, 

principal point, and lens distortion (both radial and asymmetric) [11]. However, for flight testing, 

motion of targets on the image plane is confined to localized regions (in fact, inboard image areas 

are effectively stationary to noise level), and additional lens calibration may not be necessary. A 

sensitivity analysis is suggested to determine the effects of the various lens calibration 

coefficients on the results in order to ascertain the value of additional lens calibration for a 

particular project. 
7

 

m11 = cos φ cos κ

m12 = sinω sin φ cos κ + cosω sinκ

m13 = − cosω sin φcos κ + sinω sinκ

m21 = − cos φsinκ

m22 = −sinωsin φsinκ + cosω cos κ

m23 = cosωsin φ sinκ + sinω cos κ

m31 = sin φ

m32 = −sinωcos φ

m33 = cosωcos φ

 (2) 

 With the principal distance and principal point assumed, and known X, Y, Z associated with 

each target or image patch location, the remaining unknowns in equations (1), namely ω, φ, κ and 

Xc, Yc, Zc, are then solved for with an iterative nonlinear least-squares solution, which requires 

initial start values. For the examples presented below, the FDMS targets were used as reference 

with locations that had been measured with a precision theodolite before flight testing began. An 

image from level flight was used to establish the approximate pointing angles and location of the 

video camera. The image locations were determined manually in MATLAB® by setting a cursor 

at each target location on the digital image of interest. The pixel coordinates (with the origin to 

the upper left of the image), xpix, ypix, were converted to units of millimeters, xmm, ymm, (centered at 

the middle of the image plane) with the following equations: 

 

xmm = ( xpix − xo) Sh

ymm = − ( ypix − yo) Sv

 (3) 

where xo, yo locate the approximated principal point in pixel space (352, 240), and Sh and Sv are 

the effective horizontal and vertical pixel spacings of the digital imagery. The effective pixel 

spacings Sh and Sv were adjusted from repeated photogrammetric resections until the computed 

location of the video camera from resection was in reasonable agreement with its assumed 

location (to within a tolerance of several centimeters). An additional constraint that Sh = 

(639/703) Sv was initially applied to account for the digital conversion at 704 × 480 instead of the 

more common square pixel format of 640 × 480. The horizontal pixel spacing was further 

adjusted to minimize image plane resection residuals to yield a final estimate for Sh, Sv of 

0.02484, 0.028 mm. Because differential measurements are being made from image coordinates 

that differ little from condition to condition, the exact value of the horizontal and vertical pixel 

spacings is not as critical as for multi-camera photogrammetric solutions or single-camera, singleview solutions where there is relatively large motion on the image plane (such as that which 

occurs during wind tunnel testing). 

Single-Camera, Single-View Photogrammetric Solution 

 The collinearity equations (1) can be recast in linear form as: 

 

 

a1X + a2Y + a3Z = a1X c + a2

Yc + a3Zc

a4X + a5Y + a6Z = a4Xc + a5Yc + a6Zc

 (4) 

where, 
8

( )

( )

( )

( )

( )

( ) 6 33 23

5 31 21

4 32 22

3 33 13

2 31 11

1 32 12

a y y m c m

a y y m cm

a y y m c m

a x x m c m

a x x m c m

a x x m c m

p

p

p

p

p

p

= − +

= − +

= − +

= − +

= − +

= − +

 (5) 

 If two or more cameras image a single point and a1 through a6 (containing coefficients xp, yp,

c, ω, φ, κ) and Xc, Yc, Zc are known, then the spatial coordinates X, Y, Z can be determined with 

linear least squares as 4 equations in 3 unknowns. This is the basis of photogrammetric 

intersection, or triangulation. If one of the spatial coordinates is known, such as the semispan 

location of image points (X), then a single-camera image of a point results in two equations in two 

unknowns. With X known, Y (fuselage station) and Z (vertical) can be found by Gaussian 

elimination or with the following algebraic solution: 

 

 

Y = Yc + (X − X c)(a2a6 − a5a3)

( ) a4a3 − a1a6

Z = Zc − Y − Y ( )c a1 + ( ) X − Xc a2

a3

 (6) 

 The primary advantages of this single-camera, single-view technique are: (1) only one camera 

is needed, reducing space and viewport requirements on the aircraft; (2) the base separation 

needed for multiple cameras (and requiring additional mounting locations) is not a factor; and (3) 

the constrained coordinate necessary for a single-view solution can usually be measured with 

more accuracy pre-test or pre-flight than is possible with multi-view photogrammetry during 

flight testing. When comparing multi-camera with single-camera techniques, it should be noted 

that variance in the photogrammetrically computed X-coordinate (and Y) of a multi-camera 

measurement will lead to variance in the more critical Z-values. However, with the singlecamera, single-view technique X-values are determined to greater precision with theodolites prior 

to testing and, in most cases, will propagate less error into Z-coordinates. The bias errors in Y and 

Z due to error in X tend to cancel upon the subtraction that is inherent in differential 

measurements (runway-to-flight, level-flight-to-maneuver, etc.). Additional uncertainty 

considerations for the technique when used for wind tunnel testing, including the effect of 

changes in the known semispan coordinate X due to bending, are addressed in reference [11] 

(where the known semispan coordinate is represented as Y instead of X, and X and Z are solved 

for instead of Y and Z as shown here). 

 Once the Y and Z coordinates are computed for a given semispan location X, a slope angle 

can be computed in the Y-Z plane by either least squares for multiple image coordinates, or 

directly when there are only two targets per semispan. This angle, designated as the 

videogrammetric angle θη, is given at each normalized semispan location η by: 

 

θη = tan−1 ΔZη

ΔYη

 (7) 

 Thus, the data extracted from the flight images would consist of angles at each semispan 

station (if more than one target per semispan) and Z-deflection values for each target or image 
9

patch. The reference condition (e.g., runway or level flight) Z-values or angles are then 

subtracted from the condition of interest to determine the change in deflection or angle (induced 

twist) from the reference to that condition. 

Image Processing and Targeting Considerations 

 In order to extract useful spatial information from a digital image in a timely manner, some 

variation of image processing is needed. The basic methods under consideration are location of 

discrete targets or target-like features, edge detection, and digital image correlation of natural 

features. Only results from the first 2 methods are presented here. Subpixel digital image 

correlation, although not presented here, offers perhaps the best potential for future efforts since 

its use may obviate the need for any targets to be applied to the wing. In addition, nearly global 

coverage of the wing may be possible using natural features. It is expected that for regions of the 

wing where natural features are nondescript, tape or painted targets may be necessary to obtain 

data in those regions, even when using image correlation. 

 For the resection effort outlined above, and for the examples of FDMS target deflection, it 

was simplest and most straightforward to manually select the image coordinates that correspond 

to the FDMS target housing locations [8]. Efforts to extract the image coordinates with 

binarization, segmentation, and centroiding were only marginally successful due to the very low 

contrast and low resolution of the video images. In addition, the images have enough background 

clutter surrounding a given target location to make the application of automated image processing 

difficult. The contrast of the target housings also varies drastically with time, sometimes actually 

switching sign, depending on the location of the Sun with respect to the wing. Given the mostly 

white nature of the wing, it is expected that the best chance of success with automated image 

processing would occur with flat-black targets of sufficient size to cover four or five pixels in 

image space in the horizontal and vertical directions. Black tape may be a convenient and 

effective means to apply targeting to the wing in the future. Due to the extreme foreshortening of 

the view caused by the low inclination angle of the camera to the wing of around 10°, it will be 

desirable to use elongated targets with an aspect ratio of around four or so with the long axis 

pointing toward the camera. 

 Detection of straight edges at known semispan locations was slightly more successful in 

terms of automation and data quality. Edge detection was found to partially work on the upper 

edge of the launcher rail located at the wing tip. Edge detection is illustrated in figure 8. In order 

to extract the proper edge, MATLAB® code was developed that isolated the edge detector output 

to a region surrounding the launcher rail. The code also tracked the potential edges over an 

angular range expected to include the desired edge. For figure 8 the first selection of the edge 

data is highlighted in red. The data in red are then passed through a filter that rejects data outside 

of a set fraction or number of standard deviations of the linear least squares computed angle in 

pixel space and over a range of angles. The final filtered edge data from which the angle 

computations are made are indicated in green in figure 8. With this method and operator viewing 

of the overlaid edges on each image, data segments of about 11 seconds and 31 seconds were 

analyzed from early parameter identification flights. For the 11-second data record the aircraft 

passed from level flight through a low load maneuver and then returned to level flight. The 31-

second maneuver was a high load 5-G slow wind-up turn for which FDMS data were also 

available for comparison. 
10

Examples of In-Flight Measurements 

Spanwise Wing Deflection Due to Aerodynamic Loading 

 Three hundred sixty-one individual digital images at a resolution of 704 × 480 (bitmap color 

RGB format) were extracted from a DVD image sequence recorded from a flight-hardened videomonitoring camera that views the left wing of the AAW F/A-18 research aircraft [8]. Thirty-one 

of the images were from a 1-second time history while the aircraft was on the runway. Three 

hundred thirty of the images covered the aircraft from level flight (M = 0.95, q = 750 psf, altitude 

= 15,000 ft) through a 90° bank-and-return maneuver over a time period of 11 seconds. One 

image each from the runway, level flight, and at the peak of the maneuver was selected for initial 

measurements. Because some of the FDMS target housings, especially outboard toward the 

trailing edge of the wing, were barely distinguishable from the background, the image locations 

of the targets for the three images were manually selected within MATLAB®. The red, green, 

and blue components of each digital image could be analyzed separately. The red component was 

found to have highest contrast for most of the images and was used for further analyses. The 

estimated precision of manual image coordinate location is at best a pixel or so whereas 

centroiding of high-contrast targets with proper background removal can yield a precision of 0.01 

pixel (one standard deviation). The relatively poor precision of the manual image coordinate 

selection accounts for the large scatter in the results presented below. Larger, higher contrast, 

better defined targets would allow for automated image processing, such as centroiding, and lead 

to much less data scatter. 

 Resection on the level flight image was used to establish the pointing angles and location of 

the camera in the aircraft coordinate system necessary for the single-camera, single-view solution. 

The semispan locations (X-coordinate) of each FDMS target were used to compute Y (fuselage 

station) and Z. The error in these pointing angles and location of the data camera due to resection 

are not as critical as in some photogrammetric applications. For this application, the image 

coordinates change very little from condition to condition so that bias errors in the computation of 

Y- and Z-coordinates from equations (4.6) tend to cancel when the reference condition is 

subtracted. Figure 9 shows the relative Z-deflection at locations near the 16 FDMS targets from a 

level flight condition to a maneuver [8]. The computed Z-values of the level flight image were 

subtracted from the maneuver image to arrive at the ΔZ-values. For figure 9, the deflection 

represented by the arrows does not have perspective correction. In other words, the amount of 

deflection indicated by the arrows is independent of where on the image the arrows are located. 

The most inboard targets exhibit very little deflection, as expected. The deflection increases as 

one moves outboard toward the wing tip. The housings nearest the leading edge and closest to 

the front wing spar had higher contrast, with corresponding better precision than the rest of the 

housings. The corresponding normalized (to maximum deflection) ΔZ is plotted in figure 10 

versus normalized semispan η. The solid line in figure 10 is a second-order polynomial fit to the 

seven higher contrast targets nearest the leading edge, indicated by circles, which lie along the 

front wing spar. The midchord row targets (near the rear spar) are indicated with squares and the 

targets nearest the trailing edge (aft spar) are indicated with diamonds. Although the scatter in 

these data is relatively large, it does indicate the correct trend and illustrates the potential of 

useful measurements given low-contrast non-ideal targets, even with low-resolution video. It is 

estimated that an improvement approaching a factor of 10 could be achieved in data precision 

with larger, better defined, high-contrast targets, even with relatively low-resolution video 

recording. 
11

Change in Twist at Wing Tip Due to Aerodynamic Loading 

 Plots of the wing tip deflection while on the runway, in level flight, and at maneuver 

(corresponding to manual image plane measurements on the corresponding three images) are 

shown in figure 11 [8]. Airflow direction is from left to right. The solid lines are first-order 

linear least-squares fits to each Y, Z data set for each of the three digital images. The dotted lines 

are ±1 standard deviation of the polynomial fits. The characteristic negatively induced wing twist 

due to aerodynamic loading is evident upon comparing the runway data with level flight data. 

Very little vertical deflection at the wing tip is noted when comparing runway with level flight 

data. The main effect of the maneuver is upward deflection of the wing tip and a slight increase 

in wing twist at the tip. The relatively stable image locations (and corresponding stable spatial 

object locations) of the most inboard targets give credibility that the nature of the wing tip 

deformation is effectively depicted in figure 11 rather than camera movement, which would have 

caused image plane motion of the most inboard targets as well. 

 In order to assess the precision of angle and deflection measurements, pixel coordinates were 

determined with image processing of the top edge of the launcher rail and transformed to spatial 

coordinates for 31 images of the wing (while taxiing on the runway over a 1-second period). Due 

to the relatively stable wing and background while taxiing, this data set serves as an indicator of 

close to best-case angular and deflection measurements over a short time period for this relatively 

low-contrast video resolution data set. For the runway 1-second data reference set the standard 

deviation of the measured angles was 0.020°. Note also that the standard deviations of the linear 

least-squares fits used to determine the slope angle in Y, Z spaces also had a typical value of 

0.02°. The mean angle from this 31-sample set that was subtracted out as reference for the 

following data thus has a standard deviation of about 0.004° (0.02/sqrt(31)). The standard 

deviation of the Z-measurements at midchord was 0.032 in. with a corresponding standard 

deviation of the mean Z-value used for reference of 0.006°. Background clutter (clouds, etc.), the 

location of the Sun relative to the wing, and poor contrast in general will cause these best-case 

values to worsen considerably. 

 Edge detection image processing was used to record the change in twist and deflection at the 

wing tip (top edge of the launcher rail) as a function of time [8]. The normalized change in wing 

tip twist as a function of time from level flight through maneuver and back to level flight is 

shown in figure 12 for the 11-sec data record. The data for this plot (and figure 13 as well) were 

obtained in a piecewise fashion because no single edge detector with default input parameter 

worked well for the entire time sequence. Although complete automated image processing was 

not achieved for the entire time sequence, the piecewise reduction time history is encouraging for 

future development efforts. Figure 13 shows the corresponding change in Z at the midpoint of the 

tip chord for the same time sequence as figure 12. The heavy lines in both figures 12 and 13 

show a five-sample moving average used to smooth the data. The lines plotted near zero in each 

figure show the relative precision (two times the standard deviation) of each linear fit used to 

create the plots versus time. The relative precision is noticeably worse for images with 

background clutter and poor contrast. It can be noted from figure 12 that, at the end of the 

maneuver, the wing tip twist did not return to its pre-maneuver value. This start-to-finish 

difference is several times as large as the relevant data uncertainty levels and therefore indicates 

that the wing took a “set” during the maneuver. It is known that the F/A-18 wing does this. 

When the AAW wing was load tested to quantify its torsional stiffness, as reported in reference 

12, the wing’s structural hysteresis was noted. This hysteresis is due to the aggregate effect of 

many mechanically fastened joints, each with fitting tolerance and friction. That this effect is not 

apparent in figure 13 is attributed to the fact that the vertical displacement was measured for a 

point near the wing’s elastic axis. 
12

Comparison of Videogrammetric and FDMS Deflection Data 

 A 5-G slow wind-up turn (M = 0.85, q = 600 psf, altitude = 15,000 ft) during one of the early 

parameter identification runs was selected for comparison of video and FDMS deflection data. A 

total of 930 digital video images over a time of 31-seconds were analyzed. The top edge of the 

launcher rail was used as a contrasting edge to compute the change in deflection from the start of 

the wind-up turn. The launcher rail edge data were interpolated at the chordwise stations 

corresponding to wingtip FDMS targets labeled 13, 14 and 15 for comparisons. The 

videogrammetric data as a function of acceleration in G’s is plotted in figure 14 for the 3 

interpolated chordwise stations corresponding to the 3 FDMS target locations. The differences 

between the video and FDMS deflection data are presented in figure 15. Most of the data scatter 

seen in figure 15 is due to the videogrammetric data which has not been filtered. The separation 

noted in figure 15 of the 3 chordwise stations between accelerations of 3.1 and 4.2 G’s is likely 

due to a troublesome background that caused an error in the apparent angle of the launcher rail 

edge due to non-robust image processing. The mean and standard deviation of differences 

between the video and FDMS deflection data over the 5-G slow wind-up turn are shown in Table 

1 for the 3 FDMS target locations labeled 13, 14, and 15. The units of Table 1 are in cm. 

Future Work 

 In addition to a comparison of results using a low resolution video camera versus a higher 

resolution digital still camera, future work includes the comparison of single image calibration 

using photogrammetric space resection with measuring rod calibration at finite points on the wing 

to assess the relative merits of each calibration method. The measuring rod consists of high 

contrast edges (or targets) at known locations in concert with a bubble level to ensure that the rod 

is vertical when placed at various locations on the wing. An example of a measuring rod 

calibration image is presented in figure 16. For such a calibration no photogrammetric 

information is required. The transformation from pixels to vertical deflection is determined at 

each point where the measuring rod is located. A response surface plot (2nd order may be 

sufficient) over the wing based on the individual calibration points can then be used to interpolate 

the scale factor at intermediate locations on the wing where calibration images were not recorded. 

While the measuring rod technique has the advantage of a direct calibration from pixels to 

vertical height, it does require a number of images and takes more time to complete. 

 Enhanced targeting on the wing will also be assessed during the next phase of flight testing. 

Painted black diamond targets (figure 17) will be used during the next phase of fight tests in order 

to determine the improvement in image processing automation and data quality with such larger 

higher-contrast targets. The black diamonds were sized to yield adequate pixel coverage for the 

low resolution over-the-wing video camera. The black diamonds are elongated in the direction of 

the camera look-angle to partially compensate for the fore-shortening that occurs with the camera 

at such a steep viewing angle. The black diamond targets are placed at approximately the same 

wing locations as the FDMS LED targets to facilitate comparisons with that measurement system. 

Black diamond targets were also placed on the aileron (where FDMS targets could not be placed) 

to enable the measurement of aileron deformation during next phase of flight testing. Future 

efforts will also involve the evaluation of digital image correlation. Subpixel digital image 

correlation offers perhaps the best potential for future efforts since correlation of natural features 

may obviate the need for any targets to be applied to the wing. In addition, nearly global 

coverage of the wing may be possible using natural features. It is expected that for regions of the 
13

wing where natural features are nondescript, tape or painted targets may be necessary to 

supplement image correlation. 

Conclusions 

 The adaptation of the single-camera, single-view photogrammetric method widely used in 

NASA wind tunnels is shown to be useful for the measurement of static and dynamic aeroelastic 

wing deformation of aircraft in flight. The primary advantages of the single-camera, single-view 

technique are: (1) only one camera is needed, reducing space and viewport requirements on the 

aircraft, (2) the base separation of multiple cameras (with additional mounting locations required) 

is not a factor, and (3) the constrained coordinate necessary for a single-view solution can usually 

be measured with more accuracy pre-test or pre-flight than is possible with multi-camera 

photogrammetry during in-flight data taking. Actual in-flight measurements of the AAW F/A-18 

research aircraft illustrate that useful measurements are possible from a standard resolution video 

camera with low-resolution targets. An improvement in precision by a factor of 10 is anticipated 

for high-contrast targets. Time-history examples of changes in induced twist angle and deflection 

show the improvement in measurement precision possible with image processing of lines or 

edges located at various semispan stations. Although complete automated image processing was 

not achieved for the entire time sequences presented, the results of the piecewise reduction time 

history and the agreement with FDMS results are encouraging for future developmental efforts. 

Acknowledgments 

 The support and technical assistance of team members of the Active Aeroelastic Wing 

program, the Aerostructures Branch at Dryden Flight Research Center, and the Advanced Sensing 

& Optical Measurement Branch and the Aeroelasticity Branch at Langley Research Center are 

acknowledged. The innovation of James Mills, Adam Matuszeski, Dallas Quantz, and other 

members of the AAW F/A-18 ground crew in adapting and interfacing a digital still camera 

within the constraints of flight testing are especially acknowledged. 

References 

[1] A. W. Burner and Tianshu Liu, “Videogrammetric Model Deformation Measurement 

Technique,” J. of Aircr., 38, 4, pp. 745-754, July-August 2001. 

[2] E. M. Mikhail, J. S. Bethel, and J. C. McGlone, Introduction to Modern Photogrammetry, 

chapt. 9, John Wiley & Sons, New York, 2001. 

[3] V. Michael DeAngelis and Robert Fodale, “Electro-Optical Flight Deflection Measurement 

System,” Society of Flight Test Engineers 18th Annual Symposium Proceedings, SFTE 

Technical Paper 22, pp. 1-14, Sept. 1987. 

[4] Kenneth L. Bonnema and William A. Lokos, “AFTI/F-111 Mission Adaptive Wing Flight 

Test Instrumentation Overview,” ISA 35th International Instrumentation Symposium, Paper 

#89-0084, pp. 809-839, May 1989. 
14

[5] William A. Lokos, “Predicted and Measured In-Flight Wing Deflections of a ForwardSwept-Wing Aircraft,” NASA TM-4245, 21 pp., Jan. 1990. 

[6] William A. Lokos, Catherine M. Bahm, and Robert A. Heinle, “Determination of Stores 

Pointing Error Due to Wing Flexibility Under Flight Load,” AIAA 7th Biennial Flight Test 

Conference, AIAA 94-2112, 20 pp., June 1994. 

[7] P. Williard, J. D. Hardin, and J. H. Whitehead, “Determination of In-Flight Aeroelastic 

Deformation of a Transport High-Lift System Using Optical Position Measurement 

Technology,” 1st AIAA Aircraft Engineering Technology and Operations Congress, Los 

Angles, CA, Sept. 19-21, 1995. 

[8] Burner, A. W., Lokos, W. A., Barrows, D. A., “In-Flight Aeroelastic Measurement 

Technique Development,” SPIE’s 48th Annual Meeting – International Symposium on 

Optical Engineering and Technology, San Diego, August 2-8, 2003. 

[9] E. Pendleton, K. E. Griffin, M. Kehoe, and B. Perry, “A Flight Research Program for Active 

Aeroelastic Wing Technology,” AIAA Paper No. 96-1574, 37th AIAA Structures, Structural 

Dynamics, and Materials Conference, Salt Lake City, Utah, pp. 2263-2273, April 1996. 

[10] E. Pendleton, D. Bessette, P. Field, G. Miller, and K. Griffin, “The Active Aeroelastic Wing 

Flight Research Program, Technical Program & Model Analytical Development,” J. of 

Aircr., 37, 4, pp. 554-561, July-August 2000. 

[11] A. W. Burner, Tianshu Liu, and Richard DeLoach, “Uncertainty of Videogrammetric 

Techniques Used for Aerodynamic Testing,” 22nd AIAA Aerodynamic Measurement 

Technology and Ground Testing Conference, St. Louis, Missouri, AIAA 2002-2794, 20 pp., 

June 2002. 

[12] William A. Lokos, Candida D. Olney, Natalie D. Crawford, Rick Stauf, and Eric Y. 

Reichenbach, “Wing Torsional Stiffness Tests of the Active Aeroelastic Wing F/A-18 

Airplane,” AIAA-2002-1333, 9 pp., Jan. 2002. 
15

Figure 1: Comparing wind tunnel and in-flight geometries. 

Figure 2: Relatively high-contrast 

image from wind tunnel 

test. 

Figure 3: Video image of F/A-18 

left wing. 
16

Figure 4: F/A-18 AAW research aircraft. Figure 5: F/A-18 AAW research aircraft. 

Figure 6: Camera locations on F/A-18. 

Figure 7: Planform of F/A-18 showing 

FDMS target layout (LEDs) and 

receiver and video camera 

locations. 

Figure 8: Detection of upper edge of launcher rail with image processing

FDMS LEDs 

FDMS receivers

video camera 

camera pod 

FDMS & digital cam 

video cam 
17

Figure 9: Relative Z-deflection 

overlaid on video image of 

wing. 

Figure 10: Normalized deflection 

versus semispan. 

Figure 11: Launcher rail relative position for runway, level flight, and maneuver. 
18

Figure 12: Normalized change in 

launcher rail angle versus 

time during maneuver. 

Figure 13: Normalized change in Z of 

launcher rail versus time 

during maneuver. 

Figure 14: Videogrammetric 

normalized ΔZ versus 

acceleration in G’s. 

Figure 15: Difference between 

videogrammetric and FDMS 

data (cm). 
19

Figure 16: Measuring rod calibration 

image. Figure 17: Black diamond targets on 

AAW F/A-18. 

Z13 Z14 Z15

mean 0.22 0.04 -0.17 

stnd dev 0.61 0.54 0.59 

Table 1: Mean and standard deviation of the differences between the 

videogrammetric and the FDMS Z-deflections (cm). 
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Aeroelastic Deformation: Adaptation of Wind Tunnel Measurement

Concepts to Full-Scale Vehicle Flight Testing

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 6. AUTHOR(S)

Burner, Alpheus W.; Lokos, William A.; and Barrows, Danny A.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center Dryden Flight Research Center

Hampton, VA 23681-2199 Edwards California 93523-0273

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Burner and Barrows: Langley Research Center, Hampton, VA; Lokos: Dryden Flight Research Center, Edwards, CA

An electronic version can be found at http://ntrs.nasa.gov

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14. ABSTRACT

The adaptation of a proven wind tunnel test technique, known as Videogrammetry, to flight testing of full-scale vehicles is

presented. A description is presented of the technique used at NASA's Dryden Flight Research Center for the measurement of

the change in wing twist and deflection of an F/A-18 research aircraft as a function of both time and aerodynamic load.

Requirements for in-flight measurements are compared and contrasted with those for wind tunnel testing. The methodology

for the flight-testing technique and differences compared to wind tunnel testing are given. Measurement and operational

comparisons to an older in-flight system known as the Flight Deflection Measurement System (FDMS) are presented.

15. SUBJECT TERMS

Photogrammetry/Videogrammetry; aeroelastic deformation; Transonic Dynamics Wind Tunnel

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