NASA Technical Memorandum 4767

A Feasibility Study To Control Airfoil Shape

Using THUNDER

Jennifer L. Pinkerton and Robert W. Moses

Langley Research Center • Hampton, Virginia

National Aeronautics and Space Administration

Langley Research Center • Hampton, Virginia 23681-2199

November 1997
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Summary

The objective of this study was to assess the capabilities of a new piezoelectric actuator to alter the upper surface geometry of a subscale airfoil to enhance

performance. This new piezoelectric actuator called thinlayer composite-unimorph ferroelectric driver and sensor

(THUNDER), recently developed at Langley Research

Center, is manufactured to deform out of plane when

under an applied voltage and, to date, has exhibited much

larger displacements than other piezoelectric actuators. It

was anticipated that attaching a THUNDER wafer to the

upper surface of a small airfoil and actuating it to

increase the camber of that surface when the airfoil was

at positive angles of attack (above 2°) would extend the

region of attached flow across the upper surface. Two

common characteristics of all piezoelectric actuators,

creep and hysteresis, however, pose challenges when

THUNDER is used for airfoil shaping or other positioning applications.

For this study, a subscale airfoil model was

designed, fabricated, and tested under two-dimensional

flow conditions in a small tabletop wind tunnel. Sixty

test conditions, consisting of combinations of five angles

of attack, four direct current (dc) applied voltages, and

three tunnel velocities, were studied. Results indicated

that displacements of the upper surface of the airfoil were

affected by the magnitude of the applied voltage, the tunnel velocity, the airfoil angle of attack, and the creep and

hysteresis of the THUNDER wafer. Larger magnitudes

of applied voltage produced larger wafer displacements.

Wind-off wafer displacements were consistently larger

than corresponding wind-on displacements; however,

higher velocities produced larger displacements than

lower velocities because of increased upper surface suction. Larger displacements were also recorded at higher

angles of attack because of increased upper surface suction. Creep and hysteresis of the wafer were identified at

each test condition and contributed to larger negative displacements for all negative applied-voltage conditions

and larger positive displacements for the smaller,

positive applied-voltage (+102 V) condition. An elastic

membrane used to hold the wafer onto the upper surface

hindered displacements at the larger magnitude positive

applied voltage (+170 V). Both creep and hysteresis of

the THUNDER wafer appeared bounded, based on the

analysis of several displacement cycles. These results

show that THUNDER can be used to alter the camber of

a small airfoil under aerodynamic loads. Feedback control techniques may be useful in reducing the effects of

creep and hysteresis.

Introduction

Changing the local flow field around an airfoil to

enhance overall aircraft performance has always been a

goal of aircraft designers. Historically, aircraft wings

have been designed for a single flight condition and then

modified to work for other flight conditions through the

use of conventional control surfaces (such as ailerons and

flaps), spoilers, and variable wing sweep. Variable wing

sweep affects changes in the local flow field by altering

the flow velocity perpendicular to the leading edge of the

wing. The conventional control surfaces and spoilers

affect changes in the flow field by directly varying the

camber on certain regions of the wing, thereby causing

changes in the baseline structural and aerodynamic characteristics of the entire wing. By developing a database

that relates wing sweep or a commanded aileron/flap/

spoiler deflection combination to a corresponding wing

performance, overall aircraft performance parameters,

such as lift-to-drag ratio and structural loading, may be

tailored for the different flight conditions required.

During the past decade, many researchers have also

started to look at adaptive material actuator systems for

performance-enhancing shape control. Like the conventional control surfaces, these actuator systems (in this

particular application) are designed to alter local wing

shape (through camber and/or twist) to produce favorable

structural and aerodynamic changes in the entire wing.

However, unlike the conventional control surfaces,

which have been used successfully for many years,

shape-controlling adaptive material actuator systems are

still in the development stage.

Adaptive Wing Concepts

Incorporation of leading- and trailing-edge control

surfaces on aircraft was one of the first successful innovations in wing design following the first heavier-thanair flight in 1903. Common on aircraft since the 1920's,

these camber-varying devices have been used primarily

to improve low-speed performance during takeoffs and

landings and to provide trim and maneuvering capability

during flight. Attempts to utilize such devices for broader

adaptive-camber-control purposes have also been made

many times during this century. In 1916, the Sopwith

Baby incorporated trailing-edge flaps that automatically

deflected at lower speeds and decambered at higher

speeds via a connection to restraining bungee cords.

Between 1919 and 1926, Dayton Wright Aircraft and

Army Air Services Engineering developed and flew aircraft that similarly incorporated mechanically activated

adaptive wing concepts. In 1933 and 1934, the Westland

Lysander was outfitted with independent inboard and

outboard cross-connected slats that were interconnected

with trailing-edge flaps. This concept provided lowspeed maneuvering by means of an adaptive wing that

automatically varied deflection with angle of attack. Also

in the 1930's, sailplanes began to regularly incorporate

manually controlled, camber-varying trailing-edge flaps
to optimize gliding and ascending performance. And during World War 1I, adaptive trailing-edge flaps were

included on a number of fighters, such as the P-51

Mustang, to permit high-lift maneuvering in aerial combat situations (ref. 1).

Until the 1970's, however, use of the wing control

surfaces for purposes other than maneuvering aircraft

and achieving design-point camber control for landings,

takeoffs, and trim was the exception rather than the rule.

For military aircraft, material and control limitations had

plagued further development, but breakthroughs in both

technologies in the seventies eliminated these barriers.

For commercial transports, fuel consumption concerns

provided the impetus for change. In each case, aircraft

designers in the early 1970's began to expand the airfoilshaping application of the control surfaces to improve the

off-design performance of the aircraft during the cleanwing (cruising) phases of flight (refs. 1-3). One such

technique, flap scheduling, uses predetermined flap

deflections at specific flight conditions to produce more

desirable aerodynamic shapes (ref. 2). Aircraft that have

benefited from variable-camber techniques like this one

include the Advanced Fighter Technology Integration

(AFTI)/F-111, the F-18, the X-29, and the Airbus A340

(refs. 1 and 2).

As indicated in the previous discussion, over the past

80 years, wing aerodynamic control surfaces have

proven to be an effective and efficient system for maneuvering an aircraft and regulating loads. Providing such

control is the primary function of these wing control surfaces. Therefore, any additional use of the control surfaces, such as active wing shaping for aircraft

performance enhancement, is generally a secondary

function. When control surfaces are asked to perform

both of these functions simultaneously, two issues must

be addressed: (1) not compromising the control-surface

authority available to maneuver the aircraft and (2) not

losing adequate control effectiveness (ref. 1). To date, an

actively controlled aerodynamic control surface that performs both functions well enough to remove these issues

from consideration has not been realized. Consequently,

in situations where the control surfaces are faced with a

multifunctional task, any secondary functions, such as

airfoil shaping, must be limited so that the primary function is not compromised.

There are also two design-driving issues that must be

addressed when dealing with multifunctional control surfaces: (1) producing a reliable, maintainable system and

(2) obtaining performance improvements without an

excessive increase in complexity and structural weight

(ref. 1). Because of such concerns and the multi functional limitations of the aerodynamic control surfaces,

alternatives for active wing shape control are being stud2

ied. The use of adaptive material actuators as control

effectors is one such alternative.

Adaptive material actuator systems are attractive for

performance-enhancing shape control because they offer

two advantages over the conventional control surfaces.

First, shape control can be the primary function of these

adaptive systems, and second, such shaping can be

accomplished smoothly without the introduction of flowdisturbing hinge lines.

The first attempt at active aerodynamic shape control

was conducted in the mid-1980's and involved the use of

piezoelectric actuators to generate twist and camber on

the surface of a plate (ref. 4). In the 1990's, as the

piezoelectric materials technology grew and developed,

the scope of the applications expanded from relatively

small coupon-type models to scaled wind-tunnel

models. Two examples of wind-tunnel applications are

the Piezoelectric Aeroelastic Response Tailoring

Investigation (PARTI) (refs. 5-8) and the Actively

Controlled Response of Buffet Affected Tails

(ACROBAT) (ref. 9) program, which sought to achieve

wing flutter suppression and vertical tail buffeting alleviation, respectively, through the use of piezoelectric actuation. Other studies in the 1990's, primarily analytical,

have focused on assessing the capability of the commercially available adaptive materials to create significant

skin deflections (refs. 10-16).

To date, adaptive material actuators have been tested

only in such research-related applications as those mentioned previously. Results so far indicate that, although

the currently commercially available adaptive material

actuators work well for both flutter suppression and buffeting alleviation (refs. 5-9, 17, and 18), they lack the

strength to adequately control load-dominated phenomena, such as divergence (ref. 19), and the strength and

out-of-plane displacement capability needed to create

airfoil shape variations that can significantly alter aerodynamic characteristics (refs. 10-16).

A new adaptive material technology called thin-layer

composite-unimorph ferroelectric driver and sensor

(THUNDER) (ref. 20), developed at Langley Research

Center within the past few years, has shown some promise for overcoming these barriers. In developing

THUNDER, researchers at Langley combined a new

materials technology and a new processing technique to

produce an actuator with a greatly improved out-of-plane

displacement capability. To date, much effort has been

expended toward understanding the behavior of this new

material under unloaded and statically loaded conditions.

More complex loading conditions, however, have not

been investigated, and research into the reliability and

load capacity of THUNDER has only recently been

initiated.
Goals of the Airfoil THUNDER Testing To

Ascertain Characteristics (ATTACH) Project

The purpose of the current study was to begin the

process of understanding the behavior of THUNDER

under aerodynamic loading conditions and to ascertain

the potential for using this new technology in airfoil

shaping applications. To accomplish these goals, a

two-phase test program was conceived. Phase I involved

identification of two nonlinear characteristics (creep and

hysteresis) and the reliability (in terms of performance

repeatability and fatigue) of a single wafer of

THUNDER. Phase II examined the ability of the wafer to

alter drag by changing the geometry of an airfoil. This

second phase was based on the premise that a single actuatod THUNDER wafer attached to the upper surface of

an airfoil could displace that surface enough to extend

the region of attached flow.

Figures 1 and 2 illustrate the concept of airfoil shaping using THUNDER affixed to the upper surface of a

symmetric airfoil near the leading edge and covered with

a flexible membrane. Figure 1 shows the typical flow

field with the airfoil set at a positive angle of attack

(above 2 °) and the THUNDER wafer unactuated. As

identified in the figure, the flow remains attached to the

upper surface of this nominal airfoil configuration only

across a small region near the leading edge. The flow

then separates because of the presence of a large adverse

pressure gradient (ref. 21). Figure 2 shows the anticipated flow field over the same airfoil with the

THUNDER wafer actuated up to meet the flow. With

such an increase in camber of the upper surface, the onset

of the large adverse pressure gradient would be delayed,

allowing for a longer attachment region.

This paper presents the approach and preliminary

findings of phases I and II, known collectively as the

ATTACH project. A discussion of piezoelectric materials in general, including an overview of the types of

piezoelectric actuators currently available, is presented

first, followed by detailed descriptions of the test facility,

test configurations, systems, and equipment used. Experimental results of the THUNDER technology evaluation

tests are also presented.

Piezoelectric Adaptive Materials

Piezoelectric materials, which develop a strain when

subjected to an electric field and vice versa, are currently

among the most widely used adaptive materials. Unrestrained, these materials can expand or contract freely

while under an applied voltage. However, when affixed

to a host structure, the movement of a piezoelectric material is inhibited, resulting in structural deformation and a

corresponding change in the loads for that host structure

(ref. 22). Currently, piezoelectric materials are divided

into two groups that differ by the direction in which they

are able to affect a host structure. The first group, commonly called strain actuators, exhibits an in-plane

displacement capability. The second group, a new generation of actuators, exhibits an out-of-plane displacement

capability.

The conventional configuration for an in-plane

displacement piezoelectric actuator consists of a single

piezoelectric wafer sandwiched between two electrodes,

as shown in figure 3. The relationship between an

applied electric field and the corresponding behavior of a

piezoelectric wafer is well documented (refs. 18, 22,

and 23). However, this relationship is not always ideal

because of the presence of nonlinear characteristics, such

as depoling, hysteresis, and creep. Depoling refers to the

reorientation of the dipoles within a piezoelectric material in a different direction from the original poling direction, with the degree of rotation of the dipoles affecting

the response of the actuator. As the dipoles rotate from

an original 90 ° orientation to a 0° orientation, the performance of the actuator is correspondingly reduced. Actuator response then increases as the dipoles continue

toward a complete 180 ° rotation from the original position; however, in that 90 ° to 180 ° range, the actuator

response to an applied voltage is opposite to that in the 0 °

to 90 ° range. Hysteresis is a typical characteristic of most

electromechanical devices, and it results in a residual

strain within the piezoelectric material. Creep is also a

typical characteristic of electromechanical devices that

appears as a slow deformation of the piezoelectric material when that material is subjected to a constant (zerofrequency) electric field for a prolonged period of time,

particularly under high-strain conditions. For situations

involving high frequencies, low applied voltages, and

small deformations, the effect of these nonlinear characteristics is often assumed to be negligible. In most other

cases, however, some form of correction would need to

be applied to account for them (ref. 23).

Currently, several configurations exist for the standard in-plane displacement piezoelectric actuator. The

basic configuration, called a piezoelectric patch, consists

of one or more layers of piezoelectric wafers. These

patches can be packaged in a protective coating for added

durability. Increased actuation can be obtained by grouping multiple wafers into multiple layers, usually two or

three deep. By stacking several of the actuators, an

interim-type of actuator (called a piezoelectric stack) that

possesses some out-of-plane displacement capability can

be created.

Two actuators specifically designed to have an outof-plane displacement capability are the unimorph and

the bimorph, both of which incorporate the basic in-plane

piezoelectric wafer previously discussed. As shown in
figure 4(a), a unimorph is made by bonding the wafer to

a metal shim. When a voltage is applied across the wafer,

the shim is forced to move with it, resulting in axial

buckling and an out-of-plane displacement. Figure 4(b)

illustrates the configuration for a bimorph, which is created by bonding together two oppositely poled piezoelectric wafers. In this case, an applied voltage across a

shared electrode simultaneously expands one wafer and

contracts the other, resulting in increased axial buckling

and, therefore, an even larger out-of-plane displacement

than can be obtained from the unimorph.

Recently, two actuators representing a new generation of out-of-plane displacement piezoelectric actuators

have been developed: (1) reduced and internally biased

oxide wafer (RAINBOW) (ref. 24) and (2) THUNDER.

Both devices have configurations similar to the unimorph; however, they incorporate a prestressing phase

during fabrication to set the final out-of-plane equilibrium position. Prestressing of the RAINBOW wafers is

accomplished through a high-temperature chemical

reduction of one surface. Prestressing of the THUNDER

wafers begins by surface coating a raw piezoelectric

wafer with a Langley-developed polyimide called

LaRCrM-SI. The coated wafer is then bonded on one side

to a parabolically shaped "backup," which consists of

alternating layers of a material, such as aluminum or

steel, and the LaRCrM-SI polyimide, as shown in figure 5. The new wafer is then vacuum bagged around a

form to press the layers together and cured at an elevated

temperature. Differences in the coefficients of thermal

expansion for the polyimide, which hardens at a high

temperature, and the other materials in the wafer result in

final prestressing as the wafer cools (ref. 20). The

strength and displacement capability of the THUNDER

actuators are directly proportional and inversely proportional, respectively, to the number of material layers used

in the backup. An illustration of the unrestrained movement of these out-of-plane actuators is shown in figure 6.

RAINBOW, the first of the new-generation actuators

to be developed, possesses 10 times the displacement

capability and 100 times the load capacity of the bimorphs. Comparisons between THUNDER and RAINBOW

displacements and load capacities axe not as easily

defined, however. Displacement capability comparisons

are difficult to make because THUNDER output varies

with the type and number of backup layers used. Strength

comparisons are even more difficult to make because, as

previously mentioned, THUNDER load-capacity

research is incomplete. A comparison between a 1.5-inwide, 2.5-in-long, 0.012-in-thick, 9-layer-aluminum

THUNDER wafer and a 1.25-in-diameter, 0.02-in-thick

RAINBOW wafer is provided in figure 7. As indicated in

the figure, this particular THUNDER wafer possesses

up to 13 times the displacement capability of the

RAINBOW wafer.

In general, the selection of an appropriate actuator

group for use with different types of applications is based

on four criteria: bandwidth, force, displacement capability, and ease of application. For applications seeking to

control aeroelastic phenomena, bandwidth, force, and

ease of application tend to be the major criteria; thus, inplane actuators suffice. However, for airfoil shaping,

displacement capability takes precedence. Therefore,

out-of-plane displacement actuators, such as piezoelectric stacks, RAINBOW wafers, and THUNDER wafers,

are most suited for this application. Selection of a specific actuator within the in-plane or out-of-plane

displacement actuator group is then based on consideration of the individual characteristics of the actuators,

including weight, life span (fatigue), and required

maintenance.

As in the case of the conventional control surfaces,

there are still many issues to address concerning the

use of piezoelectric adaptive-material actuators for

performance-enhancing shape control, including system

complexity, reliability, and scaling effects. However, if

such issues can be resolved, these actuators offer potential for use in future active systems, even on a full-scale

aircraft.

Test Systems and Apparatus

Wind Tunnel

The wind tunnel used for the ATI'ACH project was

the Flutter Research and Experiment Device (FRED), a

tabletop wind tunnel operated by the Aeroelasticity

Branch at Langley. This particular tunnel was selected

for the study because it had already been proven effective

for small-scale testing in 1993, when it was used to

investigate the use of in-plane piezoelectric actuators for

flutter suppression (ref. 18).

A photograph of the complete setup for the

ATTACH project, with components of the FRED wind

tunnel identified, is shown in figure 8. Figure 9 provides

a side-view schematic of the wind tunnel. As shown in

the figures, the FRED wind tunnel is an open-circuit tunnel with a 6- by 6-in. fully removable, Plexiglas I test section that also has a removable ceiling. Powered by a 2-hp

motor, the wind tunnel is capable of operating at a maximum velocity of 125 ft/sec. A single honeycomb screen

at the beginning of the contraction duct helps to smooth

the flow before it reaches the test section.

IRegistered trademark of Rohm & Haas Company.
ATTACH Testbed

The design of a testbed for the ATTACH project was

driven by four specifications. First, it had to accommodate the 1.5-in-wide, 2.5-in-long, 0.012-in-thick, 9-layeraluminum THUNDER wafer available for testing. Second, the span of the wafer had to appear infinite to the

flow (e,g., two-dimensional flow) to simplify the analysis of the data. Third, the process for integrating the testbed into the test section needed to be simple to facilitate

model changes and actuator performance checks. Finally,

restraints on the movement of the actuated wafer needed

to be minimized.

The final ATTACH testbed design is shown in figures 10 and 11. The ATTACH airfoil, shown in

figure 10, consisted of a base airfoil, the THUNDER

wafer, a thin fiberglass sheet, and a flexible latex membrane. The base airfoil was a 0.25-in-thick, 1.5-in-wide,

5-in-long Plexiglas symmetric airfoil that was positioned

at approximately midheight in the tunnel test section, as

depicted in figure 1l(a). This airfoil was supported by

two 0.25-in-thick, 10-in-long Plexiglas sidewall inserts

that extended through 85 percent of the length of the test

section, as shown in figure 11 (b), creating a nearly twodimensional flow condition between them. The

THUNDER wafer was placed 0.125 in. aft of the base

airfoil leading edge and extended from near 0 percent to

approximately 50 percent chord. Only the wafer trailing

edge was affixed to the base airfoil surface to permit relatively free expansion and contraction under an applied

voltage. To further minimize wafer restraint, 0.06-indeep notches were incorporated into the sidewall inserts

in the areas that would be traversed by the lengthwise

edges of the wafer. To smooth the airfoil-wafer interface,

the fiberglass sheet was wrapped over the upper surface

of the airfoil-wafer combination and held in place by the

latex membrane, which covered both the upper and lower

surfaces. A variable angle of attack mechanism was also

included in the design. The entire assembly (sidewall

inserts and ATTACH airfoil) could be slid easily in and

out of the removed test section, and with the test section

installed, minor adjustments could be made to the assembly by removing the test section ceiling.

For the two phases of testing mentioned earlier, two

sets of the sidewall inserts were developed. These inserts

differed only by the downstream mounting locations of

the ATFACH airfoil model in the wind-tunnel test section. For phase I, the model was positioned near the ends

of the sidewall inserts to permit maximum settling of the

flow, as shown in figure 12. For phase II, the model was

moved forward so velocity measurements could be taken

sufficiently aft of the airfoil trailing edge to allow the

wake to return to tunnel static pressure (ref. 25). This

version of the testbed is shown in figure 13.

Displacement Measurement System

Measurement of both the precise positions of the

THUNDER wafer in the tunnel at various angles of

attack and the displacements caused by applied voltages

required a very sensitive displacement measurement system. To eliminate disruptions in the flow around the airfoil, a nonintrusive, video-based measurement system

called EPIX that can detect displacements as small as

0.0004 in. was ultimately selected to monitor the

lengthwise-edge displacements of the wafer at 15 locations. These measurement locations along the wafer edge

were evenly distributed in 0.167-in. increments. The

measurement rates available from the EPIX system

included a 30-Hz sample rate for durations of up to 1 min

or a 10-Hz sample rate during continuous operation

(ref. 26). Photographs of the setup for the system, which

employed a single camera lens, a video junction box, and

a computer rack, are shown in figures 8 and 14.

The EPIX system operates by first isolating the highest contrast image in the field of view of the camera and

then producing a file that indicates the coordinates of the

points defining that image with respect to predetermined

reference positions (ref. 26). To provide the high contrast

needed to isolate the camera-side edge of the wafer, that

edge was painted with a coat of fluorescent paint and

illuminated with ultraviolet light. A photograph of the

model in the test section with the wafer edge illuminated

is shown in figure 15.

Wake Velocity Measurement System

As mentioned previously, the purpose of phase II

was to examine the drag-reducing potential of the

THUNDER wafer. To obtain the data for this phase of

testing, wake velocity measurements were taken by manually traversing a hot-film anemometer velocity probe in

0.125-in. increments through the center of the test section

3.70 in. aft of the airfoil trailing edge. This hot-film anemometer was part of a Kurz 443M air velocimeter, which

also provided an analog display for visual readouts in

meters per second.

Power Supply System

The following equipment supplied power to the

THUNDER wafer: a function generator, an APEX

Microtechnology Corporation PA85 power operational

amplifier, and two International Power direct-current

(de) power supplies. The amplifier, preset with a gain of

17, allowed maximum input voltages of +10 V; thus, the

maximum output voltages from the amplifier, which

were also the input voltages to the wafer, were limited to

+170 V. The corresponding maximum output current

was 140 mA. Wiring connections to the wafer for the

power supply system are shown in figure 16.

5
Wind-Tunnel Test Procedure

Identifying Material Characteristics

Phase I of the ATTACH project was performed to

identify the creep, hysteresis, performance repeatability,

and fatigue characteristics of the THUNDER wafer

under aerodynamic loading. The two goals for this phase

were to gain familiarity with the new THUNDER

actuator technology before applying it as a tool in airfoil

shaping and to identify variations in the response of the

wafer to different aerodynamic loading conditions.

To begin this phase of the project, a baseline for the

unconstrained performance of the wafer was determined

by applying a 2-Hz, -200-V to +400-V sine wave load to

the wafer, as depicted in figure 17. This magnitude was

selected because it represented the usable voltage range

established by the wafer poling boundaries. The resulting

wafer displacements obtained over this input voltage

range are shown in figure 18. As indicated in the figure,

the wafer cycled through a maximum out-of-plane displacement range of 0.129 in. while following a distinct

hysteresis curve. The remaining three material characteristics of interest, creep, performance repeatability, and

fatigue, were not apparent because the response was

dynamic and the load was applied for just a few cycles.

Upon completion of the baseline unconstrained performance tests, the THUNDER wafer was integrated into

the phase I testbed and installed in the test section of the

wind tunnel. Sixty conditions were then tested to identify

the effects of both applied voltage and tunnel velocity

on wafer behavior at various angles of attack. These conditions consisted of combinations of the following

parameters: five angles of attack (-2 ° , 0% +2 °, +4 °, and

+6°), four steady-state dc input voltages (-102 V,

+102 V, -170 V, and +170 V), and three tunnel velocities (wind off, 65.6 ft/sec, and 114.8 ft/sec). At each condition, the static input voltage signal pattern shown in

figure 19(a) was applied, and displacement measurements were taken with the EPIX system. The 30-Hz sample rate was used for 2 sec after 0, 0.5, 1, 2.5, 5, 7.5, 10,

10.25, 10.75, 11.25, 12.75, 15.25, 17.75, and 20.25 min

had elapsed to monitor any creeping of the material. The

15-sec increment after the first 10 min indicates the time

required to change the sign of the applied voltage. The

first 20 conditions (4 dc voltages at 5 angles of attack)

were tested with wind off and established baseline displacement references for the remaining 40 wind-on conditions. A typical plot of the wafer response to the static

input voltage signal is shown in figure 19(b). The displacement of the wafer caused by creep during the positive and negative applied-voltage periods was calculated

by determining the difference between points C and B

and F and E, respectively.

6

During this phase of testing, the reliability of the

THUNDER wafer was also examined by monitoring performance repeatability and material fatigue. Comparisons of wafer displacements measured under identical

angle of attack, applied voltage, and tunnel velocity conditions were used to estimate performance repeatability.

The presence of any THUNDER material fatigue was

identified after each day of testing by applying the maximum available input voltages (+170 V) to the wafer wind

off and comparing the resulting displacements to the corresponding displacements obtained prior to any windtunnel testing.

Identifying Airfoil Shaping Effectiveness

With the objective of determining the ability of the

THUNDER wafer to reduce drag over the airfoil,

phase II tests were conducted at the same 40 wind-on

conditions described for phase I. Through the use of the

wake velocity profile technique described in reference 25, velocity measurements were taken by manually

traversing a hot-film anemometer velocity probe in

0.125-in. increments vertically through the center of the

test section sufficiently aft of the airfoil trailing edge to

allow the wake to return to tunnel static pressure. Each

profile took approximately 4 to 5 min to complete; however, creep and hysteresis measurements could not be

correlated with the profiles because the ultraviolet lamps

used for the measurement system were removed during

this phase of testing to maximize space and eliminate the

possibility of heat exposure.

Results and Discussion

Creep, Hysteresis, Performance Repeatability,

and Fatigue Characteristics

The data obtained during phase I identified the presence of both creep and hysteresis of the wafer under

wind-off (aerodynamically unloaded) and wind-on (aerodynamically loaded) conditions. Typical wafer displacements observed in response to five cycles of the + 102-V

and +170-V applied voltage patterns are shown in

figures 22 and 21, respectively. The test conditions for

both of these examples were wind off with the base airfoil model at 0 ° angle of attack. Creep is characterized by

the increasing positive (up) and negative (down) displacements exhibited by the wafer while under constant

positive and negative applied voltages, respectively. As

previously discussed, these displacements are the B-to-C

and E-to-F displacements illustrated in figure 19(b). Hysteresis is represented by the "looping" wafer displacement patterns. The shapes of the hysteretic curves in

figures 22 and 21 differ from the one identified in

figure 18 because of the vertical offsets introduced by the

creep of the wafer. Dashed lines between the plots for the
five cycles identify these offsets. As expected, larger

magnitude voltages produced correspondingly larger displacements, with positive voltages expanding the wafer

away from the surface of the base airfoil and negative

voltages contracting the wafer down toward the surface.

The primary difference between the results obtained

for the conditions shown in figures 20 and 21 involved

the variation in wafer response to the two different constant positive applied voltages. As shown in figure 20,

for the lower voltage condition (+102 V), creep of the

wafer occurred in the same direction as the initial displacement. This result was anticipated because the voltage was well within the poling boundaries. For the higher

voltage condition (+170 V) shown in figure 21, however,

the wafer exhibited a downward creep (negative displacement), a result opposite to what is normally

expected. Typically, this type of behavior indicates that

the wafer has depoled. However, because +170 V is also

well within the poling boundaries of the wafer, this

behavior was more likely caused by the presence of the

latex membrane. For all positive applied-voltage conditions, when the wafer expanded upward, the membrane

would stretch, imposing a force on the wafer proportional to the product of the displacement and "spring constant" of the membrane. For the +170-V case, the

displacement of the THUNDER wafer apparently created

a large enough membrane force to move the wafer back

toward the surface of the base airfoil. Negative wafer displacements caused by both the-102-V and -170-V

applied voltages were increased with the membrane

present. Table 1 shows the relative effect of the membrane on the displacements achieved by the wafer. Residual strains introduced by hysteresis and creep also could

have contributed to the changes exhibited in the

displacements.

Table 1. Effect of Membrane on Wafer Displacements for WindOff Condition

Voltage,

V

+102

+170

-102

-170

Displacements, in.

Membrane

absent

+0.0242

+0.0440

--0.0105

-0.0285

Membrane

present

+0.0199

+0.0395

-0.0208

-0.0436

Difference,

percent

17.8

10.2

98.1

53.0

Wind-off versus wind-on (membrane present) wafer

displacement comparisons at 0° angle of attack are

shown in figures 22 through 25. Both creep and hysteresis were still apparent for the wind-on conditions, but the

presence of the flow contributed to typically smaller positive displacements (both before and after creep) as compared to wind off for the same positive applied voltages.

Displacements under negative applied voltages were

larger for the wind-on conditions. Figures 26 and 27

compare the displacements obtained for only the two

wind-on conditions at the +102-V and +170-V conditions, respectively, and reveal a second trend. Positive

wafer displacements were larger at the higher tunnel

velocity than at the lower velocity because of the

increased suction on the upper surface of the ATTACH

airfoil. Increased suction was also responsible for producing larger positive displacements at higher angles of

attack, as shown in figure 28. This figure presents wafer

displacements obtained at each of the five angles of

attack for the +102-V, 65.6-ft/sec flow condition. In this

case, some of the increased displacements also could

have resulted from more of the wafer being shielded

from the flow at the higher angles of attack. Figure 29

demonstrates the combined effect of increased velocity

and a higher angle of attack at the +102-V condition. As

expected, positive wafer displacements were larger at the

higher velocity than at the lower velocity. However,

because of the added influence of a +4 ° angle of attack,

the positive displacements for both velocities were larger

than those previously identified in figure 26 for the 0 °

angle of attack condition.

Comparisons of wafer displacements measured

under identical conditions at different times during

phase I identified discrepancies in the performance of the

THUNDER wafer. One such comparison for the wind

off, +102-V, 0° angle of attack condition is shown in figure 30. These measurements were made 2 weeks apart,

and as identified in the figure, they differed by as much

as 0.01 in. To minimize the introduction of such discrepancies in the creep and hysteresis data presented earlier,

only displacements measured on the same day were

compared.

After testing for 2 weeks in the tunnel, the overall

displacement performance of the THUNDER wafer

began to noticeably degrade. During subsequent examination, no visible flaws were found, but a 33-percent

(60-nF) drop in capacitance was discovered, and repoling

returned the wafer to original performance. No further

evidence of fatigue was encountered during the remaining month of testing. Thus, similar to other piezoelectric

adaptive materials, the performance of THUNDER

appears to be a function of capacitance. However, no

material property life studies were performed to solidify

this link.

Wake Velocity Effects

As a result of the effects of testing at low Reynolds

numbers (173883 and 304295 for the 65.6-ft/sec and

114.8-fffsec conditions, respectively), a possible interference from the sidewall inserts, the presence of flow
separation at higher angles of attack, and the omission of

the wafer nonlinearities, reliable quantitative profile drag

coefficients could not be obtained from the wake velocity

data. Consequently, for purposes of this feasibility study,

it was assumed that variations in drag were directly proportional to velocity changes in the wake of the model.

Comparisons of wake velocity for different test conditions, therefore, provided qualitative indications of the

drag-reducing potential of the THUNDER wafer for this

subscale model.

The wafer trends identified in phase II were consistent with those from phase I. Positive applied voltages,

which expanded the upper surface of the airfoil, had the

effect of increasing the velocity (and, therefore, the

momentum) in the wake, a result consistent with a

decrease in drag. By increasing the tunnel velocity and/or

the model angle of attack, even greater expansions of the

upper surface and, therefore, larger wake velocities were

obtained at the positive voltages. Negative applied voltages had the opposite effect on wake velocity (and, therefore, on drag). It should be noted that these wake velocity

trends were obtained using only 44 percent of the maximum unloaded capability of the wafer because of the

amplifier output voltage limitations. Thus, greater

increases in the wake velocity (or decreases in drag)

would be expected if that percentage was increased.

Concluding Remarks

Tabletop wind-tunnel tests were conducted to assess

the feasibility of using a thin-layer composite-unimorph

ferroelectric driver and sensor (THUNDER) wafer to

alter the upper surface of a small airfoil. Surface position

was measured with a nonintrusive video system at 15 stations along the chord. Upper surface position was clearly

affected by aerodynamic loading, the voltage applied to

the THUNDER wafer, and material creep and hysteresis.

Aerodynamic loads on the wafer varied with angle of

attack and affected the initial displacement of the wafer

when the voltage was applied. However, the force output

of the wafer appeared greater than the aerodynamic loads

at all times. For most cases of constant applied positive

voltages, the position of the upper surface would creep in

the positive direction from the initial positive displacement. The elastic membrane used to hold the wafer onto

the upper surface of the airfoil would hinder the positive

displacement of the wafer when the maximum amplifier

output voltage was applied. This undesirable impact may

be avoided by selection of a nonelastic membrane that

allows the wafer to expand fully. During constant applied

negative voltages, the position of the upper surface

would creep in the negative direction from the initial

negative displacement. Comparisons of the displacement

histories after each cycle of positive and negative applied

8

voltages revealed the hysteresis of the wafer. After a few

cycles, the hysteresis appeared to be bounded such that

the wafer appeared to be settling into a more predictable

displacement-voltage relationship. Based on the behavior

of the wafer during this study, the effects of creep and

hysteresis may be reduced through the use of feedback

control techniques. Because positive displacements

became larger during a constant positive applied voltage,

it is anticipated that the voltage commanded by a controller would be smaller with time. Further study is required

to verify the usefulness of feedback control in maintaining prescribed upper surface positions.

NASA Langley Research Center

Hampton, VA 23681-2199

May 21, 1997

References

1. Szodruch, J.; and Hilbig, R.: Variable Wing Camber for

Transport Aircraft. Prog. Aerosp. Sci., vol. 25, 1988,

pp. 297-328.

2. Renken, Jtirgen: Mission-Adaptive Wing Camber Control

Systems for Transport Aircraft. AIAA-85-5006, Oct. 1985.

3. Redeker, G.; Wichmann, G.; and Oelker, H.-C.: Aerodynamic

Investigations Toward an Adaptive Airfoil for a Transonic

Transport Aircraft. J. Aircr., vol. 23, no. 5, 1986, pp. 398--405.

4. Crawley, Edward F.; Warkentin, David J.; and Lazarus,

Kenneth B.: Feasibility Analysis of Piezoelectric Devices.

SSL 5-88, MIT, Jan. 1988.

5. Heeg, Jennifer; McGowan, Anna; Crawley, Edward; and Lin,

Charrissa: The Piezoelectric Aeroelastic Response Tailoring

Investigation: A Status Report. Smart Structures and

Materials 1995 Industrial and Commercial Applications of

Smart Structures Technologies, C. Robert Crowe, ed., SPIE,

Vol. 2447, Mar. 1995, pp. 2-13.

6. Lin, Charrissa Y.; Crawley, Edward E; and Heeg, Jennifer:

Open Loop and Preliminary Closed Loop Results of a Strain

Actuated Active Aeroelastic Wing. AIAA-95-1386, Apr. 1995.

7. Heeg, Jennifer; McGowan, Anna; Crawley, Edward E; and

Lin, Charrissa: The Piezoelectric Aeroelastic Response

Tailoring Investigation. International Forum on Aeroelasticity

and Structural Dynamics 1995--Proceedings, Royal

Aeronaut. Soc. (London), 1995, pp. 8.1-8.11.

8. McGowan, Anna-Maria R.; Heeg, Jennifer; and Lake,

ReneeC.: Results of Wind-Tunnel Testing From the

Piezoelectric Aeroelastic Response Tailoring Investigation.

AIAA-96-1511, Apr. 1996.

9. Moses, Robert W.: Active Vertical Tail Buffeting Alleviation

on a Twin-Tail Fighter Configuration in a Wind Tunnel.

International Forum on Aeroelasticity and Structural

Dynamics, CEAS, June 1997, pp. 269-276.
10. Ehlers, S. M.; and Weisshaar, T. A.: Static Aeroelastic

Behavior of an Adaptive Laminated Piezoelectric Composite

Wing. A Collection of Technical Papers A1AA/ASME./ASCE/

AHS/ASC 31st Structures, Structural Dynamics, and Materials

Conference, Part 3, Apr. 1990, pp. 1611-1623.

11. Ehlers, S. M.; and Weisshaar, T. A.: Static Aeroelastic Control

of an Adaptive Lifting Surface. J. Aircr., vol. 30, no. 4, JulyAug. 1993, pp. 534-540.

12. Weisshaar, Terrence: Active Aeroelastic Tailoring With

Advanced Materials. 31st Aircraft Symposium, Japan Soc.

Aeronaut. & Space Sci., Nov. 1993, pp. 34-45.

13. Muller, Mark B.: Transonic Drag Reduction Through the Use

of Induced Strain Actuators to Form an Adaptive Airfoil. M.S.

Thesis, Perdue Univ., May 1994.

14. Muller, Mark B.; and Weisshaar, Terrence A.: Maximizing

Deflections Produced by Induced Strain Beam-Like Actuators

for Transonic Drag Reduction. AIAA/ASME Adaptive

Structures Forum--Technical Papers, 1994, pp. 388--401.

15. Leeks, Tamara J.; and Weisshaar, Terrence A.: Optimizing

Induced Strain Actuators for Maximum Panel Defection.

Proceedings of the AIAA/ASME Adaptive Structures Forum,

1994, pp. 378-385.

16. Leeks, Tamara J.; and Weisshaar, Terrence A.: Optimization of

Unsymmetric Actuators for Maximum Panel Deflection

Control. Smart Structures and Materials 1995--Smart

Structures and Integrated Systems. Inderjit Chopra, ed., SPIE,

Vol. 2443, Feb.-Mar. 1995, pp. 62-74.

17. Lazarus, Kenneth B.; Crawley, Edward E; and Lin,

Charrissa Y.: Fundamental Mechanisms of Aeroelastic Control

With Control Surface and Strain Actuation. Proceedings of

the 32nd A1AA/ASME,/ASCE,/AHS/ASC Structures, Structural

Dynamics, and Materials Conference. AIAA, 1991,

pp. 1817-1831.

20.

21.

22.

23.

24.

25.

26.

Heeg, Jennifer: Analytical and Experimental Investigation

of Flutter Suppression by Piezoelectric Actuation. NASA

TP-3241, 1993.

Layton, Jeffrey B.: An Analysis of Flutter Suppression Using

Adaptive Materials Including Power Consumption. AIAA-95-

1t91-CP, 1995, pp. 299-305.

Hellbaum, Richard F.; Bryant, Robert G.; Fox, Robert L.;

Jalink, Antony, Jr., Rohrbach, Wayne W.; and Simpson,

Joycelyn O.: Thin Layer Composite Unimorph Ferroelectric

Driver and Sensor. U.S. Pat. 5,632,840, May 27, 1997.

Anderson, John D., Jr.: Introduction to Flight. Third ed.,

McGraw-Hill Book Co., 1989.

Weisshaar, Terrence: Aeroservoelastic Control Concepts With

Active Materials. ASME International Mechanical

Engineering Congress Exposition, Special Symposium on

Aeroelasticity and FluidStructure Interaction Problems, Nov.

1994.

Crawley, Edward E; and Anderson, Eric H.: Detailed Models

of Piezoceramic Actuation of Beams. J. lntell. Mater. Sys. &

Struct., vol. 1, Jan. 1990, pp. 4-25.

Haertling, Gene H.: Rainbow Ceramics--A New Type of

Ultra-High-Displacement Actuator, American Ceram. Soc.

Bull., vol. 73, no. 1, Jan. 1994, pp. 93-96.

Rae, William H., Jr.; and Pope, Alan: Low-Speed Wind Tunnel

Testing. John Wiley & Sons, 1984.

Burner, A. W.: Model Deformation Measurements at NASA

Langley Research Center. RTO Symposium on Advanced

Aerodynamic Measurement Technology, AGARD-CP-601,

1997, pp. 34-1-34-9.
Separated Attached .,_

I now _1 .ow i

"- Membrane

Figure 1. Streamline representation for nominal airfoil (wafer unactuated).

Figure 2.

Separated Attached a------

_ Membrane "_- Airfoil

Anticipated stxeamline representation with altered upper surface geometry (wafer actuated).

Electrodes

+

Applied

voltage

Figure 3. Conventional configuration for in-plane displacement piezoelectric actuator.

10
shim Applied

Piezoelectric wafer voltage

(a) Unimorph.

Shared electrode

plied

/ v_ltage f Piezoelectric wafers

/ 1i-[7:._..._-" i_ , _ ._

_ lr t -l't "_ _ T t ar "r ]

Applied

voltage

(b) Bimorph.

Figure 4. Configurations for unimorph and bimorph piezoelectric actuators.

2.5 in. )-

wafer

1.5 in.

Alternating

layers

in backup

V

0.012 in.

" ". _ Piezoelectric wafer

+ 6

I i , , , Backup material

_ (aluminum)

1' _ Polyimide

•_ (LaRCrM-sI)

Figure 5. Isometric view of THUNDER wafer with enlargement of side layering (not to scale).

Unacmamd

Figure 6. Unrestrained positive actuation of typical THUNDER wafer.

11
.10

r_

.08

-_ .06

_ .04

g

_. .02

?

8 o

o RAINBOW

50 100 150 200 250 300 350 400

Applied voltage, V

Figure 7. Comparison of out-of-plane displacements achieved by 1.5-in-wide, 2.5-in-long, 0.012-in-thick, 9-layeraluminum THUNDER wafer and 1.25-in-diameter, 0.02-in-thick RAINBOW wafer.

Amplifier rack

Wind-tunnel

test section

Figure 8. Photograph of ATTACH project experimental setup.

12
Fan

and

motor

Velocity

be _ /-- Wind-tunnel

Test section n_rtac_ti°

(11.75 in. long with (30 in. long) Screen

6- by 6-in. cross section)

• Airfoil supported by two vertical end-plate inserts

• Tunnel limits: velocity = 125 ft/sec

Figure 9. Side view of Flutter Research and Experiment Device (FRED) wind tunnel.

Fiberglass

THUNDER layer

Latex wafer _ -_

me_.__

0.25 in.

Basew_°_ _/ / _"--- 2 in'_ T

I attachment point --/ _. _ [

l" 5 in. ,1

Figure 10. Cross section of ATI'ACH airfoil model.

13
Ceiling

1.5in.

Floor

(a) Lookingdownstream.

Figure11.

Sidewall inserts

(leading edges Wind-runnel

11.75 in. _!

(b) Isometric view.

Wind-tunnel test section.

I

_"l 1.5 in.

I

11

K'

r--

,,e

Sidewall

insert

F Protractor to sidewall) (attached

Sidewall it'- Model Flow

_-Model rotation

), pivot point

Figure 12.

i_ 3.5 in.

10 in.

Aq_ACH testbed configuration for phase I.

:i

6 111.

m

I

2.5 in.

I

14
/

#

S Velocity probe

Sidewall

insert

Model _ Sidewall

\ notch ---x Flow

\ \ direction

Model rotation/

pivot point

2.5 in.

3.7 in. -_

10 in.

Figure 13. ATTACH testbed configuration for phase II.

in.

15
Figure 14. EPIX system computer rack.

16
Figure15.Photographof ATTACH model in FRED.

17
Sidewall

insert -k_

Airfoil--,_

N

"gSidewal 1

/ insert

\\\\\\\\\\ \\\\\\

Test section looking downstream

Z

1

Leads taped _ _ Y

to lowe._s___

ofairfofl_ ; ]

\

Bottom view of airfoil-wafer model

Positive lead

\

Leads taped _' II

down to THUNDER

maintai.'n _l wafer

connecuon -__ Y

(Exit area between z_ Negative lead

sidewall inserts)

Top view of airfoil-wafer model

Figure 16. Schematic of THUNDER wafer wiring connections.

>

O

o

<

4O0

2OO

0

-200

0 .5

Time, sec

1.O

Figure 17. Input voltage signal used for performance-evaluation testing.

18
Figure 18.

.a

E

_5

.12 -

.08 -

.04 --

0

--.04

-.08

-600

I _ I I I I

-400 -200 0 200 400 600

Applied voltage, V

THUNDER wafer displacements and hysteresis that resulted from dynamic actuation.

;>

_m

o

.o

e_

<

A

0

C

G

.03

..a .02

'_ .01

E 0

-.o_

e_ -.02

, i i i i -.03

5 10 15 20 25 -150 -100

Time, min

1 I

-50 0 50

Applied voltage, V

I I

100

(a) Input voltage. (b) Typical wafer response.

Figure 19. Input voltage signal used for phase I testing. Creep = B to C and E to F.

!

150

19
\

r

I

I

I

\1

A

I

I

I

I

I

4

i i

\ \

I I

I

I

t_

I

I

I" I" i"

•u! '_,uauaoaelds!G

>

.<

e_

@

@

L_

III II

i i i |

+1

I

o

@

_ •

, _

0 _

E

I,

d

t",l

i _ _ II I

i

• . ._ . .

i| I I

•u! 'luomo_lds!( I
r

il<

! I I

I

r) .,.._

<

I I

I I I I

"U! 'lu_tu_3elds!( 2

,.--, t"q _'_ ',d" t¢'_

-4-1

I

I

\ I \

"d \

, , , , _ , , × . _. _. _. _. _. _. .

I I I I

•u! 'luouaO_elds.to

C_

21
..r

u

.03

.02

.01

0

-.01

-.02

-150

0 Wind off

[] 65.6 ft/sec

I I I I I

-100 -50 0 50 100 150

Applied voltage, V

Figure 22. Wafer displacement changes caused by 65.6 ft/sec aerodynamic loading, +102 V, ot = 0%

.a

e_

_5

.03

.02

.01

0

-.01

-.02

-150

I I I

-100 -50 0 50

Applied voltage, V

0 Wind off

[] 114.8 ft/sec

I I

100 150

Figure 23. Wafer displacement changes caused by 114.8 ft/sec aerodynamic loading, +102 V, tx = 0%

Figure 24.

.a

e_

O

t5

.04

.02

-.02 0 Wind off

[] 65.6 ft/sec

--.04 I I I | I

-250 -150 -50 0 50 150 250

Applied voltage, V

Wafer displacement changes caused by 65.6 ft/sec aerodynamic loading, +170 V, _ = 0%

22
E

.04

.02

-.02

J

UI ! i

-150 -50 0 50

Applied voltage, V

O Wind off

o 114.8 ft/sec

-.04 I I

-250 150 250

Figure 25. Wafer displacement changes caused by 114.8 ft/sec aerodynamic loading, +170 V, o_ = 0%

..a

E

o

t_

_5

.03

.02

.01

0

-.01

-.02

-150

o 65.6 ft/sec

o 114.8 ft/sec

I i I I I

-100 -50 0 50 iO0 150

Applied voltage, V

Figure 26. Wafer displacements obtained at 65.6 ft/sec and 114.8 ft/sec, +102 V, o_ = 0 °.

.04

._ ,02

,,_r

e-,

o

E 0

-.02

-.04

-250

I |

-150 -50 0

65.6 ft/sec

o 114.8 ft/sec

I I I

50 150 250

Applied voltage, V

Figure 27. Wafer displacements obtained at 65.6 ft/sec and 114.8 ft/sec, +170 V, _ = 0 °.

23
.028

.024

-_ .020

.016

.012

.008

.004

0 20 40 60 80 100 120

Applied voltage, V

(a) ot = -2 °.

.028

.024

•_ .020

_ .016

_ .012

_5 .oo8

.004

0 20 40 60 80 100 120

Applied voltage, V

(b) ot=O °.

.028

.024

•-_ .020

.016 ¢

_ .012 .008

.004

I

0 20 40 60 80 100 120

Applied voltage, V

(c) (x = +2 °.

Figure 28. Effect of angle of attack on wafer displacements, +102 V, 65.6 ft/sec.

24
.028

.024

-_ .020

_ .016

8

.012

_5 .008

.004

20 40 60 80 100

Applied voltage, V

!

120

(d) a =+4 °.

.028

.024

-_ .020

i .016

_ .012

N .oo8

.004

!

0 20 40 60 80 100 120

Applied voltage, V

(e) IX=+6 °.

d

E

•028

.024

.020

.016

•012 o -2 °

.008 o 0 °

° .004 4 °

0 20 40 60 80 1O0 120

Applied voltage, V

(f) Composite of all angles of attack.

Figure 28. Concluded.

25
Figure 29.

.=A

.035

.030

.025

.020

.015

.010

.005

_65.6 ft/sec

J-_f, i , ° J114"8 ft/Tc

20 40 60 80 100 120

Applied voltage, V

Wafer displacements obtained at 65.6 ft/sec and 114.8 ft/sec, +102 V, ot = +4 °.

.03 f

i 01 F0"009in" 002 in.

-01 1 I-k_/ /1 ° Mo_ 5/s_55

-.02 _/2_9 J

-150 -100 -50 0 50 100 150

Applied voltage, V

Figure 30. Wafer displacements obtained for wind off, +102 V, cx = 0 °.

Form ,4p0rov_

REPORT DOCUMENTATION PAGE OMBNo.0704-0188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

November 1997 Technical Memorandum

4. TITLEANDSUBTITLE

A Feasibility Study To Control Airfoil Shape Using THUNDER

6. AUTHOR(S)

Jennifer L. Pinkerton and Robert W. Moses

7. PERFORMINGORGANIZATIONAME(S)ANDADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

9. SPONSORING/MONITORINGAGENCYNAME(S)ANDADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

5. FUNDING NUMBERS

WU 505-63-50-13

8. PERFORMING ORGANIZATION

REPORT NUMBER

L-17544

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM-4767

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited

Subject Category 05

Availability: NASA CASI (301) 621-0390

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The objective of this study was to assess the capabilities of a new out-of-plane displacement piezoelectric actuator

called thin-layer composite-unimorph ferroelectric driver and sensor (THUNDER) to alter the upper surface geometry of a subscale airfoil to enhance performance under aerodynamic loading. Sixty test conditions, consisting of

combinations of five angles of attack, four dc applied voltages, and three tunnel velocities, were studied in a

tabletop wind tunnel. Results indicated that larger magnitudes of applied voltage produced larger wafer displacements. Wind-off displacements were also consistently larger than wind-on. Higher velocities produced larger displacements than lower velocities because of increased upper surface suction. Increased suction also resulted in

larger displacements at higher angles of attack. Creep and hysteresis of the wafer, which were identified at each test

condition, contributed to larger negative displacements for all negative applied voltages and larger positive displacements for the smaller positive applied voltage (+102 V). An elastic membrane used to hold the wafer to the

upper surface hindered displacements at the larger positive applied voltage (+170 V). Both creep and hysteresis

appeared bounded based on the analysis of several displacement cycles. These results show that THUNDER can be

used to alter the camber of a small airfoil under aerodynamic loads.

14. SUBJECT TERMS

Piezoelectric actuators, THUNDER, Airfoil shaping, Creep, Hysteresis

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OF REPORT

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