Shape Memory Alloy Based

Morphing Aerostructures
In order to continue the current rate of improvements in aircraft performance, aircraft
and components which are continuously optimized for all flight conditions, will be
needed. Toward this goal morphing-capable, adaptive structures based on shape memory
Frederick T. Calkins alloy (SMA) technology that enable component and system-level optimization at multiple
e-mail: frederick.t.calkins@boeing.com flight conditions are being developed. This paper reviews five large-scale SMA based

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technology programs initiated by The Boeing Company. The SAMPSON smart inlet pro-
James H. Mabe gram showed that fully integrated SMA wire bundles could provide a fighter aircraft with
e-mail: james.h.mabe@boeing.com a variable engine inlet capability. The reconfigurable rotor blade program demonstrated
the ability of highly robust, controlled 55-Nitinol tube actuators to twist a rotor blade in
The Boeing Company, a spin stand test to optimize rotor aerodynamic characteristics. The variable geometry
P.O. Box 3707, chevron (VGC) program, which was the first use of 60-Nitinol for a major aerospace
Seattle, WA 93124 application, included a flight test and static engine test of the GE90-115B engine fitted
with controlled morphing chevrons that reduced noise and increased engine efficiency.
The deployable rotor tab employed tube actuators to deploy and retract small fences
capable of significantly reducing blade-vortex interaction generated noise on a rotor-
craft. Most recently, the variable geometry fan nozzle program has built on the VGC
technology to demonstrate improved jet engine performance. Continued maturation of
SMA technology is needed in order to develop innovative applications and support their
commercialization. 关DOI: 10.1115/1.4001119兴

1 Introduction additional capability to current, conventional structure, creating

multifunctional components with the ability to change shape while
The past decade has seen tremendous growth in the application
adding far less mass and requiring less power then hydraulics and
of smart materials technology to today’s commercial and military
electric motors. Shape memory alloys in particular hold tremen-
aircraft. One area that holds considerable promise for greatly im-
dous near term potential as actuation system elements, enabling
proving aircraft performance is morphing aerostructures 关1兴. Mor-
the next generation of morphing aerostructures.
phing aerostructures, sometimes referred to as smart structures,
active structures, or reconfigurable structures, increase an air-
craft’s performance by manipulating characteristics to better 2 Shape Memory Alloy Technology
match the system state to the operating conditions. The operating SMA active materials convert thermal energy into mechanical
conditions are defined by the environment and the desired aircraft energy. Heating the material above its austenitic transition tem-
task or mission. Many aircraft elements could be considered mor- perature activates a crystalline structural phase change causing the
phing aerostructures, including such common aircraft components material to revert to a previously “remembered” shape. When the
as flaps and landing gear. More exotic examples of morphing material is cooled below the transition temperature it transforms
include the rotating V22 Osprey rotor system and the B1B Lancer the Nitinol transforms into its more compliant 共lower modulus兲
variable swing wing. More specifically morphing often refers to martensitic form. The material can be easily deformed when a
continuous shape change in aircraft elements such as the wing 关2兴. load is applied during transition from austenite to martensite or
Morphing avoids compromise design points of traditional aircraft when in the final martensite state. SMA can also exhibit a two-
design. This increased design space translates into significant air- way shape memory effect 共SME兲, which results in a remembered
craft benefits including reduced operational weight, low part martensitic form as well as the austenitic form. The superelastic
count, long shelf life, aircraft adapted to changing flight condi- property is a reversible stress induced martensitic transformation,
tions allowing simultaneous increased range and payload, reduced which allows a material to fully recover after being deformed up
noise, reduced operating cost, reduced maintenance, repair time, to 10% strain.
and reconfiguration time 关3兴. Since the discovery of the shape memory effect in Nickel Tita-
Conventional morphing technology requires heavy, expensive, nium 共Ni–Ti兲, known as Nitinol, based alloys by Buehler in 1961,
and complex actuation systems, such as hydraulics or electric mo- these materials have provided great promise as actuators 关5兴. Nu-
tors, and associated structural reinforcement. These disadvantages merous commercial applications that utilize the alloy’s superelas-
make it a difficult proposition for new active structures to gain tic properties are now available, including antennas, medical
acceptance on aircraft, despite the considerable benefits they bring stents, and orthodontic wire. Some single-use applications em-
to the aircraft 关4兴. Smart materials, such as piezoelectrics, elec- ploying the alloy’s SME, such as high pressure unions for hydrau-
trostrictives, magnetostrictives, electroactive polymers, shape lic tubing, are also available. The high energy density and large
memory alloys 共SMAs兲 and polymers, and ferromagnetic shape strain capabilities of potential SMA actuators makes them particu-
memory alloys, enable unique morphing capability. Smart materi- larly attractive for aerospace applications, yet very few have
als can be fully integrated into load bearing structures, providing achieved wide spread commercial use 关6, chapter 1兴.
distributed actuation at the most advantageous point. This adds The history of SMA at The Boeing Company begins in the
1980s with an aggressive research program that resulted in numer-
ous patents and concepts for wire based spacecraft release mecha-
Contributed by the Design Automation Committee of ASME for publication in the
JOURNAL OF MECHANICAL DESIGN. Manuscript received December 1, 2008; final manu-
nisms, damping mechanisms, couplings, and bearings. In the
script received October 10, 2009; published online November 16, 2010. Assoc. Edi- 1990s government funding spurred programs that specifically de-
tor: Mary Frecker. veloped SMA aerospace actuators. Programs included the Smart

Journal of Mechanical Design Copyright © 2010 by ASME NOVEMBER 2010, Vol. 132 / 111012-1
Aircraft and Marine Project System Demonstration 共SAMPSON兲 austenitic and can thus apply large moments when attached to a
关7–9兴, which developed SMA wire bundles, and the shape structure. Flexures integrate well into aerospace structures and
memory alloy consortium 共SMAC兲 关10兴, which matured SMA ro- provide a simple method of transferring large forces and mo-
tary tube actuator technology. More recent work has seen the ments. Boeing has fabricated flexures of up to 25.4 cm 共10 in兲
maturation of SMA actuation technologies and its implementation long, 3.8 cm 共1.5 in兲 wide with complex varying cross sections
in aerospace applications. 关15兴. Such actuators can impart hundreds of pounds of force on a
Numerous studies have demonstrated that SMAs are an ideal structure. Low profile methods of attaching to a structure have
choice for actuation mechanisms for many aerospace applications been used, including threaded holes to accept bolts and simple end
关7–14兴. Simple, efficient, and low maintenance, SMA actuators brackets. This results in a low part count actuator system with
can be fully integrated and distributed into structures, reducing the reduced maintenance requirements.
added weight. This provides the opportunity to integrate actuation SMA tubes are designed to twist when thermally cycled creat-
mechanisms into current aircraft elements, thus creating active ing a solid-state high-torque rotary actuator 关10,13,14,21兴. When
multifunctional elements to give the aircraft additional capability. designed for actuation these tubes are usually thick-walled, with

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SMAs in general have a high energy density relative to other inner diameters up to 0.8 times the outer diameter. Boeing has
smart materials and conventional actuation technologies 关6兴; for developed extremely high-torque, large twist tubes. For example,
example, Nitinol has an energy density over 1000 J/kg. This high a 15 cm 共6 in兲 long, 1.25 cm 共0.5 in兲 diameter tube can output 50
work output per unit mass makes Nitinol an attractive lightweight degree twist at 90 Nm 共800 in lbs兲.
actuator. They can be considerably lighter then other electrome- Nitinol can be trained to produce complex three-dimensional
chanical actuators such as electromagnetic and electrohydraulic. shape changes. Currently, most morphing concepts exploit motion
Since these actuators are solid-state electromechanical devices, in a single dimension. Tube and flexure actuators are slightly more
complexities of hydraulics and pneumatics are avoided. Many complex in that their internal stresses are two-dimensional, but
aerospace applications require high force, thus methods to in- their primary motions are also one-dimensional. This leaves a
crease SMA actuator force output have been developed. large area of two-dimensional and three-dimensional motion vir-
The actuators discussed in the paper are all based on Nickel– tually untapped at this time. For example, a three-dimensional
Titanium 共Ni–Ti兲 family of alloys. Ni–Ti alloy systems, both bi- engine inlet, which could expand, contract, and change curvature
nary or equiatomic 共55-Nitinol, Ni–45Ti, 55% nickel by atomic to optimize engine performance, could be constructed exploiting
weight兲 and Ni-rich 共60-Nitinol, Ni–40Ti, and 57-Nitinol, Ni– Nitinol’s three-dimensional shape change capability. For a given
43Ti, respectively 60% and 57% Nickel by atomic weight兲 are the application, the goal is to seamlessly integrate the optimum SMA
most mature and best material options for near term application to actuator form into a complex aerospace structure.
aircraft 关15兴. Equiatomic Nitinol is the most common alloy with As discussed above, SMA actuators are thermally activated.
greatest commercial availability in a variety of forms. There is a The conventional view is that this thermal activation is detrimen-
long history of its successful use in commercial products. Until tal because it reduces the operational cycling rate. However, by
recently highly nickel-rich Ni–Ti alloys were not available com- taking advantage of the thermal activation, the system can be
mercially; however, research into the role of nickel in Ni–Ti alloys designed to be autonomous. The SMA actuators act as energy
has been under investigation for decades 关16–20兴. Otsuka pro- harvesting elements, using environmental thermal changes, such
vides an outstanding overview of the field 关16兴. He discusses in as the temperature difference between sea level and high altitude,
to activate the system. Thus the system is greatly simplified as no
detail the background and current understanding of the develop-
power, controls, or wiring is required.
ment and effects of the secondary nickel-rich phases in Ni–Ti
alloys. Wojcik has completed work which examines the transition
temperature, strain capability, and the effect of thermal aging on 3 Aerospace Applications
nickel-rich Ni–Ti alloys 关17,18兴.
Boeing Phantom Works pioneered the use of the nickel-rich This section describes efforts to apply SMA technology to aero-
Ni–Ti alloys for aerospace applications. The nickel-rich Ni–Ti al- space applications. This includes the development of morphing
loys have excellent thermomechanical stability, their transition aerostructure systems, the steps required to integrate the actuation
temperature can be set by a heat treat process, and they do not capability into the structure, and the analysis and testing required
require cold-work, which allows for hot forming of complex to conduct significant system-level tests, including flight tests.
shapes 关15,19兴. A comparison of 55-Nitinol and 60-Nitinol con- Five programs capture different aspects of SMA actuator design
ducted for the reconfigurable rotor blade 共RRB兲 program showed including SAMPSON smart inlet, reconfigurable rotor blade, vari-
that in tube form 55-Nitinol exhibits over twice the dynamic out- able geometry chevron 共VGC兲, deployable rotor blade aerody-
put of 60-Nitinol 关13,14兴. However, the Ni-rich alloys have shown namic device, and variable area nozzle.
much greater dimensional stability under thermal-mechanical cy- 3.1 SAMPSON Smart Inlet. The SAMPSON smart inlet was
cling, thus making them useful in flexure actuator forms 关15兴. a DARPA funded effort to demonstrate a full scale F-15 smart
Several forms of Nitinol actuators have been developed, flex- structure inlet operating in a realistic environment 关7–9兴. A shape
ures, beams, wire or tendons, cables, and coils. As actuators each changing inlet has significant benefits over fixed geometry. The
of these has advantages and disadvantages, which lead to pre- smart inlet would provide 20% increase in mission radius and
ferred actuation forms for some applications. enable a subsonic optimized strike aircraft to perform a supersonic
For many years, most actuator applications utilized SMA in intercept mission. The SAMPSON program focused on changing
wire form. Wire has many advantages with the main one being three inlet characteristics: capture area, internal duct shaping, and
availability of material and performance data. Wire actuators are lip shape. Two of the SAMPSON smart inlet components, cowl
used in tension creating a simple and efficient linear actuation rotation and lip deflection, are discussed below.
method. Direct current heating of the high resistivity wire allows The objective of the program was to replace the current F-15
for relatively rapid thermal-mechanical cycling. Some of the chal- hydraulic actuator used to rotate the cowl, changing inlet capture
lenges with wires include electrical and mechanical end connec- area, with an SMA actuator. Sixty Ni–45Ti wires 共0.17 cm
tions. Complex mechanisms are required to get large force and 共0.0675 in兲 diameter兲 were bundled together in a 7.5 cm 共3 in兲
stroke actuation systems. Bundled wire designs similar to cables diameter package to provide over 90,000 N 共20,200 lbs兲 of axial
provide an efficient method to increase force output and reduce force output. The wires were isolated along the length with Teflon
failures 关7–9兴. tubing and electrically connected to enable direct current resistive
The simplicity of the monolithic flexure actuator is a great ad- heating. The actuator is shown in Fig. 1. Two 190 cm 共75 in兲 long
vantage. Flexures can be designed to bend or straighten when bundles were attached on either side of a chain connected to a

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Fig. 1 SAMPSON Smart Inlet cowl rotation actuation system,
SMA wire bundle actuator, with inset design schematic †7‡

slider, which drives a mechanism that rotates the cowl about its
hinge point. The SMA bundles work antagonistically; when one is
activated it moves the slider and stretches the opposing bundle.
The bundles were placed in tubes, which reduced the compression
loading generated during actuation and provided a conduit for
cooling the actuators with compressed air. Fig. 3 SAMPSON Smart Inlet installed in NASA Langley 16 feet
A rigid lower lip deflection, as shown in Fig. 2, was an addition transonic tunnel †8‡
to the current inlet design and provided a 20% capture area
change when the lip was rotated 15 deg. The rigid lip was actuated
on either side of a hinge to deploy and retract it. Smart Flexskin, fixed-shape rotor blades dictates a compromise twist that is sub-
which uses SMA wires and structural supports in an elastomeric optimum in both cruise and hover. Significant increases in payload
panel, was used to actuate the lip. Forty SMA wires were embed- and range can be achieved if the rotor blade can be designed to
ded in each of two Smart Flexskins located on either side of the optimize twist at both cruise and hover flight conditions.
hinge. A rotation of plus and minus 15 deg was desired, requiring The basic concept of the RRB is illustrated in Fig. 4, which
53 cm 共21 in兲 of wire at a working 4% strain level. The wires were shows a Nitinol tube actuator assembly located inside the spar
integrated into a plate, which extended into the lip structure. near the root of the blade. The actuators were integrated into the
The SAMPSON smart inlet was tested at the NASA Langley rotor blade as structural elements controlling blade twist. The pas-
4.8 m 共16 ft兲 transonic tunnel in Hampton, VA, as shown in Fig. 3, sive torque tube transmits the torque from the actuator assembly
during two entries in 2000 and 2001. The tests demonstrated the to the tip of blade causing the blade to twist. A lightweight spring
actuators performance under realistic operating conditions. The mechanism, the strain energy shuttle, provides an energy storage
cowl rotation actuators moved the cowl 6 deg in 30 s under the element between the SMA actuator and the passive torque tube.
maximum aeroloads for 0.8 Mach flow. The lip deflection actuator The strain energy shuttle reduced the actuator system weight by
successfully rotated the lip through 23 deg in 30 s under the maxi- half. Figure 5 shows the actuator system before assembly in the
mum aeroloads. The SAMPSON program completed the first full rotor blade.
scale aerospace demonstration of a large force and displacement In 2007, the actuator was tested in the V/STOL wind tunnel
smart materials system. using a 1/4 scale three blade hub assembly mounted on the Boeing
3.2 Reconfigurable Rotor Blade (RRB). The RRB program Advanced Rotor Test Stand, see Fig. 6. The Ni–Ti actuation sys-
was funded by NAVAIR with a goal of demonstrating the poten- tem employed 55-Nitinol 共Ni-45Ti兲 rotary actuators in a quarter-
tial to improve rotorcraft performance by optimizing the configu- scale rotor blade. The wind tunnel test was a high-fidelity assess-
ration of major structures in flight 关12,13兴. Conventional design of ment of the SMA actuators. It represents one of the first attempts
to incorporate SMAs into an actuator, which provides more than
6.8 Nm 共30 in lbs兲 of torque and 3.4 Joules 共2.5 ft lbs兲 of energy

Fig. 4 Rotor blade system with actuator system „circled… at

Fig. 2 SAMPSON rigid lip deflection, installed „top… design the rotor base, antagonistic actuators, passive torque tube, and
schematic „bottom… †7‡ strain energy shuttle †12‡

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Fig. 7 VGC program development 2002 initial concept through
2005 flight test and 2006 static engine test
Fig. 5 RRB actuator system prior to installation †13‡

structural elements, an inner and outer sleeve. The VGCs are an

and is able to withstand a rotor environment. The actuator pro-
integral extension of the inner sleeve. On the finished thrust re-
vided approximately 250 twist transitions during 75 hours of test-
verser sleeve the sensor and power wiring passed through a con-
ing with no loss of performance or operational anomalies. This
duit in the outer sleeve core, exited into a cavity between the inner
test represented a new benchmark for SMA actuators and active
and outer sleeve, and was then routed into the engine’s fan case.
rotor blades.
The instrumentation wiring was terminated at a data system
3.3 Variable Geometry Chevron. One source of noise from mounted in the fan case, while the power wiring continued down
commercial high-bypass ratio turbofan engines is the turbulent the wing and into the airplane cabin. A proportional-integral-
mixing of the hot jet exhaust, fan stream, and ambient air. Serrated derivative control system employed strain gages for position feed-
aerodynamic devices, or chevrons, immersed into the flow at the back and controlled the input to surface mounted heaters on the
nacelle trailing edge have been shown to significantly lower jet actuators.
noise at take-off and reduce shock cell noise during cruise 关19兴. The flight test actuators were fabricated from a single billet of
The practical use of these devices requires a compromise between Ni–40Ti, which had been hot rolled into nominally 0.635 cm
noise reduction and engine performance. While the immersed 共0.25 in.兲 thick plate 关14兴. The flexure’s basic shape was formed in
chevrons reduce noise, their immersion also creates drag or thrust a series of water jet and wire EDM machining steps. The finished
loss. These losses result in a penalty for flights with long cruise actuators were 25.4 cm 共10 in.兲 long by 3.8 cm 共1.5 in.兲 wide with
times. The VGC was developed to utilize compact, light weight, smoothly varying thickness from 0.44 cm 共0.175 in.兲 at the middle
and robust shape memory alloy actuators to morph the chevron to 0.15 cm 共0.06 in.兲 at both ends. The tapered shape was a modi-
between a shape optimized for noise reduction at takeoff and a fied constant stress beam that minimized the size and weight of
shape at cruise that reduces shock cell noise without compromis- the actuators to fit into the available space while still providing the
ing engine performance 关10,11,20兴. The VGC program went from
initial concept to a full scale flight test in three and one-half years,
Fig. 7. The flight test validated that the technology could be safely
integrated into the airplane’s structure and flight system. It repre-
sented a large step forward in the realization of SMA actuators for
aerospace applications.
The full flight design included 14 chevrons integrated into the
trailing edge of a GE90-115B thrust reverser’s acoustic panel, as
shown in Fig. 8 关10兴. The VGC thrust reverser sleeve was fabri-
cated using modified production tooling and processes. The VGCs
were a carbon composite extension to the standard production unit
and were manufactured at Boeing’s Wichita facility 共now Spirit
Aerosystems兲 within the normal factory schedule and process
flow. The GE90-115B thrust reverser consists of two primary

Fig. 8 777–300ER flight test of variable geometry chevron

thrust reverser sleeve mounted on GE90-115B engine „top…,
close up showing cover removed and 60-Nitinol flexure actua-
Fig. 6 RRB 1/4 scale rotor blade in Boeing V/STOL tunnel †13‡ tors „bottom… †11‡

111012-4 / Vol. 132, NOVEMBER 2010 Transactions of the ASME

required austenite forces. Two holes were cut and tapped at the
midpoint of each actuator for fastening to the substrate. Two sepa-
rate heat treatments were used to set the austenitic shape and set
the actuator’s transition temperatures. Figure 8 shows a pair of
SMA flexure actuators before and after the final heat treat pro-
cesses. To maintain consistent chevron performance over the du-
ration of the calibration and flight tests, thermal-mechanical con-
ditioning methods to control actuator dimensional stability were
used. Prior to installation each actuator was conditioned for 100
thermal-mechanical cycles resulting in a very stable shape change
over many cycles.
In August 2005, Boeing tested a number of noise reduction Fig. 9 Conventional flap actuator †11‡
technologies on an All Nippon Airway 共ANA兲 777-300ER as part

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of the Quiet Technology Demonstrator 2 program. The VGC
thrust reverser translating sleeve was tested on a modified com- vides a means to operate in higher temperature environments by
mercial GE90-115B engine. The VGC system was tested on six allowing initial SMA transition without deploying the flap.
flights over five days with three different engine configurations. Springs near the front of the hinge aid in synchronizing the mo-
Autonomous operation and individual control of the 14 VGCs was tion and can be used in conjunction with a locking mechanism to
demonstrated. Nine different chevron configurations were exam- act as a fast stowage actuator.
ined in the controlled mode, investigating both community noise The double acting hinge device and the conventional flap de-
and shock cell noise performance. The system was able to vice were tested in a subsonic wind tunnel at the University of
smoothly and quickly move between immersion configurations at Cincinnati. The double acting hinge was also tested in a centrifu-
cruise conditions, allowing a parametric study of chevron shapes gal test stand. The purpose of the centrifuge testing was to induce
for shock cell noise reduction. Test configurations included both air flow, aerodynamic and centrifugal loading, characterize the
uniform and azimuthally varying immersion configurations. All actuation response, and explore design issues. The wind tunnel
instrumentation, power, gages, sensors, and controller hardware test assessed the potential acoustic benefits by measuring air flow
and software worked perfectly throughout the testing. 3D finite velocities around the airfoil and characterized the impact of de-
element modeling of the flight hardware accurately captured the vice deployment on lift and drag, see Figs. 11共a兲–11共c兲.
performance of the VGC and points toward such models as useful
3.5 Variable Area Fan Nozzle. A significant reduction in
for actuator and system design 关6兴.
noise and improved fuel consumption can be achieved by varying
3.4 Deployable Rotor Blade Aerodynamic Device. Previous the area of a commercial jet engine’s fan nozzle. A larger diameter
research has shown that small tabs on the rotor blades can reduce at takeoff and approach can reduce jet velocity reducing noise.
noise caused by blade-vortex interaction 共BVI兲; however, these Adjusting the diameter in cruise, to account for operating condi-
tabs also increase drag and decrease overall performance. The tions such as varying Mach number and altitude, can optimize fan
ideal solution is to integrate deployable tabs on the rotor blade loading and reduce fuel consumption. Two examples of variable
that can be can be extended when quiet operation is desired and area fan nozzles 共VAFNs兲 employing SMA flexure actuators have
stowed when they are not needed. Previous conventionally acti- been built, tested, and displayed. The first was a scaled version
vated rotor blade tab concepts were not pursued due to their com- used for noise tests and the second was a full scale functional
plexity and weight. Drawing on the experience from the RRB display.
program, an obvious solution was a SMA rotary tube-based Figure 12 shows a scaled variable area jet nozzle capable of a
actuator. 20% area change 关22兴. Shape memory alloy flexure actuators were
The Boeing Co. developed the robust solid-state active hinge used to position 12 interlocking panels at the nozzle exit. The
pin actuator 共AHPA兲, which uses a torque tube of trained SMA nozzle has an outlet diameter of 7 cm 共2.75 in.兲. There are 12
material as a structural hinge pin 关21兴. This actuation package can interlocking aluminum panels that make up the last 2.86 cm
reliably apply high-torque and angular displacement with a low 共1.125 in.兲 of the nozzle. SMA actuators designed to expand and
space, weight, and power burden. It uses SMA tube technology contract the nozzle when heated are attached to alternating panels.
similar to that employed by the RRB program as a solid-state The interlocks and actuator forces are designed such that each
rotary actuator. This application demonstrates the integration of a panel’s edge stays in contact with the edges of the adjoining pan-
compact, low-cost, reproducible, rugged, and efficient torsional els. A SMA actuator, made of Ni–40Ti 共60% by weight nickel兲,
actuator that can survive the harsh rotor blade environment. was attached to each panel. The actuator base is attached to the
This study investigated the use of two AHPA designs to enable support ring and the tip is attached near the free end of each panel.
the deployment of noise reduction devices on rotor blades. The A small resistive heater is bonded on the surface of each actuator.
conventional flap and the double acting hinge were developed to The expanding actuators are trained to curve away from the cen-
meet the displacement, force, and frequency requirements of a
BVI noise reduction tab within acceptable weight and size limits.
The conventional flap, shown in Fig. 9, is attractive because of
its simple, compact design, which allows the SMA actuator
mechanism to be exposed to cooling air flow when the flap is
deployed. It has two torque tubes that each control one flap. One
end of the tube is fixed to the rotor while the other is attached to
the flap. When the tubes are thermally activated they twist, open-
ing the flaps. A gear system ensures synchronized deployment
from both the upper and lower surfaces of the blade.
The double acting hinge actuator shown in Fig. 10 employs a
rotary actuator to rotate the drive component, which activates the
flap. The pinion gear synchronizes the top and bottom flap motion.
A gap between the drive component and the flap allows the tube
actuator to move to the stowed position without moving the flaps,
which are held open by a locking mechanism. This gap also pro- Fig. 10 Integrated double acting hinge actuator †11‡

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Fig. 11 „a… Boeing centrifugal test stand, „b… flow field in wind tunnel, and „c… University of Cincinnati wind tunnel †21‡

terline of the nozzle when heated pulling the panel tip with it. The consisting of a substrate, SMA flexure actuators with surface
contracting actuators are trained to bend toward the centerline mounted heaters, sensors, and covers. The substrate was aerospace
when heated, pushing the panel tip into the nozzle flow, see Fig. composite fiber reinforced plastic similar to the variable geometry
13. The actuators were 1.9 cm wide 共0.75 in.兲 ⫻ 4.4 cm long chevron thrust reverser sleeve. Each panel included a single 57-
共1.75 in.兲 and tapered from 2 mm 共0.08 in.兲 thick at the base and Nitinol 共Ni–43Ti, 57% by weight nickel兲 flexure actuator effi-
1 mm 共0.04 in.兲 thick at the tip. A closed loop control system was ciently integrated inside the space available at the end of a thrust
used to maintain a range of constant diameters with varying flow reverser sleeve. The actuators were designed to curve when heated
conditions and to vary the diameter under constant flow condi- causing the panel to bend out, opening up the end of the fan
tions. Acoustic data by side line microphones and flow field mea- nozzle. A proportional-integral-derivative control system em-
surements at several cross sections using particle imaging veloci- ployed thermal feedback via thermocouples and position feedback
metry 共PIV兲 showed that nozzle area had a significant effect on via strain gages mounted on the substrate. The control system
the flow and produced noise reduction. managed the power to a surface mounted resistance heater on each
A VAFN was displayed as part of Boeing’s Environment Ex- actuator. Individual control of each panel allowed a variety of
hibit at the 2008 Farnborough Air Show. The display included the operational modes, including the deployment of every other panel,
aft end of a full scale 777 size engine with a shape changing fan as seen in Fig. 15.
nozzle. As shown in Fig. 14, the model had 14 active panels each
4 Future Developments
The realization of the next generation of morphing aerostruc-
tures enabled by distributed and fully integrated three-dimensional
actuating and adaptive structures are within reach. Focused pro-
grams that bring together all elements of material research, system
integration, tool development, design, fabrication, controls, and

Fig. 12 Scale model variable area nozzle contracted and ex-

panded 20%

Fig. 14 Variable area fan nozzle display at Farnborough Air

Fig. 13 Variable area nozzle configuration †22‡ Show 2008

111012-6 / Vol. 132, NOVEMBER 2010 Transactions of the ASME

 industry has pushed the development of characterization standards
and fabrication methods that may be applicable to the aerospace
industry.
5 Summary
Real world applications for optimally responsive structures
based on current SMA technology, such as RRB, VGC, Sampson
smart inlet, AHPA, and VAFN, are within reach. Tests of large-
scale actuator systems, including wind tunnel tests and most im-
portantly flight tests, demonstrate the value of SMA actuators to
solve real world aerospace problems. Such tests validate that the
technology can be safely integrated into the airplane’s structure
and flight system, and represent a large step forward in the real-

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Fig. 15 VAFN panel showing flexure actuator and display with ization of SMA actuators for aerospace applications.
covers off showing SMA flexure actuator
Acknowledgment
The authors would like to thank the numerous members of The
Boeing Co. and partners whose creativity, hard work, and perse-
business case analysis are needed. Each of the programs in the last verance made these projects possible.
section required a thorough understanding of these elements and References
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Journal of Mechanical Design NOVEMBER 2010, Vol. 132 / 111012-7