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Flexfloil Shape Adaptive Control Surfaces—Flight Test and Numerical Results

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Flexfloil Shape Adaptive Control Surfaces—Flight Test and Numerical Results FLEXFLOIL SHAPE ADAPTIVE CONTROL SURFACES—FLIGHT TEST AND NUMERICAL RESULTS Sridhar Kota∗ , Joaquim R. R. A. Martins∗∗ ∗FlexSys Inc. , ∗∗University of Michigan Keywords: FlexFoil, adaptive compliant trailing edge, flight testing, aerostructural optimization Abstract shape-changing control surface technologies has been realized by the ACTE program in which the The U.S Air Force and NASA recently concluded high-lift flaps of the Gulfstream III test aircraft a series of flight tests, including high speed were replaced by a 19-foot spanwise FlexFoilTM (M = 0:85) and acoustic tests of a Gulfstream III variable-geometry control surfaces on each wing, business jet retrofitted with shape adaptive trail- including a 2 ft wide compliant fairings at each ing edge control surfaces under the Adaptive end, developed by FlexSys Inc. The flight tests Compliant Trailing Edge (ACTE) program. The successfully demonstrated the flight-worthiness long-sought goal of practical, seamless, shape- of the variable geometry control surfaces. changing control surface technologies has been Modern aircraft wings and engines have realized by the ACTE program in which the high- reached near-peak levels of efficiency, making lift flaps of the Gulfstream III test aircraft were further improvements exceedingly difficult. The replaced with 23 ft spanwise FlexFoilTM variable next frontier in improving aircraft efficiency is to geometry control surfaces on each wing. The change the shape of the aircraft wing in-flight to flight tests successfully demonstrated the flight- maximize performance under all operating con- worthiness of the variable geometry control sur- ditions. Modern aircraft wing design is a com- faces. We provide an overview of structural and promise between several constraints and flight systems design requirements, test flight envelope conditions with best performance occurring very (including critical design and test points), re- rarely or purely by chance. Several studies have sults from structural fatigue and acoustic testing, also shown the benefits of a seamless variable and CFD estimates on drag reduction. We also camber wing to optimize the performance across include a CFD-based aerostructural shape opti- various flight conditions and over a range of lift mization study to evaluate the application of this coefficients [13]. The cited benefits include in- technology to a twin-aisle commercial aircraft. creases in lift to drag ratio and better perfor- mance throughout the flight envelope, resulting 1 Introduction in fuel savings and improvements in maneuver- ability as well as operational flexibility. Ear- The U.S Air Force and NASA concluded a se- lier attempts by the U.S. Air Force, via its Mis- ries of flight tests on a Gulfstream III business sion Adaptive Wing (MAW) technology in the jet retrofitted with shape adaptive trailing edge 80s, demonstrated the aerodynamic gains of mor- control surfaces under a program called Adap- phing, but these were negated by increases in tive Compliant Trailing Edge (ACTE) [10,4] structural weight, power, and complexity of the 1 . The long-sought goal of practical, seamless, mechanisms and actuators needed [12,9]. Like- 1https://www.nasa.gov/centers/ armstrong/feature/ACTE_30_percent_less_ noise.html 1 KOTA , MARTINS wise, other attempts by researchers to achieve A shape-changing airfoil is required to sup- the same goal through use of “smart materials” port large air-loads while simultaneously adapt- suffered from similar weight and power penal- ing to different operating conditions. That is, ties and also lacked scalability. The key inno- the design has to be both flexible and strong. vation that enabled the FlexFoilTM ACTE design Flexibility and strength are usually considered to reach this goal is through exploitation of natu- antithetical in conventional engineering design ral elasticity of materials to create flexible struc- where strength is usually achieved through rigid- tures that can bend and twist with uncompro- ity. The success of the FlexFoilTM shape chang- mising strength [11]. This new design paradigm ing control surface is directly related to the un- combines the principles of computational me- derlying method of compliant design[8] of kine- chanics and kinematics, and was based on ini- matic structures (or joint-less mechanisms) with tial basic research conducted at the University of distributed compliance. The compliant design Michigan and 15 years of structural, wind tunnel, method exploits the natural elasticity of common and flight-testing conducted by FlexSys under materials, such as aluminum, steel, titanium, and Air Force SBIR Phase II and III programs. The composites. By combining the principles of con- ACTE is envisioned as a multifunctional aerody- tinuum mechanics and kinematics, we developed namic surface [3] that enables (1) airfoil shape algorithms for optimal arrangement of material to be optimized for minimum drag over a broad (skeletal configuration without joints) with built- range of flight conditions including large deflec- in mechanical advantage that enables desired tions for use in high lift conditions and cruise trim shape changes in a controlled fashion while sup- (2) actuation at high enough rates to enable load porting significant external aerodynamic loads. alleviation during maneuvers and gusts, leading Each section of the compliant structural system to lighter weight wing structures (3) spanwise shares the load more or less equally and un- twist to achieve optimal distributions and reduce dergoes the specified deformation without local induced drag. stress concentrations. The goal is to distribute the strain energy more or less equally, while ac- tively morphing the surface to desired contours and supporting the external air loads. This dis- tributed compliance enables large deformations with low stresses so that the system can be de- signed for high fatigue life. 2 FlexFoil Adaptive Complaint Trailing Edge (ACTE) As a part of Air Force SBIR Phase III program, the Air Force Research Laboratory (AFRL) pro- cured a Gulfstream III aircraft for flight-testing FlexFoilTM variable geometry control surfaces. AFRL and FlexSys teamed up with NASA’s En- vironmentally Responsible Aviation (ERA) pro- gram in 2009. Engineers at NASA’s Armstrong Fig. 1 High-lift flaps of a Gulfstream III busi- Flight Research Center have successfully modi- ness jet (NASA–Air Force test aircraft) were re- fied and instrumented the Gulfstream III and con- placed with 19 ft multi-functional variable geom- ducted a series of flight tests between Novem- etry control surfaces on each wing, including 2 ft ber 2014 and April 2015. The test aircraft was wide transition surfaces. named the SubsoniC Research Aircraft Testbed 2 FLEXFLOIL SHAPE ADAPTIVE CONTROL SURFACES (SCRAT). During Phase II of testing (ACTE aircraft flight envelope, where the red dots cor- II), NASA extended the flight envelope up to respond to the design limit load cases for which M = 0:85, twisted the controls surfaces span- the ACTE system was designed and validated wise, and conducted numerous acoustic flight through physical testing. tests. The primary trailing edge wing flaps on the Gulfstream III were replaced with 19 ft span- wise FlexFoilTM aircraft control surfaces on each wing, including 2 ft wide compliant fairings at each end. The FlexFoilTM surface’s internal mechanism had no joints to wear out, and the profile was crafted directly to the fixed portion of the wing. The shape morphing design distributes compliance throughout the structure, changing the wing camber from −9 to +40◦ on demand, as well as being able to twist spanwise along the trailing edge at up to 30 deg/second for gust- load alleviation. The control surfaces are able Fig. 2 Design loads were derived via Tranair and to generate over 11,500 lbs of lift and yet main- Star-CCM+ CFD codes. tain their unique flexibility. The surfaces are also rugged, able to withstand −650F to 1800F, harsh chemicals, and tested without failure to last five times the life cycles of a commercial air- craft. Because of its underlying mechanics, the FlexFoilTM control surface can be twisted span- wise along its length, to tailor spanwise lift dis- tribution. This capability offers two additional benefits: (1) shifting aerodynamic loads closer to the wing root thereby reducing wing stresses, hence allowing for lighter wing structures and ad- ditional fuel savings, and (2) reducing induced drag saving even more fuel. Although Airbus A350 and Boeing 787 already tailor the spanwise lift distribution using ailerons and flaps to reduce induced drag during cruise, FlexFoilTM control Fig. 3 Gulfstream III flight envelope and final de- surfaces offer a more refined and smooth span- sign limit load cases used for ACTE system de- wise variation due to it ability to morph (chord- sign (red dots). wise and spanwise) seamless surfaces. 2.1 Design Loads and Validation 2.2 Structural Tests and Model Validations Based on CFD analysis using TRANAIR and A building block approach was adopted in se- Star-CCM CFD codes on full 3D aircraft geom- quentially validating, through material character- etry at various flap positions and flight config- ization, computational analysis and physical test- urations, a comprehensive list of lift and hinge- ing, every component, sub-assembly and final as- moments was generated by NASA (Fig.2). The sembly of the ACTE flight articles. This enabled resulting load-limit cases were used to design the us to gain insights and identify potential failures FlexFoilTM ACTE system. Figure ?? shows the early in the design and development process to 3 KOTA , MARTINS avoid delays and cost increases. Nonlinear fi- flight article does not have any actuators, and nite element models of the flight article were de- therefore there is no in-flight actuation of the veloped incorporating material characterization ACTE control surfaces. However, the ground test data and validated with data from structural article (the Iron Wing) was equipped with over- tests conducted on various components and sub- sized industrial off-the-shelf actuators (weighing assemblies. Operational tests showed that the 110 lbs.) operating at much lower hydraulic pres- actuation force and strain gauge measurements sure.
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