A Feasibility Study To Control Airfoil Shape

Summary The objective of this study was to assess the capabil-ities of a new piezoelectric actuator to alter the upper sur-face geometry of a subscale airfoil to enhanceperformance. This new piezoelectric actuator called thin-layer composite-unimorph ferroelectric driver and sensor(THUNDER), recently developed at Langley ResearchCenter, is manufactured to deform out of plane whenunder an applied voltage and, to date, has exhibited muchlarger displacements than other piezoelectric actuators. Itwas anticipated that attaching a THUNDER wafer to theupper surface of a small airfoil and actuating it toincrease the camber of that surface when the airfoil wasat positive angles of attack (above 2°) would extend theregion of attached flow across the upper surface. Twocommon characteristics of all piezoelectric actuators,creep and hysteresis, however, pose challenges whenTHUNDER is used for airfoil shaping or other position-ing applications.For this study, a subscale airfoil model wasdesigned, fabricated, and tested under two-dimensionalflow conditions in a small tabletop wind tunnel. Sixtytest conditions, consisting of combinations of five anglesof attack, four direct current (dc) applied voltages, andthree tunnel velocities, were studied. Results indicatedthat displacements of the upper surface of the airfoil wereaffected by the magnitude of the applied voltage, the tun-nel velocity, the airfoil angle of attack, and the creep andhysteresis of the THUNDER wafer. Larger magnitudesof applied voltage produced larger wafer displacements.Wind-off wafer displacements were consistently largerthan corresponding wind-on displacements; however,higher velocities produced larger displacements thanlower velocities because of increased upper surface suc-tion. Larger displacements were also recorded at higherangles of attack because of increased upper surface suc-tion. Creep and hysteresis of the wafer were identified ateach test condition and contributed to larger negative dis-placements for all negative applied-voltage conditionsand larger positive displacements for the smaller,positive applied-voltage (+102 V) condition. An elasticmembrane used to hold the wafer onto the upper surfacehindered displacements at the larger magnitude positiveapplied voltage (+170 V). Both creep and hysteresis ofthe THUNDER wafer appeared bounded, based on theanalysis of several displacement cycles. These resultsshow that THUNDER can be used to alter the camber ofa small airfoil under aerodynamic loads. Feedback con-trol techniques may be useful in reducing the effects ofcreep and hysteresis.IntroductionChanging the local flow field around an airfoil toenhance overall aircraft performance has always been agoal of aircraft designers. Historically, aircraft wingshave been designed for a single flight condition and thenmodified to work for other flight conditions through theuse of conventional control surfaces (such as ailerons andflaps), spoilers, and variable wing sweep. Variable wingsweep affects changes in the local flow field by alteringthe flow velocity perpendicular to the leading edge of thewing. The conventional control surfaces and spoilersaffect changes in the flow field by directly varying thecamber on certain regions of the wing, thereby causingchanges in the baseline structural and aerodynamic char-acteristics of the entire wing. By developing a databasethat relates wing sweep or a commanded aileron/flap/spoiler deflection combination to a corresponding wingperformance, overall aircraft performance parameters,such as lift-to-drag ratio and structural loading, may betailored for the different flight conditions required.During the past decade, many researchers have alsostarted to look at adaptive material actuator systems forperformance-enhancing shape control. Like the conven-tional control surfaces, these actuator systems (in thisparticular application) are designed to alter local wingshape (through camber and/or twist) to produce favorablestructural 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 arestill in the development stage.Adaptive Wing ConceptsIncorporation of leading- and trailing-edge controlsurfaces on aircraft was one of the first successful inno-vations in wing design following the first heavier-than-air flight in 1903. Common on aircraft since the 1920's,these camber-varying devices have been used primarilyto improve low-speed performance during takeoffs andlandings and to provide trim and maneuvering capabilityduring flight. Attempts to utilize such devices for broaderadaptive-camber-control purposes have also been mademany times during this century. In 1916, the SopwithBaby incorporated trailing-edge flaps that automaticallydeflected at lower speeds and decambered at higherspeeds via a connection to restraining bungee cords.Between 1919 and 1926, Dayton Wright Aircraft andArmy Air Services Engineering developed and flew air-craft that similarly incorporated mechanically activatedadaptive wing concepts. In 1933 and 1934, the WestlandLysander was outfitted with independent inboard andoutboard cross-connected slats that were interconnectedwith trailing-edge flaps. This concept provided low-speed maneuvering by means of an adaptive wing thatautomatically varied deflection with angle of attack. Alsoin the 1930's, sailplanes began to regularly incorporatemanually controlled, camber-varying trailing-edge flaps