This paper discusses the design, manufacturing and test of a new kind of adaptive airfoil with Shape Memory Alloy actuation. An antagonistic arrangement of SMA wires is used in a Post-buckled Precompressed (PBP) kind of actuator that is being employed in an adaptive flap system. SMA actuators are typically used either antagonistically and/or arranged to move structural components with linearly varying resistance levels, like springs. This generally means that large percentages of strain energy are spent doing work on passive structure (rather than performing the task at hand, like moving a flight control surface or resisting air loads etc.). Post-Buckled Precompressed (PBP) actuators on the other hand are arranged so that the active elements do not waste energy fighting passive structural stiffnesses. Most (if not all) of the PBP actuators of the past have used piezoceramic elements and are highly prone to tensile failure on convex faces. Because SMA actuators are far more tolerant of tensile stresses than piezoceramics, a switch of actuator type is a natural progression of technology. With the Post-buckled Precompressed mechanism, the power consumption to hold deflections is reduced by one if not two orders of magnitude. Because aircraft often require flight control surfaces to be held in a given position for extremely long times to trim the vehicle, conventional SMA’s are essentially non-starters for many classes of aircraft. For the reason that PBP actuators balance out air and structural loads, the steady-state load on the SMAs is essentially negligible, when properly designed. Experiments showed that the SMAPBP actuator shows tip rotations on the order of 45° which is nearly triple the levels achieved by piezoelectric PBP actuators. The paper opens with a short survey on the history of flap systems actuated by adaptive materials and delves into actuation theory. In the following the author gives a detailed explanation of the design concept and the manufacturing of the airfoil. A NACA0012 airfoil with a chord length of 150 mm was used to prove the concept of the adaptive flap system. The paper continues with a description of the test setup, the CFD model assumptions and the results of wind tunnel tests. The architecture and the employment of a closed loop position feedback system to overcome the nonlinear behavior of the SMA material and the PBP mechanism is also discussed.
Design, Manufacturing and Test of a High Lift Secondary Flight Control Surface with Shape Memory Alloy Post-Buckled Precompressed Actuators